Functional guidance in intracranial tumor surgery

Objective

Navigated transcranial magnetic stimulation (nTMS) is a newly evolving technique. Despite its supposed purpose of preoperative mapping of the central region, little is known about further applications as well as the accuracy compared to more commonly used modalities like direct cortical stimulation (DCS) and functional MRI (fMRI).

Methods

We examined 30 patients with tumors in or close to the precentral gyrus as well as in the subcortical white matter motor tract using nTMS with the Nexstim eXimia system. Data was sent to the neuronavigation system and correlated with intraoperative direct cortical stimulation.

Results

In the cases of lesions of the precentral gyrus, preoperative motor cortex characterization correlated well with intraoperative DCS with a deviation of 4.5 ± 3.5 mm. When comparing nTMS with fMRI however, deviation was quite larger with 9.6 ± 7.9 mm for upper and 15.0 ± 12.8 mm for lower extremity. In patients with subcortical lesions DTI fiber tracking was performed using nTMS mapping as seed region, which resulted in a subjectively more specific presentation of the corticospinal tract compared to conventional fiber tracking and caused less interobserver variability.

Conclusion

Navigated TMS correlates well with DCS as the best established standard despite many factors, which are supposed to contribute to inaccuracy of the method. Moreover, nTMS-aided DTI fiber tracking is user-independent and, therefore, a method for further standardization of DTI fiber tracking.