Discussion

As far as we know, this is the first study to quantify retinal sensitivity after ILM peeling using the accurate technique of OCT/SLO eye tracking microperimetry. A new pattern was used to ensure adequate density of the measurement points in the macular area. As MH only affects the centre of the macula, the measurement points, except for the central point, were located in the formerly normal area of the retina. Therefore, comparison of the data for eyes whose MH was closed and which underwent ILM peeling to the data for eyes without peeling gives a strong indication of the effect of peeling on the normal retina. We found that mean retinal sensitivity was significantly lower (in dB) in eyes that underwent peeling than in those that did not. Postoperative microscotomas were also significantly more frequent in eyes that had undergone peeling.

Although, as stated in the Introduction section, a desire to increase the anatomic success rate has been the main motive for including ILM peeling in the treatment of several diseases, including MH, functional outcomes such as postoperative scotomas and reduced retinal sensitivities should also be considered in the risk–benefit analysis. Even if there are no intraoperative complications, ILM peeling can cause anatomic changes in the retina, such as a DONFL appearance of the fundus, which has frequently been reported.8–11 The DONFL appearance consists of numerous arcuate striae slightly darker than the surrounding retina. On OCT it is visible in the form of defects in the optic nerve fibre layer.12 Whether or not the presence of a DONFL adversely affects retinal function is still controversial. In a study using the conventional Humphrey perimetry 10-12 program, no scotomas were detected in any of 20 eyes which had undergone idiopathic MH surgery with ILM peeling.9 The results of microperimetry evaluations are conflicting: a few authors found no changes after peeling,13–14 while a few others showed the presence of microscotomas.10–11 This discrepancy is partly due to the different technologies used. VA, standard Humphrey visual field testing and even some microperimetry techniques might miss some of the subtle defects that the present SD-OCT/SLO approach detected. As stated in the Methods section, the new OCT/SLO microperimetry device used in our study to test retinal sensitivity is, with its OCT and eye tracking system, one of the most precise microperimetry methods available. Moreover, we used a customised pattern with a high density of central points, to obtain dense coverage of the central area of the macular region. With this new method, mean macular sensitivity was found to be lower by about 3.4 dB in eyes with ILM peeling than in those without. It should be remembered that the decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity, and that a 3 dB change is a change in power ratio by approximately a factor of two. In other words, after ILM peeling, the retina needed more than twice as much light to see the spot as before peeling.

Five of the eight eyes that underwent peeling exhibited at least one paracentral absolute microscotoma (one eye exhibited four). Note that the presence of these microscotomas was revealed by 29 pinpoint measurements. Consequently, these eyes may have had many more paracentral microscotomas than were revealed with the present number of measurement points. These absolute microscotomas seem strongly correlated to ILM peeling, as they were not found in any of the eyes that did not undergo peeling.

Moreover, in eyes that underwent peeling, more than a third of the points measured had a sensitivity of <10 dB, compared to about one in 10 of the points for unpeeled eyes. All these abnormalities may reduce the quality of vision, even if they do not actually reduce VA. These results for microscotomas are in line with those reported in two studies using the Rodenstock SLO-105 device for microperimetry and covering the central 8–10° of the visual field with a Goldmann II size stimulus.10–11 In one of these studies,10 paracentral scotomas were found in 56% of the eyes, which had undergone vitrectomy with ILM removal for idiopathic MH without the use of any dye. In an earlier study of eyes that underwent surgery for idiopathic MH, also with ILM removal,11 the same authors had shown the occurrence of paracentral scotomas in areas which had been tested and found to be normal before surgery, but which thereafter exhibited a DONFL appearance.11

The cause of the development of microscotomas after peeling has still not been established. The direct trauma caused by the forceps when gripping the ILM may not be the main cause of all these microscotomas, because first, operations were performed by experienced surgeons (RT, AG, PM) accustomed to exercise caution when peeling off the ILM, and second, because the deterioration in retinal sensitivity was too diffuse to be explained by a direct retinal trauma. An effect of dyes, which were only used for peeled eyes, cannot be completely excluded, but as the three different dyes used did not include indocyanine green (ICG), the only dye that has been demonstrated to have a toxic effect on ganglion cells, this hypothesis seems unlikely. Retinal sensitivity deterioration and microscotomas might, like the development of a DONFL appearance, be due to deterioration of the retina, especially of the Müller cells, whose endfeet are closely connected to the ILM and may be affected by ILM peeling.15 Deterioration of other cells is also possible, either directly, due to the stretching caused by the peeling, or indirectly, due to Müller cell deterioration.

As the present work is a retrospective study, it has some limitations, including the different periods that elapsed between surgery and the postoperative examination, and the different sizes of the MH. However, the preoperative characteristics of the two groups of patients did not exhibit any differences that might have caused postoperative differences in retinal sensitivity (tables 1 and 2). In this study, there was also a difference between the maximal best corrected visual acuity (BCVA) reached in the eyes that underwent ILM peeling and those that did not. BCVA was significantly better (lower logarithm of the minimum angle of resolution (log MAR)) in the unpeeled than the peeled group, probably because, in our department, we usually peel off the ILM for MH ≥400 μm, which have a lower preoperative VA, and these larger MH may therefore have a lower maximal postoperative VA than eyes with a smaller MH, whose ILM is not usually peeled off. However, the size of the MH did not affect retinal sensitivity outside the hole, or the presence of paracentral microscotomas after ILM peeling, because the area in which paracentral microscotomas were present was much larger than that of the MH. As in this series we did not have eyes with ILM peeled without staining or operated on by less experience surgeons, the effects of these factors could not be studied. Indeed, less experience and lack of staining may cause more scotomas due to greater difficulty grasping the ILM and direct trauma to the retina.

In the absence of a prospective trial confirming the present results, the findings of this study should be evaluated and compared with the benefit of ILM peeling. From this point of view, the demonstrated benefit of peeling in increasing the postoperative closure rate of large surgically treated MH surpasses the risks. However, in many unproved indications for ILM peeling, or even in small MH for which it seems at least possible to achieve a high closure rate with procedures other than ILM peeling,4–7 the probable reduction in retinal sensitivity after peeling should be taken into account. In these cases, ILM peeling should, in our opinion, be reserved for rare cases of first surgery failure or of recurrence, as this strategy would ensure the best quality of vision for the remaining large majority of these patients.

In summary, ILM peeling may reduce retinal sensitivity and cause postoperative microscotomas. Our retrospective study justifies a larger randomised study to confirm the present results. Such confirmation should lead to more rigorous selection of eyes to undergo ILM peeling. In the meantime, the present results justify the avoidance of such peeling for unproved indications, if its potential benefit seems small. However, if peeling is decided upon, the surface peeled should be limited to the bare minimum.