Background and Objective With its relevance for everyday function, upper-limb-coordination is an insufficiently explored domain for digital-motor-outcomes in ataxia. Leveraging the trial-ready quantitative-motor (Q-Motor) system, we developed and validated a comprehensive task battery to capture upper-limb-ataxia.

Methods Cross-sectional single-center-study (46 patients with predominantly degenerative cross-genotype cerebellar-ataxias (mean-age: 50 years; mean-SARA: 12 points), and 48 age- and sex-matched controls. Q-Motor assessments with a force transducer and an electromagnetic position sensor comprised both existing bilateral-tasks particularly promising for ataxia (foot-tapping, finger-tapping, diadochokinesia, grip-lift) (47 parameters/side), and tasks newly designed to capture ataxia (multi-directional two-dimensional-target-pointing, spiral-drawing) with the dominant hand (450 parameters, including spatial, temporal, spatiotemporal, and texture measures).

Results Target hits per second (AUC: 0.97) and frequency of foot-tapping, finger-tapping or diadochokinesia (AUC: 0.91-0.94) were excellent discriminators between ataxia and controls. Target hits per second and finger-tapping-frequency correlated strongly with the SARA (Spearman rho: -0.87 and -0.81), and even specifically with the sum of its upper limb items (-0.85 and -0.72). For diadochokinesia, temporal variability showed the strongest correlation with SARA (0.71) and its upper-limb composite (0.67, all p<0.001). Parameters in upper-limb tasks had moderate to strong correlations with activities-of-daily-living-function (FARS ADL, |rho|: 0.5-0.6), and dexterity as assessed by the 9-hole peg-test (0.53-0.78, all p<0.001).

Discussion and Conclusion Severity of upper-limb ataxia can be captured by Q-Motor with physiologically interpretable and functionally relevant measures of a finger-tapping, diadochokinesia, and a novel two-dimensional target-pointing-task of interest also for HD. Longitudinal assessments are ongoing to explore their responsiveness to change in a clinical trial setting.