Stojanovska, Jadranka MD, MS; Cascade, Philip N. MD; Chong, Suzanne MD, MS; Quint, Leslie E. MD; Sundaram, Baskaran MD
Aortic abnormalities are common cardiovascular malformations, accounting for 15% to 20% of all congenital cardiovascular diseases.1 Advances in imaging technology, such as thin-section computerized tomography (CT) with short scanning times, have made obtaining images with exquisite isotropic resolution possible. Moreover, these malformations are frequently encountered as incidental findings, due to the increasing use of cross-sectional imaging, especially multidetector computerized tomography (MDCT) and magnetic resonance imaging (MRI), for unrelated indications.
Congenital aortic arch anomalies result from errors in the embryologic development of the branchial arches, including errors of involution or migration, or abnormal persistence of vascular structures. Most arch abnormalities consist of errors of laterality or aberrations in the level of interruption of the primitive branchial arches, which determine the presence or absence of aberrant supra-aortic branches.1 Strong associations of arch anomalies with chromosomal and genetic abnormalities are supported by studies demonstrating a deletion within chromosome 22q11 with DiGeorge and velocardiofacial syndromes. A total of 24% of patients with DiGeorge syndrome have conotruncal cardiac anomalies such as tetralogy of Fallot (TOF), either with or without pulmonary atresia, truncus arteriosus, interruption of the arch, and/or isolated anomalies of laterality or branching of the arch.2,3
Aortic arch anomalies can be discovered when there are symptoms of airway or esophageal compression produced by vascular rings,4 or anomalies can be found incidentally on imaging studies obtained for other reasons. An understanding of the normal embryologic development of the arch, coupled with knowledge of the imaging features of malformations, may aid both adult and pediatric radiologists in making correct interpretations of these anomalies. This article describes the embryologic development of the normal aortic arch and its malformations, along with imaging features of the more common anomalies that are encountered in children and adults.
In the past, barium esophagography was used to diagnose vascular rings in patients with dysphagia by depicting the presence of deep and constant esophageal indentation at the level of the arch. In addition, aortic arch anomalies were sometimes found incidentally during catheter angiography performed to investigate congenital heart lesions. At present, chest radiography (CR) and echocardiography (ECHO) are the first-line imaging modalities used in diagnosing congenital aortic arch anomalies, particularly in children. Exquisite anatomic display of the cardiovascular structures is now made possible by technological advancements in MDCT and MRI. ECHO, MDCT, and MRI provide valuable morphologic information about the configuration of the aortic arch and its branches. Echocardiographic Doppler interrogation, flow pattern evaluation with MRI, and intravenous gadolinium contrast-enhanced MRI provide a noninvasive assessment of functional changes, such as assessing flow directions and quantifying flow velocities and pressure gradients associated with congenital heart disease.5 Contrast-enhanced magnetic resonance angiography is reported to be superior to transthoracic ECHO and unenhanced MRI in the diagnosis of aortic arch anomalies.6 ECHO suffers from operator- and patient-dependant factors, and also from limited access to the entire thoracic aorta. Similarly, the disadvantages with CT imaging are ionizing radiation, iodinated contrast material in patients with impaired renal function, and limited ability to assess functional changes. Hence, at present, cardiovascular MRI with or without intravenous gadolinium is increasingly accepted as the noninvasive gold standard test to assess cardiovascular morphology.
EMBRYOLOGIC DEVELOPMENT OF THE AORTIC ARCH
The Rathke Diagram
Martin Heinrich Rathke (1793 to 1860), a renowned embryologist, is credited with much of the work leading to the understanding of the embryologic development of the branchial arches; the classic Rathke diagram is shown in Figure 1. Development of the branchial apparatus begins during the second week of gestation and is completed by the seventh week. The apparatus consists of 6 branchial arches in the wall of the foregut. The branchial arches are numbered 1 to 6 from cephalad to caudad. Each of the branchial arches connects paired dorsal and ventral aortas. Although the classic Rathke diagram shows 6 aortic arches, in reality the arches appear and disappear at different times.
