fourth pharyngeal arch

**Overview** Pharyngeal (branchial) arches form in weeks 4-5 of development. Each arch has a core of mesenchyme (neural crest and mesoderm) and contains a cartilage, muscle, nerve, and aortic arch artery. The 4th arch is the first of the arches that merges with the 6th (the 5th regresses), so the 4th and 6th together form the larynx.

**Derivatives (mnemonic: 4th arch = "Superior laryngeal", 6th = "Recurrent laryngeal")** - **Cartilage:** Thyroid cartilage, cricoid cartilage, arytenoid, corniculate, and cuneiform cartilages. Some sources assign the cricoid, arytenoid, corniculate, and cuneiform to the 6th arch. The thyroid cartilage is the key 4th arch structure. The epiglottis is also attributed to the 4th arch in some sources. - **Muscles:** Most pharyngeal constrictors, cricothyroid, and levator veli palatini. (Mnemonic: "Constrictors, Cricothyroid, Levator veli palatini".) Remember that the 4th arch muscles are not the stylopharyngeus (3rd arch, CN IX) and not the intrinsic laryngeal muscles other than cricothyroid (6th arch, recurrent laryngeal nerve). - **Nerve:** Superior laryngeal branch of the vagus nerve (CN X). The external branch supplies the cricothyroid muscle. The internal branch is sensory above the vocal folds. - **Artery (aortic arch):** - Left: arch of the aorta (between the left common carotid and left subclavian arteries). - Right: proximal part of the right subclavian artery.

**Clinical correlations** - **Superior laryngeal nerve injury** (for example, during thyroidectomy when ligating the superior thyroid artery) causes loss of cricothyroid function. Result: voice fatigue, inability to reach high pitch, and monotone voice. The recurrent laryngeal nerve (6th arch) injury instead causes hoarseness and, if bilateral, airway compromise. - **Aberrant right subclavian artery (arteria lusoria):** Caused by abnormal involution of the right 4th aortic arch and right dorsal aorta, with persistence of the distal right dorsal aorta. The artery arises from the left side and passes behind the esophagus, causing possible dysphagia (dysphagia lusoria). - **Right-sided aortic arch / double aortic arch:** Abnormal persistence or regression of 4th arch segments. A double aortic arch forms a vascular ring that can compress the trachea and esophagus (stridor, dysphagia in infants). - **Interrupted aortic arch (type B):** Interruption between the left common carotid and left subclavian arteries (4th arch region). It is associated with DiGeorge syndrome (22q11.2 deletion), in which the 3rd and 4th pharyngeal pouches also fail to develop (thymic aplasia, hypocalcemia from absent parathyroids). - **Pharyngeal pouch relationship:** The 4th pharyngeal pouch forms the superior parathyroid glands (and the ultimobranchial body/parafollicular C cells). The 3rd pouch forms the thymus and inferior parathyroid glands.

**High-yield comparisons** - 1st arch: CN V (V2/V3), Meckel cartilage, muscles of mastication; maxillary artery. - 2nd arch: CN VII, Reichert cartilage, muscles of facial expression; stapedial artery and hyoid artery. - 3rd arch: CN IX, greater horn of hyoid, stylopharyngeus; common carotid and proximal internal carotid. - 4th arch: CN X (superior laryngeal), laryngeal cartilages, pharyngeal constrictors, cricothyroid, levator veli palatini; aortic arch and right subclavian. - 6th arch: CN X (recurrent laryngeal), intrinsic laryngeal muscles (except cricothyroid); proximal pulmonary arteries and ductus arteriosus.

**Board pearls** - 4th arch = superior laryngeal nerve = cricothyroid; 6th arch = recurrent laryngeal nerve = all other intrinsic laryngeal muscles. - The right recurrent laryngeal nerve hooks around the right subclavian artery (4th arch derivative), and the left hooks around the ligamentum arteriosum (6th arch derivative). - A non-recurrent laryngeal nerve can accompany an aberrant right subclavian artery.