Terminology

The aorta consists of thoracic and abdominal components. There are 5 segments of the thoracic aorta. The aortic root is a short segment that includes the aortic valve, the annulus, and the sinuses of Valsalva. The ascending aorta extends from the root to the origin of the right brachiocephalic artery. The proximal aortic arch is the segment from the origin of the right brachiocephalic artery to the left subclavian artery. The posterior aortic arch (the aortic isthmus) extends from the origin of the left subclavian artery to the ligamentum arteriosum. This portion of the arch is often slightly narrower than the descending aorta. The descending thoracic aorta begins at the ligamentum arteriosum and ends at the diaphragm. The proximal aspect of the descending aorta is sometimes slightly prominent relative to the adjacent segments. Mild localized bulging of the posterior aortic arch or the superior aspect of the descending aorta is termed the aortic spindle. The abdominal aorta consists of suprarenal and infrarenal segments.

Vascular Rings

The most common vascular rings are double aortic arch and right aortic arch with an aberrant left subclavian artery. Unusual arch anomalies that can cause a complete vascular ring include circumflex aorta, right aortic arch with mirror image branching and a left-sided ligamentum arteriosum, and left aortic arch with an aberrant right subclavian artery and a right-sided ligamentum arteriosum (Table 12-1).¹

The most important determinants of the clinical presentation of a vascular ring are the severity of tracheal compression and the occurrence of associated anomalies such as a congenital heart lesion. Some patients present in the neonatal period with life-threatening respiratory distress. At the other end of the spectrum are patients who remain asymptomatic into adulthood, with the lesion discovered incidentally. Symptoms are present during infancy in most patients with double aortic arch or right aortic arch with a left ductus arteriosus. Pulmonary artery sling (origin of the left pulmonary artery from the right pulmonary artery) is another great vessel anomaly that, although not a vascular ring, is usually symptomatic during infancy.

The most common clinical manifestations of a vascular ring are stridor and wheezing. The stridor is most severe during inspiration. Severely affected infants tend to hold their neck hyperextended. Physical activity may exacerbate the respiratory symptoms. These patients are prone to recurrent lower respiratory tract infections. Dysphagia is a less consistent finding in patients with an arch anomaly. Infants with a tight vascular ring may have poor feeding, fatigability, or episodes of aspiration. Older patients occasionally present with food impaction in the esophagus at the site of the vascular compression.¹

The imaging evaluation of the child with a suspected arch anomaly generally consists of standard chest radiographs, esophagography, and MR or CT. The location of the aortic arch can usually be demonstrated on properly positioned frontal chest radiographs. The arch appears as a soft-tissue density aortic knob that causes slight indentation on the tracheal air column. The presence of a vascular ring is suggested on chest radiographs by abnormal narrowing of the inferior aspect of the trachea, anterior bowing of the trachea as viewed on the lateral projection, and thickening of the retrotracheal soft tissues. Esophagography provides important anatomic information with regard to compression of the esophagus by components of the aorta, anomalous great vessels, and the ductus arteriosus or ligamentum arteriosum. A vascular ring is indicated by abnormal esophageal compression in association with tracheal narrowing. MR or CT provides specific depiction of the pathological vascular anatomy.^1–3

Double Aortic Arch

Double aortic arch is the most common symptomatic vascular ring. This anomaly is a result of embryological persistence of both the left and right fourth aortic arches. The ascending aorta divides anterior to the trachea into right and left limbs that encircle the trachea and esophagus. The ascending portion of the arch is anterior to the trachea, and the descending portion courses posterior to the esophagus. Vascular structures thereby encircle the esophagus and trachea. The clinical manifestations of double aortic arch predominantly relate to tracheal obstruction and associated tracheomalacia. There are no significant hemodynamic sequelae.

