Abstracts

RATIONALE: This study examines the current state-of-the-art of intraoperative motor evoked potential (MEP) monitoring, a relatively new and actively evolving field.
METHODS: A survey questionnaire filled out by centers performing intraoperative monitoring.
RESULTS: Surveys had been returned by 27 centers at the time of submission of this abstract. Of these, 24 indicated that they perform MEP monitoring, with an aggregate total of 1,456 cases per year.
No center used transcranial magnetic stimulation.
Fourteen centers elicited MEPs with transcranial electrical stimulation (TCES), all using brief pulse trains (3 - 7 pulses/train, interpulse interval 1.5 - 3.0 msec) and reporting that this is their preferred stimulation technique. Many centers excluded patients with cochlear implants, cardiac pacemakers, metallic implants, prior craniotomy or skull fracture, or a history of seizures from TCES. No center reported seizures or other adverse effects from TCES. Twelve centers recorded myogenic responses following TCES, either alone (10 centers) or in combination with spinal cord or peripheral nerve recordings; two centers recorded only spinal cord responses following TCES.
Eighteen centers utilized spinal cord stimulation with electrodes either inside or outside the spinal canal. Six of these recorded responses only from peripheral nerve and/or spinal cord (this technique will also pick up responses mediated by the dorsal columns); the other 12 monitored myogenic responses, either alone or in combination.
The reported percentage of cases in which MEPs were successfully monitored ranged from 10% to 100% but the distribution was highly skewed; only five centers reported less than 80% success rates. Monitoring failures were predominantly attributed to technical factors such as anesthesia, neuromuscular blockade, electrical artifacts, and misplaced or dislodged electrodes.
Almost all centers monitored both MEPs and somatosensory evoked potentials (SSEPs), and 18 centers answered questions about the frequency of intraoperative changes in these measures. SSEP changes without MEP changes occurred at 10 centers; MEP changes without SSEP changes occurred at 11 centers. The maximum frequency reported for each of these was 10% of the monitored cases.
CONCLUSIONS: Many different protocols are currently being used for MEP monitoring. Transcranial magnetic stimulation is no longer a technique of choice. Multipulse transcranial electrical stimulation appears to be a safe and effective technique.
Dysfunction limited to just the motor pathways or to just the somatosensory pathways of the spinal cord, while infrequent, clearly occurs. Therefore, both SSEPs and MEPs should be used for optimal intraoperative monitoring of the spinal cord.
Disclosure: Honoraria - Bio-Logic Systems Corporation.