Number of Edb motor units estimated using an adapted multiple point stimulation method: normal values and longitudinal studies in ALS and peripheral neuropathies

Objective: To validate the adapted multiple point stimulation (AMPS) method to estimate the number of motor units (MUNE) from the extensor digitorum brevis (Edb) muscle. Methods: Twenty controls (10 young and 10 old) were examined on both sides and 10 patients with amyotrophic lateral sclerosis (ALS) and 5 with acute peripheral neuropathy (PN) were examined longitudinally on one side during a series of repeated electrophysiological sessions. Results: In the controls, the median MUNE and size of the motor unit action potentials (S-MUAPs) were found to be age-related (411 and 70 μVms in the young group; 164 and 142 μVms in the old group; P<0.01), with a coefficient of variation of MUNE values of 27% and 20%, respectively. In the ALS group, the median MUNE value at diagnosis was 31 (P<0.05 vs. controls), and during a mean follow-up period of 11.5 months a continuous decrease in the MUNE value was seen, together with an initial increase, followed by a later decrease in 4 cases, in S-MUAP size. In the PN group, the MUNE value was initially similar to that in controls, but then decreased, accompanied by an increase in S-MUAP size, and then showed a progressive increase, together with a decrease in S-MUAP size. Conclusions: AMPS, a MUNE method developed in the upper extremity, also appears to be a useful procedure for quantifying changes in the MUNE value in the Edb muscle without specific software in order to study age-related changes or changes in patients with ALS or PN.