Amyotrophic Lateral Sclerosis: Electrophysiologic Study (Number of Motor Units and Rate of Decay of Motor Units)

Recently described methods for estimating the number of motor units in the extensor digitorum brevis and thenar muscles have provided a quantitative means for assessing the rate of loss of motor units with aging and disease. In amyotrophic. lateral sclerosis, repeated motor unit estimates showed a very rapid initial loss of motor units with slowing in the late stages. Early functional failure in giant units was suggested by abnormal decremental responses to repetitive stimulation, and late terminal collapse by the finding of small shrunken motor units. Accompanying these changes were mild slowing of the motor conduction velocity and prolongation of the motor terminal latencies. Contrary to most previous studies, digital sensory action potentials were reduced.