WS5.2. Intraoperative Monitoring of Motor Evoked Potentials: Physiology and Applications

Abstract

Intraoperative monitoring (IOM) of motor evoked potentials (MEP) represents a significant advance for evaluation of descending motor tract integrity. For spinal decompression, there is increasing evidence of its efficacy, but ongoing research is performed to improve sensitivity and specificity. To that end, there is also ongoing debate for warning criteria of MEP changes from transcranial electrical stimulation (TES). It is presently unclear if a cutoff for ensuing neurological deficit should be MEP disappearance or a value between this and 50% amplitude reduction. Interim adjunctive strategies include use of ipsilateral MEPs, modified scalp stimulating positions and multimodality monitoring. Some investigators have proposed a more directed motor threshold to obtain supramaximal MEP voltage in an individual subject. The indications of MEP monitoring have expanded from spinal surgery to central nervous system procedures involving cortical, subcortical and brainstem regions. Corticobulbar MEPs can provide localization and evaluate functional integrity of cranial nerve nuclei. Awake craniotomy can combine monopolar MEP mapping with other modalities to preserve speech and motor functions. The evidence of MEP monitoring for scoliosis to date has been encouraging for the prevention of neurological compromise, as a result of deformity correction. IOM of MEPs is pertinent here as a non-invasive surgical adjunct for these neurologically intact patients. Recently, a study has suggested that in susceptible individuals, intraoperative suppression of MEPs may rarely occur unpredictably, independent of surgical or anesthetic intervention, but anesthetic factors may be overall more significant. Side effects of TES are rare, but seizures, bite injury, lip laceration, mandibular fracture, scalp burns and cardiac complications should be anticipated. Improved knowledge in the use of intravenous anesthetic agents and pulse train cortical stimuli has allowed for more efficacious MEP monitoring. However, recent studies suggest that often, suboptimal MEPs were elicited, especially during IOM of diseased spinal cord. Various central and peripheral facilitatory techniques will be discussed.