Abstract 1 Erik J. Kobylarz, 2 Mark H. Bilsky, 1 Sonia K. Sandhu, 1 Edward A. Avila, and 1 Jonathan D. Victor ( 1 Department of Neurology and Neuroscience, Weill Medical College of Cornell University, New York, NY ; and 2 Department of Neurosurgery, Memorial Sloan Kettering Cancer Center, New York, NY ) Rationale: Transcranial electrical motor evoked potential (MEP) intraoperative monitoring can help reduce morbidity from spine surgery. However, it poses some risks, including intraoperative seizures. Transcranial electrical and magnetic stimulation at sufficiently high intensities have both been shown to produce epileptiform afterdischarges, as well as seizures. The purpose of this study is to determine the effects of transcranial electrical stimulation on the electroencephalogram (EEG), and the possible utility of the EEG during MEP monitoring for spine surgery. Methods: Twenty-seven patients with no history of seizures or anticonvulsant treatment and no known intracranial lesions underwent intraoperative MEP monitoring during spine tumor resection and instrumentation surgery. An electrode montage surrounding the MEP scalp stimulation site (C1, C2) was used for EEG monitoring (Cz-Fpz, C3-Fpz, C4-Fpz, Fz-Fpz, F3-Fpz, F4-Fpz, Pz-Fpz). Baseline, post-stimulus and inter-stimulus EEGs were recorded with the stimulation amplifiers temporarily disconnected to decrease electrical noise. MEP stimulation settings varied between patients: 250–600V, 4–6 pulse trains at 1.5 to 2 ms intervals. Electrical stimulation consistently caused a polyspike burst in all EEG channels lasting several seconds. The pure midline EEG derivations returned to baseline more rapidly after transcranial stimulation than those involving C3, C4, F3 and F4. For this study an afterdischarge was defined as a spike-and-aftercoming slow wave, not looking like artifact, that is separated in time from the polyspike burst that immediately follows the electrical stimulation. Results: Baseline EEGs revealed no epileptiform activity or electrographic seizures. In four of 27 patients, there were single or brief bursts of afterdischarges lasting up to several seconds, following transcranial MEP stimulation. In all cases, there was spontaneous return of the EEG to normal baseline activity. No patients demonstrated clinical or electrographic seizures prior to or after transcranial MEP stimulation during the intraoperative monitoring period. Conclusions: Concurrent electroencephalography during motor evoked potential stimulation is practical and occasionally reveals afterdischarges. This suggests that concurrent EEG monitoring may improve the safety of MEP monitoring.