Aniseikonia is a binocular condition in which the same image in one eye is perceived as being different in size and/or shape compared with the other eye.1,2 It is most commonly associated with optical magnification differences in anisometropia or antimetropia, oblique astigmatism, aphakia, pseudophakia, and corneal refractive surgery.1–4 Aniseikonia causes symptoms such as asthenopia, general fatigue, headaches, impaired fusion and stereopsis, reading difficulties, rivalry, diplopia, and distorted space perception.1,3
Retinal conditions or interventions that occur either monocularly or asymmetrically such as epiretinal membrane, macular edema, macular hole, vitreomacular traction, retinoschisis, reattached retinal detachment, and other retinal surgeries, can also cause aniseikonia.5–10 Aniseikonia that is retinally induced is presumably due to forces (either stretching or compression) causing an abnormal distribution of the retinal receptors.7 When the retinal receptors are stretched apart, the image of the object stimulates fewer receptors and is perceived by the patient as being smaller (micropsia). When the retinal receptors are compressed closer together, the image of the object stimulates more receptors and appears larger (macropsia). If one eye is not suppressed, the inability to integrate the different-sized images from each eye causes symptoms and in some cases, small-angle strabismus may occur.7 Retinally induced aniseikonia, unlike optically induced aniseikonia, has been reported to be heterogenous in magnitude over the retinal area affected and varies as a function of the visual field angle.6,8–10 The compensatory mechanisms of changing the magnification properties of the spectacle lenses or prescribing contact lenses to reduce the aniseikonia may be less helpful.
The purpose of this study is to report the clinical findings for patients who developed binocular vision difficulties attributed to retinally induced aniseikonia.
PATIENTS AND METHODS
The clinical records of patients referred to the author from 2006 to 2012 and diagnosed as having retinally induced aniseikonia were identified and reviewed. This study was approved by the institutional review board of the University of Alabama at Birmingham. Data extracted from the records included patient demographics, visual symptoms, ocular history, best-corrected visual acuity, manifest refraction, ocular alignment using the alternate cover test with prisms at distance and near, cyclotorsion using the double Maddox rod test, sensory fusion status using the Worth four-dot test at distance and near and the Randot stereotest, assessment of aniseikonia, and recommended treatment. Aniseikonia was measured at approximately 40 cm in both the vertical and horizontal meridians using either the computerized Aniseikonia Inspector version 1 (Optical Diagnostics, Culemborg, the Netherlands) or the New Aniseikonia Test (Handaya Co Ltd. Tokyo, Japan). The target size for the Aniseikonia Inspector was 10.3 cm (visual field angle ∼ 14.5 degrees) and the target size for the New Aniseikonia Test was 4 cm (visual field angle ∼ 5.7 degrees). Size lenses or afocal magnifying lenses (lenses having zero power with optical magnification based on the combination of front or base curve and center thickness) were used diagnostically to determine whether the visual symptoms could be improved subjectively by minimizing the aniseikonia.
A summary of the clinical details for 12 patients with retinally induced aniseikonia is shown in Table 1. Four patients (cases 3, 4, 5, and 9) described in previous articles are included.11,12 Eleven patients were male. The age range was 44 to 76 years. The basic complaint was image size differences between the eyes. Seven patients perceived the image of the affected or more involved eye as being smaller (micropsia), and five perceived it as being larger (macropsia). The amount of aniseikonia measured using either the Aniseikonia Inspector (visual field angle ∼ 14.5 degrees) or the New Aniseikonia Test (visual field angle ∼ 5.7 degrees) ranged from 1.7 to 11.3% and 1.5 to 13.3% in the vertical and horizontal meridians, respectively. The conditions likely causative for the aniseikonia included epiretinal membrane, retinal detachment surgical repair, and age-related macular degeneration. Diplopia was reported by 10 patients, and prism lenses had been prescribed previously by the referring practitioner for seven. The diplopic images were perceived by most patients as being superimposed. One patient manifested strabismus, a small hypertropia at distance; cyclotorsion (≤5 degrees) occurred in five patients; and 10 patients had metamorphopsia. Corrected visual acuity in the affected eye was 20/50 or better.
Detailed case reports for three representative patients are presented.
A 68-year-old man was referred for intermittent diplopia, ocular fatigue, and headaches.11 The patient described the diplopia as being both horizontal and vertical and reported the image seen by his right eye as being much larger than the image seen by his left eye. He found it easier to read when covering one eye. The onset was several years ago, with increased severity in the previous 2 years. Ocular history included bilateral pseudophakia, epiretinal membrane peel in the right eye, and treatment for bilateral glaucoma. The referring clinician prescribed prism lenses, which provided no relief.
