Brain surgery in motor areas: the invaluable assistance of intraoperative neurophysiological monitoring

AIM: Surgery for tumors in the central and precentral region, as much as for insular tumors, places at risk the functional integrity of the motor cortex and the subcortical motor pathways. These procedures may therefore benefit from the assistance of intraoperative neurophysiological monitoring (INM). INM consists of "mapping" and true "monitoring" (the continuous "on-line" assessment of the functional integrity of neural pathways) techniques. In spite of the large interest in mapping techniques, monitoring techniques have received less attention. We describe our experience with intraoperative neurophysiological mapping and monitoring of motor tracts during surgery for brain gliomas in or near motor areas, in order to support the feasibility and reliability of monitoring as an essential adjunct to mapping during surgery in these areas. METHODS: Between September 2000 and January 2002, 51 patients were surgically treated for brain gliomas located in the precentral gyrus (45.1\%), the postcentral gyrus (23.5\%), anterior to the precentral gyrus (15.6\%), or in the insula (15.6\%). INM of the motor system consisted of monitoring muscle motor evoked potentials (mMEPs) recorded via needle electrodes inserted into the controlateral upper and lower extremity muscles and elicited by transcranial multipulse electrical stimulation (TES). Once the dura was open and the central sulcus was identified using the phase reversal technique, mMEPs were elicited by direct stimulation of the motor cortex (DCS). Motor mapping was performed with a monopolar electrode using the same stimulation parameters as used for monitoring except for much lower intensity (up to 20 mA). RESULTS: Ninety-eight percent of the patients exhibited recordable baseline mMEPs. The success rate of the phase reversal technique was 95.8\%. Eight patients presented disappearance of mMEPs during tumor removal. Using corrective measures, all intraoperative changes in mMEPs were reversed in time to prevent an irreversible complete injury to the motor system and no patient lost mMEPs at the end of the operation. At discharge, 66\% of the patients remained at their preoperative status, 4\% improved, and 24\% had a mild worsening as compared to the preoperative status assessed using the Medical Research Council scale; 6\% of the patients presented a moderate to severe supplementary motor area syndrome. CONCLUSION: Monitoring techniques significantly implement the reliability and effectiveness of INM since these provide: 1) continuous "on-line" assessment of the functional integrity of motor pathways with higher chance to early detect a progressive mechanical or vascular injury to the neural tissue, as compared to mapping techniques; 2) lower risk to induce intraoperative seizures and strong muscular twitches as compared to the single pulse mapping technique; 3) possibility to monitor motor pathways using TES also when there is no direct access to the motor cortex