Metamorphopsia and Interocular Suppression in Monocular and Binocular Maculopathy

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Metamorphopsia is a key symptom of macular disease, particularly age-related macular degeneration (AMD) (de Jong 2006), that is often measured with Amsler grids at home and in the clinic, even though the test lacks both sensitivity and specificity (Crossland & Rubin 2007, Schuchard 1993). We address issues of monocular viewing and fixation compliance with a computerized version of the Amsler grid and binocular eye-tracking. Control of monocular viewing and compliant fixation enabled accurate quantification of the area and location of metamorphopsia in each eye. An eight-item questionnaire was administered to assess metamorphopsia patient reported outcomes (Alster et al. 2005, Arimura et al. 2011), and we measured interocular suppression with a dichoptic task with stereo shutter glasses. All participants were recruited with the criterion of no reported foveal vision loss and binocular calibration was used to track either eye monocularly. Those not accurately tracked were excluded from the study.

Of the 74 macular disease patients who viewed the Amsler grid monocularly with each eye, 95% (70/74) experienced distortion in at least one eye, and 27% (20/74) experienced distortion in both eyes. Of the 34% (25/74) of participants diagnosed with binocular macular pathology, 80% (20/25) perceived distortions in each eye. Despite this high prevalence, most patients report no impact of distortion on the metamorphopsia questionnaire (Figure 1A), and the overall area of the Amsler grid affected by distortion was not correlated with the score. There were no differences between total metamorphopsia score across the nine disease groups (see Figure 1B). Participants with foveal distortion in both eyes (25%, 18/74) had a significantly higher score (3.7) than those who experienced metamorphopsia in only one eye (1.9), (p = .04, non-parametric Mann-Whitney test, Figure 1C left). We also found a significant correlation with LogMAR acuity in the affected eye and the total metamorphopsia questionnaire score (r = 0.189, p = 0.031).

Figure 1.

Figure 1A. The response distributions across all participants for the eight questions with four similar response categories. Figure 1B. The average total questionnaire score for each diagnosis tested, including wet age related macular degeneration (AMD), dry AMD, epiretinal membrane (ERM), macular edema, macular scar, central serous retinopathy (CSR), and retinal detachment. Red dots represent outliers. Figure 1C Left panel shows box plots for the difference in questionnaire score between patients who experienced binocular foveal distortion and those who did not. Figure 1C Right panel shows a similar comparison for patients who experienced suppression and those who did not. The black asterisks represent the mean of each distribution and the horizontal black lines represent the medians. Outliers are displayed as black points.

In a post hoc analysis, an assessment to detect inter-ocular suppression was administered to 49/74 participants who reported distortion in only one eye with our computerized Amsler grid. A non-overlapping circle and cross were presented to each eye dichoptically with stereo shutter glasses. Suppression was classified when observers were unable to perceive both targets simultaneously even when the unseen target was moved. We found robust suppression of the affected eye in 59% (29/49) of monocular participants, and those who did not suppress scored higher (2.5) on the metamorphopsia questionnaire than those who did suppress (2.2), but not significantly so (p =0.3, Figure 1C right).

Thus, distortions must be foveal and binocular to be noticeable in everyday life since monocular distortions are often suppressed by a less impaired eye. This lack of awareness may be critical when distortion is used as a screening and progression-monitoring tool or as a post-operative outcome measure (Jensen O 1998, Wang et al. 2005, Wittich et al. 2005), especially given the importance of self-referral for AMD. Our findings should warn patients and clinicians against a false sense of security when the progression of monocular vision pathology seems stable when assessed with Amsler grids or patients reported outcomes that may be (self-) administered binocularly. In these cases, the patient may not be aware of spatial distortion in everyday activities, and hence fail to report pathological vision impairment to a clinician.