Research

PURPOSE: EPID M-MEP may enhance SSEP spinal cord monitoring because of reported SSEP false-negatives. This MEP method requires more stringent anesthesia and placement of epidural electrodes. We compared MEP to SSEP with regard to ease of acquisition, reproducibility and sensitivity to incipient and complete spinal cord conduction failure.
METHOD: 228 deformity corrections were included. The anesthetic regimen included propofol/narcotic infusion. A >50% reduction in the monitored response, not explained by obvious technical failure or anesthetic change, indicated remedial increase BP or change in instrumented position and, if no improvement, an elective Stagnara wake-up test.
RESULTS: Monitorable potentials were acquired in 95.6% EPID M-MEP, 95.1% SSEP, 94.3% both and 2.6% neither. EPID M-MEP demonstrated a better signal/noise ratio and faster acquisition times than SSEP, thus loss of signal was always identified first by the MEP. Combined MEP and SSEP loss occurred in 7 cases: 3 were true positives (paraplegic on elective wake-up) with significant post-op improvement, 4 recovered MEP/SSEP during the course of an elective negative wake-up with concomitant increased BP. MEP loss with preserved SSEP in 2 other patients recovered by increase BP in one case and change of instrumented position in the other. No patient woke up with a new myelopathic deficit when monitored potentials were normal.
CONCLUSION: EPID M-MEP detects complete spinal cord conduction block (paraplegic on elective wake-up) before SSEP. Incipient spinal cord conduction failure (EP loss and recovered by increase BP, instrumentation change, or performance of a wake-up test) is recognized with greater sensitivity by MEP than SSEP.