Comparison of monopolar and bipolar stimulation protocol for mapping of motor cortex

Objectives: Direct electrical cortical stimulation is the gold standard technique for intraoperative mapping of the motor cortex. The stimulation can be done in monopolar or bipolar configuration and with high or low-frequency stimulation protocols. However, there is a paucity of literature comparing monopolar and bipolar configurations for motor cortex identification. Therefore, this study aimed to compare the relative effectiveness of monopolar and bipolar configurations for cortical motor mapping with the high-frequency multi-pulse stimulation protocol. Materials and Methods: Forty-one patients requiring cortical motor mapping during neurosurgery (awake craniotomy or under general anaesthesia) were included in the study. Intraoperative cortical motor mapping by direct electrical stimulation with monopolar and bipolar probes was done at similar sites for comparison. The intraoperative findings were correlated with patient outcomes in terms of pre-operative and post-operative motor power status. Results: The median threshold current intensity (TCI) and delivered current density were significantly lower with monopolar, 10.0 (7.7–18.0) mA) compared to bipolar 18.5 (10.0–28.5) mA) configuration, respectively. Out of the 41 patients, 28 patients were positively mapped with monopolar, whereas only 14 were with bipolar configuration (50% false-negative motor mapping with bipolar configuration). Conclusion: The TCI for stimulation of the motor cortex is lower in monopolar configuration with a high-frequency protocol. The bipolar configuration, when used in a high-frequency stimulation protocol, can result in high false-negative mapping (maximum current 30 mA).