Study on the relationship between peripheral nerve fiber types and levodopa usage in Parkinson's disease

The aim of this study is to comprehensively determine the types of affected fibers in Parkinson's disease (PD) patients by employing nerve conduction studies (NCS), sympathetic skin response (SSR) examinations, and current perception threshold (CPT) testing and to analyze the correlation between levodopa use and nerve involvement.

This retrospective study included 36 clinically diagnosed PD patients who were recruited between January 2018 and April 2019. All patients underwent NCS, SSR testing, and CPT sensory examinations. Additionally, the PD patients were assessed for disease staging using the Hoehn and Yahr (H-Y) scale.

Fifteen patients were included in the tremor-dominant subtype, ten patients in the rigid-dominant subtype, and eleven patients in the mixed subtype. Eleven patients were using levodopa, while twenty-five patients had never used any anti-Parkinson's medication. Ten patients (28%) showed abnormal sympathetic skin responses (SSR). The CPT examination revealed sensory abnormalities in twenty-four patients (67%), with eighteen patients (75%) experiencing sensory hypersensitivity and six patients (25%) experiencing sensory hypoesthesia. Twelve patients (33%) had normal CPT results. Among the patients with abnormal CPT findings, seven cases (29%) involved large myelinated fiber damage, twenty-two cases (92%) involved small myelinated fiber damage, and nineteen cases (79%) involved unmyelinated fiber damage. The rate of sensory abnormalities was 64% (7/11) in the levodopa group and 68% (17/25) in the non-levodopa group, with no statistically significant difference between the two groups.

The incidence of abnormal CPT findings in PD patients was higher than that of abnormal SSR responses, suggesting that nerve fiber damage primarily affects small fiber nerves (SFN).