Intraoperative Neuromonitoring for the Lower-Extremity Region Using Motor-Evoked Potential With Direct Cortical Stimulation in Brain Tumor Surgeries.

Purpose: To evaluate the motor function of the lower extremity (LE), we used direct cortical stimulation motor-evoked potential (D-MEP) monitoring with a single six-contact subdural strip electrode placed in the interhemispheric fissure.

Methods: Intraoperative neuromonitoring using D-MEPs in the LE was performed in 18 cases (16 patients) for brain tumor surgery from December 2018 to April 2023 with a follow-up period of at least 3 months. After dural opening, a single six-contact subdural strip electrode was placed inside the interhemispheric fissure. To identify the central sulcus, phase reversal was recorded using somatosensory evoked potentials. Next, direct cortical stimulation was applied to the primary motor cortex. The baseline waveform was defined as a reproducible waveform of 30 µV or higher, and a significant decrease of ≥50% in the amplitude resulted in a warning during surgery.

Results: The success rate of central sulcus identification in the LE was 66.7% (12/18 cases). Direct cortical stimulation motor-evoked potential monitoring could record stable contralateral motor-evoked potentials of the tibialis anterior, gastrocnemius, and abductor hallucis in 16 of 18 cases (88.9%). The mean intensity of stimulation for D-MEPs was 20.5 ± 9.9 mA, and the 16 cases showed no significant reduction in amplitude. Seventeen cases showed no deterioration of motor function of the LE at 1 and 3 months postoperatively. In the remaining case with unsuccessful D-MEP, paralysis of the LE worsened at 1 and 3 months postoperatively.

Conclusions: The placement of electrodes in the interhemispheric fissure on the primary motor cortex of the LE enabled motor-function monitoring in the LE with D-MEPs, suggesting that D-MEP-based monitoring may be a reliable approach.