Intraoperative subcortical motor evoked potential stimulation: how close is the corticospinal tract?

Abstract

T he aim of surgery for primary and secondary intrinsic brain tumors is complete tumor resection to improve prognosis and control symptoms, and thereby improve the patient's quality of life. 24,39,45,47 However, the goal of maximum resection should be met without any new surgery-related neurological deficits arising, 13 and all efforts should be made to reduce patients' risk for neurological damage. Therefore, lesions in or near motor-eloquent regions, such as the precentral gyrus or the corticospinal tract (CST), represent particularly difficult situations in neurosurgery, and precautions need to be taken to prevent postoperative damage while maximizing resection.Intraoperative electrophysiological mapping and monitoring of the motor cortex and subcortical pyramidal tract obJect Subcortical stimulation is a method used to evaluate the distance from the stimulation site to the corticospinal tract (CST) and to decide whether the resection of an adjacent lesion should be terminated to prevent damage to the CST. However, the correlation between stimulation intensity and distance to the CST has not yet been clearly assessed. The objective of this study was to investigate the appropriate correlation between the subcortical stimulation pattern and the distance to the CST. methods Monopolar subcortical motor evoked potential (MEP) mapping was performed in addition to continuous MEP monitoring in 37 consecutive patients with lesions located in motor-eloquent locations. The proximity of the resection cavity to the CST was identified by subcortical MEP mapping. At the end of resection, the point at which an MEP response was still measurable with minimal subcortical MEP intensity was marked with a titanium clip. At this location, different stimulation paradigms were executed with cathodal or anodal stimulation at 0.3-, 0.5-, and 0.7-msec pulse durations. Postoperatively, the distance between the CST as defined by postoperative diffusion tensor imaging fiber tracking and the titanium clip was measured. The correlation between this distance and the subcortical MEP electrical charge was calculated. results Subcortical MEP mapping was successful in all patients. There were no new permanent motor deficits. Transient new postoperative motor deficits were observed in 14% (5/36) of cases. Gross-total resection was achieved in 75% (27/36) and subtotal resection (> 80% of tumor mass) in 25% (9/36) of cases. Stimulation intensity with various pulse durations as well as current intensity was plotted against the measured distance between the CST and the titanium clip on postoperative MRI using diffusion-weighted imaging fiberitracking tractography. Correlational and regression analyses showed a nonlinear correlation between stimulation intensity and the distance to the CST. Cathodal stimulation appeared better suited for subcortical stimulation. coNclusioNs Subcortical MEP mapping is an excellent intraoperative method to determine the distance to the CST during resection of motor-eloquent lesions and is highly capable of further reduc...

Continuous subcortical motor evoked potential stimulation using the tip of an ultrasonic aspirator for the resection of motor eloquent lesions

“…7,12,13,19,21 Until recently, the spatial and temporal stimulation capabilities of scMEPs were limited since the resection had to be stopped to apply stimulation with a handheld probe in the resection cavity, thereby estimating the location and distance to the CST. 6,15,16,17,18,21,22 This interruption of surgical flow is not only inconvenient and time consuming but may also increase the risk of mechanical injury to the CST. 18,21 The increased risk is attributable to the fact that at presumably safe distances from the CST, resection is mostly guided by cortical MEPs.…”

“…11,22 During tumor resection, MEPs were elicited every 1-15 seconds depending on the location of resection. Subcortical stimulation with the trainof-five method was delivered using either a concentric monopolar handheld probe (Inomed Medizintechnik), as previously described, 22 or the tip of the ultrasonic surgical aspirator.…”

“…3,8,9,14,18,21,22 Subcortical MEP (scMEP) stimulation is an additional intraoperative neurophysiological monitoring (IOM) tool, which is very helpful in assessing the proximity of the stimulation point to the CST. Thus, it supports the surgeon in determining where the resection should be stopped to avoid injury to the CST.…”

“…Thus, it supports the surgeon in determining where the resection should be stopped to avoid injury to the CST. 3,4,7,12,13,15,18,19,21,22,26 However, the use of scMEP stimulation has always involved an interruption in the resection, a change to a handheld stimulation probe for mapping the resection borders. Therefore, a balance between safety (frequent halting of the resection and changing the instruments for mapping) and a continuous resective workflow (infrequent changing of the instruments for mapping) was necessary.…”

Neuro control – using neuro monitoring data to control networked active instruments

“…A nerve monitor for instance is capable of monitoring the activity of specific nerves intraoperatively. With the use of a stimulation probe it enables the localization of a nerve and to a certain degree also the identification of the distance between nerve and probe [2], [3]. Nerve identification is only available as long as the stimulation with the probe is ongoing, so that the surgeon has to remember the nerve's position.…”