The relationship of the thyroid gland to the recurrent laryngeal nerve and to the external branch of the superior laryngeal nerve is of major surgical significance because damage to these nerves leads to disability in phonation or to difficulty breathing (4). Both nerves are branches of the vagus nerve.
Recurrent Laryngeal Nerve The right recurrent laryngeal nerve arises from the vagus nerve, loops posteriorly around the subclavian artery, and ascends behind the right lobe of the thyroid (Figs. 3 and 4a). It enters the larynx behind the cricothyroid muscle and the inferior cornu of the thyroid cartilage and innervates all the intrinsic laryngeal muscles except the cricothyroid. The left recurrent laryngeal nerve comes from the left vagus nerve, loops posteriorly around the arch of the aorta, and ascends in the tracheoesophageal groove posterior to the left lobe of the thyroid, where it enters the larynx and innervates the musculature in a similar fashion as the right nerve. Several factors make the recurrent laryngeal nerve vulnerable to injury, especially in the hands of inexperienced surgeons (4,6).
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The presence of a non-recurrent laryngeal nerve (Fig. 4b). Nonrecurrent nerves occur more on the right side (0.6%) than on the left (0.04%) (5). They are associated with vascular anomalies such as an aberrant takeoff of the right subclavian artery from the descending aorta (on the right) or a right-sided aortic arch (on the left). In these abnormal positions, each nerve is at greater risk of being divided.
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Proximity of the recurrent nerve to the thyroid gland. The recurrent nerve is not always in the tracheoesophageal groove where it is expected to be. It can often be posterior or anterior to this position or may even be surrounded by thyroid parenchyma. Thus, the nerve is vulnerable to injury if it is not visualized and traced up to the larynx during thyroidectomy.
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Relationship of the recurrent nerve to the inferior thyroid artery. The nerve often passes anterior, posterior, or through the branches of the inferior thyroid artery. Medial traction of the lobe often lifts the nerve anteriorly, thereby making it more vulnerable. Likewise, ligation of this artery, practiced by many surgeons, may be dangerous if the nerve is not identified first.
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Deformities from large thyroid nodules (6). In the presence of large nodules the laryngeal nerves may not be in their “correct” anatomic location but may be found even anterior to the thyroid (Fig. 5). Once more, there is no substitute for identification of the nerve in a gentle and careful manner.
Figure 4a. A) Normal anatomy of the recurrent laryngeal nerve. Note that on the right side the recurrent laryngeal nerve hooks around behind the subclavian artery, while on the left side this nerve passes around behind the aortic arch before ascending in the neck.
Figure 4b. B) When there is a vascular anomaly of the right subclavian artery, the recurrent laryngeal nerve no longer "recurs" around this artery but proceeds from the vagus nerve in a more transverse direction to the larynx. In such a situation, the nerve is much more likely to be damaged during operation unless care is taken to visualize its course in the neck. (From Skandalakis et al, with permission.)
Figure 5. Recurrent laryngeal nerve displacements by cervical and substernal goiters. Such nerves are at risk during lobectomy unless the surgeon anticipates the unusual locations and is very careful. Rarely, the nerves are so stretched that spontaneous palsy results. After careful dissection and preservation, functional recovery may occur postoperatively. (From Thompson NW, Demers M: Exposure is not necessary to avoid the recurrent laryngeal nerve during thyroid operations. In Simmons RL, Udekwu AO (eds): Debates in Clinical Surgery, Chicago, Year Book, 1990.)
On each side, the external branch of the superior laryngeal nerve innervates the cricothyroid muscle. In most cases, this nerve lies close to the vascular pedicle of the superior poles of the thyroid lobe (7), which requires that the vessels be ligated with care to avoid injury to it (Figure 6). In 21% the nerve is intimately associated with the superior thyroid vessels. In some patients the external branch of the superior laryngeal nerve lies on the anterior surface of the thyroid lobe, making the possibility of damage during thyroidectomy even greater (8). In only 15% of patients is the superior laryngeal nerve sufficiently distant from the superior pole vessels to be protected from manipulation by the surgeon. Unfortunately, many surgeons do not even attempt to identify this nerve before ligation of the upper pole vessels of the thyroid (9).
