The difference in perceived image sizes between the two eyes in the presence of isometropia is most commonly acquired due to conditions such as unilateral pseudophakia1,2, macular disease,3 and tractional retinal diseases such as epiretinal membrane, surgical repair of retinal detachment, or after refractive surgery.4,5 These patients often have had normal binocular vision or had been long adapted to their visual condition. Changes in the visual inputs after disease or surgery that affect the relative image sizes of the two eyes differentially can cause symptomatic aniseikonia. These symptoms include eyestrain, headache, or generalized discomfort with distance or near viewing and sometimes a perception of different image size between the two eyes or perception of distorted images.6,7 Unless significant astigmatism is induced, aniseikonia induced by refractive or cataract surgery is generally regular across retinal loci and in all meridians. In these cases, determining the magnitude of the aniseikonia is uncomplicated. In tractional retinal disease, the aniseikonia is due to the dragging of the photoreceptors, therefore it may vary substantially across retinal loci and in different meridians. This makes measurement of aniseikonia difficult and reduces the prognosis for success with eikonic lenses8 that are designed to reduce overall perceived image size differences. This case report illustrates that despite these drawbacks, reducing aniseikonia in a patient with retinally induced aniseikonia can improve patient binocularity and comfort for the long term.
Three different tests for measuring aniseikonia were used in this study: the Space Eikonometer (American Optical Corporation, no longer manufactured), the New Aniseikonia Test (NAT) (Handaya Co., Tokyo, Japan), and the Aniseikonia Inspector (AI) (Optical Diagnostics, Culemborg, the Netherlands). The Space Eikonometer assesses aniseikonia in vertical, horizontal, and oblique meridians using stereopsis cues for measurement of image size difference.6,7,9 NAT10 consists of a series of anaglyphic red/green half-circles of different relative sizes in a direct comparison format with a central fixation cross. The test is administered in normal room illumination with multiple stimuli per page. The booklet can be held in different orientations to determine the degree of aniseikonia present in various meridians. The AI is a computerized program that allows the assessment of aniseikonia and the design of eikonic lenses.11,12 The AI Version 1 used for the current case report, is a direct comparison test using computer-generated anaglyphic pairs of half-circles similar to the NAT. One pair of anaglyphic half-circles is presented at a time, and the patient views the screen under dim room illumination to reduce peripheral fusional stimuli. The targets are presented in four meridians, 45, 90, 135, and 180°, to determine the percent magnification in each meridian.
A 63-year-old white male was referred to the pediatrics and binocular vision specialty clinic for symptoms of perceived image size difference secondary to retinal detachment and scleral buckle repair in the right eye 6 months prior. Ocular history included bilateral cataract extraction with intraocular lens implantation earlier in the year.
The patient reported that the right eye image appeared smaller than the left eye when compared monocularly. Binocularly, he complained of fatigue with near work after approximately 15 minutes. He reported that distance vision did not create as much of a problem for him.
The patient's presenting spectacle prescription was OD −0.25 −1.00 × 175 with 0.75Δ base down and OS −0.25 −0.75 × 175 with a +2.50 progressive addition add. The patient had previously been prescribed up to 1.5Δ base down OD for his symptoms, but he preferred the lesser amount of vertical prism that was in his presenting glasses. Base curves were 6.25 OU, and center thicknesses were 2.5 mm OD and 2.8 mm OS. Visual acuities with this prescription were 20/25+ OD and 20/20+ OS.
Cover test at near with his habitual bifocal correction revealed an 8Δ exophoria and a variable magnitude right hyperphoria measured as low as 2Δ on one day and as high as 6Δ on a subsequent visit. Because of the variation in magnitude, comitancy testing was performed at the second visit. This included the Parks 3-Step with cover test, and Maddox rod subjective measures in vertical and horizontal meridians in nine gaze positions. On both tests, the deviation was found to be comitant. With the Wesson card, the patient reported no fixation disparity for vertical and horizontal targets. Sensory testing with the Worth Four-Dot revealed fusion at near and diplopia at distance in full illumination, even though he did not complain of diplopia in day-to-day life and had fewer visual complaints for distance vision. With his habitual prescription, he achieved 140 sec arc local stereopsis on the Randot stereotest but was unable to identify any of the global forms.
