Physiology of the fasciculation potentials in amyotrophic lateral sclerosis: which motor units fasciculate?

We set out to study whether in amyotrophic lateral sclerosis (ALS) fasciculation potentials (FPs) arise from the most excitable motor units (MUs). We studied 70 patients with ALS and 18 subjects with benign fasciculation syndrome (BFS). Of the 56 eligible ALS patients, 31 had signs of reinnervation in the right first dorsal interosseous muscle selected for study, and 25 did not. Two needle electrodes were placed in different MUs in each studied muscle. We defined the most excitable MU as that first activated by minimal voluntary contraction. In muscles without reinnervation, the recording site with most frequent FPs had a higher probability of showing the first recruited MU (p < 0.001). No significant difference was found in other patients or in BFS subjects. In very early affected muscles, fasciculating MUs are the most likely to be recruited volitionally. This probably represents hyperexcitability at lower motor neuronal level.