Anatomy: Recurrent Laryngeal Nerve
Vagus Nerve: The recurrent laryngeal nerve is a branch of the vagus nerve, so an understanding of the vagus nerve is needed to understand its branches. The vagus nerve (Cranial Nerve X) is a mixed cranial nerve with motor, sensory, special visceral and autonomic functions. It is the longest cranial nerve in the body but also has a large and complicated territory of innervated end organs. Branches of the vagus nerve innervate the muscles of the larynx and pharynx, sensation of the external tympanic membrane and portions of the ear, special taste sensation from the root of the tongue and epiglottis, and the innervation of the parasympathetic cardiovascular and gastrointestinal systems. After it exits from the skull base via the jugular foramen, the vagus nerve then courses through the carotid sheath posterior to the carotid artery and internal jugular vein. This anatomy, while variable, variable giving off several branches during its descent including the auricular branch to the ear, pharyngeal branches to the pharyngeal plexus, the superior laryngeal nerve (with external and internal branches), cardiac branches, and finally the recurrent laryngeal nerve.
Non - Recurrent Laryngeal Nerve: The recurrent laryngeal nerve has a distinct course on each side. On the right the recurrent laryngeal nerve typically branches from the vagus nerve anterior to the subclavian artery, loops posteriorly around it, and ascends in the tracheoesophageal groove. The presence of an aberrant right subclavian artery has critical implications for the recurrent laryngeal nerve anatomy, as it is strongly associated with a right non-recurrent laryngeal nerve. When there is an aberrant right subclavian artery, the altered vascular anatomy eliminates the looping point, and the nerve branches directly from the cervical vagus nerve more superiorly in the mid- to lower neck — most often at the level of the cricoid cartilage or just above the inferior pole of the thyroid gland. From its origin, it courses horizontally and medially toward the larynx, usually passing superficial to or between branches of the inferior thyroid artery before entering the larynx just posterior to the cricothyroid joint at the level of the inferior cornu of the thyroid cartilage. The nerve lies medial to the carotid artery, lateral or anterolateral to the trachea, and may run parallel to the inferior thyroid vein. Its transverse path brings it into proximity with the ligament of Berry, where it can be tethered and more vulnerable to injury during vessel ligation or thyroid or parathyroid retraction. The ligament of Berry is a dense condensation of the thyroid capsule that firmly anchors the posterior aspect of each thyroid lobe to the cricoid cartilage and the upper tracheal rings.
On the left, the recurrent laryngeal nerve still branches anterior to the aortic arch, looping posterior to the ligamentum arteriosum and arch before ascending in the same tracheoesophageal groove. Both sides enter the larynx deep into the inferior constrictor at the level of the cricothyroid joint. The left recurrent laryngeal nerve, and in this case right non-recurrent laryngeal nerve, provides motor innervation to all intrinsic laryngeal muscles except the cricothyroid muscle and sensory innervation to the mucosa below the vocal folds, cervical trachea, and upper esophagus.
Aberrant Right Subclavian Artery:
An aberrant right subclavian artery is the most common aortic arch anomaly, present in approximately 0.5–2% of the population. In this variant, the normal brachiocephalic trunk is absent, and the aortic arch gives rise to four separate vessels. From proximal to distal this includes the right common carotid artery, left common carotid artery, left subclavian artery, and finally the aberrant right subclavian artery. The aberrant right subclavian artery originates from the posterior aspect of the distal aortic arch or proximal descending thoracic aorta, just distal to the origin of the left subclavian artery and typically opposite the T3–T4 vertebral body. It courses obliquely to the right, most frequently passing posterior to the esophagus between T2–T5, though it may pass between the esophagus and trachea (~15%) or even less commonly anterior to the trachea (~5%). On barium swallow this could cause a posterior impression on the esophagus. A Kommerell diverticulum, present in 15–30% of cases, appears as a focal saccular or fusiform dilation at the origin. It is important to recognize this as it is prone to aneurysmal degeneration, rupture, or dissection.
Clinical Relevance:
Surgery: From a surgical perspective, the recurrent laryngeal nerve is highly vulnerable during thyroidectomy, parathyroidectomy, central neck dissection, tracheostomy, esophagectomy, anterior cervical spine surgery, and cardiothoracic procedures involving the aortic arch. Injury can result in ipsilateral vocal fold paralysis causing hoarseness, dysphonia, aspiration risk, or, if bilateral, airway obstruction. Recognition of an aberrant right subclavian artery and a potential non-recurrent laryngeal nerve is essential for thoracic, vascular, head and neck, and endocrine surgeons. Preoperative cross-sectional imaging should alert the surgical team to the high likelihood of this variant on the right. During thyroidectomy or parathyroidectomy, careful dissection of the vagus nerve in the mid-neck, early identification of a medial branch, and the use of intraoperative nerve monitoring are key to preventing iatrogenic injury. In both vascular and cervical surgery, awareness of these fixed anatomic relationships to the carotid sheath, inferior thyroid vessels, cricothyroid joint, and ligament of Berry can prevent potentially devastating complications.
Radiology: Radiographically, the non-recurrent and recurrent laryngeal nerve itself are rarely directly visualized but its course can be inferred at its branch points, anatomic landmarks, and within the tracheoesophageal groove on the left. Pathologic processes such as tumor, infection, or postsurgical changes can obscure fat planes or distort this region on cross sectional imaging. If there is clinical concern for new onset vocal cord paralysis it is important that cross sectional imaging protocols extend inferiorly below the artic arch to cover the entire course of the nerves course on the left. Imaging signs of unilateral vocal cord paralysis include medial deviation and thickening of the ipsilateral aryepiglottic fold, dilation of the ipsilateral piriform sinus, anteromedial rotation of the arytenoid cartilage, and enlargement of the laryngeal ventricle. On the left, the recurrent laryngeal nerve is particularly susceptible to compression by mediastinal masses, aortic aneurysms, or pulmonary artery enlargement. On the right the aberrant right subclavian artery should alert the radiologist to the altered anatomic course of the non-recurrent laryngeal nerve. On barium swallow exams it will produce a posterior impression on the esophagus.
