Analysis of double discharges in amyotrophic lateral sclerosis

Double discharges of motor units (MUs) occurring during sustained voluntary muscle contractions are observed occasionally in healthy muscles and more frequently in disorders of the neuromuscular system. In healthy subjects, double discharges are generated in motoneurons (MNs) and are considered to be a sign of their increased excitability. Therefore, an analysis of their firing pattern may provide information on the state of MNs in neuromuscular diseases, particularly in amyotrophic lateral sclerosis (ALS), whose etiology remains to be disclosed. Firing patterns of MUs capable of firing double discharges were analyzed in brachial biceps of 14 patients with ALS (184 MUs) and 8 healthy control subjects (102 MUs). The incidence of MUs capable of firing double discharges was significantly higher in ALS patients (28.8%) than in controls (3.9%). The majority of doublet interval durations (range 4-8 ms) as well as firing patterns of doubling MUs did not differ between subject groups. Although our data confirm the hyperexcitability of the MN pool in ALS, analysis of firing characteristics of doubling MUs indicates that doublet generation is governed by the same mechanism as in controls, that is, by delayed depolarization. Our findings may provide insight into MN function in ALS.