Diagnostic Procedures in Ophthalmology 2nd Edition
Diagnostic Procedures in OPHTHALMOLOGY Diagnostic Procedures in OPHTHALMOLOGY SECOND EDITION HV Nema Former Profes
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Diagnostic Procedures in
OPHTHALMOLOGY SECOND EDITION
HV Nema
Former Professor and Head Department of Ophthalmology Institute of Medical Sciences Banaras Hindu University Varanasi, Uttar Pradesh, India
Nitin Nema
MS Dip NB
Assistant Professor Department of Ophthalmology Sri Aurobindo Institute of Medical Sciences Indore, Madhya Pradesh, India
JAYPEE BROTHERS MEDICAL PUBLISHERS (P) LTD New Delhi • Ahmedabad • Bengaluru • Chennai • Hyderabad Kochi • Kolkata • Lucknow • Mumbai • Nagpur • St Louis (USA)
Published by Jitendar P Vij Jaypee Brothers Medical Publishers (P) Ltd Corporate Office 4838/24 Ansari Road, Daryaganj, New Delhi - 110 002, India, +91-11-43574357 (30 lines) Registered Office B-3 EMCA House, 23/23B Ansari Road, Daryaganj, New Delhi 110 002, India Phones: +91-11-23272143, +91-11-23272703, +91-11-23282021, +91-11-23245672, Rel: +91-11-32558559 Fax: +91-11-23276490, +91-11-23245683 e-mail: [email protected], Website: www.jaypeebrothers.com Branches • 2/B, Akruti Society, Jodhpur Gam Road Satellite Ahmedabad 380 015 Phones: +91-79-26926233, Rel: +91-79-32988717 Fax: +91-79-26927094 e-mail: [email protected] • 202 Batavia Chambers, 8 Kumara Krupa Road, Kumara Park East Bengaluru 560 001 Phones: +91-80-22285971, +91-80-22382956, +91-80-22372664 Rel: +91-80-32714073, Fax: +91-80-22281761 e-mail: [email protected] • 282 IIIrd Floor, Khaleel Shirazi Estate, Fountain Plaza, Pantheon Road Chennai 600 008 Phones: +91-44-28193265, +91-44-28194897, Rel: +91-44-32972089 Fax: +91-44-28193231 e-mail: [email protected] • 4-2-1067/1-3, 1st Floor, Balaji Building, Ramkote Cross Road Hyderabad 500 095 Phones: +91-40-66610020, +91-40-24758498, Rel:+91-40-32940929 Fax:+91-40-24758499 e-mail: [email protected] • No. 41/3098, B & B1, Kuruvi Building, St. Vincent Road Kochi 682 018, Kerala Phones: +91-484-4036109, +91-484-2395739, +91-484-2395740 e-mail: [email protected] • 1-A Indian Mirror Street, Wellington Square Kolkata 700 013 Phones: +91-33-22651926, +91-33-22276404, +91-33-22276415 Rel: +91-33-32901926, Fax: +91-33-22656075, e-mail: [email protected] • Lekhraj Market III, B-2, Sector-4, Faizabad Road, Indira Nagar Lucknow 226 016 Phones: +91-522-3040553, +91-522-3040554 e-mail: [email protected] • 106 Amit Industrial Estate, 61 Dr SS Rao Road, Near MGM Hospital, Parel Mumbai 400012 Phones: +91-22-24124863, +91-22-24104532, Rel: +91-22-32926896 Fax: +91-22-24160828 e-mail: [email protected] • "KAMALPUSHPA" 38, Reshimbag, Opp. Mohota Science College, Umred Road Nagpur 440 009 (MS) Phone: Rel: +91-712-3245220, Fax: +91-712-2704275 e-mail: [email protected] USA Office 1745, Pheasant Run Drive, Maryland Heights (Missouri), MO 63043, USA, Ph: 001-636-6279734 e-mail: [email protected], [email protected] Diagnostic Procedures in Ophthalmology © 2009, HV Nema, Nitin Nema All rights reserved. No part of this publication should be reproduced, stored in a retrieval system, or transmitted in any form or by any means: electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the editors and the publisher. This book has been published in good faith that the material provided by contributors is original. Every effort is made to ensure accuracy of material, but the publisher, printer and editors will not be held responsible for any inadvertent error(s). In case of any dispute, all legal matters to be settled under Delhi jurisdiction only. First Edition: 2002 Second Edition: 2009 ISBN 978-81-8448-595-0 Typeset at JPBMP typesetting unit Printed at Replika Press
Contributors Jorge L Alió
Director, Vissum Institute of Ophthalmology of Alicante Alicante, Spain
Sonal Ambatkar
Glaucoma Service Aravind Eye Hospital Tirunelveli, Tamil Nadu, India
Francisco Arnalich
Sreedharan Athmanathan
MD, DNB
Virologist LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India MD
Professor Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Tinku Bali
Consultant Department of Ophthalmology Sir Ganga Ram Hospital, New Delhi, India
Rituraj Baruah
Senior Registrar Lady Hardinge Medical College New Delhi, India
Jyotirmay Biswas
Ex-Fellow Sankara Nethralaya Chennai, Tamil Nadu, India
Taraprasad Das
Director LV Prasad Eye Institute Bhubaneswar, Orissa, India
Vissum Institute of Ophthalmology of Alicante Alicante, Spain
Mandeep S Bajaj
Surbhit Chaudhary
MS, FAMS
Munish Dhawan
Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Lingam Gopal
MS, FRCS
Chairman Medical Research Foundation Sankara Nethralaya, Chennai Tamil Nadu, India
AK Grover
MD, FRCS
Chairman Department of Ophthalmology Sir Ganga Ram Hospital New Delhi, India
Roshmi Gupta
Consultant, Narayana Nethralaya Bengaluru, Karnataka, India
Sanjiv Gupta
Head, Ocular, Pathology and Uveitis Sankara Nethralaya, Chennai Tamil Nadu, India
Stephen C Hilton
Ambar Chakravarty
Santosh G Honavar
MS, FRCP
Honorary Professor and Head Department of Neurology Vivekananda Institute of Medical Sciences Kolkata, West Bengal, India
West Virginia University Morgantown, USA MD, FACS
Director Department of Ophthalmic Plastic Surgery and Ocular Oncology, LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Diagnostic Procedures in Ophthalmology Anjali Hussain
Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Subhadra Jalali
Head Smt Kanuri Santhamma Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Sadao Kanagami
FOPS
Professor Kitasato University School of Medicine Teikyo, Japan
Sangmitra Kanungo
Shahnawaz Kazi
Fellow Sankara Nethralaya Chennai, Tamil Nadu, India
R Kim
Head Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology Madurai, Tamil Nadu, India
Parmod Kumar
Glaucoma Imaging Centre New Delhi, India
S Manoj
Consultant Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology, Madurai, Tamil Nadu, India
S Meenakshi
Amit Nagpal
Consultant Pediatric Ophthalmology Sankara Nethralaya Chennai, Tamil Nadu, India Consultant Sankara Nethralaya, Chennai Tamil Nadu, India
A Narayanaswamy
Consultant Sankara Nethralaya Chennai, Tamil Nadu, India
Rajiv Nath
Professor Department of Ophthalmology KG Medical University Lucknow, Uttar Pradesh, India
Tomohiro Otani
Professor Department of Ophthalmology Gunma University School of Medicine Maebashi, Japan
Nikhil Pal
Senior Resident Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Rajul Parikh
Consultant, Sankara Nethralaya Chennai, Tamil Nadu, India
David Piñero
K Kalyani Prasad
Consultant Krishna Institute of Medical Sciences Hyderabad, Andhra Pradesh, India
Leela V Raju
Monongalia Eye Clinic Morgantown, USA
VK Raju
MD, FRCS, FACS
Clinical Professor Department of Ophthalmology West Virginia University Morgantown, USA
LS Mohan Ram
D Opt, BS
LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Contributors R Ramakrishnan
Professor and CMO Aravind Eye Hospital Tirunelveli, Tamil Nadu, India
Manotosh Ray MD, FRCS
Associate Consultant National University Hospital Singapore
Pukhraj Rishi
Monica Saha
Department of Ophthalmology KG Medical University Lucknow, Uttar Pradesh, India
Chandra Sekhar
Director LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Harinder Singh Sethi MD, DNB, FRCS
Senior Research Associate Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Yog Raj Sharma
Deependra Vikram Singh
Devindra Sood
Consultant, Glaucoma Imaging Centre New Delhi, India
MS Sridhar
S Sudharshan
Kallakuri Sumasri
B Optm
Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
T Surendran
MS, M Phil
Professor Dr RP Centre for Medical Sciences AIIMS, New Delhi, India
Vice Chairman and Director Pediatric Ophthalmology Sankara Nethralaya Chennai, Tamil Nadu, India
Rajani Sharma MD (Ped)
Garima Tyagi B Opt
Savitri Sharma
Vasumathy Vedantham
Senior Resident Department of Pediatrics AIIMS, New Delhi, India MD
Head Jhaveri Microbiological Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Tarun Sharma MD, FRCS
Director Retina Service, Sankara Nethralaya Chennai, Tamil Nadu, India
Senior Resident Dr RP Centre for Ophthalmic Sciences, AIIMS New Delhi, India
Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India MS, DNB, FRCS
Consultant, Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology Madurai, Tamil Nadu, India
L Vijaya
Head Glaucoma, Sankara Nethralaya Chennai, Tamil Nadu, India
Preface to the Second Edition The goal of this second edition of Diagnostic Procedures in Ophthalmology remains the same as that of the first—to provide the practicing ophthalmologists with a concise and comprehensive text on common diagnostic procedures which help in the correct and speedy diagnosis of eye diseases. Like other disciplines of medicine, the knowledge of ophthalmology continues to expand and a number of newer and sophisticated investigative procedures have been introduced recently. Extensive and detailed information on recent diagnostic approaches is available in resource textbooks or online to ophthalmologists. To search these is time consuming, tiring and at times not practical in a busy clinical practice set-up. Therefore, this ready reckoner has been conceptualized. The book covers most of the basic and well-established diagnostic procedures in ophthalmology. It starts with visual acuity and describes color vision and color blindness, slit-lamp examination, tonometry, gonioscopy, evaluation of optic nerve head in glaucoma, perimetry, ophthalmoscopy and ophthalmic photography. Most of these procedures are considered basic and carried out routinely but to obtain an evidence-based diagnosis, a correct procedure for the examination must be followed. Corneal topography is very useful in detection of corneal pathologies such as early keratoconus, pellucid marginal corneal degeneration, corneal dystrophies, etc. It guides the ophthalmic surgeon to plan appropriate refractive surgery. Recent development in the application of wavefront technology can reduce different types of optical aberrations and may provide supervision and improve results of the LASIK surgery. A new chapter on Confocal Microscopy is included. Confocal microscopy, a noninvasive procedure, allows in vivo observation of normal and pathogenic corneal microstructure at a cellular level. It can identify subclinical corneal abnormalities. Procedures like Fundus Fluorescein Angiography and Indocyanine Green Angiography are invaluable diagnostic tools. They are not only useful in the diagnosis, documentation and followup but also in monitoring the management of the posterior segment eye diseases. With the development of high quality fundus camera and digital imaging, utility of both techniques has significantly increased. Ultrasonography, as a diagnostic procedure, has immense importance in the modern ophthalmology. Both A-scan and B-scan ultrasonography are dynamic procedures wherein diagnosis is made during examination in correlation with clinical features. Three-dimensional ultrasound tomography allows improved visualization and detection of small ophthalmic lesions. Ultrasound biomicroscopy is a method of high frequency ultrasound imaging used for evaluating the structural abnormality and pathology of the anterior segment of the eye both qualitatively and quantitatively. It is very helpful in understanding the pathomechanism of various types of glaucoma.
Diagnostic Procedures in Ophthalmology Optical Coherence Tomography is a noninvasive, cross-sectional imaging technique which provides objective and quantitative measurements that are reproducible and show very good correlation with clinical picture of retinal pathology especially macula. Presently, OCT is often used in assessment of optic nerve head damage in glaucoma. One must remember that imaging technique alone may not contribute to a correct diagnosis. It is complementary to clinical examination. Therefore, results of imaging should always be interpreted in conjunction with clinical findings and results of other relevant tests. Electrophysiological tests are often ordered to assess the functional integrity of the visual pathway and in evaluating the cause of visual impairment in children. Multifocal ERG and multifocal VEP are newer techniques still under evaluation. It is claimed that multifocal ERG can distinguish between the lesions of the outer retina and the ganglion cells or optic nerve. Results of electrophysiological tests should never be analyzed in isolation but always be correlated with clinical findings to establish a definitive diagnosis. Etiological diagnosis of infectious keratitis and uveitis has been more vexing and often fraught with pitfalls. Collection of samples from eye, their microbiological work-up and interpretation of laboratory results have been described in chapters on keratitis and uveitis. Role of optical coherence tomography in the diagnosis and management of complications of uveitis is also discussed. A number of new chapters such as: Retinopathy of Prematurity, Localization of Intraocular Foreign Body, Comitant Strabismus, Incomitant Strabismus, Dry Eye, Epiphora, Proptosis and Neurological Disorders of Pupil have been added in the second edition of the book. Retinopathy of prematurity is one of the important causes of childhood blindness. Risk factors, documentation, staging, classification, screening procedure and management of the disease are briefly described. Precise localization of intraocular foreign body is a tedious procedure but is critically important for its removal and management. Computerized tomography and magnetic resonance imaging have replaced old cumbersome radiological methods for localization of intraocular foreign bodies, metallic and wooden. Strabismus often has an adverse effect on psychological functioning, personality trait and working capabilities of an individual. Patients with strabismus suffer from low self-esteem and have problem in social interaction. Therefore, early correction of strabismus is necessary for improving the quality of the life of the patient. The chapter on comitant strabismus presents various methods for examination and measurement of deviations. Incomitant strabismus, though less common, is more troublesome. It usually results from cranial nerves (III,IV,VI) paralysis. Restrictive strabismus may be associated with interesting clinical ocular syndromes. Dry eye is one of the most common external ocular diseases seen by ophthalmologists. Prevalence of dry eye is on rise mainly due to an environmental pollution, change in lifestyle and increase in aging population. Should dry eye be considered a disorder of tear film and excessive tear evaporation or a localized immune-mediated inflammatory response of ocular surface? Besides the controversy, what is more important is an early diagnosis of dry eye and its proper management.
Preface to the Second Edition Epiphora is an annoying symptom. It may occur either in infants or adults. An understanding of anatomy and physiology of the lacrimal apparatus is necessary for the evaluation of epiphora. A number of invasive and noninvasive tests are available to investigate patients with epiphora and localize site of obstruction in the lacrimal passage. Proptosis has a varied etiology. It may occur due to ocular, orbital and systemic causes. Generally, proptosis requires interdisciplinary cooperation amongst ophthalmologists, neurologists, oncologists, ENT surgeons, internists and radiologists. Investigation of patients with proptosis should begin with simple standard noninvasive techniques and, if necessary, progress to more elaborate and invasive procedures. Ultrasonography, CT and MRI are of immense value in the diagnosis. Examination of pupil (size, shape and pupillary reactions) is essential in neurological disorders. Typical pupillary signs can help in localizing lesions in the nervous system. Characteristics of Adie tonic pupil and Argyll-Robertson pupil and a detailed evaluation of the third cranial nerve palsy are described in the last chapter. Most of the contributors who have vast experience in their respective fields have written chapters for this book. To make the reader familiar, they have not only described diagnostic procedures but also given characteristic findings of eye disorders with the help of illustrations. The book has expanded greatly as many new chapters with numerous illustrations are added. We hope the book should be of great help to the practicing ophthalmologists and clinical residents providing a practical resource to investigative procedures in ophthalmology. HV Nema Nitin Nema
Preface to the First Edition The word diagnosis comes from a Greek word meaning to distinguish or discern. Besides history and clinical examination of the patient, diagnostic tests are required to aid in making correct diagnosis of eye diseases. The role of diagnostic technology is not inferior to that of a clinician's acumen. A correct diagnostic report helps in differentiating functional from organic and idiopathic from non-idiopathic diseases. The number of diagnostic tests available to an ophthalmologist has increased significantly in the last two decades. Both selective and non-selective tests are presently used for the clinical and research purposes. Non-selective approach to testing is costly and does not provide useful information. In order to be useful, diagnostic tests have to be properly performed, accurately read, and correctly interpreted. The ordering oculist should always compare the results of test with the clinical features of the eye disease. The main aim of the book—Diagnostic Procedures in Ophthalmology is to provide useful information on diagnostic tests, which an ophthalmologist intends to perform or order during his clinical practice. Some of the procedures described in the book, assessment of visual acuity, slit lamp examination, tonometry, gonioscopy, perimetry and ophthalmoscopy, are routine examinations. However, the technique of proper examination and interpretation of findings to arrive at a correct diagnosis must be known to the practising ophthalmologist or optometrist. Procedures like ophthalmic photography, evaluation of optic nerve head, fundus fluorescein angiography and indocyanine green angiography are invaluable because they not only help in the diagnosis and documentation but also help in monitoring the management of eye disease. Corneal topography gives useful data about corneal surface and curvature and contributes to the success of Lasik surgery to a great extent. The role of A-scan ultrasonography in the measurement of axial length of the eye and biometry cannot be over emphasised. B-scan ultrasonography is needed to explore the posterior segment of the eye when media are opaque or an orbital mass is suspected. Ultrasound biomicroscopy (UBM) and Optical coherence tomography (OCT) are relatively new non-invasive tools to screen the eye at the microscopic level. UBM helps in understanding the pathogenesis of various forms of glaucoma and their management. OCT obtains a tomograph of the retina showing its microstructure incredibly similar to a histological section. It helps in the diagnosis and management of the macular and retinal diseases. Electrophysiological tests allow objective evaluation of visual system. They are used in determination of visual acuity in infants and in the diagnosis of the macular and optic nerve disorders. What diagnostic tests should be ordered in the evaluation of the patients with infective keratitis or uveitis? Chapters on Diagnostic Procedures in Infective Keratitis and Diagnostic Procedures in Uveitis provide an answer. The experts who have credibility in their fields have contributed chapters to the book. Not only the procedures of diagnostic tests are described but to make the reader conversant, characteristic findings in the normal and the diseased eye are also highlighted with the help of illustrations. The book should be of great help to the practising ophthalmologists, resident ophthalmologists, optometrists and technicians as it provides instant access to the diagnostic procedures in ophthalmology. We are indebted to all contributors for their excellent contributions in short time in spite of their busy schedule. Mr JP Vij deserves our sincere thanks for nice publication of the book. HV Nema Nitin Nema
Acknowledgements The publication of the second edition of Diagnostic Procedures in Ophthalmology is possible with the help and cooperation of many colleagues and friends. We wish to express our gratitude to all the contributing authors for their time and painstaking efforts not only for writing the comprehensive and well illustrative chapters but also updating and revising them to conform the format of the book. We are indebted to Prof JL Alió, Dr Vasumathy Vadantham and Dr Tarun Sharma for contributing chapters on a short notice because the initial contributors failed to submit their chapters. Our grateful thanks go to Dr Mahipal Sachdev for persuading Dr Manotosh Ray to write a chapter on Confocal Microscopy. Mrs Pratibha Nema deserves our deep appreciation; without her patience, tolerance and understanding, this book would not have become reality. Finally, Shri Jitendar P Vij (Chairman and Managing Director), Mr Tarun Duneja (DirectorPublishing) and supporting staff of M/s Jaypee Brothers Medical Publishers (P) Ltd, New Delhi especially deserve our sincere thanks for their cooperation and keen interest in the publication of this book. HV Nema Nitin Nema
Contents 1. Visual Acuity ..................................................................................................................... 1 Stephen C Hilton, Leela V Raju, VK Raju
2. Color Vision and Color Blindness ........................................................................... 12 Harinder Singh Sethi
3. Slit-lamp Examination ................................................................................................... 33 Harinder Singh Sethi, Munish Dhawan
4. Corneal Topography ....................................................................................................... 46 Francisco Arnalich, David Piñero, Jorge L Alió
5. Confocal Microscopy ...................................................................................................... 84 Manotosh Ray
6. Tonometry .......................................................................................................................... 95 R Ramakrishnan, Sonal Ambatkar
7. Gonioscopy ...................................................................................................................... 106 A Narayanaswamy, L Vijaya
8. Optic Disk Assessment in Glaucoma ................................................................... 115 Rajul Parikh, Chandra Sekhar
9. Basic Perimetry .............................................................................................................. 128 Devindra Sood, Parmod Kumar
10. Ophthalmoscopy ............................................................................................................. 151 Pukhraj Rishi, Tarun Sharma
11. Ophthalmic Photography ............................................................................................ 165 Sadao Kanagami
12. Fluorescein Angiography ............................................................................................ 181 R Kim, S Manoj
13. Indocyanine Green Angiography ............................................................................ 200 Vasumathy Vedantham
14. A-scan Ultrasonography .............................................................................................. 216 Rajiv Nath, Tinku Bali, Monica Saha
15. B-scan Ultrasonography .............................................................................................. 239 Taraprasad Das, Vasumathy Vedantham, Anjali Hussain Sangmitra Kanungo, LS Mohan Ram
Diagnostic Procedures in Ophthalmology 16. Ultrasound Biomicroscopy in Ophthalmology .................................................... 259 Roshmi Gupta, K Kalyani Prasad, LS Mohan Ram, Santosh G Honavar
17. Optical Coherence Tomography .............................................................................. 269 Tomohiro Otani
18. Electrophysiological Tests for Visual Function Assessment ........................ 279 Subhadra Jalali, LS Mohan Ram, Garima Tyagi, Kallakuri Sumasri
19. Diagnostic Procedures in Infectious Keratitis ................................................... 316 Savitri Sharma, Sreedharan Athmanathan
20. Diagnostic Procedures in Uveitis ........................................................................... 333 Jyotirmay Biswas, Surbhit Chaudhary, S Sudharshan, Shahnawaz Kazi
21. Retinopathy of Prematurity: Diagnostic Procedures and Management .... 353 Yog Raj Sharma, Deependra Vikram Singh, Nikhil Pal, Rajani Sharma
22. Localization of Intraocular Foreign Body ............................................................ 362 Amit Nagpal, Lingam Gopal
23. Comitant Strabismus: Diagnostic Methods ......................................................... 369 Harinder Singh Sethi, Pradeep Sharma
24. Incomitant Strabismus ................................................................................................. 395 S Meenakshi, T Surendran
25. Diagnostic Procedures in Dry Eyes Syndrome .................................................. 405 MS Sridhar
26. Evaluation of Epiphora ............................................................................................... 412 AK Grover, Rituraj Baruah
27. Diagnostic Techniques in Proptosis ...................................................................... 426 Mandeep S Bajaj, Sanjiv Gupta
28. Neurological Disorders of Pupil ............................................................................. 441 Ambar Chakravarty
Index .................................................................................................................................................. 461
STEPHEN C HILTON, LEELA V RAJU, VK RAJU
Vision is the most important of all senses. Approximately 80% of the information from the outside world is incorporated through the visual pathway. Loss of vision has a profound effect on the quality of life. The process of vision includes: 1. Central resolution (visual acuity) 2. Minimal light sensitivity 3. Contrast sensitivity 4. Detection of motion 5. Color perception 6. Color contrast 7. Peripheral vision (spatial, temporal and motion detection). In the normal clinical settings, we measure only one of these functions – central resolution at high contrast (visual acuity).1
Definition and Terminology of Visual Acuity The most basic form of visual perception is detection of light. Visual acuity is more than just detecting light. It is the measurement of the ability to discriminate two stimuli separated in space at high contrast compared with the background. The minimal angle of resolution that allows a
human optic system to identify two points as different stimuli is defined as the threshold of resolution. Visual acuity is the reciprocal of the threshold of resolution.2 Clinically, discriminating letters in a chart determine this, but this task also requires recognition of the form and shape of the letters, which are processes that also involve higher centers of visual perception. Discrimination at a retinal level may, therefore, be determined by less complex stimuli, such as contrast sensitivity gratings. Theoretically, the maximum resolving power of the human retina could be derived from an estimate of the angle of approximately 20 seconds of arc because this represents the smallest unit distance between two individually stimulated cones. Thus the resolving power of the eye could be much greater than what is measured by visual acuity charts.3 Cones have the highest discriminatory capacity, but rods can also achieve some resolution. The greater the distance from the fovea the level of visual acuity falls off rapidly. At a 5° distance from the foveal center, visual acuity is only one quarter of foveal acuity.4 Luminance of test object, optical aberrations of the eye and the degree of adaptation of the observer also influence the visual acuity.5
Diagnostic Procedures in Ophthalmology Visual thresholds can be broadly classified into three groups: 1. Light discrimination (minimum visible, minimum perceptible) 2. Spatial discrimination (minimum separable, minimum discriminable) 3. Temporal discrimination (perception of transient visual phenomena such as flickering stimuli). Many clinical tests can assess many visual functions simultaneously. In a healthy observer in best focus, the resolution limit, or as it is usually called, the minimum angle of resolution (MAR), is between 30 seconds of arc and one minute of arc. Clinically, we use Landolt C and Snellen E to assess visual acuity. The minimum discriminable hyper-acuity or vernier-acuity is another example of spatial discrimination. The eye is capable of subtle discrimination in spatial localization, and can detect misalignment of two line segments in a frontal plane if these segments are separated by as little as three to five seconds of arc, considerably less than the diameter of a single foveal cone. The mechanism subserving hyper-acuity is still being investigated.
Charts and Scales to Record Visual Acuity The function of the eye may be evaluated by a number of tests. The cone function of the fovea centralis is assessed mainly by measurement of the form sense, the ability to distinguish the shape of objects. This is designated as central visual acuity. It is measured for both near and far, with and without the best possible correction of any refractive error present. Because only cones are effective in color vision and because they are concentrated in the fovea, the measurement of the ability to recognize colors is also a measurement of foveal function. The function of the peripheral retina which
contains mainly rods, may be assessed by peripheral visual field.1 Visual acuity is the first test performed after obtaining a careful history. Measurement of the central visual acuity is essentially an assessment of the function of the fovea centralis. An object must be presented so that each portion of it is separated by a definite interval. Customarily, this interval has become one minute of an arc, and the test object is one that subtends an angle of five minutes of an arc. A variety of test objects has been constructed on this principle, so that an angle of five minutes is at distances varying from a few inches to many feet5 (Figs 1.1 and 1.2). The most familiar examination chart is Snellen chart (Fig. 1.3). Conventionally, reading vision is examined at 40 cm (16 inches). The testing distance of a preferred near distance chart
Fig. 1.1: Snellen letters subtend one minute of arc in each section, the entire letter subtends five minutes of arc
Fig. 1.2: Each component of Snellen letters subtend one minute of visual angle the entire letter subtends five minutes of visual angle at stated distance
Fig. 1.4: ETDRS chart
Fig. 1.3: Snellen chart
should be observed accurately. The Snellen notation is simply an equivalent reduction for near, maintaining the same visual angle. Most of the Snellen-based distance acuity charts are also commercially available as 'pocket' charts to check the near acuity at a preferred distance for every patient or at a defined distance for clinical trial purposes including ETDRS (Fig. 1.4) and Snellen letter "E". The Jaeger notation is a historic enigma and Jaeger never committed himself to the distance at which the print should be used. The numbers on the Jaeger chart simply refer to the numbers on the boxes in the print shop from which Jaeger
selected his type sizes in 1854. They have no biologic or optical foundation. Clinically, Jaeger's charts (Fig. 1.5) are widely used. Central visual acuity is designated by two numbers. The numerator indicates the distance between the test object and the patient; the denominator indicates the distance at which the test object subtends an angle of five minutes. In the United States these numbers are given in inches or feet, whereas in the Europe the designation is in meters. The test chart commonly used in the United States has its largest test object one that subtends an angle of five minutes at a distance of 200 feet (6 m). Then there are test objects of 100, 80, 70, 60, 50, 40, 30, 20 and 15 feet. If the individual is unable to recognize the largest test object, then he or she should be brought closer to it, and the distance at which he or she recognizes it should be recorded. Thus, if the individual recognizes the test object that subtends a five minute angle at 200 feet when he or she is at 12 feet, the visual acuity is recorded as 12/200. This is not a fraction but indicates two physical
Diagnostic Procedures in Ophthalmology
Fig. 1.5A: Jaeger's type near vision chart
Fig. 1.5B Fig. 1.5B: Near vision chart: Music type and numericals
Fig. 1.6: Broken C, letter E and pictures of familiar objects for testing visual acuity in illiterates and children
measurements, the test distance and the size of the test object. The most familiar test objects are letters or numbers. Such tests have the disadvantage of requiring some literacy on the part of the patient. Additionally, there is a variation in their ability to be recognized. "L" is considered the easiest letter in the alphabet to read and "B" is considered the most difficult. To obviate this difficulty, broken rings (Fig. 1.6) have been devised in which the break in the ring subtends one minute angle, and the ring subtends a five minute angle. Similarly, the letter "E" may be arranged so that it faces in different directions (Fig. 1.6). These test objects are easier to see than letters, eliminate some of the difficulties inherent in reading, and
can be used in the testing of illiterates and persons not familiar with the English alphabet. A variety of pictures (Fig. 1.6) have also been designed for testing the visual acuity of children. When a person is unable to read even a top letter, he or she is asked to move toward the chart or a chart can be brought closer. The maximum distance from which he or she recognizes the top letter is noted as the nominator. When visual acuity is less than 1/60, the patient is asked to count fingers from close at hand (CF at 20 cm). When a patient cannot even count fingers, the patient is asked if he or she can see examiner's hand movements (HM positive). When hand movements are not seen we have to record whether the perception of light (LP) is present or absent by asking the patient if he or she sees the light. Standard illumination should be used for the acuity chart (10 to 20 foot candles for wall charts). When a patient is examined with the Snellen chart in a dark room, the subject sees a high contrast and glare-free target. But in real circumstances, contrast and glare reduce visual acuity, and even more so in a pathological conditions. The contrast sensitivity function of a subject may be affected even when Snellen acuity is normal. The contrast sensitivity tests are more accurate in quantifying the loss of vision in cases of cataracts, corneal edema, neuroophthalmic diseases, and retinal disorders. A patient with a low contrast threshold has a high degree of sensitivity; therefore, a healthy young subject may have a threshold of 1%, and a contrast sensitivity of 100% (inversely proportional). It is important to have adequate lighting when testing visual acuity so that it does not become a test of contrast sensitivity.
