Dual Vagus Nerve: a Rare Anatomical Entity

Abstract

Duplicate cranial nerves are fundamentally rare anatomical variants. Few case reports have documented cranial nerve duplication. Proper understanding of the anatomy is important to prevent iatrogenic injury during any surgical procedure. We present a case report of duplicate vagus nerves identical in size and thickness identified during neck dissection procedure. A 44-year-old gentleman diagnosed with Squamous Cell Carcinoma of Right Parotid Gland underwent Total Parotidectomy with Elective Neck Dissection. During carotid sheath microdissection, two parallel nerve tracts were identified. The two nerves were identical in size and width. Proximal dissection confirmed that the two nerves were independent of one another and neither was a branching segment. This is the one of the few reported cases of duplicate vagus nerves identified. The authors would like to highlight the rare anatomical variant of vagus nerve and emphasize on knowledge of proper surgical anatomy to prevent inadvertent injury to cranial nerve and any vital structures during surgical procedure.

Introduction

Vagus nerve duplication is a rare anatomical finding with very few literatures documentation available. Knowledge of the variants of the vagus nerve and other potential nerve fibres within the carotid sheath is critical to avoid iatrogenic injury during any surgical procedures. The vagus nerve which is the tenth cranial nerve, originates from medulla and innervates the laryngeal aspect of the neck, thorax, and abdomen. This case report will discuss an exceedingly rare variant of vagus nerve anatomy as well as the intraoperative findings during a total parotidectomy surgery.

Case Presentation

A 44-year male presented to us with a chief complaint of swelling in his right ear region since 1 month. On clinical examination, 5*3 cm hard fixed swelling was noted in right preauricular region. The swelling presented clinically as an ulcerative lesion involving the skin surface. The histopathological impression from lesion revealed Invasive Squamous Cell Carcinoma.

CECT (Contrast Enhanced Computed Tomography) Neck was done and it revealed bulky lesion in right parotid gland of size 55 * 36 *32 mm epicentered in the preauricular region/retromandibular region. CECT revealed extension of lesion medially till the parapharyngeal space with infiltration of right sternocleidomastoid muscle along the anterior aspect and tumour involvement of Internal Jugular Vein was seen. No bony destruction/ erosive changes were seen. No cervical lymphadenopathy was noted. Therefore, the patient was planned for Total Parotidectomy with Comprehensive Neck dissection.

The patient was placed in a supine position with the head in the midline and the neck slightly extended. Circumtumoral Incision was placed in right pre auricular region with extension into cervical neck crease. Subplatysmal flap was raised extending superiorly till lower border of mandible and inferiorly till clavicle for performing comprehensive neck dissection. Sternocleidomastoid muscle was divided inferiorly 2–3 cm superior to clavicle. The external jugular vein was divided. The greater auricular nerve was divided as it crosses the sternocleidomastoid muscle, posterior to the external jugular vein. We Identified and skeletonise the posterior belly of the digastric muscle. Intraoperatively the tumour encased the right internal jugular vein and the right external carotid artery. There was need for carotid sheath dissection. The tumour was removed by ligating and dividing the right internal jugular vein and right external carotid artery. Right comprehensive neck dissection was performed.

After diving the external carotid artery and while performing microsurgical dissection of the carotid sheath to gain access to free the inferior margin of the tumour, additional unidentified nerve tissue was found running parallel and medially to where the vagus nerve was identified. This duplicated nerve did not appear to branch from the vagal trunk nor exit the sheath but rather ran parallel to the course of the vagus nerve. Neither of the trunks appeared to give off branches within the carotid sheath. The parallel course and the proximity of the unidentified nerve make this structure likely to be a duplicated vagus nerve (Fig. 1).

Fig. 1.

Vagus nerve accompanied by duplicated parallel trunk

The patient’s post operative histopathology report mandated further need of Adjuvant Radiotherapy. However, the patient developed pulmonary complications on follow up and the patient suffered cardiopulmonary arrest thereafter.

Discussion

Vagus nerve duplication is an extremely rare phenomenon. There have been case reports of duplicated second, fifth, sixth, and eleventh cranial nerves but there are very few literatures on duplication of the vagus nerve which have been reported till date [1, 2]. Proper anatomical knowledge of the carotid sheath is critical for performing any surgical procedure. Vagus Nerve injury occurs during the dissection of the carotid artery from the internal jugular vein, thus leading to stretching, inadvertent clamping, or transection.

The vagus nerve has both afferent and efferent fibres, providing somatic, sympathetic, and parasympathetic functions [3]. The vagus nerve also has a more extensive (vague) course though head, neck, thorax, and Abdomen. It has complex distribution than any other cranial nerve. It emerges as 8–10 rootlets from the post olivary sulcus of the medulla, caudal to the glossopharyngeal nerve, to cross the cerebellomedullary cistern to enter the pars vascularis of the jugular foramen. The vagus nerve expands into its superior ganglion within this foramen. As the vagus nerve exits the skull base, it expands into its inferior ganglion. The vagus nerve then descends into the carotid sheath, located between the internal jugular vein (IJV) and the internal carotid artery to the upper border of thyroid cartilage, and then passes between the IJV and common carotid artery to the root of the neck. At the root of neck, the vagus nerve enters the thorax, crossing over by the 1st part of subclavian artery on the right side, inclining behind brachiocephalic vessels. The left vagus travels between the left common carotid and left subclavian arteries on the left side.

The cervical portion the vagus nerve receives its predominant blood supply via the vagal artery. About 2–3 cm from its origin, the vagal artery bifurcates into ascending and descending branches. The artery runs on the anteromedially to the nerve.

