Retinal Nerve Fibre Layer Thinning and Ganglion Cell Complex Degeneration in Parkinson's Disease: Emerging Biomarkers for Neurodegeneration and Cognitive Decline

Parkinson's disease (PD) is characterised by progressive neurodegeneration extending beyond the central nervous system to affect retinal structures. Recent advances in optical coherence tomography (OCT) have revealed significant alterations in retinal nerve fibre layer (RNFL) thickness and ganglion cell complex (GCC) integrity in PD patients, suggesting potential biomarker applications for disease monitoring and progression assessment. This comprehensive review examines the current evidence for RNFL thinning and ganglion cell layer degeneration in PD, evaluating their correlation with visual disturbances, cognitive decline, and disease progression whilst assessing their potential as non-invasive biomarkers for clinical practice. A systematic analysis of recent literature was conducted, focusing on spectral-domain OCT studies investigating retinal structural changes in PD patients compared to healthy controls. Particular attention was given to meta-analyses, longitudinal studies, and investigations correlating retinal changes with clinical outcomes. Consistent evidence demonstrates significant RNFL thinning in PD patients, particularly in the inferior quadrant, with mean thickness reductions of approximately12-15 μm compared to controls. Ganglion cell-inner plexiform layer (GCIPL) thinning shows even greater sensitivity, with parafoveal GCIPL demonstrating twice the rate of degeneration in PD patients compared to controls. These changes correlate significantly with visual acuity, cognitive function, and disease severity, whilst appearing early in the disease course, often preceding clinical manifestations. RNFL and GCIPL measurements represent promising non-invasive biomarkers for PD diagnosis, progression monitoring, and cognitive decline prediction. The correlation between retinal neurodegeneration patterns and clinical outcomes supports the integration of OCT assessments into routine PD management protocols. Future research should focus on establishing standardised measurement protocols and validating these biomarkers across diverse populations and disease stages.