Nerve conduction studies in amyotrophic lateral sclerosis

We studied 137 ulnar nerves and abductor digiti minimi (ADM) muscles in 70 patients with amyotrophic lateral sclerosis (ALS), and correlated the resul ts with ADM strength graded on the Medical Research Council (MRC) scale, to address the potential value of a standardized neurophysiological assessmen t of this nerve-muscle system. The ulnar nerves of 35 normal subjects match ed for age, gender, and height served as controls. Reduced compound muscle action potential (CMAP) amplitude and area in the ADM muscle recordings cor related strongly with weakness. Distal motor latency, proximal conduction t ime, and F-wave frequency were abnormal with minimally detectable weakness. In weaker ADM muscles, conduction velocities and F-wave latencies were als o abnormal. Conduction block was never observed and sensory potentials were normal. An "ALS neurophysiological index" was derived from these ulnar ner ve studies and consisted of the expression: (CMAP amplitude/DML) x F freque ncy -, where F frequency was expressed as the number of F responses recorde d in 20 trials. This index was strongly correlated with ADM weakness (r=0.7 4, P < 0.001). Neurophysiological studies restricted to a single nerve-musc le system, the ulnar nerve/ADM, appear potentially useful in objectively as sessing change in ALS. (C) 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 34 4-352, 2000.