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 results with ADM strength graded on the Medical Research Council (MRC) scale, to address the potential value of a standardized neurophysiological assessment of this nerve-muscle system. The ulnar nerves of 35 normal subjects matched for age, gender, and height served as controls. Reduced compound muscle action potential (CMAP) amplitude and area in the ADM muscle recordings correlated strongly with weakness. Distal motor latency, proximal conduction time, and F-wave frequency were abnormal with minimally detectable weakness. In weaker ADM muscles, conduction velocities and F-wave latencies were also abnormal. Conduction block was never observed and sensory potentials were normal. An "ALS neurophysiological index" was derived from these ulnar nerve studies and consisted of the expression: (CMAP amplitude/DML) x F frequency -, where F frequency was expressed as the number of F responses recorded in 20 trials. This index was strongly correlated with ADM weakness (r = 0.74, P < 0.001). Neurophysiological studies restricted to a single nerve-muscle system, the ulnar nerve/ADM, appear potentially useful in objectively assessing change in ALS.