Responsiveness of the motor function measure in patients with spinal muscular atrophy.

Objective: To assess the ability of the Motor Function Measure (MFM) to detect changes in the progression of spinal muscular atrophy (SMA).

Design: Observational, retrospective, multicenter cohort study.

Setting: Seventeen departments of pediatric physical medicine.

Participants: Volunteer patients with SMA (N=112) aged 5.7 to 59 years with no treatment other than physical therapy and nutritional or respiratory assistance.

Interventions: Not applicable.

Main Outcome Measures: The distributions of the MFM scores (total score and 3 subscores) were analyzed per SMA subtype. The relationships between scores and age were studied. The slopes of score changes (reflecting MFM responsiveness) were estimated in patients with at least 6 months' follow-up and 2 MFMs. Hypothetical sample sizes for specific effect sizes in clinical trial scenarios are given.

Results: In 12 patients with SMA type 2 and 19 with SMA type 3 (mean ± SD follow-up, 25.8 ± 19mo), there was a moderate inverse relationship between age and the MFM total score. Patients with less than 6 months' follow-up showed little score changes. Patients with longer follow-ups showed a slow deterioration (-0.9 points/y for type 2 and -0.6 points/y for type 3). Substantial responsiveness was obtained with the MFM Dimension 2 subscore (proximal and axial motricity) in patients with SMA type 2 (standardized response mean [SRM]=1.29), and with the MFM Dimension 1 subscore (standing and transfers) in patients with SMA type 3 aged 10 to 15 years (SRM=.94).

Conclusions: If further confirmed by larger studies, these preliminary results on the relative responsiveness of the MFM in SMA will foster its use in monitoring disease progression in patients who participate in clinical trials.