The 6 branchial aortic arches normally develop into the thoracic aorta and its branches (Figs. 1, 2A, B). The first 2 arches involute before development of the sixth arch, and the fifth arch is atretic or never fully develops. The third arch and portions of the ventral and dorsal aortic arches contribute to the head and neck arteries. The fourth arch becomes the aortic arch, and the pulmonary arteries develop from the sixth branchial arches. On the right side, the dorsal contribution of the sixth arch disappears, and on the left it persists as the ductus arteriosus. The intersegmental arteries migrate and form the subclavian arteries.
The Edwards Hypothetical Double Arch
Another concept that helps to understand the development of thoracic aortic anomalies is the Edwards hypothetical double arch (Fig. 3). Jessie E. Edwards, considered by many to be the father of modern pediatric cardiology, developed a diagram of a hypothetical double arch system to help explain the derivation of thoracic arch anomalies. The diagram shows a double aortic arch with a ductus arteriosus on each side. Specific anomalies can be explained by showing the effect of interrupting the double arch at different locations.
Classification of Aortic Arch anomalies
Aortic arch anomalies can be classified according to the Edwards hypothetical double arch system, although many modifications of this classification system have been proposed.7–9 Anatomical classification is based on the absence, course, or position of the aortic arch. As a result, arch anomalies may be characterized as interrupted, right sided, left sided, or double in configuration, and also characterized based on the order or pattern of branching of the great vessels. Anomalies may be left-sided aortic arch, right-sided aortic arch, double aortic arch, or cervical aortic arch (Table 1).
Aortic arch anomalies can also be classified based on their clinical presentation or morphology. Many arch anomalies are asymptomatic. However, vascular rings and anomalous isolation of the subclavian, carotid, or brachiocephalic arteries can cause clinical symptoms. On the basis of the clinical presentation, arch anomalies can be classified as (1) asymptomatic cases; (2) cases with clinical symptoms caused by tracheobronchial and/or esophageal compression by vascular rings; and (3) cases in which there is isolation of aortic arch branches and alteration of normal blood flow with a “steal” phenomenon from the cerebral circulation.
LEFT AORTIC ARCH
Based on the branching pattern of the great vessels, the left aortic arch can be divided into 3 groups:
a. Normal branching
b. Aberrant right subclavian artery (ARSA)
c. Isolation of the left subclavian artery
As described above, the development of normal anatomy depends on the involution and persistence of different arches in the 6 pairs of dorsal branchial arches. The normal left aortic arch develops when the hypothetical arch is interrupted distal to the right subclavian artery. The fourth branchial arch becomes the aortic arch, whereas portions of the third arch contribute to the head and neck arteries (Figs. 4A, B).
The left aortic arch is located to the left of the trachea on a frontal chest radiograph. Cross-sectional imaging demonstrates a left-sided arch with a normal branching pattern. The order of the branches, from right to left, is as follows: the brachiocephalic artery that branches into right subclavian and right common carotid arteries, the left common carotid artery, and finally the left subclavian artery. The diameter of the brachiocephalic artery is larger than the left common carotid and subclavian arteries.
LEFT AORTIC ARCH WITH ARSA
The most common anomaly associated with the left aortic arch is the ARSA with a left ductus arteriosus. This anomaly occurs in approximately 1% to 2% of patients10 and occurs when there is a break in the primitive right arch between the right common carotid and subclavian arteries (Figs. 5A, B). Thus, the ARSA is the last aortic arch branch, a branch that travels from the left aortic arch, behind the esophagus, to perfuse the right upper extremity. A diverticulum of Kommerell is often evident at the origin of the ARSA. This diverticulum is thought to be a remnant of the posterior portion of the original dorsal left arch. The anomaly is usually asymptomatic. In some patients, tortuosity and dilatation of the ARSA or Kommerell diverticulum can cause compression of the esophagus with resultant dysphagia.
