Anal muscle electromyograms differ in amyotrophic lateral sclerosis and Shy-Drager syndrome

Electromyography (EMG) of anal sphincter muscles was different in patients with amyotrophic lateral sclerosis (ALS) and Shy-Drager syndome. In 30 patients with ALS, EMG of the external sphincter muscle was essentially normal, with no signs of denervation. In eight cases of Shy-Drager syndrome, however, motor unit potentials of the anal sphincter had highly polyphasic forms of long duration and high amplitude. In the Shy-Drager syndrome, there seems to be specific damage of lower motor neurons that innervate the external sphincter muscle of the anus.