Facial nerve communication with ansa cervicalis – An unusual anatomical variation

ABSTRACT

Neural connections of the seventh cranial nerve with its neighboring nerves are common and well documented; however, communication with ansa cervicalis is as yet unknown. We present a case with such a connection found during cadaveric dissection, with hitherto unknown consequences. In this specimen, after giving the marginal mandibular and cervical branches, the cervicofacial division continued distally to communicate with the distal loop of ansa cervicalis. Presence of such connection may result in facial muscle paralysis on injury to the ansa or strap muscle paralysis on injury to the facial nerve, depending on the direction of nerve fibers. Such unusual connections bring to light the need for extreme care during surgeries in the neck to safeguard any such connections and when using the ansa as donor.

Introduction

Ansa cervicalis is formed as a loop with contributions to the superior root from C1, C2 ventral rami and to the inferior root from C2, C3 ventral rami. The superior root travels with the hypoglossal nerve and supplies the superior belly of omohyoid, sternohyoid, and sternothyroid muscles before joining the inferior root to complete the loop. It is responsible for innervation of the infrahyoid musculature.[1,2] Facial nerve arises from the pontomedullary sulcus and after coursing through the internal auditory meatus to reach the medial wall of tympanic cavity, it exits the cranium via stylomastoid foramen.[3]

The seventh nerve and its branches have known communications to various nerves present in proximity. Incidence varies from 20% to 82.3%.[4] Some of these arborizations include the following:

• Main trunk with glossopharyngeal nerve

• Main facial nerve with the auricular branch of vagus

• Upper division with auriculotemporal nerve

• Marginal mandibular branch with mental nerve

• Marginal mandibular branch with sensory transverse cervical nerve

• Cervical branch with transverse cervical nerve[3,4,5]

Classifications of ansa according to its various neural connections with vagus and hypoglossal nerves (Jelev’s classification)[1,6] have been described.

An anastomosis of facial nerve with ansa cervicalis or a direct communication with ventral rami of C2–C3 cervical nerves, however, has never been described, to the best of our knowledge. A systematic review of eight articles by Hwang et al.[4] after a literature search of 832 articles, regarding communications between the facial and vestibulocochlear, glossopharyngeal nerves and the cervical plexus, did not mention any such findings. Even Bergman’s Comprehensive Encyclopedia makes no mention of any such connections.[3,7]

Case Report

This unusual anatomical variant was found on dissection of a cadaveric head in our Department of Anatomy, carried out to study the anatomy of extratemporal facial nerve after obtaining ethical clearance from the Institutional Ethics Committee (No.: INT/IEC/2020/SL1590). Dissection was carried out under magnification, using a modified Blair’s incision with Al-kayat–Bramley’s C-shaped extension.

It was dissected bilaterally. On complete dissection of facial nerve on the right, from its exit from the stylomastoid foramen up to its branches innervating the facial mimetic muscles, various anomalies and anatomic variations were noted. Figure 1 shows the dissection in progress.

Figure 1.

Dissection under progress

Length of the main facial nerve trunk was 19.6 mm. Two minor trunks were present. The main trunk trifurcated within the parotid gland and divided into further branches – two temporal, one zygomatic, three buccal, three marginal mandibular, and two cervical branches. The middle division of the main trunk subsequently joined branches of the temporofacial division. An arborization pattern not fitting the classical six Davis patterns was found.[8] There were communications between the branches of upper and lower divisions among themselves, but not between the divisions.

The anomalies and anatomic variations noted were as follows:

1. A cervical branch to the platysma, running in a more superficial plane than the rest of the branches. The cervical branch also crossed the angle of mandible, running 6 mm anterior to it.

2. A branch from the main trunk joining a temporal branch distally

3. Trifurcation of the main nerve trunk with branches of middle division joining the branches of upper division

4. Two accessory trunks

5. Cervical branch with a communicating nerve with the ansa cervicalis.

These can be seen in Figure 2.

Figure 2.

Dissected right hemiface with facial nerve and its branches painted for better visualization. The top of the image is the cranial end of the cadaver and the sub-superficial musculoaponeurotic system (SMAS) skin flap is retracted medially towards the nose. The anomalies noted are marked as: (A) a cervical branch to the platysma, running in a more superficial plane than the rest of the branches, crossing lower border of mandible; (B) a branch from the main trunk joining a temporal branch distally; (C) trifurcation of the main trunk with branches of middle division joining branches of upper division; (D) a communicating nerve with ansa cervicalis: The lower division after giving the marginal mandibular and cervical branches is seen to continue inferiorly into the neck. When traced distally, it is seen to join the inferior root of ansa cervicalis

After dividing into three marginal mandibular and two cervical branches, the cervicofacial division continued distally to communicate with the inferior loop of ansa cervicalis, as seen in Figure 3.

