gms | German Medical Science

Objective: Preserving functionality is of significant importance during neurosurgical resection of brain tumors. Particularly within the temporo-parieto-occipital junction a large white matter pathway network of higher brain functions including arithmetic processing (AP) is located. Recently, the feasibility of mapping AP by navigated repetitive transcranial magnetic stimulaton (nrTMS) in healthy volunteers has been shown. The present study aims to correlate nrTMS-based DTI fiber tracking (DTI FT) of AP pathways with postoperative functional patient outcome.

Methods: We performed preoperative nrTMS mapping of AP by presenting simple arithmetic tasks and applying disruptive nrTMS stepwise over the whole hemisphere in 15 patients with parietal brain tumors. We used AP-positive cortical sites in terms of nrTMS as regions of interest (ROI) for DTI FT. Tumor resection was blinded to nrTMS results. In 8 patients we additionally performed postoperative DTI FT of AP based on preoperative nrTMS data and postoperative DTI data. Pre- and postoperatively patients underwent a standardized number processing and calculation test (NPCT).

Results: DTI FT of AP pathways was feasible in all cases. In 6 of 8 cases (75%) the loss of subcortical AP fibers (mean 62.6 ± 15.9%) correlated with worsened postoperative NPCT results. In one case the postoperative DTI FT did not show a loss of fibers and the pre- and postoperative NPCT results were similar. In one case the postoperative DTI FT showed more fibers than preoperatively but the postoperative NPCT result was worse.

Conclusion: The loss of subcortical AP fibers correlated with postoperative NPCT results in 7 of 8 cases (87.5%). Despite only presenting a low number of cases nrTMS-based DTI FT might be a useful tool for the intraoperative visualization of AP pathways. However, the reliability of the present results has to be confirmed in a larger series and by intraoperative mapping data.