An Independent C1 Nerve Root Variant of the Ansa Cervicalis: A Case Report

Abstract

The ansa cervicalis (AC) is a neural loop within the carotid triangle of the anterior neck. The loop is traditionally formed by nerve roots C1–C3 of the cervical plexus and extends multiple motor branches. The current case was discovered during a routine dissection as an anatomical variation of the right AC in an 86-year-old Caucasian male cadaver. In this variation, the C1 nerve root did not form the typical loop with the C2 and C3 nerve roots, but instead, remained independent, traveling deep to the superior belly of the omohyoid muscle to supply the sternothyroid muscle. Because no loop was formed, the anatomy of the current case was not an ansa according to the Latin origin of the word, meaning handle or loop. The AC is an important anatomical landmark within the neck and is implicated in laryngeal reinnervation and respiratory nerve stimulation for patients with sleep apnea. The current anatomical variant contributes to a relatively limited catalog of identified anomalies. Knowledge of new AC variations can guide future surgical interventions and further develop the current base of knowledge surrounding the neuromuscular structures of the head and neck.

1. Introduction

The ansa cervicalis (AC) is a neural structure within the anterior neck region, providing significant motor innervation to muscles involved in swallowing, vocalization, and neck stabilization. The neural loop of the AC is composed of two major roots, superiorly by C1 fibers and inferiorly by the C2 and C3 fibers of the ventral rami of the cervical plexus (C1–C3). These superior and inferior roots join distally near the carotid bifurcation, forming a communicating loop from which various motor branches arise [1]. Motor components of the AC exist to innervate three of the four infrahyoid muscles of the neck (sternothyroid, sternohyoid, and omohyoid), while the thyrohyoid is innervated directly by C1 fibers branching from the hypoglossal nerve. These muscles play an important role in swallowing and depression of the hyoid bone [2].

As the AC serves a multitude of functions, the disruption of the structure can result in various disabilities, ranging from difficulty speaking to neck pain [1]. The dysfunction of the infrahyoid muscles due to AC injury can lead to significant changes in vocal tone, the ability to produce speech, and swallowing deficits [3]. In addition, the AC plays a crucial role in surgical procedures of the neck, as iatrogenic injury can lead to critical postoperative complications [1]. Therefore, healthcare professionals must be aware of the AC’s anatomy and function to ensure optimal patient outcomes during both clinical and surgical interventions.

Beyond the native motor and sensory functions, the AC is useful as a viable donor nerve for reconstructions. Historically, the AC has been used as a donor nerve in laryngeal reinnervation following damage to the recurrent laryngeal nerve (RLN) [4]. The RLN is a distal branch of the vagus nerve that is in close proximity to the thyroid gland and innervates multiple endothyroid muscles important for speech production [5]. One review reported damage to the RLN in up to 1% of thyroid surgeries, rendering it a major source of malpractice litigation following thyroidectomy, leading to use of the AC for reinnervation a technique under frequent consideration [6]. Recent research has also implicated the AC as a potential target for respiratory nerve stimulation in patients with obstructive sleep apnea that has not responded to the more common hypoglossal nerve stimulation [7,8].

The complex branching of the AC renders it prone to anatomical variation, and the implications of damage to this structure render it important to understand its function and form. The current case describes a unilateral, right-sided variation of the AC in which the C1 root from the cervical plexus does not join roots C2 and C3, traveling independently to innervate the sternothyroid muscle.

2. Materials and Methods

2.1. Disclaimer Regarding Use of Human Donor Patients

The cadaver referenced in this study was obtained through a university anatomical donation program through which the patient provided written informed consent for the utilization of their remains for the purposes of medical education and research such as this. The dissection of the donor body, specimen preparation, and variation discovery were performed following established university protocols of good practice. The authors of this case report offer their sincerest thanks to all those who have participated in the anatomical donation program. Their donation aids in the education of future physicians and the discovery of anatomical variations and anomalies such as this one, which can improve patient care in the future and further our knowledge in the fields of anatomy and medicine as a whole [9].

2.2. Anatomical Dissection

The current case report was discovered during routine dissection in an 86-year-old Caucasian male human body donor. After removing the skin and superficial fascia bilaterally from the anterior cervical region, the right sternocleidomastoid muscle was reflected superiorly to expose the anterior carotid and muscular triangles of the right neck. The extraneous tissue was removed, the courses of the hypoglossal, vagus, and C1, C2, and C3 nerve roots were noted, and the AC was identified. Following the identification of the anatomical variation, the left cervical region was dissected and investigated to determine if the variation was bilateral.

