Spontaneous rhythmic motor unit potentials in the carpal tunnel syndrome.

In 18 out of 164 consecutive patients with carpal tunnel syndrome ongoing spontaneous rhythmic motor unit potentials were found in the lateral thenar muscles. The frequency ranged from 3 to 9 Hz with an average of 6 Hz. The spontaneous rhythmic motor unit potentials appeared to be doublets in many cases. Ischaemia usually caused a marked increase of spontaneous activity within 1 to 2 minutes. Directly after ischaemia the spontaneous rhythmic motor unit potentials subsided for some time. Variable periods of suppression also occurred after forceful muscle contraction and during repetitive supramaximal stimulation of the median nerve. In four patients where the median nerve had been blocked at the level of the elbow with lidocaine the spontaneous activity remained, indicating that the site of origin was distal, probably at the compression site at the carpus.