Author Response: “Metamorphopsia” Assessment

Ugarte and Williamson address the need to distinguish between dysmetropsia, which affects the entire visual field, and micropsia and macropsia, which affect only a part of the visual field, the latter condition being confined largely to the center of the visual field. They refer to our recent study ¹ of patients with macular holes, in which the objective was to assess centrosymmetric metamorphopsia before and after surgical treatment of macular holes in patients with a healthy fellow eye. We do not propose that our test has any relevance outside the confines of this situation, in which it is known beforehand that the patient has an idiopathic macular hole. We have shown that metamorphopsia is centrosymmetric in eyes with a macular hole, ² and it is therefore reasonable and of considerable practical utility to reduce the test to one of centrosymmetric image distortion. 

Ugarte and Williamson address the need to clarify the dimensions of the test stimuli. Test stimuli ranged from being identical with the reference stimulus to an extra width of up to 1.5° of visual angle. “Width” thus refers to the diameter of the retinal image produced by the test stimulus which was increased in size by increments of 0.5°. Thus, for each of the five reference stimuli, the patient had four test stimuli of different sizes to choose from, the objective being to choose the one that best matched the reference stimulus. This gave a total of 20 combinations. 

Ugarte and Williamson correctly note that a stimulus diameter of 2.34° would correspond to 673.92 μm on the retina. However, they make an incorrect conclusion about the stimulus that is being projected within the hole, by comparing the average disparity at 1° with the macular hole diameter range. The quoted 0.34° of disparity at 1° eccentricity is a mean value for 42 patients with a mean hole diameter of 438 μm (range, 199–735 μm). They wisely suggest titrating the test stimulus to the hole size by projecting targets of different size before performing the test. We fully agree. We must point out, however, that this methodology is actually already an integral part of the test, as described in our previous article. ³ Specifically, there were 48 instances in which the reference stimulus, ranging in angular subtense from 1.0° to 3.0°, could not be matched by a test stimulus, presumably because the reference stimulus fell entirely within the central scotoma in the eye with the macular hole and therefore could not be seen. Ugarte and Williamson correctly point out that the vertical and horizontal components of size difference have not been quantified separately. However, we do not make any conclusions about the vertical and horizontal component of size perception. We do find evidence to support the assumption that the displacement of photoreceptors around a macular hole is more or less uniformly radial, following a meridional direction away from the center of the fovea, and is of roughly equal magnitude in all directions. ^2,4  

Because the stimuli we used were semicircular and had to be matched along their opposing vertical surfaces such that, together, they appeared to form a full circular target, our method may have been biased toward measuring photoreceptor displacement along the vertical meridian. Our finding that reports of micropsia were actively expressed by only a subset of patients expressing distorted vision could be explained by the fact that micropsia is harder to recognize by the patient than simply image distortion.