Electrophysiological estimation of motor units in limb-girdle muscular dystrophy and chronic spinal muscular atrophy

Abstract

A comparative clinical and electrophysiological study of the extensor digitorum brevis (EDB) muscle of patients with limb-girdle muscular dystrophy and patients with chronic spinal muscular atrophy is presented. A firm diagnosis was based upon extensive clinical and laboratory findings. The results are compared with those obtained in control subjects. The mean duration of the illness was greater in the limb-girdle muscular dystrophy group. The EDB muscle shows extreme hypertrophy in nearly all patients with limb-girdle muscular dystrophy while it is usually atrophic and weak in patients with chronic spinal muscular atrophy. Hypertrophy of the EDB muscle in limb-girdle muscular dystrophy was also indicated by the high amplitude of the maximal evoked response of this muscle in muscular dystrophy; whilst small amplitude maximal evoked responses were obtained in chronic spinal muscular atrophy. It is proposed that hypertrophy of the EDB muscle may be a useful clinical criterion in differential diagnosis of cases with progressive proximal muscular atrophy. The hypertrophy of the EDB muscle in limb-girdle muscular dystrophy is considered similar to the compensatory hypertrophy of the gastrocnemius muscle seen in Duchenne muscular dystrophy. The estimated mean amplitude of motor units has a tendency to be higher in limb-girdle muscular dystrophy as compared with control values; it was greatly enhanced in chronic spinal muscular atrophy. Finally, the number of motor units was within normal limits in the limb-girdle muscular dystrophy group whilst a pronounced reduction was observed in chronic spinal muscular atrophy. Our findings indicate that: (a) the EDB muscle behaves differently in the 2 diseases; (b) there is no loss of functioning motor units in limb-girdle muscular dystrophy; and (c) there is no electrophysiological evidence to support the neural hypothesis proposed lately as accounting for limb-girdle muscular dystrophy.