Resection of intramedullary spinal cord tumors carries a high risk for surgical damage to the motor pathways. This surgery is therefore optimal for testing the performance of intraoperative motor evoked potential (MEP) monitoring. This report attempts to provide evidence for the accurate representation of patients' pre- and postoperative motor status by combined epidural and muscle MEP monitoring during intramedullary surgery.
The authors used transcranial electrical motor cortex stimulation to elicit MEPs, which were recorded from the spinal cord (with an epidural electrode) and from limb target muscles (thenar, anterior tibial) with needle electrodes. The amplitude of the epidural MEPs and the presence or absence of muscle MEPs were the parameters for MEP interpretation. A retrospective analysis was performed on data from the resection of 100 consecutive intramedullary tumors and MEP data were compared with the pre- and postoperative motor status.
Intraoperative monitoring was feasible in all patients without severe preoperative motor deficits. Preoperatively paraplegic patients had no recordable MEPs. The sensitivity of muscle MEPs to detect postoperative motor deficits was 100% and its specificity was 91%. There was no instance in which a patient with stable MEPs developed a motor deficit postoperatively. Intraoperative MEPs adequately represented the motor status of patients undergoing surgery for intramedullary tumors. Because deterioration of the motor status was transient in all cases, it can be considered that impairment of the functional integrity of the motor pathways was detected before permanent deficits occurred.