Complex fasciculations and their origin in amyotrophic lateral sclerosis and Kennedy's disease

Complex fasciculations are common in patients with amyotrophic lateral sclerosis (ALS). Combined fasciculations, defined as a complex fasciculation consisting of two or more components that occur independently but also in combination with another component, also occur in ALS. To test the hypothesis that combined fasciculations originate at the supraspinal level, we analyzed 2681 fasciculation potentials from 17 patients with definite or probable ALS by the El Escorial criteria. Results were compared with 304 fasciculation potentials recorded from 6 patients with Kennedy's disease, in which the upper motoneurons are spared. The mean firing frequency of the fasciculations was 24.4 +/- 25.6 per minute in ALS, significantly higher than the 2.9 +/- 3.4 per minute found in Kennedy's disease (P < 0.0001). In ALS, the mean combination ratio (the number of times that a combined fasciculation occurred divided by the total number of fasciculations) was 4.6 +/- 8.3% (range 0-33). Fourteen of 17 ALS patients had combined fasciculations, but only one combined fasciculation was found in a patient with Kennedy's disease. Combined fasciculations are distinctive in ALS, and we hypothesize that they are triggered by a supraspinal mechanism reflecting dysfunction of descending motor pathways.