Longitudinal Evaluation of Polyneuropathy in Atypical Parkinsonian Syndromes.

Background: In Parkinson's disease (PD), a higher prevalence of polyneuropathy (PNP) is increasingly recognized, although the causal association is still under debate. In contrast, PNP in atypical parkinsonian syndromes (APS) has been insufficiently addressed, despite preliminary evidence suggesting elevated prevalence. Methods: Nerve conduction studies were performed on 13 patients with multiple system atrophy (MSA) and 9 patients with progressive supranuclear palsy (PSP) at baseline. PNP was diagnosed according to standard electrophysiological criteria after exclusion of common secondary causes. Comprehensive clinical evaluation included motor and non-motor assessments over two years of follow-up. Results: At baseline, PNP was present in 53.8% of MSA patients and 66.7% of PSP patients. MSA patients with PNP showed greater motor symptom severity (UPDRS III score; p = 0.046) and worse cognitive performance (MoCA; p = 0.044) compared to those without PNP. Over two years, a significant reduction in the tibial nerve amplitude was observed exclusively in MSA patients (p = 0.039), paralleling disease progression. Conclusions: This study provides the first longitudinal evaluation of clinical and electrophysiological PNP progression in MSA and PSP. A high comorbidity of PNP in patients with APS could contribute to motor and sensory impairments in these patients. Our findings indicate that PNP progression may reflect disease progression in MSA. Given the limited sample size, larger-scale longitudinal studies are needed to further investigate biomarker potential of PNP in APS and to clarify differences in peripheral nerve involvement between synucleinopathies and tauopathies.