Continuous Monitoring of Corticobulbar Motor-Evoked Potentials of Vagus Nerve during Brainstem or Skull Base Surgery

Objective: Skull base surgery can lead to dysphagia as a result of intraoperative damage to brainstem centers and/or cranial nerves involved in swallowing.

Study Design: Consecutive patients undergoing neurological procedures were continuously monitored for vagus nerve activity by transcranial electrical stimulation using endotracheal tube surface electrodes. In addition, postoperative swallowing function was evaluated.

Patients and Methods: Fifteen consecutive patients undergoing neurological procedures for skull base lesions were enrolled. We used an endotracheal tube to record EMG signals from the vocalis muscle. Motor neurons were stimulated using corkscrew electrodes placed subdermally on the scalp at C3 and C4. The operator received a warning when a motor evoked potential (MEP) disappeared and/or its amplitude decreased to <50% of the control level. Swallowing was assessed clinically by grading criteria.

Results: Seven patients who had consistent MEPs, keeping >50% of the control level during surgery, were neurologically intact and retained swallowing function after surgery. Three patients had transiently deteriorated MEPs to <50% of the control level and had worse swallowing after surgery; all of these patients recovered. Five patients exhibited persistently deteriorated MEPs to <50% of the control level and presented with dysphasia, but three recovered.

Conclusion: Vagus nerve monitoring with endotracheal tube electrodes by transcranial electrical stimulation is safe and provides continuous information on nerve activity during surgery. This method is useful for preventing intraoperative injury to the brainstem corticobulbar tracts or the vagus rootlets in brainstem or skull base surgery.