Upper Limb Ataxia Among Residents With Chronic Inorganic Arsenic Exposure: A Quantitative Pilot Study.

Background Organic arsenic compounds can cause cerebellar ataxia, but the neurological profile of chronic inorganic arsenic exposure remains unclear. In Toroku, Japan, a historically exposed community, many residents report dizziness, yet it is unknown whether chronic inorganic arsenic exposure is associated with cerebellar dysfunction. We focused on upper limb ataxia as an accessible and quantifiable manifestation of cerebellar function in this setting. Methods We conducted a retrospective pilot study of residents who had lived within 1,000 meters of the Toroku mine roaster before 1962 and were chronically exposed to airborne inorganic arsenic. All participants belonged to this exposed cohort; no unexposed control group was included. Participants aged 66-85 years who could perform a spiral-drawing task were classified as certified with chronic arsenic poisoning (C-CAP) or non-certified (NC) participants, based on Japanese government criteria. Upper limb coordination was quantified using the validated smartphone application TREMOR AI (Densan Software, Co., Ltd., Miyazaki, Japan), which computes deviation area (mm²) and percentage spiral length (%). An experienced neurologist, blinded to TREMOR AI results, assessed clinical cerebellar ataxia and peripheral neuropathy. Group comparisons used the Mann-Whitney U test, Fisher's exact test, and rank biserial correlations. Results Thirty-two residents were included (17 C-CAP and 15 NC). C-CAP participants tended to be older than NC participants, but demographics and metabolic factors were otherwise comparable. Clinically evident cerebellar ataxia was rare in both groups, whereas distal symmetric peripheral neuropathy was common (64.7% vs 60.0%). No significant between-group differences were observed in % spiral length (p = 0.484, r = 0.127) or deviation area (p = 0.970, r = 0.010), indicating no large differences in upper limb ataxia between certified and non-certified exposed residents. Conclusions Within this chronically exposed Japanese cohort, chronic inorganic arsenic poisoning was not primarily characterized by prominent upper limb ataxia on either clinical examination or smartphone-based spiral analysis. The combination of frequent distal symmetric neuropathy and age-related vestibular decline may contribute to dizziness in some residents, supporting a multisensory deficit hypothesis that warrants testing in future vestibular and postural studies.