The NAT was administered at a distance of approximately 45 cm. The patient reported a match in perceived image sizes with 11% magnification of the right eye image in the vertical meridian, indicating a significant amount of aniseikonia at this retinal locus and orientation. The AI Version 1 was also administered at about 45 cm and revealed significantly less aniseikonia, measuring only 1.8% in the vertical, 1.5% in the horizontal, and −0.7% in the diagonal meridians for the right eye. The Space Eikonometer measures were variable ranging from 1 to 3% × 090 and 2 to 4% × 180. Declination was not measured.
A series of trial size lenses was then used to assess the patient's subjective symptoms empirically. Size lenses are afocal magnifiers which create shape magnification by specifying base curve, center thickness, and refractive index, as described in the discussion. Shape magnification increases with increasing base curve, center thickness, and refractive index. The patient's initial impression was no improvement in subjective symptoms with a 5% size lens over the right eye. He found that a 10% size lens provided clearer and more comfortable vision. However, it is generally recommended that no greater than about 5% magnification difference be designed into lenses7 because the lens thickness and base curve required to would be excessive. Therefore, a 5% afocal magnifier was fitted over the right eye for an in-office trial. After 5 to 10 minutes, his aniseikonia measured with the NAT measured 6% vertically showing no adaptation to the 5% size lens. With the AI, aniseikonia measures decreased to 0% in the vertical, 2.5% in the horizontal, and −2.8% in the diagonal meridians in the right eye.
Stereopsis was reassessed with the Randot stereotest, using a 3% size lens over the right eye. This magnitude was selected because it was estimated that it could be manufactured in the current spectacle frame. The patient demonstrated reliable improvement in binocular integration indicated by the improvement in random dot global stereopsis with the size lens in place. Results are summarized in Table 1. The patient also reported subjective improvement in comfort with binocular viewing after wearing the size lens over the right eye for 10 to 20 minutes.
Aniseikonia data on this patient were variable despite a alert, observant, and careful observer. This is likely due to the retinal nature of his aniseikonia causing differences in receptor spacing and therefore perceived image size at different field angles and/or eccentricity.8 After extensive discussion with the patient, he was cautiously enthusiastic about trying eikonic lenses to reduce some of the overall aniseikonia, with the conservative goal of reducing symptoms. Single-vision reading glasses were selected as a starting point. After consulting with the optical laboratory, it was determined that the steepest base curve and maximum center thickness combination that could be cut with the existing eyewire size were 10.50 D and 5.75 mm, respectively. This would result in 3.5% greater shape magnification of the right lens compared with the left lens. The final reading prescription is shown in Table 2 (mentioned in Appendix—available at http://links.lww.com/OPX/A94 for an explanation of how to design an eikonic prescription).
Fig. 1 is a photograph of the final lens prescription showing the relative thickness of the right and left lenses. Figs. 2 and 3 show the patient wearing his eikonic prescription. The patient believed that the cosmesis was acceptable.
A follow-up consultation was conducted by phone, and the patient reported a short adaptation time. The patient reported comfort after 10 minutes of wear. After several days of wear, he reported that he could use his new glasses intensively for over 2 hours of near work on multiple occasions. Previously, he experienced fatigue after just 15 minutes of near work. The patient acknowledged that this is not a perfect solution for him but one that permitted much more comfortable near viewing for longer periods of time than before, and he requested eikonic design in single-vision distance glasses as well. One year later, he was still very satisfied with the vision from his reading glasses and distance glasses and requested progressive addition lenses designed with the same magnification. Measurements were taken with the AI Version 1 through the patient's eikonic reading glasses. Vertical, horizontal, and diagonal measures OD were −0.5, −0.8, and −2.1%, respectively. New progressive addition lenses were designed with the same shape magnification as his original reading glasses. After several weeks of wear, he reported that he still used the single-vision near glasses for computer work and reading but that he used the progressive addition lenses with magnification the rest of the time.
