4th aortic arch

The aortic arch and its vessels are derived from the pharyngeal arch arteries (arches 3-6 in embryonic development). The 4th pharyngeal arch artery is critical: the LEFT 4th arch forms the aortic arch (between left common carotid and left subclavian origins); the RIGHT 4th arch normally regresses in humans but forms the right subclavian artery (anomalously, when it arises from the distal aortic arch on the right, it creates an aberrant right subclavian artery—ARSA). Abnormalities of the 4th arch create vascular rings and slings: (1) Double aortic arch (both right and left 4th arches persist)—forms a ring around esophagus/trachea; presents with dysphagia (difficulty swallowing solid foods, food impaction) and/or tracheal compression (stridor, respiratory symptoms); (2) Right aortic arch with left ligamentum arteriosum—the aortic arch arises on the right (right 4th arch forms arch) with the left subclavian arising as the last branch from the descending aorta; the left ligamentum arteriosum (remnant of left 6th arch/ductus arteriosus) connects the pulmonary artery to descending aorta, forming a ring; (3) Aberrant right subclavian artery—anomalous origin of right subclavian from distal aortic arch, passes behind esophagus; usually asymptomatic but can cause dysphagia if symptomatic. These anomalies are detected by imaging (chest X-ray showing right aortic arch, CT/MRA, esophagography with barium showing impressions on esophagus/trachea). Diagnosis: imaging + endoscopy/bronchoscopy. Treatment: surgical division of ring/sling if symptomatic. On boards: vascular rings are rare but tested; know that 4th arch abnormalities cause them and present with dysphagia/stridor.