RLN Branching Patterns

inferior laryngeal nerve crosses posterior to the common carotid artery and enters the larynx posterior to the CT joint [13]. This aberrant pathway is explained by abnormal embryo-logical development. Normally, the recurrent laryngeal nerves are pulled caudad in the neck and chest around the sixth arches of the aorta. The left sixth arch remains as the ductus arteriosus, and later the ligamentum arteriosum. The right sixth arch normally resorbs, allowing the right RLN to pass inferior to the right fourth aortic arch, which becomes the subclavian artery. In rare cases, the right fourth aortic arch also resorbs during embryonic development. The consequence is that the right subclavian artery arises from the descending aorta and the right inferior laryngeal nerve tracks from the vagus directly to the larynx without looping aroundavascular structure [26-28].

As the RLN ascends in the neck it courses from lateral to medial until it reaches the tracheoesophageal (TE) groove prior to entering the larynx. Ardito et al. [29] studied 2626 RLNs during 1543 thyroidectomies over 10 years and found that 61.4% of right RLNs reach the TE groove and ascend in this sulcus, 37.8% remain lateral to the TE groove then enter the larynx, and 0.6% course anterolateral to the TE groove prior to entering the larynx. This variability, especially when the nerve travels anterolateral to the TE groove, may pose significant risk for iatrogenic injury.

The left recurrent laryngeal nerve branches from the main trunk of the vagus at the level of the aortic arch as it passes anterior to the branching points of the left common carotid and subclavian arteries [13]. The RLN loops beneath the aorta lateral to the ligamentum arteriosum which is a sixth aortic arch derivative [28]. The nerve then courses cranially and medially towards the larynx. As the RLN ascends in the left side of the neck it courses toward the TE groove. Ardito et al. found that 67.3% of nerves reach the TE groove, 31% remain lateral to the TE groove, and 1.6% course anterolateral to the TE groove [29]. There are only a few reports of non-recurrent left inferior laryngeal nerves in the literature; these have been associated with a right aortic arch with anomalous left subclavian artery, essentially the mirror image of the

The recurrent laryngeal nerve provides multiple branches along its course from the main trunk of the vagus. As the right RLN recurs around the subclavian artery and the left RLN recurs around the aorta, each supplies branches to the deep cardiac plexus [13]. During the ascent through the neck each nerve also supplies the trachea and esophagus with sensory and motor branches [30,31]. Finally, it supplies the cricopharyngeus muscle prior to entering the larynx where it provides motor function to the intrinsic laryngeal muscles except for CT and sensation to the glottis, subglottis, and proximal trachea. By the time the inferior laryngeal nerve courses around the CT joint and enters the larynx, it is often already divided into anterior and posterior branches.

Nemiroff and Katz [32] reviewed the ex-tralaryngeal branching patterns of 153 RLNs in patients undergoing thyroid or parathyroid surgery. They found that 41.2% of the nerves branched prior to entering the larynx. There was no difference in incidence among right and left sides; in 14.5% of patients bilateral branching was present. The branching occurred between 0.6 and 4.0 cm from the inferior border of the cricoid cartilage, although 73% of the nerves which branched did so between 1.2 and 2.3 cm from the cricoid cartilage. A later series by the same authors [33] reviewed 1177 RLNs in surgical patients. They found 63% of nerves bifurcated or trifurcated (11 nerves tri-

furcated) more than 0.5 cm from the inferior border of the cricoid cartilage, and 23.6% of all patients had bilateral bifurcation. Page et al. [34] had a lower incidence of branching in their series of 251 thyroidectomies (403 nerves) with 23.41% of right nerves branching and 15.15% of left nerves branching prior to entering the larynx. Moreau et al. [35] found 10 bifurcating RLNs in 34 dissected cadavers. Other investigators report RLN branching ranging from 30 to 100% of nerves [34-40]. It is not always clear in these reports whether branches to extralaryn-geal structures are considered when labeling a nerve as branching.

Most of the RLN branching prior to the larynx results in bifurcation. There are patients in whom there are more than two branches of RLN prior to entering the larynx. Nemiroff and Katz had only 4 nerves out of 153 which trifurcated [32]. Sun et al. [31] examined 50 cadaver larynges and found between two and five cervical branches of the RLN. There were two branches in 17%, 3 branches in 48%, 4 branches in 25%, and 5 branches in 10% of the nerves examined. It is important to note that the foregoing numbers refer to branches which terminate both within and outside of the larynx. Regarding branches of the RLN entering the larynx, Sun et al. [31] found 94 nerves with two or more branches (two nerves with four branches), and only six nerves with one trunk entering the larynx. Of the 100 nerves studied by Sun et al. [31], the branching pattern in 87 was of tree-branching type, and the remaining 13 nerves demonstrated anastomoses of multiple branches and/or anastomoses with branches from the cervical sympathetic chain to form a looptype configuration. In Ardito et al.'s review of 2626 RLNs, 1891 nerves branched, 1856 of the nerves bifurcated, 23 nerves trifurcated, and 14 nerves had multiple small branches before entering the larynx [29]. The discrepancy among these reports may be related to cadaver [31] vs intraoperative [29, 32] studies, although this is unlikely to fully account for the vast disparity among the multiple studies.

Previous investigators noted a predominance of anterior and posterior branching of the laryngeal portion of the RLN. These branches were designated motor and sensory, respectively, based purely on anatomical location [25]. Contemporary RLN anatomists prefer to classify the anterior and posterior branches as adductor and abductor branches; both likely contain sensory and motor fibers [17,18,41].

In light of the variation in incidence and type of extralaryngeal branching patterns, the surgeon's best approach to prevent inadvertent trauma to this crucial structure is to assume there is greater than one branch of the nerve. The pattern of branching may also play a role in the configuration of the true vocal fold in the setting of paresis or paralysis [32,33]. Whether one or more branches are injured would result in selective denervation with or without synkinesis of specific intrinsic muscles of the larynx. Whatever the case, it is certain that variability in neural anatomy and compensatory laryngeal adjustments make the relationship of vocal fold position to pattern of neural injury virtually impossible to decipher clinically. The Wagner-Grossman hypothesis, which related vocal fold position to the integrity of cricothyroid innervation, and Semon's law, which related it to different vulnerability of abductor and adductor innervation, dominated the clinical discourse on this subject for decades. Careful work has shown that both are invalid [42,43].

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