The examination revealed best-corrected visual acuities of 20/25 in each eye. With the Amsler grid, a small scotoma was noted superiorly in the left eye, whereas the right eye showed no defect. With the patient’s spectacles (right eye: +0.25 – 3.75 × 173, left eye : –0.75 –2.00 × 173), which included 1.5 prism diopters (PD) base down left eye and 2 PD base out, orthophoria at distance and 6 PD exophoria at near were measured. Using the double Maddox rod test, 5-degree left eye incyclotorsion was measured. The patient fused the Worth four-dot test at both distance and near and had 70 arc seconds stereopsis.
Aniseikonia measurements using the Aniseikonia Inspector (visual field angle: ∼ 14.5 degrees) averaged 1.9% image disparity in the vertical meridian and 3.6% image disparity in the horizontal meridian, with the left eye requiring magnification. The aniseikonia was likely not due to anisometropia because the spherical equivalent refractive errors were almost similar (right eye: –1.62 D, left eye: –1.75 D). It was presumed that the epiretinal membrane peel was causing the aniseikonia. The patient used a 1.5% size lens over the left spectacle lens to read for approximately 45 min, which lessened his visual symptoms. Based on this observation and because the amount of aniseikonia was not excessive, the spectacle lens parameters, including base curve, center thickness, index of refraction, and vertex distance, were altered to minimize the aniseikonia by magnifying the smaller image in the left eye and minifying the image in the right eye. One month later, the patient reported absence of diplopia images appeared the same size. However, he was bothered by blurred vision. Visual acuities were 20/30 in the right eye and 20/25 in the left eye. No change in refraction, fusional status, and ophthalmoscopic results were found. Treatment with a Bangerter filter was attempted and was subsequently rejected by the patient. He continued to experience aniseikonic symptoms.
A 74-year-old man was referred for long-standing diplopia.12 The retired engineer also reported the image from his right eye as being approximately 50% larger than the image from his left eye. When reading, the print continually changed size, creating rivalry between the eyes. Frequent spectacle lens changes by the referring practitioner did not alleviate his symptoms. A small prism (1 PD base down) was ground in the right spectacle lens. Ocular history included retinal detachment repair 15 years earlier in the right eye and pseudophakia in the right eye. The examination revealed habitual visual acuities of 20/30 (–0.50 –1.75 × 90) in the right eye and 20/20 (+1.25 – 2.25 × 85) in the left eye. With the Amsler grid, metamorphopsia was noted in the right eye. Orthophoria existed at both distance and near. The patient fused the Worth four-dot test at distance and near and had stereopsis of 200 arc seconds. Aniseikonia measured using the New Aniseikonia Test (visual field angle: ∼ 5.7 degrees) averaged 11.3% image disparity in the vertical and 9.6% image disparity in the horizontal meridian, the left eye requiring magnification. This was opposite to what would be predicted from the patient’s anisometropic spectacle prescription (right eye spherical equivalent: –1.37 D, left eye spherical equivalent: +0.12 D). Size lenses (up to 9.5%) before the left eye did not eliminate the aniseikonic symptoms. A Bangerter filter was prescribed over the left spectacle lens.
A 62-year-old patient had undergone retinal detachment surgery 6 months earlier in his left eye. Previous history included bilateral pseudophakia. Since the retinal surgery, he has noted that the image seen by his left eye is 25 to 30% smaller than that seen by his right eye. He also experienced occasional diplopia. He described rivalry between a normally sized and undistorted image and a minified and distorted image. The patient uses store-bought glasses for near. Examination showed corrected visual acuities of 20/25 (–0.50 – 0.75 × 105) in the right eye and 20/30 (plano – 0.50 × 67) in the left eye. Metamorphopsia in the left eye was detected using the Amsler grid. Cover testing showed 4 PD exophoria and 2 PD left hyperphoria at distance and 8 PD exophoria and 2 PD left hyperphoria at near. Using the double Maddox rod test, 4.5-degree left excyclotorsion was measured. The patient fused with the Worth four-dot test at both distance and near and had no measureable stereopsis. The average amount of aniseikonia measured with the New Aniseikonia Test (visual field angle: ∼ 5.7 degrees) was 4%, with the left eye requiring the magnification. With a 4% size lens before his left eye, stereopsis improved to 200 arc seconds. Using the patient’s subjective responses, a –4 D contact lens was placed on his left eye, with a + 4 D spectacle lens placed in a trial frame to minimize the aniseikonia. The Galilean telescopic system had minimal effect on reducing the patient’s symptoms. Bangerter filters were demonstrated to the patient as possible treatment, but the patient decided against treatment at that time.
