Recording of Myogenic Motor Evoked Potentials (mMEP) Under General Anesthesia

Summary

This chapter reviews possible reasons for the difficulties encountered obtaining well-delineated myogenic motor evoked potentials (mMEP) under general anesthesia. Three possible levels for anesthetic suppression are discussed: the cortical level, the spinal motor neuronal level, and the neuromuscular junction. A technique of facilitating mMEP at the spinal level is described. By combining peripheral stimulation, as for the H-reflex, with transcortical stimulation, the mMEP can be facilitated. The degree of facilitation is compared in 12 anesthetized and 8 nonanesthetized patients.

By combining transcranial electrical stimulation of the cortex with a well-timed peripheral stimulus, we were able to elicit reproducible muscle responses under general anesthesia and a typical surgical degree of muscle relaxation. Adding the peripheral stimulus resulted in a biphasic facilitatory effect in patients without anesthesia, but the second phase of increased excitation was missing in patients under anesthesia. In a small sub-group of patients the effects of transcranial magnetic and electrical stimulation were compared. In cases in which magnetic stimulation worked it provoked a more variable degree of facilitation than transcranial electrical stimulation.

The possible mechanisms of anesthetic suppression of mMEP are discussed, as are possible ways to circumvent suppression, including a change of stimulus parameters, the use of facilitation techniques, and a change in anesthesia regimen. It is concluded that, although there are limitations on the use of H-reflex facilitation in transcranial motor stimulation, this technique is useful to reduce the incidence of false-negative mMEP in comatose patients. Magnetic transcranial stimulation is not particularly useful in comatose patients.

This work and M. Taniguchi were financially supported by a grant from the Deutsche Forschungsgemeinschaft (DFG Schr 285-1/2).