Parafoveal retinal function of epiretinal membrane foveoschisis: a pilot comparison among ERM phenotypes using MP-3 microperimetry

To compare structural and functional outcomes among three epiretinal membrane (ERM) phenotypes—ERM without ectopic inner foveal layers (EIFL), ERM with EIFL, and ERM foveoschisis (ERM-F)—and to explore the anatomical and functional characteristics of ERM-F in a single-center, retrospective pilot study. Thirty-two eyes of 32 patients undergoing pars plana vitrectomy were categorized as ERM (n = 16), EIFL (n = 11), or ERM-F (n = 5). Swept-source OCT quantified central macular thickness (CMT) and morphology (cystoid spaces, retinal surface wrinkling, ellipsoid zone disruption, posterior vitreous detachment). Functional assessments included best-corrected visual acuity (BCVA, logMAR), parafoveal retinal sensitivity measured by MP-3 microperimetry within the central 5°, and M-CHARTS (horizontal/vertical). Assessments were performed at baseline and 1, 3, and 6 months postoperatively. Baseline demographic characteristics, fixation stability, and metamorphopsia scores were similar across groups. Baseline BCVA and CMT were worse in EIFL than in ERM or ERM-F (both p < 0.05). Cystoid spaces were more frequent in EIFL and ERM-F than in ERM (p < 0.001). CMT decreased in ERM and EIFL (both p < 0.01) but not in ERM-F. BCVA improved postoperatively in the ERM and EIFL groups, and no significant intergroup differences in BCVA were observed at any postoperative time point. In contrast, MP-3 microperimetry demonstrated persistently lower retinal sensitivity in the EIFL group compared with the ERM group at all postoperative time points (all p < 0.05), while ERM-F showed parafoveal sensitivity numerically close to ERM by 6 months. Metamorphopsia (M-CHARTS) did not differ among groups; only ERM showed significant postoperative improvement in horizontal scores. Despite characteristic parafoveal schisis, ERM-F demonstrated postoperative BCVA and parafoveal sensitivity comparable to typical ERM, while EIFL remained functionally worse. MP-3 can reveal focal deficits not captured by BCVA, underscoring the value of multimodal assessment. As an exploratory, hypothesis-generating pilot study, these findings suggest that ERM-F may represent a relatively milder tractional phenotype and warrant confirmation in larger, prospective studies.