“Metamorphopsia” Assessment

We read with interest the article by Krøyer et al. ¹ entitled “Metamorphopsia Assessment before and after Vitrectomy for Macular Hole.” The authors assessed the interocular differences in perceived image size in 42 patients before and after vitrectomy (with or without internal limiting membrane peel) and gas tamponade for macular hole (stage 2 or 3). This study complements earlier work in which the same methodology was used in patients with unilateral macular hole. ² We feel the term “metamorphopsia” has been incorrectly used throughout both articles. Metamorphopsia is defined by the Oxford English Dictionary as “an affection of the sight characterized by distortion of things seen.” The authors measured the difference in image size perception between the eye with a macular hole and the fellow eye (with no disease), by presenting a red and a green semicircle to each eye by means of dissociation with red/green filters. We believe that the method used quantifies, not distortion (“the twist, wrench or turn to one side, or out of the straight position”), but dysmetropsia (“an alteration of the size of perceived objects”). Therefore, it would be wrong to say that this test “evaluates the degree of metamorphopsia” or “quantifies visuospatial distortion.” ¹  

Our group has been involved in measuring differences in image size perception in patients with unilateral macular disease (i.e., epiretinal membranes ³ and previous macula-off retinal detachment ⁴ ), using a modified version of the new aniseikonia test designed by Awaya et al. ⁵ This test is based on the presentation of a red and a green semicircle to each eye after dissociation with red/green goggles, the same methodology as that used by Krøyer et al. ¹ The results published by Krøyer et al. ¹ were obtained by projection of images 10° around fixation in the healthy eye and up to 11.5° in the eye with the macular hole. These dimensions are not what the original test was designed for. Therefore, to demonstrate the precision of their measurements, the authors should have assessed the test validity and reliability for these sizes. Nowhere in their articles is such an assessment indicated. The analysis of their data might therefore have led the authors to draw erroneous conclusions. 

Green semicircles (reference stimulus) of 2°, 4°, 6°, 8°, and 10° diameter and centered at the fovea were projected in the good eye. In the eye with the macular hole, the size of the red semicircles (test stimulus) was increased “to an additional width of up to 1.5°.” We wonder whether by “width” they mean the radius or diameter of the semicircles. Furthermore, the increment steps of the test stimulus are not mentioned, making the interpretation of their results very difficult. 

They report a difference between the size of the image perceived by the eye with the macular hole and the healthy eye of 0.34° at 1° eccentricity and 0.2° at 3° and 5° of eccentricity, before surgery. The test semicircle had to have a diameter of 2.34° to be perceived of a size equal to the 2° reference stimulus seen by the good eye. A diameter of 2.34° would correspond to 673.92 μm on the retina. The range of the minimum inner diameter of the macular hole in their patients was 199 to 735 μm. With this range, the 2.34° diameter semicircle would have been projected within the hole. Their results may therefore be misleading. To demonstrate that the images were seen completely by the patient, the authors should have projected targets of different sizes before performing the test. In addition, the vertical and horizontal components of size difference have not been quantified separately. The size perception difference is likely to vary with the meridian. In fact, all 41 of their patients reported image distortion, whereas only 10 of the 41 experienced micropsia. This outcome has important consequences for the conclusions of their paper, which may be incorrect. 

In conclusion, while we agree with the importance of assessing visual distortion and dysmetropsia in patients with macular disease, we are of the opinion that the methodology used by these authors does not measure metamorphopsia.