Early ophthalmologic features of Parkinson's disease: a review of preceding clinical and diagnostic markers.

Non-motor symptoms in Parkinson's disease are an important cause of morbidity and may even precede the onset of the motor features of the disease. Visual abnormalities are among the most frequent non-motor symptoms observed during the early stages of the disease. Some of the visual symptoms of Parkinson's disease can likely be explained by the presence of dopaminergic neurons within the retina, where the progressive loss of dopamine and the accumulation of α-synuclein within the retinal layers leads to visual dysfunction, while some are caused by abnormalities in cortical visual processing. Many of these visual symptoms can be overlooked or go unrecognized. We review the visual symptoms in Parkinson's disease, including visual-processing and ocular motility abnormalities, stereopsis deficits, and visual hallucinations, focusing on the early stages of the disease. We focus on the reciprocal influence between the visual symptoms and the progression of the disease, analyzing the influence of dopaminergic therapy on the visual abnormalities. Finally, we discuss the possible role of some of these visual symptoms as possible markers or early diagnostic signs of the disease.