The Interpretation of Muscle Motor Evoked Potentials for Spinal Cord Monitoring

Abstract

Objective: To provide a summary of the intraoperative monitoring of muscle motor evoked potentials (MEPs) based on the presence-absence concept during neurosurgical operations along the spinal cord.

Method: Expert review.

Discussion: The measurable parameters of MEPs, such as signal amplitudes and thresholds vary considerably both during a single surgery in a single individual patient as well as between individuals and operations. The presence or absence of responses irrespective of stimulus intensity and response amplitude is much more clearly defined. The correlation of intraoperative MEP data to clinical findings preoperatively and postoperatively so far is best if a presence-absence paradigm is used. The most reliable correlation of postoperative motor deficits is with the disappearance of previously present MEPs, not with the deterioration of amplitudes or the elevation of thresholds. However, in intraoperative decision making an elevation of threshold, without signal loss may still be considered a practical warning sign as it may be a subclinical injury indicator, and may therefore induce a change in surgical strategy. This may be considered a minor warning criterion. A practical concept of the combined use of MEPs with D-wave recordings produced a neurophysiological pattern, which correlates with a reversible motor deficit: Disappearance of MEPs correlates with transient motor deficits if the D-wave amplitude is preserved above an approximate value of 50% of its baseline. Disappearance of the D-wave correlates to paraplegia.

Conclusions: To date, the best correlation of muscle MEP data to clinical deficits lies in the assessment of disappearance of a previously present MEP regardless of thresholds or amplitudes. Increase in stimulus thresholds for MEPs or to a lesser degree decrement of signal amplitudes may be considered subclinical injury indicators without correlation to neurological dysfunction and thus is considered a minor warning criterion.