Transcranial electrical motor evoked potential monitoring for brain tumor resection

OBJECTIVE: This study was designed to examine whether transcranial electrical motor evoked potential (MEP) monitoring is safe, feasible, and valuable for brain tumor surgery. METHODS: Fifty consecutive patients undergoing brain tumor resection were studied, using nitrous oxide/propofol anesthesia. MEPs were continuously recorded throughout surgery, using a Sentinel 4 evoked potential system (Axon Systems, Inc., Hauppauge, NY). The MEPs were elicited by transcranial electrical stimulation (train of 5; stimulation rate, 0.5-2 Hz; square wave pulse with a time constant of 0.5 ms; stimulation intensity, 40-160 mA) through spiral electrodes placed over the primary motor cortex and were recorded by needle electrodes inserted into the contralateral orbicularis oris, biceps, abductor pollicis brevis, and anterior tibialis muscles. When MEP amplitudes decreased by more than 50%, MEP stimulation was repeated, with increased stimulation intensity, and MEP changes were reported to the surgeon. The motor function of each patient was examined before and after surgery, using a reproducible scale. The relationship between MEP amplitude decreases and worsening motor status was analyzed using linear regression. RESULTS: Preoperative neurological examinations revealed mild to moderate motor deficits (2/5 to 4/5) for 38% of patients (19 of 50 patients). Most of the patients (96%) exhibited recordable baseline MEPs. Persistent MEP decreases of more than 50% were noted for eight patients (16%) (11 muscles). The MEPs were completely abolished in two patients (three muscles). The degree of postoperative worsening of motor status was correlated with the degree of intraoperative MEP amplitude reduction (r = -0.864; P < 0.001). CONCLUSION: Persistent intraoperative MEP reductions of more than 50% were associated with postoperative motor deficits. The degree of MEP amplitude reduction was correlated with postoperative worsening of motor status. Transcranial electrical MEP monitoring is feasible, safe, and valuable for brain tumor surgery.