gms | German Medical Science

Objective: Neurosurgical interventions in the brainstem have a high risk for new postoperative deficits due to the close spatial relation and density of vital neuronal structures. Reliable preoperative methods to evaluate factors that correlate with these risks are necessary for informed therapeutic decision making. The nTMS based tractography has already taken an important role in preoperative planning in brainstem cavernoma surgery, but has not been investigated in brainstem tumors yet. The aim of this study is to establish a reliable methodology to visualize the descending motor fibers in patients with brainstem tumors and to evaluate the potential for presurgical risk stratification of this method.

Methods: All patients with brainstem tumors, diagnosed by means of MRI, were examined preoperatively with navigated transcranial magnetic stimulation (nTMS) and diffusion tensor imaging (DTI). [TP1] The MEP-positive spots were used as seed points for the DTI fiber tracking of the descending motor tracts. Somatotopic fiber tracking was performed at a fractional anisotropy (FA) value of 75% of the individual FA threshold. Then, distance measurements between the brainstem tumor and the corticospinal tract (CST) and corticobulbar tract (CBT) were performed.

Results: 18 patients were enrolled in this study, with 11 patients undergoing surgical resection and 6 receiving stereotactic biopsy. The diagnoses consisted of glioma (WHO I-II° 28%, III-IV°: 44%), meningioma (22%) and metastasis (5%). Mapping of the motor cortex and tract reconstruction for hand, leg, and facial function was successful in all patients. The somatotopy of corticospinal and corticobulbar tracts was also clearly depicted on the brainstem level. In all the patients with a CST to tumor distance of 0-1mm, only a partial resection due to MEP loss during surgery could be accomplished. None of the patients developed a new motor deficit postoperatively.

Conclusion: NTMS-based fiber tracking allows for an individually tailored surgical planning and risk stratification. The somatotopic tract visualization on the brainstem level might serve as a valuable instrument to increase safety in brainstem tumor surgery and expand surgical indication in these patients.