Neurophysiological features of fasciculation potentials evoked by transcranial magnetic stimulation in amyotrophic lateral sclerosis

Abstract

We report 13 patients with amyotrophic lateral sclerosis in whom fasciculation potentials (FPs) driven by transcranial magnetic stimulation (TMS) were recorded. A total of 18 different FPs were analyzed. TMS-driven fasciculations had a simple morphology and were stable. Complex potentials were never cortically driven. Recruitment by a slight voluntary contraction was verified in 7 of 13 tested FPs. FPs were driven by threshold stimuli in 7 of 10 patients and by stimuli 5% below threshold in 3 of 6. Mapping demonstrated that FPs were driven in an area close to the center of gravity of the muscle cortical area. In one case FPs were evoked from most of the cortical representation area of a very weak muscle. Three other patients with profuse fasciculations associated with other clinical conditions were also studied. No TMS evoked fasciculation was observed in this group. The results of this systematic study suggest that cortically evoked FPs arise centrally, at spinal cord or even more proximally, and can represent a marker of increased corticomotor excitability, which is predominant at an earlier phase but can persist as the disease progresses.