TMS in Neurosurgery: 1 year experience with navigated TMS for preoperative analysis

Objective: Navigated transcranial magnetic stimulation (nTMS) is the only painless non-invasive method that allows for functional testing analogous to the procedure of direct electrocortical stimulation (DCS). The challenge with nTMS is achieving precise, repeatable application of the TMS impulse. We report on our findings with two different nTMS systems and outline the possible error sources.

Methods: Initially, a sensor-based navigation system was used for guidance of the coil which allowed projection of the magnetic impulse focus onto the cortical surface as well as visualization of the angle at which the impulse struck the cortex. We then started to use a system which additionally allowed to display the exact strength and extent of the induced electrical field in dependence of the depth under the coil, the tilting angle and the rotation of the coil. Intraoperatively direct cortical stimulation was performed in all cases (n=15).

Results: Both systems allowed to establish a cartography of the peritumoral cortical motor function. The results were confirmed by direct electrical stimulation in all cases. The exact knowledge of the strength and the extent of the induced electrical field added significantly to the reproducibility and the spatial resolution of the motor mapping.

Conclusion: nTMS is a reliabel method for non-invasive analysis of cortical function before rolandic tumor surgery. Real-time visualization of the induced electrical field increases the accuracy and reliability of the method.