The Retina as a Proxy for Brain Neurodegeneration: A Narrative Review on OCT-Based Retinal Imaging in the Early Detection of Alzheimer's and Parkinson's Disease.

Neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), are major causes of cognitive and motor decline, yet early diagnosis remains challenging due to asymptomatic phases and limited non-invasive biomarkers. This narrative review systematically synthesized studies on retinal imaging in AD and PD. Published studies were identified through searches of PubMed, MEDLINE, Google Scholar, and reference lists, focusing on Optical Coherence Tomography (OCT), OCT Angiography (OCTA), and Spectral-Domain OCT (SD-OCT) assessing retinal structural and vascular changes. Data were extracted on retinal layer thickness, vascular parameters, and diagnostic metrics. Findings indicate that both diseases consistently exhibit thinning of inner retinal layers, particularly the retinal nerve fiber layer (RNFL) and ganglion cell-inner plexiform layer (GCIPL). In AD, studies reported progressive inner retinal thinning across disease stages, sometimes accompanied by outer retinal and retinal pigment epithelium changes. In PD, thinning was observed predominantly in RNFL and GCIPL, correlating with disease duration and motor severity. Microvascular alterations were described in both disorders, with disease-specific spatial patterns reported across studies. Overall, retinal imaging emerges as a non-invasive, high-resolution, and cost-effective tool for early detection, differential assessment, and longitudinal monitoring of neurodegenerative diseases. These findings support the translation of retinal biomarkers into clinical practice for improved disease management.