Factors Affecting Visual Acuity Factors affecting visual acuity may be classified as physical, physiological and psychological.
Visual Acuity Uncorrected refractive error is a common cause of poor acuity. Physical factors include illumination and contrast. Increased illumination increases visual acuity from threshold to a point at which no further improvement can be elicited. In the clinical situation this is 5-20 foot candles. When contrast is reduced more illumination is required to resolve an object. Beyond a certain point, illumination can create glare. Therefore, visual acuity is recorded under photopic condition and one wants to evaluate best visual acuity at the fovea. Physiological conditions include pupil size, accommodation, light-dark adaptation and age.2
Pupil Size The pupil size has great influence on visual acuity. Visual acuity decreases if pupils are smaller than 2 mm due to diffraction. Pupil diameters larger than 3.5 mm increase aberration. Variation in pupil size changes acuity by altering illumination, increasing depth of focus, and modifying the diameter of the blur circle on the retina.
Accommodation An accommodation creates miosis, which could account for small hyperopic prescriptions being rejected for distance viewing in younger individuals. It is worth while to discuss the role of a pinhole in obtaining the best visual acuity in the clinical setting. The optimum pinhole is 2.5 mm in diameter. A pinhole in an occluder (Fig. 1.7) may be introduced in a trial frame with the opposite eye occluded. Single pinhole device is not adequate. The patient must be able to find a hole, therefore, multiple pinholes are preferred. If the patient is older or infirm, or has tremors, he is asked to read only a single letter from each line as we proceed down the chart to record the vision.
B Figs 1.7A and B: Occluder with multiple holes
Many patients have been referred for neuro-ophthalmologic consultation because of painless loss of vision in one eye only. The best visual acuity may be 20/60 in the affected eye but when properly tested with the pinhole, the acuity may improve to 20/20. This indicates that the macula and optic nerve are functioning normally. When the patient's vision is improved with pinhole one knows the problem is a refractive one and simply need the change in glasses. If the patient's vision is less when looking through the pinhole; it indicates that the patient has either an organic lesion at macula, or a central scotoma, or functional amblyopia. A patient with 20/400 vision that improves with pinhole to 20/70 indicates that the improvement is refractive, but some pathology may also be present.
Visual Acuity Testing in Young Children Early determination of vision loss and refractive error is an essential component of assessing the infant's ultimate visual development potential. The visual acuity of a newborn as measured by preferential looking is in the range of 30 minutes of arc (20/600); acuity rapidly improves to six minutes of arc (20/120) by three months. A steady but modest improvement to approximately three minutes of arc (20/60) occurs by 12 months of age. One minute of arc (20/20) is usually obtained at the age of three to five years.6 The examination is generally performed on the parent's lap. The room should never be totally darkened because this may provoke anxiety. Objective retinoscopy remains the best method of determining a child's refraction. Other clinical methods involve estimation of fixation and following behavior. A test target should incorporate high contrast edges. For infants younger than six months the best target represents the examiner's face. For the child of six months and older, an interesting toy can be used. After assessment of the binocular fixation pattern, the examiner should direct attention to differences between the two eyes when tested monocularly. Objection to occlusion of one eye may suggest abnormality with the less preferred eye.7 Three common methods are used for determining resolution acuity: 1. Behavioral technique (preferential looking Fig. 1.8) 2. Detecting optokinetic nystagmus (OKN Fig. 1.9) 3. Recording visual evoked potentials (VEP Fig. 1.10). It is desirable to measure the visual acuity of children sometime during their third year to detect strabismic or sensory amblyopia and to recognize the presence of severe refractive errors.
Fig. 1.8: Preferential looking test chart
Fig. 1.9: OKN drum
In this group of preschool children, visual acuity testing is easier to perform with the use of the following charts: 1. Allen and Osterberg charts (Fig. 1.11) 2. Illiterate E chart 3. Landolt broken ring.
Fig. 1.10: VEP testing
Contrast is defined as the ratio of the difference in the luminance of these two adjacent areas to the lower or higher of these luminance values. The amount of contrast a person needs to see a target is called contrast threshold. The contrast sensitivity is assessed by using the contrast sensitivity chart. It has 5-8 different sizes of letters in six or more shades of gray. Some contrast sensitivity charts contain a series of alternating black and white bars; 100 line pairs per mm is equivalent to space of one minute between two black lines. The alternating bar pattern is described as spatial frequency. The contrast sensitivity is measured in units of cycles per degrees (CPD). A cycle is a black bar and white spaces. To convert Snellen units to units of cycles per degree, divide 180 by Snellen denominator. Contrast sensitivity measurements differ from acuity measurements; acuity is a measure of the spatial resolving ability of the visual system under conditions of very high contrast, whereas contrast sensitivity is a measure of the threshold contrast for seeing a target.8
Visual Acuity in Low Vision Patients
Fig. 1.11: Allen and Osterberg chart
Contrast Sensitivity A general definition of spatial contrast is that it is a physical dimension referring to the lightdark transition at a border or an edge of an image that delineates the existence of a pattern or object.
Individual near acuity needs are different among different population groups. For low vision patients these differences are magnified. Two persons with the same severe visual impairment may exhibit marked differences in their ability to cope with the demands of daily living. Visual acuity loss, therefore, is the aspect that must be addressed in individual rehabilitation plans. Colenbrander9 subdivides several components of visual loss into impairment aspects (how the eye functions), visual ability (how the person functions in daily living), and social/economic aspects (how the person functions in society (Table 1.1).
20/12.5 20/16 20/20 20/25 20/32 20/40 20/50 20/63 20/80 20/100 20/125 20/160 20/200 20/250 20/320 20/400 20/500 1.6in 20/630 1.2in 20/800 1in 20/1000 20/1250 1cm 20/1600 1cm 20/2000 1cm
Normal vision
Mild vision loss
Moderate vision loss
Profound vision loss
Near-blindness
Total Blindness
No visual reading must rely on talking books or other
Marginal with aids Uses magnifiers for spot reading, but may prefer talking books for leisure
Slower than normal with reading aids High-power magnifiers (restricted field)
Near-normal with appropriate reading aids Low-power magnifiers and large-print books
Normal reading speed Reduced reading distance No reserve for small
Normal reading speed Normal reading distance Reserve capacity for small print
Vision substitution aids
Vision enhancements aids
In this range, residual vision tends to become unreliable, though it nonvisual sources may still be used as an adjunct to vision substitution skills.
In the EU, many benefits start at this level. The WHO includes this range in its blindness category.
In the United States, persons in this range are considered legally blind and qualify for tax-break disability benefits.
In the United States, children in this range qualify for special educational assistance
Many functional criteria (whether for a driver's license or for cataract surgery) fall within the range
Note that normal adult vision is better than 20/20
Social and economic aspects (how the person functions in society)
(From Colenbrander A. Preservation of vision or prevention of blindness [editorial]? Am J Ophthalmol 2002;133:2. p.264.)
4in 3in 2.5in 2in
10in 8in 6in 5in
25in 20in 16in 12.5in
63in 50in 40in 32in
Newsprint (1 M)
Ranges (ICD-9-CM)
Statistical estimate of reading ability
Visual ability aspects/functional vision (how the person functions-daily living skills)
Impairment aspects (how the eye function)
TABLE 1.1: RANGES AND ASPECTS OF VISION LOSS
10 Diagnostic Procedures in Ophthalmology
Summary Both distance and near visual acuities are recorded for each eye with and without spectacles. Distance visual acuity is recorded at a distance of 20 feet or in a room of at least 10 feet using mirrors and projected charts. Near visual acuity can be recorded using reduced Snellen or equivalent cards at 40 cm. Acuity performance, like any other human performance, is subject to impairment depending on ocular and general health, emotional stress, boredom, and a variety of drugs acting both peripherally and centrally. The examiner must provide encouragement and must have patience. For clinical studies the ETDRS charts are recommended because near vision is often more important in the daily life of older or infirm patients. Reading charts or other near vision testing charts should be used as part of the routine assessment of the visual acuity. Visual acuity measurement is often taken for granted. Many pitfalls make this most important assessment subject to variability.10Ambient illumination, aging bulbs, dirty charts or slides, small pupils, and poorly standardized charts are just
some of the factors that can lead to erroneous results. A little care in ensuring the proper environment for testing can significantly improve accuracy.
References 1. Newell FW. Ophthalmology Principles and Concepts. St Louis, Mosby, 1969. 2. Moses RA (Ed). Adlers Physiology of the Eye. St Louis, Mosby, 1970. 3. Scheie H. Textbook of Ophthalmology. Philadelphia, WB Saunders, 1977. 4. Duane TD. Clinical Ophthalmology. New York, Harper and Row, 1981. 5. Michaels DD. Visual Optics and Refraction. St Louis, Mosby, 1985. 6. Vander J. Ophthalmology Secrets. Hanley and Belfus. 7. Borish I. Clinical Refraction. Professional Publisher, 1970. 8. Owsley C. Contrast Sensitivity. Ophthalmic Clinics of North America 2003;16:173. 9. Colebrander A. Preservation of Vision or Prevention of Blindness? Am J Ophthalmol 2003; 133:263. 10. Kniestedt, Stamper RL. Visual Acuity and its Measurements. Ophthalmic Clinics of North America 2003; 16:155.
HARINDER SINGH SETHI
Color Vision and Color Blindness
Color vision examination is an essential part of screening before a person is taken up for a job. A person who is color vision defective may go through life quite unconscious of his color deficiency and without making any incriminating mistakes, differentiating objects by their size, shape and luminosity, using all the time a complete color vocabulary based on his experience which teaches him that color terms are applied with great consistency to certain objects and to certain achromatic shades, until circumstances are arranged to eliminate these accessory aids and then he realizes that his sensations differ in some way from the normal. Various tests have been developed to enable screening of anomalous subjects with color deficiency from a much larger group of normal subjects.
main characteristics of color namely hue, saturation, and brightness. Hue is a function of wavelength. It depends on what the eye and brain perceive to be the predominant wavelength of the incoming light. An object's "hue" is its "color." Saturation refers to the richness of a hue as compared to a gray of the same brightness. Saturation is also known as "chroma." Brightness correlates to the ease with which a color is seen, other factors being equal. Brightness is a subjective term referring to the sensation produced by a given illuminance on the retina. The spectral wavelengths of different colors are as follows: violet 430 nm; blue 460 nm; green 520 nm; yellow 575 nm; orange 600 nm and red 650 nm. The concept of white light is vague, most agreeable definition is, white surface is one which has spectral reflection factors independent of wavelength (in the visible spectrum) and greater than 70%.
Color Vision Color is a sensation and not a physical attribute of an object. Color is what we see and is result of stimulation of retina by radiant energy in a small band of wavelengths of the electromagnetic spectrum usually considered to span about one octave, from 380 nm to 760 nm. There are three
Factors Affecting Color Vision Crystalline Lens The lens absorbs shorter wavelengths; in young, wavelengths of less than 400 nm and in old people up to 550 or 600 nm are absorbed by
Color Vision and Color Blindness the lens resulting in defective color vision on shorter wavelength side.
Retinal Distribution of Color Vision The center of the fovea (1/8 degree) is blue blind. Trichromatic vision extends 20-30° from the point of fixation. Peripheral to this red-green become indistinguishable up to 70-80° and in far peripheral retina all color sense is lost although cones are still found in this region. In the central 5°, macula contains carotenoid pigment, xanthophyll. The molecules of the pigment are arranged in such a way that they absorb blue light polarized in the radial direction. If one looks at a white card through linear polarizer, one will see two blue sectors separated by two yellow sectors the figure is called Haidinger's brushes. Macular pigment may also be seen as in homogeneity in the field of blue or white light called Maxwell's spot.
Wavelength Discrimination The normal observer is able to detect a difference between two spectral lights that differ by as little as 1 nm in wavelength in the regions of 490 nm and 585 nm. In the region of violet and red a difference of greater than 4 nm is necessary.
Hue, Saturation and Lightness Hue is the extent to which the object is red, green, blue or yellow. Saturation is the extent to which a color is strong or weak. Lightness is self explanatory attribute, for example, yellow by color is light.
Illumination Illumination affects color vision of low illuminances, the errors increase due to poorer discrimination for most of the hue range while
testing color vision. An illuminance of 400 lux (± 100 lux) would be practical value for most clinical applications.
Bezold-Burcke Effect von Bezold (1873) and Burcke (1878) discovered independently the phenomenon named after them, that variation of the luminance levels modifies hues.
Color Constancy; Aperture Colors and Surface Colors Color constancy is a phenomenon in which color of the objects can be recognized unchanged in spite of possible differences in the illumination. Aperture colors are colors that alter due to change in illumination. Surface colors do not vary with illumination. Extrafoveal vision favors the appearance of aperture colors and foveal vision that of surface colors.
Complementary Wavelengths Complementary wavelengths are those which, when mixed in appropriate proportions, give white.
Simultaneous Color Contrast Color contrast is visually demonstrated by observing the color of a spot in a surround. The general rule is that the color of the spot tends toward the complementary of the color of the surround.
Successive Color Contrast Successive color contrast is more commonly described as colored after images, when one stares at a red spot for several seconds and then looks at a gray card one sees a green spot on
Diagnostic Procedures in Ophthalmology the card. The after image tends toward the complementary of the primary image (StilesCrawford effect). The light entering near the edge of the pupil is less effective than light entering at the center of the pupil because of the shape of the receptors and the fact that they are embedded in a medium of different refractive index. This effect is wavelength-dependent.
Color Triangle Color triangle can be drawn to describe the trichromacy of color mixtures and is useful for deciding which bands of wavelength are indistinguishable from each other. Three reference wavelengths are chosen, i.e. 450 nm, 520 nm and 650 nm and are placed at vertices of X, Y and Z of a triangle, the position of other wavelengths is determined. A color triangle does not describe the color of a band of wavelengths unless other circumstances are defined.
Theories of Color Vision This is a complex topic as no theory explains the phenomenon of color vision fully. Few important theories are given below:
Young-Helmholtz Theory (Trichromatic Theory) Young's concept is that there are three types of retinal receptors with different spectral sensitivities. Young's principal colors are red, green and violet. Young's hypothesis was not followed up until it was revived by Helmholtz in 1852. The Young's theory may be summarized as follows: a. At some stage of visual receptor mechanism there are three different types of sensory apparatus G1, G2, G3. These receptors must be same for everyone but they may not be same at the fovea as at the periphery.
b. Each of these receptors is characterized from the spectral point of view by particular function of wavelengths which may be denoted by G and the response G1 of a receptor for radiation with a spectral energy distribution Eλ may be supposed to have the form. G1 = Sgi Eλ dλ. c. Sensation of color is a function of the relative values of the three responses G1. d. Sensation of light is a function of a linear combination of the three responses. Fundamental sensations By determining approximately the coordinate of the confusion points of dichromats Arthur Konig in1893 established a system of fundamental sensations and identified red, green and violet as fundamental colors. Blue was also identified as fundamental color in addition to red and green by Gothelin.
Granit's Theory of Color Vision Granit divides retina into receptor units, each unit comprising groups of cones and rods which are connected with a single ganglion cell or several ganglion cells which synchronize their discharges. These units are classified as "dominators" or "modulators". The dominators which are numerous have a spectral sensitivity curve which indicates that they are responsible for the sensations of luminosity. Modulators show a selective sensitivity which makes them responsible for color discrimination. Granit's theory does not explain the fact of trichromatism.
Hering's Theory of Color Vision (Opponent Color Theory) Hering assumed six distinct sensations arranged in three opposing pairs: white-black; yellow-blue and red-green; he explains three pairs as being
Color Vision and Color Blindness due to opposing actions of light on three substance of the retina, a catabolism producing warm sensation (white, yellow, red) and an anabolism the cold ones. This theory is clearly a psychological concept and aims at explaining complex percepts than the intermediate effect of the stimuli.
Anatomy of Color Vision The understanding of visual pathway is complex and not evident fully. There are two types of photoreceptors in the retina: rods and cones. Approximately 120 million rods are responsible for night and peripheral vision. Rods contain a photopigment called rhodopsin, a chemical variant of vitamin A and a protein called opsin that serves at very low levels of illumination. Rods have their maximum density about 5 degrees from the fovea and cannot distinguish one color from another. The fovea itself is essentially rod-free containing only cones. Approximately 7 million cones are responsible for central and color vision. Cones have their maximum density within 2 degrees of the center of the fovea. Both types of receptors diminish in number toward the retinal periphery.
Cones In the retina three types of cones responsible for the red, green and blue sensations have been isolated. Three types of cone pigments in the human retina absorb photons with wavelengths between 400 nm and 700 nm. Color vision is mediated by these three cone photoreceptors referred to as long, middle, and short wavelengthsensitive (LWS, MWS, SWS) cones. The long wavelength-sensitive (LWS) cones (sometimes called "red" or "red-catching") contain a pigment called erythrolabe, which is best stimulated by a wavelength near 566 nm. Medium wavelengthsensitive (MWS) cones ("green" or "green-
catching") contain the pigment chlorolabe, which has a maximal sensitivity to a wavelength near 543 nm. Short wavelength-sensitive (SWS) cones ("blue" or "blue-catching") contain cyanolabe, which have maximal sensitivity at 445 nm. The blue cones are absent in the center of the macula. Trichromatic vision perception occurs in central 30º field. It is not uncommon to hear the cones referred to as blue, green, and red cones, but such nomenclature is misleading because the L-cones are more sensitive to blue lights than they are to red lights. The spectral sensitivities of the three cone pigments overlap somewhat. For example, light of 540 nm and 590 nm stimulate both green (MWS) and red (LWS) receptors yet we can easily distinguish between these two wavelengths as "green" and "yellow." If the human retina contains all three cone pigments in normal concentrations, and has normal retinal function, the subject is a trichromat. Any color the trichromat sees can be matched with a suitable mixture of red, green, and blue light.
Color Coded Cells Two types of color coded cells are found at peripheral levels (ganglion cells and lateral geniculate body) of the visual system and they have been named opponent color cells and double opponent color cells. More complex types are found at more central levels (striate cortex). Opponent color cells: An opponent color cell is one that gives only polarity of response for some wavelengths and opposite polarity of response for other wavelengths. Opponent color cells are concerned with successive color contrast. Double opponent color cells: These are cells opponent for both color and space. The response may be onto red light, off to green light in the center of the receptive field and off to red light, onto green light in the periphery of the receptive
Diagnostic Procedures in Ophthalmology field. Double opponent cells are concerned with simultaneous color contrast.
Congenital vs Acquired Color Deficiencies
Simple, complex and hypercomplex cells: In rhesus monkey striate cortex there are a variety of cells that are specific for both color and orientation. They have been categorized as color sensitive simple, complex and hypercomplex cells. Simple cells have a bar-flank double opponent arrangement to their receptive fields. Complex color coded cells respond to color boundaries of the appropriate orientation and the response is independent of the part of the receptive field being stimulated. The edge of hypercomplex cells must be short. Opponent color cells are found among ganglion cells of the retina and lateral geniculate body. Double opponent cells with centersurround or flank receptive fields are present in the input layer IV of the striate cortex. Complex and hypercomplex color coded cells are also found in the striate cortex in layers II, III, V and VI. Vaetichin in 1953 recorded a negative slow potential from fish retinae called "S-potential" of two types: L-type (luminosity type) and Ctype (chromaticity type). Mitarai in 1961 regarded horizontal cells as responsible for S-potentials of L-type and Muller's fibers for those of C-type. The properties of S-potentials support the Herings opponent color theory more than the trichromatic theory of Young.
Congenital color vision deficiencies can be distinguished functionally from acquired deficiencies in a number of ways. Congenital deficiencies typically involve red-green confusions, whereas acquired deficiencies, more often than not, are a blue-yellow (Köllner's rule). Also, because some of the most common congenital defects are linked to the X-chromosome, they are more prevalent in males than females. Acquired defects, in contrast, are not related to gender except by gender differences to trauma or toxic exposure. Acquired color deficiencies are more likely to be asymmetric between the two eyes than are hereditary defects; they are also less likely to be stable with time. Congenital defects are usually easier to detect with standard clinical color vision tests, but some acquired ones can be more subtle and thus are difficult to diagnose. Finally, those with acquired color deficiencies are also more likely to display color-naming errors because, unlike those with congenital deficiencies, they lack the life-long experience with defective color perception.
Anomalies of Color Vision Deficiency of color vision first was described by Dalton in1794, the founder of the atomic theory, who himself was color blind; hence the term daltonism was coined. The color deficiency is of two types: (1) congenital and (2) acquired. In clinical evaluation of color vision it is important to distinguish between acquired and congenital defects.
Congenital Color Vision Deficiency The color vision anomalies commonly being X-linked are relatively common (8%) in men and rare in women (Fig. 2.1). Nearly all congenital color defects are due to absence or alteration of one of the pigments in photoreceptors. Congenital color deficits may be divided into classes according to whether the patients are red deficient (protans), green deficient (deuterans) or blue deficient (tritans). The term anopia is used for absolute deficiency and anomaly for relative deficiency (Tables 2.1 and 2.2). Anomalous trichromats are people who generally require three wavelengths to match
Color Vision and Color Blindness TABLE 2.1: CLASSIFICATION OF COLOR BLINDNESS Congenital: Males (8%), Females (0.4%) classically X-linked recessive inheritance pattern, always bilateral (a) Achromatopsia Cone monochromats Rod monochromats (b) Dyschromatopsia Dichromats - Deuteranopia - Protanopia - Tritanopia Anomalous trichromats - Protanomaly - Deuteranomaly - Tritanomaly
Acquired Unilateral Bilateral
Disease Glaucoma Hypertensive retinopathy Diabetic retinopathy AMD Lesions of visual pathway Alcohol-nicotine
Red-green defect Blue-Yellow defect Red-green defect Blue-Yellow defect Acquired defect Blue-Yellow Blue-Yellow Blue-Yellow Blue-Yellow Red-Green Red-Green
TABLE 2.2: VARIOUS TYPES OF COLOR DEFICIENCY
Anomalous trichromats Dichromats Monochromats
Red deficient
Green deficient
Blue deficient
Protanomaly Protanopia Rod monochromat
Deuteranomaly Deuteranopia
Tritanomaly Tritanopia Blue monochromat
Fig. 2.1: Inheritance pattern of congenital color vision defects
another wavelength but do not accept the color matches made by normal people, Lord Rayleigh in 1881 discovered trichromacy. Anomalous trichromats have three classes of cones but one is abnormal. Protanomalous people lack the red receptors and instead they have two pigments both peaking in the range of the normal green. Similarly the deuteranomalous people lack green receptors. Dichromats require only two wavelengths to match another wavelength and will accept the color matches made by normal people. The dichromats have two classes of cone receptors with normal spectral sensitivity, the third class being absent. Measurements of their pigments can be made by reflection densitomer and cone processes isolated by colored backgrounds confirm the findings. Protanopes have normal green and blue cones, red cones being absent. Deuteranopes have normal red and blue cones and tritanopes normal red and green cones.
Diagnostic Procedures in Ophthalmology Protans color deficient subjects are easier to test and classify than deuterans and tritans; because the red cone pigment is quite sensitive to green wavelengths and both red and green cone pigments are quite sensitive to blue wavelength covering the green and blue range, in deuterans and tritans, as the sensitivity of visual pigment does not fall off sharply on the short wavelength side of the peak. Monochromatics can be blue cone monochromatics and rod monochromatics. Blue cone monochromatics have normal blue cone pigment but no red or green cone pigment. In rod monochromatism only 500 nm pigment is present in the retina and all three cones pigments are absent. Genetics of congenital color deficiencies The protans and deuterons are commonly sexlinked recessive. About 1% males are protanopes, 1% protanomalous, 1% deutaranopes and 5% deuternomalous. The incidence of color vision deficiency (red-green) in females is 0.4%. The gene for tritans is autosomal incompletely dominant. Rod monochromatism is very rare; occurs 1 in 30,000, autosomal recessive and thus an increased incidence is seen in consanguineous offsprings.
Acquired Deficiency of Color Vision Koellner formulated that lesions in the outer layers of the retina give rise to a blue-yellow defect, while lesions in the inner layers of the retina and the optic nerve gives rise to red green defect. However, the correlation is not always true. Some patients with lesions in the cerebral cortex may have color deficits. These may involve naming of the colors or perception of colors.
Factors Responsible for Deficiency of Color Vision Ocular Diseases a. Squint amblyopia: Francois by means of clinical tests stated that color vision deficiencies in squint amblyopia do not correspond to the classical type of acquired deficiencies but rather approximate the normal color sense of eccentric retinal positions. b. Glaucoma: Primary glaucoma and ocular hypertension cause tritan-type of defect. c. Diabetic retinopathy: Diabetic retinopathy may cause color deficiency which may vary from a mild loss of hue discrimination to moderate blue-yellow color vision deficiency. In severe cases of diabetic retinopathy the defect may resemble tritanopia. d. Retinal disorders: Blue-yellow deficits are found in senile macular degeneration, myopia, retinitis pigmentosa, siderosis bulbi and chorioretinitis. e. Optic nerve disorders: In one study about 57% of patients with resolved optic neuritis were found to have color vision defects. Red-green defects have been found in cases of multiple sclerosis and optic atrophy. Tobacco amblyopia causes red-green defect. f. Color vision after laser photocoagulation: After argon-laser photocoagulation there may be overall loss of hue discrimination and color deficiency, mostly of blue-yellow.
Drugs Many drugs are known to cause deficiency of color vision. They can cause more than one type of color deficiency (Table 2.3).
Color Vision and Color Blindness TABLE.2.3: DRUGS CAUSING COLOR DEFICIENCY Drugs
Type of color deficiency
Chloroquine, Indomethacin, oral contraceptives, antihistaminics, estrogens, digitalis and butazolidin.
Blue-yellow
Ethyl alcohol, Ethambutol
Red-green
Tri- and bicyclic antidepressants
Mixed type
Systemic Disorders Besides diabetes, a few systemic disorders are known to be associated with defective color vision. Following diseases may cause color deficiency: a. Cardiovascular disease: Patients with heart diseases have been found to have blueyellow deficiency. b. Turner's syndrome: Red-green color deficiency is usually encountered in the syndrome.
Color Vision Testing The main objective for testing the color blindness is to determine the exact nature of the defect and whether the color deficiency is likely to be a source of danger to the community and/or to the individual, if given a particular job.
Types of Color Vision Tests Color Confusion Tests Pseudo-isochromatic (PIC) plates are example of color confusion tests (Figs 2.2 and 2.3). PIC Tests are designed on the basis of the color confusions made by persons with color defects. In these a symbol or figure in one color is placed on a background of another color so that the figure and background are isochromatic for the color-defective person. PIC tests are used primarily as screening tests to identify those with an inherited color defect, although, some of the
C Figs 2.2A to C: A Ishihara pseudo-isochromatic plates, B Transformation plate seen as "3" by patients with anomalous red-green color defect, C "Vanishing" or "disappearing" digit type
Fig. 2.3: City University test
tests permit a diagnosis of type and severity. Because the inventory of PIC tests is extensive, only the more commonly used tests are described here. The most widely used test, Ishihara pseudoisochromatic plates, is a screening test used to determine the presence of X-linked congenital (red/green) color deficiency. Most screening tests are designed to give a quick, accurate assessment of red/green deficiencies. The Ishihara test is not designed to detect tritan disorders or acquired color defects unless the optic neuropathy is severe.
Arrangement Tests The arrangement tests require the observer to place colored samples in sequential order on the basis of hue, saturation, or lightness or to sort samples on the basis of similarity. One of the earliest tests of this nature that is still available but is rarely used today is the Holmgren Wool test. In this matching test, 46 numerically coded comparison schemes of yarn are selected to match three test colors: yellow-green, pink, and dark red. The comparison schemes differ from the test schemes in being lighter or darker. The test is
not accurate for screening or classification and is not recommended for clinical use. It is of historical significance as an early occupational test. The clinical arrangement tests that are in use today are colored papers mounted in black plastic caps. The caps are placed in order according to specific instructions, and the order is recorded as the sequence of numbers printed on the underside of the caps. Results are plotted on score forms for analysis and interpretation and quantitative scores computed. The tests are standardized for CIE standard illuminant C. The Farnsworth-Munsell Dichotomous-15 (D-15) and the FM-100 test are examples of hue discrimination based on arrangement tests utilizing color chips mounted in a circular cap that subtend exactly 1.5 degrees at a test distance of 50 cm. This ensures that the observations of the subject are made with the central rod free retina. The D-15 contains 15 colored chips and the FM-100 contains 85 chips. The chips have identical brightness and saturation and differ from one another. Farnsworth-Munsell tests reveal the type of defect, but not the severity.
Color Matching Tests The spectral anomaloscope and PickfordNicolson anomaloscope are used for color matching examinations. They can provide the examiner with information on the severity of a particular color vision defect. The Nagel anomaloscope is the most widely used. It consists of a spectroscope in which two halves of a circular field are illuminated respectively by monochromatic yellow (589 nm) and a mixture of monochromatic red and green (670 nm and 546 nm, respectively). The observer is asked to match the two halves of the circle with the three primary colors available. The most widely used color vision tests are the pseudo-isochromatic plates and the D-15
Color Vision and Color Blindness panel due to their ease of use and relative low cost. The Nagel anomaloscope and FM-100 tests are usually only found in academic or research settings. All color vision tests have specific requirements for lighting, viewing distance, and viewing time. It is important for the examiner to be familiar with the test requirements and score sheets before conducting a color vision test, otherwise the results may be inaccurate.