A study using conventional CR in patients with proven ARSA reported a pattern of abnormalities.11 On the posteroanterior radiographs (n=101), findings included an oblique edge extending to the right from the aortic knob (60%); demonstration of ARSA through the tracheal air column (43%), with sharp margins (29%) or as a tubular opacity without sharp margins (14%); and a “mass” effect at the medial right clavicular area (32%). Lateral chest radiographic findings (n=89) included a retrotracheal opacity in the Raider triangle (79%), aortic arch obscuration (62%), and a posterior tracheal imprint (49%). Findings on contrast esophagrams include anterior displacement of the esophagus with an impression on the posterior wall of the esophagus due to the retroesophageal course of the ARSA. Cross-sectional examinations show a left-sided aorta with ARSA coursing posterior to the esophagus and trachea, sometimes originating from an aortic diverticulum of the Kommerell. The trachea may be narrowed by the crossing ARSA (Figs. 6A–D).12
Patients with this anomaly occasionally have associated congenital abnormalities such as hypoplastic left heart syndrome, coarctation of the aorta, and atrioventricular canal defects.13 Aortic pathologies such as aneurysms, dissections, and arch branching abnormalities have been reported with ARSA.14,15
Treatment and Prognosis
This anomaly is usually found incidentally, with treatment not required in asymptomatic patients. Occasionally, the aortic diverticulum of the Kommerell becomes aneurysmal, and may require surgical resection.
ISOLATION OF THE SUBCLAVIAN ARTERY
This anomaly occurs when 2 breaks in the primitive arch leave the subclavian artery separated from the aortic arch. This anomaly generally occurs on the side contralateral to the arch, and hence is more commonly associated with a right aortic arch (RAA).
Imaging of the aorta shows loss of continuity between the subclavian artery and the aortic arch. The isolated subclavian artery fills by collaterals derived from the vertebral artery or from the ductus arteriosus.
Association With Other Congenital Anomalies
Isolation of the subclavian artery may be associated with intracardiac anomalies, such as TOF, or with great vessel anomalies, such as interrupted aorta or patent ductus arteriosus (PDA).16,17
Treatment and Prognosis
The treatment includes surgical correction by reconnecting the isolated subclavian artery to the aorta.
The RAA has a reported prevalence in adults of 0.04% to 0.1%, based on a necropsy series.18 However, on the basis of our cardiothoracic imaging practice using MDCT and MRI, we believe that the incidence is probably higher. The RAA was first described 2 centuries ago.18 The RAA results from dissolution of the left dorsal aortic root instead of the right dorsal root. The RAA anomaly can be associated with congenital cardiac anomalies, esophageal atresia, and tracheoesophageal fistula.19,20 The prevalence of RAA in patients with esophageal atresia is 1.8%.21 Operative complications during repair of an esophageal atresia and tracheoesophageal fistula have been reported to be higher with the RAA (25%) compared with the left aortic arch (11%).19 Therefore, correct preoperative identification of the side of the aortic arch is important in surgical planning.
Several classifications for congenital RAA have been proposed based on the arrangement of the arch vessels, the relationships with the esophagus, or the presence of congenital heart anomalies.22 The most common classification scheme identifies 3 main subgroups of the RAA as follows:
1. RAA with mirror image branching (type I)
2. RAA with an aberrant left subclavian artery (ALSA) (type II)
3. Rigth aortic arch with isolation of the left subclavian artery (type III)
Type II RAA with an ALSA is the most common of the 3 types.23,24 There are additional subtypes based on the site of the ductus arteriosus. For example, the ductus arteriosus may be on the left, on the right, or bilateral. An uncommon variant of the RAA, the “circumflex retroesophageal aorta” occurs when the RAA crosses the midline posterior to the esophagus before descending on the contralateral side, resulting in the ascending and descending aorta on opposite sides of the thoracic vertebrae. The RAA can also be associated with a complete vascular ring, as formed by a patent ductus or ligamentum arteriosum contralateral to the ascending aorta. These types of rings can cause symptoms of tracheoesophageal compression.25 The presence of a mirror image RAA can be associated with congenital heart disease such as pulmonary atresia with ventricular septal defect (VSD) in 46.4%, TOF in 32%, and double-outlet right ventricle with right atrial isomerism in 14% of cases.26
RAA WITH MIRROR IMAGE BRANCHING (TYPE I)
In the embryogenesis of the type I RAA, the break occurs in the left fourth arch between the left ductus and the descending aorta (Figs. 7A, B). Regression of the dorsal left arch between the left ductus arteriosus and the descending aorta produces mirror image branching of the aortic arch. A left-sided ductus arteriosus may connect the left pulmonary artery to the left subclavian portion of the brachiocephalic artery or to the descending aorta. This arch branching pattern is the mirror image of the conventional branching pattern of the normal left aortic arch, as the first branch of the arch is a left brachiocephalic artery, followed by the right common carotid artery, and finally the right subclavian artery. Usually, there is left ductus arteriosus and no vascular ring. In rare cases, there are bilateral ducti, or a right ductus is present.18 The descending aorta of the mirror image RAA initially descends on the right, before crossing over to the left of the thoracic spine to exit the thorax through the diaphragmatic hiatus.
