- Pachymetry: Measurement of the thickness of the cornea – a test done to determine if your cornea is suitable for LASIK or collagen cross linkage procedure in patients with keratoconus or conical cornea
- Keratometry: Measurement of corneal curvature, usually done in the centre of the cornea. This measurement is handy to add to other measurements to calculate amount of LASIK laser required to bring your spectacle number to zero; also one of the measurements required to calculate the Intra-ocular lens power suitable for your eye in case you have a cataract.
- AS-OCT: Anterior Segment Optical Coherence Tomography. Something like a CT scan of the front portion of the eye. Comes in handy for a variety of diseases of the cornea and anterior segment
- UBM – Ultra Biomicroscopy: This is an ophthalmic ultrasound using a special probe, to view in detail structures in the anterior segment, up to and including the crystalline lens of the eye and an inserted IOL. Also useful to examine the angle of the eye (UBM Gonioscopy) to find out if it is narrow, closed or open. This may influence decision making about glaucoma surgery in some cases.
- B-scan: An ultrasound test to examine structures in the back of the eye, especially if the view of the back of the eye is hampered due to opacities in the normally clear media, such as a cataract or haemorrhage in the posterior compartment(vitreous haemorrhage), so that ophthalmoscopy examination is not possible. Also useful in lesions of the extraocular muscles and any space occupying lesions in the orbit, outside the eye.
- A-scan: Also called A scan biometry. In this the examiner determines the front to back measurement of the eye using ultrasound. He then feeds in the keratometry reading obtained from the keratometer. An inbuilt computer in the A- scan biometry machine, using one of several complex formulae, then calculate the power of the intra-ocular lens implant required for your eye
- OCT: Exactly like the AS-OCT talked about earlier; it gives you colour coded pictures of the back of the eye including individual layers of the retina. Therefore , if there is bleeding into the back of the eye or fluid collection in the retina, the OCT can tell you in which layer of the retina or whether it is just in front of or just behind the retina. It can also measure retinal thickness .This is helpful in planning treatment in conditions like diabetic macular oedema or monitoring efficacy of intravitreal injections given for wet age related macular degeneration.
- Corneal Topography: This gives you several ‘maps’ of the cornea; an invaluable tool in assessing suitability of the cornea for LASIK and useful in detection of keratoconus and in fitting of contact lenses. It provides information on the curvature of the cornea at various points of the cornea, the ‘power’ of the cornea etc.
- Aberrometry: This is often used by the LASIK surgeon in patients unhappy after a successful LASIK – it detects minute flaws called “higher order aberrations” in your eye’s optical system which prevents you from enjoying ‘super’ vision after LASIK. Some LASIK machines which are “wave-front guided” routinely use data from aberrometers to fine tune the LASIK treatment to reduce these aberrations post LASIK surgery.
- Visual Field Analysis: Mechanical devices that measured your field of vision were called perimeters. They have been largely replaced by Visual Field Analysers (VFA). These have inbuilt computers for storing and analysing your visual fields. They can compare fields done over a period and analyse whether the patient’s field is worsening or remaining the same. The routine test is a little cumbersome and requires your co-operation, lasting around 30-40 minutes, though for simple screening faster tests are available in the same machine. VFA is ordered in cases of glaucoma both for diagnosis and for judging efficacy of treatment. They are also invaluable in some neurological disorders causing drop in vision like in stroke etc.
- Fluorescein Fundus Angiography (FFA): In this test a dye, fluorescein Sodium is injected into the vein of your forearm. It takes around 10 seconds for the dye to spread into your retinal vasculature. This can be viewed by the observer using an ophthalmoscope, though more commonly, a digital camera called a fundus camera is used to take multiple photos of the retina shortly after dye injection.Depending on the areas of perfusion (dye seen) or non-perfusion (dye not seen) or leakage of dye from the vessels into the retina, decisions are made in cases of diabetic retinopathy, age-related macular degeneration etc. about whether to do laser or give some injections or treat with medications .
- HRT/GDX : These are fancy names for computer software in which images of the optic disc and retina are analysed. Accurate measurements are made of optic nerve size, cup/disc ratio, retinal nerve fibre thickness. This is a useful adjunct to diagnose glaucoma and monitor its progression.
- Electro-retinography (ERG): This test is used to find out the function of the retina. This comes in useful when the retina appears normal on ophthalmoscopy but patient has poor vision and also when the retina is not clearly seen due to media opacities. If the ERG is poor or absent, the eye surgeon may decide that the visual prognosis is poor and further surgery to clear the media opacities like a corneal transplant for corneal opacities or a cataract extraction for a mature cataract is not warranted.
- M-ERG or Multifocal ERG. This is the newest version of the ERG. In this a colour map is generated with the area of maximum cone function (macula) showing a steep elevation while the area with no cones (optic nerve ) showing up as flat and blue in colour. In diseases like macular degeneration, the central hill is considerably flattened, making diagnosis a ‘no-brainer’.
- Visual Evoked Potential (VEP): In this test, either a very bright light (Flash VEP) or a checker board pattern is shone into the patient’s eye. The time taken for this to be perceived by the visual area of the brain (situated at the back of the head) and the intensity of the signal is recorded by sensitive electrodes placed at the back of the scalp. This measures the integrity of the visual pathway from the eye right up to the brain. In conditions like optic neuritis, where the optic nerve which carries signals from the eye to the brain is inflamed, the VEP will be reduced. On the other hand, a patient who is feigning blindness will have a normal VEP.
Can a person who has undergone one or more of these tests donate their eyes?
None of these tests is life threatening. However, it is proved that every patient who undergoes any of these tests will die some day, maybe months, years or decades later! If a patient were to die due to natural or accidental causes years after any of these tests were performed, the author does recommend donation of the eyes!