Lantern Tests Lantern tests are used only for occupational purpose. Different types of lantern tests are in use in different countries. The FALANT is used in the United States by marine and aviation authorities; the Holmes Wright Type A is used in the United Kingdom by aviation authorities; and the Holmes Wright Type B is used in Australia, the United Kingdom and other Commonwealth countries by marine authorities. The Edridge-Green Lantern is included in the United States Coast Guard requirements, but it is surpassed by the FALANT. Electroretinography (ERG) and microspectrophotometry may be used in special circumstances.
Test Conditions Lantern testing is performed after dark adaptation but all other tests require artificial daylight conditions. Light adaptation is critical for anomaloscopy and especially for FM-100 hue testing, but a color neutral glare-free background and correct illumination are more important. Reliable results can be obtained with an artificial daylight source (such as a Macbeth Sol source) or fluorescent lighting with a color temperature between 5850 and 6850 degrees Kelvin and good color rendering index (Ra over 90). If appropriate artificial light is not available then skylight is a good source. The illumination should be
between 250 and 350 lux (approximately 1.5 meters below twin fluorescent globe). A failed Ishihara test under incandescent globe is a failure of the examiner to observe basic principles, not a failure of the subject. A pass on the other hand is still a pass and is statistically the more likely outcome. The viewing geometry should be with the light 45 degrees to the surface and the subject viewing the pages at 90 degrees to the surface. Newly printed books sometimes have differential reflectance between pigments so when tilted back and forth in the light by an anomalous observer they may provide luminance clues. Appropriate optical correction for the 65 cm viewing distance must be available if required. Experienced testers know that some people read the small identifying numbers on the bottom of each page and give a memorized response. Cheating can be prevented by covering these identifying numbers with a secret label.
Clinical Significance of the Various Tests Lantern testing is entirely vocational since around 5% of males fail and these include all those with a severe anomaly but a relatively unpredictable group from those with the milder anomalies. Anomaloscopy is the gold standard for clinical testing, while the D-15 and FM-100 tests have both clinical and vocational applications (diamond sorters and croupiers). A common vocational test battery should consist of: • Ishihara plates 2 - 17 from the 38 plate series • D-15 color sorting test (3 or more cross over errors is a failure) • Lantern testing.
Pseudo-isochromatic Color Plates The most common use of plate tests is to identify
Diagnostic Procedures in Ophthalmology persons with congenital color defects. Pseudoisochromatic plates (for example, AO-HRR, Ishihara, Dvorine,Tokyo Medical College, SPP1) provide efficient screening of congenital redgreen defects (efficiency 90-95%). Other tests have been designed to detect achromatopsia (Sloan Achromatopsia test), to differentiate incomplete achromatopsia from complete achromatopsia (Berson blue cone monochromatism plates), to detect acquired defects (SPP-2), or to detect color confusion (City University test). Plate tests have the advantages of being relatively inexpensive, easily available, simple to use, and appropriate with children and persons who are illiterate. They are only suitable for screening purpose, however, they neither provide a quantitative evaluation of color vision nor distinguish the type and severity of the color vision defect. Plate tests are designed to distinguish congenital color-defective from color-normal observers, but they do not evaluate the wide range of abilities and aptitudes of observers with normal color vision to distinguish colors. Given individual differences in prereceptoral filters and normal photo pigment polymorphisms, no plate test can be 100% effective in screening. When used improperly (nonstandard illuminant, binocular viewing, colored lenses not removed from observer), their efficiency can diminish dramatically. The viewing distance required for pseudoisochromatic plates is 75 cm or approximately 30 inches. Proper refractive correction should be provided to the patient in order for them to see the plates clearly. Viewing time for each plate should be no more than 4 seconds. Undue hesitation can be a sign of a slight color deficiency.
color in red-green color defectives (Fig. 2.2B). There are three editions –- a 16 plate series, 24 plate series and a 38 plate series. The 10th edition of Ishihara has 38 plates. It is best to use the larger series because there are relatively few reliable plates in the smaller series. Both 24 set and 38 plate series set consist of two groups of plates — a group for those who are literate / numerate which starts from plate 1 at the front of the book, and a group for illiterates / innumerate in which the colored pattern is a meandering path of connected dots between two X symbols. The second group is arranged so as to commence with the last page of the book and proceed in reverse order. The group of plates for innumerate are seldom used because they are not as easy or reliable to score, but they are based on the same colorimetric principles as the set for numerates. It is not necessary to use both types in the one subject. From a colorimetric perspective there are four different types of test plate employed in both the 38 and 24 plate series preceded by a demonstration plate that is not for scoring. In the large series plates 1 and 38 are both for demonstration only, while in the smaller series plates 1 and 24 are for demonstration. If the subject fails viewing the demonstration plate do not proceed with the test. The following description applies to the numerate plates in the 38 plate series. The different types of plates in the test are:
Ishihara Pseudo-Isochromatic Plates (Confusion Charts)
Transformation plates (Fig. 2.2B): Anomalous color observers give different responses to color normal observers. In these plates, one number is seen by a normal trichromat and another (different) number is seen by a color deficient person. Those with true total color blindness cannot read any numeral. These are the plates numbered 2 to 9 inclusive.
The Ishihara color vision charts are developed by Shinobu Ishihara in 1917. This test is based on the principle of confusion of the pigment
Disappearing digit (Vanishing) plates (Fig. 2.2C): The normal observer is meant to recognize the colored pattern. On these plates, a number can
Color Vision and Color Blindness be seen by a normal trichromat but nothing can be seen by the color deficient person. These are plates 10 to 17 inclusive in the 38 plate series. Hidden digit plates: The anomalous observer should see the pattern. The number on a hidden digit design cannot be seen by a normal trichromat but can be seen by most people with red/green deficiencies. Those people with total color blindness cannot see any numeral. These are plates 18 to 21 inclusive. Qualitative plates: These are intended to classify protan from deutan and mild from severe anomalous color perception. The plates are numbered 22 to 25.
Procedure of Testing The plates are designed to be appreciated correctly in a room which is lit adequately by daylight. Introduction of direct sunlight or the use of electric light may produce some discrepancy in the results because of an alteration in the color values of the charts. It is suggested that when it is convenient only to use electric light, it should be adjusted as far as possible to resemble the effect of natural daylight. The plates are held 75 cm from the subject and tilted at right angles to the line of vision. A missed/ misread plate must be reread (may be in a random order). The findings should be recorded on the Ishihara color vision test and interpretation marking chart (Table 2.4). A correct response to the Ishihara introductory plate is expected and demonstrates suitable visual acuity to perform the test and rules out malingering. • Plates 1-25 have numerals and each answer should be given without more than 3 seconds of delay. • Plates 26-38 are tracings for use in illiterates, and windings lines between the two Xs are traced with a dry soft brush. Each tracing should take less than 10 seconds.
• Each eye should be tested separately (as should be done for all color vision tests). The recommendations of the test state that of the first 21 plates if 17 or more plates are read correctly by an individual his color sense should be regarded as normal. If 13 or less plates are correctly read then the person has a redgreen color defect. It is rare to have persons who read 14-16 plates correctly.
Hardy, Rand, Rittler (H-R-R) Plates Hardy, Rand, Rittler (H-R-R) plates are another type of pseudo-isochromatic (PIC) plate test. This test is similar to the Ishihara test except that the H-R-R plates classify and quantify the type of color defect whether protan, deutran, or tritan (blue/yellow). H-R-R plates have colored symbols/shapes rather than numbers. This makes H-R-R plates a good choice for children and illiterates. Since it is capable of detecting tritan disorders, this test is especially useful when an acquired color vision defect is suspected. Lighting, viewing distance, and viewing time are the same as that of testing with Ishihara plates. The first four (non-numbered) plates of the H-R-R series are for demonstration only (similar to the Ishihara "12"). The first six (numbered) plates are screening plates. Color vision is deemed "normal" and no further testing needs to be done if the subject gives correct responses to the screening plates. If there is an incorrect response to one or more of the screening plates, the examiner must follow the directions on the scoring sheet and show additional plates to the subject in order to specifically classify the color vision defect.
City University Color Vision Test The City University test (Fig. 2.3) was developed by Fletcher. It consists of 10 black charts each of which has 5 color dots. One of the dots is
TABLE 2.4: INTERPRETATION AND MARKING OF THE ISHIHARA COLOR VISION TEST Number of plate
Normal person
Person with red-green deficiency
Protan Strong
Person with total color blindness and weakness
Deutan Mild
(2)6
The mark x shows that the plate cannot be read. Blank space denotes that the reading is indefinite. The numerals in parenthesis show that they can be read but they are comparatively unclear
Color Vision and Color Blindness located in the center being encircled with 4 other dots so that a subject has to match the central color dot with one of the 4 other dots.
American Optical Company Plates The American Optical Company (AOC) plates, a screening test for protan and deutan defects, appears to be a composite of other tests. In addition to a demonstration plate, there are 14 test plates that include 6 transformation and 8 vanishing plates. The figures are single- and double-digit Arabic numerals. There are at least two different fonts used on different plates. Five or more errors on the 14 test plates constitute failure of the test. Plates with double-digit numbers are failed if the response to either digit is incorrect.
Dvorine The Dvorine is another widely used screening test for protan and deutan defects. The test booklet contains both PIC plates and a Nomenclature test, which is a unique and valuable feature of this test. The plates are presented in two sections: 15 plates with Arabic numerals and 8 plates with wandering trails, with 1 demonstration plate in each section. Any symbol missed is an error. Three or more errors in the first section constitute a failure. The Dvorine Nomenclature test is used to assess color naming ability. There are eight discs (2.54 cm in diameter) of saturated color and eight discs of unsaturated or pastel colors, which include red, brown, orange, yellow, green, blue, purple, and gray. A rotatable wheel allows the presentation of one disc at a time. Color-naming aptitude adds another dimension to a color vision assessment, and the results are appreciated by patients and employers curious to know the impact of a color defect on the ability to name colors.
Tritan Plate (F-2) The Tritan plate, or F-2, is a single plate that Farnsworth designed to screen for tritan color defects. It is a good test and it can also be used for screening for red-green (protan-deutan) defects. The test is performed by a vanishing plate consisting of outlines of two interlocking squares with different chromaticities on a purple background. One square is purple-blue and vanishes for patients with the red-green defects; the other square is green-yellow and vanishes, or is seen less distinctly compared with the purple-blue square, for the tritan. Persons with normal color vision see both squares, but the green-yellow one is more distinct.
Arrangement Tests Farnsworth-Munsell 100-Hue Test (Pigment Matching Test) Farnsworth-Munsell test (Fig. 2.4A) is a psychotechnical test, which quantifies a person's ability to discriminate hues of pigment color. This simple and useful test consists of 85 colored chips that are designed to approximate the minimum difference between the hues that a normal observer can distinguish (1-4 nm). Color deficient
Fig. 2.4A: Farnsworth-Munsell 100-hue test
Fig. 2.4B: Farnsworth-Munsell 100-hue test results from four subjects: A Normal; B Protan defects; C Deutan defects; D Tritan defects
persons make characteristic errors in arranging the chips. The results are recorded on a circular graph. The greater the error arranging the chips, the farther the score is plotted from the center of the circle (Fig. 2.4B). Automated score for FM 100-hue test is also available. The currently available standard version consists of 85 knobs with pigment-colored paper
on top arranged in 4 horizontal panels. Each panel has 2 knobs fixed at its 2 ends. The subject is required to arrange the knobs in each panel in such a manner that the colors of the knobs appear to be changing gradually from one end of the panel to another. Generally recommended time for arranging each panel is 2 minutes. The time spent on
Color Vision and Color Blindness arranging the each panel is recorded. Scores of a knob/cap is the sum of the differences between the number of that cap and the number of the caps adjacent to it on either side. Sum of the scores of the entire set of knob / caps goes to make the total error score (TES). Then, the scores of each knob are plotted on a circular graph. By plotting the scores in a graph, it is seen that characteristic patterns are obtained in specific defects (Fig. 2. 4 B). The test is capable of detecting all types of color deficiencies. The test results show that: 1. Average discrimination lies between 20 to 100 total error score, 2. Superior discrimination is below 20 total error score, and 3. Low discrimination is more than 100 total error score.
Farnsworth D-15 Test The Farnsworth D-15 test (Fig. 2.5) consists of single box of 15 colored chips. The test can be carried out more rapidly than the 100-hue test. Viewing distance required is 50 cm or approxi-
mately 20 inches. Unlimited testing time is usually allowed but the subject may be told he/ she has two minutes to complete the test in order to prevent dawdling. The object of the test is to arrange the caps in order using the fixed reference cap as a starting point. The subject is instructed to take the cap which most closely resembles the fixed reference cap, and place it next to it; then find the cap that most closely resembles the cap he just placed, and place it next to it. Once the subject has arranged all the caps, the lid is closed and the box flipped over. The examiner then scores the test based on the order in which the subject placed them (the caps are numbered on the bottom). The examiner then connects the numbers on the score sheet in the order in which the patient placed the caps. The score is either "passing" or "failing." A circular pattern on the score sheet indicates passing, a criss-crossing or lacing pattern indicates failing. The D-15 panel uses only saturated colors, therefore, subtle defects such as those seen with an anomalous trichromat may be missed. The D-15 is useful for detecting dichromacy, in particular, tritan defects which are often associated with eye diseases and drug toxicity. The disadvantage with this test is that minor defects are not detected. Dichromatic subjects will generally form a series of parallel or crisscrossing lines with at least two lines crossing the chart in the same direction. The type of deficiency is indicated by the index line most nearly parallel to the crossover lines.
Lanthony Desaturated D-15 Test
Fig. 2.5: Farnsworth D-15 color test kit
The Lanthony desaturated D-15 test (Fig. 2.6) is similar to the Farnsworth D-15 except that the color on chips is much less saturated. This makes the hue circle smaller and the arrangement task more difficult. It is especially useful for detecting subtle acquired color deficiencies.
Diagnostic Procedures in Ophthalmology are much more difficult to administer than pseudo-isochromatic plates and arrangement tests. The first anomaloscope was designed by Nagel and is based on the color match known as the Rayleigh equation, that is, R + G =Y. Because of their relatively high price, anomaloscopes are rarely used in private practice.
Nagel Anomaloscope (Spectral Matching Test)
Fig. 2.6: Lanthony desaturated D-15
The Sloan Achromatopsia Test The Sloan Achromatopsia test is a matching test designed for rod monochromats described by Sloan in 1954. The test consists of seven plates, each with a different color: gray, red, yellowred, yellow, green, purple-blue, and red-purple. Each plate includes 17 rectangular strips forming a gray scale from dark to light in 0.5 steps of the Munsell value. In the center of each rectangle is a colored disc that has the same Munsell value from one end of the gray scale to the other. The patient's task is to identify the rectangle that matches the lightness of the colored disc. This is a difficult task for persons with normal color vision because of the color difference, but it is readily and precisely accomplished by complete achromats, who see the colors as grays of different lightness. There are normative data for both persons with normal color vision and achromats.
Nagel (1970) constructed anomaloscope for studying the color vision defects. It is based on the color match known as the Rayleigh equation, that is Red (R) + Green (G) = Yellow (Y). The Nagel anomaloscope (Fig. 2.7) assesses the observer's ability to make a specific color match. In anomaloscope, the observer is asked to match a mixture of red and green wavelengths to a yellow. This instrument consists of a source of white light, which is split into spectral colors by a prism. These colors are viewed through a telescope. The field of vision consists of a circle divided into two halves. The lower half projects a spectral Yellow (Sodium line) and this has to be matched by a mixture of Red (Lithium line) and Green (Thallium line) in the other half. The ratio of the two component lights can be controlled by press buttons on the base of the telescope on a scale of 0 – 73, where 0 is pure green, and 73 is pure red. The readings are interpreted as follows: the red/green mix
Anomaloscopes Anomaloscopes are instruments that assess the ability to make metametric matches. The results are used for definitive diagnosis and quantitative assessment of color vision status. Anomaloscopes
Fig. 2.7: Nagel anomaloscope
Color Vision and Color Blindness proportions can be expressed in the form of an Anomaly Quotient (AQ). Normal observers have AQ between 0.7 and 1.4; higher AQs indicate deuteranomaly (AQ usually >1.7), whereas lower AQs indicate protanomaly. A major advantage of the Nagel anomaloscope is that it can distinguish between dichromatic and anomalous trichromatic vision by measuring the balance of red and green wavelengths in the mixture field.
Pickford-Nicolson Anomaloscope The Pickford-Nicolson anomaloscope can be used for three different matches or colorimetric equations: The Rayleigh equation [R + G = Y], The Engelking equation [B + G = CY] and The Pickford - Lakowski equation [B + Y = W]. The matching field is presented on a screen for free viewing at a variety of distances, and there are no intervening optics between the patient and the matching field. The size of the field is changed by selecting different apertures: the largest is 2.54 cm (1 inch) in diameter and the smallest, 0.48 cm (3/16 inch). Different colors are obtained by inserting broadband filters. The Pickford-Lakowski equation is used to assess the consequence of senescent changes in the spectral transmission of the ocular media (yellowing of the lens), it also has value in examining acquired color defects. The Engelking equation is used for diagnosis of the blue - yellow or tritan color defects. Individual variability in density of the macular pigment and lens pigmentation affects both the Engelking and PickfordLakowski equations and, accordingly, confounds the interpretation of an individual result.
navigational aids are extensively used. Lantern tests are performance-based, and they do not diagnose, classify, or grade the level of color vision defect. Rather, they attempt to determine whether the person is capable of performing the color signal recognition tasks with adequate proficiency to maintain safety standards. There are two types of lantern tests, those that use actual signal light filters and those that use simulations of signal lights.
Farnsworth Lantern Test (Falant) In the United States, the Farnsworth Lantern (Falant) is the standard lantern test (Fig. 2.8). It simulates marine signal lights under a variety of atmospheric conditions. Two lights are presented in a vertical display in any of the nine possible combinations of three colors—red, green, and white—in the two positions. A subject must average eight out of nine correct responses to
Lantern Tests In marine, rail, and airline transportation, and in the armed forces, colored signals and
Fig. 2.8: Holmes-Wright Lantern
Diagnostic Procedures in Ophthalmology pass the test. White lights are particularly problematic, especially for milder color defects. It is reported that the test is not representative of actual field conditions.
Edridge-Green Lantern Test The Edridge-Green Lantern (Fig. 2.9) is an instrument used for testing the ability of a person to recognize color of transmitted light. It was built to simulate the light of railway traffic signals, as they are visible from a distance. The apertures represent the equivalents of five and half-inch railway signals at 600, 800 and 1000 yards, respectively when viewed from 20 feet distance. Usually two apertures 1.3 and 13 mm are used, set of filters showing signal red, yellow, green and blue colors are shown, each color being shown twice for each aperture size.
Other Tests Electroretinography Use of electroretinography (ERG) in the modem era is more useful for detection of color vision deficiencies for two reasons: (i) new methods allow to separate and observe accurately the photopic and scotopic components of ERG with the possibility of better study of cone activity and (ii) with the use of computer averaging, picking up of oscillatory potentials is more easy.
Microspectrophotometry In spectrophotometry, an individual cone of a dissected retina is aligned under a small spot of light and its absorption is measured at various wavelengths. The most direct evidence of Young's trichromatic theory (3 classes of cones) comes from spectrophotometry. The results of microspectrophotometry confirm three groupings with peak sensitivities at 437-458 nm, 520-542 nm and 562-583 nm.
Color Vision Deficiencies and Everyday Life
Fig. 2.9: Edridge-Green Lantern
The recommendations of the test state that a candidate should be rejected if he calls 1. Red as Green 2. Green as Red 3. White light as Green or Red or vice versa 4. Red-Green or White light as Black. Any candidate who makes any other errors should be tested with other test.
Many tasks depend on our ability to discriminate color. Selecting products at the grocery store, matching paint colors or items of clothing, or connecting color-coded wiring all depend on efficient color vision. Color vision deficiencies can seriously affect an individual's ability to learn, to work at a chosen occupation and move effectively in the world. Young children are expected to learn color names early in their educational experience and color is frequently used to categorize educational materials. Good color vision is also important for students of art, chemistry, biology, geology and geography. A child with deficient color vision will have disadvantage on such tasks as
Color Vision and Color Blindness color naming, coding, and matching. Color vision testing should be done for all children as early as possible, and certainly prior to starting school. If a color deficiency is present, the child's school, teacher, and parents should be informed so that methods of instruction can be modified to meet their visual needs. Teachers and parents can help the child in a number of different ways. First, images and utensils such as crayons, pencil and pens can be labeled with words or symbols. Second, discrimination between items of different color can be facilitated by the use of high luminance contrast. For example, it would be better to use white chalk on a black or green chalkboard or a dark marker on a white board than combinations that provide less luminance contrast. The level of luminance contrast in colored materials can be determined quite easily by making a black and white photocopy of them or by converting them to black and white on your computer. Third, children should be taught common objects by their usual color (e.g. "bananas are normally yellow and the sky is blue"). Occupations vary in their requirement of color identification. For some, good color judgment is desirable but not necessary. For others, knowledge of one's color vision is critical. Examples where good color judgment can be critical for careers include a painter, safety officer, dermatologist, pharmacist, cartographer, coroner, chemist, buyer of textiles, food inspector, electrician, and marine navigator. Color perception failures in such jobs could be costly, even disastrous.
Enhancing Performance with Filters The color performance of the patients with color deficiency can be sometimes enhanced using colored filters. By absorbing wavelengths selectively, these filters help the observer to differentiate stimuli based on their relative brightness. For example, a red object viewed through a green filter or a green object viewed through a red filter will appear much darker.
For example the X-chrom lens is a red contact lens worn on one eye that absorbs shorter wavelengths and passes longer ones. By comparing the relative brightness in eye with the X-chrom lens to that in the eye without it, a dichromat's ability to distinguish red from green can be enhanced. While such monocular comparisons may be useful in specific applications, the user remains a dichromat and is unlikely to find the approach practical for everyday use.
Summary Ophthalmic personnel are frequently asked to perform color vision testing. Knowing whether a congenital or acquired defect is suspected can help determine which color vision test should be administered. All color vision tests have specific requirements for lighting, viewing distance, viewing time, and scoring. It is important to be familiar with the various testing and scoring guidelines in order to provide the requesting doctor with accurate and useful information.
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Diagnostic Procedures in Ophthalmology 7. Duke-Elder S. Congenital colour defects. In System of Ophthalmology. Henry Kimpton, London 1964; Vol III (Part 2): 661-68. 8. Duke-Elder S. Colour vision. In System of Ophthalmology. Henry Kimpton, London 1968; 4: 617-51. 9. Edridge-Green. Physiology of Vision, London 1920. 10. Farnsworth D. Protan, deutan and tritan. J Opt Soc Amer 1943;33:568. 11. Farnsworth D, Reed. Small-field Tritanopia. USN Submar Med Res Lab Rep No 19, 1944. 12. Farnsworth D. Manual of the FarnsworthMunsell 100-hue test for the Examination of Color Discrimination. 1949; revised 1957, pp 1-7. 13. Francois J, Verriest G. Acquired diseases producing colour vision defects. Vision Res 1961;1:201. 14. Francois J. La discrimination chromatique dans amblyopie strabique. Documents Ophthal 1967;23:318. 15. Geddes. Prevalence of colour vision deficients. Br J Psychol 1946;37:30. 16. Georgia Antonakon Chrousos. Ocular findings in Turner's syndrome: A perspective study. Ophthalmology 1984;91:926. 17. Gouras P. Identification of cone mechanism in monkey ganglion cells. J Physiol 1968;199:533. 18. Hardy LH, Rand G, Rittler MC. Comparison of HRR with other tests. Arch Ophthalmol 1954; 51:216. 19. Hart WM Jr. Acquired dyschromatopsias. Surv Ophthalmol 1987;32:10. 20. Hart WM Jr. Colour vision. In Adler's Physiology of the Eye. Mosby, St Louis 1992;708-27. 21. Hering. Zur Lehre vom Lichtsinne Wien, 1878 cited by Duke-Elder ref 6. 22. Holmgren. Holmgren's wool test. Ann Rep Smithsonian Inst 1877; 131. 23. Ishihara S. Test for Colour Blindness Manual of Ishihara Plates, 1917, 5th ed. Tokyo 1925. and 14th ed. 1959, Kanehara Shuppan Co Ltd, Tokyo – Kyoto, Japan. 24. John A, Fleishman, Roy W Beck. Defects in visual function after resolution of optic neuritis. Ophthalmology 1987;94:1029. 25. Kinnear PK, Sahraie A. New Farnsworth-Munsell 100-hue test norms of normal observers for each year of age 5-22 and for age decades 30-70. Br J Ophthalmol 2002;86:1408-11. 26. Ladd-Franklin. Tetrachromatic Theory. Z Psychol Physiol Sinnes 1893;4:211.
27. Maxwell C. Fundamental response curve of the cone pigment. Trans Roy Soc Edin 1885; 21(2): 275. 28. Michael CR. Colour vision mechanisms in monkey striate cortex: Simple cells with dual opponent colour receptive fields. J Neurophysiol 1978;41:1233. 29. Michael CR. Colour sensitive complex cells in monkey striate cortex. J Neurophysiol 1978;4: 1250. 30. Michael CR. Colour sensitive hypercouplex cells in monkey striate cortex. J Neurophysiol 1979; 42:726. 31. Miller SJH. Colour blindness or achromatopsia. In Parsons' Diseases of the Eye. 18th ed. Edinburgh, Churchill Livingstone, 1900, 269-70. 32. Mitarai G. Glia-neuron interactions and Adaptional mechanisms of the retina. ln Jung R, Kormaluber H (Eds). The Visual System: Neuroplysiology and Psychophysics 1961. 33. Nakamura K. New color vision test to evaluate faulty color recognition. Jpn J Ophthalmol 2002; 46: 601-06. 34. Neitz J, Jacobs GH. Polymorphism in normal color vision. Vision Res 1990;30:62. 35. Newton I. Composition of white light. Phil Trans 1672;6:3075. 36. Nigel W Daw: Colour vision: Adler's Physiology of the Eye, Robert Moses (Ed). St Louis, Mosby, 1981. 37. Pearlman AL, Birch J, Meadows JC: Cerebral colour blindness:An aquired defect in hue discrimination. Amer Neurol 1979;5:253. 38. Rushton WAH. A cone pigment in the protanope. J Physiol 1963;168:345. 39. Swanson WH, Cohen JM. Color vision. Ophthalmol Clin N Am 2003;16:179-203. 40. Taylor WOG. Effects on employment of colour vision defectives. Br J Ophthalmol 1971;155: 753-760. 41. Vola JL, Leprince G. 100-Hue at mesopic level. Mod Probl Ophthal 1978;19:67-70. 42. Wald G. Defective colour vision and its inheritence. Proc Nat Acad Sci USA 1966;55:1347. 43. Wiesel TN, Hubel DH: Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. J Neurophysiol 1966;29:1115. 44. Young T. A course of lectures on natural physiology. Phil Trans 91, 43, 92, 12, 387, 1801-07. 45. Yves le Grand. Light Colour and Vision. London: Chapman and Hall, 1957.
Slit-lamp Examination
HARINDER SINGH SETHI, MUNISH DHAWAN
The slit-lamp is one of the important examining tools of ophthalmologists. Clinical ophthalmologists all over the world routinely use a slit-lamp to examine their patients. A raw slit-lamp was introduced in the early 1900s, but presently, it is a sophisticated instrument (Fig. 3.1). One of the most important advantages of slit-lamp
examination is that one can examine the eye structure in three dimensions (3D). There are three basic requirements for appreciation of depth with a slit-lamp. The first depends upon the clinician possessing a third grade of binocular vision called steriopsis. The second involves the direction of the incoming light source, and is dependent upon the fact that the light beam can be moved so it comes in from one side or the other. The third involves the shape of the slit and is dependent upon the fact that the light source can be moved separately from the oculars.
Fig. 3.1: Slit-lamp
One of the first individuals to apply microscopy to the living eye was Purkinje, who studied the iris with an adjustable microscope by illuminating the field of view. The uniocular slit-lamp was born years later when Louis de Wecker combined an eyepiece objective and adjustable condensing lens within a tube. It was improved by Siegfried Czapski, who added binocularity to the microscope. However, none of the units had sufficient and adjustable illumination. Allvar Gullstrand, an ophthalmologist and 1911 Nobel laureate developed a true slit-lamp to
Diagnostic Procedures in Ophthalmology Illumination System
Fig. 3.2: Allvar Gullstrand
illuminate the eye (Fig. 3.2). Then Henker and Vogt improved upon Gullstrand's device in 1910s by creating an adjustable slit-lamp and combining Czapski's microscope with Gullstrand's slit-lamp illumination. The modern slit-lamp is a tool capable of stereoscopically examining optical sections of the anterior segment of the eye in great detail. Vogt used the slit-lamp biomicroscope to study a vast array of eye diseases and documented his findings in a publication, "Lehrbuch und Atlas der Spaltlampenmikroskopie des Leibenden Auges" in 1930s. Besides examination of the anterior segment of the eye, the slit-lamp, in conjunction with certain contact lenses, is often used to examine the anterior chamber angle and posterior segment of the eye.
Optics of Slit-lamp The slit-lamp is a compound microscope with an objective lens and an eyepiece. The two main components of the modern slit-lamp are the illumination system and observation system (Fig. 3.3).
The illumination system of most slit-lamps consists of two different designs. The first design, the Haag-Streit type illumination, allows de-coupling in the vertical meridian. Such vertical de-coupling is particularly useful when performing gonioscopy to minimize reflections and for indirect funduscopy to gain increased peripheral views. The second design, the Zeiss type illumination system, does not allow decoupling in the vertical meridian. The Zeiss illumination is light and compact and makes the slit-lamp easy to use. In either case, the illumination systems are capable of producing a homogenous and aberration-free beam of white light. Most slit-lamps have halogen bulbs to yield shorter wavelengths of light, which allows better visualization of smaller structures compared with longer wavelengths of light (i.e. tungsten bulbs). A condensing lens first focuses the light onto slit aperture. This light is again focused by another lens onto the eye after being reflected by tilted mirror. Blue and green (redfree) filters are available in slit-lamp to study fluorescein staining pattern and microaneurysm and nerve fiber layer.
