John P. Leonetti, M.D., Victor P Mokarry, M.D.,Zhaomin Fan, M.D., Patricia Warf, R.N., M.S.,

and Elizabeth Hudson, R.N.

Vagal Nerve Monitoring

during Parapharyngeal

Space Tumor Removal

Intraoperative monitoring of motor cranial nerves

has contributed greatly to the advancement of contempo-rary skull base surgery. Evoked electromyographic activ-ity of specific cranial nerves can be interpreted by theoperating surgeon in such a way as to allow continuousalterations in tumor dissection technique, thus maximiz-ing the potential for anatomic as well as functional neuralpreservation. Unfortunately, techniques for monitoring thevagus nerve during lateral or posterior skull base surgeryhave not been as clearly defined as those methods used inthe preservation of other cranial nerves.1

Iatrogenic injury to the vagus nerve may result inalterations in voice quality, voice strength, swallowing,and airway capacity.2 These untoward side effects are

compounded by multiple ipsilateral injuries to the adjacentcranial nerves IX and XII. Any surgical procedure involv-ing the posterior cranial fossa, the jugular foramen, the

infratemporal fossa, the parapharyngeal space, the cervicalneck, or the thyroid gland may jeopardize the vagus nerve

as well as these adjacent lower cranial nerves. Functional,or at least anatomic, preservation of the vagus nerve istherefore crucial in minimizing postoperative patient mor-bidity related to associated aerodigestive tract derangement.

The purpose of this article is to review the anatomy ofthe vagus nerve as it relates to neoplasms situated in theparapharyngeal space. A technique for intraoperative vagalnerve monitoring is outlined and our results in the man-

agement of 10 patients are provided.

ANATOMY

The course of the extracranial vagus nerve begins atthe skull base. The nerve exits through the pars nervosa

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Skull Base Surgery, Volume 4, Number 4, October 1994 Departments of Otolaryngology (J.PL., V.PM., Z.E) and Electrophysiologic Monitoring(PW, E.H.), Loyola University Medical Center, Maywood, Illinois Reprint requests: Dr. Leonetti, Department of Otolaryngology-Head and NeckSurgery, Loyola University Medical Center, 2160 South First Avenue, Maywood, IL 60153 Copyright X) 1994 by Thieme Medical Publishers, Inc.,381 Park Avenue South, New York, NY 10016. All rights reserved.

SKULL BASE SURGERYNOLUME 4, NUMBER 4 OCTOBER 1994

portion of the jugular foramen. As the vagus nerve de-scends in the neck it initially lies medial to the internaljugular vein and then moves posteriorly between the veinand the internal carotid artery. At the level of the thyroidcartilage, the nerve passes between the vein and the com-mon carotid artery to enter the root of the neck.

At this point the course of the nerve is different inthe right and left sides of the neck. The right vagus nervedescends posterior to the internal jugular vein to cross thesubclavian artery. The right recurrent nerve branches atthis time and curves backward and behind the subclavianartery. The recurrent laryngeal nerve ascends into theneck in the tracheoesophageal groove. On the left side, therecurrent laryngeal nerve arises from the vagus nerve atthe level of the aortic arch. The recurrent laryngeal curvesinto the aortic concavity and ascends along the left sideof the trachea. After traveling in the groove between theesophagus and the trachea both recurrent laryngeal nervespass medial to the surface of the thyroid gland and enterthe larynx between the thyroid and the cricoid cartilages.The recurrent laryngeal nerve supplies all the intrinsiclaryngeal muscles and therefore is essential for normalspeech, swallowing, and respiration.3

