Psychometric Properties of MFM32 in Myotonic Dystrophy Type 1

OBJECTIVE: To examine the psychometric properties of the Motor Function Measure 32 (MFM32) in adults with myotonic dystrophy type 1 (DM1). MFM32 is an instrument for evaluating function in neuromuscular diseases that have not been adequately assessed in DM1. DESIGN: Cross-sectional study using video recording of the performance for evaluation of intra- and inter-rater reliability. SETTING: Data collection was performed at 5 different sites (3 neurologic departments, 1 rehabilitation center, 1 center for rare disorders) and in several of the participants' home. PARTICIPANTS: Convenience sample of 86 adults (N=86) with DM1. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The MFM32. The objective was to assess internal consistency, intra and inter-rater reliability, measurement error, convergent validity, and floor and ceiling effects of MFM32 in adults with DM1. We analyzed the total percentage score and the 3 domain scores: D1 (standing and transfers), D2 (axial, proximal motor function), and D3 (distal motor function). RESULTS: =0.86-0.99). SEM of the total percentage score was 1.59 for the intra-rater reliability and 1.66 for the inter-rater reliability. Minimal detectable change was 4.42 for the intra-rater reliability and 4.6 for the inter-rater reliability. Convergent validity was medium to high. There was a tendency for a ceiling effect on the total percentage score, as well as on some of the domain percentage scores. CONCLUSIONS: The MFM32 has a high internal consistency, intra- and inter- rater reliability, and convergent validity in adults with DM1. Calculated SEM and minimal detectable change were low. There is a ceiling effect of MFM32, especially in the distal function domain and among the participants with high motor function.