Morphology and Morphogenesis of Individual Anomalies Double aortic arch is the tightest form of vascular ring.1–10 It refers to the presence of two aortic arches, one on each side of the trachea and esophagus (Fig 47.3) Both the left and right aortic arches of the hypothetical model persist, without regression of any segment An arterial duct, more frequently the left than the right, persists, although cases with bilateral ducts have rarely been described.11 During fetal life, when the arterial duct is patent, the composite arrangement of the two arches and a patent arterial duct produces a “9” or “6” configuration at fetal echocardiography.12,13 Each aortic arch gives rise to common carotid and subclavian arteries In the majority of the cases with double aortic arch, both arches are patent Usually the right arch is larger than the left arch, or less commonly, the two arches are equally sized The left arch is dominant in less than 20% of cases In general, the apex of the larger arch is higher than the smaller arch Occasionally, a segment of one arch may be atretic, mostly on the left The atretic segment is almost always distal to the subclavian artery, although an atretic strand may also be found between the common carotid and subclavian arteries The atretic segment cannot be visualized by any imaging modality Therefore it is difficult to differentiate a double aortic arch with an atretic segment distal to the origin of the left subclavian artery from a right aortic arch with mirror-image branching Similarly, the double aortic arch with an atretic segment between the origins of the left common carotid and left subclavian arteries is difficult to differentiate from the right aortic arch with aberrant left subclavian artery and left arterial duct In the setting of a double aortic arch, the subclavian and common carotid arteries that arise from the patent and atretic arches almost always show a symmetric arrangement.14 The patent part of the atretic left aortic arch tends to have a more posterior position than the left brachiocephalic artery arising from the right aortic arch An inferior kink of the proximal part of the common trunk for the subclavian and common carotid arteries in the presence of a diverticular outpouching from the descending aorta is a telltale sign of the presence of an atretic segment between the kink and the apex of the diverticulum.15 The descending aorta is left sided in just over twothirds of patients with double aortic arch, being right sided in almost all the rest and only rarely occupying a neutral midline position FIG 47.3 Computed tomograms showing a complete double arch, are seen from behind and above (A) and below (B) The double arch encircles the trachea and esophagus, with the right arch dominant The reformatted image in the coronal plane (C) shows narrowing of the trachea due to compression by the dominant right aortic arch (RAA) The trachea is slightly bent to the left LCCA, Left common carotid artery; LPA, left pulmonary artery; LSA, left subclavian artery; RCCA, right common carotid artery; RPA, right pulmonary artery; RSA, right subclavian artery Right aortic arch with aberrant left subclavian artery results from abnormal persistence of the right aortic arch and abnormal regression of the left arch between the origins of the left common carotid and left subclavian arteries, the left subclavian artery taking its origin from the distal part of the left aortic arch (Figs 47.4A–C and 47.5A–B) The distal remnant of the left aortic arch, along with the aberrant left subclavian artery, produce the retroesophageal component of the ring It has previously been described that the aberrant artery may course either between the trachea and esophagus or in front of the aorta.1 It is currently usually believed that arteries that do not take a retroesophageal course are collateral arteries.2 The persistent arterial duct is usually left sided, connecting the left pulmonary artery to the distal remnant of the left aortic arch.1–7,10,16 This combination is the second most common type of ring reported in most series During fetal life, when the arterial duct is widely patent, this combination is characterized by a U-shaped vascular loop that encircles the trachea and esophagus from behind.12,17–20 This U-shaped loop consists of the ascending aorta, right aortic arch, distal remnant of the left aortic arch, left-sided arterial duct, and pulmonary trunk Although the vascular loop looks open anteriorly, a vascular ring is completed by the underlying heart This configuration changes dramatically with closure of the arterial duct after birth The left limb of the Ushaped loop disappears with ductal closure, while the distal remnant of the left aortic arch persists as a diverticular outpouching, with the left subclavian artery arising from its apex The diverticular outpouching is called the diverticulum of Kommerell.21–24 Flow through this distal remnant is from the left-sided arterial duct into the descending aorta in the fetal circulation but switches its direction with ductal closure so that the aberrant left subclavian artery is supplied from the descending aorta in postnatal circulation Therefore the presence of a diverticulum of Kommerell postnatally is indicative of the presence of an arterial ligament between the apex of the diverticulum and the left pulmonary artery This vascular ring is usually not as tight as that produced by the double aortic arch The severity of the esophageal and, to a certain extent, tracheal compression varies with the size of the diverticulum When this type of anomaly is associated with significant obstruction of the pulmonary outflow tract, as in tetralogy of Fallot, the diverticulum of Kommerell may be absent or inconspicuous This is because the flow of blood through the left arterial duct was reduced, or even reversed, during fetal life Therefore the distal remnant of the left aortic arch does not persist as a diverticular outpouching after ductal closure.12 Postnatally, an arterial ligament is suspected when the proximal left subclavian artery is tethered inferiorly toward the left pulmonary artery The right-sided aortic arch with aberrant origin of the left subclavian artery is occasionally associated with persistence of the right arterial duct or even absence of arterial ducts bilaterally The latter combination is typically seen in tetralogy of Fallot with pulmonary atresia and pulmonary arterial supply via major aortopulmonary collateral arteries This combination forms an incomplete encirclement around the right side of the trachea and esophagus The right aortic arch with aberrant origin of the left brachiocephalic artery is rare.25,26 It results from abnormal regression of the left aortic arch proximal to the origin of the left common carotid artery The persisting arterial duct is usually left sided, completing a vascular ring