In approximately 75% of patients with double aortic arch, the right arch is dominant and the descending aorta is on the left. The right arch in this situation usually extends more superiorly than the left arch. The larger right arch curves behind the esophagus to join the smaller left arch and form a left-sided descending aorta. A dominant left arch with a right descending aorta occurs in approximately 20% of patients with a double arch. The arches are approximately equal in size in approximately 5% of patients. The ductus arteriosus in patients with double aortic arch is most often left-sided. The ductus/ligamentum is not an integral component of the vascular ring, however.¹

In some patients with double aortic arch, there is regression of at least a portion of the left-sided arch (double aortic arch with partial atresia of the left arch). The segments remain in fibrous continuity, thereby maintaining an anatomic vascular ring. The descending aorta in these patients is frequently on the right. A portion of the left aortic arch sometimes persists and forms a diverticulum that gives rise to the left common carotid artery and left subclavian artery. An additional potential variation is left arch atresia between the left common carotid and left subclavian arteries, resulting in origins of the left common carotid artery from the ascending aorta and the left subclavian artery from an aortic diverticulum. Atresia of the right side of a double arch can occur, but is rare.

The severity of symptoms in patients with double aortic arch is variable, with the age at presentation largely determined by the degree of tracheal narrowing. Those patients who are symptomatic as infants most often present with wheezing, stridor, and respiratory distress, which are sometimes severe. Apnea and cyanosis may occur. Older children may present with wheezing, stridor, chronic cough, dyspnea, and recurrent pneumonia. The cough in children with double aortic arch has a harsh, brassy character (“seal bark”). Dysphagia is not a major characteristic in the clinical presentation of double aortic arch. Approximately 20% of patients with double aortic arch have associated cardiac anomalies, most often cyanotic. The most commonly associated lesions are tetralogy of Fallot and ventricular septal defect. Other associations include common arterial trunk, tricuspid atresia, and D-transposition of the great arteries. Approximately 15% of patients with double aortic arch have a chromosome band 22q11 deletion. There is a weak association between double aortic arch and the VACTERL (vertebral, vascular, anal, cardiac, tracheoesophageal, renal, limb) and CHARGE (coloboma, heart anomaly, choanal atresia, retardation, genital, ear) syndromes.^1,2

Frontal and lateral chest radiographs of patients with double aortic arch usually show narrowing of the trachea just above the level of the carina. Tracheal obstruction sometimes leads to symmetric hyperinflation. On the lateral view, there is anterior bowing of the mid to distal portion of the thoracic segment of the trachea and at least some degree of narrowing. Lateral radiographs may also show prominence of the retrotracheal soft tissues because of the anomalus vessel (Figure 12-1). On the frontal view, bilateral soft-tissue densities represent the aortic arches, with the bulge on the right usually larger than that on the left (Figure 12-2). In other patients, the nondominant (usually the left) arch is too small to produce a radiographic density. In this situation, the radiographic appearance may mimic that of a right-sided arch with a left descending aorta. However, if substantial tracheal narrowing is observed, the finding should be considered suspicious for a vascular ring.²

Esophagography of patients with double aortic arch shows prominent bilateral and posterior esophageal indentations. On the frontal view, the indentations sometimes have the appearance of a reverse S: the upper curve of the S is produced by the large right arch, while the lower curve is a result of the smaller left arch. The prominent posterior indentation of the esophagus viewed on a lateral or steep oblique projection represents the retroesophageal component of the arch. This indentation is more prominent and more horizontal than that of an anomalous right subclavian artery with a left aortic arch. The presence of a complete ring is confirmed by visualization of concomitant tracheal narrowing anteriorly (Figure 12-3).^4,5

MRI or contrast-enhanced CT provides excellent depiction of the pathological vascular anatomy in patients with double aortic arch, and demonstrates the effects on the airway (Figure 12-4). On axial images just superior to the arch, the subclavian and carotid arteries produce a characteristic appearance, with these 4 vessels evenly spaced around the trachea; this is termed the 4 artery sign.³ This finding is not, however, specific to double aortic arch, as it also occurs with other arch anomalies that have independent origins of these 4 vessels. In patients with partial atresia of the left arch, CT and MR show lack of patency of the atretic segment. As described above, the patterns of origin for the left common carotid artery and left subclavian artery are variable in these patients, depending on the site of atresia.

