S2-2. Intraoperative monitoring of motor evoked potential in aneurysm surgery: Transcranial or direct cortical stimulation?

Object: We examined usefulness of transcranial (TC) stimulation for motor evoked potential (MEP) in aneurysm surgery. Data of TC MEP were compared with those of direct cortical (DC) MEP which has been considered as gold standard for supratentorial lesions. Method: Clinical materials were consecutive 122 patients (ruptured 43, unruptured 79) who underwent aneurysmal clipping under both TC and DC MEP monitoring. TC MEP was recorded every 10 min and stimulation strength was set +20% to threshold. After dural opening, the strip electrode with 16 electrodes was inserted subdurally. Stimulation strength was set +2 mA to threshold. Results: TC MEP was recorded in all patients, while DC MEP could not be recorded in 15 patients mainly due to subdural resistance. Stimulation threshold of TC MEP was decreased after craniotomy and increased after CSF suction. In 14 patients both MEPs disappeared after temporary occlusion or aneurismal clipping. In 12 both MEP recovered after release of temporary occlusion or clipping. In remaining 2 patients MEP re-appeared but amplitudes were significantly lower than the control. Consequently these two patients developed transient and slight hemiparesis. Conclusion: TC MEP was recorded in all cases and revealed parallel changes with DC MEP, suggesting TC MEP alone is enough to monitor MEP.