METHODOLOGY

One hundred and seventeen patients with tumors ofthe skull base were referred to the Loyola Center forCranial Base Surgery between July 1992 and January1994. Of the 31 patients with tumors adjacent to the vagusnerve, 24 patients had lesions primarily located in theparapharyngeal space. This study included only the 10patients, out of this group of 24, who demonstrated nor-

mal preoperative vocal cord mobility, thus confirming a

functionally intact vagus nerve on the involved tumor side.The medical records of these 10 patients were ana-

lyzed with respect to the history of symptoms, physicalexamination, and any evidence of preexisting speech or

swallowing difficulties. All patients underwent surgicalremoval of the parapharyngeal neoplasm via a lateral,transcervical approach utilizing intraoperative vagal nerve

monitoring. The surgical technique was reviewed via oper-ative reports and the postoperative results were ascer-

tained through inpatient and outpatient medical records.Postoperative vocal cord function was recorded by indi-rect or direct laryngoscopy at 1 month and at 6 months inthe outpatient videolaryngoscopy laboratory. The histol-ogy ofeach tumor was obtained from pathological reports.

Indications for Vagal Nerve Monitoring

Each patient presented with a tumor of greater than3.0 cm, situated in the parapharynx, adjacent to or arisingfrom the vagus nerve. All patients demonstrated normal

214 vocal cord mobility on the involved tumor side. The deci-

sion to utilize vagal nerve monitoring was based upon theextent and location of the tumor as seen on magneticresonance imaging and on the likelihood of intraoperativedistortion of the nerve at the time of tumor resection.

Monitoring Technique

Routine intraoral intubation was performed and gen-eral anesthesia was administered using a combinationtechnique of low-dose inhalational isoflurane and intra-venous narcotic infusion without muscle relaxants. Withthe patient in the supine position and the head slightlyextended, direct laryngoscopy was performed by a mem-ber of the Otolaryngology Department. An anterior com-missure laryngoscope was employed to maximize theview of both vocal cords and the subglottic space (Fig. 1).

The cricothyroid membrane is located in the anteriormidline of the neck by gentle finger palpation. A bipolarhook-wire electrode (L.A. Diagnostics) is introduced viaa 21-gauge, 1.5-in. needle in the midline of the crico-thyroid membrane in an anterior to posterior directionuntil the lumen of the subglottic larynx is entered (Fig. 2).Entry of the airway is "felt" during insertion by the de-crease in tissue resistance and is confirmed by the assistantperforming the laryngoscopy. The needle tip is then an-gled toward the vocal cord to be monitored, the needle isadvanced into the vocalis (thyroarytenoid) muscle, thehook wire is implanted in the vocal cord, and the insertionneedle is removed (Fig. 3). The electrode is taped to theanterior neck skin at the exit point.

The bipolar electrode is connected into a headbox usedfor stimulating and recording. Also connected into theheadbox are standard subdermal electroencephalogramelectrodes placed in the forehead for use as a ground andin the ipsilateral shoulder as an anode for stimulating.

The electromyogram system currently used is a NerveIntegrity Monitoring Unit (Xomed-Treace). This is a two-channel system that allows monitoring of either both left-and right-sided muscles or a single side, depending on theindication. The Prass Monopolar Stimulating probe isused for identification and stimulation of the vagus nerve.The stimulus intensity used is 0.2 to 0.35 mA (constantcurrent). Stimulus duration is 0.1 milliseconds and stim-ulus frequency is 4.1 per second. The display screen on theNIM is 15 seconds and the verticle display is 500 RuV.

We have noted that after the placement of the electro-myogram electrode the baseline is sometimes elevatedanywhere from 30 to 100 RuV This increase may last a fewminutes or, depending on the amount of tumor involve-ment, throughout the case.

Surgical Technique

Following electrode placement for cranial nervemonitoring the patient is positioned supine with the head

VAGAL NERVE MONITORING-LEONETTI ET AL

Figure 1. Direct laryngoscopyfor visual inspection of the subglot-tic larynx during hook-wire elec-trode placement into the vocalismuscle.

turned away from the tumor side and slightly extended atthe neck. A transverse neck incision is utilized with eithera pre- or postauricular extension. Control of the carotidartery and jugular vein is first obtained above and belowthe tumor, and all lower cranial nerves are identifiedadjacent to the lesion. In most cases the operating micro-scope is utilized to enhance the tumor dissection awayfrom the vagus nerve and the great vessels in the neck.Judicious monopolar stimulation of the vagus nerve isutilized, at the surgeon's discretion, during tumor removalto assist in the accurate mapping of the nerve throughoutthe operative bed. Stimulation thresholds are determinedprior to and immediately following tumor removal.