Left Aortic Arch

Anomalous Right Subclavian Artery

A left aortic arch with an anomalous origin of the right subclavian artery (aberrant right subclavian artery; arteria lusoria) is the most common malformation of the great vessels. The prevalence of this anomaly in the general population is approximately 0.5%. The configuration of the aortic arch is normal. The anomalous right subclavian artery arises as the last branch of the arch and courses obliquely from left to right across the midline dorsal to the esophagus. Rarely, the anomalous subclavian artery courses between the trachea and the esophagus; these patients usually have coexistent cardiac anomalies.

An anomalous right subclavian artery in conjunction with a normal left aortic arch is nearly always asymptomatic, and can be considered an inconsequential developmental variation. There may be rare instances in which this anomaly is associated with mild dysphagia in adults (dysphagia lusoria). There also are rare instances in which the combination of an aberrant right subclavian artery and a tortuous right common carotid artery causes tracheal compression in children. An anomalous right subclavian artery sometimes occurs in those tetralogy of Fallot patients who have a left aortic arch. When an anomalous right subclavian artery occurs in patients with coarctation of the aorta, it often arises distal to the site of the coarctation. This results in an elevated blood pressure in the left upper extremity and decreased blood pressure in the right upper extremity. The blood pressure in the lower extremities is similar to that of the right upper extremity in these patients.

Anomalous right subclavian artery with a left aortic arch is most often detected as an incidental finding during esophagography performed for another purpose. On the frontal view, the subclavian artery produces a characteristic oblique esophageal indentation that is lower on the left and higher on the right. This indentation often parallels the course of the left main bronchus and is located 2 to 3 cm above the bronchus. The lateral view shows a posterior indentation on the esophagus (Figure 12-5). This anomaly is differentiated from a vascular ring or other more significant arch anomaly by the lack of substantial lateral esophageal impressions, the mild degree of the posterior esophageal impression, the characteristic oblique course, and the lack of associated tracheal narrowing. CT and MR demonstrate the vascular anatomy in patients with anomalous right subclavian artery, and serve in selected patients to exclude the presence of a vascular ring (Figure 12-6).⁶

Left Circumflex Aortic Arch

Left circumflex aortic arch is an extremely rare anomaly in which the aorta ascends to the left of the trachea, crosses dorsal to the esophagus, and descends to the right of the spine. If the ductus arteriosus/ligamentum arteriosum is right-sided, a vascular ring results. The components of the vascular ring in this situation are the left-sided segment of the aortic arch (on the left), the dorsal retroesophageal portion of the arch (posteriorly), the ductus arteriosus (on the right), and the pulmonary artery (anteriorly). The anomalous right subclavian artery that is usually present in these patients does not form part of the ring. However, if there is a diverticulum of Kommerell (at the aortic origin of the vessel), this can contribute to compression of the airway. The major importance of an anomalous right subclavian artery in patients with left circumflex aortic arch relates to its association with a right-sided ductus arteriosus.^7–9

The embryological basis for left circumflex aortic arch likely involves preservation of the proximal left fourth arch, persistence of a segment of the right sixth arch (right-sided ductus arteriosus), and persistence of the distal portion of the right fourth arch (the right dorsal arch). The distal portion of the right dorsal aortic arch may persist in these patients as the diverticulum of Kommerell, from which the right subclavian artery arises.

The presence of, and severity of, symptoms in patients with left circumflex aortic arch relate to the specific anatomy and the completeness of the vascular ring. Those patients with a right-sided ductus or ligamentum arteriosum have a complete ring, and may have feeding difficulties and respiratory distress as infants. The distal portion of the trachea is displaced to the left and the right main bronchus may be compressed. A hypoplastic right lung sometimes accompanies this anomaly. If the ductus is located on the left, the ring is incomplete; these patients may be asymptomatic or suffer mild dysphagia as a consequence of compression of the esophagus by the retroesophageal segment of the aortic arch.

