gms | German Medical Science

Objective: In glioma surgery, preservation of fibers originating in the precentral gyrus is still challenging. Preoperative DTI based tractography allows an approximation of the course of fibers but is intraoperatively limited due to brain shift. Therefore the intraoperative cortical and subcortical elicitation of motor evoked potentials (MEP) is commonly used to localize and continuously evaluate in real time the function and integrity of these fibers. The disappearance of MEP or the increase of the motor threshold is a strong predictor for postoperative morbidity. The correlation between stimulating current and distance from the stimulation site to the stimulated fibers is evaluated.

Methods: In 11 patients with deep white matter lesions adjacent to the corticospinal tract (CTS), fiber tracking of the CTS was done before and after operation.In order to do thi, fMRI of the motor cortex and diffusion tensor imaging (12 diff. directions) were acquired employing a 3 Tesla MR-scanner. Fiber tracking was performed according to the anatomical and functional data sets with separate segmentation of the CTS relating to the foot-, hand- and tongue-representations. The resulting bundle was confirmed by anatomical landmarks and segmented three-dimensionally (iPlan 2.5Cranial, BrainLab^®).

During and at the end of tumor resection monopolar subcortical stimulation (5 pulses, cathodal, 0.5 ms monophasic pulsewidth, ISI=4ms) in the resection cavity was performed alongside of the course of fibers, starting with 20mA and decreasing to the lowest current with a MEP response. After postoperative tractography and fusion with anatomical imaging, the shortest distance between stimulation site and the CTS bundles were measured by three-dimensional measurement and correlated with the corresponding current values (e.g. MEP of the M. tibialis ant. with CTS from the foot representation).

Results: MEPs elicited by subcortical stimulation were achievable in all patients, ranging from 3 mA to 14 mA. 21 stimulation sites could be correlated with distances varying from 0.1 mm to 22.2 mm. The resulting mean value is 1.09 mm/mA (Max: 2.77mm/mA; Min: 0.03mm/mA; SD: 0.85).

Conclusions: Subcortical elicited MEPs reliably localize the CST. A correlation between stimulating current and distance from the tumor cavity to the CTS points toward a value of 1.09 mm/mA, but has to be considered with precaution because of the small number of patients included, the widespread single results and the limitations due to fiber tracking.