Observation System The second main component of slit-lamps is the observation system. Modern slit-lamp microscopes can magnify images between X5 and X25, with some microscopes allowing magnification to X40 and even X100. Magnification is generally achieved by three methods: • Flip-type • Galilean rotating barrel, and • Continuous zoom system. However, magnification of the slit-lamp is less important than its resolution. The resolution of a slit-lamp is dependent on the wavelength of light used, the refractive index between the
Figs 3.3A to F: A The binocular eyepieces provide stereoscopic vision and can be adjusted to accommodate the examiner's interpupillary distance. The focusing ring can be twisted to suit the examiner's refractive error. B The illumination arm can be swung 180 degrees side to side on its pivoting bases allowing the examiner to direct the light beam anywhere between the nasal and temporal aspect of the eye. The dimension of the light beam can be varied in height and width with the levers. C The patient positioning frame consists of two upright metal rods to which are attached a forehead strap and a chin rest. D The joystick allows for focusing by shifting forward, backward, laterally or diagonally. The joystick can also be rotated to lower or elevate the light beam. The locking screw located at the base secures the slit-lamp from movement when it is not in use. E Knurled knob is slit-beam height adjuster, Flip lever controls filters, from left to right: bright, dim, red-free. F ON/OFF power switch provides high or low options in light intensity
eye and objective, the working distance, and the diameter of the objective lens. In practice, the first three of these factors are not easily modifiable, but the objective lens diameter can be modified to increase resolution. However, a very large diameter lens can introduce optical aberrations. The observation system is also influenced by the proximity of the patient's eye to the examiner's eyes. This necessitates a convergence system for binocular viewing, and most modern slit-lamp biomicroscopes are designed with 10 to 15 degrees of convergence to minimize eye strain to the examiner.
Clinical Procedure Before using the slit-lamp, it is important to ensure that the instrument is correctly set up. The following points should be checked: • The eyepieces should be focused for the observer for his/her own refractive error. Often a little more minus correction is required than the observer's actual refractive error due to proximal accommodation and convergence. • The pupillary distance (pd) is adjusted for the observer (perhaps the pd should be
slightly less than that usually measured to account for proximal convergence). Check that the slit-lamp is parallel on the runners of the table. Check that the observation and illumination systems are coupled, and the slit-beam is of even illumination and has sharply demarcated edge (otherwise irregularity of the beam may be falsely interpreted as irregularity of tissues). The locations of the controls are known. The observer and patient are comfortable in the mid-travel of the slit-lamp. Mid-travel is the location of the slit-lamp when it is half-way up or down.
The slit-lamp examination is conducted in a semi dark room. Patient is seated in front of slit-lamp on an adjustable stool and his head is steadied by placing chin on chin-rest and his forehead rests on the bar of head-rest. As with any technique, a general routine should be followed, in most cases when examining the eye and adnexa, a large field of view is used initially and then focus in on detail when required with higher magnification. The examination should be commenced using the X10 eyepieces and the lower powered objective. Use the lowest voltage setting on the transformer. Select the longest slit-length by means of the appropriate lever. Adjust the chin-rest so that the patient's eyes are approximately level with the black marker on the side of the head rest. Adjust the height of the slit-lamp until the slit-beam is centered vertically on the patient's eye. Focus the slit-beam on the eye by moving the joystick either towards or away from the patient. Coarse positioning can be effected without using the microscope but critical focusing should be carried out whilst viewing through the microscope. The angulation between the observation arm and the illumination arm is adjusted. In addition,
accessories like a fixation light, Hruby lens, an applanation tonometer, camera or CCTV can be attached. Laser system can also be attached to a slit-lamp utilizing its optics for laser delivery.
Examination Techniques The various techniques of slit-lamp examination are: 1. Diffuse illumination 2. Direct focal illumination a. Narrow beam (optic section) b. Broad beam (parallelepiped) c. Conical beam 3. Indirect illumination 4. Retroillumination a. Direct b. Indirect 5. Specular reflection 6. Sclerotic scatter 7. Oscillatory illumination 8. Tangential illumination.
Diffuse Illumination Diffuse illumination (Fig. 3.4) is a good method for observing the eye and adnexa in general.
Fig. 3.4: Diffuse illumination
Slit-lamp Examination The beam width is kept at maximum and magnification is kept low and light is thrown at an obtuse angle. It gives an overview of lids, conjunctiva, cornea and lens. Detail examination is not possible with diffuse illumination. Its main purpose is to illuminate as much of the eye at once for general observation. A broad beam of light is directed at the cornea from an angle of approximately 45 degrees. Position the microscope directly in front of the patient's eye and focus on the anterior surface of the cornea. Low to medium magnification (X7-X16) should be used which allows the observer to view as many of the structures as possible. When viewing the eye with achromatic light one should note on gross inspection, any corneal scar, tear debris, irregularities of Descemet's membrane or pigmentary changes in the epithelium. These findings are investigated more thoroughly with other types of illumination.The diffuse illumination mode is also used with cobalt blue filter after fluorescein staining. Fluorescein staining is also used to evaluate positioning of contact lenses, tear breakup time (TBUT), and staining of the cornea for corneal ulcer. Diffuse, wide-beam, illumination together with the red free (green) filter is helpful when viewing the bulbar conjunctiva, and episcleral blood vessels. With the aid of the red free filter small hemorrhages, aneurysms and engorged vessels stand out well.
Direct Focal Illumination Direct focal illumination is the most commonly used method of viewing tissues of the anterior segment of the eye. The focused slit is viewed directly by the observer through the microscope (Fig. 3.5A). The magnification can be increased (X10 to X40) to view any areas of interest in greater detail.
Fig. 3.5A: Direct illumination: the light source is positioned off to one side, and a bright slit-beam is shone directly onto the object to be studied. The light is scattered in all directions by the object, and some of this scattered light finds its way back to the oculars, where it can be observed by the examiner
Direct/focal illumination can be used with different types of beams: a. Narrow beam (optic section) b. Conical beam c. Broad beam (parallelepiped).
Narrow Beam Narrow beam optical section is used primarily to determining the depth or elevation of a defect of the cornea, conjunctiva or locating the depth of an opacity within the lens of the eye (Fig. 3.5B). With the optic section, it is possible to detect corneal thickness, site of foreign body, scars and opacities, the depth of anterior chamber and location of cataracts. The biomicroscope should be directly in front of the patient's eye, the illumination source at about 45 degrees and the illumination mirror in "click" position. The slit-width is almost closed (0.5-1.0 mm wide by 7-9 mm high). Set the magnification on low to medium (X7-X10) and focused on the patient's closed lid. The
Fig. 3.5B: Direct illumination: Narrow beam (optic section)
thickness of the eyelid (about 1 mm) means focusing on the cornea is accomplished with only slight movement of the joystick. With eyes open, give the patient a point of fixation such as the fixation light, or the top of the examiner's opposite ear. Once the cornea is in sharp focus, scan the cornea from temporal limbus to nasal limbus. To maintain a clear, distortion-free view, the illumination source is always moved to the opposite side when crossing the mid-line of the cornea. With a clearly focused optic section slightly temporal to the center of the cornea, magnification is increased to X16, then to X20, and brightness is also increased. Try to note the following: 1. The front surface bright zone is the surface of the tears, 2. The next dark line is the epithelium, 3. The next brighter thin line is Bowman's membrane, 4. The gray wider granular area is the stromal zone, and 5. The last bright inner zone is the endothelium To attain an optic section of the crystalline lens, the angular separation of the illumination source is reduced until the light beam just grazes the edge of the pupil and the vertical height is reduced to approximate the pupil size. This alignment can easily be accomplished from
outside the biomicroscope. When the beam cuts just across the edge of the pupil, the crystalline lens will appear sectioned. By focusing the biomicroscope with joystick with one hand and controlling the direction or angle of the light source with the other hand, the different layers of the lens can be brought into focus. The anatomical location of lens opacities can be visualized. Furthermore, the degree of nuclear opalescence and color can be evaluated and graded. Medium or high magnification gives the best details of lens. Van Herick's technique for grading the anterior chamber angle uses an optic section placed near the limbus with the light source always at 60 degrees (Figs 3.6A and B). The biomicroscope is placed directly before the patient's eye. This technique only allows an estimate of the temporal and nasal angles. The classification of the angle grades and risk of angle closure are summarized in Table 3.1. Split limbal technique: It can be used for an estimation of the superior and inferior angles (Fig. 3.7). The slit-lamp and illumination system
Fig. 3.6A: Van Herick angle estimation method
Fig. 3.6B: Split limbal technique for assessing anterior chamber angle depth
Slit-lamp Examination TABLE 3.1: CLASSIFICATION OF ANTERIOR CHAMBER ANGLE BASED ON VAN HERICK ANGLE OF THE ANTERIOR CHAMBER ESTIMATION METHOD Angle grade
Risk of angle closure
Cornea to angle ratio
Wide open angle incapable of closure. Iris to cornea angular separation equals to 35-45°
Anterior chamber depth (shadow) is equal to or greater than corneal thickness
Moderately open angle incapable of closure. Iris to corneal angular separation equals to 20-35°
Anterior chamber depth (shadow) is between 1/4 and 1/2 of the corneal thickness
Moderately narrow angle closure possible. Iris to corneal angular separation equals to 20°
Anterior chamber depth (shadow) is equal to 1/4 of the corneal thickness
Extremely narrow angle, closure chance high. Iris to corneal angular separation equals to 10°
Anterior chamber depth (shadow) is equal to less than 1/4 of the corneal thickness
Basically closed angle. Iris to corneal angular separation is 0°
Anterior chamber depth (shadow) is nil or only a very narrow slit
are in click position aligned directly in front of the patient. The beam width is that of an optic section which is focused on the limbalcornea junction thus splitting the cornea and limbus. Then view the arc of light through the cornea and that falling on the iris without the aid of the slit-lamp. The angular separation seen at the limbus-corneal junction is an estimation of the anterior chamber angle depth in degrees.
Conical beam Examination of the anterior chamber for cells or flare must be performed before either dilation or applanation tonometry. High magnification (X16-X20) and high illumination may be needed. High illumination is used to detect floating aqueous cells and flare by the Tyndall effect (particles of dust floating in a sun light beam). The traditional method of locating and grading cells and flare is to reduce the beam to a small circular pattern with the light source 45 to 60 degrees temporally and directed into the pupil. The biomicroscope is positioned directly in front of the patient's eye with high magnification and with as bright illumination as the patient will permit. The examiner always allows a
period of time to dark adapt. The conical beam is focused on a dark zone lying between the cornea and the anterior lens surface. This zone is normally optically empty and appears totally black. Flare (protein escaping from dilated vessels) makes the normally optically empty zone appear gray or milky when compared to the normal eye. Cells will reflect the light and can be seen as white dots. The techniques used may be either to oscillate the light source with the joystick from left to right while focused in the anterior chamber or to focus from the posterior cornea to the anterior lens while oscillating the light source.
Broad beam (parallelepiped) A parallelepiped is one of most common types of illumination used (Fig. 3.7). It is used in combination with a number of different types of illuminations. The biomicroscope should be placed directly in front of the patient's eye, the illumination source at about 45 degrees and the illumination mirror in "click," position. A parallelepiped is essentially an optic section with 2.0-4.0 mm slit-width and variable height. The parallelepiped presents a three dimensional
Fig. 3.7: Broad beam (parallelepiped)
view of the cornea or the crystalline lens. The three dimensional view permits observation of distinguishable details within the crystalline lens "zones of discontinuity". As with the optic section, the angle between the illumination source and biomicroscope may be varied to expose more corneal epithelium, stroma and endothelium. The whole cornea should be scanned using a parallelepiped. When scanning the cornea, a clear undistorted view must be maintained by positioning the light source to the opposite side when crossing the mid-line of the cornea. Both normal and abnormal findings can be seen when scanning the cornea with varied levels of magnifications and brightness. Look for the following findings: 1. Tear debris is usually related to allergies or occasionally with infections. 2. Corneal nerves are white thread-like structures that bifurcate and trifurcate and are located anywhere within the cornea. 3. Blood filled vessels extend from the limbus onto or into the cornea, and may be diagnostic of chronic or acute insult or inflammation. 4. Ghost vessels extend from the limbus into the cornea. They are empty of blood and diagnostic of past deep corneal inflammation.
5. Corneal scars are white in color and diagnostic of some past corneal damage, ulcer, abrasion or foreign body. 6. Corneal striae are white usually vertical thread-like twisting lines found in the Descemet's membrane and posterior stroma. They are diagnostic of poor fitting soft contact lens and diabetes. 7. Endothelial pigmentation, when heavy and located vertically on the endothelium, is known as Krukenberg's spindle, it may be diagnostic of iris atrophy and pigmentary glaucoma. Transillumination of the iris may reveal transillumination iris defects (TIDs). Scanty and very fine pigment deposits are commonly seen and are not pathological.
Indirect Illumination Indirect illumination means looking at tissue outside the area which is directly illuminated and can be used in conjunction with most of the above techniques. Corneal opacities, corneal nerves and limbal vessels are easily seen under indirect illumination as glare is reduced. Examine always directly as well as indirectly illuminated areas of the structure. To use this type of illumination place the biomicroscope directly in front of the patient's eye and the illumination light source at about 45 degrees. Make sure the illumination mirror is in "click" position. Use a parallelepiped beam sharply focused on a given structure like the cornea. The light passes through the cornea and falls out of focus on the iris. The dark area just lateral or proximal to the parallelepiped is the indirect or proximal zone of illumination. This is the area of the cornea which one surveys through the biomicroscope. This type of illumination is helpful in detection of microcystic edema, faint corneal infiltrates and irregularities of the corneal epithelium and tears. Because it utilizes
Slit-lamp Examination direct, indirect and retroillumination simultaneously, one should consider it to be as important as any other type of illumination.
Retroillumination Retroillumination is another form of indirect viewing. The light is reflected off the deeper structures, such as the iris or retina, while the microscope is focused to study the more anterior structures in the reflected light (Figs 3.8A to D). It is used to study the cornea in light reflected from the iris, and the lens in light reflected from the retina. Structures that are opaque to
light appear dark against a light background (e.g. corneal scars, pigment, and lens opacity). Portions that scatter light appear lighter than the background (e.g. edema of the epithelium, corneal precipitates). This method is useful for examining the size and density of opacities, but not their location. Retroillumination uses a parallelepiped that bounces unfocused light off one structure while observing the back of another. The alignment and angular separation of the biomicroscope to the illumination source will vary. The light source beam is reflected off another structure like the iris, crystalline lens or retina while the
Figs 3.8A and B: Retroillumination: This technique allows the observer to view a clear structure with light that has been transmitted through, rather than just bounced off it. A Light from the slit-lamp is shone through the pupil, reflected off the fundus, and transmitted through the lens and cornea. B Light is reflected off the iris and transmitted through the cornea
Figs 3.8C and D: Retroillumination
Diagnostic Procedures in Ophthalmology biomicroscope is focused on a more anterior structure. For retroillumination or transillumination of the iris or crystalline lens a low to medium magnification (X7-X10) is used. A slitwidth 1-2 mm wide and 4-5 mm high is used with the biomicroscope and light source placed in direct alignment with each other. They are both positioned directly in front of the eye to be examined. Focus the slit just off the edge of the iris and on the front of the lens. If there are defects or atrophy of the iris they will be seen as a retinal "orange" glow coming back through each defect or hole. Patients who have numerous endothelial pigment deposits must have their iris transilluminated. The cornea is probably the most common structure viewed on retroillumination. Keratic precipitates will appear white in direct illumination but dark by retroillumination. This technique is valuable for observation of deposits on the corneal endothelium and invading blood vessels.
Sclerotic Scatter Sclerotic scatter examination uses the principle of total internal reflection (Fig. 3.9). Slit-lamp is set to a low X6-X10 magnification and a narrow vertical-slit (1-1.5 mm in width) is directed in line with the temporal or nasal limbus. A halo of light will be observed around the limbus as light is internally reflected within the cornea, but scattered by the sclera. Presence of corneal opacities, edema or foreign bodies will be made visible by the scattering light, appearing as bright patches against the dark background of the iris and pupil. Even minute nebular opacities can be picked up.
Specular Reflection Specular reflection is achieved by positioning the beam of light and microscope in such a position so that the angle of incidence is equal
Fig. 3.9: Sclerotic scatter: A bright, wide-slit is shone directly at the limbus; most of the light is trapped within the cornea through total internal reflection, and, therefore, the cornea appears dark. When the light hits the opposite limbus or anything abnormal located within the corneal substance, it will scatter; some of the scattered light is directed back to the oculars, the abnormality is visible to the observer
to the angle of reflection. The light can be reflected from either the anterior or posterior corneal surface. Note that the reflected light should pass through only one eyepiece, and, therefore, this method is monocular. Any roughness or irregularity as induced by the presence of corneal guttata is visible due to irregular reflection of light. A parallelepiped is used to view the endothelial cells of the cornea. The cells are seen only by one eye and they appear in the opposite direction of the illumination light source. A parallelepiped is used for specular reflection. The angle between the illumination source and the biomicroscope should be approximately 60 degrees and a high magnification and high illumination must be used. Place the biomicroscope directly in front of the patient's eye and the illumination light source at 45-60 degrees. Just off the limbus, obtain a sharply focused parallelepiped of the
Slit-lamp Examination cornea. Slowly advance the parallelepiped across the cornea until a dazzling reflection of the filament is seen within the biomicroscope. This reflection is only seen by one eye. Keeping the reflected light within the field of view of biomicroscope, the focus is moved back toward the endothelial cells. There will be a point where two images of the filament are seen, one bright, and the other ghost-like or copper-yellow in color. When the biomicroscope is focused on the ghost-like filament a mosaic of hexagonal cells are seen. It should be noted that even with X40 magnification the endothelial cells do not look as large as most texts show. They resemble the appearance of the dimpled surface of an orange peel or basketball. When the slit-lamp illumination system and the biomicroscope are at equal angles of incidence and reflection, the endothelium of cornea is viewable. Both front and back surfaces of the crystalline lens can also be viewed by using the specular reflection.
Oscillatory Illumination In oscillatory illumination, a beam of light is rocked back and forth by moving the illuminating arm or rotating the prism or mirror. This method may be used to determine occasional aqueous floaters and the extent of opacities in the crystalline lens.
Tangential Illumination In tangential illumination iris is examined under very oblique illumination while the microscope is aligned directly in front of the eye. It is useful for examining tumors of the iris.
Clinical Application Slit-lamp biomicroscopy is very useful in the diagnosis of eye diseases. It should routinely be performed in almost all diseases of the eye.
1. Eyelids and lashes: A low magnification, with a long and fairly narrow beam should be used to scan the eyelashes and lid margins. The examination can reveal the presence of crusted material, lash loss, erythema and flaking suggestive of blepharitis. 2. Conjunctiva: For examination of conjunctiva, pull the lower lid away from the globe with hand and look at the palpebral and bulbar conjunctiva. One may find foreign body, purulent material, injection, conjunctival follicles, pinguecula or pterygium. Try to see the entire cul-de-sac while the patient looking up. The upper lid must be everted to examine the upper palpebral conjunctiva. 3. Cornea: A narrow beam should be directed approximately 45 degrees at the cornea. Scan the entire corneal surface, moving lids and beam appropriately while trying to evaluate the epithelium, stromal thickness and endothelium. Note any defects, opacities or pigment dusting on the endothelium. If defects are seen or suspected, instill a topical anesthetic and fluorescein stain. Make the beam as large as possible and flip the cobalt blue filter on. Examine the epithelium for areas of bright yellow-green staining. The staining represents an epithelial defect. 4. Anterior chamber: The depth of the anterior chamber can be determined by comparing the corneal thickness to the space between the posterior surface of the cornea and the iris surface. The beam should be directed at approximately 45 degrees and just inside the temporal limbus. An anterior chamber depth of less than 1/4 of the corneal thickness is considered a narrow-angle. A search for flare should also be made. 5. Iris: The iris is generally screened with a narrow-beam with full height. It should be fairly flat and free of masses. Small
Diagnostic Procedures in Ophthalmology pigmented nevi are common, but should be flat. The pupillary margin should be round. A slight extension of the posterior pigment around the margin is common but the presence of vessels on the iris is abnormal (rubeosis iridis). 6. Lens: The anterior capsule, cortex, nucleus, and posterior capsule of the lens are scanned with a narrow and full beam of the slit-lamp. When opacity in the lens is present, localize its depth within the lens. Pupillary dilatation facilitates the localization. If the pupils are dilated, widen the beam slightly, lower the height and direct the beam in a straight line toward the retina between the microscope and the eye near the pupillary border. It results in retroillumination and focus on the lens to find iris defects or lens opacities. 7. Anterior vitreous: Anterior vitreous is seen with a narrow beam. Small proteinaceous strands are normal, but cells, blood or opacities in the vitreous are abnormal and warrant investigations.
Fig. 3.10: Goldmann applanation tonometer
Slit-lamp Attachments Besides routine examination of the eye, the slitlamp with the help of its attachments is used for various investigative procedures. Important slit-lamp attachments with their use are mentioned below: Goldmann tonometer (Fig. 3.10) is used for applanation tonometry. Pachymeter (Fig. 3.11) is used for measurement of corneal thickness. Gonioscope (Figs 3.12A to C) is used for visualization of the angle of the anterior chamber. Hruby lens is used for funduscopy. Digital camera for fundus photography (Fig. 3.13).
Fig. 3.11: Corneal pachymeter mounted on slit-lamp
B Fig. 3.13: Slit-lamp with digital camera
C Figs 3.12A to C: Goldmann gonioscopes: A Singlemirror, B Double-mirror, C Three-mirror
1. Fingeret M, Casser L, Woodcombe HT. Atlas of Primary Eye Care Procedures. Norwalk, Appleton & Lange, 1990. 2. Waring GO, Laibson PR. A systematic method of drawing corneal pathologic conditions. Arch Ophthalmol 1977:95:1540-42.
FRANCISCO ARNALICH, DAVID PIÑERO, JORGE L ALIÓ
Corneal Topography
The cornea is the most important refractive element of the human eye, providing approximately two-thirds of optical power of the eye, accounting for about 43-44 diopters at the corneal apex. Because its surface is irregular and aspherical, it is not radially symmetric, and simple measurement techniques are inadequate. The great upsurge in refractive surgery led to a need for improved methods to analyze corneal surface and shape since refraction and keratometric data alone were insufficient to predict surgical outcomes. Understanding and quantifying corneal contour or shape has become essential in planning modern surgical intervention for refractive surgery, as well as in corneal transplantation. It is also very valuable for assessing optical performance of the eye. The different methods for evaluating the anterior surface of the cornea, developed over several centuries, have, in the present era, led to the modern corneal topographers.
History of Corneal Measurement In 1619 Scheiner analyzed corneal curvature by matching the image of a window frame reflected onto a subject's cornea with the image produced by one of his calibrated spheres.
Fig. 4.1: Helmholtz ophthalmometer
Keratometer In 1854 Helmholtz described the first true keratometer, which he called an ophthalmometer (Fig. 4.1). With some minor improvements, it is still being used clinically for calculating refraction, intraocular lens power and contact lens fitting. This apparatus is based on the tendency of the anterior corneal surface to behave like a convex mirror and reflect light. The projection of four points, or mires, onto the cornea, creates a reflected image that can be converted into a
Corneal Topography corneal radius, "r", using a mathematical equation that considers distance from the mire to cornea (75 mm in the keratometer), image size and mire size (64 mm in keratometer). The corneal radius can be transformed into dioptric power using the formula: DP= (index of refraction of the lens - 1)/ r The standard keratometric index represents the combined refractive index of the anterior and posterior surfaces of the cornea, considers the cornea as a single refractive surface, and is 1.3375. Thus, the equation can be simplified to: DP= 337.5/ r Although keratometers are still common in ophthalmology clinics, they do have specific limitations that need to be considered in order to avoid misleading conclusions. 1. Most traditional keratometers measure the central 3 mm of the cornea, which only accounts for 6% of the entire surface. 2. It assumes that the cornea is a perfectly sphero-cylindrical surface, which it is not. The cornea is aspheric in shape, flattening between the center and the periphery. Usually the central corneal curvature is fairly uniform, and this is the reason why it can be used to calculate corneal power in normal patients. However, this is not true in some pathogenic conditions like ectatic disorders or after refractive surgery. 3. The keratometer provides no information as to the shape of the cornea either inside or outside the contour of the mire. Several corneal shapes can all give the same keratometric value so this apparatus is of little use should it become necessary to reconstruct the whole corneal morphology.
reflections of a series of illuminated concentric rings (known as Placido's rings) first time in 1880 (Fig. 4.2). In 1896 Gullstrand developed a quantitative assessment of photokeratoscopy. The keratoscope, like a keratometer, projects an illuminated series of mires onto the anterior corneal surface, usually consisting of concentric rings. The distance between the concentric rings or mires gives the observer an idea of the corneal shape. A steep cornea will crowd the mires, while a flat cornea will spread them out. Surface irregularity is seen as mire distortion. When a photographic camera is attached to the keratoscope, it becomes a photokeratoscope, which gives semi-quantitative and qualitative information about the paracentral, midperipheral and peripheral cornea. Based on the mathematical equation, it is possible to calculate corneal power from object size. Still, photokeratoscopy gives limited information on the central area, which is not covered by the mires.
Fig. 4.2: Placido's rings
Keratoscopy and Photokeratoscopy
Videokeratoscopy
Goode presented the first keratoscope in 1847. Placido is credited to photograph the corneal
Modern corneal topographers are based on videokeratoscopy. A video camera is attached
Diagnostic Procedures in Ophthalmology to the keratoscope, and the information is analyzed by a computer that displays a colorcoded map of power distribution or corneal curvature of the anterior corneal surface (Fig. 4.3). It overcomes some of the limitations of other methods, since it measures larger areas of the cornea, with larger number of points thus increasing resolution. Computer technology makes it possible to create permanent records and conducts multiple data analyses.
Q is asphericity, a parameter that is used to specify the type of conicoid. For a perfect sphere this parameter takes the value of zero (Q=0), for an ellipsoid with the major axis in the X-Y plane (oblate surface) the asphericity is positive (Q>0), for an ellipsoid with the major axis in the Z axis (prolate surface) asphericity is negative (-1 1.4 while those with clinical keratoconus had central corneal power > 47.8 D or I-S > 1.9. However, using only these simple measurements for a diagnosis could create specificity problems. To solve the specificity problem, the new strategy must be able to detect and consider the unique characteristics of keratoconus maps, such as local abnormal elevations. The Keratoconus Prediction Index, developed by Maeda et al, is calculated from the Differential Sector Index (DSI), the Opposite Sector Index (OSI), the Center/Surround Index (CSI), the SAI, the Irregular Astigmatism Index (IAI), and the percent Analyzed Area (AA). This method partially overcomes the specificity limitation. Maeda et al also developed the neural network model, based on artificial intelligence. It is a much more sophisticated method for classifying corneal topography and detecting different corneal topographic abnormalities; it employs indexes that were empirically found to capture specific characteristics of the different corneal pathologies, including keratoconus. Further modifications in neural network approach developed by Smolek and Klyce supposedly produce 100% accuracy, specificity and sensitivity in diagnosing keratoconus.
Corneal Aberrometry: Fundamentals and Clinical Applications Whenever a point object does not form a point image on the retina, as it should be in an ideal optical system, one encounters an optical aberration. Although one may feel that he is measuring the total refractive error, when refracting a patient, one is actually only
considering two components of a whole host of refractive components in the optics of the eye. However, these two components — sphere and cylinder do constitute the main optical aberrations of an eye. Even in a normal eye with no subjective need for refraction, optical aberrations can be detected. Since the cornea has the highest refractive power, more than 70% of the eye's refraction, it is the principal site of aberrations, although the lens and the tear film may also contribute to aberrations.
Fundamentals Measuring Total Wavefront Aberration It is possible to express ideal image formation by means of waves. An ideal optical system will provide a spherical converging wave centered at the ideal point image. However, in practice, the resulting wavefront, differs from this ideal wavefront. The deviation from this ideal wavefront is called wavefront aberration, and the more it differs from zero, the more the real image differs from the ideal image and the worse the image quality. Ocular wavefront sensing devices use four main technologies to determine the resulting or output wave: 1. The Shack-Hartmann method is the most widely used and is inspired by astronomy technology. It consists of analyzing the wave emerging from the eye after directing a small low energy laser beam. This reflected wave is divided by means of a series of small lenses (lenslet array) which generates focused spots. The position of spots is recorded and compared to the ideal one 2. The Tscherning technique uses typically a grid that is projected onto the retina. The distortion of the pattern is analyzed and used to calculate the wavefront aberration of the eye.
Diagnostic Procedures in Ophthalmology 3. The ray tracing system is similar to the Tscherning technique. However, instead of a grid, a programmable laser serially projects light beams that forms spots on the retina at different locations. 4. The spatially resolved refractometer evaluates the wavefront profile using the subjective patient response. This technology is not practical for clinical use.
Measuring Corneal Wavefront Aberration It is known that 80% of all aberrations of the human eye occur in the corneal area and only 20% of aberrations originate from the rest of the ocular structures. The effect of corneal aberrations is especially important after corneal surgery such
as keratorefractive procedures or penetrating keratoplasty, since the anterior corneal surface is the only one modified. The corneal wavefront aberration, which is the component of the total ocular wavefront aberration attributed to the cornea, can be derived from the corneal topographic height data. Specifically, the calculation of wavefront aberrations is performed by expanding the anterior corneal height data into a set of orthogonal Zernike polynomials (Fig. 4.19).