The characterizing features of left circumflex aortic arch on chest radiographs are a descending aorta on the right side of the spine and the aortic knob located to the left of the trachea. The right lung is small in many of these patients. The retroesophageal segment of the aorta produces a large posterior indentation on the esophagus that is readily demonstrable with esophagography. Frequently, the esophageal impression follows an oblique course, from a more cephalad location on the left to a caudad orientation on the right. There is some degree of rightward displacement of the trachea and esophagus, as well. MR shows the horizontal segment of the aortic arch to pass posterior to the trachea and esophagus, with the descending portion coursing inferiorly to the right of the midline. Although the ligamentum arteriosum usually cannot be visualized on MR, the presence of an aberrant right subclavian artery (arising as the last arch vessel) indicates that the ductus is on the right and that a complete ring is present.¹⁰

Right Aortic Arch

Right Aortic Arch With an Aberrant Left Subclavian Artery

Right aortic arch with an aberrant left subclavian artery is one of the more common forms of vascular ring. The aortic arch ascends anterior and to the right of the trachea, then courses posterior to the esophagus, and descends to the left of the spine or at the midline. The first branch of the arch is the left common carotid artery, followed by the right common carotid artery and then the right subclavian artery. The left subclavian artery arises anomalously from the descending portion of the aorta. In many patients, the origin and proximal segment of the left subclavian artery are enlarged; this is termed the diverticulum of Kommerell. Embryologically, this represents the posterior portion of the otherwise obliterated left aortic arch. The ductus arteriosus in patients with this anomaly nearly always arises from the descending aorta, the left aortic diverticulum, or the proximal aspect of the left subclavian artery. The ductus communicates with the left pulmonary artery. Although the anomalous left subclavian artery does cause some degree of compression along the posterior wall of the esophagus, it is the ductus arteriosus or ligamentum arteriosum that actually completes the vascular ring of this anomaly.¹

The vascular ring produced by a right aortic arch with an aberrant left subclavian artery and a left ligamentum arteriosum is relatively loose, and many of these patients are asymptomatic. Those patients who are symptomatic tend to have milder manifestations than those patients with a double aortic arch. Potential findings of tracheal or esophageal narrowing include wheezing, stridor, cough, and recurrent respiratory infection. Older children may complain of dysphagia alone.

Standard chest radiographs of children with right aortic arch and aberrant left subclavian artery show the position of the arch as a right-sided indentation in the tracheal air column. The aortic knob is usually located somewhat higher than is typical for a normal left aortic knob. Occasionally, there is a round density along the left upper mediastinum, slightly below the level of the aortic knob; this represents the diverticulum of Kommerell or a portion of the descending aorta. The lateral chest radiograph frequently shows anterior bowing and/or narrowing of the inferior aspect of the trachea.²

Esophagography of patients with right aortic arch and an anomalous left subclavian artery demonstrates a pronounced posterior indentation on the esophagus, best visualized on the lateral projection (Figure 12-7). The ligamentum arteriosum, diverticulum of Kommerell, and aberrant left subclavian artery all contribute to this posterior indentation. On the frontal view, there is an impression along the right side of the esophagus because of the right aortic arch. The presence of a ring is indicated by an indentation along the left side of the esophagus at approximately the same level. The left-sided indentation is a result of the diverticulum of Kommerell, the aberrant left subclavian artery, and/or the ligamentum arteriosum. This constellation of findings often produces a “reverse S” configuration, similar to that of double aortic arch. The lateral esophageal indentations tend to be less pronounced with this anomaly than with double aortic arch. Also, tracheal narrowing is mild or absent in most patients with a right arch and aberrant left subclavian artery.¹¹

MR and CT provide a definitive diagnosis of right aortic arch with aberrant left subclavian artery. The right-sided location of the arch is easily appreciated on cross-sectional imaging studies (Figure 12-8). A key finding is the anomalous origin of the left subclavian artery as the last branch of the arch. When present, the diverticulum of Kommerell appears as an outpouching from the arch at the origin of the left subclavian artery (Figure 12-9). The ligamentum arteriosum is not visible on MR or CT; if the ductus arteriosus is patent, it can sometimes be demonstrated on MR angiography or contrast-enhanced CT. MR and CT generally provide accurate characterization of the tracheobronchial compression in these patients (Figure 12-10).^12,13

