Content Validity of the Spinocerebellar Ataxia Composite Score as a Measure of Disease Progression in Patients with Spinocerebellar Ataxia

Spinocerebellar ataxia (SCA) composite score (SCACOMS) is a statistically-derived composite measure comprising weighted items that are sensitive to change during early-stage disease. SCACOMS items and weights include the functional Scale for the Assessment and Rating of Ataxia Gait (12%), Stance (17%), Sitting (8%), and Speech (10%) items, and the Clinician Global Impression of Change (CGI) (53%). The content validity of SCACOMS is yet to be established. Semi-structured qualitative interviews were conducted with individuals with SCA (N = 24) and healthcare professionals (HCPs) who treat SCA (N = 2) to evaluate the relevance and weighting of SCACOMS items for assessment of SCA progression. Interviews were audio recorded, transcribed, coded, and analyzed using ATLAS.Ti v23 software, following established methods. SCACOMS items measured all sign and symptom concepts reported by > 50.0% of individuals with SCA, including difficulties with speech (79.2%), balance (75.0%), and gait (66.7%). Of SCACOMS items, individuals with SCA ranked Gait as most important for assessing disease progression (45.8%) and CGI as least important (58.3%). When weighting SCACOMS items, individuals with SCA assigned the highest weight to Gait (mean [standard deviation], 32.8% [13.24]) and the lowest weight to Sitting (12.9% [7.98]). HCPs varied the item weights dependent on SCA severity. Agreement with the statistically-derived weighting of SCACOMS varied, with 41.6% of individuals with SCA indicating that CGI weighting was too high. Overall, all participants indicated that SCACOMS could detect meaningful changes and/or disease stabilization. This study supports the content validity of SCACOMS in SCA; however, SCACOMS item weights may warrant adjustment.