To investigate the relationship between changes in body mass index (BMI) and patient-reported function in ambulatory adults with Spinal Muscular Atrophy (SMA).

SMA is an autosomal recessive motor neuron disorder with a broad phenotypic spectrum ranging from severe, infantile onset to milder, late onset with preserved ambulatory ability. Recent studies have demonstrated clear progressive functional decline even in adults with milder forms. SMA results not only in the loss of muscle strength but also muscle wasting that may increase risk for weight gain and adiposity. Here we tested the hypothesis that changes of BMI would be associated with changes in function. 

Data was collected at baseline and year 10 in genetically-confirmed ambulatory adults. Assessments performed included maximal voluntary isometric contraction testing (MVICT), forced vital capacity (FVC), 6-minute walk test (6MWT), compound muscle action potential (CMAP), and SMA Functional Rating Scale-Revised (SMAFRS). Weight and height were also measured to calculate BMI. Multiple linear regression (MLR) analysis was performed with the change in SMAFRS as the dependent variable. 

8 adults (62.5% male, mode SMN2 copy # = 4) were analyzed. Mean change in BMI was 1.75 ± 3.16 kg/m² with 62.5% showing increase. To reduce bias in the MLR model, change in FVC, 6MWT, and CMAP were removed due to high multicollinearity. Of the remaining factors, change in BMI alone was significant and had an inverse relationship with change in SMAFRS (p = 0.0206; β = -1.174). The R² and adjusted R² were 0.6186 and 0.5550, respectively.

Increases of BMI were associated with reduced self-reported function over a 10-year period. Our findings suggest altered body composition and weight are modifiable factors that can be targeted in SMA. Thus, an emphasis on weight surveillance and management throughout adulthood is a critical component of patient-centered care aimed at maximizing long-term functional independence.