PURPOSE: To determine whether the outer nuclear layer (ONL) could serve as a quantitative biomarker for photoreceptor function in pachychoroid pigment epitheliopathy (PPE) and pachychoroid neovasculopathy (PNV). Additionally, to assess the impact of pachychoroid features, the presence of neovascularization, and a history of exudation on photoreceptor function. METHODS: One hundred and one eyes with pachychoroid features and thirty-four normal control eyes were retrospectively analyzed. The study groups were categorized into PNV and PPE subgroups based on the presence of neovascularization detected on optical coherence tomography angiography. PNV eyes were further stratified based on a history of exudation. The average thicknesses of individual retinal layers and choroidal thickness were measured, along with best-corrected visual acuity (BCVA). The relationship between BCVA and each retinal/choroidal layer thickness was analyzed. RESULTS: ONL thickness differed significantly among the 3 groups, with the thinnest in the PNV group and the thickest in the control group (p = 0.001), revealing a linear correlation to BCVA. The PPE group exhibited thicker ONL and better BCVA than the PNV group without a history of exudation (p <
0.001 and p = 0.015, respectively). In PNV eyes, those with a history of exudation had thinner ONL and worse BCVA compared to those without such a history (p = 0.003 and 0.008, respectively). CONCLUSIONS: ONL thickness serves as a quantitative biomarker of photoreceptor function in PNV and PPE. Photoreceptor function declines in eyes with pachychoroid features, the presence of neovascularization, and a history of exudation. The development of neovascularization or exudation in PNV or PPE should be treated promptly to salvage photoreceptors.