Ophthalmic symptoms as biomarkers for prodromal and early Parkinson’s disease: a scoping review

INTRODUCTION: Non-motor symptoms of Parkinson's disease (PD), particularly visual disturbances, often appear before a formal diagnosis and may serve as early signs or prodromal features of the disease. This review aims to evaluate ocular biomarkers associated with prodromal and early-stage PD, highlighting ophthalmic indicators that could support earlier diagnosis and aid in tracking disease progression. MATERIAL AND METHODS: PubMed and Scopus databases were systematically searched for all records available from their inception through May 2024, in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement extension for Scoping Reviews (PRISMA-ScR) guidelines. Two distinct PICO (PICO stands for Population, Intervention, Control, Outcome) frameworks were employed to guide the selection criteria: one for people with early Parkinson's disease and another for individuals at risk of prodromal PD. RESULTS: 30 studies were included - 4 solely on prodromal cohorts (RBD), 18 on patients with PD and 8 studies with both prodromal and clinical cohorts. Studies of people with RBD have shown abnormalities in pupillary light reflex, eye movements, and color discrimination. People with early PD showed impaired contrast sensitivity, impaired color discrimination, retinal structural changes, including retinal thinning (in the inner nuclear layer and ganglion cell-inner plexiform layer, altered pupillary responses, visual hallucinations, and ocular movement disorders). CONCLUSIONS: Impairment of both color and contrast vision, as well as changes in optical coherence tomography (OCT), correlate with disease severity and cognitive decline. They can serve as an early, safe, noninvasive biomarker of neurodegeneration.