Introduction: Subcortical stimulation is a method to evaluate the distance from the stimulation site to the motor tract (CST) and to decide whether a resection should be terminated. It is believed that each 1mA of stimulation intensity needed to elicit a MEP response corresponds to 1 mm distance from the CST. However, this assumption has not been properly substantiated
Methods: Monopolar subcortical stimulation was performed in addition to continuous MEP monitoring in 19 consecutive patients with a motor-eloquent lesion. The functional boundaries of the resection were identified with the help of subcortical stimulation. At the end of the resection, the point at which a MEP response was still elicitable with minimal stimulation intensity was marked with a titanium clip. The intensities at this point for different stimulation variants were documented using i) cathodal or anodal stimulation, ii) 0.3, 0.5 and 0.7 ms pulse width. The distance between the CST (based on the postoperative DTI data) and the titanium clip was measured.
Results: Subcortical stimulation was successful in all patients. There were no new permanent postoperative deficits. The anodal stimulation intensity was 3 - 20 mA and the stimulation charge 0.9 - 11.9 (mA X ms). The cathodal stimulation intensity was 3 - 17 mA and the stimulation charge 0.9 - 9.8 (mA X ms). The measured distance between the CST and titanium clip was 2.6 - 21.9 mm. A correlation analysis between stimulation charge and distance between the titanium clip and the CST revealed a weak correlation, linear (R2 = 0.40) and algorithmic (R2 = 0.53). However, a linear correlation between the stimulation intensity at 0.3 ms pulse duration and the distance between the titanium clip and the CST (mm = 1.42 + 0.81 x mA, R2=0.72, p <0.001) was revealed.
Conclusions: The subcortical stimulation is an excellent intraoperative method to determine the distance to the CST during resection of motor eloquent lesions. This should minimize the risk of injuring the CST. With a pulse width of 0.3 ms there is a linear correlation between stimulation intensity and distance to the CST.
Patient Care: In minimizing postoperative new neurological deficits.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of subcortical stimulation , 2) Discuss, in small groups how this methodology could change there surgical approach to motor eloquent lesions