gms | German Medical Science

Objective: Navigated transcranial magnetic stimulation (TMS) is a widely used method to preoperatively assess hand knob location in patients with brain tumors related to the precentral gyrus. TMS induces an electric field in the brain that varies depending on coil position and orientation. The distance between hand areas determined by TMS and intraoperative direct cortical electrical stimulation (DCES) ranges between 5 and 15 mm. We used a newly developed software to determine motor-related areas before intraoperative DCES and evaluate its accuracy in the prediction of motor areas.

Method: In 10 patients with brain tumor of different origin in relation to the hand knob, we performed a high resolution MRI. After creating an individual 3D finite element head model, navigated TMS was performed on a grid of 25 points. Each point was stimulated 10 times and simultaneous electromyography was recorded from the first dorsal interosseus muscle (FDI). Electric field simulations were conducted to determine areas of most probable stimulation during TMS. Based on these computational predictions, a grid of nine points (5mm spacing) with the center point located on the predicted “hotspot” of the FDI muscle was used for DCES during surgery.

Results: The comparison of motor evoked potentials during direct cortical stimulation showed a significant effect of spatial location (p < 0.05). A comparison of computationally predicted motor areas with TMS show an improved overlap with direct cortical stimulation results seen in traditional TMS methods to determine motor areas.

Conclusions: Navigated TMS is a very promising tool to preoperatively map motor cortex representation in relation to brain tumors near the precentral gyrus. The spatial precision of TMS to determine the motor representation can be improved using new computational methods. We validated these methods using MRI, TMS and DCES to precisely determine motor representation in patients with precentral tumors.