Insights Into Retinal Pathologies in Neurological Disorders: A Focus on Parkinson's Disease, Multiple Sclerosis, Amyotrophic Lateral Sclerosis, and Alzheimer's Disease.

Neurological diseases are central nervous system (CNS) disorders affecting the whole body. Early diagnosis of the diseases is difficult due to the lack of disease-specific tests. Adding new biomarkers external to the CNS facilitates the diagnosis of neurological diseases. In this respect, the retina has a common embryologic origin with the CNS. Retinal imaging technologies including optical coherence tomography (OCT) can be used in the understanding and processual monitoring of neurological diseases. Retinal imaging has been recently recognized as a potential source of biomarkers for neurological diseases, increasing the number of studies in this direction. In this review, the association of retinal abnormalities with Parkinson's disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), and Alzheimer's disease (AD) is explained. Structural and functional abnormalities in retina as a predictive marker may facilitate early diagnosis of diseases. Although not all retinal abnormalities are predictive of neurologic diseases, changes in the retinal layers including retinal pigment epithelium and plexiform layers should suggest the risk of PD, MS, ALS, and AD.