P70-S Effect of motor unit firing rate on cutaneous silent period in a patient with ALS

Abstract

Background Cutaneous silent period (CuSP) is a brief suppression of voluntary contraction that follows strong electrical stimulation of a cutaneous nerve. It is known that the level of voluntary contraction has little influence on the CuSP duration. On the other hand, in several experiments with single motor unit (MU) recordings it was shown that CuSP duration depends on the discharge rate of a single motor neuron. Materials and methods We have observed a 63-year-old woman with definite amyotrophic lateral sclerosis (ALS) according to El Escorial criteria, clinical signs of both upper and lower motor neuron involvement and bulbar onset. The number of functioning MUs in the right abductor pollicis brevis muscle estimated by adapted multi-point stimulation method showed very few remaining high-amplitude MUs. Several CuSP recordings were performed at different levels of voluntary contraction which were mostly provided by change of motor unit firing rate. Results CuSP duration varied little between different recordings. The onset latency was normal, but CuSP duration was increased significantly. At the same time, CuSP duration was similar between recordings with different motor unit discharge rates. Conclusion We suggest that intrinsic electrophysiological characteristics (e.g. effects of collateral reinnervation) of the remaining motor neurons may be the underlying cause of CuSP duration consistency regardless of MU firing rate. Lengthening of CuSP can be both associated with changes that motor neurons undergo during the course of the disease or inhibitory effects of supraspinal origin.