Somatosensory-Evoked Potentials

Early attempts at monitoring relied solely on recording SSEPs. Reports of false-negative outcomes when using only SSEP monitoring illustrated the need for multimodality monitoring.^6–8 SSEPs remain the standard technique for intraoperative monitoring. Newer techniques such as EMG and MEP have been developed and are used as an adjunct to SSEP monitoring.

Motor-Evoked Potentials

Because of the well-reported limitations of SSEP monitoring, the recording of MEPs has been advocated. Myogenic transcranial MEP offers the advantage of assessing the entire length of the corticospinal tract from the cortex to the distal extremity beyond the neuromuscular junction. The most significant issue when recording MEPs is the anesthetic regimen: total intravenous anesthetic without neuromuscular blockade needs to be utilized to provide the optimal environment for recording MEPs. This requires experience on the part of the surgical and anesthetic teams. For example, surgical exposure may be somewhat more challenging and the depth of anesthesia may require closer vigilance.

Electromyography

Spontaneous electromyographic recordings provide real-time data that are sensitive to surgical manipulation or compression.² Myotomes are selected for recording based on the operative level and the nerve roots most at risk. Burst or train activity is considered significant and is thought to represent ongoing compression or stretch. Spontaneous EMG is sensitive but not specific.⁹ As well as recording spontaneous electromyographic activity, a number of other applications have used electromyographic recordings to determine the proximity of nerve roots. Direct nerve root testing with EMG recording aids in the dissection of nerve roots off intradural tumors especially in the conus region. Triggered EMG has been used to aid in placement of pedicle screws.

Evidence for Somatosensory-Evoked Potentials

Most of the earlier literature predominantly evaluated the role of SSEPs alone in spinal surgery. As far back as 1982, Grundy and colleagues¹¹ reported a study showing that a wake-up test was not necessary provided SSEPs were monitored and stable. The authors prospectively studied the effects of moderate hypotension on 24 patients undergoing spinal fusion with Harrington rod instrumentation. Five of the 24 patients had alterations in their SSEPs and required an intraoperative wake-up test, all of which had normal results.

Epstein and coworkers¹² evaluated the role of SSEPs in cervical surgery by comparing the outcomes of patients who were monitored and operated on over a 3-year period (1989–1991) with those who were not monitored and had been operated on between 1985 and 1989. No instances of quadriplegia in the 100 patients who were monitored versus eight in the 218 who were not monitored were reported. The authors conclude that the reduction of neurologic deficit was attributed in part to early SSEP detection of vascular or mechanical compromise and to the immediate alteration of anesthetic or surgical technique in response to SSEP changes. Kombos and coworkers¹³ report similar findings in a prospective evaluation of the impact of SSEP monitoring during anterior cervical surgery. In a prospective study of 100 patients, they deduce that SSEP monitoring was easy to perform and helped to increase the safety during anterior cervical surgery. Monitoring of both cortical and subcortical sites for SSEP responses has been shown to increase the reliability of SSEPs during spinal surgery.¹⁴

Evidence for Motor-Evoked Potentials

Concerns over false-negative results when using SSEPs alone have led to the proposal of alternative strategies, with either monitoring of MEPs alone or used in combination with other modalities.⁶ Hilibrand and investigators¹⁵ analyzed the data of 427 patients who underwent anterior or posterior cervical spine surgery over a 2-year period. Twelve of their patients had loss of amplitude of MEPs, of which 10 had complete reversal of the loss after prompt intraoperative intervention and the remaining 2 had a new postoperative deficit. The sensitivity and specificity for MEPs was 100% in their series, whereas SSEP had only a sensitivity of 25%, although it was 100% specific. The authors conclude that transcranial MEPs appeared to be superior to conventional SSEPs for identifying evolving motor tract injury during cervical spine surgery.

Difficulties in obtaining and maintaining MEP responses with transcranial stimulation has led some authors to propose direct spinal cord stimulation to obtain neurogenic MEPs in certain pathologies.¹⁶ Komanetsky and colleagues¹⁷ compared two methods of stimulation when obtaining neurogenic MEPs. They prospectively compared spinous process stimulation with percutaneous stimulation in obtaining neurogenic MEPs in 184 patients. Both methods were found to be sensitive to neurologic deficit. When responses were obtained, the percutaneous method was found to be sufficiently reliable to obviate the need for the spinous process method.

Numerous studies have addressed the difficulties in obtaining reliable predictors for postoperative C5 palsy.¹⁸ Tanaka and colleagues¹⁸ evaluated the usefulness of transcranial MEPs for prediction of the occurrence of postoperative C5 palsy after cervical laminoplasty. They prospectively evaluated 62 consecutive patients, three of which developed postoperative transient C5 palsy. No critical decrease in amplitude occurred in any of the 62 patients. Because of this, the authors conclude that postoperative C5 palsy after cervical laminoplasty was not associated with an intraoperative injury (Table 7-1).

Evidence for Electromyography

There are fewer Level I or II evidence articles in the literature that validate the use of intraoperative EMG monitoring in spinal surgery. Dimopoulos and investigators¹⁹ conducted a prospective randomized trial to correlate the findings of intraoperative EMG with immediate postoperative pain in patients undergoing lumbar microdiscectomy (Level II). They found no correlation between intraoperative electromyographic findings and postoperative pain.

Krassioukov and coauthors²⁰ examined the neurologic outcomes of 61 patients, most of whom were treated for spinal/spinal cord tumours (61%) or adult tethered cord syndrome (25%). Patients underwent multimodal neurophysiologic monitoring with EMG monitoring of the lower-limb muscles, external anal sphincter (EAS), external urethral sphincter (EUS), and lower-limb SSEPs. Spontaneous electromyographic activity was observed in the lower-extremity muscles and/or EAS and EUS in 51 cases (84%). In addition to spontaneously recorded electromyographic activity, electrically evoked EMG activity was also used as an intraoperative adjunct. The presence of electrically evoked EMG activity in structures encountered duringmicrodissection altered the plan of treatment in 24 cases (42%).

Similar findings were reported by Paradiso and colleagues,¹ who examined the use of intraoperative monitoring in tethered cord syndrome. Posterior tibial nerve SSEPs were found to have high specificity, but low sensitivity, for predicting new neurologic deficits. In contrast, continuous EMG showed high sensitivity and low specificity. Evoked EMG accurately identified functional neural tissue.

Multimodality Monitoring

To overcome the limitations of individual monitoring modalities, many teams have explored the value of multimodal monitoring to obtain a more robust real-time assessment of intraoperative neural function. Pelosi and researchers²¹ investigated the combined monitoring of MEPs and SSEPs in 126 spinal operations. Combined monitoring was successfully achieved in 104 operations; it was possible only to monitor a single modality in 18 patients (16 SSEPs, 2 MEPs). No response to either modality could be recorded in two patients. The authors report that combined monitoring was superior to single-modality techniques both for increasing the number of patients in whom satisfactory monitoring could be achieved and for improving the sensitivity and predictivity of monitoring.

Gunnarsson and coauthors⁹