Right Aortic Arch With Mirror-image Branching

Right aortic arch with mirror-image branching is a relatively common anomaly in which the aorta ascends anterior to the trachea, courses over the right main bronchus, and descends to the right of the vertebral column. The first branch of the arch is the left brachiocephalic artery, which courses to the left of and anterior to the trachea. The right common carotid and the right subclavian arteries constitute the second and third branches of the arch. In most patients, the descending thoracic aorta crosses over the spine and exits the thorax through the aortic hiatus in a normal left-sided location. The ductus arteriosus in patients with right aortic arch and mirror-image branching is most often located on the left, usually arising from the left brachiocephalic artery near its origin and connecting to the left pulmonary artery. Occasionally, there is a right-sided ductus; there are rare instances of bilateral duct ductus arteriosi. Rarely, a left-sided ligamentum arteriosum extends from the descending aorta to the left pulmonary artery, thereby forming a complete vascular ring. The other forms of right aortic arch with mirror-image branching do not constitute complete rings.¹

Approximately 95% of patients with right aortic arch and mirror-image branching have congenital heart disease. Most of the associated lesions are cyanotic. This arch anomaly occurs in approximately 25% of patients with tetralogy of Fallot, 35% of those with truncus arteriosus, and 20% of individuals with double-outlet right ventricle. Other less-common associations include tricuspid atresia and transposition of the great arteries with pulmonary valvular stenosis. This aortic arch configuration is present in the great majority of individuals with situs inversus and mirror-image dextrocardia. Of those patients without associated intracardiac anomalies, most either have a vascular ring formed by a ductus extending from the descending aorta to the left pulmonary artery or have anomalies of the proximal aspect of the left pulmonary artery. Some of these patients have a chromosome 22q11 deletion.^14–16

Chest radiographs of patients with right aortic arch and mirror-image branching show leftward deviation of the trachea. There may be minimal impression on the contrast-filled esophagus, without substantial narrowing. MR or CT shows a right-sided aortic arch and great vessel orientation that is the mirror image of a normal left arch (Figure 12-11).¹⁷

With the rare form of aortic arch and mirror-image branching in which a vascular ring is formed by a left ductus or ligamentum that extends from the descending aorta to the left pulmonary artery, esophagography shows a pronounced right-sided impression (the right aortic arch), a posterior impression (the anomalous ductus or ligamentum), and a small left-sided impression (the left pulmonary artery) that is slightly inferior to the arch impression. MR of these patients shows the right-sided aortic arch, as well as the great vessel anatomy. Although the ligamentum arteriosum is not directly visualized on MR, the presence of an anomalous ligamentum is suggested if there is a diverticulum arising from the proximal aspect of the descending aorta. MR may also show anterior displacement of the trachea adjacent to the diverticulum.^18–20

Right Aortic Arch With Isolated Left Subclavian Artery

Right aortic arch with an isolated left subclavian artery is a rare anomaly in which the left subclavian artery does not directly connect to the aorta or the brachiocephalic artery. In utero, blood flow to the left subclavian artery is via a left ductus arteriosus. When the ductus closes in the neonate, antegrade flow ceases in the proximal aspect of the subclavian artery. Blood then flows in a retrograde manner from the ipsilateral vertebral artery to supply the subclavian artery and the left upper extremity; this represents a congenital subclavian steal syndrome. This anomaly sometimes occurs in association with tetralogy of Fallot.¹⁵

Right aortic arch with isolated left subclavian artery is not a vascular ring and does not cause respiratory symptoms or swallowing dysfunction. The major clinical manifestations are a result of the poor arterial supply to the left upper extremity. These patients sometimes develop ischemic changes in the left arm or hand. Clinical examination shows diminished pulses.

The diagnosis of right aortic arch with isolated left subclavian artery is suggested when radiographs show a right-sided aortic arch in a patient with clinical signs of poor left upper-extremity perfusion. MR or CT angiography confirms absence of the proximal segment of the left subclavian artery. Retrograde flow in the left vertebral artery can be documented with Doppler sonography, transcatheter angiography, or phase-contrast MRI. Aortography shows failure of opacification of the left subclavian and vertebral arteries on the initial images, and delayed visualization as contrast-opacified blood courses in a retrograde manner from the left vertebral artery. Additional neck and chest vessels may supply collateral blood flow to the left upper extremity.^21,22