Individuals with the mildest spinal muscular atrophy (SMA) phenotype, type 3, are able to walk independently, but their residual weakness causes gait impairments and fatigue, and ultimately can result in loss of ambulation. This study aimed to examine longitudinal changes in the six-minute walk test (6MWT) beyond one year in a larger cohort of SMA patients and to explore if changes in ambulatory function are associated with age or SMA subtype.Data from three prospective natural history studies were used. Seventy-three participants who performed the 6MWT more than once, at least 6 months apart, were included. The mean age of the participants was 13.5 years (range 2.6-49.1), with 52 having disease onset before age 3 years (type 3A). At baseline, type 3A participants walked a shorter distance on average (257.1 m) than type 3B participants (390.2 m) (difference = 133.1 m, 95% confidence interval [CI] 71.8-194.3, p < 0.001). Distance walked was weakly associated with age (r = 0.25, p = 0.04). Linear mixed effects models were used to estimate the mean annual rate of change. The overall mean rate of change was -7.8 m/year (95% CI -13.6 –2.0, p = 0.009) and this did not differ by subtype (type 3A: -8.5 m/year, type 3B: -6.6 m/year, p = 0.78), but it did differ by age group (< 6: 9.8 m/year; 6-10: -7.9 m/year; 11-19: -20.8 m/year; ≥ 20: -9.7 m/year; p = 0.005). The results showed an overall decline on the 6MWT over time, but different trajectories were observed depending on age. Young ambulant SMA patients gain function but in adolescence, patients lose function.
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