In the case presented, partial correction of the patient's aniseikonia resulted in a reduction in symptoms and objective improvement in binocular integration indicated by stereopsis measures. This improvement in binocular integration with reduction in aniseikonia is consistent with visually evoked potential (VEP) recordings in induced aniseikonia reported by Katsumi et al.13 In this study, as the amount of induced aniseikonia was increased, the amplitudes of binocular VEPs declined relative to the amplitudes of monocular VEPs, with no binocular summation evident beyond 5% induced aniseikonia.
The improvement in binocularity and symptoms in the current case report was found despite significant variability in the measures of aniseikonia with the different tests administered. This variability may be due to at least two factors: the test methods and their inherent differences as described later in the text and the different retinal locus being measured with the different tests.
Both the NAT and the AI have been reported to underestimate induced aniseikonia to varying degrees under different test conditions in different meridians, questioning the validity of these tests.14–18
McCormack et al.14 measured induced aniseikonia with afocal magnifiers using the direct comparison method with the NAT. The NAT significantly underestimated the induced aniseikonia in both vertical and horizontal meridians. They hypothesized that, as the NAT booklet presents six target pairs per page and therefore six fusion locks per page, sensory fusion mechanisms may cause rescaling of binocular correspondence in the presence of these “binocular textures.” They then created computer-generated circles that mimic the NAT except that only a single anaglyphic pair and fixation lock were presented on a screen to reduce binocular stimulation. Under these conditions, underestimation of aniseikonia was much less and not statistically significant. They hypothesized that the underestimation of induced aniseikonia with the NAT is likely due to a sensory fusion response to binocular textures, which they feel is different than true adaptation to aniseikonia. They concluded that the NAT is not a good screening test.
Yoshida et al.18 measured aniseikonia in myopic individuals who were corrected with a spectacle lens on one eye and contact lens on the other using the NAT and a phase difference haploscope. In a phase difference haploscope, the opposite halves of a circle are rapidly alternated between the two eyes, rather than being presented simultaneously. This rapid alternation results in a perception of a whole circle.19 The size of each eye's half-circle is adjusted until they are perceived as a perfect circle. The NAT was presented in two formats. One was the standard clinical format with six half-circle pairs per page and the other was a modified format with only one half-circle pair per page to reduce fusional content. They reported that the aniseikonia measured with the NAT was 1.4% less than when measured with the phase difference haploscope, with no significant difference in the two presentation formats. Because of this, they concluded that the underestimation is clinically insignificant with the NAT.
In 2007, de Wit20 reported on the validity of the AI Version 1 finding an average slope for measured versus induced aniseikonia to be −0.98 in the vertical meridian and −0.89 for the horizontal meridian, indicating a small amount of under-correction only in the horizontal meridian. Two other studies15,17 reported significant underestimation of the induced aniseikonia measured by AI Version 1 with greater underestimation in the horizontal meridian (Table 3). Antona et al.17 concluded that rescaling from peripheral fusional stimuli does not appear to account for the underestimation in the horizontal meridian. A proposed explanation for the underestimation in the horizontal meridian is the incidence of horizontal phorias and/or fixation disparity that may cause instability and affect measurements.
Version 2 of the AI incorporates changes to control for fixation disparity and eye movements that can affect measurements. While Version 1 presents continuously viewed anaglyphic half-circles, Version 2 presents anaglyphic rectangular pairs that can be presented in brief intervals and includes an improved central fixation target to reduce eye movements during testing. Version 2 uses a two-alternative-forced-choice response method to plot a psychometric function to determine the magnitude of aniseikonia. Fullard et al.16 compared Version 1 and Version 2 of the AI for reliability and repeatability. Despite the changes, Fullard et al.16 reported a significant amount of variability for both versions in both vertical and horizontal meridians in light and dark conditions. They reported that the best repeatability and validity for the AI is with Version 1 in the vertical meridian, measured in the dark.