Zernike Polynomials For a quantitative description of the wavefront shape there is a need for a more sophisticated analysis than conventional refraction, as the latter only divides the wavefront in two basic terms:
Fig. 4.19: Corneal wavefront analysis derived from height topography data
Fig. 4.20: Zernike polynomial expansion
sphere and cylinder. One can obtain more information by breaking down the wavefront into terms which are clinically meaningful, besides the sphere and the cylinder. For this purpose, a standard equation has been universally accepted by refractive surgeons and vision scientists, known as Zernike polynomials. Zernike polynomials are equations which are used to fit the wavefront data in a three dimensional way. The wavefront function is decomposed into terms that describe specific optical aberrations such as spherical aberration, coma, etc. (Fig. 4.20). Each term in the polynomial has two variables, ρ (rho) and θ (theta), where ρ is the normalized distance of a specific point from the center of the pupil, and θ is the angle formed between the imaginary line joining the pupillary center with the point of interest and the horizontal. According to that, we can imagine that aberrations are strongly influenced by pupil size, and, therefore, aberrometric measurements should be related to the diameter of the patient's pupil. Zernike terms (Znm) are defined using a double index notation: a radial order (n) and an angular
frequency (m). When talking about first, second, third order aberrations we point to indicate the radial order (n). Each radial order involves n + 1 term. There are an infinite number of Zernike terms that can be used to fit an individual wavefront. However, for clinical practice, terms up to the 4th radial order are usually considered: 1. Zernike terms below third order can be measured and corrected by conventional optical means. The first order term, the prism, is not relevant to the wavefront as it represents tilt and is corrected using a prism. The second order terms represent low order aberrations that include defocus (spherical component of the wavefront) and astigmatism (cylinder component). Wavefront maps that measure only defocus and astigmatism can be perfectly corrected using spectacles and contact lenses. 2. After the second radial order comes high order aberrations. These are not measured by conventional refraction or auto refraction. The aberrometer is the only method available that can quantify these complex kinds of distortions. 3. Third order terms describe coma and trefoil defects. 4. Fourth order terms represent tetrafoil, spherical aberration and secondary astigmatism components. Because spherical and coma aberrations refer to symmetrical systems and the eye is not rotationally symmetrical, the terms spherical-like and coma-like aberrations are normally used (Fig. 4.21).
Wavefront Maps Wavefront map describes the optical path difference between the measured wavefront and the reference wavefront in microns at the pupil entrance. The wavefront error is derived mathematically from the reconstructed wavefront
Fig. 4.21: Spherical-like and coma-like wavefront aberration maps
by one of the techniques described above. It is plotted as a 2D or 3D map for qualitative analysis in a similar fashion to corneal topography maps. In wavefront error maps, each color represents a specific degree of wavefront error in microns (Fig. 4.22) and like corneal topography maps, it is necessary to consider the range and the interval of the scale.
Optical and Image Quality In order to evaluate the impact of aberrations on visual quality following quantitative parameters have been defined (Fig. 4.23): Peak to valley error (PV error): This is a simple measure of the distance from the lowest point to the highest point on the wavefront and is not
Fig. 4.22: Corneal wavefront aberration maps that include all kind of aberrations including low and high order
the best measurement of optical quality since it does not represent the extent of the defect.
results and it is linked to the RMS by the Maréchal formula.
Root mean square error (RMS error): This measure is by far the most widely used. In a simple way, the RMS wavefront error is a statistical measure of the deviation of the ocular or corneal wavefront from the ideal (Table 4.1). In other words, it describes the overall aberration and indicates how bad individual aberrations are.
Point spread function (PSF): This is the spread function observed on the retina when the object is a point in infinity. PSF measures how well one object point is imaged on the output plane (retina) through the optical system. In the eye, small pupils (approximately 1 mm) produce diffraction-limited PSFs, because of the pupil border. In larger pupils, aberrations tend to be the dominant source of degradation.
Strehl ratio: This represents the ratio of the maximum intensity of the actual image to the maximum intensity of the fully diffracted limited image, both being normalized to the same integrated flux. This ratio measures optical excellence in terms of theoretical performance
Modulation transfer function, Phase transfer function and Optical transfer function: Sinusoidal gratings greatly simplify the study of optical systems, because irrespective of the amount of eye aberra-
Fig. 4.23: Visual quality summary obtained with the CSO topographer. It is possible to visualize the wavefront map (gray scale), Strehl ratio, PSF and MTF function
TABLE 4.1:
REFERENCE VALUES FOR CORNEAL ABERRATIONS IN THE NORMAL POPULATION
Pupil (mm)
Total RMS
Astigmatism RMS
Spherical aberration
Coma RMS
Sphericallike RMS
Comalike RMS
RMS: root mean square, Coma primary coma: terms Z3±1, Spherical aberration primary spherical aberration: term Z40 Spherical-like: terms fourth and sixth order, Coma-like: terms third and fifth order Reference: Vinciguerra P, Camesasca FI, Calossi A. Statistical analysis of physiological aberrations of the cornea. J Refract Surg 2003; 19 (Suppl): S265-9.
Corneal Topography tions, sinsusoidal grating objects always produce sinusoidal grating images. Consequently, there are only two ways that an optical system can affect the image of a grating, by reducing contrast or by shifting the image sideways (phase-shift). The ability of an optical system to faithfully transfer contrast and phase from the object to the image at a specific resolution are called respectively the modulation transfer function (MTF) and the phase transfer function (PTF). The eye's optical transfer function (OTF) is made up of the MTF and the PTF. A high-quality OTF is, therefore, represented by high MTF and low PTF.
Clinical Applications Aberrometers allow practitioners to gain a better understanding of vision by measurement of high order aberrations. These aberrations reflect a refractive error that is beyond conventional spheres and cylinders. There may be a large group of patients whose best corrected visual acuity (BCVA) may improve significantly on removal of the optical aberrations and this new refractive entity has been called aberropia. Reduced optical quality of the eye produced by light scatter and optical aberrations may actually be the root cause of blurred vision associated with dry eye syndrome and tear film disruption. Measurement of these aberrations can also be helpful in keratoconus, orthokeratology, post graft fitting, irregular astigmatism or when refractive surgery has reduced the patient's optical quality. Customized ablations are the future step in laser technology that should address not only spherical and cylindrical refractive errors (loworder aberrations), but also high-order aberrations such as trefoil and coma (Fig. 4.24). Thus, vision can be optimized to the limits determined by pupil size (diffraction) and retinal structure and function.
Clinical Uses of Corneal Topography Pathological Cornea Corneal topography is a very important tool in the detection of corneal pathologies, especially ectatic disorders. Screening for these anomalies or their potential development is a critical point in preoperative evaluation for refractive surgery. Keratorefractive procedures are contraindicated in these abnormal corneas.
Keratoconus Keratoconus is characterized by a localized conical protrusion of the cornea associated with an area of corneal stromal thinning, especially at the apex of the cone. The typical associated topographic pattern is the presence of an inferior area of steepening (Fig. 4.25). In advanced cases, the dioptric power at the apex is at or above 55 D. In a small group of patients, the topographic alterations may be centered at the central cornea. In these cases there may be an asymmetric bowtie configuration, and normally the inferior loop is larger than the superior loop (Fig. 4.26). Keratoconic corneas have three common characteristics that are not present in normal corneas: 1. An area of increased corneal power surrounded by concentric areas of decreasing power 2. An inferior-superior power asymmetry 3. A skewing of the steepest radial axes above and below the horizontal meridian. Keratoconus suspects are problematic. They may signal impending development of a clinical keratoconus, but they may also represent a healthy cornea. The lack of ectasia in the fellow cornea does not indicate that the keratoconus suspect will not progress to true keratoconus. In these cases the ideal management is close follow-up of the signs of keratoconus in order to check on their stability, and a thorough analysis of the videokeratographic indexes.
B Figs 4.24A and B: Customized ablation designed according to corneal aberration for the correction of aberrations induced by a decentered ablation. There is a large amount of coma: axial map A and customized ablation designed B with the ORK-CAM software (Schwind)
Pellucid Marginal Degeneration Pellucid marginal degeneration is characterized by an inferior corneal thinning between 4 and 8 O'clock positions above a narrow band of clear thinned corneal stroma. The ectasia is extremely peripheral and it presents a crescent-shaped morphology. This pattern has a classical
"butterfly" appearance that results in a flattening of the vertical meridian and a marked againstthe-rule irregular astigmatism (Fig. 4.27).
Keratoglobus Keratoglobus is a rare bilateral disorder in which the entire cornea is thinned out most markedly
Fig. 4.25: Keratoconus topography pattern
near the corneal limbus, in contrast to the localized central or paracentral thinning of keratoconus. It is very difficult to obtain reliable and reproducible measurements in these cases due to the high level of irregularity and the poor quality of the associated tear film. Reliable topographic examinations show an arc of
peripheral increase in corneal power (steepening) and a very asymmetrical bow- tie configuration.
Terrien's Marginal Degeneration In Terrien´s marginal degeneration there is a flattening over the areas of peripheral thinning.
Fig. 4.26: Keratoconus with an asymmetric bow-tie configuration
When thinning is restricted to the superior and/ or inferior areas of the peripheral cornea, there is a relative steepening of the corneal surface approximately 90 degrees away from the midpoint of the thinned area. Therefore, high against-the-rule or oblique astigmatism is a
common feature, as this disorder involves more frequently the superior and/or inferior peripheral cornea. If the area of thinning is small or if the disorder extends around the entire circumference of the cornea, central cornea may remain relatively spared with a spherical configuration.
Fig. 4.27: Pellucid marginal degeneration topography pattern
Fig. 4.28: Corneal astigmatism induced by a pterygium
Diagnostic Procedures in Ophthalmology Pterygium Pterygium is a triangular encroachment of the conjunctiva onto the cornea usually near the medial canthus. When the lesion continues to grow out onto the cornea, it could lead to a high degree of astigmatism. When the growth of pterygium is about 2 mm or more, a flattening of the cornea at the axis of the lesion occurs (Fig. 4.28). This produces a marked with-the-rule astigmatism, even of more than 4 D. The evolution of the pathology and the surgical outcome could be monitored by changes in corneal topography.
Postoperative Cornea in Refractive Surgery Keratorefractive procedures attempt to alter the curvature of the central and mid-peripheral cornea, and usually have a minimal effect on the corneal periphery. The area in which the curvature is modified is called the optical zone. This tends to be surrounded by a small zone of altered curvature before normal cornea is
reached at the periphery. The corneal effect of surgery could be determined by analyzing the difference map between the preoperative and postoperative measurements.
Postradial Keratotomy (RK) Radial keratotomy (RK) corrects myopia by placing a series of radial incisions (nearly full corneal thickness) leaving a central clear zone (optical zone). These incisions cause a flattening of the central cornea due to retraction of the most anterior collagen fibers and the outward pressure of the intraocular force. This area of flattening is surrounded at an approximately 7 mm zone by a bulging ring of steepening called the paracentral knee. This increases asphericity and corneal irregularity. A very typical finding in these corneas is a topographic pattern with a polygonal shape. Depending on the number of incisions made, squares, hexagons or octagons can be seen. The angles of the polygons correspond closely to the central ends of the incisions (Fig. 4.29).
Fig. 4.29: Polygonal pattern in a postradial keratotomy cornea
Corneal Topography Postastigmatic Keratotomy (AK) Astigmatic keratotomy is a simple modification of the radial keratotomy that is used to correct astigmatism. Rather than placing incisions radially on the cornea, incisions are strategically placed on the steepest meridian. The incisions induce a flattening in that meridian, but provoke steepening in the perpendicular meridian, in a process called coupling. Coupling results from the presence of intact rings of collagen lamellae that run circumferentially around the base of the cornea. With the surgery, these rings become oval in the operated meridian and transmit forces to the untouched meridian. The stigmatic change achieved is the sum of the flattening in one meridian and the steepening in its perpendicular meridian.
Postphotorefractive Keratotomy Photorefractive keratotomy (PRK) is a procedure which uses a kind of laser (excimer laser, a cool
pulsing beam of ultraviolet light) to reshape the cornea. To correct myopia, the excimer laser flattens the central cornea by removing tissue in that area. However, the optical zone needs to be steepened to correct hyperopia. This is achieved by removing an annulus of tissue from the mid-periphery of the cornea. The topographic pattern in myopic corrections shows a flattening of the central cornea, oblate profile (Fig. 4.30). The treatment zone is usually easily delineated by the close proximity of adjacent contours at its edge. Hyperopic corrections have a pattern of central steepening surrounded by a ring of relative flattening at the edge of the treatment zone (more prolate profile) (Fig. 4.31). In astigmatic treatment, the treatment zone is oval. Inadequate ablations during surgery can be detected postoperatively by analyzing the resulting corneal topography. Decentrations can only be identified by a relatively asymmetric localization of the treatment area (Fig. 4.32). Other
Fig. 4.30: Topographic pattern after a myopic ablation
Fig. 4.31: Topographic pattern after a hyperopic ablation
Fig. 4.32: Pattern of decentered myopic ablation
Fig. 4.33: Central island after myopic photoablation
complicated patterns that may lead to severe vision disturbances are the presence of focal irregularities or central islands (Fig. 4.33) produced by an inhomogeneous laser beam or an irregular process of corneal healing.
Postlaser in situ Keratomileusis Postlaser in situ keratomileusis (LASIK) is an excimer laser procedure like PRK, but in this case tissue is ablated under a superficial corneal flap in order to minimize the influence of the epithelium. The topographic patterns for myopic and hyperopic corrections are the same as in PRK (Figs 4.30 and 4.31). Although the ablation is covered by a flap of corneal tissue, surface irregularities and central islands may still occur. Decentration may also occur in a LASIK ablation, depending on the patient's ability to maintain eye fixation during surgery (Fig. 4.34). Epithelial in-growth at the periphery of the flap-stromal interface produces an area of steepening surrounded by an area of marked flattening
making the corneal surface more irregular (Fig. 4.35).
Postlaser Thermal Keratoplasty In laser thermal keratoplasty (LTK), a Holmium laser is used to heat corneal stromal collagen in a ring around the outside of the pupil. The heat causes the tissue to shrink, producing a zone of localized flattening centered on the spot, and a surrounding zone of steepening. This bulging effect of the central cornea makes it possible to correct hyperopia. The typical topographic pattern shows the central corneal steepening and a ring of flattening overlying the spots.
Postintrastromal Corneal Rings Implantation Intrastromal rings are small segments or rings, made of a plastic-like substance, that are inserted into the periphery of the cornea to correct small degrees of myopia or hyperopia. They act as spacers and by changing the orientation of the
B Figs 4.34A and B: Topographic patterns of LASIK decentered ablations after myopic treatment A and after hyperopic treatment B
B Figs 4.35A and B: Topographic analysis in a post-LASIK cornea with an epithelial in-growth at the inferonasal area: placido rings image A, and axial map B
collagen lamellae, depending on their shape and position, flatten or steepen the central cornea. Intrastromal rings could also be used to reduce the corneal steepening and astigmatism associated with keratoconus (Fig. 4.36).
Postkeratoplasty Keratoplasty topographies exhibit a wide variety of patterns, depending on the type of keratoplasty
performed, the quality of the surgical procedure, whether sutures are still in place in the cornea, and the time elapsed after the procedure. Sutures usually induce a central bulge in the corneal graft and its removal results in a decrease of the astigmatic component. The prolate configuration after keratoplasty is the most frequent pattern with a high degree of irregularity (Fig. 4.37). There can be multiple regions of abnormally high or low power, or both simultaneously in
Fig. 4.36: Management of keratoconus by intrastromal rings
the map. Irregular astigmatism over the entrance pupil may be detrimental to optimum visual acuity in the keratoplasty patient.
Contact Lens-induced Corneal Warpage or Molding Corneal warpage is characterized by topographic changes in the cornea following contact lens wear (most frequently in wearers of hard or RGP lenses) as a result of the mechanical pressure exerted by the lens. There are at least 4 different forms of noticeable topography change that usually
occur mixed with one another: (i) peripheral steepening, (ii) central flattening, (iii) furrow depression, and (iv) central molding or central irregularity (Fig. 4.38). Inferior corneal steepening (pseudokeratoconus) is caused by a superiorly riding contact lens that flattens above the visual axis with an apparent steepening below. The topographic image could appear similar to keratoconus, but both conditions are easily differentiated. In corneal warpage, the shape indexes do not indicate any keratoconic condition, and the flat K is not as steep as in keratoconus.
Corneal Topography Other Uses of Corneal Topography
Fig. 4.37: Topographic pattern after penetrating keratoplasty
Corneal topography is a diagnostic tool, but it is also essential before all refractive procedures, to enable the surgeon to understand the refractive status of an individual eye, and plan the optimum refractive treatment. The corneal topography is also used for the following purposes: 1. To guide removal of tight sutures after corneal surgery (keratoplasty, cataract surgery, etc.) that are causing steepening of the cornea (Fig. 4.39). 2. To help in the designing the astigmatic keratotomy.
Fig. 4.38: Corneal warpage
Fig. 4.39: Superior corneal steepening caused by a tight suture
3. To guide contact lens fitting: Selection of the probe lens and design of the lens. 4. To calculate the keratometry values for the calculation of the required power of an intraocular lens for implantation. This is an important issue in corneas that have undergone refractive surgery, because it is more difficult to estimate the real keratometric values in order to avoid over or under corrections. 5. To evaluate the effect of a keratorefractive procedure.
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2. Bogan SJ, Waring GO, Ibrahim O, Drews C, Curtis L. Classification of normal corneal topography based on computer-assisted videokeratography. Arch Ophthalmol 1990;108:945-9. 3. Boyd BF, Agarwal A, Alio JL, Krueger RR, Wilson SE. (Eds). Wavefront analysis, aberrometers and corneal topography. Highlights of Ophthalmology, 2003. 4. Cairns G, McGhee CNJ. Orbscan computerized topography: Attributes, applications, and limitations. J Cataract Refract Surg 2005;31:20520. 5. Corbett M, O'Brart D, Rosen E, Stevenson R. Corneal topography: principles and applications. BMJ Publishing Group, 1999. 6. Corneal Topography. American Academy of Ophthalmology. Ophthalmology 1999;106:162838. 7. Courville CB, Smolek MK, Klyce SD. Contribution of ocular surface to visual optics. Exp Eye Res 2004;78:417-25. 8. Dabezies OH, Holladay JT. Measurement of corneal curvature: keratometer (ophthalmo-
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18. Molebny VV, Panagopoulou SI, Molebny SV, Wakil YS, Pallikaris IG. Principles of ray tracing aberrometry. J Refract Surg 2000;16:S572-75. 19. Rabinowitz YS. Keratoconus. Surv Ophthalmol 1998;42:297-319. 20. Rabinowitz YS, Nesburn AB, McDonnell PJ. Videokeratography of the fellow eye in unilateral keratoconus. Ophthalmology 1993;100: 181-86. 21. Rao SK, Padmanabhan P. Understanding corneal topography. Curr Opin Ophthalmol 2000;11:248-59. 22. Thibos LN, Applegate RA, Schwiergerling JT, Webb R. Standards for reporting the optical aberrations of eyes. J Refract Surg 2002;18:S652-60. 23. Vincigerra P, Camesasca FI, Calossi A. Statistical Análysis of phisiological aberrations of the cornea. J Refract Surg 2003;19(suppl):265-69. 24. Wang L, Koch DD. Corneal Topography and its integration into refractive surgery. Comp Ophthalmol Update 2005;6:73-81. 25. Wilson SE, Ambrosio R. Computerized corneal topography and its importance to wavefront technology. Cornea 2001;20:441-54. 26. Wilson SE, Klyce SD. Advances in the analysis of corneal topography. Surv Ophthalmol 1991;35: 269-77. 27. Wilson SE, Lin DT, Klyce SD, Insler MS. Terrien's marginal degeneration: corneal topography. Refract Corneal Surg 1990;6:15-20.
MANOTOSH RAY
Confocal microscopy, one of the most advanced imaging technologies, offers several advantages over conventional wide-field optical microscopy. It has the ability to control the depth of field, eliminate or reduce the background information away from the focal plane and the capability to collect serial optical sections from thick specimens. The basic key to the confocal approach is the use of spatial filtering techniques to eliminate out-of-focus light or glare. There has been a tremendous interest in confocal microscopy in recent years, due in part to the relative ease with which extremely high quality images can be obtained. Confocal microscopy has enhanced the ability to image the cornea in vivo. The application of this technology permits the acquisition of images of high spatial resolution and contrast as compare to conventional microscopy. Confocal microscope employs an oscillating slit aperture in an ophthalmic microscope configuration, especially suitable for the analysis of cell layers of cornea. It can focus through the entire range of a normal cornea from epithelium to endothelium. A series of scan shows: (a) epithelium, (b) corneal nerves, (c) keratocytes, (d) endothelium and (e) a computer generated slice of cornea. There are distinct advantages
of confocal microscope over the regular microscope. When focused on a transparent tissue like cornea with regular microscope, the unfocused layers affect the visibility of the focused layer. Confocal microscope, on the other hand, can focus on different layers distinctly without affecting the quality of the image.
Optics A halogen light source passes through movable slits (Nipkow disk). A condenser lens (front lens) projects the light to the cornea. Only a small area inside the cornea is illuminated to minimize the light scattering. The reflected light passes through the front lens again and is directed to another slit of same size via beam-splitter. Finally the image is projected onto a highly sensitive camera and displayed on a computer monitor (Fig. 5.1). The confocal microscope utilizes a transparent viscous sterile gel that is interposed between front lens and cornea to eliminate the optical interface with two different refractive indices. The front lens works on 'Distance Immersion Principle'. The working distance (distance between front lens and the cornea) is
Confocal Microscopy performed, a graphic shows the depth coordinate on the 'Z' axis and the level of reflectivity on the 'Y' axis. The graphic also displays the distance between two images along the anteroposterior line. This simultaneous graphic recording is called 'Z' scan graphic. The reflectivity on 'Z' scan is entirely dependent on the tissue being scanned. A transparent tissue displays low reflectivity whereas a higher reflectivity is obtained from an opaque layer. Therefore, different corneal layers would display different reflectivity on 'Z' scan. The corneal endothelium displays the maximum reflectivity while stroma is the lowest. An intermediate reflectivity is obtained from epithelial layers. A typical 'Z' scan of entire normal cornea shows high endothelial reflection curves followed by low stromal reflection and then late intermediate reflectivity from superficial corneal epithelium. Thus confocal miscroscopy enables to perform corneal pachymetry or even can measure the distance between two corneal layers.
Fig. 5.1: Optics of confocal microscope
1.92 mm. The back and forth movement of the front lens enables scanning of the entire cornea starting from anterior chamber and corneal endothelium to most superficial corneal epithelium. Use of standard X40 immersion lens gives magnified cellular detail and an image field of 440 × 330 μm. Other lenses (e.g. X20) delivers wide field but less distinct cell morphology. Newer model (Confoscan 2.0) captures 350 images per examination at a rate of 25 frames per second. Thickness of the captured layers varies from 3 to 5 microns depending on scanning slit characteristics. In addition, every recorded image is characterized by its position on the 'Z' axis of the cornea. Every time a confocal scan is
Confocal Microscopy of Normal Cornea This is a noninvasive technique of imaging of corneal layers that provides excellent resolution with sufficient contrast. A well-executed scan can visualize the corneal endothelium, stroma, subepithelial nerve plexus and epithelial layers distinctly. The limitations are non-visualization of normal Bowman's layer and Descemet's membrane since these structures are transparent to this microscope. However, it is possible to view these structures when they are pathologically involved. Eyes with corneal opacity or edema can also be successfully scanned.¹ The quality of image depends on: (a) centration of the light beam, (b) stability of the eye, and (c) optimum brightness of the illumination.
Diagnostic Procedures in Ophthalmology Epithelium Corneal epithelium has five to six layers. Three different types of cellular component are recognized in the epithelium. • Superficial (2-3 layers): flat cells • Intermediate (2-3 layers): polygonal cells • Basal cells (single layer): cylindrical cells. The superficial epithelial cells appear as flat polygonal cells with well-defined border, prominent nuclei and uniform density of cytoplasm. The main identifying features of superficial epithelial cells are nuclei, which are brighter than surrounding cytoplasm and usually associated with perinuclear hypodense ring (Fig. 5.2). The intermediate epithelial cells are similar polygonal cells as superficial layers but the nuclei are not evident. Basal cell layers are smaller in size and appear denser than other two layers (Fig. 5.3). The nucleus is not evident in basal layers also.
Fig. 5.3: Basal epithelial cells. High cell density with well demarcated cell borders
Now deep vertical fibers derive from deep corneal plexus to run anteriorly to form subbasal and subepithelial nerve plexus. Small nerve fibers from subbasal plexus terminate at the superficial epithelium. This complex anatomy was not possible to visualize in vivo until the advent of corneal confocal microscope. Generally, the nerve fibers appear bright and well contrasted against a dark background (Fig. 5.4). Confocal microscopy can visualize the orientation, tortuosity, width, branching pattern and any abnormality of the corneal nerves.²
Fig. 5.2: Superficial epithelial cells with prominent nuclei
Subepithelial Nerve Plexus Corneal nerves originate from long ciliary nerve, a branch of ophthalmic division of trigeminal nerve. Nerve fibers from long ciliary nerve form a circular plexus at the limbus. Radial nerve fibers originate from this circular plexus and run deep into the stroma to form deep corneal plexus.
Fig. 5.4: Subepithelial nerve fibers
Confocal Microscopy Stroma Corneal stroma represents 90% of total corneal thickness. It has three components: a. Cellular stroma: Composed of keratocytes and constitutes 5% of entire stroma. b. Acellular stroma: Represents the major component (90-95%) of stroma. The main component has regular collagen tissue (Type-I, III, IV) and interstitial substances. c. Neurosensory stroma: Represented by stromal nerve plexus and nerve fibers originating from it. The keratocyte concentration is much higher in the anterior stroma and progressively decreases towards the deep stroma. Generally, the keratocyte count is approximately 1000 cells/ mm² in anterior stroma while the average value drops to 700 cells/mm² in the posterior stroma. Confocal image of stroma shows multiple irregularly oval, round or bean-shaped bright structures that represent keratocyte nuclei. These nuclei are well contrasted against dark acellular matrix (Fig. 5.5). Anterior stromal keratocyte nuclei assume rounded bean-shaped morphology while the same in rear stroma are more often irregularly oval. A bright highly reflective keratocyte represents a metabolically activated
keratocyte of a healthy cornea. In a normal healthy cornea collagen fibers and interstitial substances appear transparent to confocal microscope and impossible to visualize. It is possible to identify stromal nerve fibers in anterior and mid stroma. These nerve fibers belong to deep corneal plexus and appear as linear bright thick lines. The stromal nerve fiber thickness is greater than epithelial nerves. Occasionally, nerve bifurcations are also clearly visible.
Endothelium Endothelium is a non-innervated single layer of cells at the most posterior part of cornea. Endothelial cell density is maximal at birth and progressively declines with age. Normal endothelial cell count varies from 1600 to 3000 cells/mm² (average 2700 cells/mm²) in a normal healthy adult.2-4 However, cornea can still maintain the integrity till the cell count declines below 300-500 cells/mm².
Fig. 5.6: Hexagonal endothelial cells in a healthy cornea
Fig. 5.5: Stromal keratocytes with bright oval-shaped nuclei
Homogeneous hexagonal cells with uniform size and shape represent healthy endothelial cells. Increasing age and endothelial assault cause pleomorphism and polymegathism. Confocal microscopy easily identifies endothelial cells. These cells appear as bright hexagonal and polygonal cells with unrecognizable nucleus. The
Diagnostic Procedures in Ophthalmology cell borders are represented by a thin, nonreflective dark line (Fig. 5.6). A X20 objective lens provides wide field with less magnification. It is possible to perform cell count and study the minute details of cellular morphology.
Confocal Microscopy in Corneal Pathologies Keratoconus Keratoconus is a non-inflammatory ectatic disorder of the cornea characterized by a localized conical protrusion associated with an area of stromal thinning. The thinning is most apparent at the apex of the cornea. The steep conical protrusion of the corneal apex causes high myopia with severe irregular astigmatism. Other features of keratoconus include an iron ring, known as Fleischer's ring that partially or completely encircles the cone.5 The cone appears as 'oil drop' reflex on distant direct ophthalmoscopy due to internal reflection of light. Deep vertical folds oriented parallel to the steeper axis of the cornea at the level of deep stroma and Descemet's membrane are known as Vogt's striae. An acute corneal hydrops appears when there is a break in the Descemet's membrane. The corneal edema usually subsides after few months leaving behind scar and flattening the cornea. The corneal nerves become more readily visible due to thinning of the cornea. High irregular astigmatism precludes adequate spectacle correction. In the early stages, use of contact lenses may improve the visual acuity. However, contact lens fitting can be extremely difficult and in advanced cases it ceases to improve visual acuity optimally forcing the patient to rely on only options left, corneal transplantation. The most effective way to identify early cases of keratoconus is computerized corneal topography that has become a gold standard
for diagnosis and follow-up of the disease in recent years.6,7 Confocal microscopy is a relatively newer investigative modality to assess the keratoconic cornea. Morphological changes in keratoconus are mostly confined to the corneal apex and depend on the severity of the disease. Rest of the cornea may appear normal. The typical polygonal shape of superficial epithelial cells is lost. They appear distorted and elongated in an oblique direction with highly reflective nuclei (Fig. 5.7). Cell borders are not distinguishable. There may be areas of basal epithelial loss as evident by a linear dark non-reflective patch in confocal microscopy. The subepithelial nerve plexus generally appears normal. However, the sub- basal nerve fibers are curved and take the course of stretched overlying epithelium. Corneal stroma is also affected by keratoconus. The confocal images of stroma are highly specific. The characteristic stromal changes are multiple 'striae' represented by thin hyporeflective lines oriented vertically, horizontally or obliquely (Fig. 5.8). These are confocal representation of Vogt's striae.8 In advanced stages of keratoconus, the keratocyte concentration is reduced in anterior stroma. The shape of the keratocytes is also altered. Occasionally, highly reflective bodies
Fig. 5.7: Obliquely elongated superficial epithelium in keratoconus
Confocal Microscopy but with progression of the disease they can involve the posterior stroma as well. Confocal microscopy reveals highly reflective, bright, dense structures in the anterior and midstroma. Keratocytes are not involved. Depth of stromal involvement may be ascertained by using 'Z' scan function. This is an added advantage over other contemporary investigations that enables surgeon to plan for surgical modalities. Confocal microscopy is also useful in differential diagnosis and follow-up of the disease.