Clinically significant aniseikonia (> 2% image size difference) occurs in approximately 1 to 3.5% of the general population and is associated with either anisometropia, oblique astigmatism, aphakia, pseudophakia, or corneal refractive surgery.1–4,11 In this study, 12 patients with aniseikonia attributed to retinal pathology and/or retinal surgery are presented. The prevalence of retinally induced aniseikonia is unknown because it is not routinely tested in the clinic. With epiretinal membrane being a common cause and occurring in 1.9% of patients aged <60 years, 7.2% of patients aged 60 to 69 years, and in 1.6% of patients aged 70 to 79 years,13 retinally induced aniseikonia may be an increasingly important cause of visual symptoms with the aging of the population.
The mechanism for retinally induced aniseikonia is presumed to be either compression of the photoreceptors causing macropsia or stretching of the photoreceptors causing micropsia. The retinal condition can often be predictive of whether micropsia or macropsia develops. Benegas et al.7 reported macropsia in the affected eye for five of six patients with unilateral epiretinal membranes and micropsia in the affected eye for a patient with vitreomacular traction. Ugarte and Williamson6 reported macropsia occurring in the affected eye for 11 of 14 patients with unilateral epiretinal membrane and micropsia in three. For patients with successful macula-off rhegmatogenous retinal detachment surgical repair, Ugarte and Williamson10 reported micropsia of the operated eyes in all four of their patients. Macular edema and macular holes have been associated with micropsia.14,15 In the present study, three of four patients with epiretinal membrane perceived macropsia in the affected eye, and six of seven patients with retinal detachment repair perceived micropsia. The patient with age-related macular degeneration experienced macropsia.
Ten patients in the present case series had varying degrees of metamorphopsia, which was documented using the Amsler grid. Retinal image distortion alone can cause severe visual symptoms and makes it more difficult to accurately measure the amount of aniseikonia. It is impossible to tell the relative roles of aniseikonia or metamorphopsia in causing these patients’ symptoms. The major symptom for all 12 patients was image size difference. As suggested by Benegas et al.,7 it is likely that there is an additive effect of aniseikonia and metamorphopsia in these situations.
Binocular vision impairment can be a consequence of aniseikonia.16–18 Jiménez et al.16 reported significant decline in random-dot stereopsis when using size lenses of 3 and 5% to induce aniseikonia in patients with normal binocular vision. Eleven patients in the present study had 3% or more aniseikonia in the vertical and/or horizontal meridian. One patient manifested a small vertical strabismus at distance (case 12) at the time of referral. Another patient (case 10) had previously undergone extraocular muscle surgery for strabismus caused by a scleral buckle and was orthophoric at the time of referral. Seven patients maintained some stereopsis. Two patients (cases 8 and 3) with 7 and 11% aniseikonia had stereopsis of 140 and 200 arc seconds, respectively. Another patient (case 6) elicited stereopsis of 200 arc seconds when compensating for his aniseikonia with a size lens. The aniseikonia for the 12 patients was likely larger than that measured because both the New Aniseikonic Test and the Aniseikonia Inspector tend to slightly underestimate the amount of aniseikonia.19–24 For example, a measured 3% aniseikonia may, in fact, be 3.5%, and a measured 8% aniseikonia may, in fact, be 8.5%. In addition, stereopsis was measured with the Randot stereotest using contour-based targets. Retinally induced aniseikonia may cause less disruptive effect with contour-based stereotests than random-dot stereotests, the former being less affected by distortion.25
Treatment for aniseikonia involves either designing iseikonic lenses by manipulating spectacle lens parameters or prescribing contact lenses.1,11,26 In my experience, aniseikonia can be altered up to 2 to 3% by changing the base curve or front surface, center thickness, index of refraction, and/or vertex distance of spectacle lenses.1,11 For larger amounts of aniseikonia, spectacle-contact lens combinations can be prescribed. The latter, however, may cause optically induced anisophoria.
