Spinal cord injury (SCI) and nerve root injury are feared complications in spine surgery, and spinal surgeons have long sought ways to minimize the intrinsic dangers of operating near the neural elements. In order to avert complications and optimize outcome, intraoperative neuromonitoring (IONM) is used to provide a real-time assessment of neurologic function. With the growing popularity of minimally invasive surgery (MIS), the surgeon increasingly relies on intraoperative imaging and IONM because of the reduced visualization inherent to minimally invasive procedures.

Open and MIS surgery employ similar principles of neuromonitoring. The four most commonly employed techniques during spinal procedures are somatosensory evoked potentials (SSEP), transcranial motor evoked potentials (tcMEPs), spontaneous electromyographs (sEMG), and triggered EMG (tEMG).¹ When IONM is utilized, baseline measurements serve as the patient’s own control for any deviations that may arise. Detection of a significant change in waveform patterns compared to the preoperative standard constitutes a potential neurologic injury. This chapter provides an overview of each of the above-mentioned techniques.

Monitoring of SSEPs evaluates the conduction of the dorsal columns of the spinal cord, and thus provides sensory information. SSEPs are recordings of summated signals that enter the spinal cord through numerous segments and then undergo averaging and amplification, and the intraoperative amplitude and latency are compared to baseline recordings. An amplitude decrease of more than 50% and/or a latency prolongation or greater than 10% compared to the baseline values constitutes a significant change and warrants evaluation for potential neurologic injury.², ³, ⁴, ⁵, ⁶, ⁷, ⁸ A change in SSEPs has been reported to have a sensitivity up to 92% for cord dysfunction.⁹ However, it is not an effective method for monitoring individual nerve roots, since the summation and amplification of the SSEPs may disguise signals that arise from an individual nerve root injury. Additionally, SSEP monitoring may require 3 to 5 minutes or more to detect a neurologic change.¹⁰, ¹¹, ¹², ¹³

Transcranial motor evoked potentials monitor the corticospinal tracts, which involves electrically stimulating the cortical motor areas in the brain by electrodes applied to the overlying scalp region. Because continuous transcranial stimulation to elicit and record distal compound muscle action potentials (CMAPs) is not practical, tcMEPs provide interval snapshot data of the motor function of the spinal cord.⁹,¹³,¹⁴ This is in contrast to SSEPs that are continuous recordings. Also, tcMEPs have the advantage of providing direct live feedback because no signal averaging is involved. Motor evoked potentials are recorded distally as myogenic motor responses in the form of CMAPs from electrodes placed over key peripheral muscles. A decline in CMAP amplitude of greater than 75% compared to baseline reference values is indicative of potential motor tract injury; this change has been reported to have sensitivity of up to 99% in complex thoracolumbar cases.⁶,¹⁵,¹⁶