Repetitive nerve stimulation often shows responses with an abnormal decrement in patients with amyotrophic lateral sclerosis (ALS), suggesting instability of the neuromuscular junction; however, the pathophysiology and response characteri stics of this instability are not clear. We evaluated response variability of 47 single motor units from 16 patients with ALS and 51 units from 10 normal subjec ts, acquired by delivering threshold stimuli sporadically at 0.5 Hz or less. In addition, in 46 other different single motor units obtained from 21 patients wit h ALS, variability was studied at both 1 and 3 Hz stimulation rates. Motor uni ts from patients with ALS were significantly more variable than those from norma l subjects, even when their larger amplitude was accounted for. This increased v ariability was not rate dependent. Response variability is a critical measure in the statistical method of motor unit number estimation and is attributed to var iability in the number of units activated; the fact that variability of single m otor units varies with disease state may be a potentially confounding factor in the application of the technique.
展开▼
