These discoveries may mean that neurologists may eventually be able to use a simple eye scan to detect Parkinson's disease in its earliest stages, before problems with movement begin."
As the study was small, however, the researchers acknowledged a larger project is needed to confirm if a thinning retina and the loss of dopamine-producing cells are linked. 
Other downfalls included the fact the scans honed in on a limited part of the retina, and the participants were not followed up over a long period of time.
"If confirmed, retina scans may not only allow earlier treatment of Parkinson's disease but more precise monitoring of treatments that could slow progression of the disease as well,” said Lee.
https://www.newsweek.com/parkinsons-disease-could-be-predicted-eye-scan-study-suggests-1084915