The patient in case 4 responded favorably to diagnostic size lenses in the clinic and was treated with iseikonic lenses. Although he reported similar image sizes and no diplopia with the new lenses, he had blurred vision which could not be substantiated by either change in refractive error or ophthalmoscopy. Increased off-axis aberrations induced by the iseikonic lens design may have caused the blurred vision. Another possibility for the unsatisfactory results is that retinally induced aniseikonia, unlike optically induced aniseikonia, can be heterogenous across the retinal area affected.6,8–10,14 The amount of aniseikonia varies as a function of the visual field angle, or angular distance from the line of sight. de Witt and Muraki8 described a patient with unilateral epiretinal membrane, whose aniseikonia varied from 23 to 2.5% for visual field angles ranging from 0.36 to 5.7 degrees. Higher visual field angles usually result in lower aniseikonia. Field-dependent aniseikonia has been associated with retinal detachment repair, retinoschisis, and central serous chorioretinopathy.9,10,14 During follow-up, the patient in case 8, with a history of retinal detachment surgical repair, had his aniseikonia re-measured using the New Aniseikonia Test at both 40 cm (visual field angle ∼ 5.7 degrees) and 20 cm (visual field angle: ∼ 11.4 degrees). The amount of aniseikonia was 8.5 and 3.5% for the smaller and larger visual field angles, respectively. The aniseikonia values in Table 1 were measured at approximately 40 cm, resulting in different visual field angles for the Aniseikonia Inspector (∼ 14.5 degrees) and the New Aniseikonia Test (∼ 5.7 degrees).
The patient in case 11 was treated 4 years later with rigid contact lenses by a practitioner in another state, which improved visual acuity in the right eye to 20/20 (written communication with Mark Smith, OD, January 13, 2012). According to the practitioner’s records, the patient’s aniseikonic symptoms lessened somewhat for distance but continued for near. Quantification of the aniseikonia was not determined. Because of dry eye, the patient wears the contact lenses only 3 h/d and uses glasses the remainder of the day.
These cases suggest that for retinally induced aniseikonia, unlike optically induced aniseikonia, either changing the magnification properties of the spectacle lenses or prescribing contact lenses may not be as helpful. Anecdotal case reports have reported some success.27 Because of its field dependency, correcting the aniseikonia in only part of the visual field with iseikonic lenses has been recommended.8,9 The long-term effect of this treatment on a series of patients has not been reported.
Another treatment option involves using occlusion filters. Bangerter occlusion filters or foils (Reyser Optic AG. St. Gallen, Switzerland) are graded partially occlusive filters that have been used for amblyopia treatment in children28–30 and for adults with intractable diplopia.12,31,32 They vary in strength from the most dense, which produces marked reduction in visual acuity, to the barely occlusive, in which nearly normal visual acuity is possible. The translucent filters are applied to the spectacle lens before the affected eye and allow acceptable cosmesis. By degrading image quality and inducing a functional central scotoma, Bangerter filters should eliminate the rivalry between foveal image size disparities. With less dense filters, peripheral fusion and stereopsis can be maintained.31 Bangerter filters were recommended for seven patients. The filter with minimum density filter that eliminated the aniseikonic symptoms was prescribed.12,31,32 The blur induced by the filters may not be acceptable for some patients, especially patients who have undergone retinal surgery to improve visual acuity.
There are limitations to this case series. First, it was retrospective and the number of patients reported is small. All patients were referred for tertiary care and follow up by the author was limited. Second, aniseikonia was measured with the New Aniseikonia Test or the Aniseikonia Inspector at one test distance, at a fixed visual field angle, was likely underestimated, and influenced by metamorphopsia. At other visual field angles, the amount of aniseikonia would likely have been different. Recent investigators8,9 have used computerized forced- choice methods of measurement having limited target presentation time at varying visual field angles rather than direct- comparison methods of measurements at fixed visual field angles. Finally, the clinical course and long-term treatment effect of retinally induced aniseikonia are unknown. Interestingly, it has been reported to persist even after surgical intervention to remove its cause.7 Three patients with epiretinal membrane (cases 1, 4, and 9) underwent pars plana vitrectomy and internal limiting membrane peel. Although visual acuity improved, aniseikonia persisted.
In summary, this case series suggests that retinally induced aniseikonia is an increasingly important cause of binocular visual symptoms in the aging population. Prospective studies to determine its incidence, clinical course, and the most effective treatment need to be performed.
Robert P. Rutstein
School of Optometry, University of Alabama at Birmingham 1716
University Boulevard Birmingham, AL 35294-0010
I thanks Dr. Tammy Than for reviewing the manuscript. Dr. Rod Fullard assisted with the calculation of the visual field angles for the Aniseikonia Inspector and New Aniseikonia Test.
I no proprietary or commercial interest in any of the products or materials mentioned in the manuscript.
Received: April 23, 2012; accepted June 28, 2012.
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