Figure 2. Midline cricothyroidmembrane insertion of a 21-gauge,1.5-in, needle for hook-wire elec-trode placement into the vocalismuscle.

RESU LTS

Ofthe 10 patients with tumors of the parapharynx andintact vocal cord mobility on the ipsilateral side whounderwent intraoperative vagal monitoring, 6 were maleand 4 were female and the age range was 28 to 62 years.The tumor histology included 4 patients with carotid bodytumors, 2 patients with glomus vagale lesion, 2 individ-uals with vagal schwannomas, and 2 cases of metastaticsquamous cell carcinoma.

The vagus nerve was purposely resected in the 2patients with schwannomas of the involved nerve and in 1

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Figure 3. Bipolar hook-wireelectrode implanted in the right vo-calis muscle.

patient with a carotid body tumor in whom no discernibledissection plane could be established between the tumorand the vagus nerve. All 3 of these patients demonstratedthe expected ipsilateral postoperative vocal cord paralysis.

The vagus nerve was anatomically preserved in theremaining 7 patients. In this group of patients, 4 demon-strated normal vocal cord movement at 1 month and 3 hada vocal cord paralysis noted at 1 month. However, all 3patients regained vocal cord mobility by the 6th post-operative month. This 10-patient series is summarized inTable 1.

Two of the 3 patients with vagal nerve sacrifice re-quired secondary surgical intervention in the form ofvocalcord medialization (laryngoplasty) in order to restorefunctional voice capacity. The remaining patient in thisgroup developed adequate contralateral vocal cord com-pensation without surgical intervention. None of the 7patients whose vagus nerves were preserved required ad-ditional laryngeal surgery, although 4 patients utilizedvoice and speech therapy during the postoperative period.None of the 10 patients required tracheotomy or feedinggastrostomy in the postoperative period.

CASE PRESENTATION

A 34-year-old male physician was referred with a6-month history of a slowly progressive right-sided neckmass and ipsilateral pulsatile tinnitus. His physical exam-ination demonstrated a nondiscrete 3-cm mass over theright carotid bifurcation with an audible bruit upon aus-cultation. Indirect laryngoscopy demonstrated normal vo-cal cord mobility, and the remainder of the head, neck, andcranial nerve examination was unremarkable.

A magnetic resonance imaging study demonstrated a5-cm vascular carotid body tumor in the right neck (Fig.4). This lesion was resected through a transcervical ap-proach with intraoperative vagal nerve monitoring (Fig.5). Complete tumor removal was accomplished and neuralintegrity was confirmed by postresection stimulation atthe 0.3-mA level with a 400-,uV response.

The patient experienced hoarseness with mild dys-phagia in the early postoperative period and a completeright vocal cord paralysis was noted by indirect laryngos-copy. He was discharged on the 4th postoperative day with

Table 1. Ten Patients with Parapharyngeal Space Tumors Resected during lntraoperative Vagal Nerve MonitoringCase Patient Age Diagnosis Intraop 1 mo 6 mo

1 M.K. 34 Carotid body tumor Saved - +2 J.C. 54 Carotid body tumor Saved - +3 D. B. 46 Schwannoma Sacrificed -

4 R. B. 44 Schwannoma Sacrificed -

5 D.W. 62 Glomus vagale Saved + +6 W. D. 59 Carcinoma Saved + +7 J.B. 28 Carotid body tumor Saved + +8 L.W. 29 Carotid body tumor Sacrificed9 B. E. 61 Carcinoma Saved + +10 J.M. 56 Glomus vagale Saved - +

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a normal diet and voice therapy instructions. His vocalcapacity improved gradually, and he was noted to haveregained normal vocal cord movement on the previouslyparalyzed side by the 6th postoperative month.

