recurrent laryngeal nerve

The recurrent laryngeal nerve arises from the vagus nerve and has an asymmetric anatomical course due to embryological development of the aortic arches. The left RLN loops around the arch of the aorta (specifically posterior to the ligamentum arteriosum, a remnant of the ductus arteriosus), giving it a longer intrathoracic course, while the right RLN loops around the right subclavian artery, staying higher in the neck/thorax. Both nerves ascend in the tracheoesophageal groove to innervate the larynx.

Functionally, the RLN supplies motor innervation to all intrinsic laryngeal muscles except the cricothyroid muscle (innervated by the external branch of the superior laryngeal nerve). These muscles control vocal cord adduction, abduction, and tension, which are essential for phonation, airway protection during swallowing, and coughing. The RLN also provides sensory innervation to the mucosa below the vocal folds.

Clinical significance: Due to its long and recurrent course, the RLN is susceptible to injury during surgical procedures, particularly thyroidectomy (a classic board-relevant complication), parathyroidectomy, anterior neck surgery, and thoracic surgeries involving the aortic arch (e.g., patent ductus arteriosus ligation, aortic aneurysm repair, mediastinal masses).

Unilateral RLN injury typically causes hoarseness due to ipsilateral vocal cord paralysis, with the cord often assuming a paramedian position. Bilateral RLN damage can cause life-threatening airway obstruction due to bilateral paramedian vocal cord paralysis, requiring emergency intervention (e.g., tracheostomy).

A left-sided lung tumor (particularly at the pulmonary hilum) or an enlarged left atrium (e.g., from mitral stenosis) can compress the left RLN as it courses near the aortic arch, causing hoarseness—an important clinical pearl for USMLE (e.g., 'Ortner syndrome' referring to hoarseness from cardiovascular pathology compressing the RLN). Aortic aneurysms and mediastinal lymphadenopathy (e.g., from lung cancer) are also classic causes of RLN palsy tested on boards.

Surgeons performing thyroidectomies must carefully identify and preserve the RLN, as it runs closely posterior to the thyroid gland near the inferior thyroid artery, making this a critical anatomical landmark.