gms | German Medical Science

Objective: Navigated transcranial magnetic stimulation (nTMS) combined with diffusion tensor imaging (DTI) is used preoperatively in patients with eloquent-located brain lesions and allows analyzing non-invasively the spatial relationship between the tumor and functional areas (e.g. the motorcortex and the corticospinal tract [CST]). Combining neurophysiological data and data provided by nTMS-based fiber tracking (FT) can be used to stratify patients with motor associated tumors into high and low risk cases with impact on the further treatment strategy. In this study, we examined the diffusion parameters FA (fractional anisotropy) and ADC (apparent diffusion coefficient) within the CST in different locations and analyzed their usefulness for predicting the patients’ motor outcome.

Methods: Prospectively collected data of 30 patients undergoing bihemispheric nTMS mapping prior to surgery of motor eloquent high-grade gliomas were analyzed. The following data were scrutinized for both hemispheres after tractography based on nTMS-motor positive cortical seeds and a 2^{nd region} of interest in one layer of the caudal pons defined by the color-coded FA-map (ROI): the pre- and postoperative motor status (after day of discharge und 3 months), the closest distance between the tracts and the tumor (TTD), the fractional anisotropy (FA) and the apparent diffusion coefficient (ADC). The latter as an average within the CST as well as specifc values in different locations (peritumoral, mesencephal, pontine).

Results: Eight (27%) out of 20 (67%) patients treated as high risk cases suffered from a paresis that did not recover over time whereas no patient deteriorated postoperatively whose tumor loation had been determined as uncritical (p = .027). Lower average FA-values within the whole affected CST are significantly associated with deteriorated postoperative motor function (p = .002) where higher average ADC-values are significantly related to an impaired postoperative motor status (p = .026). Detailed analysis within the CST revealed that the diffusion parameters are especially disturbed on a peritumoral level and that the degree of their impairment correlates with motor deficits (FA p = .007, ADC p = .007). No significant segmental variation was seen in the healthy hemisphere.

Conclusion: Navigated TMS-based motor DTI not only offers data for analysing the spatial relationship between tumor and functionally essential tissue but also provides information on the structural integrity of the tracts, which improves the prediction of postoperative motor outcome. Further prospective studies are needed to validate the nTMS-based prediction of the patient’s outcome.