Navigated Transcranial Magnetic Stimulation and Diffusion Tensor Imaging Tractography in Insular Glioma Surgery

Background and objectives: The surgical resection of insular gliomas is associated with a high rate of postoperative morbidity as they grow close to descending motor fibers and lenticulostriate arteries. It is believed that intraoperative perforator infarctions are the determining factor for patients' postoperative outcome, while the majority of patients with intraoperative ischemic events do not develop postoperative motor deficits. This study aims to evaluate whether navigated transcranial magnetic stimulation (nTMS) and nTMS-based fiber tracking could be valuable for the preoperative assessment of patients with insular gliomas.

Methods: Thirty-two patients with insular gliomas were presurgically examined by nTMS. The resting motor threshold and cortical representation areas of legs, hands, and face were identified on both hemispheres. Motor evoked potential positive stimulation points were then used as a region of interest for diffusion tensor imaging tractographies. Somatotopic fiber tracking was performed enabling analyses of the spatial relation between tumor and cortico-spinal tract (CST) as well as the extraction of fiber tract integrity, measured by fractional anisotropy and the apparent diffusion coefficient.

Results: The performance of nTMS mappings of the motor cortex and reconstruction of descending motor fibers for legs, hands, and facial functioning was successful in all patients. Higher preoperative resting motor threshold ratios and a distance between tumor and CST of <3 mm were associated with a permanent deterioration in motor function (P = .029 and P = .007). Shorter distances between CST and tumorous tissue were correlated with lowered peritumoral fractional anisotropy values, suggesting alterations in fiber tract integrity. Lower interhemispheric peritumoral fractional anisotropy ratios showed an association with new postoperative motor deficits (P = .017).

Conclusion: nTMS-based diffusion tensor imaging tractography enables somatotopic tract visualization and provides a valuable tool for preoperative planning, intraoperative orientation, and individual risk stratification. Thus, it may be beneficial to increase safety in insular glioma resection surgery.