Levodopa, methylmalonic acid, and neuropathy in idiopathic Parkinson disease.

Objective: Peripheral neuropathy (PN) is thought to be coincidental in patients with idiopathic Parkinson disease (IPD). We sought to examine the prevalence of PN in a population of IPD patients and a potential relationship to levodopa use and fasting methylmalonic acid (MMA) levels.

Methods: In a prospective cohort study, IPD patients randomly selected from a comprehensive database were compared to control subjects regarding the presence and severity of PN using clinical and electrophysiological measures. IPD severity was determined using the Unified Parkinson's Disease Rating Scale (UPDRS). We determined the relation of levodopa use with serum levels of cobalamin, MMA, and homocysteine (Hcy). We also explored the association between presence and severity of PN and age, duration of IPD, cumulative levodopa dosing, cobalamin, MMA, and Hcy levels.

Results: Fifty-eight randomly selected IPD patients were compared to 58 age- and sex-matched controls. PN was present in 55% of IPD patients and 9% of controls. Patients with IPD had greater prevalence of PN and fasting MMA/Hcy levels than controls. IPD patients with PN were older and exhibited higher UPDRS scores, fasting MMA/Hcy levels, and cumulative levodopa exposure. PN severity in IPD subjects positively correlated with both levodopa exposure and MMA levels.

Interpretation: IPD patients have a higher prevalence of PN than controls. Although causality is not established, levodopa exposure is associated with MMA elevation and sensorimotor neuropathy in IPD patients. Cobalamin replacement concurrent with levodopa therapy should be considered to protect against development of PN in IPD patients.