Oculo-visual abnormalities in Parkinson's disease: Possible value as biomarkers.

Parkinson's disease (PD) is one of the most common neurodegenerative disorders and the second leading cause of dementia worldwide. With an aging population, the prevalence of the disease has dramatically increased. Clinical management has advanced through recent developments in dopaminergic imaging and genetic risk profiling. However, early and accurate diagnosis of the disorder remains a challenge, largely because of the lack of noninvasive and inexpensive reliable diagnostic tests. Besides the well-studied cerebral neurodegeneration that underlies the cardinal symptoms of PD (ie, bradykinesia, tremor, rigidity, and postural instability), ocular changes have also been described in PD, including visual dysfunction, pupil abnormality, lens opacity, and retinal neuronal loss and dysfunction. These ocular pathological processes are related to α-synuclein deposition, and dopamine deficiency in the retina-mirroring the defining pathological features of PD in the brain. Together, these observations support the notion that the eye can serve as a window to the brain, providing clinicians with noninvasive methods to visualize disease. This review focuses on recent advances in the characterization of ocular changes in PD and their promising use as biomarkers in the eye, which can be potentially used for aiding in early diagnosis, tracking disease progression, and valuating novel therapeutic strategies.