Posterior Polymorphous Dystrophy Fig. 5.8: Advanced keratoconus: vertical striae in the stroma
with tapering ends are visible in anterior stroma near the apex. The nature of these abnormal bodies is not yet known but it may be due to altered keratocytes. The corneal endothelial changes vary from none to occasional pleomorphism and polymegathism.
Corneal Dystrophies Corneal dystrophies are inherited abnormalities that affect one or more layers of cornea. Usually both eyes are affected but not necessarily symmetrically. They may present at birth but more frequently develop during adolescence and progress gradually throughout life. Some forms are mild, others severe.
Granular Dystrophy This is an autosomal dominant bilateral noninflammatory condition that results from deposition of eosinophilic hyaline deposits in the corneal stroma.9 It specifically affects the central cornea and eventually can cause decreased vision and eye discomfort. Initially, the lesions are confined to superficial stroma
Posterior polymorphous dystrophy (PPD) is a rare inherited disorder of the posterior layer of the cornea. It is a bilateral disorder with early onset, although early stage diagnosis is rare since most of the affected individuals remain asymptomatic. The characteristic endothelial changes are small vesicles or areas of geographic lesions. In fact, endothelial cells lining of the posterior surface of the cornea have epitheliallike features.10,11 These cells can also cover the trabecular meshwork, leading to glaucoma in some patients. Most severe cases may develop corneal edema due to compromised pump function of the endothelial cells. Confocal microscopy shows multiple round vesicles at the level of Descemet's membrane and endothelium.12 PPD usually distorts the normal flat profile of the endothelial cells and present large dark cystic impressions on confocal scan. The endothelial cells surrounding the lesion appear large and distorted.
Fuchs Endothelial Dystrophy Fuchs endothelial dystrophy is a chronic bilateral hereditary (variable autosomal dominant or sporadic) disorder of corneal endothelium. It typically presents after the age of 50 and more
Diagnostic Procedures in Ophthalmology common in females. There is a loss of endothelial cells that results in deposition of collagen materials in Descemet's membrane (guttata). Corneal guttata is the hallmark of this disease. The integrity of corneal endothelium is essential to maintain the metabolic and osmotic function of the entire cornea. Corneal edema in Fuchs dystrophy initially involves the posterior and mid-stroma. As the disease advances, the edema progresses to involve the anterior cornea; resulting in formation of bullous keratopathy. Confocal microscopy is useful to visualize the corneal guttata. This technique has a distinct advantage over conventional specular microscopy that fails to visualize the endothelium when there is significant corneal edema.13 The corneal guttata appears dark with bright central reflex (Fig. 5.9).14 In advanced stage the endothelial morphology altered completely but it is still possible to identify the distorted cell borders.14 In the early stages of bullous keratopathy, intraepithelial edema is seen as distorted cellular morphology with increased reflectivity. It can also identify the bullae in the basal epithelial layer.
Fig. 5.9: Distorted endothelium in Fuchs endothelial dystrophy
Laser in situ Keratomileusis Laser in situ keratomileusis (LASIK) is one of the latest techniques of excimer laser refractive
surgery that is currently being successfully used by refractive surgeons for the correction of various types of refractive errors. LASIK has become the technique of choice to correct myopia and hyperopia with or without astigmatism.15 LASEK is a modification of photorefractive keratectomy (PRK) where excimer laser is used to ablate superficial corneal stroma after the epithelium has been removed. LASIK involves the use of microkeratome to prepare a hinged corneal flap of uniform thickness. The excimer laser is subsequently used to ablate the mid-corneal stromal bed and thereafter the flap is reposited to its original position without applying any suture. After LASIK, the healing of corneal tissue occurs quickly since there is minimal damage to the corneal epithelium and the Bowman's membrane. Traditionally, the cornea is evaluated with slit-lamp biomicroscopy and computerized corneal topography both pre- and postoperatively. Confocal microscopy adds newer dimensions to the commonly employed investigations. Functional outcome of LASIK depends on many factors including the biomechanics, healing process and the inflammatory response of the flap interface that is created between the epithelial flap and stromal bed. Confocal scan is useful in evaluation of following parameters. • Corneal flap thickness • Interface study a) Healing process b) Inflammatory response c) Abnormal deposits • Corneal nerve fiber regeneration, and • Residual stromal thickness. A well-designed flap is the key to successful outcome of LASIK. Thinner flaps are more at risk from flap complications. A few studies with confocal microscopy had suggested that actual flap thickness after LASIK is consistently lower than predicted thickness.16 The reasons are not
Confocal Microscopy yet known. However, corneal edema that may be caused by microkeratome cut and suction may play an important role. Postoperative scarring and tissue retraction could be other possible factors. Using a 'Z' scan, it is possible to identify the interface that corresponds to a very low level of reflectivity. The flap thickness is obtained by measuring the distance between high reflective spike from the front surface of the cornea and the low reflective interface (Fig. 5.10).
white bodies (Fig. 5.11). Microstriae are present at the Bowman's layer. Excessive interface microstriae and bright particles may lead to astigmatism and eventually poor outcome after LASIK. These microstriae can be imaged with confocal microscope even when the slit-lamp examination is unremarkable.
Fig. 5.11: Bright highly reflective particles at the flap-stroma interface
Fig. 5.10: Measurement of flap thickness in LASIK
The interface usually appears as a hyporeflective space in between relatively hyperreflective cellular stroma. Interface can easily be imaged by confocal microscope. Typically, the keratocyte concentration is lower than normal in the interface. Bright particles and microstriae are consistently visible in the interface. These bright particles most probably originate from microkeratome blade and represented by highly reflective
Diffuse lamellar keratitis (DLK) also known as sands of Sahara syndrome, is a noninfectious inflammation of the interface. The etiology is not known but it is assumed to be toxic or allergic in nature. In confocal scan DLK appears as diffuse and multiple infiltrates in the interface with no anterior or posterior extension. Subepithelial nerve fibers are affected by LASIK. No nerve is visible in immediate postoperative period. However, the regenerating nerve fibers appear as thin irregularly branching line when confocal scan is performed 5-7 days after surgery. The residual stromal thickness can also be measured using 'Z' scan technique as described while evaluating the epithelial flap.
Corneal Grafts Confocal microscope is a useful tool to followup the corneal grafts and to diagnose the
Diagnostic Procedures in Ophthalmology abnormal changes that may occur postoperatively. It provides images at the cellular level to identify any pathological changes even before it becomes clinically evident. It can also be used to assess the donor cornea. Corneal graft survival is entirely dependent on optimum number of healthy endothelial cells. Endothelial cell loss occurs rapidly after corneal transplantation17. Majority of cell loss takes place during the first two postoperative years.18 Several studies had suggested that endothelial cell loss is much higher after corneal grafting when the primary indications are bullous keratopathy or hereditary stromal dystrophy in compare to keratoconus and corneal leukomas.19,20 Another interesting fact is that endothelial cell loss is greater when corneal transplantation is performed on phakic eyes than on aphakics.21 Confocal microscopy scores over conventional specular microscopy while evaluating endothelial cell characteristics especially in eyes with stromal edema. Endothelial morphology in confocal scan has been described earlier. Immediate postoperative period, endothelium looks normal and healthy. However, as time progresses, endothelial cell density decreases as evidenced by pleomorphism and polymegathism. Occasionally, a bright preendothelial deposits appear, the significance of which is not yet known (Fig. 5.12). Reinnervation after grafting is another issue well addressed by confocal microscopy. First sign of innervation that starts few months after keratoplasty is visible at the periphery of the graft stroma. However, complete innervation may take many years to develop. Regenerated nerve fibers look similar to that found in a normal cornea. Occasionally, they may take a tortuous and convoluted course depending on age (e.g. older patients) and primary indications of keratoplasty (e.g. bullous keratopathy, corneal dystrophies).
Fig. 5.12: Pleomorphism, polymegathism and preendothelial deposits in a corneal graft
It is well known that allograft rejection is one of the most common causes of graft failure. Graft rejection can be classified as epithelial, subepithelial and endothelial rejection, of which the endothelial rejection is the worst. Confocal
Fig. 5.13: Co-existence of degenerated and normal endothelial cells in early endothelial allograft rejection
Confocal Microscopy features of epithelial rejection are distorted basal epithelial cells with altered subepithelial reflectivity. Subepithelial rejection is identified by discrete opacities underneath the epithelial layer.22 Endothelial rejection, on the other hand, is characterized by coexistence of normal looking and degenerated endothelial cells, focal endothelial cell lesions and bright highly reflective microprecipitates (Fig. 5.13).23
Intracorneal Deposits Sources of intracorneal deposits can be exogenous or endogenous. They can involve various layers of cornea individually or in combination.
inclusion bodies located at the basal epithelial layer.24 Confocal microscopy adds newer dimensions to the existing knowledge. It demonstrates involvement of entire cornea, although vortex keratopathy is primarily a corneal epithelial pathology. The characteristic features are presence of highly reflective, bright intracellular deposits at the basal epithelial layer (Fig. 5.14). Overlying epithelium is usually normal. In advanced cases these microdeposits may extend to the stroma and eventually to the endothelium.25 Stromal keratocyte density is often reduced.
Exogenous sources: • Long-term use of contact lenses • Refractive surgery • Vitreoretinal surgery using silicone oil • Drugs: Amiodarone, Chloroquine Endogenous sources: • Wilson's disease • Hyperlipidemia • Fabry's disease • Hemosiderosis The clinical diagnosis is based on slit-lamp biomicroscopy and systemic features in selected cases. The knowledge of confocal features in these disorders is limited except in drug induced keratopathies.
Fig. 5.14: Intracellular deposits at basal epithelial layer in amiodarone toxicity
Vortex Keratopathy
Vortex keratopathy known as cornea verticillata is characterized by whorl-like corneal epithelial deposits. It can be induced by various drugs, e.g. amiodarone (used for cardiac arrhythmias) and anti-malarials (chloroquine, hydroxychloroquine). Clinically, vortex keratopathy is manifested as golden-brown opacities at the inferior corneal epithelium. On electron microscopy, they appear as intracytoplasmic lysosom-like lamellar
Ophthalmic investigations and instrumentations have come long way over the past decades. Confocal microscope is one of those wonderful innovations in recent time. It is becoming more popular everyday and its indications are expanding. Confocal microscopy is truly an exciting tool that can be useful for the clinical diagnosis, follow-up and analysis of many corneal lesions.
Diagnostic Procedures in Ophthalmology Acknowledgement I would like to thank Aria Mangunkusumo and Vanathi Ganesh for their help.
References 1. Weigand W, Thaer AA, Kroll P, et al. Optical sectioning of the cornea with a new confocal in vivo slit-scanning videomicroscope. Ophthalmology 1995;102(4):485-92. 2. Oliveira-Soto L, Efron N. Morphology of corneal nerves using confocal microscopy. Cornea 2001;20(4):374-84. 3. Tuft SJ, Coster DJ. The corneal endothelium. Eye 1990;4:389. 4. Nucci P, Brancato R, Mets MB, et al. Normal endothelial cell density range in childhood. Arch Ophthalmol 1990;108:247. 5. Gass JD. The iron lines of the superficial cornea: Hudson-Stahle line, Stocker's line, and Fleischer's ring. Arch Ophthalmol 1964;71:348. 6. Maguire LJ, Bourne WM. Corneal topography in early keratoconus. Am J Ophthalmol 1989; 108:107. 7. Maguire LJ, Lowry J. Identifying progression of subclinical keratoconus by serial topography analysis. Am J Ophthalmol 1991;112:41. 8. Somodi S, Hahnel C, Slowik C, et al. Confocal in vivo microscopy and confocal laser-scanning fluorescence microscopy in keratoconus. Ger J Ophthalmol 1996;5(6):518-25. 9. Werner LP, Werner L, Dighiero P. et al. Confocal microscopy in Bowman's and stromal corneal dystrophies. Ophthalmology 1999;106(9):16971704. 10. Hirst LW, Waring GO. Clinical specular microscopy of posterior polymorphous endothelial dystrophy. Am J Ophthalmol 1983;95(2):143-55. 11. Mashima Y, Hida T, Akiya S, et al. Specular microscopy of posterior polymorphous endothelial dystrophy. Ophthalmic Paediatr Genet 1986; 7(2):101-07. 12. Chiou AG, Kaufman SC, Beuerman RW, et al. Confocal microscopy of posterior polymorphous endothelial dystrophy. Ophthalmologica 1999;213(4):211-13.
13. Chiou AG, Kaufman SC, Beuerman RW, et al. Confocal microscopy in cornea guttata and Fuch's endothelial dystrophy. Br J Ophthalmol 1999;83(2):185-89. 14. Rosenblum P, Stark WJ, Maumenee IH, et al. Hereditary Fuch's dystrophy. Am J Ophthalmol 1980;90:455. 15. Reviglio VE, Bossana EL, Luna JD, et al. Laser in situ keratomileusis for the correction of hyperopia from +0.50 to +11.50 diopters with Keracor 117C laser. J Refract Surg 2000;16(6):71623. 16. Durairaj VD, Balentine J, Kouyoumdjian G, et al. The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis. Ophthalmology 2000;107(12): 2140-43. 17. Harper CL, Boulton ML, Marcyniuk B, et al. Endothelial viability of organ cultured corneas following penetrating Keratoplasty. Eye 1998;12(5):834-38. 18. Vasara K, Setala K, Ruusuvaara P. Follow up study of corneal endothelial cells, photographed in vivo before eneucleation and 20 years later in graft. Acta Ophthalmol Scand 1999;77(3):27376. 19. Obata H, Ishida K, Murao M, et al. Corneal endothelial cell damage in penetrating keratoplasty. Jpn J Ophthalmol 1991;35(4):411-16. 20. Abott RL, Fine M, Guillet E. Long-term changes in corneal endothelium following penetrating keratoplasty. A specular microscopic study. Ophthalmology 1983;90(6):676-85. 21. Ing JJ, Ing HH, Nelson LR, et al. Ten-year postoperative results of penetrating keratoplasty. Ophthalmology 1998;105(10):1855-65. 22. Cohen RA, Chew SJ, Gebhardt BM, et al. Confocal microscopy of corneal graft rejection. Cornea 1995;14(5):467-72. 23. Cho BJ, Gross SJ, Pfister DR, et al. In vivo confocal microscopic analysis of corneal allograft rejection in rabbits. Cornea 1998;17(4):417-22. 24. Ghose M, McCulloch C. Amiodarone induced ultrastructural changes in human eye. Can J Ophthalmol 1984;19:178-86. 25. Ciancaglini M, Carpineto P, Zuppardi E, et al. In vivo confocal microscopy of patients with amiodarone induced keratopathy. Cornea 2001;20(4):368-73.
R RAMAKRISHNAN, SONAL AMBATKAR
Tonometry in reference to the eye is the measurement of intraocular pressure (IOP). A tonometer is an instrument that exploits the physical properties of the eye to permit measurement of pressure without the need to cannulate the eye. The first practical tonometer was invented by Maklakov in 1885. Fick is credited with inventing a second applanation tonometer employing a fixed area produced by an adjustable force. This instrument was a forerunner of the Goldmann applanation tonometer (1954) which is today considered the most accurate clinical tonometer. From a functional standpoint, a normal IOP is one that does not result in optic nerve damage. All eyes do not respond similarly to a particular IOP, therefore, a normal pressure cannot be represented as a specific measurement. Various studies of IOP distribution have shown a mean IOP of 15.5 ± 2.6 mm Hg and the upper limit has been demonstrated to be 2 standard deviations above the mean IOP that is 20.5 mm Hg.
Types of Tonometers The physical properties of a normal cornea determine the limits of accuracy of tonometry. When the cornea is deformed by a tonometer,
the resulting fluid displacement causes the remainder of the globe to distend. The tendency of the wall of the eye is to resist stretching, and deformation of the cornea raises the IOP. Tonometers in which the IOP is negligibly raised during tonometry (less than 5%) are termed as low-displacement tonometers. The Goldmann tonometer displaces only 0.5 μl of aqueous humor and raises IOP by only 3%. Tonometers that displace a large volume of fluid and consequently raise IOP significantly are termed as highdisplacement tonometers. In a normal eye IOP becomes more during Schiøtz tonometry. Highdisplacement tonometers are mostly less accurate than low-displacement tonometers.
Types of Tonometry Tonometry can be broadly classified into 2 types, direct and indirect.
Direct Method A catheter is inserted into the anterior chamber of the eye and the other end is connected to a manometric device from which the pressure is calculated. Though this is the most accurate
Diagnostic Procedures in Ophthalmology method available, it is not feasible in human being because of its invasive nature.
Indirect Method It is based on eyes response to an applied force.
The shape of corneal deformation is truncated cone. It displaces large intraocular volume so conversion tables based on empirical data is used to estimate IOP. The prototype is Schiøtz tonometer.
Applanation Tonometers Palpation Method Intraocular pressure (IOP) is estimated by response of eye to pressure applied by finger pulp (indents easily/firm to touch). The indirect methods can be broadly divided into contact and non-contact methods. Basic types of contact tonometers differ according to shape and magnitude of deformation.
Contact Tonometers IOP measurement is performed by deforming the globe and correlating the force responsible for deformation to the pressure within the eye. Both indentation and applanation tonometers effect a deformation of globe but the magnitude varies (Fig. 6.1).
Applanation tonometers are used to measure force necessary to flatten a small, standard area of cornea. The shape of corneal deformation is simple flattening. The shape is constant so IOP is derived from a mathematical calculation. They are of 2 types on the basis of variable that is measured. Variable force: Area of cornea on applanation held constant, force varies. Prototype is Goldmann tonometer. Variable area: Force applied to cornea held constant, area varies. Prototype is Maklakov tonometer. The volume displacement is sufficiently large to require a conversion table.
Noncontact Tonometer Noncontact tonometer measures time required to deform a standard area of corneal surface in response to a jet of air.
Schiøtz Tonometer
Fig. 6.1: A Deformation of globe during indentation tonometry, B Deformation of globe during applanation tonometry
Indentation Tonometer Indentation tonometer is used to measure the amount of deformation or indentation of the globe in response to a standard weight applied to the cornea or the area flattened by a standard force.
Schiøtz tonometer (Fig. 6.2) consists of metal plunger that slides through a hole in a concave metal plate. The plunger supports a hammer device connected to needle that crosses a scale. The extent to which cornea is indented by plunger is measured as the distance from the foot plate curve to the plunger base and a lever system moves a needle on calibrated scale. The indicated scale reading and the plunger weight are converted to an IOP measurement. More the plunger indents the cornea, higher the scale reading and lower the IOP
Tonometry generated an empirical formula for linear relationship between the log function of IOP and the ocular distension. This formula has 'C' a numerical constant, the coefficient of ocular rigidity which is an expression of distensibility of eye. Its average value is 0.025. Technique: Patient should be in supine position, looking up at a fixation target while examiner separates the lids and lowers the tonometer plate to rest on the anesthetized cornea so that plunger is free to move vertically (Fig. 6.3). A fine movement of needle on scale is in response to ocular pulsations. Scale reading is an average of the extremes of these excursions. The 5.5 gm weight is initially used. If scale reading is 4 or less, additional weight is added to plunger. Conversion table is used to derive IOP in mm Hg from scale reading and plunger weight. The instrument is calibrated before each use to check scale (reading is zero). Fig. 6.2: Schiøtz tonometer
The standard instrument has following characteristics: Foot plate has concavity of 15 mm radius of curvature. The tonometer weighs 11 gm. Plunger has 3 mm diameter, a weight of 5.5 gm including the force of the lever rests on top of the plunger. Additional weights are added to plunger to increase it to 7.5, 10, or 15 gm. The scale reading is zero when plunger extends 0.05 mm beyond foot plate curve. Each scale unit represents 0.05 mm protrusion of the plunger.
Fig. 6.3: Technique of tonometry
Basic concept: The weight of tonometer on the eye increases the actual IOP (Po) to a higher level (Pt). The change in pressure from Po to Pt is an expression of the resistance of the eye (scleral rigidity) to the displacement of fluid. Determination of Po from a scale reading Pt requires conversion which is done according to Friedenwald conversion tables. Friedenwald
Sources of error: Accuracy is limited as ocular rigidity varies from eye to eye. As conversion tables are based on an average coefficient of ocular rigidity; eye that varies significantly from this value gives erroneous IOP. High ocular rigidity is seen in high hyperopia, long-standing glaucoma, age-related macular degeneration, and vasoconstrictor therapy. Low ocular rigidity
Diagnostic Procedures in Ophthalmology is found in high myopia, advanced age, miotics, use of vasodilators, after RD surgery (vitrectomy, cryopexy, scleral band) and intravitreal injection of compressible gas. The variable expulsion of intraocular blood during Schiøtz tonometry may influence IOP measurement. Repeated measurements lower IOP. Either a steeper or a thicker cornea causes greater displacement of fluid during tonometry and gives a falsely high IOP measurement.
Variable Force Applanation Tonometers
Fig. 6.4: Goldmann applanation tonometry
Goldmann Applanation Tonometer (GAT) Basic concept: Based on Imbert-Fick law, an external force (W) against a sphere equals the pressure in the sphere (P) times the area flattened (applanated) by external force (A) W = P × A Cornea being aspherical, wet, and slightly inflexible fails to follow the law. Moisture creates surface tension (S) or capillary attraction of tear film for tonometry head. Lack of flexibility requires force to bend the cornea (B) which is independent of internal pressure. The central thickness of cornea is about 0.55 mm and the outer area of corneal flattening differs from the inner area of flattening (A1). It is this inner area which is of importance.
Fig. 6.5: Biprism in the Goldmann tonometer
When A1 = 7.35 mm2, S balances B and W =P. This internal area of applanation is achieved when the diameter of the external area of corneal applanation is around 3.06 mm. Grams of force applied to flatten 3.06 diameter of the cornea multiplied by 10 is directly converted to mmHg.
biprism (Fig. 6.5) which is used to applanate cornea. Two beam splitting prisms within applanating unit optically convert circular area of corneal contact in 2 semicircles. Edge of corneal contact is made apparent by instilling fluorescein while viewing in cobalt blue light. By manually rotating a dial calibrated in grams, the force is adjusted by changing the length of a spring within the device. The prisms are calibrated in such a fashion that inner margin of semicircles touch when 3.06 mm of the cornea is applanated. Biprism is attached by a rod to a housing which contains a coil spring and series of levers that are used to adjust the force of the biprism against the cornea.
Instrument: Instrument is mounted on the end of a lever hinged on the slit-lamp (Fig. 6.4). Examiner views through the center of plastic
Technique: Cornea is anesthetized, tear film is stained with sodium fluorescein. Cornea and biprism is illuminated by a cobalt blue light.
Modified Imbert-Fick Law is W + S = PA1 + B
Tonometry Fluorescein facilitates visualization of tear meniscus at margin of contact. Fluorescent semicircles are viewed through the biprism. Force against the cornea is adjusted until the inner edges overlap. Ocular pulsations create excursions of semicircular tear meniscus and IOP is read as the median over which arc glides. This is the end point (Fig. 6.6) at which a reading can be taken from a graduated dial which indicates grams of force applied to tonometer and so this number is multiplied by 10 to obtain IOP in mm Hg.
Figs 6.7A and B: Vertical misalignment. To minimize this, tonometer biprism should be rotated so that axis of least corneal curvature is opposite the red line on the prism holder. Other method is to obtain measurements with mires oriented horizontally and vertically and to average these readings
6. More than 6 D astigmatism produces an elliptical area on applanation that gives erroneous IOP. 4D with-the-rule and against-the-rule astigmatism underestimate and overestimate IOP, respectively. 7. Mires may be distorted on applanating on irregular corneas.
Fig. 6.6: End point recording of IOP
Sources of error in applanation tonometry 1. Inadequate concentration of fluorescein in precorneal tear film gives hypofluorescence. 2. Fluorescein may lose fluorescence in acidic solution (quenching of fluorescence) causing underestimation of IOP. 3. Wider meniscus or improper vertical alignment gives higher IOP readings (Figs 6.7A and B). 4. Thin corneas underestimate and thick corneas overestimate IOP. 5. For every 3D increase in corneal curvature, IOP raises about 1 mm Hg as more fluid is displaced under steeper corneas causing increase in ocular rigidity.
Effect of central corneal thickness (CCT): Variations in corneal thickness change the resistance of the cornea to indentation so that this is no longer balanced entirely by the tear film surface tension thus affecting the accuracy of IOP measurement. A thinner cornea may require less force to applanate it, leading to underestimation of true IOP while a thicker cornea would need more force to applanate it, giving an artificially higher IOP. The Goldmann applanation tonometer was designed to give accurate readings when the CCT was 520 μm. As shown by Ehlers et al, there can be under estimation or overestimation of IOP when the corneal thickness is less or more than 520 micron, respectively. They interpolated that deviation of CCT from 520 μm yields a change in applanation readings of 0.7 mm Hg per 10 μm. IOP measurements are also modified after PRK and LASIK. Thinning of the central cornea is believed to give lower readings on applanation.
Other Variable Force Applanation Tonometers Hand-Held Goldmann-Type Tonometers Perkins Tonometer Perkins tonometer (Fig. 6.8) uses same prisms as Goldmann but is counterbalanced so that tonometry is performed in any position (Fig. 6.9). The prism is illuminated by battery powered bulbs. The force on the prisms is adjusted manually. Being portable it is practical when measuring IOP in infants / children and for use in operating rooms.
Fig. 6.9: Tonometry with Perkins tonometer
Mackay-Marg Tonometer
Fig. 6.8: Perkins tonometer
Draeger Tonometer Draeger tonometer is similar to Perkins but uses different set of prisms and operates with a motor adjusting the force on these prisms.
Basic concept: Force is required to keep the flat plate of a plunger flush with a surrounding sleeve against the pressure of corneal deformation. Tonometer incorporates a 1.5 mm diameter plunger affixed to a rigid spring that extends 10 μm beyond the plane of surrounding rubber sleeve. Movement of plunger is electronically monitored by a transducer and recorded on a moving paper strip. When the tonometer is placed against cornea, the tracing that represents the force applied to the plunger begins to rise. At 1.5 mm of corneal area applanation, tracing reaches a peak and the force applied = IOP + force required to deform the cornea. At 3 mm flattening, force required to deform cornea is transferred from plunger to surrounding sleeve, creating a dip in tracing corresponding to IOP. Flattening of >3 mm of area gives artificial elevation of IOP. It is accurate in eyes with scarred, edematous and irregular corneas.
Tonometry Other Mackay-Marg-type Tonometers: CAT 100 Applanation and Biotronic Tonometers They have an internal logic program which automatically selects the acceptable measurement and 3 or more good IOP readings are averaged and displayed on screen.
Tonopen Tonopen (Fig. 6.10) is a portable and battery operated tonometer. It has the same principle as that of Mackay-Marg tonometer. The tip has a strain gauge that is activated when in contact with cornea. The built-in microprocessor logic circuit senses a trough force and records until an acceptable measurement is achieved. Four to ten such measurements are averaged to give a final IOP which is displayed.
The probe tip is applied perpendicularly to cornea until it is just indented. An audible click indicates that the measurement is acceptable. The process is repeated 4-10 times until a beep indicates a statistically valid average reading.
Pneumatonometer Pneumatonometer or pneumatic tonometer is like Mackay-Marg tonometer. It has a core sensing mechanism for measuring IOP while force required to bend the cornea is transferred to surrounding structure. The sensor is a air pressure like electronically controlled plunger in Mackay-Marg tonometer. It can also be used for continuous monitoring of IOP. It gives significantly higher IOP estimates.
Constant Force Applanation Tonometry Maklakov Applanation Tonometer With Maklakov applanation tonometer IOP is estimated by measuring the area of cornea flattened by a known weight. It consists of a dumb-bell-shaped metal cylinder with flat end plates of polished glass on either end with a diameter of 10 mm. Tonometers weighing 5, 7.5, 10, and 15 gm are used to measure the IOP. Crossaction wire handle to support instrument on the cornea is used. A thin layer of dye is spread onto the bottom of either end plate and the instrument is brought in contact with anesthetized cornea in supine position for 1 second. A circular white imprint on end plate corresponds to the area of corneal flattening. Area is measured and IOP is read from conversion table in the column corresponding to the weight used.
Noncontact Tonometer Fig. 6.10: Tonometry with tonopen
Noncontact tonometer (NCT) was introduced by Grolman. A puff of room air creates a
Diagnostic Procedures in Ophthalmology constant force that momentarily flattens the cornea. The time from an internal reference point to the moment of flattening is measured and converted to IOP. The corneal apex is deformed by a jet of air. The force of air jet which is generated by a solenoid activated piston increases linearly over time.
normal, accuracy decreases with increase in IOP and in eyes with abnormal cornea or poor fixation. New NCT, Pulsair is a portable hand held tonometer.
Devices under Investigation Flush fitting silastic gel contact lens instrumented with strain gauges that measures changes in meridional angle of corneoscleral junction caused by variations in IOP. A similar device using a pressure transducer is made in form of a cylindrical guard ring applanation tonometer. A scleral gauge is embedded in an encircling scleral band to measure the distension of globe. An instrument using suction cups for recording IOP up to 1 hour in supine position is under investigation.
Fig. 6.11: Tonometry with noncontact tonometer
Original NCT has 3 subsystems: 1. Alignment system: It aligns patient's eye in 3 dimensions. 2. Optoelectronic applanation monitoring system: It comprises transmitter, receiver and detector, and timer. a. Transmitter directs a collimated beam of light at corneal apex. b. Receiver and detector accept only parallel coaxial rays of light reflected from cornea. c. Timer measures from an internal reference to the point of peak light intensity. 3. Pneumatic system: It generates a puff of room air directed against cornea. When the reflected light is at peak intensity, the cornea is presumed to be flattened. The time elapsed is directly related to the force of jet necessary to flatten the cornea and correspondingly to IOP. NCT is accurate if IOP is nearly
Comparison, Calibration and Sterilization of Different Tonometers Comparison Goldmann Applanation Tonometer (GAT) In eyes with regular corneas, GAT is generally accepted as the standard against which other tonometers must be compared. Even with GAT, inherent variability must be taken in account.