A RAA on a frontal chest radiograph can mimic a right superior mediastinal mass. Cross-sectional imaging shows that the left brachiocephalic artery arises as the first branch off the aortic arch, which then divides into a left common carotid and a left subclavian artery. The ductus arteriosus either arises from the undersurface of the aortic arch opposite the right subclavian artery or from the bifurcation of the left brachiocephalic artery. The left brachiocephalic artery takes an anterior course in relation to the main pulmonary artery without compromising the trachea and the esophagus. The descending aorta courses from the right lateral aspect of the upper thoracic spine to the left of the lower thoracic spine. A previous study reported that these patients usually have associated cyanotic congenital cardiac anomalies, and therefore a detailed evaluation of the cardiac chambers is needed.18
Cyanotic heart disease, such as TOF, pulmonary atresia with VSD, tricuspid atresia, and truncus arteriosus, is reported to occur in approximately 75% of patients with a mirror image RAA.27 Cantinotti et al26 reported the presence of conotruncal abnormalities in a significant number of patients with a mirror image RAA. However, it is important to note that the study finding was based on a retrospective review of pediatric cardiac MRI examnations conducted specifically to identify aortic arch anomalies in a referral center. In our experience with MDCT and MRI, we have observed that the mirror image RAA can be found in many patients without any significant associated congenital heart diseases. The mirror image RAA anomaly is not associated with symptoms of tracheoesophageal compression, and can be found as an incidental finding in asymptomatic patients.
Treatment and Prognosis
The mirror image RAA associated with cyanotic congenital heart disease requires the repair of cardiac anomalies. The isolated mirror image RAA without associated cardiac anomalies does not require surgical treatment.
RAA WITH AN ALSA (TYPE II)
The RAA with an ALSA is associated with congenital heart disease, generally reported in 5% to 10% of cases.27,28 The break in the theoretical double arch occurs between the left common carotid and the left subclavian artery (Figs. 8A, B). The ALSA arises either as a last branch from a RAA or from an aortic diverticulum known as the diverticulum of Kommerell, which is a remnant of the dorsal left aortic arch.29 Patients with a ligamentum arteriosum to the right of the midline have no tracheal or esophageal compressive symptoms, because there is no complete ring to cause compression. Airway compression in the RAA with ALSA is often associated with either a Kommerell diverticulum or a midline descending aorta. In addition, an aortic diverticulum seems to play an important role in esophageal indentation in patients with an RAA with ALSA.30 Pediatric patients with a RAA and a left ligamentum arteriosum have a vascular ring that can compress the trachea and esophagus. In adults, this anomaly is usually asymptomatic unless there is aneurysmal dilatation of the diverticulum of the Kommerell. In RAA with ALSA configuration, the order of branching pattern from proximal to distal is the left common carotid artery, right common carotid artery, right subclavian artery, and left subclavian artery. The ALSA passes from the right into the left hemithorax posterior to the esophagus and trachea, and together with the ligamentum arteriosum forms a vascular ring. A Kommerell diverticulum at the origin of the ALSA can be clinically important because it can add to the complexity of corrective surgery and increase surgical morbidity and mortality.31
Imaging findings of the RAA with ALSA include widening of the right superior mediastinum with an absence of the normal left aortic arch contour on frontal CR and a well-defined area of increased opacity in the retrotracheal space (also known as the Raider triangle) on lateral CR. A barium esophagogram may show anterior displacement of the esophagus with a characteristic diagonal impression on the posterior aspect of the esophagus at the level of the fourth thoracic vertebral body, due to the obliquely coursing ALSA. MDCT or MRI may be used to confirm the diagnosis, demonstrating any coexisting cardiac or vascular anomalies and delineating the exact anatomy.
