Structural and functional changes in the retina in Parkinson’s disease

Parkinson’s disease is caused by degeneration of dopaminergic neurons, originating in the substantia nigra pars compacta, and characterized by bradykinesia, rest tremor and rigidity. In addition, visual disorders and retinal abnormalities are often present and can be identified by decreased visual acuity, abnormal spatial contrast sensitivity or even difficulty in complex visual task completion. Because of their early onset in de novo Parkinson’s disease patients, the anatomical retinal changes and electrophysiological modification could be valuable markers even at early stages of the disease. However, due to the concomitant occurrence of normal aging, the relevance and specificity of these predictive values can be difficult to interpret. This review examines retinal dysfunctions arising in Parkinson’s disease. We highlight the electrophysiological delays and decreased amplitude in the electroretinography recorded in patients and animal models. We relate this to coexisting anatomical changes such as retinal nerve fiber layer and macular thinning, measured using optical coherence tomography, and show that functional measures are more consistent overall than optical coherence-measured structural changes. We review the underlying chemical changes seen with loss of retinal dopaminergic neurons, and the effect of levodopa treatment on the retina in Parkinson’s disease. Finally, we consider whether retinal abnormalities in Parkinson’s disease could have a role as potential markers of poorer outcomes and help stratify patients at early stages of the disease. We emphasize that retinal measures can be valuable, accessible, and cost-effective methods in the early evaluation of Parkinson’s disease pathogenesis with potential for patient stratification.