3. Results

3.1. Anatomical Variant of the Right Ansa Cervicalis

In the right anterior triangle of the neck, we identified the hypoglossal nerve (Figure 1A, black arrow), along with nerve roots C1–C3 of the cervical plexus (Figure 1A, yellow, red, and blue arrows, respectively), which typically join distally to form the AC (Figure 1A). Two branches of C1 emerged medially from the hypoglossal nerve, one diving under the mylohyoid (Figure 1B, pink double-headed arrow) and another innervating the thyrohyoid (Figure 1B, brown double-headed arrow). Upon further dissection, it was discovered that the third and most lateral branch of C1, which typically forms the superior root of the AC, remained isolated throughout the carotid triangle (Figure 1B, yellow arrows). Rather than joining the inferior root of the AC (IRAC, Figure 1, purple arrow) near the carotid bifurcation, this branch traveled independently, anterior to the common carotid artery, under the superior belly of the omohyoid (Figure 1C, yellow arrows), before branching to supply the sternothyroid and sternohyoid muscles (Figure 1C, green arrowheads). The fibers from C2 and C3 (Figure 1A,B, red and blue arrows, respectively) joined to form the inferior root of the AC (Figure 1A–C, purple arrows), which continued independently until it divided into multiple smaller branches, deep to the tendon separating the superior and inferior belly of the omohyoid (Figure 1B, green arrowheads).

3.2. Bilateral Comparison of the Left Ansa Cervicalis

The hypoglossal nerve was identified (Figure 2A,B, red arrow), along with the associated branching C1 fibers. As with the right dissection, the most medial of the C1 branches emerged from the hypoglossal to dive under the mylohyoid (Figure 2A, yellow arrowhead), and the next branch emerged to innervate the thyrohyoid (Figure 2A, yellow double arrowhead). Unlike the anatomy on the right, however, the most lateral C1 branch (Figure 2A, yellow arrow) combined with the C2 and C3 roots (Figure 2A,B; blue and green arrow, respectively) to form a single trunk (Figure 2A–C, purple arrows). This single trunk continued a vertical course superficial to the longus colli, between the common carotid artery and internal jugular vein, until branching at its terminal end. Three terminal branches were visualized and believed to supply the sternothyroid, sternohyoid, and omohyoid (Figure 2C, green arrowheads). During the dissection of the left carotid triangle, the omohyoid muscle was reflected; therefore, the branch innervating it was believed to have been cut (Figure 2B). The anatomy of this left carotid triangle is similar to short AC variants but not completely consistent with previous cases. Rather than a long superior root forming an ansa distally with multiple terminal branches emerging from the loop, this anatomy shows the three cervical roots joining almost simultaneously into a single trunk that descends to innervate distal infrahyoid muscles.

A simplified diagram of the typical anatomy of the AC (Figure 3A) and the anatomy of the anatomical variant discovered in the right AC can be found below. The diagram highlights the anomalous nerve branch in orange, with traditional anatomic features illustrated in yellow Figure 3B).

4. Discussion

The AC is an important neural structure within the neck that stems from the ventral rami of the cervical plexus, commonly the first three or four cervical spinal nerves. The major components of a true AC are a superior and inferior root that converge to form a loop within the carotid triangle [1]. Due to the complex branching of the AC and its descent from the cervical plexus, there are a variety of reported anatomical variations within human subjects. Of note, all but one anatomical variation of the AC examined during a literature review for the current study was found to be unilateral [10].

Variations in the length of and branching pattern of the distal ansa loop have been described previously by Chhetri and Caloit [11,12]. The most common form is the double classic form, in which two cervical branches contribute to the inferior root of the AC, and accounted for 40% of cases [11,12]. This structure can be seen in Figure 3A. The anatomy of the left AC in our cadaver is similar to the double short form described by Chhetri, with one single terminal branch descending the carotid triangle [11]. With the vagus nerve traveling near the AC, some cases have shown nerve variants in which the cervical roots join the vagus nerve before emerging again to innervate the infrahyoid muscles [10,13,14,15,16]. The term “vagus ansa” can be used when the cervical nerve roots, typically C1 or C2, coalesce with the vagus nerve before reemerging as a single root that forms an ansa loop with the inferior cervical roots, C2 and/or C3 [13,17]. Other vagal variants have been described in which the cervical roots join the vagus nerve but never form a structural loop, instead extending various branches along the course of the vagus nerve to innervate various infrahyoid muscles [10,14,15]. During the literature review, our team described these as absent ansa vagal variants.

