4. Usefulness of new protocol for stable monitoring of transcranial MEP in aneurysm surgery

Abstract

We examined the usefulness of a new protocol for stable monitoring of transcranial (TC) motor evoked potential (MEP) in aneurysm surgery. Data of TC stimulation MEP were compared with those of direct cortical (DC) stimulation MEP which had been considered the gold standard in aneurysm surgery. The study included 177 patients (ruptured 66, unruptured 111) who underwent aneurysmal clipping between September 2009 and April 2013. In all patients both TC and DC stimulations were used for MEP monitoring. After introduction of anaesthesia, TC MEP was recorded every 10 min. Stimulation strength of TC MEP was set + 20% of stimulation threshold, since stimulation thresholds fluctuated due to craniotomy and aspiration of cerebrospinal fluid. After dural opening, a strip electrode with 16 contacts was inserted subdurally. Stimulation strength of DC MEP was set + 2 mA of stimulation threshold. Thereafter both stimulations were used to record MEPs every ten minutes. In both stimulations, electromyograms of contralateral thenar muscle were recorded. TC MEP was recorded in all patients, while DC MEP could not be recorded in 18 patients (10%), mainly due to subdural resistance. The threshold of TC MEP was decreased after craniotomy and increased after CSF suction significantly. In 22 patients both MEPs disappeared after temporary occlusion or aneurysmal clipping. In 20 of these patients both MEPs recovered after release of temporary occlusion or clipping. In the remaining 2 patients MEPs did not recover. Consequently these two patients developed permanent hemiparesis (MMT 4/5). TC MEP was recorded in all cases and paralleled changes with DC MEP, suggesting TC MEP alone is enough to monitor MEP. Our new protocol of TC MEP is useful for stable monitoring in aneurysm surgery.