Despite the controversy about the validity and variability in the measurements of aniseikonia using the NAT and the AI, these tests are clinically useful in identifying the presence of aniseikonia and finding an approximation of the magnitude. The definition of aniseikonia is a difference in perceived retinal image size that includes the physiological and cortical processes involved in vision.6 Therefore, any factors that alter the perceived image size from what is physically induced with afocal magnifiers such as rescaling and adaptation should not be considered to invalidate the test unless they are unique to the testing condition. de Wit20 has argued that the visual system is able to adapt to some of the induced aniseikonia, and as eikonic prescriptions generally under-correct aniseikonia for cosmetic reasons, this underestimation of the amount may represent a useful minimum when considering correction.
The measured values of aniseikonia can be used to confirm a diagnosis of aniseikonia, identify which eye requires additional magnification, and serve as a starting point in the management of aniseikonia. AI Version 2 also has the ability to measure at two retinal loci located 6.8° and 3.2° from the fovea, which can provide additional information to the clinician about aniseikonia in etiologies involving retinal traction. Based on these measures and knowing the physical and cosmetic limitations of lens design, which generally restricts the maximum amount of magnification that can be designed into lenses to no more than about 5%,7 an afocal magnifier of an appropriate magnification power can be selected. A set of afocal magnifiers can be easily designed and manufactured by a laboratory for a minimal cost, using a nomograph corresponding to the material being used. The nomograph is a graphic representation of the formula for shape magnification (Fig. 4), allowing the user to find combinations of thickness and front curve that result in a range of shape magnifications. Table 4 shows base curves and center thicknesses for afocal magnifiers of 1 to 5% magnification using CR-39. As afocal magnifiers are additive for small powers,21 this set can be used to produce up to 9% magnification. The afocal magnifier can then be placed over the patient's existing spectacles or in a trial frame for a short period to assess the patient's subjective response. In some cases, the patient may need to wear the magnifier for a few days to determine whether there is an appreciable improvement in symptoms. Other clinical tests that can be performed to determine whether the symptoms are due to aniseikonia include monocular occlusion to determine whether symptoms are relieved and a provocative test placing the afocal magnifier over the opposite eye to determine whether symptoms are exacerbated.6,7
Designing eikonic lenses for the reduction of aniseikonia has been described previously22 and is summarized in the Appendix. Base curve, center thickness, and refractive index can be selected using a nomograph to minimize the shape magnification in the eye with the larger perceived image and increase shape magnification in the eye with the smaller perceived image. When selecting the frame, eye size should be kept minimal to minimize edge thickness producing a lighter and thinner lens.
Alternatively, the AI can be used to design eikonic lenses. This program allows the user to easily change lens parameters, including base curve, center thickness, refractive index, and even vertex distance and bevel position in cases of anisometropic aniseikonia. As the parameters are changed, the interactive program shows the effect on magnification. An interactive drawing shows the lens shape, which helps the practitioner decide which combination of parameters is cosmetically acceptable.
Acquired aniseikonia can be disturbing because it generally occurs in mature visual systems that may not be able to adapt to the perceived image size changes induced by disease or surgery. This case shows that even when aniseikonia is substantial, variable in magnitude, and irregular due to retinal disease, reducing the overall aniseikonia to some degree can be sufficient to improve binocularity and patient comfort noticeably for the long term. The underestimation of induced aniseikonia reported for currently available clinical tests does not preclude their use as clinical tools in the management of symptomatic aniseikonia.
University of Houston, College of Optometry
505 J. Davis Armistead Bldg.
Houston, Texas 77204-2020
I thank Dr. Gregory Stephens who advised in the management of this patient and provided helpful comments concerning the manuscript. Dr. Casey Packer collected patient data as part of his fourth year internship at the University Eye Institute. Sally Martin, our in-house certified optician, worked with the lab to determine parameters for final lens designs. I also thank Dr. Bruce Wick for his helpful comments on the manuscript.
Informed consent was received for publication of the figures in this article.
The appendix, which gives an explanation of how to design an eikonic prescription is available at http://links.lww.com/OPX/A94.
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aniseikonia; afocal lenses; eikonic lenses; retinal traction