Figure 3.

Same specimen showing detailed course of the communicating branch. The top of the image is the cranial end of the cadaver. Dashed red line drawn along the continuation of lower division of facial nerve coursing inferiorly towards the neck. It was seen to join the inferior root of ansa cervicalis formed from C2 and C3 ventral rami. Labels: (1) sub-SMAS skin flap retracted; (2) lobule of pinna retracted superiorly; (3) sternocleidomastoid muscle, divided in the middle and upper half retracted medially, label placed on lower half; (4) deep lobe of parotid gland, superficial lobe removed to visualize facial nerve

The classical loop of ansa is seen in Figure 4.

Figure 4.

The image shows the dissected right hemiface and neck with the top of the image being the cranial end of the cadaver. Pinna seen at top loft corner of image. Distal course of the facial nerve branch communicating with ansa cervicalis is seen in same specimen. The classical loop of ansa is seen, marked with red dashed line, into which the communicating branch is seen to join (blue dotted line)

Discussion

Even though rare descriptions of neural connections between the branches of facial nerve and transverse cervical sensory nerve are present, a connection with ansa cervicalis is unknown to the best of our knowledge.[3,5,7]

Shoja et al. published an extensive review of literature about neural interconnections between the trigeminal, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves and the C1–C4 spinal nerves. They described presence of connections of facial nerve with cutaneous branches of cervical plexus like greater occipital, lesser occipital, greater auricular, and transverse cervical nerves.[5]

A review article by Hwang et al. elucidating connections between the facial nerve and the cervical plexus found seven articles mentioning such connections including the marginal mandibular and cervical branches to transverse cervical and greater auricular nerves. However, no arborization with ansa cervicalis was found.[4]

The third edition of Bergman’s Comprehensive Encyclopedia, which is considered an exhaustive reference on human anatomical variations, does not mention about this anomalous connection either.[3,7]

Similarly, there are no documented innervations to strap muscles from any cranial nerve nucleus.[6]

In adults, the ansa cervicalis is derived from the first, second, and third cervical spinal nerves, which supply the infrahyoid muscles of the neck. Embryologically, during the fourth week of development, the second pharyngeal arch develops in the neck region. It contributes significantly to the formation of neck and facial structures. These structures migrate to a higher level in later part of development. Facial nerve is the neural component of the second pharyngeal arch, and hence, this might have possibly caused nerve fibers to communicate between the facial nerve and ansa cervicalis in early embryogenesis.[9]

Presence of neural interconnections of facial with other nerves is not routinely considered in surgical training. Unusual connections, as seen in this case, bring to light the need for extreme vigilance during dissection, to safeguard any anomalous arborizations, injury to which may result in muscle paralysis. However, it must be mentioned that communication of nerve branches does not allow any statement about a possible fiber exchange or fiber quality (motor, sensory) per se. Therefore, even though predicting the consequences of injury is currently a speculation, it does not undermine the need for prudence.

Variations in contributing roots, course, and distribution of ansa require modifications in the procedures relating to this nerve.[10] Ansa is used as a donor nerve for facial reanimation as well as for laryngeal reanimation in patients of recurrent laryngeal nerve palsy because of redundancy in supply to strap muscles of the neck, as well as no discernable loss of function or cosmetic concern.[1,6,11,12,13] Iatrogenic injury to ansa can occur during various surgeries including thyroplasty, cervical lymph node dissection, arytenoid adduction, carotid endarterectomy, facial reanimation, and infrahyoid myocutaneous flap reconstruction.[1,13]

However, in case of presence of such connections, wherein the ansa may be contributing to innervation of facial mimetic muscles, using it as donor nerve or inadvertent injury during surgery, must be avoided to prevent complications.[14,15] A nerve stimulator may be able to elicit such contribution intraoperatively. In case redundancy is detected however, such connections can be useful as donors, providing an extra length of nerve.

Also, the clinical features and examination after nerve injury may yield unexpected findings due to these neural arborizations.

Awareness of the variant presented and knowledge of other neural connections are crucial for surgeons working in the neck region. This lends itself to patient and vigilant dissection to prevent chances of any postoperative morbidities.

Declaration of patient consent

The authors certify that appropriate patient consent was obtained.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.