Weakness and fatigue in diverse neuromuscular diseases

Weakness and fatigue are captured by the 6-minute walk test, but the relationship between these symptoms is uncertain. Comparison across neuromuscular diseases has not been examined. A cohort study of 114 patients with spinal muscular atrophy, Duchenne/Becker muscular dystrophy, myasthenia gravis, and energy failure syndromes were included. Percent-predicted distance on the 6-minute walk test was computed from normative values to determine weakness. Fatigue was determined by the decrement in distance from the first to sixth minute. Weakness was seen across all groups (61.9%) but significant fatigue was seen only in spinal muscular atrophy (21.0%). Other groups showed little fatigue. Correlation between weakness and fatigue was significant only in spinal muscular atrophy (R = -0.71; P < .001). Longitudinally, distance walked declined only in Duchenne/Becker muscular dystrophy. In spinal muscular atrophy, weakness did not change, but fatigue increased significantly. These findings suggest independent mechanisms underlying weakness and fatigue in diverse neuromuscular conditions.