The human retina in Alzheimer's disease: Pathology, mechanisms, and biomarkers.

Alzheimer's disease (AD) is characterized by progressive neurodegeneration and synaptic dysfunction that begins decades before clinical symptoms emerge. While AD research has traditionally focused on the brain, increasing evidence suggests that the retina undergoes pathological remodeling that shares features with cerebral changes. Advances in retinal imaging, including optical coherence tomography (OCT), OCT angiography, and hyperspectral approaches, have identified structural, vascular, and functional abnormalities in individuals with mild cognitive impairment (MCI) and early-stage AD. This supports the potential utility of the retina as a non-invasive biomarker for detecting neurodegenerative processes. Furthermore, postmortem studies have demonstrated accumulation of amyloid-β and phosphorylated tau, increased vulnerability of retinal ganglion cells (RGC), synaptic alterations in the inner plexiform layer (IPL), and significant activation of glial cells and complement-mediated inflammatory pathways. Melanopsin RGCs appear to be selectively affected, suggesting a mechanistic link between retinal pathology and the circadian or sleep disturbances commonly observed in AD. This review synthesizes human clinical data from imaging, histopathological, and proteomic studies supporting retinal involvement in AD, with emphasis on convergent mechanisms, including mitochondrial dysfunction, oxidative stress, microglial activation, and synaptic degeneration. Key limitations and sources of variation in current retinal biomarker studies, including cohort heterogeneity, comorbid ocular disease, and methodological variability, are discussed, and future directions are outlined to strengthen retinal diagnostics and therapeutic monitoring of visual system dysfunction in AD.