The recently published study cited below shows evidence that various measurements of eye function (especially contrast sensitivity and the thickness of the inner plexiform layer in the retina) correlated very strongly with a summary measure of clinical variables associated with dementia risk in people with Parkinson disease (PD). We have long known that the back of the eye develops essentially as part of the brain, and that patients with PD tend to have subtle vision problems. Some of these problems (like accommodation insufficiency or impaired color discrimination) tend to improve with levodopa. The cell layer thinning in the retina presumably indicates a more permanent, disease-related signal.
These findings are exciting as potentially adding to our ability to predict cognitive decline in PD. However, the importance of this test will depend on two kinds of further research: (1) confirmation in a longitudinal follow-up study, and (2) development of treatments to forestall or treat PD-related dementia.