The Retina as a Biomarker for Parkinson's Disease: A Systematic Review

Parkinson's disease is the second most common neurodegenerative condition in the world. Due to the absence of a single definitive diagnostic test, there has been increasing emphasis on identifying reliable biomarkers. This systematic review investigates the potential use of the retina as a biomarker for Parkinson's disease, with a focus on its utility for diagnosis, early detection, or monitoring disease progression. We conducted a comprehensive search using systematic review methodology and tools across multiple databases (PubMed, Embase via OVID and Cochrane), limiting publications to the last five years, in the English language, and to human studies. Of the 13 studies submitted to critical appraisal after systematic filtering, 11 used optical coherence tomography (OCT), 4 used optical coherence tomography angiography (OCT-A), 3 used contrast sensitivity, 7 used best corrected visual acuity (BCVA), 2 used electroretinography (ERG), and 2 visually evoked potential (VEP) to compare between Parkinson's disease patients and healthy controls. The results varied across different techniques, with OCT and OCT-A showing inconsistent statistical significance in multiple studies. Contrast sensitivity demonstrated statistical significance, while BCVA showed no significant difference. ERG and VEP each exhibited some degree of statistical significance. Among the techniques, contrast sensitivity, ERG, VEP, and vessel density (measured with OCT-A) showed the most consistent statistical significance as potential biomarkers. These findings provide early evidence supporting the retina's potential as a biomarker for Parkinson's disease.