Smartphone-derived digital motor measures to monitor progression in idiopathic REM sleep behavior disorder

Sensitive and scalable biomarkers are critical for tracking progression during the prodromal phase of Parkinson's disease, particularly in idiopathic REM sleep behavior disorder (iRBD). We evaluated the Roche PD Mobile Application version 2, a smartphone-based platform, in 51 individuals with polysomnography-confirmed iRBD, 89 patients with early Parkinson's disease, and 22 healthy controls over 12 months. Participants completed daily active tasks generating validated digital summary measures. Adherence was high (73%). Baseline digital bradykinesia scores discriminated between groups (p < 0.001) and were higher in phenoconverters versus non-converters (p = 0.003; Cohen's d = 1.10). Over 50 weeks, bradykinesia (Cohen's d = 0.50) and speech (Cohen's d = 0.79) scores worsened significantly. Sample size modeling showed that digital bradykinesia required 132 participants per arm to detect a 50% treatment effect, fewer than the best-performing clinical measure. These findings support digital bradykinesia as a sensitive endpoint for prodromal Parkinson's disease trials.