Decreased dopamine in the retinas of patients with Parkinson's disease.

Dopamine and it metabolites dihydroxyphenylacetic acid and homovanillic acid were measured in the retinas of eight patients with Parkinson's disease who died. They were divided into two groups according to their last dose of levodopa therapy. One group of three patients had not received levodopa therapy for at least 5 days before death, and the other group of five patients had received therapy 2-15 hours before death. Each patient was matched with controls for delay between death and freezing. In the three patients without levodopa therapy, the retinal dopamine content was lower than normal. In the five patients who received levodopa therapy before death, the retinal dopamine content was similar to that in the controls. This study is the first direct evidence to the authors' knowledge that retinal dopamine concentration is decreased in Parkinson's disease, as it is in the nigrostriatal pathway.