Definition

A vascular ring is an anomaly of aortic arch development that results in complete encirclement of the trachea and the esophagus by vascular structures. Some of the vascular components of the ring may not be patent but rather consist of fibrous remnants such as the ductal ligament or an atretic segment of the aorta [1].

Incidence

Because some vascular rings may not cause symptoms, their true prevalence is difficult to ascertain. They are clearly a rare anomaly and most evidence indicates that they represent approximately 1–3% of congenital cardiovascular anomalies [2]. In most but not all surgical case series, a double aortic arch is the most common vascular ring followed by a right aortic arch with an aberrant left subclavian artery and a left ductal ligament [3–8]. The latter type of vascular ring is likely the most common in the general population as it is typically looser and therefore patients may not meet criteria for surgery or may not be diagnosed. Usually, vascular rings are isolated abnormalities, although they may occur with other congenital heart disease, most commonly ventricular septal defect and tetralogy of Fallot [6].

Etiology

The etiology of most vascular rings is unknown. Chromosome 22q11.2 deletions are associated with isolated aortic arch anomalies, including vascular rings, and also with conotruncal cardiac defects and noncardiac abnormalities [9].

Morphology and classification

Developmental considerations

Knowledge of the morphogenesis of the aortic arch system provides a framework for understanding and classifying the wide variety of vascular rings [10,11]. In the early embryo, six paired arches form to connect the truncus arteriosus of the embryonic heart tube to the paired dorsal aortae, which fuse to form the descending aorta (Figure 32.1). In humans, the arches develop sequentially and persist or regress but are never all present simultaneously. In normal development, the first arches contribute to the external carotid arteries (Table 32.1). The second arches regress rapidly and only a portion remains to form the stapedial and hyoid arteries. The third arches become the common carotid arteries and the proximal portion of the internal carotid arteries. The left fourth arch forms that part of the definitive left aortic arch between the left carotid and left subclavian arteries. The right fourth arch is incorporated into the proximal right subclavian artery. The fifth arches regress. For the sixth arches, the proximal parts form the branch pulmonary arteries and the distal portions join the pulmonary vascular tree to the descending aorta via bilateral ductus, with the right usually regressing completely to leave a left ductus arteriosus. Between the sixth arches (the ductus) and the descending aorta are the paired dorsal aortae, which connect to the descending aorta. The seventh intersegmental arteries arise from the dorsal aortae and become the left subclavian and the distal right subclavian arteries. When the right dorsal aorta regresses, as is usual, the definitive left aortic arch is all that remains.

Embryonic structure	Outcome
Truncus arteriosus	Proximal ascending aorta and pulmonary root
Aortic sac	Distal ascending aorta, innominate artery, and arch to the origin of the left common carotid artery (proximal aortic arch)
1st arch	Portion of the external carotid artery
2nd arch	Portions of the hyoid and stapedial artery
3rd arch	Common carotid artery and proximal internal carotid artery
4th arch:
Left	Aortic arch segments between the left common carotid artery and left subclavian arteries (distal aortic arch)
Right	Proximal right subclavian artery
5th arch	Complete involution
6th arch:
Left	Proximal portion becomes the proximal left pulmonary artery; distal portion becomes the ductus arteriosus
Right	Proximal portion becomes the proximal right pulmonary artery; distal portion involutes
Left dorsal aorta	Aortic isthmus distal to the left subclavian artery
Right dorsal aorta	Cranial portion becomes the right subclavian artery distal to the contribution from the right 4th arch; distal portion involutes
Left 7th intersegmental artery	Left subclavian artery
Right 7th intersegmental artery	Distal right subclavian artery

In conjunction with embryologic anatomy, the hypothetical double‐arch model originally introduced by Edwards [12], and modified, simplified, and redrawn multiple times since, helps one to understand various arch anomalies (Figure 32.2). In this model, the ascending aorta divides into two arches, one passing to the right of the trachea and esophagus and the other to the left. These arches join posteriorly to form the descending aorta. From each arch there is a segment that gives rise to the right and left common carotids as the first branches on either side and the right and left subclavian arteries as the second branches on either side. A ductus arteriosus arises from the proximal aspect of each subclavian segment. This model allows anomalies of the aortic arch to be conceptualized as variations in regression of segments of the hypothetical double arch. A further simplified line drawing is easily sketched and can be used in daily practice to depict almost every known arch anomaly (Figure 32.3).

Anatomy

Double aortic arch

In a double aortic arch, both of the embryonic right and left arches persist, arising from the ascending aorta, passing on both sides of the trachea and esophagus, and joining posteriorly to form the descending aorta, thereby completely encircling the trachea and esophagus (Figures 32.3a, 32.4, and 32.5, Video 32.1–32.3). A ductal ligament usually contributes to the ring and is most often left‐sided. The carotid and subclavian arteries arise separately from each arch and are usually symmetrically positioned around the trachea. The right arch is larger than the left in approximately 75% of cases and also typically higher [13]. Occasionally, a segment of an arch (usually the left) may be atretic with a fibrous cord either between the carotid and subclavian arteries or distal to the left subclavian artery. In such cases, the aortic arch branching pattern evident on imaging studies may mimic other aortic anomalies.

