Patients

The present study was approved by the Institutional Review Board (IRB) of Seoul National University Hospital (IRB No. 1408-128-607). We screened 3,148 limbs from 787 patients between January 2008 and December 2013. Surgery was performed by three experienced neurosurgeons. The following inclusion criteria for patients were applied: having undergone cervical spinal surgery with intraoperative MEP monitoring, and received total intravenous anesthesia with propofol and remifentanil. The exclusion criteria were as follows: patients who received inhaled anesthesia during surgery (8 limbs from 2 patients), MEP loss related to position changes (8 limbs from 2 patients), or not being able to monitor MEPs from any muscle (27 upper and 228 lower extremities). We analyzed each limb of the enrolled patients independently. The final analytical samples comprised 116 limbs from 30 patients with IM tumors (IM group) and 2,761 limbs from 693 patients with NIM tumors (NIM group).