Keywords vagus nerve - endotracheal tube surface electrodes - neuromonitoring - vagal nerve
stimulation
Introduction
Intraoperative identification and functional neuromonitoring of the vagus nerve have
been used in brain stem and skull base surgery. The use of endotracheal tube (ETT)
surface electrodes for vagal nerve monitoring opened new avenues for intraoperative
neuromonitoring in thyroid surgeries as well as neurosurgical cases operating in close
proximity to the vagal nerve nucleus or roots. This is the first reported case of
use of ETT surface electrodes for vagal nerve identification during vagal nerve stimulation
procedure.
Case Report
A 10-year-old boy weighing 26 kg presented with a history of medically refractory
seizures and drop attacks for 6 months. He had a history of hypoxic injury leading
to bilateral temporal gliosis. The patient was planned for vagal nerve stimulation
and was posted for electrode and stimulator placement under general anesthesia with
endotracheal intubation.
Anesthesia was induced with intravenous fentanyl 2 µg/kg and propofol 2mg/kg. Intravenous
atracurium 0.5mg/kg was given after confirmation of adequacy of ventilation. Child
was intubated with 6 mm ID cuffed ETT and fixed at 16 cm at the level of incisors.
To enable easy vagal nerve identification, we planned for ETT surface electrode monitoring
of electromyography (EMG) activity of vocalis muscle. Due to unavailability of NIM-EMG
ETT (nerve integrity monitor-electromyography ETT) at our institute, we prepared the
ETT surface electrodes by placing two single needle electrodes at the level of the
vocal cord guide ([Fig. 1A ]). These were precisely fixed to the ETT with a sterile adhesive transparent tape
ensuring that the needle tips are covered and atraumatic ([Fig. 1B ]). During intubation, the ETT was meticulously placed such that the electrodes are
in contact with the vocal cords ([Fig. 2 ]). A paired electrode was also placed in the cricothyroid muscle to monitor the muscle
activity ([Fig. 3 ]). Ground electrodes were placed over the acromion process. The triggered EMG activity
of vocalis and cricothyroid muscle was monitored using nerve integrity monitor (NIM-Neuro
3.0) using a 1 mA stimulation current. Anesthesia was maintained using total intravenous
anesthesia with fentanyl and target control infusion of propofol (target plasma concentration
of 1.5–2 µg/mL). Further doses of neuromuscular blockers and inhalational agents were
avoided.
Fig. 1 (A ) Photograph representing the needle electrodes attached to endotracheal tube (ETT).
(B ) Photograph representing the needle electrodes attached as ETT surface electrodes.
Fig. 2 Diagrammatic representation of endotracheal tube surface electrodes placed in contact
with the vocal cords. (Created with BioRender.com).
Fig. 3 Photograph showing endotracheal tube surface electrodes and paired electrode in cricothyroid
muscle.
During surgical neck dissection, the left vagus nerve was identified by a sinusoidal
wave with an audible signal on NIM-Neuro 3.0 ([Fig. 4 ]). The vagal nerve stimulator electrodes were anchored to the nerve. A repeat stimulation
of the left vagus nerve confirmed the functional integrity at the end of the procedure.
The child was extubated on table without any new focal neurological deficits
Fig. 4 Photograph of the recorded sinusoidal wave during vagus nerve identification.
Discussion
ETT surface electrode has been in use for recurrent laryngeal nerve monitoring during
thyroidectomy and parathyroid surgeries to predict postoperative vocal cord mobility.[1 ] It has been recently used for vagal nerve mapping and evaluation of functional status
intraoperatively in several neurosurgical procedures like skull base surgeries and
surgeries involving brain stem. Such electrophysiological monitoring of vagus nerve
perioperatively had two goals; one was to identify the vagal nerve rootlets and nucleus
intraoperatively especially when the anatomy is distorted by space-occupying lesion.
The other goal is evaluation of functional status.[1 ]
A prospective study done by Julien et al found that use of surface electrodes is as
sensitive as monitoring by intramuscular electrodes and was helpful for nerve identification
and nerve integrity testing perioperatively. They concluded that it was a simple,
noninvasive and effective method with stimulation thresholds similar to those reported
with the use of intramuscular electrodes.[2 ] Mikuni et al found that the placement of the ETT surface electrodes on the bilateral
vocal cords could be easily performed under direct vision (direct laryngoscopy). Also,
when there occurred a change in tube positioning during surgery, re-establishing electrode
contact was relatively easy with the guidance of impedance and X-ray images.[1 ] Ito et al found that use of ETT surface electrodes along with transcranial electrical
stimulation enables continuous real-time monitoring of nerve integrity in brain stem
or skull base surgeries.[3 ]
Conclusion
Hence, a noninvasive ETT surface monitor for vagal nerve identification and evaluation
of functional status can be used during vagal nerve stimulation procedure. This would
be useful in identification of the nerve in patients with altered neck anatomy due
to previous radiation therapy or burns contracture. The electrical identification
in conjunction with visual assessment would help prevent inadvertent damage to the
nerve during neck dissection.
