Given the different clinical characteristics of LMH patients with ERMs, surgical efficacy remains controversial. To figure out whether surgery is appropriate for all LMH patients, we conducted this meta-analysis. The results suggested that although BCVA improved in all patients after surgery, BCVA in LHEP patients was lower than that in T ERM patients, and REZ rate was not significantly different between the two groups.
The existence of ERM has been detected for several decades, while its pathogenesis and category remain unclear. T ERM is tractional and the retinal surface under it is usually plicate. The LMH with T ERM always has short diameter and shallow cleft, limited in the inner part of retina and described as “high hat” by Govetto et al. In contrast, LHEP is an atypical ERM without traction, having larger and deeper LMH under it [21]. In a previous study, glial cells and hyalocytes were found in both membranes, while α-SAM was only found in T ERM, explaining the tractional ability of T ERM [22].
The origin of LHEP is not clear but there are two main theories. In one theory, as LHEP has abundant clusters of fibrous long-spacing collagen, fibroblasts and hyalocytes, the posterior detachment of vitreous body might induce both anterior and tangential traction and therefore plays a role in the formation of ERM [16, 22]. However, in another theory, cystic spaces in LHEP formed by leakage of fluid from retinal vessels within it suggested that LHEP might originate from the middle retinal layers since they share the same characteristics [3]. The proof that the yellow color of LHEP is xanthophyll mainly produced by muller cells, further certifies the relationship between LHEP and the middle retinal layers [23].
Based on the results of present studies, the postoperative BCVA recovery in LHEP patients is less than those without LHEP. Besides, previous studies also indicated that LMH or FTMH eyes with LHEP had worse visual outcomes after surgery [8, 13, 14, 24], probably attributable to the severer destruction of the retina in eyes with LHEP than without LHEP. However, there are also some studies reported similar surgery outcomes of eyes with and without LHEP [10, 12, 15, 16]. As there are no significant differences in preoperative BCVA (<20/40), surgery method (ILM and ERM peeling) and time of follow-up (>6 months) among these two kinds of studies, more studies with longer follow-up are required to determine the relationship between the poor visual outcomes and LHEP. Though many studies have reported the positive association between the REZ and postoperative BVCA [25, 26], no significant differences of REZ existed between with and without LHEP groups in present study. Previous studies reported that although the defection of ellipsoid zone before surgery was associated with worse postoperative BCVA, its restoration showed no direct association with functional recover [5, 27]. Additionally, several influential factors, including the physical conditions of patients, different preoperative BCVA, and time of follow-up, should also be taken into consideration. In the previous study that had higher REZ rate in LHEP patients, the preoperative BCVA of patients were lower than 20/40 [13], while in other two studies, the preoperative BCVA had wilder range [14, 15].
Patients from six of the eight included studies underwent standard pars plana vitrectomy and conventional ERM and ILM peeling [13-17, 19]. However, the other two studies operated a new surgical method on patients [18, 20]. Comparing these two surgical methods, the difference between the two surgical methods lies in the disposition of ERM and ILM. Instead of peeling the membrane, the new method double inverted the ERM and flapped ILM [28, 29]. This kind of surgery can preserve LHEP and promote the recovery of LHM. A previous study indicated that the development of LHEP might be a part of recovery progress of LMH [15]. This new surgical method and its positive outcomes further raise doubts in the necessity of taking surgery for LHM patients with ERM. Thus, whether surgery is necessary for patients with LHEP to get functional and morphological restoration requires more study to prove.
However, our study has some limitations. Firstly, a limited number of studies involved would inevitably lead to bias, and no subgroup analysis was made based on different surgical methods. Thus, more studies are required to obtain more convincing results. Secondly, only 5 studies reported the number of cataract patients, and the number of patients with combined phacoemulsification and intraocular lens implantation may influence the outcome of BCVA. Thirdly, different choices of the gas for the final tamponade, such as air, sulphur hexafluoride, or perfluoropropane gas, might have a distinct influence on the surgical results.
In conclusion, this study pooled the postoperative outcomes of LMH patients with and without LHEP and found that the postoperative BCVA of patients without LHEP was better than patients with LHEP, and REZ showed no significant difference between the two groups.