The Application of Transcranial Electric Motor-Evoked Potential Monitoring as an Early Indicator of Emerging Thoracic Spinal Cord Ischemic Injury**

Introduction: Somatosensory evoked potential (SSEP) monitoring has enjoyed widespread application for assessing global spinal cord function during corrective thoracic spine surgery for scoliosis and trauma. Because the SSEP is dorsal column mediated, however, it can be insufficiently sensitive to identifying inadequate perfusion or impending ischemia to the anterior spinal cord. Transcranial electric motor evoked potentials (TceMEPs), on the other hand, permit real-time monitoring of the cortico-spinal tracts and as such, provide instantaneous assessment of changes in spinal cord function secondary either to surgical insult or inadequate spinal cord blood flow from hypotension. The purpose of this study was to: 1) determine the sensitivity of TceMEP monitoring as an early indicator of impending spinal cord injury during corrective thoracic spine surgery, 2) explore the correlation between changes in TcMEPs, SSEPs, and subsequent neurologic sequelae, and 3) identify factors associated with the loss of TcMEPs suggestive of an impending spinal cord ischemic injury.

Methods: A retrospective chart review was done for all adult patients who underwent thoracic or thoraco-lumbar spine surgery over a two-year period between January 2000 and December 2001 at a single institution by three orthopaedic spine surgeons. Perioperative neurophysiological monitoring included recording of posterior tibial nerve cortical and subcortical SSEPs and TceMEPs recorded from hand, leg and foot muscles by a single group of surgical neurophysiologists. Monitoring commenced following intubation, but prior to positioning and continued throughout the entire operative procedure. All patients who experienced an intraoperative unilateral or bilateral loss of TcMEPs or SSEPs for at least ten minutes were identified. The time course for monitoring changes and the temporal relationship of any intraoperative events were recorded. Hospital and office charts were also reviewed to obtain baseline diagnosis, preoperative, immediate postoperative and follow-up neurologic data.

Results: Of 125 cases reviewed, significant intraoperative TceMEP amplitude decreases which prompted surgical alert were noted in seven (5.6%). A concomitant significant (>75%) SSEP amplitude loss was noted in only one of these patients and lagged behind the loss of TceMEPs by 20 minutes. Of the remaining six patients, TceMEP amplitude diminished by >75% during placement of corrective instrumentation in four, whereas that for the two patients was secondary to acute hypotension (MAP< 50 mm Hg). SSEP amplitudes never changed across all six of these patients. When TceMEP amplitude was significantly depressed, intraoperative intervention included reversal of corrective forces or complete instrumentation removal, raising mean arterial blood pressure to at least 90 torr and administering high-dose methylprednisolone (NASCIS-2 protocol). Improvement in spinal cord motor function occurred in seven of the eight patients following surgical and/or medical intervention. None of these presented with post-operative neurologic deficit. The one patient who lost transcranial motor evoked potentials after placement of thoracic pedicle screws, followed 20 minutes later by complete SSEP loss, never regained neurphysiological function and awoke with dense paraplegia as predicted. One additional patient showed a SSEP loss alone; that is, no TceMEP loss, after placement of a sublaminar hook, thus representing a pure dorsal column insult.

Conclusions: These data demonstrate that transcranial electric motor evoked potentials are a much more sensitive and earlier indicator of impending spinal cord ischemic injury then somatosensory evoked potentials.

** The FDA has not cleared a drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed in an “off-label” use).