Three-dimensional Motion Capture Discriminates Extremely Mild Cerebellar Ataxia from Parkinsonism in the Upper Limbs

Upper-limb motor symptoms in spinocerebellar degeneration (SCD) and Parkinson's disease (PD) frequently overlap, making clinical differentiation challenging in mild cases using the finger-to-nose test (FNT) or the diadochokinesis test (DDKT). This study aimed to identify quantitative kinematic markers that distinguish mild SCD from mild PD using three-dimensional motion capture. Twenty-six individuals per group-mild SCD, mild PD, and normal controls (NCs)--underwent motion analysis during FNT forward, FNT backward, and DDKT. Key parameters including segmental velocity ratios during FNT and rotational angle parameters during DDKT were analyzed. Significant parameters were validated in an additional cohort of 20 patients with moderate-to-severe SCD and 23 with moderate-to-severe PD, and their associations with motor rating scores were examined. During FNT-forward, the velocity ratio in the final third of the movement was significantly higher in patients with mild SCD than in those with mild PD and NCs and correlated with dysmetria scores in the moderate-to-severe SCD group. During DDKT, patients with mild PD showed smaller rotation angles and a greater decline over time than did patients with mild SCD and NCs. This reduction correlated with the akinesia scores in the moderate-to-severe PD group. Three-dimensional motion capture identified disease-specific kinematic features that were undetectable by visual inspection. Mild SCD was characterized by an increased terminal velocity during FNT-forward, reflecting subtle dysmetria, whereas mild PD showed reduced rotation amplitude during DDKT, reflecting early akinesia. These markers may support the early differential diagnosis and upper-limb motor assessment.