Intraoperative Neurophysiologic Sensorimotor Mapping-A Review

Abstract

The main goal of neurological surgery for supratentorial lesions remains maximal resection with preservation of function of the nearby eloquent cortical and subcortical structures. Many authors [1-9] have emphasized the positive impact of aggresive removal of gliomas on the survival rate and the quality of life in both adults and children. Others [10-12] have found a positive correlation between incomplete resection of an epileptic focus and poor seizure control outcome in epilepsy surgery. However, maximal resection of supratentorial lesions is in many circumstances difficult to achieve due to the close proximity of functionally normal eloquent cortex. Despite advanced neuroimaging techniques and sensitive microscopes, visual inspection has oſten suboptimal resolution for distinguishing between normal and abnormal tissue, especially in cases of distorted anatomy and infiltrative pathology. More so, abnormal brain tissue, as appreciated by visual inspection, can retain normal function [13-15]. Thus, functional cortical mapping is essential for safe resection of lesions in the vicinity of eloquent cortex. This usually entails a multimodality approach, including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), diffusion tensor imaging (DTI) and neurophysiologic studies as well as co-registration techniques that optimally utilize all the available data [16,17]. This article offers an overview of the neurophysiologic sensorimotor mapping. Probably one of the most important advantage this method has over the neuroimaging techniques is allowing live assessment of the cortical function and direct intraoperative feedback to the surgeon. Unlike fMRI, neurophysiologic techniques also allow subcortical mapping [18] and continuous monitoring of the sensorimotor pathways during the actual resection; also, its results are much less affected by the perilesional hemodynamic changes. Last, neurophysiologic mapping offers increased localizing specificity, when compared to other techniques [19,20]. Sensorimotor neurophysiologic mapping consists of two parts: first, contralateral (to the craniotomy side) median somatosensory evoked potentials (SSEPs) phase reversal technique is employed with the attempt to localize the central sulcus (CS). Identification of the latter reveals to the surgeon and neurophysiologist the presumed location of the primary motor cortex. In the second part of the motor mapping, the surgeon stimulates the precentral regions, in order to identify the motor strip. Direct electrical stimulation is applied subdurally or epidurally and triggered muscle motor evoked responses (mMEPs) and/or evoked clinical responses are recorded and/or observed in the contralateral hemibody muscles in anesthetized or awake patients. Triggered responses at the lowest current amplitude will help delineate the primary motor cortex. Additionally, once the motor strip is identified, its continuous stimulation allows monitoring of the primary cortex and corticospinal tract throughout the resection. Similarly, recording of the thalamocortical SSEPs helps identification of the somatosensory cortex and monitoring of the large fiber sensory pathways during lesionectomy. Last, in awake patients, somatosensory cortex cortex can be further delineated by eliciting sensory symptoms by electrical stimulation of the postcentral region