Objective To observe and analyze the correlation between ectopic foveal inner layer (EIFL) and the EIFL-based idiopathic epiretinal membrane (ERM) staging system and the anatomic and functional prognosis of ERM eyes post pars plana vitrectomy (PPV). MethodsA retrospective study. From January 1, 2020 to October 30, 2023, 345 eyes of 330 patients diagnosed with idiopathic ERM in Department of Ophthalmology of Peking University People's Hospital and treated with standard transciliary flat three-channel 25G PPV combined with ERM and internal limiting membrane exfoliation were included in the study. Among them, 96 were males (111 eyes) and 234 were females (234 eyes). The mean age was (66.8±7.7) years. All study eyes received standard three-port 25G PPV combined with ERM and internal limiting membrane peeling. All study eyes underwent best corrected visual acuity (BCVA) and optical coherence tomography (OCT) examinations. BCVA was performed using a standard logarithmic visual acuity chart and converted to logarithm of the minimum angle of resolution visual acuity for statistical analysis. EIFL thickness and central foveal thickness (CFT) on OCT were measured. ERM eyes were grouped into stage Ⅰ, Ⅱ, Ⅲ and Ⅳ according to ERM staging scheme based on EIFL; disorganization of the retinal inner layers (DRIL) of study eyes were assessed and grouped into no, mild and severe groups. The correlation between ERM staging as well as EIFL thickness and the anatomical and functional prognosis 6 months post-PPV were analyzed. ResultsAmong 345 study eyes, 12, 87, 174 and 72 eyes were stage Ⅰ-Ⅳ ERM respectively, 63 with no DRIL, 216 with mild DRIL and 66 with severe DRIL. Among the 153 eyes with macular edema, the edema subsided in 66 eyes (43.1%, 66/153) 6 months after the operation. Eighty-seven eyes (56.9%, 87/153) did not regress. The edema subsided 6 months after the operation was not significantly correlated with the ERM stage before the operation (χ2=3.331, R=−0.145, P=0.304) or the degree of DRIL (χ2=0.655, R=−0.108, P=0.445). The results of the correlation analysis showed that logMAR BCVA 6 months after the surgery was positively correlated with the degree of DRIL before the surgery (Tau-b=0.236), ERM stage (Tau-b=0.194), CFT (r=0.383), and EIFL thickness (r=0.317) (P＜0.05). There was no significant correlation with the thickness of the outer nuclear layer before the operation (r=0.004, P＞0.05). Preoperative ERM stage (Tau-b=0.303, P＜0.001) and DRIL severity (Tau-b=0.238, P=0.001) were positively correlated with CFT at 6 months after surgery. ConclusionThe ERM stage and EIFL thickness before the operation are positively correlated with logMAR BCVA and CFT 6 months after the operation.