Validation of the Italian version of the SBMA Functional Rating Scale as outcome measure

The Spinal and Bulbar Muscular Atrophy Functional Rating Scale (SBMAFRS) is an established rating instrument used to assess the functional status of patients with Spinal and Bulbar Muscular Atrophy (SBMA). Our aim was to validate an Italian version of the scale. We administered the SBMAFRS to sixty SBMA patients during routine follow-up of clinical evaluations. To estimate the test stability, the scale was re-administered to a subset of 39 randomly selected patients after 8 weeks. The patients underwent clinical evaluation including 6-min walk. Psychometric analysis included reliability assessment and factorial analysis. To evaluate convergent validity, correlations between SBMAFRS items and muscular force assessed by manual testing, ALSFRS total score and subscales scores, and forced vital capacity, were performed. Internal consistency as measured by Cronbach's alpha (total scale 0.85) was high. Test-retest reliability assessed by Spearman's rho was also high. Principal component analysis with varimax rotation yielded a four-factor solution accounting for approximately 79 % of the variance. The scale total score and subscales score were strongly correlated with respective items and subscores of the ALSFRS, with respiratory function and with the 6-min walk test. In conclusion, we performed an Italian validation of the only existing disease-specific Functional Rating Scale for SBMA patients. This scale will be a useful tool not only in the clinical practice but also as an outcome measure in upcoming clinical trials.