sixth pharyngeal arch

**Embryology overview** Each pharyngeal arch has a core of mesoderm/neural crest mesenchyme with its own cartilage, muscle, nerve, and aortic arch artery. The 6th arch develops around weeks 4–5 and is the most caudal arch. The 5th arch never forms or regresses in humans.

**Derivatives of the 6th arch (mnemonic: "Arches 4 and 6 = larynx and vagus")** - **Nerve:** Recurrent laryngeal nerve (branch of CN X). Supplies motor to all intrinsic laryngeal muscles except the cricothyroid, and sensation below the vocal folds. - **Muscles:** All intrinsic muscles of the larynx except the cricothyroid. These include the posterior cricoarytenoid (the only vocal cord abductor), lateral cricoarytenoid, transverse and oblique arytenoids, thyroarytenoid, and vocalis. - **Cartilages:** Cricoid, arytenoid, corniculate, and cuneiform cartilages. The thyroid cartilage comes from the 4th arch, with some 3rd/4th arch contribution depending on the source. The 4th and 6th arch cartilages fuse. - **Aortic arch artery:** The proximal part forms the proximal pulmonary arteries. On the left, the distal part forms the ductus arteriosus. On the right, the distal part degenerates.

**Contrast with the 4th arch** - The 4th arch nerve is the superior laryngeal nerve (CN X), whose external branch supplies the cricothyroid. It also supplies the pharyngeal constrictors, levator veli palatini, and palatopharyngeus (the palatoglossus is a CN X-innervated muscle, but it is a tongue muscle). - The 4th arch artery forms the left aortic arch and the right proximal subclavian artery.

**Why the recurrent laryngeal nerve "recurs"** The nerves travel with their arch arteries. As the heart and great vessels descend, the 6th arch artery pulls the recurrent laryngeal nerve down with it. - **Left RLN:** Loops under the ligamentum arteriosum (the 6th arch remnant of the ductus) and the aortic arch. - **Right RLN:** Loops under the right subclavian artery. The right 6th arch distal artery regresses, so the nerve hooks around the 4th arch derivative instead. - **Non-recurrent laryngeal nerve:** The right RLN can be non-recurrent when there is an aberrant right subclavian artery (arteria lusoria), because the 4th arch artery is lost and the nerve passes directly to the larynx.

**Clinical significance** - **Recurrent laryngeal nerve injury** causes hoarseness. It occurs with thyroidectomy, aortic arch aneurysm, left atrial enlargement (Ortner syndrome), and mediastinal or lung (apical) tumors. Unilateral injury causes hoarseness. Bilateral injury causes stridor and airway compromise from unopposed cricothyroid action (adducted cords). - **Patent ductus arteriosus (PDA):** Failure of the left 6th arch distal artery to close after birth. It causes a continuous machine-like murmur. It is associated with congenital rubella and prematurity. Treat with indomethacin or ibuprofen (inhibit prostaglandins) to close it. Prostaglandin E1 keeps it open in ductal-dependent lesions. - **Ligamentum arteriosum** is the fibrous remnant of the ductus arteriosus. - **Vascular rings** involve abnormal persistence of arch arteries (usually the double aortic arch, from failed regression of the right 4th arch). They cause dysphagia and stridor. - **Pharyngeal arch syndromes:** Mostly involve the 1st/2nd arches (Treacher Collins, Pierre Robin). 3rd/4th pouch defects cause DiGeorge syndrome (22q11 deletion).

**High-yield pearls** - Arch derivatives are tested by nerve: 1st = CN V3, 2nd = CN VII, 3rd = CN IX, 4th = CN X (superior laryngeal), 6th = CN X (recurrent laryngeal). - Arches contain: cartilage (skeletal), muscle, nerve, and artery. The pouches (endoderm) and clefts (ectoderm) are separate structures. - Posterior cricoarytenoid = "safety muscle" of the larynx, the only abductor of the vocal cords. It is lost in RLN injury.