Preoperative functional mapping for rolandic brain tumor surgery: comparison of navigated transcranial magnetic stimulation to direct cortical stimulation.

Background: Transcranial magnetic stimulation (TMS) is the only noninvasive method for presurgical stimulation mapping of cortical function. Recent technical advancements have significantly increased the focality and usability of the method.
Objective: To compare the accuracy of a 3-dimensional magnetic resonance imaging-navigated TMS system (nTMS) with the gold standard of direct cortical stimulation (DCS).
Methods: The primary motor areas of 20 patients with rolandic tumors were mapped preoperatively with nTMS at 110% of the individual resting motor threshold. Intraoperative DCS was available from 17 patients. The stimulus locations eliciting the largest electromyographic response in the target muscles ("hotspots") were determined for both methods.
Results: The nTMS and DCS hotspots were located on the same gyrus in all cases. The mean ± SEM distance between the nTMS and DCS hotspots was 7.83 ± 1.18 mm for the abductor pollicis brevis (APB) muscle (n = 15) and 7.07 ± 0.88 mm for the tibialis anterior muscle (n = 8). When a low number of DCS stimulations was performed, the distance between the nTMS and DCS hotspots increased substantially (r = -0.86 for APB). After the exclusion of the cases with < 15 DCS APB responses, the mean ± SEM distance between the hotspots was only 4.70 ± 1.09 mm for APB (n = 8).
Conclusion: Peritumoral mapping of the motor cortex by nTMS agreed well with the gold standard of DCS. Thus, nTMS is a reliable tool for preoperative mapping of motor function.