Schiøtz Tonometer Studies indicate that Schiøtz reads lower than GAT even when the postural influence on IOP is eliminated by performing measurements in supine position. The magnitude of difference between the two tonometers and the influence of ocular rigidity are such that Schiøtz indicates only that the IOP is within a certain range and is of limited value even for screening purposes.
Tonometry Perkins Applanation Tonometer Perkins applanation tonometer compares favorably against GAT. In one study, difference between readings with the two instruments was 1.4 mmHg. It is subject to the same influence of corneal thickness as the GAT. It is useful in infants and children and is accurate in horizontal as well as vertical position.
Draeger Applanation Tonometer Comparative studies of Draeger applanation tonometer with GAT have given inconsistent results because of its more complex design. Draeger tonometer is more difficult to use than the Perkins. Patient's acceptance to Draeger tonometer is poor.
Mackay-Marg Tonometer (MMT) Highly significant correlation is found between MMT and GAT readings. The average mean MMT values are often higher than GAT.
Mackay-Marg Type Tonometers Tonopen has compared favorably against manometric readings in human autopsy eyes but it may cause a significant increase in IOP during measurements. It has good correlation with GAT readings within normal IOP ranges. But most studies indicate that tonopen under estimates IOP in the higher ranges and over estimates in the lower range.
Pneumatic Tonometer Pneumatic tonometer correlates well with GAT readings. However, it gives significantly higher IOP estimates.
Noncontact Tonometer Noncontact tonometer is reliable within the normal IOP range, although its reliability is
reduced in higher IOP ranges and is limited by abnormal corneas or poor fixation. Corneal thickness has greater influence on NCT than on GAT. The hand-held pulsair NCT has compared favorably with Goldmann applanation readings in normal and glaucomatous eyes. It tends to read lower IOP above the normal range.
Tonometry on Irregular Corneas Accuracy of GAT and Maklakov-type applanation tonometers and NCT is limited in eyes with irregular corneas. MMT is considered to be accurate in scarred or edematous corneas. As MMT applanates a small surface area, the effects of corneal resistance to deformation and surface tension of tears are less than that with GAT. Pneumotonometer has also been shown to be useful in eyes with diseased cornea. Tonopen compared favorably with MMT on irregular corneas in a study.
Tonometry over Soft Contact Lens MMT, pneumtonometer and tonopen can measure the IOP through bandage contact lens with reasonable accuracy although soft contact lenses of different powers create a bias with tonopen. Applanation tonometers are affected by the power of the contact lens with high water content and correction tables are developed to compensate it. The power of soft contact lenses influences the difference in IOP between the paired readings by NCT.
Tonometry over Gas Filled Eyes Intraocular gas significantly influences scleral rigidity rendering indentation tonometry unsatisfactory. Pneumatonometer underestimates GAT readings in gas filled eyes while Tonopen compared favorably with GAT readings.
Diagnostic Procedures in Ophthalmology Calibration of Goldmann Applanation Tonometer It is essential that Goldmann applanation tonometer (GAT) should be calibrated periodically, at least monthly. Following checks are necessary: • Check position 0: Turn the zero calibration on the measuring drum downwards by the width of one calibration marking, against the index marker. When the feeler arm is in the free movement zone, it should then move itself against the stop piece in the direction of the examiner. • Check position 0.05: Turn the zero calibration on the measuring drum upwards by the width of one calibration marking, against the index marker. When the feeler arm is in the free movement zone, it should then move itself against the stop piece in the direction of the patient. • Check position at drum setting 2: For checking this position, check weight is used. Five circles are engraved on the weight bar. The middle one corresponds to drum position 0, the two immediately to the left and right to position 2 and the outer ones to position 6. One of the marks on the weight corresponding to drum position 2 is set precisely on the index mark of the weight holder. Holder and weight are then fitted over the axis of the tonometer so that the longer part of the weight points towards the examiner. • Check position 1.95: The feeler arm should move towards the examiner. Check position 2.05.The feeler arm should move in the direction of the patient. • Check at measuring drum setting 6: Turn the weight bar to scale calibration 6, the longer part shows in the direction of the examiner. • Check position 5.9/6.1 as performed for drum setting 2.
Sterilization Schiøtz Tonometer The tonometer is disassembled between each use and the barrel is cleaned with 2 pipe cleaners, the first soaked in alcohol and the second dry. The foot plate is cleaned with alcohol swab. All surfaces must be dried before reassembling.
Goldmann Applanation Tonometer A variety of techniques are described for disinfecting the tonometer. Applanation tip should be soaked for 5-15 min in diluted sodium hypochlorite, 3% H2O2 or 70% isopropyl alcohol or by wiping with alcohol, H2O2, povidone iodine or 1: 1000 merthiolate. Other methods of sterilization include: 10 min of rinsing in running tap water, wash with soap and water, cover the tip with a disposable film, and exposure to UV light.
Tonopen Tip is protected by a disposable latex cover.
Pneumatonometer Tip should be cleaned with an alcohol sponge, taking care to dry the surface before use. Alternative is the use of disposable latex cover over the tip.
Bibliography 1. Armaly MF. On the distribution of applanation pressure. I. Statistical features and the effect of age, sex, and family history of glaucoma. Arch Ophthalmol 1965;73:11. 2. Bengtsson B. Comparison of Schiøtz and Goldmann tonometry and tonography in a population. Acta Ophthalmol (Copenh) 1972;50: 455.
Tonometry 3. Craven ER, et al. Applanation tonometer tip sterilization for adenovirus type 8. Ophthalmology 1987;94:1538. 4. Drance SM. The coefficient of scleral rigidity in normal and glaucomatous eyes. Arch Ophthalmol 1960;63:668. 5. Dunn JS, Brubaker RF: Perkins applanation tonometer, clinical and laboratory evaluation. Arch Ophthalmol 1973;89:149. 6. Durhan DG, Bigliano RP, Masino JA: Pneumatic applanation tonometer. Trans Am Acad Ophthalmol Otolaryngol 1965;69:1029. 7. Finlay RD. Experience with the Draeger applanation tonometer. Trans Ophthalmol Soc UK 1970;90:887. 8. Forbes M, Pico GJ, Goldmann B: A noncontact applanation tonometer description and clinical evaluation. Arch Ophthalmol 1974;91:134. 9. Friedenwald JS. Standardization of tonometers decennial report. Trans Am Acad Ophthalmol Otolaryngol 1954;58. 10. Friedenwald JS. Contribution to the theory and practice of tonometry. Am J Ophthalmol 1937; 20:985. 11. Friedman E, et al. Increased scleral rigidity and age-related macular degeneration. Ophthalmology 1989;96:104. 12. Glouster J, Perkins ES. The validity of the ImbertFick law as applied to applanation tonometry. Exp Eye Res 1963;2:274. 13. Grolman B. Non-contact applanation tonometry. Optician 1973;166:4. 14. Hollows FC, Graham PA: Intraocular pressure, glaucoma, and glaucoma suspects in a defined population. Br J Ophthalmol 1966;50:570. 15. Imbert A. Theories ophthalmotonometers: Arch Ophthalmol 1885;5:358. 16. Kaufman HE, Wind CA, Waltman SR. Validity of Mackay-Marg electronic applanation tonometer in patients with scarred irregular corneas. Am J Ophthalmol 1970;69:1003. 17. Khan JA, et al. Comparison of Oculab Tono-Pen readings obtained from various corneal and scleral locations. Arch Ophthalmol 1991; 109: 1444. 18. Krieglstein GK, Waller WK. Goldmann applanation versus hand-applanation and Schiøtz indentation tonometry. Graefes Arch Clin Exp Ophthalmol 1975;194:11. 19. Kronfeld PC. Tonometer calibration empirical validation: the committee on standardization of
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34. 34. 36. 37.
tonometers. Trans Am Acad Ophthalmol Otolaryngol 1957;61:123. Langham ME, McCarthy E. A rapid pneumatic applanation tonometer: comparative findings and evaluation. Arch Ophthalmol 1968;79:389. Macro FJ, Brubakar RF. Methodology of eye pressure measurement. Biorheology 1969;6:37. Markiewitz HH. The so-called Imbert Fick law (Correspondence). Arch Ophthalmol 1960;64:159. McMillan F, Forster RK. Comparison of MackayMarg, Goldmann, and Perkins tonometers in abnormal corneas. Arch Ophthalmol 1975;93:420. Moses RA. Fluorescein in applanation tonometry. Am J Ophthalmol 1960;49:1149. Moses RA. The Goldmann applanation tonometer. Am J Ophthalmol 1958;46:865. Pepose JS, et al. Disinfection of Goldmann tonometers against human immunodeficiency virus type I. Arch Ophthalmol 1989;107:983. Perkins ES. Hand-held applanation tonometer. Br J Ophthalmol 1965;49:591. Petersen WC, Schlegel WA. Mackay-Marg tonometry by technicians. Am J Ophthalmol 1973;76:933. Posner A. Practical problems in the use of the Maklakov tonometer. EENT J 1963;42:82. Posner A. An evaluation of the Maklakov applanation tonometer. EENT J 1962;41:377. Rootman DS, et al. Accuracy and precision of the Tono-Pen in measuring intraocular pressure after keratoplasty and epikeratophakia in scarred corneas. Arch Ophthalmol 1988;106:1697. Schmidt T. The clinical application of the Goldmann applanation tonometer. Am J Ophthalmol 1960;49:967. Schwartz NJ, Mackay RS, Sackman JL. A theoretical and experimental study of the mechanical behavior of the cornea with application to the measurement of intraocular pressure. Bull Math Biol 1966;28:285. Schields MB. The noncontact tonometer: Its value and limitations. Surv Ophthalmol 1980;24:211. Starrels ME. The measurement of intraocular pressure. Int Ophthalmol Clin 1979;19:9. Stepanik J. Tonometry results using a corneal applanation 3.53 mm in diameter. Klin Monatsbl Augenheidkld 1984;184:40. Ventura LM, Dix RD. Viability of herpes simplex type I on the applanation tonometer. Am J Ophthalmol 1987;103:48.
A NARAYANASWAMY, L VIJAYA
Gonioscopy
Gonioscopy, the visualization and assessment of the anterior chamber angle, is an essential procedure in the diagnosis and management of glaucoma. The term gonioscopy was coined by Trantas in 1907. Subsequently, Goldmann introduced the gonioprism, and Barkan mastered the art of gonioscopy and highlighted its role in the management of glaucoma. All cases of glaucoma should undergo a routine and periodic gonioscopic evaluation. The procedure is fairly easy to perform, but experience is needed in accurate assessment and interpretation.
Optical Principles The anatomy of the eye is such that the angle recess is not visualized by routine instrumentation due to total internal reflection of rays emerging from the angle recess. The gonioscope was evolved to overcome this optical problem of critical angle (Fig. 7.1).
Types of Gonioscopy Direct Gonioscopy Direct gonioscopy is performed with the aid of
Fig. 7.1: Optical principles of gonioscopy: a: Incident light from the angle exceeds critical angle resulting in total internal reflection and preventing visibility of the recess. b and c: The gonio lens optically eliminates the cornea as shown in the schematic diagrams and allows visibility of the angle
concave contact lenses (e.g. Koeppe) placed over an anesthetized cornea with the patient in supine position and the space between the lens and cornea filled with normal saline or methyl cellulose as a coupling agent. Viewing is achieved directly using a hand-held biomicroscope and an illuminator. Alternatively, the operating microscope can be used to evaluate the angle of the anterior chamber by making appropriate adjustments. Koeppe's lenses are available in diameters of 16 mm and 18 mm allowing easy
Gonioscopy TABLE 7.1: CONTACT LENSES USED FOR GONIOSCOPY Type
1. Koeppe
Diagnostic lens—50 diopter concave lens available in two sizes for infants (16 mm) and adults (18 mm) Surgical lens—available in various sizes and has blunted edges allowing access for goniotomy Surgical and diagnostic lens Surgical lens for goniotomy Diagnostic lens for evaluating neonatal angle
2. Barkan 3. Thorpe 4. Swan-Jacob 5. Layden Indirect
1. Goldmann single mirror and three mirrors 2. Zeiss and Posner four mirrors 3. Sussman four mirrors 4. Ritch trabeculoplasty lens
Diagnostic and therapeutic lenses, provide excellent images with good magnification and globe stability Ideal diagnostic lenses, patient friendly and very valuable in evaluating narrow angles and to perform indentation gonioscopy Hand held four mirrors similar advantages as the Zeiss lenses Four-mirrored lens with pairs inclined at 59 and 62 degrees. One of each set has a convex lens over it providing magnification—both diagnostic and therapeutic
use in pediatric patients. This technique can be practiced both in the outpatient clinic as well as in the operation theatre. A major advantage of this method is that it allows simultaneous comparison of different quadrants of the angle. Apart from the diagnostic value, lenses like the Swan Jacob, Barkan and Thorpe aid in surgical intervention (Figs 7.2 and 7.3).
Fig. 7.2: Koeppe's lenses
Indirect Gonioscopy Indirect gonioscopy employs reflecting prisms (e.g. Goldmann lens) mounted in a contact lens and angulated at appropriate degrees to evaluate the angle structures using the slit-lamp. The most popular lenses are the Goldmann type, Zeiss, Posner and Sussman four mirrors (Table 7.1). Goldmann lenses (Fig. 7.4) are of two types: (i) Single mirrored—has a mirror angulated at 62°, (ii) Three-mirrored lens—has mirrors at 59° (tongue-shaped, used to evaluate the angle), 67°
Fig. 7.3: Surgical lenses: Barkan and Thorpe
Fig. 7.4: Goldmann lenses three and single mirror
Diagnostic Procedures in Ophthalmology (midsized, used to view midperipheral fundus) and 73° (long, used to view peripheral fundus and ciliary body). The central wall has a diameter of 12 mm and radius of curvature of 7.38 mm. A newer modified version with 8.4 mm radius of curvature eliminates the need of using a coupling solution. The three mirrors also aid in retinal evaluation and laser therapy. Zeiss lens (has under holder), or Posner (has a screw-in handle) four mirror has mirrors angulated at 64° and are amongst the most popular gonioscopy lenses. The Zeiss four mirror (Fig. 7.5) eliminates the need for rotation to evaluate the angle and it's radius of curvature is 7.8 mm, closer to the corneal curvature, thereby eliminating the need for a viscous coupling agent. The diameter of the lens is 9.0 mm which aids in dynamic or compression gonioscopy, an important technique in evaluating narrow angles and angle-closure glaucomas.
4. Adequate anesthesia is ensured using either 0.5% topical proparacaine or 4.0% lignocaine. 5. The patient and examiner should be in a comfortable posture with adequate support to examiner's forearm and elbow to make sure of good control and minimal pressure over the eye throughout the procedure. 6. The lens is held in the examiner's left hand for evaluating the right eye and vice versa. 7. The three-mirror gonioscope is filled with viscous solution and inserted as shown in Figure 7.6. The four-mirror is applied directly (Fig. 7.7).
Fig. 7.6: The inferior rim of three mirror gonioscope is inserted in the lower fornix with patient in upgaze as shown and swiftly tilted on to the cornea preventing loss of any coupling fluid
Fig. 7.5: Zeiss four mirror lens
Protocol for a Routine Gonioscopy 1. Explain the procedure to the patient. 2. Reassure the patient and ensure cooperation. 3. Corneal surface is examined to rule out any contraindication for gonioscopy (abrasion, infection, significant corneal edema or opacity).
Fig. 7.7: The four mirror gonioscope is applied gently and directly on to the cornea. Fingers rested over cheek to ensure adequate support and prevent inadvertent pressure over the globe
Gonioscopy 8. The patient is asked to maintain a straight gaze once the lens is in situ. 9. Low, but adequate illumination, and small beams are focused on the mirror, with viewing and illumination maintained in the same axis. The illumination arm is moved paraxial when needed to evaluate the nasal and temporal recesses. Magnification and illumination can be increased when needed to evaluate finer details like new vessels and foreign bodies. One quadrant can be evaluated at a time with the three mirror by sequential rotation while with the four mirror gonioscope all four quadrants can be evaluated without rotation and with minimum adjustments of the slit-lamp. Always remember the opposite quadrant (e.g. with mirror at 7 o'clock, the 1 o'clock angle) is being evaluated and the image is reversed but not crossed. Other dynamic maneuvers like compression and over the hill evaluation are subsequently done. Over the hill maneuver involves asking the patient to look in the direction of the mirror; which in turn gives access to viewing angle recess over the convex iris. Compression techniques will be dealt with subsequently. 10. Disinfection of lenses is necessary prior and after every use because of the potential of transmitting infection. Lenses can be swiped dry with bacillocid (2% gluteraldehyde) or alternatively lenses can be rinsed with soap solution and water and allowed to dry.
Gonioscopic Anatomy and Interpretation Repeated and routine normal gonioscopic studies are essential in adding to one's experience in evaluating a pathological angle. A methodical evaluation of each structure either from iris plane
Fig. 7.8: Gonioscopic landmarks of a normal angle: 1 Iris root, 2 Ciliary body band, 3 Scleral spur, 4 Trabecular meshwork, 5 Schwalbe's line, 6 Schlemm's canal, 7 Parallelopiped effect
to Schwalbe's line (Fig. 7.8) or from iris plane to Schwalbe's should curtail errors in interpretation. To start with, from the peripheral iris plane one can follow upwards to the insertion of iris root. The contour of iris has several variations. The normal adult eye has a slightly convex contour. The same may be exaggerated in hyperopic eyes, where in the anterior segment it is crowded. A flat iris configuration is commonly associated with myopia and aphakia. A flat iris configuration with a peripheral convex roll or hump of iris that lies in close relation to the trabecular meshwork and can be seen in phakic normal eyes which often mimics a narrowangle and is referred to as plateau iris configuration. Contours could also be concave and are associated with high myopes and pigment dispersion syndrome. The insertion of iris root, may vary from a posterior, anterior or high insertions, thereby determining the visibility of the ciliary body band and the contour and depth of angle recess. The ciliary body band is composed of the anterior end of ciliary muscle
Diagnostic Procedures in Ophthalmology and is seen as a slate gray or dark brown uniform band when insertion of iris root is posterior, anterior and high insertions preclude its view. An unusually wide ciliary body band may be seen in myopes and aphakes and may be confused with angle recession, but comparative gonioscopy and other signs of trauma help to distinguish between the normal and the pathological. The next anterior transition is the scleral spur, the most prominent and most important landmark, identification of which is vital in terms of orientation of the angle. The scleral spur is the posterior lip of the scleral sulcus and is attached to the ciliary body band posteriorly and to the corneoscleral portion of trabecular meshwork anteriorly. It is visible as a glistening opaque white line between the ciliary body band and trabecular meshwork, however, identification at times may be difficult when the trabeculum is nonpigmented. The scleral spur may be obscured in the presence of dense pigmentation of angle structures like in posttraumatic or postsurgical situations. Iris processes, which are fine uveal strands arising from anterior iris surface and running upto the corneoscleral meshwork may also prevent a good view especially when they are prominent, as seen in congenital glaucomas. The spur is not visible in the presence of peripheral anterior synechiae or appositional angle-closure on routine gonioscopy. The trabecular meshwork has a posterior functional, more pigmented portion and a less functional nonpigmented anterior portion. The corneoscleral part of the meshwork extends from the scleral spur to the Schwalbe's line. The pigmentation of the meshwork varies with the kind of eyes, age and other pathological conditions. Brown eyes and adult eyes tend to have a deeper pigmentation compared to blue eyes and younger individuals. A nonpigmented trabecular meshwork may often present a tricky situation as far as accurate assessment is concerned, since its color and texture seems to
merge with the scleral spur. However, a careful evaluation reveals it to be a more translucent and less white structure. The parallelopiped effect is a useful adjunct that can be used in situations wherein the landmarks are indistinguishable. This effect causes a narrow-slit beam of light that is reflected from the anterior and posterior corneal surfaces to collapse at the Schwalbe's line. Once this point is identified the other landmarks can be assessed based on the distance from the line. The Schlemm's canal is usually not visible, but can be seen through a less pigmented posterior trabeculum when reflux blood fills up either due to raised episcleral venous pressure, or rarely as a normal phenomenon. Excess pressure over the globe especially with a threemirror gonioscope can also cause artifactual filling up of the Schlemm's canal with blood. Schwalbe's line as described before represents the peripheral termination of the Descemet's membrane. Usually optically identified by the parallelopiped method, it also at times appears as a prominent white ridge known as posterior embryotoxon, a misnomer. This ridge is better appreciated when the patient looks in the direction of the mirror and is more prominent in the temporal quadrants. The line may occasionally be pigmented and is referred to as Sampaolesi line as seen in pseudoexfoliation and pigment dispersion syndrome.
Pediatric Eye The pediatric eye has definite but subtle variations in its anatomy. The iris contour in a newborn is usually flat and its insertion is posterior to scleral spur with the anterior extension of ciliary body band visible. This contour does eventually become convex as the angle recess develops in 6-12 months. The trabecular meshwork is nonpigmented and appears thick and translucent. Congenital glaucomas present with
Gonioscopy TABLE 7.2: CLASSIFICATION SYSTEMS FOR GONIOSCOPY System
Scheie (1957)
Shaffer (1960)
System basis
Extent of angle structures visualized
Angular width of recess Insertion of iris root
Spaeth (1971) Angular approach to the recess Configuration of peripheral iris
Angle structures and classification All structures visible Angle recess not seen Ciliary body band not seen Posterior trabeculum obscured Only Schwalbe's line visible
Wide open Grade I narrow Grade II narrow Grade III narrow Grade IV narrow
Wide open (30°-45°) Moderately narrow (20°) Extremely narrow (10°) Partly or totally closed
Grade Grade Grade Grade
Anterior to Schwalbe's line Behind (posterior) to Schwalbe's line At scleral spur Deep into ciliary body band Extremely deep 0-40 degrees
Regular (slightly convex) Quirk (posterior bowing) Steep
r q s
3-4, closure impossible 2, closure possible 1, closure probable 0, closure present
anterior insertions of the iris directly on to the trabeculum and at times the anterior iris stroma sweeps upward in a concave fashion to insert onto the trabecular meshwork.
Grading and Recording of Gonioscopic Findings Though multiple individual variations in assessment and grading gonioscopic details are being followed, it is important to follow a certain protocol of documentation, which aids in follow up of the disease process. Among the systems described (Table 7.2), the Spaeth's system is thought to be complete as it covers details with regard to angle width, iris insertion and configuration. Any gonioscopic data should contain: (a) width of angle recess, (b) iris contour and insertion of iris root, (c) degree of pigmentation and (d) presence of abnormal structures in each quadrant. Figure 7.9 shows a wide-open angle (Shaffer's grade IV or Speath's D40r) with regular iris contour and deep recess.
Fig. 7.9: Gonio-photograph of a grade IV Shaffer's angle (corresponds to Spaeth—D40r). (a) Iris root, (b) Ciliary body band, (c) Scleral spur, (d) Trabecular meshwork. Iris contour is regular with a deep recess
All the landmarks—iris root, ciliary body band, scleral spur and trabecular meshwork are visible. When insertion of iris occurs at scleral spur, the peripheral iris appears slightly convex, the angle of the anterior chamber still remains open (Shaffer's grade III or Speath's C30r, Fig. 7.10).
Fig. 7.10: Gonio-photograph of a grade III Shaffer's angle (corresponds to Spaeth—C30r). Landmarks are visible upto scleral spur with a mild iris convexity
Compression Gonioscopy Compression or indentation gonioscopy is a simple and invaluable technique that one needs to know to assess narrow angles (Fig. 7.11) and chronic angle-closure situations. It helps distinguish appositional angle-closure from synechial angle-closure. The technique employs exerting external pressure over the cornea using the Zeiss, Posner or Sussman four mirror lenses; thereby forcing the lens iris diaphragm posteriorly and allowing to visualize the hidden angle recess (Fig. 7.12). The technique involves a routine assessment of all quadrants, following which, if one subsequently decides the angle is narrow, each
Fig. 7.12: The same angle on compression widens to reveal landmarks upto scleral spur
quadrant is re-evaluated using a narrow slitbeam (to prevent miosis causing artifactual opening of the angle recess), pressure is applied directed towards the center of the eye. This results in deepening of the anterior chamber in the area of recess caused by bowing back of peripheral iris along with stretching of the limbal scleral ring and straightening of the angle recess; following this one can see structures that were not visible earlier, or confirm the presence of peripheral anterior synechiae. Corneal folds often distort the view but this can be minimized with appropriate technique in application of pressure. The physiological principles involved in compression gonioscopy have been depicted in Figure 7.13. Compression may not be effective when intraocular pressures are beyond 40 mm Hg as this limits the expansion of the limbal scleral ring.
Common Gonioscopic Findings and their Variations Peripheral Anterior Synechia (PAS)
Fig. 7.11: The photograph shows a narrow angle visible upto the Schwalbe's line
The peripheral anterior synechia is a pathological term referring to the adhesions of peripheral iris to the anterior angle structures, most often the functional trabecular meshwork, or rarely,
Gonioscopy arising from the peripheral iris surface and branching out in an arborizing and lacy pattern onto the corneoscleral portion of trabecular meshwork. Varying amounts of peripheral anterior synechiae may also be associated depending on the stage of disease process.
Pigmentation Fig. 7.13: Compression gonioscopy: a: The narrow angle appears closed on a routine gonioscopy, b: Compression fails to allow visibility of angle structures due to PAS, c: Compression widens the recess and allows a view of all structures in the absence of PAS
extending to the Schwalbe's line. Typically seen associated with primary angle-closure glaucoma, uveitic and other secondary angle-closure glaucomas, PAS may often be confused with iris processes—which are normal fine lacy cords of uveal tissue extending from the peripheral iris to the trabecular meshwork. PAS on the other hand are broad adhesions commonly localized to quadrants with areas in between widening with indentation technique of gonioscopy. An angle that is closed 360° may often present a dilemma but one can follow the slit-beam from the posterior surface of the cornea which normally does not meet the beam on the iris directly in an angle that is open but instead lies alongside the other. A direct continuation of the beam without a break is suggestive of a closed-angle. Clinical correlation and experience will often help overcome this hurdle.
The trabecular meshwork has a varying amount of pigmentation varying from 0 to 4, which is a subjective grading that correlates to none (0), faint (1), average (2), heavy (3), and very heavy (4). Pigmentation increases with age under normal physiological conditions. Excessive pigmentation is usually pathological and is associated with pseudoexfoliation syndrome, pigment dispersion syndrome, traumatic and uveitic glaucomas.
Other Abnormal Findings A variety of surprises may be hidden in the angle recess. Blood in Schlemm's canal appears as a uniform linear reddish hue just anterior to pigmented trabecular meshwork and is associated with raised episcleral venous pressure. It can also be observed under normal conditions and as an artifact when excess external pressure is exerted during gonioscopy. Pseudoexfoliative material, microscopic hyphema and hypopyon can be visualized. Foreign bodies and emulsified silicone oil globules are among the other things that can be picked up by a careful gonioscopy.
Blood Vessels Normally all vessels in the angle are restricted to the ciliary body band and iris root and do not extend to the scleral spur or trabecular meshwork. Anomalous vessels are not rare, they, however, can readily be distinguished from neovascularization which are vessels usually
Conclusion In conclusion, the diagnostic basis of any glaucoma should be in correlation to the gonioscopic findings whenever possible. The management and prognosis of the disease depends on a complete diagnosis that includes
Diagnostic Procedures in Ophthalmology a routine and periodic gonioscopic evaluation. Gonioscopy widens our scientific understanding of the disease process and guides us to manage the disease more effectively.
Bibliography 1. Epstein DL. Chandler and Grant's Glaucoma (3rd edn). Philadelphia: Lea and Febiger, 1986. 2. Fellman RL, Spaeth GL, Starita RJ. Gonioscopy: Key to successful management of glaucoma. In Focal points Clinical Modules for Ophthalmologists, San Francisco, AAO 1984.
3. Kanski JJ, James AM, John FS. Glaucoma—A Colour Manual of Diagnosis and Treatment (2nd edn). London, Butterworth-Heinemann, 1996. 4. Kolker AE, Hetherington J Jr. Becker-Shaffer's Diagnosis and Therapy of the Glaucomas (5th edn). St Louis, Mosby, 1985. 5. Neil TC, Diane CL. Atlas of Glaucoma. Martin Dunitz, 1998;39. 6. Palmberg P. Gonioscopy. In Ritch R, Shields MB, Krupin T (Eds). Glaucomas (2nd edn). St Louis, Mosby, 1996. 7. Shields MB. Aqueous humor dynamics. II. Techniques for evaluating. In: Textbook of Glaucoma (3rd edn). Baltimore, Williams and Wilkins, 1992.
Optic Disk Assessment in Glaucoma
RAJUL PARIKH, CHANDRA SEKHAR
An estimated 67 million people worldwide have glaucoma in the year 2000. At least 50% do not know that they have the disease since it is usually without symptoms.1,2 Rapid advances in imaging technologies such as confocal scanning laser ophthalmoscopy, scanning laser polarimetry and optical coherence tomography for detection of early glaucomatous damage have only moderate sensitivity and specificity.3-5 New psychophysical procedures such as short wavelength automated perimetry, frequency doubling perimetry and motion automated perimetry which are targeted at specific visual functions have been shown to be more sensitive and specific than standard automated perimetry for identifying early glaucomatous damage. 6-8 However, these techniques may not be available to all clinicians and have the limitations of all subjective tests. Several studies have shown that abnormalities in the appearance of the optic disk may precede visual field defects.9,10 Conventional stereoscopic clinical evaluation and imaging of the optic disk with fundus photographs is still the most frequently used and sensitive means of diagnosing glaucoma. 11 With some training, it is possible to clinically evaluate optic nerve head and retinal nerve fiber layer stereoscopically and detect early glaucomatous damage. The aim
of this communication is to describe the morphological changes of the optic nerve in glaucoma, highlight the techniques of clinical evaluation of the optic disk and discuss the differential diagnosis.
