Ellen R. Grass Lecture: Motor evoked potential monitoring

Because somatosensory evoked potentials (SSEPs) recorded from the brain are carried solely within the dorsal columns of the spinal cord, SSEP monitoring may fail to detect damage to the spinal cord motor pathways, and techniques for directly monitoring the motor pathways have been developed. Transcranial magnetic brain stimulation is useful for extraoperative evaluation of the motor system, but anesthetic effects on cortical synaptic activity limit its usefulness for intraoperative monitoring. Another proposed monitoring technique, rostral spinal cord stimulation with recording from peripheral nerves, predominantly reflects retrograde conduction within the dorsal columns; like SSEPs, this technique may fail to detect motor pathway damage. The true motor evoked potential (MEP) monitoring technique preferred by most institutions involves transcranial electrical brain stimulation. This stimulates the corticospinal tract axons directly, producing a D-wave in them. There may be additional corticospinal tract volleys, reflecting indirect activation of the pyramidal cells via synapses from cortical interneurons; these I-waves are largely suppressed by anesthesia. Temporal summation at the alpha motor neuron synapse is necessary to elicit myogenic MEPs (M-waves) under anesthesia; this is accomplished with brief trains of stimuli that produce multiple pyramidal tract volleys. High stimulus intensities are required to stimulate the brain through an intact skull; with proper precautions, this is acceptably safe. Myogenic MEPs are quite sensitive to anesthesia and show considerable run-to-run variability. Therefore, criteria for evaluating MEPs differ from those for SSEPs, and the anesthetic regimen and degree of neuromuscular blockade must be carefully controlled. SSEPs should be monitored concurrently with MEPs, because the dorsal columns may be affected without compromise of the motor tracts, and also because this provides redundancy, with two independent assessments of spinal cord function.