What is the anatomic course of the recurrent laryngeal nerve (RLN)?

Anatomic Course of the Recurrent Laryngeal Nerve

The recurrent laryngeal nerve (RLN) is a mixed nerve with sensory and motor fibers that originates from the thoracic portion of the vagus nerve, with the left and right nerves following different anatomical paths to reach the larynx. 1

Origin and General Course

• The RLN is a branch of the vagus nerve containing both sensory and motor fibers that innervate the intrinsic laryngeal muscles (except the cricothyroid muscle) and provide sensory innervation to the vocal folds and upper esophagus/trachea 1, 2
• The right RLN follows a more superficial course along the lateral esophageal edge, while the left RLN loops around the aortic arch and ascends in the tracheoesophageal groove in a deeper position 1
• Both nerves ultimately penetrate the cricopharyngeal membrane to innervate the laryngeal muscles 1

Specific Anatomical Path

• The RLN typically runs in the tracheoesophageal groove, with the left RLN more commonly found inside the groove (71.7% of cases) compared to the right RLN 3
• When outside the tracheoesophageal groove (28.3% of cases), the nerve is most commonly found lateral to the groove (64.7% of cases) 3
• The RLNs appear as 2-3mm thick compact slack cords that sinuously pass upward within the lateral peritracheal and sometimes periesophageal loose connective tissue 4

Important Anatomical Landmarks

• Three critical anatomical landmarks for identifying the RLN during surgery include: 1, 3

  1. Zuckerkandl tubercle (a bulge of thyroid tissue from the lateral thyroid lobe)
  2. Inferior thyroid artery and its branches
  3. Berry's ligament (a posteromedial thickening of perithyroid fascia adherent to thyroid capsule)

• The RLN is most often located superficial to Berry's ligament (88.3% of cases), but can also be found deep to it (8.4%) or piercing through it (3.3%) 3

Relationship with the Inferior Thyroid Artery

• The relationship between the RLN and inferior thyroid artery (ITA) is highly variable and critical for neural identification during surgery 1
• On the right side, the RLN is typically anterior to the ITA (67.9% of cases), while on the left side, the RLN is more commonly posterior to the ITA (67.9% of cases) 5
• In approximately 3-5% of cases on both sides, the RLN may cross between branches of the ITA, complicating dissection and hemostasis 5

Branching Pattern

• The RLN typically gives off 8-14 branches that supply both the esophagus and trachea 4
• Extra-laryngeal branching is common, with 89.1% of right RLNs demonstrating 2-5 extra-laryngeal branches and 74.6% of left RLNs showing 2-3 extra-laryngeal branches 5
• The anterior extralaryngeal branches are generally motor fibers that may be stretched during Berry's ligament dissection 1

Important Anatomical Variations

• Non-recurrent laryngeal nerve is a rare variant (incidence 0.3-1.6%) observed more frequently on the right side 1

• Two types of non-recurrent laryngeal nerve anomalies exist: 1

  1. Type I: originates from vagus nerve above the laryngotracheal junction and descends into larynx, mimicking a branch of the superior thyroid pedicle vessels
  2. Type II: arises from vagus nerve below the laryngotracheal junction and runs parallel to the inferior thyroid artery

• A non-recurrent laryngeal nerve often occurs in conjunction with an anomalous (retro-esophageal) right subclavian artery 1

• Approximately 23% of RLNs follow an abnormal intraoperative trajectory, which is associated with a higher risk of nerve injury during surgery 6

Clinical Significance

• Knowledge of RLN anatomy is crucial for surgeons performing thyroid and parathyroid procedures to prevent nerve injury 1
• RLN injury can result in vocal fold immobility, causing dysphonia (80% of cases), breathing difficulties (75%), and dysphagia with aspiration risk (56%) 1, 2
• The risk of permanent RLN injury during thyroidectomy is 0.3-3%, with transient injury rates of 6-8% 2
• Re-operative procedures carry a significantly higher risk of RLN injury (up to 30%) 1, 2

Understanding these anatomical details is essential for surgeons to minimize the risk of RLN injury during neck procedures, particularly thyroidectomy, which can significantly impact patient morbidity and quality of life.