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Interaction of random electromyographic activity with averaged sensory evoked potentials

Section of Electromyography, Mayo Clinic and Mayo Foundation, Rochester, MN.

We averaged sensory nerve action potentials (SNAP) of the median nerve recorded at the wrist, forearm, and elbow with up to 6,000 trials in the presence of quantitated levels of background EMG activity in six normal volunteers. The SNAP could be recorded reproducibly with averaging when the EMG amplitude was up to 50 times the SNAP amplitude. EMG amplitudes of greater than 100 times the SNAP amplitude produced continuous variation in the averaged waveform that did not stabilize, probably because of the quasi-random, large, triphasic potentials that make up the EMG. Monitoring and reduction of background EMG activity can improve reliability of somatosensory evoked potential recording.

Address correspondence and reprint requests to Dr. Jasper R. Daube, Mayo Clinic, 200 First Street SW, Rochester, MN 55905.

Presented in part at the 43rd annual meeting of the American Academy of Neurology, Boston, MA, April 1991.

Received October 8, 1991. Accepted for publication in final form January 2, 1992.