Decrement in surface-recorded motor unit potentials in amyotrophic lateral sclerosis.

Background: A decrement to repetitive stimulation sometimes occurs in ALS. The ability to isolate individual surface-recorded motor unit potentials (SMUPs) indirectly allows the analysis of the decrement by comparing the effect of different rates of stimulation on the size of the isolated SMUP.

Methods: The median or ulnar nerve or both were studied in 11 patients with reduced compound muscle action potential (CMAP) amplitude. Isolated SMUPs were identified and studied using standard electrophysiologic techniques. One- and 3-Hz stimulation frequencies were compared in isolated SMUP by collecting > or =100 responses to a nonvarying stimulus. Changes in individual SMUP size and activation with change in stimulus frequency were compared with the size of the SMUP and with the CMAP decrement to a train-of-four stimulus.

Results: Fifteen studies were obtained from the 11 subjects. Eight studies showed a decrement of the total CMAP to a train-of-four stimulus. Twenty-nine well-defined, isolated SMUPs were studied. SMUP size reductions of 1 to 16% occurred with an increase in the stimulus frequency. No conduction block of individual SMUP occurred with the SMUP size reduction. No consistent change in variance was found. The SMUP size reduction was related to the size of the CMAP and the CMAP train-of-four decrement and less so to the size of the SMUP.

Conclusion: The decrement of the total compound muscle action potential to repetitive stimulation appears to arise from individual surface-recorded motor unit potentials and does not result from loss of activation of the main motor axon. The probable mechanism lies at the nerve terminal and neuromuscular junction. No consistent change in variance occurs with different rates of stimulation.