A review of intraoperative monitoring for spinal surgery

Abstract

Background: Intraoperative neurophysiologic monitoring (IONM) is a technique that is helpful for assessing the nervous system during spine surgery.

Methods: This is a review of the field describing the basic mechanisms behind the techniques of IONM. These include the most often utilized trancranial motor evoked potentials (Tc-MEPs), somatosensory evoked potentials (SSEPs), and stimulated and spontaneous EMG activity. It also describes some of the issues regarding practices and qualifications of practitioners.

Results: Although the anatomic pathways responsible for the Tc-MEP and SSEP are well known and these clinical techniques have a high sensitivity and specificity, there is little published data showing that monitoring actually leads to improved patient outcomes. It is evident that IONM has high utility when the risk of injury is high, but may be only marginally helpful when the risk of injury is very low. The monitoring team must be well trained, be able to provide the surgeon feedback in real time, and coordinate activities with those of the surgical and anesthesia teams.

Conclusions: Although IONM is a valuable technique that provides sensitive and specific indications of neurologic injury, it does have limitations that must be understood. Maintaining a high quality of practice with appropriately trained personnel is critical.

Keywords: Intraoperative neurophysiologic monitoring; motor evoked potentials; somatosensory evoked potentials; spine.

Figure 1

Illustration of the responses to a single-pulse transcranial electrical stimulation in the (a) awake and (b) anesthetized patients. (c) Shows the effect of multipulse stimulation in the anesthetized patient

Figure 2

Illustrations of the anatomy underlying the upper (a) and lower (b) somatosensory evoked potentials

Figure 3

Illustration of the physiology of the (a) F-wave and (b) H-reflex

Figure 4

Illustration of the effect that the a priori probability P(I) has on the probability that a warning is associated with true injury when a warning is made with every injury (100% sensitive) and varying levels of false-positive warnings P(W/NI). W = warning, I = injury, NI = no injury, P(W/NI) is the conditional probability of a warning when there is no injury