Right aortic arch with an aberrant left subclavian artery

In the hypothetical double‐arch paradigm, a right aortic arch with an aberrant left subclavian artery is the result of regression of the left aortic arch segment between the left common carotid and subclavian segments (Figure 32.3e). As a result, the left subclavian artery originates as the last branch from the aortic arch, at a relatively posterior location, coursing behind the esophagus to the left arm. A left ductal ligament originates from a bulbous dilation at the base of the left subclavian artery (termed the diverticulum of Kommerell) and attaches to the proximal left pulmonary artery, effectively pulling the diverticulum and left pulmonary artery towards each other, compressing the esophagus and trachea and forming a vascular ring (Figure 32.6, Video 32.4–32.6). Rarely, the ductus arteriosus is right‐sided and connects the right pulmonary artery to the right‐sided aortic arch and thus no vascular ring is formed. In such cases, there is no diverticulum of Kommerell and the caliber of the left subclavian artery is uniform throughout.

Right aortic arch with mirror image branching and a left ductus arteriosus

A right aortic arch with mirror image branching has the first branch off the arch as the innominate artery followed by the right carotid and the right subclavian arteries (Figure 32.3d, Video 32.7). Typically, this is the end to the mirror image symmetry of the normal left aortic arch as the ductus arteriosus usually is left‐sided, arising anteriorly from the base of the innominate artery (Video 32.8 and 32.9) rather than posteriorly from the descending aorta. Less frequently, the ductus arises from the descending aorta and connects to the proximal right pulmonary artery, yielding the true mirror image of the normal left aortic arch. Because in both of these cases there is no left‐sided encirclement of the trachea and esophagus, neither forms a vascular ring. On occasion, however, the ductus or ligament arises from the right‐sided descending aorta, stemming from a retroesophageal diverticulum, courses leftwards, and then connects to the proximal left pulmonary artery. A vascular ring is thereby formed from the right aortic arch, retroesophageal ductal diverticulum, and left‐sided ductal ligament (Figure 32.7). Embryologically, this ring results from regression of a segment of left dorsal aorta between the left seventh intersegmental artery (destined to become the subclavian artery) and the left sixth arch (destined to become the ductus arteriosus) (Figure 32.3f).

Right aortic arch with a left descending aorta and a left ductus arteriosus

In this type of vascular ring, there is a right aortic arch that curves leftwards to pass posterior to the esophagus and trachea and then joins a left‐sided descending aorta. A left ductus arteriosus or ductal ligament connects the descending aorta and the proximal left pulmonary artery, completing the vascular ring (Figure 32.8, Video 32.10–32.12). Note that unlike the more common type of right aortic arch in which the aorta descends to the right of the spine for some distance before gradually crossing to the left of the spine at about the level of the diaphragm, in this anomaly the aortic arch itself crosses to the left and is posterior to the esophagus, after which it gives rise to the ductus arteriosus. Because of the course of the aorta, some authors have called this a “circumflex retroesophageal right aortic arch.” The arch vessel branching pattern may be either the left carotid artery followed by the right carotid, right subclavian, and left subclavian arteries or the left innominate followed by the right carotid and right subclavian arteries. Developmentally, depending on the branching pattern, this anomaly results from either regression of the left fourth arch or left dorsal aorta distal to the left subclavian artery (Figure 32.3g). The distal arch is composed of the retroesophageal right dorsal aorta and the persistent left sixth arch (ductus) completes the ring.

Left aortic arch with a right descending aorta and right ductus arteriosus

This anomaly is nearly a mirror image version of the right aortic arch with a left descending aorta and left ductus arteriosus as mentioned in the previous section, but is less common. A left aortic arch courses posterior to the esophagus and trachea and to the right, leading to a right‐sided descending aorta. A ductus or ductal ligament connects the descending aorta and the proximal right pulmonary artery, completing the vascular ring (Figure 32.9). Developmentally, this anomaly results from the regression of the right fourth arch (segment between the right common carotid and right subclavian segments), persistence of the left dorsal aorta passing retroesophageally to a descending aorta beginning to the right of the spine, and persistence of the right sixth arch (Figure 32.3h).

Pathophysiology

Vascular rings can cause varying degrees of compression of the trachea and esophagus. Mild tracheal compression may be asymptomatic. More significant involvement in younger patients may manifest as stridor, dyspnea, and a barking cough, all of which are worse during feeding or exertion [4–6,8]. “Reflex” apnea lasting seconds or even minutes may be triggered by feeding. Older children may have a history of chronic cough or wheezing, sometimes misdiagnosed as asthma. Symptoms related to esophageal compression are less frequent and less well defined. They include vomiting, choking, and nonspecific feeding difficulties in infants, and dysphagia and slow eating in older children.

Imaging

The diagnosis of vascular rings requires a high index of suspicion because of the relative infrequency of this entity compared with other conditions that cause respiratory distress in children, such as asthma, respiratory infection, and gastroesophageal reflux. Once a vascular ring is suspected, diagnostic imaging studies should be obtained with the goals of (i) identifying the cause of a patient’s symptoms by demonstrating the relevant vascular and airway anatomy and (ii) preoperative planning. If video‐assisted thoracoscopic surgery is being considered, it is important to determine whether the ring can be released by dividing ligamentous (nonpatent) or hypoplastic portions. This requires the diagnostician to have a high level of certainty regarding vessel caliber and patency throughout the ring. For both thoracoscopic and open surgery, the diagnosis must be established confidently enough to determine whether a right, left, or midline approach is appropriate.

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