The RAA with an ALSA is less commonly associated with congenital heart disease in comparison with type I mirror image RAA.32 Ramaswamy et al10 have reported that conotruncal anomalies occur more frequently with ALSA from a RAA than ARSA from a left aortic arch.
Early surgical treatment of the Kommerell diverticulum is recommended because of the risk of potential aneurysm rupture. Surgery entails resection of the diverticulum and reimplantation of the ALSA into the aorta by means of left thoracotomy.33
RAA WITH ISOLATION OF THE LEFT SUBCLAVIAN ARTERY (TYPE III)
The RAA with isolation of the left subclavian artery is the least common type of 3 subgroups of abnormalities associated with the RAA. Isolation of the left subclavian artery occurs if the break in the aortic arch vascular ring takes place proximal and distal to the left subclavian artery (Figs. 9A, B).34 This anomaly is very rare and is observed in only 0.8% of patients with a RAA.35 The isolated left subclavian artery may be connected either to the pulmonary artery by the ductus arteriosis or to the left vertebral artery. The ductus may or may not be visualized. The left subclavian artery may be perfused by collateral vessels from the external carotid artery, vertebral artery, thyrocervical trunk, thyroid artery, internal thoracic artery, costocervical trunk, or axillary artery.
Isolation of the left subclavian artery may be clinically suspected in a patient with a RAA and diminished blood pressure in the left upper extremity. Patients with this anomaly may present with symptoms of arm ischemia, although patients are sometimes asymptomatic.
The first branch to arise from the RAA is the left common carotid artery, followed by the right common carotid artery and the right subclavian artery. The left subclavian artery communicates with either the left vertebral artery or the left pulmonary artery. The latter artery is connected by the left PDA (Fig. 10). On angiography, the isolated left subclavian artery may show delayed opacification relative to the aortic arch. The angiography may also show a retrograde flow from the left vertebral artery or collateral vessels from other craniocervical vessels.36 A congenital pulmonary steal has also been reported in a case of isolation of a left brachiocephalic artery, RAA, and patent left ductus arteriosus.37
A RAA with isolation of the left subclavian artery is reported as having associated congenital heart disease in 59% of cases, with TOF as the most common abnormality.38 Other reported associations include contralateral PDA, bilateral ductus arteriosus, coarctation of the aorta, left pulmonary artery stenosis, VSD, atrial septal defect, atrioventricular septal defect, dextrocardia, aortic atresia, double-outlet right ventricle, and venous anomalies.
Treatment and Prognosis
Surgical correction of the isolated subclavian artery may be performed at the same time as corrective surgery for associated congenital heart disease. In patients with no congenital heart disease, left carotid-subclavian bypass may alleviate the symptoms related to vertebral steal phenomenon.39
DOUBLE AORTIC ARCH
Double aortic arch is an uncommon vascular anomaly that develops when there is no break in the hypothetical double arch and both fourth arches and dorsal aortas persist (Fig. 11). As a result, 2 aortic arches connect the ascending and descending aortic segments. The ascending aorta bifurcates anterior to the trachea, with one arch coursing to the left and the other to the right. The arches rejoin into a single descending aorta posterior to the esophagus, thus forming a vascular ring. The 2 complete arches may be equal in size, or 1 arch may be hypoplastic.40 The RAA is dominant in 75% of patients with a double aortic arch,41 and, on occasion, the smaller arch may be atretic (usually the left aortic arch). Atresia of one of the double arches (the distal segment of the arch is a nonpatent fibrous cord) is known as the incomplete double aortic arch, and it is an unusual cause of a symptomatic vascular ring.42 When present, a ductus arteriosus is almost always on the left side. The incomplete double aortic arch with a ductus arteriosus forms a complete ring around the trachea and esophagus, causing compressive symptoms. On occasion, the anomaly presents incidentally in an adult. However, on close questioning, many adults give a history of dysphagia.
