Outcomes with Bipolar vs. Monopolar Cortical and Subcortical Motor Mapping in Craniotomies for Brain Tumors

Introduction: Proper resection of diffuse glial brain tumors must strike a delicate balance between volume of residual tumor and postoperative deficits, as both significantly impact a patient’s overall survival. Since their conception, stimulation probes have remained the gold standard for intraoperative assessment of motor and speech function.

While both bipolar and monopolar stimulation probes are commonly employed, their use is far from standardized; thus, a better understanding of how these probes may influence surgical outcomes is needed. The senior author’s (IFP) practice presents a unique opportunity to directly evaluate patient outcomes following the use of solely either a bipolar or monopolar stimulator for motor mapping, as they were never used in conjunction.

Methods: 300 consecutive patient records of motor mapping cases were retroactively reviewed. Volumetric analysis was performed using either BrainLab or Slicer3D.

Results: There was no significant difference in the occurrence of permanent neurologic deficits (7.6% vs. 3.3%, p = .317) or extent of resection (EOR) of non-enhancing tumors (p = .453) when using a bipolar or monopolar stimulator for intraoperative motor mapping. EOR of enhancing tumors was significantly higher with the bipolar stimulator (92.7% vs. 83.5%; p = .010).

Notably, use of a monopolar stimulator resulted in higher rates of subcortical motor pathway identification (78% vs. 42.1%; p = .008) and fewer intraoperative seizures (13.3% vs. 32.8%; p = .002).

Conclusion : Both bipolar and monopolar stimulators are safe for use alone in cortical and subcortical motor mapping. The monopolar stimulator may be more reliable for identifying subcortical motor fibers and results in significantly lower rates of intraoperative seizures, thereby reducing the risk of aborted procedures. While use of a bipolar stimulator led to a higher EOR of contrast-enhancing tumors, it may also be associated with higher rates of postoperative deficits, in part due to less consistent identification of subcortical motor fibers. Recent literature suggests that a combination of the two modalities (bipolar for cortical mapping, monopolar for subcortical mapping), may lead to a safer and more complete overall resection.