Significance of cathodal stimulation for motor evoked potential monitoring

[Objective]: Intraoperative motor evoked potential (MEP) monitoring is essential to reduce the risk of neurosurgical procedures damaging the motor system. The standard technique of recording MEP uses high-frequency anodal monopolar stimulation of the primary motor cortex. The present study investigated the possibility that cathodal stimulation allows MEP recording at lower intensities than does anodal stimulation in certain patients. [Methods]: Motor threshold (MT) was measured and compared for anodal and cathodal stimulation in 65 patients in Kitasato University Hospital. The patients were divided into 3 groups based on the polarity of the lower MT: anodal less than cathodal stimulation (Group A), cathodal less than anodal stimulation (Group B), and anodal similar to cathodal stimulation (Group C). Magnetic resonance (MR) imaging findings of hyperintensity extending to the precentral knob accompanied by brain tumor were evaluated to correlate with the polarity of lower MT. [Results]: There were 32 patients classified as Group A, 23 patients as Group B, and 10 patients as Group C. MR imaging showed positive findings in 3 of 32 patients in Group A, 15 of 23 patients in Group B, and 1 of 10 patients in Group C. [Conclusions]: Cathodal stimulation allows MEP recording at lower intensities than does anodal stimulation in a significant number of patients. MR imaging findings of hyperintensity extending to the precentral knob may be indicative of appropriate cases for cathodal stimulation.