Methods of Optic Disk Examination Traditionally, the direct ophthalmoscope has been used for the evaluation of the optic nerve head. Though it has the advantage of providing a magnified view of the optic nerve head, it, however, lacks stereopsis and can result in missing of subtle changes. Therefore, the use of the direct ophthalmoscope is to be strongly discouraged. A variety of contact and noncontact lenses are available which allow stereoscopic view of the fundus at the slit-lamp. Contact lenses such as Goldmann lenses are relatively uncomfortable for the patient, take longer time and the coupling fluid can cause transient blurring and difficulty in obtaining good quality fundus photographs. Noncontact lenses include +60D, +78D, +90D and Volk superfield lenses (Fig. 8.1). These provide excellent stereoscopic and magnified view of the optic disk.
Fig. 8.1: Noncontact lenses: +60D, +78D and Volk superfield lenses
It is important to draw the appearance of the optic nerve head based on these methods. Though drawing of the optic disk suffers from the disadvantage of being subjective in nature, this does offer a quick and inexpensive method of evaluation of the optic nerve head in patients with glaucoma during follow-up. In addition, photographs may not be possible in all cases such as patients with rigid miotic pupils and those with significant media opacities. However, wherever possible, photographs are an indispensable adjunct to clinical evaluation.
Fig. 8.2: Disk margin (black arrow) and cup margin (white arrow)
Features of Glaucomatous Disk Damage Cup-Disk Ratio Early studies by Armaly et al have reported that the vertical and horizontal cup-disk diameter ratios are useful for the quantification of glaucomatous optic neuropathy and for early detection of glaucoma.12 The ratio has limited value in the identification of glaucomatous damage, because of the wide variability in the size of the optic cup in the normal population. Disk margin is defined by inner edge of white scleral ring (outer arrows), and the optic cup is the level at which neuroretinal rim (NRR) steeps (inner arrow) (Figs 8.2 and 8.3). A large cupdisk ratio can be normal if the optic disk is large13 and a small cup-disk ratio may be glaucomatous if the optic disk is small14 (Fig. 8.4). The problem with estimating cup-disk ratio as a measure of
B Figs 8.3A and B: Vertical disk diameter and horizontal disk diameter
Fig. 8.5: Measurement of disk diameter with slitlamp biomicroscopy with use of noncontact lenses
B Figs 8.4A and B: Cup-disk ratio in relation to optic disk size. A Optic disk is small with small cup and still has inferior notch (white arrow) with nerve fiber layer defect (black arrows) B Cup-disk ratio in a large disk
glaucomatous damage is that it is difficult to decide if the cup is physiological in a large disk or pathological in a small or normal-sized disk. In a recent study by Garway-Heath et al, vertical cup-disk diameter ratio corrected for the optic disk size was the best variable to separate between normal subjects and patients of ocular hypertension with retinal nerve fiber layer defect.15 Therefore, in the clinical description of the optic nerve head, it is important to state the vertical cup-disk diameter ratio in combination with the estimated disk size. The disk diameter can be easily measured by adjusting the slit-lamp beam height to the edges of the disk while viewing
the disk with a 60D lens (Fig. 8.5).16 The measurement by this method is roughly equal to the measurement obtained by the planimetry of disk photographs by Litmann's correction. Measurements can also be made with other lenses by multiplying the measured value with the appropriate magnification factor, Goldmann contact lens X1.26 and Volk superfield lens X1.5.16 It is important to differentiate contour cupping from color cupping. The margin of the cup should be determined by the bend of the small vessels
Fig. 8.6: Asymmetry of cupping in relation to asymmetry of disk size. The left optic disk is larger than right optic disk and has a larger optic cup
Fig. 8.7: HRT print out of the same optic disks shown in Fig. 8.6 showing asymmetry of optic disk cup in relation to disk area
across the disk rim and not by the central area of disk pallor.
Asymmetry of Optic Disk Cupping Asymmetry of cupping is seldom seen in normal eyes and until proven otherwise, must be taken as an indication of early glaucomatous damage. However, while assessing asymmetry, it is important to rule out asymmetry of the disk size, which may be due to anisometropia. This can result in difference in the cup-disk ratio between two eyes, in the absence of glaucoma (Figs 8.6 and 8.7).
Neuroretinal Rim Evaluation Glaucomatous damage can be diffuse, focal or a combination of both. Diffuse damage results in symmetrical enlargement of the cup. Focal damage usually involves a particular area of the rim. Normally, according to the ISNT rule, the inferior rim is the thickest followed by the superior, the nasal and then the temporal (Fig. 8.8).17 During optic nerve head evaluation, one
Fig. 8.8: Shows ISNT rule, the inferior rim is the thickest followed by the superior, the nasal and then the temporal
must look carefully for any areas of thinning of the neuroretinal rim or for notching or in other words extension of the cup into the rim tissue. If the cup is especially deep in the notch, it is known as a pseudo-pit. Notching and pseudopits are usually seen at the superior or inferior poles. The width of the notch tends to correspond to the extent of the visual field defect (Figs 8.9A and B, and 8.10A and B). Optic rim pallor is another important indicator of glaucomatous disk
B Figs 8.9A and B: Relation between neuroretinal rim notch and visual field defect. The optic disk photograph shows inferior notch (black arrow) with corresponding superior arcuate field defect
B Figs 8.10A and B: Relation between inferior notch (here inferior notch is wider than the one seen in Fig. 8.9) and visual field defect. The optic disk photograph shows neuroretinal notch (black arrow) with corresponding superior arcuate field defect
Optic Disk Assessment in Glaucoma damage. In the glaucomatous optic disk, the pale and translucent atrophic tissue may replace the normal pink color of the neuroretinal rim which can result in a field defect in the corresponding opposite hemisphere.
Vascular Changes Splinter hemorrhages on the optic disk are a common finding in glaucoma patients (Fig. 8.11). Various studies have shown that disk hemorrhages in association with localized nerve fiber layer defects and notches of the neuroretinal rim are more common among patients of normal tension glaucoma.18, 19 A possible explanation for the difference in frequency has been suggested by Jonas et al. They stated that the amount of blood leaking out of a vessel into the surrounding tissue depends on the intraocular pressure when the bleeding occurs.19 High transmural pressure gradient in normal pressure glaucoma leads to larger disk hemorrhages. Also, since the absorption rate of disk hemorrhages depends on the size of the disk bleed, the hemorrhages in patients of normal pressure glaucoma may take a longer time to disappear and thus have a higher chance to be detected than the disk
Fig. 8.11: Disk hemorrhage
hemorrhages in patients of high pressure glaucoma.20 Hemorrhages in glaucoma usually appear as splinter-shaped or flame-shaped hemorrhages on the disk surface21 (Fig. 8.11). They usually precede neuroretinal rim changes and visual field defects. The defects corresponding to the location of the hemorrhage may be expected to appear weeks to year later.22 The presence of disk hemorrhages is considered an indication for the enhancement of treatment of glaucoma.
Configuration of Vessels The retinal vessels on the optic nerve head can provide clues about the topography of the disk. Nasalization of the vessels and baring of circumlinear vessels can be seen in glaucoma as well as in other diseases of the optic nerve. Bayoneting of the vessels can be seen if the rim is absent or very thin. This causes the vessels to pass under the overhanging edge of the cup and then make a sharp bend as they cross the disk surface. This convoluted appearance of the vessels is called 'bayoneting'.
Peripapillary Atrophy The zone closer to the optic nerve head with retinal pigment epithelium (RPE) and choroidal atrophy and baring of sclera is called zone β. The more peripheral zone with only RPE atrophy is called zone α (Fig. 8.12). A highly significant correlation has been reported between the location of peripapillary atrophy and visual field defects.23 Sometimes, these changes may represent a congenital anomaly, especially in myopic eyes. However, appearance of these changes de novo or their presence in small, nonmyopic disks should be viewed with suspicion. Peripapillary atrophy may be focal or circumferential (Figs 8.13 and 8.14).
Fig. 8.12: Peripapillary atrophy. The diagram shows atrophic zone closer to the optic nerve head called zone β and the more peripheral zone called zone α
Fig. 8.13: Peripapillary atrophy: Localized in the temporal area of the disk
Retinal Nerve Fiber Layer Abnormalities Examination of the nerve fiber layer is often useful in detecting early glaucomatous damage among patients of ocular hypertension with normal disk appearance and normal visual fields. The neuroretinal rim is formed by axons converging from the retina to the scleral canal. Since the axons are spread out in a thin layer in the retina, even minor losses of the axons can be observed in the retinal nerve fiber layer. In healthy eyes, the nerve fiber layer appears opaque with
Fig. 8.14: Peripapillary atrophy: generalized
radially oriented striations. The small retinal blood vessels have a blurred and crosshatched appearance, as they lie buried in the nerve fiber layer. The best way to see the nerve fiber layer defect is through a dilated pupil with a stereoscopic lens, at the slit-lamp, using white or green light and a wide-slit beam. In the presence of nerve fiber layer atrophy, the small retinal blood vessels become more clearly visible and appear unusually sharp, clear and well focused (Fig. 8.15). The fundus in the affected area appears darker and deeper red in contrast to the silvery or opaque hue of the intact nerve fiber layer. Defects may be in the form of a wedge shape arising from the disk margin and widening towards the periphery, are pathological (Fig. 8.16), while slit-like defects narrower than the adjacent blood vessels may be physiological. Diffuse areas of atrophy are less common in early glaucoma and more difficult to identify.
Myopic Changes vs Glaucoma Myopic disks can present difficulty in evaluation for glaucoma due to the tilted disks, peripapillary atrophy and shallow cupping. One needs to
Fig. 8.15: Retinal nerve fiber layer defect: Wedge-shaped RNFL defect can be seen between two black arrows. It is more easily marked in red free photograph
Fig. 8.17: Myopic disk with primary open-angle glaucoma Fig. 8.16: Retinal nerve fiber layer defect. Wedge-shaped RNFL defect reaching up to optic disk margin
carefully examine the disk to look for changes in the contour of the blood vessels, as well delineate the disk margin from the peripapillary changes (Fig. 8.17).
Differential Diagnosis In addition to glaucoma, other abnormalities can cause excavation and or pallor of the optic disk
and it is, therefore, important to rule out these possibilities before making the diagnosis of glaucoma.
Physiological Cupping Assessment of the size of the optic disk, careful examination of the neuroretinal rim and the retinal nerve fiber layer can help distinguish physiological cupping from glaucomatous damage in most cases.
Diagnostic Procedures in Ophthalmology Optic Nerve Coloboma Optic nerve colobomas typically demonstrate enlargement of the papillary region, partial or complete excavation, blood vessels entering and exiting from the border of the defect and a glistening white surface. The visual field defects can be in the form of generalized constriction, centrocecal scotomas, altitudinal defects, arcuate scotomas, enlargement of the blind spot and ring scotomas that can mimic those found in glaucomatous eyes. Morning glory syndrome is a variant of optic disc coloboma and is characterized by a large excavated disk, central core of white or gray glial tissue surrounded by an elevated annulus of variably pigmented subretinal tissue (Fig. 8.18). The retinal vessels appear to enter and exit from the margins of the disk, are straightened and often sheathed.
Fig. 8.19: Optic disk photograph showing congenital optic disk pits
in about one-third. Involvement is usually unilateral in about 80% cases and the optic disk is larger on the involved side. Approximately 55-60% of the eyes have a field defect in the form of arcuate scotomas, paracentral scotoma, altitudinal defect, generalized constriction and nasal or temporal steps.24 In the absence of other indicators of congenital anomaly (like associated fundus coloboma, the differential diagnosis may be difficult and the absence of progression on follow-up may be the only indicator that the patient has a congenital anomaly and not glaucoma. Fig. 8.18: Optic disk photograph showing characteristic morning glory syndrome
Congenital Optic Disk Pit Congenital optic disk pits appear gray or yellowish-white, round or oval, localized depression within the optic nerve (Fig. 8. 19). They are located within the temporal aspect of the disk in over half of the cases and centrally
Anterior Ischemic Optic Neuropathy A history of acute visual loss, initial swelling of the optic disk, absence of marked cupping, rise in ESR, presence of centrocecal scotoma or altitudinal defects can help differentiate it from glaucoma (Fig. 8.20). In the late stages the cupping in some cases may be exactly the same as is seen in glaucoma.
Fig. 8.20: Anterior ischemic optic neuropathy. The right-sided optic disk photograph is from patients with longstanding AION showing typical glaucomatous cupping
B Figs 8.21A and B: A Optic disk photograph showing significant cupping, but with out of proportion pallor. B Visual field defect showing a temporal hemianopia suggestive of pituitary tumor
Diagnostic Procedures in Ophthalmology Neurological Causes Pallor disproportionate to cupping, normal intraocular pressure or unusual history of onset, progression and age should arouse suspicion of a neurological disorder causing optic disk damage (Fig. 8.21). Presence of visual field defects that respect vertical midline and the pattern of the field defects should be able to suggest the possible site of the intracranial lesion.
Summary In summary, the optic disk evaluation in glaucoma is best done stereoscopically at the slit- lamp with a dilated pupil using one of the 60D, 78D or 90D lenses. Changes in the neuroretinal rim, optic disk hemorrhages, peripapillary atrophy and nerve fiber layer defects are more important features than the cup-disk ratio. The cup-disk ratio is to be documented and interpreted along with the disk size and not in isolation. The diagnosis of glaucoma will depend on the presence of a visual field defect that correlates with the anatomic changes on the optic nerve head and the peripapillary retina.
References 1. Quigley HA. Number of people with glaucoma worldwide. Br J Ophthalmol 1996;80:389-93. 2. Dandona L, Dandona R, Srinivas M, et al. Openangle glaucoma in an urban population in southern India: the Andhra Pradesh Eye Disease Study. Ophthalmology 2000; 107(9): 170209. 3. Zangwill LM, Bowd C, Berry CC, Williams J, Blumenthal EZ, SanchezGoleans CA, Vasilie C, Wainreb RN. Discriminating between normal and glaucomatous eyes using the Heidelberg retina tomograph, GDx nerve fibre analyser and optical coherence tomograph. Arch Ophthalmol 2001;119:985-93.
4. Bowd C, Zangwill LM, Berry CC, Blumenthal EZ, et al. Detecting early glaucoma by assessment of retinal nerve fibre layer thickness and visual functions. Invest Ophthalmol Vis Sci 2001;42:2001-03. 5. Medeiros FA, Zangwill LM, Bowd C, Weinreb RN. Comparison of the GDx VCC scanning laser polarimeter, HRT II confocal scanning laser ophthalmoscope, and stratus OCT optical coherence tomograph for the detection of glaucoma. Arch Ophthalmol 2004;122;827-37. 6. Johnson CA, Adams AJ, Casson EJ, Brandt JD. Blue-on-yellow perimetry can predict the development of glaucomatous field loss. Arch Ophthalmol 1993;111:645-50. 7. Bayer AU, Maag KP, Erb C. Detection of optic neuropathy in glaucomatous eyes with standard visual fields using a battery of short wave-length automated perimetry and pattern electroretinography. Ophthalmology 2002;109: 1009-17. 8. Sample PA, Bosworth CF, Blumenthal EZ, Girkin C, Weinreb RN. Visual function-specific perimetry for indirect comparison of different ganglion cell populations in glaucoma. Invest Ophthalmol Vis Sci 2000;41:1783-90. 9. Quigley HA, Dunkelberger GR, Baginski TA, et al. Chronic human glaucoma causing selectively greater loss of larger optic nerve fibers. Ophthalmology 1988;95:357-63. 10. Sommer A, Pollack I, Maumenne AE. Optic disc parameters and onset of glaucomatous field loss: I Methods and changes in disc morphology. Arch Ophthalmol 1979;97:1444-48. 11. Greaney MJ, Hoffman DC, Garway-Heath DF, et al. Comparison of optic nerve imaging methods to distinguish normal eyes from those with glaucoma. Invest Ophthalmol Vis Sci 2002; 43(1):140-45. 12. Armaly MF, Saydegh RE. The cup/disc ratio. Arch Ophthalmol 1969;82:191-96. 13. Jonas JB, Zach F-M, Gusek GC, Naumann GOH. Pseudoglaucomatous physiologic large cups. Am J Ophthalmol 1989;107:137-44. 14. Jonas JB, Fernandez MC, Naumann GOH. Glaucomatous optic nerve atrophy in small disks with low cup-to-disc ratios. Ophthalmology 1990;97:1211-15. 15. Garway-Heath DF, Ruben ST, Viswanathan A, Hitchings R. Vertical cup/disk ratio in relation to optic disk size: its value in the assessment
of the glaucoma suspect. Br J Ophthalmol 1999; 82:1118-24. Jonas JB, Dichtl A. Advances in the assessment of the optic disc changes in early glaucoma. Cur Opi Ophthalmol 1995;6:61-66. Jonas JB, Gusek GC, Naumann GOH. Optic disc, cup and neuroretinal rim size, configuration, and correlations in normal eyes. Invest Ophthalmol Vis Sci 1991;29,1151-58, Invest Ophthalmol Vis Sci 1993;32. Kitazawa Y, Shirato S, Yamamoto T. Optic disc hemorrhage in low-tension glaucoma. Ophthalmology 1986;93:853-57. Jonas JB, Budde WM. Optic nerve head appearance in juvenile-onset chronic highpressure glaucoma and normal-pressure glaucoma. Ophthalmology 2000;107:704-11.
20. Jonas JB, Xu L. Optic disc hemorrhages in glaucoma. Am J Ophthalmol 1994;118:1-8. 21. Drance S.M, Fairclough M, Butler DM, Kottler MS. The importance of disc haemorrhage in the prognosis of chronic open-angle glaucoma. Arch Ophthalmol 1977;95:226-28. 22. Heijl A. Frequent disc photography and computerized perimetry in eyes with optic disc haemorrhage. Acta Ophthalmol 1986;64: 274-81. 23. Jonas JB. Naumann GOH. Parapapillary chorioretinal atrophy in normal and glaucoma eyes. II. Correlations. Invest Ophthalmol Vis Sci 1989;30:919-26. 24. Brown GC. Congenital fundus abnormalities. In: Duane TD (Ed). Clinical Ophthalmology 1991, Philadelphia, J.B. Lippincott.
DEVINDRA SOOD, PARMOD KUMAR
Basic Perimetry
Visual field is a part of space, seen at any given moment. Changes in the visual field are produced by a number of disease conditions which can affect the visual system and often manifest through changes in the visual field. Hence, it is essential to determine the extent of the visual field for the diagnosis and management of these conditions. The visual field is usually perceived with both eyes. It is, however, measured separately for each eye. The normal visual field extends up to 50 degrees superiorly, 70 degrees inferiorly, 60 degrees nasally and 90 degrees temporally. After defining the visual field for each eye, the two can be compared with each other for asymmetry or compared to a normal reference test for any abnormality and be examined together to look for patterns suggestive of disease conditions. Perimetry is the science of measuring the peripheral vision ("Peri"= peripheral and "-metry" = measurement). Perimetry involves placing the eye at the center of curvature of a hemispherical or arc-shaped instrument. The test objects have a constant angular size and are at a constant distance from the eye. The visual field has been compared to an island of vision in a sea of blindness by Traquair in 1930. This island of vision is a three dimensional structure. The
x and y co-ordinates represent the location of points on the visual field. At the fovea, the x and y co-ordinates are 0,0. The location of all points on the visual field are described along the x and y axis, with respect to fixation (Fig. 9.1). The blind spot is 15 degrees temporal to fixation. The z axis represents the height of the "hill island of vision" at a given co-ordinate (x,y) and corresponds to the retinal sensitivity at that point. Greater the sensitivity at a given point, greater is the height of the island
Fig. 9.1: A point on the island of vision is marked along the x and y axis
Basic Perimetry of vision. Since sensitivity is maximum at the fovea, the height of "the hill island of vision" (z) is also maximum at the fovea. The retinal sensitivity drops to sea level 15 degrees temporal to fixation (blind spot).
Types of Perimetry Kinetic Perimetry Perimetry aims to draw the map of the island of vision, such that it is a true representation for each eye and also aims to present it in a way which is clinically useful. Earlier methods defined the outer limits of the visual field by moving objects from the non-seeing area to the center. This technique of perimetry, called kinetic perimetry, it utilizes a moving object of a fixed size and intensity (e.g. Tangent screen or Goldmann perimeter) to define the boundary of the island of vision at a fixed height. This line representing the outer boundary for a given size of the test stimulus is called isopter. An isopter is synonymous to a horizontal slice through the hill island of vision. Manual kinetic perimetry allows large areas to be traversed in a fairly short order. One can move quickly over areas of little interest and spend relatively more time in examining critical regions. Equipment is inexpensive and durable. Since the perimetrist is constantly communicating with the patient, the patient is more comfortable. However, reproducible and reliable examinations require technical skill and early or subtle changes are more likely to be overlooked on manual kinetic perimetry. Isopters which are stylized representations of the visual field, making quantification and statistical analysis difficult.
Static Perimetry The outer boundary of the island of vision can also be determined by measuring the retinal
sensitivity (z) at each point (x,y). This technique of perimetry is called static perimetry because the test location is fixed, while the intensity of the test object of known size is varied, e.g. Tubinger, Octopus and Humphrey perimeters. Static perimetry provides a vertical slice through the hill island of vision. Because of the difficulty, inability and a potential for lack of reproducibility with kinetic perimetry, static perimetry is preferred for detecting and following subtle non-geographic defects in the diagnosis and follow-up of glaucoma patients. One can perform effective static perimetry with the tangent screen or the Goldmann perimeter. However, manual static perimetry is tedious, cumbersome and at times boring. Both the patient and the examiner find it difficult to concentrate for 30 to 90 minutes at a stretch. Automated /computerized perimetry presents targets at a random sequence undecipherable by the patient. It can test the same patient with the same methodology year after year and still does not get bored. Kinetic testing is difficult to computerize particularly with regard to the decisions regarding same speed and direction of presentation. A static test, on the other hand is relatively straight forward, since the target does not move, the machine has only to choose a site, target intensity and then record whether the patient responds, yes or no. Computers have revolutionized perimetry by allowing precise repetition and meticulous attention to detail, testing the patient's response under optimal conditions repeatedly by allowing a binary yes/no answer from the patient. All this makes perimetry tailor made for computerization.
Stimulus Presentation During static visual field measurement the stimulus can be presented by projection or nonprojection. In the projection system a simple
Diagnostic Procedures in Ophthalmology computer video monitor is used to present dark or light combinations of stimuli against a diffuse background. This system has the advantage of being more flexible and allows kinetic color perimetry. Drawbacks pertain to the mechanical aspects of presenting and moving the test target such as mechanical failures, periodic maintenance and servicing. Also, the combination of mirrors, shutters and the rotational unit produces an unsuitable clinking noise with each projection. This was used to advantage in earlier models, to assess the reliability of a given field (false positives). Newer models elicit the false positive response by omitting the light stimulus and assessing the pace of the patient to the rhythm of the testing. In the non-projection system stimuli are generated by the turning on and off of Light Emitting diodes (LED) which are placed into the surface of the perimetric bowl. Advantages of LEDs include silent operation, no moving parts, multiple stimuli presentation and inexpensive and durable equipment. However, in the LED system stimuli are fixed in the bowl surface at the time of manufacture, inability to vary stimulus size and color, test site location or resolution pattern. Further fixed LED positions cannot be expanded to accommodate new programs. LEDs have a condensed light output. Slight variations in positioning and mounting of the LED result in different directional light intensities. All LEDs need to be calibrated individually. This needs to be done at the factory and on a routine basis. In the non-projection system a high resolution, flat video monitor can also be used to present the stimuli. In this method, the patient fixes on a pseudo-infinite target and stimuli are presented throughout the visual field. With this method of presentation, test site location is infinitely variable, kinetic perimetry is possible, and stimulus presentation is without the audible click. Additionally the video monitor projection does away need for a perimeter bowl and the projection
device allows greater flexibility and durability. They also occupy less space. However, video monitor systems are able to assess only the central 30 degrees. Projected stimuli are usually white and of variable size and intensity. The size of the stimuli in automated perimeters is similar to that used for Goldmann perimeter. There are five different sizes designated by Roman numerals I to V. One very often uses stimulus size III. Failure to recognize target size III necessitates testing with stimulus size V. However, tests using stimulus size V cannot be processed statistically by STATPAC 2 on the Humphrey perimeter. In static perimetry, the patient has to respond to a stimulus of predetermined size, color and location projected for a fixed duration at a given intensity level. The patient responds with the button in two ways: stimulus seen or stimulus not seen. Any such response is only suggestive but not actual proof, that the light was seen or not seen. For a stimulus of a fixed size and location to be seen depends on its intensity. This probability of a stimulus of fixed size and location when plotted against the intensity of the stimulus is called probability of seeing curve. That is to say, the intensity of the stimulus where it is seen 50% of the time and missed 50% is called threshold. Similarly, the intensity at which the stimulus is seen 95% of the projected times, is called suprathreshold. A low intensity stimulus which is seen only 5% of the times when projected is called infrathreshold.
Bracketing Determining the threshold for each point in the field would require thousands of stimulii of varying intensity. However, the number of stimuli for threshold determination has been conveniently reduced by a testing algorithm which is also accurate. At a given point on the visual field, the patient responds to a given stimulus
Fig. 9.2: The probability of seeing curve
Fig. 9.3: Threshold determination at the point P (Staircase technique)
intensity (P1). The intensity of the stimulus is then decreased in steps of 4 dB till the stimulus is not seen (3). The threshold lies between 2 and 3. The intensity of the stimulus is then increased in steps of 2 dB till the patient is able to perceive the stimulus. Herein the threshold for the point is lying between 4 and 5, and is a more accurate assessment of the threshold value at that point. This technique of threshold determination is called 4-2 bracketing (Staircase technique). In the Octopus perimeter, the thresholding strategy continues, until a third reversal, in steps of 1 dB, called 4-2-1 algorithm (Fig. 9.3). Normal threshold values are dependant on the location of the point on the visual field and also the age of the patient. Fovea, the most sensitive point of the visual field corresponds to 0 degree of eccentricity. As the point moves from the fovea, the threshold value (sensitivity) decreases by 0.3 dB for every
Fig. 9.4: Effect of location and age on threshold
Diagnostic Procedures in Ophthalmology degree of eccentricity outside the macula. Sensitivity drops to zero, 15 degrees temporal to fixation (blind spot). Sensitivity also decreases with the age; 0.6-1dB per decade of life. Since, threshold is age-related, the patients date of birth should be correctly entered as the results are compared to age-matched normals. The intensity of light which reflects off the surface is expressed as apostillbs (unit of luminance). The sensitivity of the human visual system varies from 1 to more than 1,000,000 apostillbs (asb). The maximum stimulus intensity of the Octopus Field Analyzer is 1000 asb and for the Humphrey Field Analysis is 10,000 asb. Hence, large numbers representing the listed sensitivity on the printout would be cumbersome. A convenient way of expressing threshold values is in terms of a relative logarithmic scale where the intensity of the stimulus is varied by powers of 10 1 1dB = ———————— log unit (asb) Increasing dB numbers on the printout imply that dimmer stimuli have been perceived. Thresholds corresponding to a dimmer stimulus mean greater retinal sensitivity. In a report of the measured thresholds, large dB values correspond to better sensitivity and small dB numbers indicate reduction in sensitivity.
Testing Strategy With the inherent ability to vary the intensity of the light stimulus, static perimeters explore the visual field in three ways: 1. Suprathreshold screening. 2. Threshold related screening. 3. Full threshold determination. Suprathreshold screening: Very bright stimuli (suprathreshold) intense enough to be seen easily by most normal people are presented. The patient has simply to respond (yes / no) to the presence
of the target. The role of such examinations is related to quick screening of large populations and also to define gross pathology quickly. However, such examinations can miss early changes suggestive of glaucoma. Threshold related screening: Herein, the intensity of the light presented is 5dB brighter than the actual threshold at the test point in question. This allows the entire field to be screened quickly. Threshold related screening is at best a variant of suprathreshold tests which allow for an approximation of the true sensitivity of the visual field. It can be used as a screening test for detection and follow-up of known pathologies. Threshold determination: A more time consuming way of determining the sensitivity of the visual field is by determining the threshold value at each point by the bracketing technique described earlier. After presenting a light stimulus the machine waits for a yes / no response. If the stimulus is not seen, the intensity of the light seen is increased in steps of 4dB till it is visible (machine records this as suprathreshold level). Subsequently, light stimuli are decreased in steps of 2dB till the stimulus is not seen (infrathreshold). The actual threshold is between the suprathreshold and infrathreshold.
Newer Strategies Threshold determination at each point of the visual field is tedious and time consuming. Because by definition threshold is tested by the staircase algorithm, where every patient can see only half of the stimuli presented, newer techniques aim to make the procedure as short as possible, to ensure that the patient maintains concentration and thus provides better reliability. Swedish Interactive Thresholding Algorithm (SITA) is similarly based on the fact that a response at one location has implications at the point tested
Basic Perimetry and also its neighboring points. Just as one tested point is normal, other points on the visual field are likely to be normal too. Tendency Oriented Perimetry (TOP) is available on the Octopus perimeter and takes advantage of each response of the patient five-fold. It tests and adjusts the location where the stimulus is presented and assesses the threshold of the four neighboring locations by interpolation. Several threshold tests are available on the two commonly available Octopus and Humphrey perimeters. In each test a certain number of points can be tested. The number of points tested in a given test is actually a compromise between the time applied and precision, which depends on the type of damage looked for as well as the diagnostic and therapeutic implications resulting therein. The response at each thresholded point is compared with a group of normal individuals. The likelihood of such a response in this population of normal patients is expressed as a probability symbol for each tested point. These probability symbols increase in significance from a set of 4 dots to a black box, p
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