Visual impairment in Parkinson's disease

Parkinson's disease (PD) is a complex neurological disorder characterized by motor and nonmotor symptoms. The onset of visual symptoms may occur many years before motor ones. Growing evidence shows a strong involvement of the visual system, including corneal changes, retinal layer thinning, and alterations of gray and white matter. These modifications can explain the wide range of visual symptoms, including visual acuity loss, eye movements and pupillary defects, electrophysiology alterations, and visual processing impairment. Moreover, visual symptoms and visual system involvement also can be detected in other forms of parkinsonism, such as Lewy body dementia and multiple system atrophy. However, the deep characterization of common features and differences between each parkinsonism, in terms of visual system changes, is still missing. The main aim of the present chapter is to describe the main alterations reported in the literature of the visual system in PD.