Intraoperative Mapping and Monitoring of Motor Cortex—A New Paradigm

Abstract

Surgery for tumors near the sensorimotor cortex poses risks to sensory and motor pathways. Sensory pathways are monitored successfully with somatosensory-evoked potentials (SEP), but motor pathways pose different challenges. Historically, electrical stimulation with prolonged 60 Hz trains was used to map the motor cortex, but this method has a high incidence of induced seizures. Transcranial motor-evoked potentials (tcMEP) can monitor spinal motor pathways. However, transcranial stimulation is inappropriate for intracranial tumors, since the current may activate deep corticospinal fibers, bypassing a superficial injury. In contrast, brief high-frequency trains as used for tcMEP, applied directly to the cortical surface at lower intensity, avoid this problem and do not appear to induce seizures. Once the motor cortex has been mapped, an electrode can be left in place and used for continuous monitoring of corticospinal tract function during tumor resection.