Imaging findings of a double aortic arch can be variable, depending on the size of the arches and the presence or absence of an atretic segment (Figs. 12, 13). Chest radiographic findings of a double aortic arch characteristically show a midline trachea with slight bilateral indentations caused by the bilateral arches. The lateral chest radiograph may show constriction of the tracheal air column at the level of the arch, implying a tight ring. Similarly, a barium esophagram may also show rounded extrinsic impressions on each side of the esophagus, caused by the adjacent double aortic arch. Cross-sectional imaging and catheter angiographic images depict 4 great vessels arising independently from the double aortic arch. The right common carotid artery arises anterior to the right subclavian artery and the left common carotid and subclavian arteries arise as they conventionally would, from a normal left aortic arch. In most cases, imaging depicts a larger RAA that extends posterior to the trachea, with a smaller left aortic arch joining the RAA inferiorly to form the descending aorta. The descending aorta then follows a normal course down to the diaphragmatic hiatus. A double aortic arch with an atretic left arch segment can at times be impossible to differentiate from a RAA with an ALSA.9
The double aortic arch anomaly usually occurs without associated cardiovascular anomalies. However, VSD, TOF, conus truncus arteriosus, transposition of the great arteries, and pulmonary atresia may occur in conjunction with a double aortic arch. Approximately 20% of patients with a double aortic arch anomaly have associated chromosomal abnormalities.41
Treatment and Prognosis
The treatment of symptomatic patients with a double aortic arch is surgical correction with division of the vascular ring to relieve compression of the trachea and esophagus. The long-term prognosis for patients with a repaired, uncomplicated double aortic arch is excellent.
CERVICAL AORTIC ARCH
The aortic arch is termed a “cervical arch” when it is shifted cranially from its usual mediastinal position at the level of the fourth thoracic vertebral body to a location extending above the clavicles. The cervical aortic arch may be on the left or right side. This anomaly is usually an incidental finding, although patients may present with a pulsatile neck mass or with symptoms arising from compression of the trachea or esophagus. There are several theories regarding the formation of the cervical arch. One theory proposes that the cervical arch is derived from the second or third embryonic arch, instead of the fourth arch. Another theory suggests that there may be failure of caudal migration of a fourth arch. The third proposed theory is that the third and fourth arches fuse, with a lack of caudal migration.43
Anomalies of the cervical aortic arch branching pattern are common, and include mirror image RAA, aberrant subclavian artery with a Kommerell diverticulum, and separate origins of the internal and external carotid arteries from the aortic arch. In some patients, the ipsilateral vertebral artery arises directly from the cervical aortic arch.
Felson and Strife44 described many variations of anatomy in their commentary on cervical aortic arches. There may be variations such as absent contralateral brachiocephalic artery, anomalous origin of the contralateral subclavian artery, less constant configuration of the ipsilateral subclavian/carotid arteries, variations in the ductus/ligamentum arteriosum attachments, aortic diverticulum, and variable relation between the thoracic spine and descending aorta (Figs. 14A, B). Some patients may have a stenotic or atretic origin of the contralateral subclavian artery. Congenital heart disease is encountered rarely.
A cervical arch may be associated with cardiovascular abnormalities including TOF, pulmonary atresia with VSD, double-outlet right ventricle, VSD, and PDA.43,45–48
Treatment and Prognosis
Most patients with a cervical arch are asymptomatic. Patients may present with a pulsatile neck mass in the supraclavicular region with a murmur and thrill on clinical examination; in addition, there may be obstructive symptoms related to the airway and/or esophagus. Complications may arise, such as aneurysm formation or a constricting vascular ring. Corrective surgery for a symptomatic cervical aortic arch varies according to the highly variable anatomy of patients with this condition.49
Understanding the embryologic development and imaging features of the normal aortic arch and its anomalous variants can enable radiologists to make a more informed diagnosis of aortic arch malformations and associated cardiac lesions.
The authors thank Brian Rhee, medical illustrator, Department of Radiology, University of Michigan, Ann Arbor, Michigan, for creating the schematic illustrations in Figure 1, Figures 2A and B, Figure 3, Figures 4A and B, Figures 5A and B, Figures 7A and B, Figures 8A and B, Figures 9A and B, and Figure 11.
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