Comparison of Navigated Transcranial Magnetic Stimulation to Direct Electrical Stimulation for Mapping the Motor Cortex Prior to Brain Tumor Resection

Abstract

There are two main indications for mapping of the motor cortex in patients eligible for surgery with rolandic tumors. First, mapping is indicated if the functional anatomy (i.e. the exact spatial relationship between the tumor and the presumed essential motor areas) remains unclear after anatomical imaging. The reasons for this can be the mass effect of the tumor or infiltrative growth. Second, mapping is indicated if there is a discrepancy between the imaging results and the clinical findings (for example, a large tumor within the primary motor cortex but no noticeable motor deficits). In such cases, the functional anatomy may have changed due to tumor-induced plasticity. In either of these two scenarios (which may occur separately or together), motor mapping provides elucidation of the functional anatomy and the state of the motor system. This chapter presents an overview of the possibilities and limitations of transcranial magnetic stimulation (TMS) and direct electrical stimulation (DES) for mapping of the cortical motor topography in the neurosurgical setting. The intriguing feature of TMS is that it is the only painless non-invasive method that allows for direct electrical stimulation of the brain. Findings in basic research have recently been backed up by current studies that TMS is a relevant tool for performing stimulation mapping procedures, which were previously only possible with direct electrical stimulation of the brain during surgery. All relevant studies comparing TMS to DES for mapping of the motor cortex are summarized and commented in this chapter.