Hammer et al. during a cadaveric study of 35 human donors in 2015 observed that cervical vagus nerve branching was observed in 29% of all cases (26% unilaterally, 3% bilaterally) [4]. Right-sided branching (22%) was more common. The left side branching (12%) is usually seen at the level of the fourth and fifth cervical vertebra and the Right was common on the level of the second to fifth vertebra on the right side.

Vagus nerve injury results in dysphagia and dysphonia. The site of the deficit affects the overall symptom. A unilateral deficit results in a weak, breathy voice, difficulty in maintaining a normal length of voicing, loss of voice with attempts at projection, and difficulty in swallowing with risk of aspiration. Patients often feel short of breath due to difficulty in adjusting laryngeal resistance with incomplete glottic closure. A high vagal lesion results in more pronounced symptoms including incomplete paresis of the soft palate and a transient inability to swallowing than a recurrent laryngeal nerve (RLN) deficit. The vagus nerve has several branches: the pharyngeal branches supply motor function to the pharynx and palate except for the tensor veli palatini; the superior laryngeal nerve (SLN) branch has both internal and external branches. The internal branch of the SLN supplies sensory and parasympathetic fibres to the epiglottis, tongue base, aryepiglottic folds, and the larynx to the level of the glottis. The external branch of the SLN innervates the cricothyroid muscle and inferior pharyngeal constrictors. Lesion in SLN produces anaesthesia of upper part of larynx and affects pitch control and swallowing. These combined sensory and motor deficits result in a higher likelihood of aspiration and more complex phonatory problems. It communicates with the intercarotid plexus, monitoring carbon dioxide and oxygen tensions [5] (Fig. 1).

Clinical Evaluation of Vagus Nerve Injury

Clinical assessment of vocal cord paralysis includes examination of palatal movement, assessment of vallecular or pyriform sinus pooling, and subjective evaluation of voice quality, vocal range, and maximum phonation time (MPT). MPT varies significantly. Range below 12 s is associated with a breathy voice with difficulty sustaining phrasing with connected speech. Suspect Pharyngeal or palatal weakness if there is significant pooling of secretions. The oculocardiac reflex can be tested as part of the clinical evaluation. Oculocardiac reflex also known as Aschner Phenomenon involves placing pressure on the eye extra ocular muscles on the side of the lesion and monitoring the pulse. Triggering factors include mechanical stimulation, facial trauma, or regional blocks [6]. Bradycardia is most seen if OCR is activated. Cardiac arrhythmias and asystole are documented as fatalities. The pulse will become slower if the site of the lesion is distal to the communication of the vagus nerve with the intercarotid plexus (proximal to the take-off of both the SLN and the RLN).

Injury of pharyngeal muscles result in Dysphagia. Full assessment of dysphagia is integral for appropriate treatment planning. High vagal lesions typically result in failure of coordination of the oral and pharyngeal phases of swallowing, with poor coordination of the relaxation of the upper oesophageal sphincter, and with pooling and overflow into the larynx.

Diagnostic Examination of Vagus Nerve Injury

Multiple various factors can result in vagal injury including trauma, surgical complications, compression, or disease. Diagnostic Imaging studies include static (CT, MRI), or dynamic (modified barium swallow [MBS]) radiographic studies. Flexible Endoscopic Evaluation of Swallowing with Sensory Testing (FEESST), or laryngopharyngeal sensory testing, invented by Dr. Jonathan E. Aviv is an outpatient procedure utilised for testing motor and sensory functions in patients with swallowing difficulties [7].

A Modified Barium Swallow study is useful to assess and is an instrumental examination to document dysphagia [8]. Sensory deficits and nasopharyngeal reflux are more apparent on a MBS. The MBS examines oral, pharyngeal, and oesophageal function. It identifies the cause of aspiration (i.e., when, and how the event occurs during the swallow hierarchy) and allows assessment of the efficacy of treatment and management strategies.

Treatment of Vagus Nerve Deficits

Treatment options include voice or swallowing therapy; amplification devices, injectable substances to improve glottic closure, surgical medialization, treatment of velopharyngeal incompetence, and treatment of cricopharyngeal hypertonicity or dysfunction. Voice or swallowing therapy should be considered a complementary treatment regardless of any other interventions. Injection Augmentation is preferred over Teflon to restore bulk and form. These are minimal invasive approaches which can be performed under Local anaesthesia [9]. Complications including foreign body giant cell granulomas and persistent vocal deficits are well established with Teflon injections. Treatment of velopharyngeal incompetence can improve speech and swallowing function. This treatment may involve working with a palatal prosthesis. In addition, surgical options such as a palatal adhesion technique may be useful for this unilateral deficit. Cricopharyngeal hypertonicity can be treated by myotomy or botulinum toxin.

Conclusion

The conventional belief that the vagus nerve has a definite anatomical path with no cervical branching in the carotid sheath is inaccurate. This case report is an example of a doubled vagus nerve. There appears to be no other nerve than the vagus nerve. The vagus nerve has a stable anatomical path in the carotid sheath with no cervical branching. There are very few cases reported in the literature and hence proper knowledge of the anatomy is important to prevent inadvertent damage to the vagus nerve.

Declarations

Informed Consent

Written informed consent was obtained from the patient for publication of this article.

Research Involving Human Participants and/or Animals

It is a clinical case report. No research was done on human participants/or animals.

Conflicts of Interest

The authors declare that there were no potential conflicts of interests.