A prospective comparison of neurogenic mixed evoked potential stimulation methods: utility of epidural elicitation during posterior spinal surgery.

ABSTRACT

STUDY DESIGN: A prospective study of 50 patients who underwent posterior thoracic or thoracolumbar instrumented spinal surgery from June 1998 through June 1999. OBJECTIVES: To highlight the advantages and disadvantages of neurogenic mixed evoked potential (NMEP) data obtained with three methods of stimulation epidural, spinous process, and percutaneous. SUMMARY OF BACKGROUND DATA Reports in the literature have established the efficacy of epidural, spinous process, and percutaneous stimulation of the NMEP response. The three methods have not been compared for reliability, sensitivity, and specificity. METHODS: The study group consisted of 50 patients who underwent posterior thoracic or thoracolumbar instrumented spinal surgery from June 1998 through June 1999. Somatosensory-evoked potentials were used to monitor upper and lower extremities. An attempt to obtain NMEPs was made in all patients by using percutaneous (PERC-NMEP), spinous process (SP-NMEP), and epidural (EPI-NMEP) stimulation. These data were evaluated for reliability, sensitivity, and specificity. The number of minutes monitored in the postcorrection period were calculated for each method, and stimulus intensities were noted. RESULTS: In the current study, PERC-NMEPs were obtained in 88% of the patients and were maintained in 91% of those cases, SP-NMEPs were obtained in 96% and maintained in 77%, and EPI-NMEPs were obtained in 100% and maintained in 88%. Data collection continued in the postcorrection period for 46 minutes for PERC-NMEPs, 19 minutes for SP-NMEPs, and 23 minutes for EPI-NMEPs. The study group had no true-positive or false-negative findings. CONCLUSIONS: Results showed that EPI-NMEPs provide reliable data in a greater number of patients than either SP-NMEPs or PERC-NMEPs. However, PERC-NMEP data are readily maintained during and after the critical time window after corrective spinal maneuvers. The NMEPs elicited with both percutaneous and epidural stimulation have a useful role in an intraoperative spinal cord-monitoring protocol.