title: Motor evoked potential monitoring of the vagus nerve with transcranial electrical stimulation during skull base surgeries.
abstract: OBJECTIVE
  Dysphasia is one of the most serious complications of skull base surgeries and results from damage to the brainstem and/or cranial nerves involved in swallowing. Here, the authors propose a method to monitor the function of the vagus nerve using endotracheal tube surface electrodes and transcranial electrical stimulation during skull base surgeries.
  
  
  METHODS
  Fifteen patients with skull base or brainstem tumors were enrolled. The authors used surface electrodes of an endotracheal tube to record compound electromyographic responses from the vocalis muscle. Motor neurons were stimulated using corkscrew electrodes placed subdermally on the scalp at C3 and C4. During surgery, the operator received a warning when the amplitude of the vagal motor evoked potential (MEP) decreased to less than 50% of the control level. After surgery, swallowing function was assessed clinically using grading criteria.
  
  
  RESULTS
  In 5 patients, vagal MEP amplitude permanently deteriorated to less than 50% of the control level on the right side when meningiomas were dissected from the pons or basilar artery, or when a schwannoma was dissected from the vagal rootlets. These 5 patients had postoperative dysphagia. At 4 weeks after surgery, 2 patients still had dysphagia. In 2 patients, vagal MEPs of one side transiently disappeared when the tumors were dissected from the brainstem or the vagal rootlets. After surgery, both patients had dysphagia, which recovered in 4 weeks. In 7 patients, MEP amplitude was consistent, maintaining more than 50% of the control level throughout the operative procedures. After surgery all 7 patients were neurologically intact with normal swallowing function.
  
  
  CONCLUSIONS
  Vagal MEP monitoring with transcranial electrical stimulation and endotracheal tube electrode recording was a safe and effective method to provide continuous real-time information on the integrity of both the supranuclear and infranuclear vagal pathway. This method is useful to prevent intraoperative injury of the brainstem corticobulbar tract or the vagal rootlets and to avoid the postoperative dysphagia that is often associated with brainstem or skull base surgeries.
authors.affiliation: Department of Neurosurgery, Chubu Rosai Hospital, Nagoya, Japan. eito754@yahoo.co.jp
authors.forename: Eiji
authors.initials: E
authors.lastname: Ito
authors.forename: Masahiro
authors.initials: M
authors.lastname: Ichikawa
authors.forename: Takeshi
authors.initials: T
authors.lastname: Itakura
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Ando
authors.forename: Yuka
authors.initials: Y
authors.lastname: Matsumoto
authors.forename: Keiko
authors.initials: K
authors.lastname: Oda
authors.forename: Taku
authors.initials: T
authors.lastname: Sato
authors.forename: Tadashi
authors.initials: T
authors.lastname: Watanabe
authors.forename: Jun
authors.initials: J
authors.lastname: Sakuma
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Saito
mesh_terms: D000328:Adult; D000368:Aged; D020295:Brain Stem Neoplasms / Q000601:surgery; D004558:Electric Stimulation / Q000379:methods*; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D019291:Skull Base / Q000601:surgery*; D019292:Skull Base Neoplasms / Q000601:surgery; D014630:Vagus Nerve / Q000502:physiology*