Correlates of Fatigability in Patients With Spinal Muscular Atrophy

OBJECTIVE: To determine the associations between fatigability and muscle strength, motor function, neuromuscular junction (NMJ) function, and perceived fatigue in spinal muscular atrophy (SMA), we assessed 61 patients with SMA. METHODS: Fatigability was defined as the inability to continue a 20-minute submaximal repetitive task of either walking or proximal or distal arm function and expressed as drop-out on the Endurance Shuttle Test Combined Score (ESTCS). We assessed muscle strength with the Medical Research Council (MRC) sum score, motor function with the Hammersmith Functional Motor Scale Expanded (HFMSE) and Motor Function Measure (MFM), NMJ function with repetitive nerve stimulation of the accessory and ulnar nerve, and perceived fatigue with the PROMIS Fatigue Short Form questionnaire in 61 children and adults with SMA types 2-4. We applied Cox regression analysis to explore the associations between fatigability and these factors. RESULTS: The hazard of drop-out on the ESTCS decreased 0.8%, 2%, and 1.3% for each point increase in the MRC sum score, the HFMSE score, and the MFM percentual score, respectively. However, we observed prominent fatigability with preserved muscle function and vice versa in 13%-16% of patients. We did not find an association between NMJ dysfunction of the accessory (p = 0.37) and ulnar nerve (p = 0.063) and fatigability, which could be due to a large number of missing values. Perceived fatigue in SMA was comparable to reference values and was not associated with fatigability (p = 0.52). CONCLUSION: Fatigability in SMA is associated with, yet not equivalent to, muscle strength and motor function.