The present study found the surgery outcomes of LMH patients with and without LHEP had significant difference. Patients with LHEP were more likely to gain restored ellipsoid zone after surgery but had lower postoperative BCVA.
The existence of ERM has been detected for several decades, while the pathogenesis and category of it are still unclear. T ERM is tractional and the retinal surface under it is usually plicated. The LMH associated with T ERM always has short diameter and shallow cleft, limited in the inner part of retina and described as “high hat” by Andrea et al. In contrast, LHEP is an atypical ERM with no traction and the LMH associated with it is larger and deeper [13]. In a previous study, glial cells and hyalocytes were found in both these two kinds of membranes, while α-SAM was only found in T ERM, explaining the tractional ability of T ERM [14].
The origin of LHEP is not clear but there are two main theories. Firstly, as LHEP has abundant clusters of fibrous long-spacing collagen, fibroblasts and hyalocytes, it is believed that the posterior detachment of vitreous body, which induce anterior and tangential traction, plays a role in the formation of ERM [14, 15]. Secondly, the finding of the cystic spaces in LHEP, which is due to the leakage of fluid from retinal vessels within it, suggesting LEHP possesses characteristics similar to that of the middle retinal layers and probably originates from there [3]. The proof that yellow color of LHEP is xanthophyll mainly produced by muller cells further certifies the relationship between LHEP and the middle retinal layers [16].
According to the results of present study, less BCVA could be regained in patients with LHEP after surgery compared with those without LHEP. In some previous studies, BCVA improved in both patients with or without LHEP [17, 18], while in other studies BCVA had no change among patients with LHEP [15, 19]. Moreover, in present study, surgery benefited patients with LHEP more than patients without LHEP in the restoration of ellipsoid zone. Hence, it can be concluded that more patients with LHEP gained ellipsoid zone restoration, while had worse postoperative BCVA. This phenomenon may can be explained by previous studies, which reported that though the defection of ellipsoid zone before surgery was associated with worse postoperative BCVA, its restoration showed no direct association with functional recover [5, 20]. Furthermore, a study explored the natural process of LMH and regarded it as a stable disease with no significant BCVA change without surgery [7]. Though the present results showed that the surgery of LMH with LHEP was not helpful for improving visual acuity, more studies comparing LHEP cases with and without surgery are necessary.
All studies included operated standard pars plana vitrectomy with traditional ERM and ILM peeling. However, in recent years, a new surgery method has been developed for LHM patients with ERM. Comparing with the regular surgery method, the main difference is about the disposition of ERM and ILM. Instead of peeling the membrane, in the new method, ERM was double inverted and ILM was flapped [21, 22]. This kind of surgery can retain LHEP and promote the recovery of LHM, and a study indicated that the development of LHEP might be a recovery progress of LMH [19]. This newly surgical method and its positive outcomes further raise a doubt in the necessity of taking surgery for LHM patients with ERM.
There are some limitations in this study that need to be mentioned. Firstly, the number of studies involved is small and more studies are required for extended exploration. Secondly, the number of patients with cataract were only stated in two studies, and the different number of patients with combined phacoemulsification and intraocular lens implantation can influence the outcome of BCVA. Thirdly, there were slight differences in the choice of the gas for the final tamponade, either air, sulphur hexafluoride (SF6) or perfluoropropane (C3F8) gas.
In conclusion, this study pooled the postoperative outcomes of LMH patients with and without LHEP and found that though more LHEP patients can gain EZR, the postoperative BCVA of them was worse than patients without LHEP.