Intraoperative Neurophysiologic Monitoring and Mapping of the Motor System During Surgery for Supratentorial Lesions under General Anesthesia

The resection of supratentorial tumors may be associated with functional morbidity, particularly when the lesions are located near blood vessels or near the eloquent cortices and tracts (e.g., the motor cortex). New postoperative functional deficits during tumor resections might be caused by different patterns of injury. During surgery involving the insula, deficits are frequently caused by ischemic insult rather than mechanical injuries of the fiber tracts. During surgery in the paracentral region and close to the posterior limb of the internal capsule, direct mechanical injury to the motor cortex (M1) and the corticospinal tract (CST) may be of major concern. Therefore, motor preservation requires both mapping of the M1 and the CST (cortical stimulation, somatosensory-evoked potential [SSEP] phase reversal, subcortical mapping techniques) and continuous monitoring by motor-evoked potential (MEP) recordings. Both techniques can be performed with the patient under general total intravenous anesthesia. Stable MEP recordings allow for safe completion of surgery, whereas MEP deterioration due to surgical causes should lead to early surgical intervention. Restoration of the MEP signals may prevent the occurrence of permanent new deficits. Subcortical mapping techniques may even allow estimating the distance to the CST, thus providing functional guidance during tumor resection. Other functions such as language, vision, somatosensory perception, and even cognitive functions may be mapped and monitored in awake procedures or by other neurophysiologic and imaging methods. The following chapter focuses on intraoperative neurophysiological methods to preserve the motor system during surgery of supratentorial surgery