Optimizing the MDS-UPDRS Part III for early-stage Parkinson’s: early supportive evidence for a limb-related bradykinesia/rigidity sub-score

The MDS-UPDRS was not specifically developed for early-stage Parkinson's disease (PD). This study investigated a sub-score of MDS-UPDRS Part III to assess bradykinesia and rigidity, which would be more targeted and clinically meaningful to assess disease progression in early-stage PD. A cross-sectional Rasch model was applied to the Parkinson's Progression Markers Initiative (PPMI) untreated PD cohort data (N = 423) to define an adequate item set. Additionally, we evaluated how the daily life items from MDS-UPDRS Part II relate to bradykinesia and rigidity severity. A sub-score of 15 MDS-UPDRS Part III items focusing on limb-related bradykinesia and rigidity demonstrated good measurement properties in early-stage PD. Items adequately targeted to the study sample, and a meaningful hierarchy supported the validity of the scale. The scale was invariant to symptomatic treatment intake. Onset of clinical signs was represented as a milestone in the emergence of impairment in patients' daily functioning.