LMH patients with different ERM have different clinical features, and thus the surgical efficacy for different LMH patients is still under study. To figure out whether surgery is proper for all LMH patients, we conducted this meta-analysis and our results suggested that the though the postoperative BCVA of all patients improved, that of LHEP patients was lower than that of T ERM patients, and no significant difference was found in the rate of REZ among the two groups.
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 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 Andrea et al. In contrast, LHEP is an atypical ERM with no traction and the LMH with LHEP is larger and deeper [21]. 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 [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, it is believed that the posterior detachment of vitreous body, which induces anterior and tangential traction, plays a role in the formation of ERM [16, 22]. In another, the finding of cystic spaces in LHEP, which is formed by leakage of fluid from retinal vessels within it, suggests that LHEP possesses characteristics as same as the middle retinal layers and probably originates from there [3]. The proof that yellow color of LHEP is xanthophyll, which is mainly produced by muller cells, further certifies the relationship between LHEP and the middle retinal layers [23].
According to the results of present studies, less BCVA was regained in patients with LHEP after surgery compared with those without LHEP. This result is in consistence with most of the previous studies, which also found LMH or FTMH eyes with LHEP had worse visual outcome after surgery, compared with eyes without LHEP [8, 13, 14, 24]. This may be because the destroy of retina is severer 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 is required for detecting whether poorer visual outcome is associated with LHEP.
Though many studies have reported the positive association between the REZ and postoperative BVCA [25, 26], no significant differences of REZ existed in the present study between with and without LHEP groups. Previous studies reported that despite the defection of ellipsoid zone before surgery was associated with worse postoperative BCVA, its restoration showed no direct association with functional recover [5, 27]. Meanwhile, several influencing factors also need to be taken into consideration, including the basic characteristics of patients, different preoperative BCVA and time of follow-up. In the study, which showed more eyes with LHEP get restored after surgery, patients all had preoperative BCVA lower than 20/40 [13], while in other two studies patients had wider range of preoperative BCVA [14, 15].
Six of eight included studies had patients with standard pars plana vitrectomy with traditional ERM and ILM peeling [13-17, 19]. However, a new surgery method was used in other two studies [18, 20]. Comparing these two surgical methods, the main difference exists in the disposition of ERM and ILM. Instead of peeling the membrane, in the new method, ERM is double inverted and ILM is flapped [28, 29]. This kind of surgery can retain LHEP and promote the recovery of LHM. A previous study indicated that the development of LHEP might be a recovery progress of LMH [15]. This newly surgical method and its positive outcomes further raise a doubt 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 proof.
There are some limitations in this study that need to be mentioned. Firstly, the number of studies involved was limited and no subgroup analysis was made based on different surgical methods. Thus, more studies are required for obtaining a more convictive result. Secondly, the number of patients with cataract were only stated in five studies, and the different number of patients with combined phacoemulsification and intraocular lens implantation may influence the outcome of BCVA. Thirdly, there were differences in the choice of the gas for the final tamponade, either air, sulphur hexafluoride or perfluoropropane gas, and might have slight influence to 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 there was no significant difference of REZ among the two groups.