DISCUSSION

Figure 4. Axial magnetic resonance imaging studydemonstrating a large carotid body tumor filling the rightparapharyngeal space.

Figure 5. lntraoperative viewof a right carotid body tumor withpreresection stimulation of the pos-teriorly displaced vagus nerve usingan insulated monopolar stimulator(Black probe).

A variety of complex surgical approaches have beenadvocated in the management of tumors of the posteriorcranial fossa and lateral skull base. Lesions near the jugu-lar foramen or the carotid sheath can be surgically re-moved, but the price paid by the patient related to lowercranial nerve injury may be devastating. Vagal nerve dam-age, combined with injury to the glossopharyngeal andhypoglossal nerves, can cause dysphagia and airway in-competence requiring temporary or possibly permanentgastrointestinal alimentation or tracheotomy, respectively.Any method for monitoring the motor portion of the vagalnerve that can assist in the anatomic preservation of thenerve without jeopardizing patient safety may signifi-cantly impact on the successful recovery of patients under-going lateral skull base or posterior fossa surgery.

Tumors of the parapharyngeal space may distort ordisplace the vagus nerve within the carotid sheath from thecervical neck inferiorly to the jugular foramen superiorly.Gradual growth of benign tumors in, this region results insevere splaying of the vagal nerve fibers; however, neuralintegrity and function are often preserved.4 Ipsilateralvocal cord paralysis may occur but go unnoticed by thepatient as contralateral vocal cord compensation occurs.

The technique for vagal nerve monitoring used in thisgroup of patients involves direct intramuscular placement

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of hook-wire electrodes into the laryngeal muscles inner-vated by the recurrent division of the vagus nerve. None ofthe 10 patients demonstrated intramuscular hematomas ofthe vocal cord with this technique. Neural stretching,direct mechanical trauma, or thermal injury are reflectedby visual and audible burst or train (sustained) activity.Such adverse responses can alert the surgeon to the poten-tial for neural injury, and appropriate measures can beundertaken to minimize such neural trauma. Judicious useof the monopolar stimulator or stimulator-dissector instru-ments can also be used to accurately map the course ofdistorted vagal nerve filaments throughout the tumor bed.No adverse cardiac arrhythmias or blood pressure changesoccurred, despite direct vagal nerve stimulation.

Three of 10 patients in this series had their vagalnerve sacrificed, despite the use of intraoperative monitor-ing. Two of these had vagal nerve schwannomas, in whichcase the vagus nerve was replaced by tumor despite nor-mal preoperative vocal cord function. Some schwan-nomas, however, can arise from the outer sheath of thevagus nerve, thus making tumor removal and neural pre-servation feasible. We therefore advocate the use of vagalmonitoring in all such cases.

Of the seven patients with anatomically preservedvagus nerves, four patients (57%) demonstrated normalvocal cord movement at 1 month, and all seven showednormal vocal cord mobility at 6 months. In addition, none

ofthese patients required secondary laryngeal surgery andall developed normal swallowing capacity.

SUMMARY

Intraoperative vagal nerve monitoring can be achievedby the simple, inexpensive, and safe method of needleelectrode placement through the cricothyroid membraneinto the vocalis muscle under direct laryngoscopic vision.Anatomic and often functional vagal nerve preservationcan be accomplished during the surgical resection of ad-vanced tumors of the parapharyngeal space.

REFERENCES

1. Kartush JM, LaRouere MJ, Graham MD, Bouchard KR, Audet BV:Intraoperative cranial nerve monitoring during posterior skullbase surgery. Skull Base Surg 1:85-92, 1991

2. May M: Rehabilitation of the crippled larynx. Laryngoscope 90:1-18, 1980

3. Williams PL: Gray's Anatomy. 37th ed. New York: Churchill Liv-ingstone, 1989

4. Biller HF, Lawson W, Som P: Glomus vagale tumors. Ann OtolRhinol Laryngol 98:21-26, 1989

The authors would like to thank Mrs. Jacqueline Burns forher help in the preparation and critique of the manuscript.

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