MUSCLE FATIGUE IN SPINAL MUSCULAR-ATROPHY

We previously reported that patients with spinal muscular atrophy do n ot lose muscle strength over time as measured quantitatively. However, we noted that many patients with spinal muscular atrophy suffer from what they called fatigue. We wondered if we could measure fatigue duri ng a single maximal voluntary contraction, whether fatigue might incre ase with time, independent of muscle strength, and whether increasing fatigue might correlate with loss of function in some patients. We mea sured fatigue during: a single maximal voluntary contraction in a coho rt of patients having spinal muscular atrophy using quantitative stren gth testing. We included only patients with spinal muscular atrophy ag ed 5 years or older, so they could follow instructions regarding muscl e contraction, and who were followed for at least 2 years. Seventy-six children with spinal muscular atrophy and 24 untrained individuals, a ged 5 to 57 years (mean, 16.8 years), were studied. There was no disce rnible abnormal fatigue in patients with spinal muscular atrophy compa red to untrained controls using our methodology. Thus, spinal muscular atrophy may not be associated with fatiguability. Moreover, spinal mu scular atrophy does not appear to cause progressive muscle fatigue wit h age or loss of function. It is possible that fatigue was undetectabl e by our methods. An alternative explanation is that what patients des cribe as fatigue may be caused by factors outside the neuromuscular sy stem. Such factors may include chronic respiratory insufficiency with hypoventilation and carbon dioxide retention as well as chronic malnut rition and negative nitrogen balance.