Common variations in the structure of the AC are well summarized by Jelev into five morphological classifications based on the previous literature [18]. Within this classification system, an absent ansa exists when the superior and inferior roots do not converge to form a loop, represented by type I and type V. Type I has distinctly separate hypoglossal and cervical components, with only the cervical components from C2/C3 descending to innervate the infrahyoid muscles. Type V has distinctly separate hypoglossal and vagal components, in which the cervical components from C1,2, and 3 converge onto the vagal nerve before emerging as a single root to continue their descent to the infrahyoid muscles [18]. Type V is an absent ansa vagal variant.

One report identified a small C1 nerve branching from the hypoglossal nerve to innervate the distal one-third of the sternocleidomastoid muscle [19]. To our knowledge, there has not been a case reporting a major C1 nerve root descending independently to innervate the omohyoid muscle without anastomosing with the inferior root of the AC. Previously reported absent ansa variants involve some form of union between the anomalous branch and the vagus or hypoglossal nerves. What renders our report unique is that the two medial C1 branches from the hypoglossal nerve and the inferior root of the AC, follow traditional anatomic form and courses, but the branch of C1 that would normally form the upper root of the AC travels through the carotid triangle without any connections to nearby nerves (Figure 3). This case is most like Jelev type I, with the addition of this independent C1 branch [18].

The importance of reporting anatomical variations of the AC is paramount due to its use in the clinical setting for laryngeal reinnervation, respiratory nerve stimulation in patients with sleep apnea, and other head and neck procedures of this region [4,7]. Iatrogenic damage to the AC may occur during operations involving the head and neck region, such as in the setting of a total thyroidectomy, especially when there is an unknown variation from normal anatomy. Unfortunately, the rate of vocal paralysis following these procedures is alarmingly high, with some reports showing up to 23% of vocal paralysis occurred due to iatrogenic causes [20]. Another rare but potentially relevant pathology involving the AC is the development of a schwannoma. Schwannomas of the AC are very uncommon, with as few as six cases reported. However, schwannomas of other nearby nerves can put the AC at risk of iatrogenic or collateral damage [21,22]. In the current case report, three nerves travelled independently through the cervical region: the vagus nerve, combined inferior root of the AC, and the anomalous independent superior root. This arrangement of nerves further complicates the anatomy of the already complex carotid triangle, increasing the risk of inadvertent damage. It is for this reason that it is vital to understand the variations in anatomical structure that can occur in the AC.

The use of the AC for laryngeal reinnervation is due to its similar size and proximity to the recipient site. Unknown variants of the AC may preclude its use in graft harvest or place interconnected structures at risk of further damage [23]. In one case, a patient underwent a parotidectomy for adenoid cystic carcinoma with planned upper-division facial nerve resection to be repaired using the AC. Following the identification of a vagal ansa variant, surgery was aborted to preserve vagal nerve function [24]. Without careful exploration of the associated structures, unexpected anatomical variation can predispose to complications. The lack of a true ansa loop can also complicate reinnervation procedures. The sternohyoid branch of the AC loop is the most commonly used nerve for laryngeal reinnervation [25]. In the current case, no loop is formed, and the terminal branches are unusually short as they emerge near their target muscle groups. This has the potential to complicate reinnervations by shortening the length of donor nerves and potentially paralyzing other muscles that may not be implicated or affected when traditional anatomy is present. Awareness of new anatomical variations is necessary for successful nerve dissection with current surgical approaches and can guide future education and development of novel surgical treatments for conditions involving the neuromuscular structures of the anterior neck.

When considering the current case report, there are some limitations that we should be aware of. Regarding the dissection, this case report was discovered during a student anatomy course for first year medical students, therefore the quality of the dissection could have been improved. This is primarily the reason some of the dissection images are not as clear as they could be. Secondly, the cadaver used was preserved with formalin and not fresh frozen, therefore some detail and biomechanical properties of the tissue have been altered. This is not particularly relevant to our study of the nerves in the neck region but is worth noting. Finally, image quality could be improved, however, the cadaver was replaced shortly after the initial discovery and pictures were taken of the anomalous AC.

5. Conclusions

The current case report describes a novel anatomical variation in the structure of the right AC in an 86-year-old male cadaver. The structure differs from the typical anatomy and previously published variants in three specific ways:

• There is no ansa according to the true Latin derivative of the word (meaning “loop”);
• The root composed of C2 and C3, which would typically form the inferior root of the AC, descends independently to innervate the omohyoid and sternohyoid muscles;
• A C1 root exists that does not connect with the vagus nerve or C2/C3 branch, traveling independently to innervate the sternothyroid muscle.

This current case report adds to a relatively limited set of reported anatomical variants of the AC. This anatomical variant is clinically important in any surgical procedure of the anterior neck to avoid iatrogenic nerve injury and must be considered when planning to use the AC as a donor nerve for reconstruction.