A variant nerve that mimics the left recurrent laryngeal nerve: a case study in human anatomy

We describe a variant nerve in a human cadaver patient that parallels the course of the left recurrent laryngeal nerve (RLN). Like the normal left RLN, the variant nerve branches from the vagus nerve and wraps around the arch of the aorta, but it passes anterior and medial to the ligamentum arteriosum (= fetal ductus arteriosus) instead of behind it like the normal RLN. After recurring around the aorta, the variant nerve joins the esophageal plexus and also appears to connect to the cervical sympathetic chain. The bilaterally paired RLNs supply innervation not only to the larynx but also to the upper parts of the trachea and esophagus, in particular those parts derived from the 4^th and 6^th pharyngeal arches. We hypothesize that in this case, some of the nerve fibers to the trachea and esophagus were pulled down into the torso by the 4^th embryonic aortic arch (= the arch of the aorta in adults), but passed cranial to the 6^th embryonic aortic arch (= fetal ductus arteriosus). From where it recurs around the aorta to join the esophageal plexus, the variant nerve is very similar to the pararecurrent nerve in dogs, so there is at least a partial precedent in another placental mammal. Understanding the relationships of the embryonic pharyngeal and aortic arches and their adult derivatives is crucial for correctly identifying the RLN, especially when imposter nerves, like the one documented here, are present.