Avoidance of Cerebral Infarction in the Territory of Perforating Arteries Using Intraoperative Motor Evoked Potential in Aneurysm Surgery: Current Status and Limitation

Object: Intraoperative monitoring of motor evoked potential (MEP) enables us to detect blood flow insufficiency (BFI) in the anterior choroidal artery (AChA) and the lenticulostriate artery (LSA). We examined the current status and limitation of MEP monitoring to avoid cerebral infarction in the territory of perforating arteries in aneurysm surgery. Methods: We studied 150 consecutive patients with ruptured (n＝53) or unruptured (n＝97) aneurysms who underwent clipping between September 2009 and August 2012. All patients were fully anesthetized and both transcranial stimulation MEP (TC-MEP) and direct cortical stimulation MEP (DC-MEP) were recorded. The stimulation threshold of TC-MEP was checked at 10-min intervals. The strength of TC stimulation was changed and set at ＋20% of the stimulation threshold because it changed after craniotomy and cerebrospinal fluid (CSF) aspiration. After dural opening, we subdurally inserted a strip electrode with 16 electrodes. The DC stimulation strength was set at ＋2 mA of the stimulation threshold. Electromyograms of the contralateral thenar muscle were obtained. After temporary occlusion of the parent artery or clipping of the aneurysm, both TC- and DC-MEPs were obtained at 1-min intervals until release of temporary occlusion or 20 min after clipping. Results: While TC-MEP was recorded in all patients, DC-MEP could not be recorded in 18 patients (12%). Although the strip electrode was placed subdurally, in nine patients no recordings could be made at maximal 25 mA stimulation (unknown reasons, n＝7; preexisting hemiparesis (MMT 3/5), n＝2). In another eight patients, we encountered subdural resistance, and one patient presented with a chronic subdural hematoma. The stimulation threshold of TC-MEP was significantly decreased after craniotomy and increased after CSF aspiration. In 143 patients, TC-MEP did not change during surgery; three patients developed transient hemiparesis (MMT 4/5) due to infarction of the genu of the internal capsule. In the other 17 patients, TC-MEP disappeared. In 15 of these, it disappeared after temporary occlusion of the parent artery or aneurysmal clipping but reappeared after release of the temporary occlusion or re-clipping; one patient whose MEP amplitude recovered to 50% of the control developed transient hemiparesis (4/5). Another two patients whose MEP disappeared until the end of surgery developed permanent hemiparesis (4/5). Conclusions: TC-MEP, which was recorded in all 150 patients, cha