Monitoring of cortical evoked potentials during surgical procedures on the cervical spine

We reviewed the results of intraoperative monitoring of short-latency cortical evoked potentials in 81 patients who underwent surgical procedures of the cervical spine. Of these patients, 43 had (1) baseline somatosensory evoked potentials from stimulation of the upper and lower extremities, (2) intraoperative monitoring for at least 3 hours, and (3) absence of an intrinsic lesion of the spinal cord. Major reductions in amplitude of the ulnar and tibial cortical evoked potentials occurred shortly after induction of anesthesia in 12% of patients. Latencies gradually increased during the operation as the concentration of volatile anesthetic agents increased. In patients with preoperative evidence of cervical cord damage, the cortical responses showed more fluctuations and could be lost without major changes in the concentration of the anesthetic agent or surgical manipulation. In one patient, intraoperative monitoring of the cervical cord function prevented a postoperative neurologic deficit. Ulnar and tibial cortical evoked potentials were successfully monitored throughout operation in all patients in whom baseline scalp responses could be obtained, but careful monitoring of anesthetic effect is necessary for proper interpretation.