I read the paper by Kang and associates, in which they described the fate of small outer foveolar defects after surgery for macular hole (MH), with great interest. They noted that outer foveolar defects are observed in 45.8% of MH patients on initial optical coherence tomography (OCT) after surgery and concluded that the defect gradually decreases in size and eventually disappears completely.
I agree with their published observations using Stratus OCT (Model 3000; Carl Zeiss Meditec, Dublin, California, USA). However, their conclusion that the defect disappears completely raises some questions. In their report, the defect disappeared in all cases within an average of 6 months of follow-up after surgery. However, this seems inconsistent with previous histologic findings that indicated that photoreceptor defects persist long after surgery. This persistence also has been documented in previous studies using spectral-domain OCT (SD OCT).
Kang and associates defined outer foveolar defects as lucencies of the outer foveal layer and presented a case representing the sequential changes of outer foveolar defect in their Figure 1. However, different mechanisms may have played roles at different stages of the postoperative alteration of outer foveolar defects. The decrease of the outer foveolar defect shown in the second and third rows of Figure 1 may indicate recovery of subretinal detachment by absorption of subretinal fluid, an interpretation that would be consistent with previous findings. However, the shrinkage of the defect seen between the third and fourth rows of Figure 1 may be related to the recovery of the photoreceptor layer. Other mechanisms such as sliding or regeneration of photoreceptors may explain the shrinkage of the defect, but still remain unclear. In addition, the bottom row of Figure 1, showing complete disappearance of the defect, seems actually to be the scan of fellow eye, perhaps included by mistake.
To confirm the disappearance of a small defect, it must be guaranteed that successive scans are obtained of the same area. Kang and associates observed changes of defects with Stratus OCT scans, which cannot be guaranteed to have been obtained from the same location. Kang and associates suggested further higher-resolution studies using SD OCT to obtain more information and may have illustrated SD OCT scans in Figure 2, although it was not clear that these scans were obtained from the same patient. When using SD OCT scanning protocols for 128 B-scans (512 A-scans per B-scan) in an area of 6 × 6 mm, 128 B-scans are spaced approximately 47 μm apart. Therefore, small defects smaller than 47 μm may be missed by B scans obtained with SD OCT. Moreover, patients may adapt to defects and learn fixate using recovered healthy photoreceptors adjacent to the defect after surgery. These mechanisms may make it more difficult to detect small defects long after surgery, because OCT scans are affected by patient fixation. To confirm that photoreceptor defects actually have disappeared completely, rather than become hidden as a result of the above causes, it is necessary to conduct SD OCT scans that offer higher resolution in smaller macular areas.