gms | German Medical Science

Objective: Surgical resection of brainstem tumors is still associated with a high risk of postoperative motor deficit due to complex anatomy and possible impingement of the corticospinal tracts by the tumor. The current study aims at establishing a reliable and objective way to visualize descending motor pathways with navigated TMS-based fiber tracking.

Method: For this pilot study 10 patients with brainstem tumors were examined with nTMS prior to surgery. We depicted the resting motor threshold (RMT) and recruitment curve (RC) for the FDI muscle. Using the MEP responses at 105% RMT and 160% RMT the representation area of hand and leg function was determined. Coordinates for MEP responses were imported into the fiber tracking software and set as seed points for tractography. Then the individual FA threshold, which is the highest FA value leading to visualization of tracts at a predefined minimum fiber length of 110 mm, was determined. Lastly, fiber tracking was performed at a fractional anisotropy value of 75% of the individual FA threshold.

Results: Mapping of the motor cortex and corticospinal tract reconstruction was successful in all patients. Tractography results facilitated decision for surgical approach and intraoperative orientation resulting in a favorable postoperative outcome (modified ranking scale: 0 (30%), 1 (20%), 2 (30%), 4 (20%)). A comparison of nTMS results for RMT and RC in accordance to clinical status allowed for an individual evaluation of possible affected corticospinal tracts.

Conclusions: nTMS based fiber tracking represents an objective visualization method based on functional data. These results provide a valuable instrument for preoperative planning as well as intraoperative orientation and risk stratification.