Small case-control study finds that peak pupil constriction velocity—unaffected by symptomatic dopaminergic therapy—correlates with autonomic symptoms in early, drug‑naïve Parkinson's disease, while other pupil parameters poorly distinguish patients from controls.

Offers a noninvasive, medication‑resistant physiological biomarker of autonomic dysfunction useful for patient stratification or monitoring in trials, but provides limited mechanistic insight or immediate therapeutic targets.

We examined pupil-light reflex in 25 drug-naïve patients with newly diagnosed Parkinson's disease and compared them to 29 healthy controls. We also evaluated effects of symptomatic treatment and correlations with clinical scores. Besides a smaller pupil diameter at maximum constriction, no pupil parameter robustly distinguished patients from controls. Following initiation of symptomatic therapy, baseline pupil diameter and constriction amplitude increased, whereas peak constriction velocity remained unchanged and correlated with patient-reported autonomic symptoms. These findings suggest peak constriction velocity may reflect autonomic dysfunction less influenced by medication. Further studies should clarify the pupil-light reflex's clinical utility in Parkinson's disease.