Introduction
The aortic arch, the arterial duct, and the right and left pulmonary arteries have a close spatial relationship with the major airways and the esophagus. Due to this close proximity, abnormalities in the size, position, and/or pattern of branching pattern of these structures may cause obstruction to the trachea, bronchi, or esophagus. Many of the abnormalities of position and branching, along with the so-called pulmonary arterial sling, are characterized by a complete or partial encirclement of the trachea and esophagus, or the trachea in isolation, by a composite vascular structure. Although the terms ring and sling have been used somewhat loosely, we use vascular ring to denote a complete encirclement of the airway and esophagus. Incomplete encirclement of the airway and esophagus will be referred to as a partial vascular ring . Pulmonary artery sling is discussed as a separate entity. Vascular rings and slings are the classic anomalies that cause symptoms and signs of obstruction to the airways and esophageal compression. However, not all rings and slings result in clinically recognizable symptoms and signs. Conversely, other vascular abnormalities do not form a ring or sling and yet may produce significant obstruction of the airways and esophagus. This chapter discusses these various anomalies of the aortic arch and pulmonary arteries that may cause obstruction to the central airways.
Anomalies of the Aortic Arch
The obstructive lesions within the arch, such as coarctation or interruption, are not generally included in a discussion of abnormalities in position and/or branching of the aortic arch. Insight into the mode of development of the arterial trunks and their pattern in the fetal circulation is tremendously helpful in understanding the prenatal and postnatal features of the various malformations that involve the aortic arch.
Hypothetical Model of the Double Aortic Arch
The normal and abnormal development of the components of the aortic arch can best be understood by referring to the model introduced in 1948 by the pioneer pathologist, Jesse E. Edwards ( Fig. 47.1 ). The model illustrates a relatively late stage of development, in that the distal intrapericardial outflow tract has been divided into the ascending aorta and pulmonary arterial trunk, and the descending aorta occupies a neutral position. The earlier stages of development are not discussed, in part because the precise morphogenesis of the outflow tracts has yet to be clarified. In the hypothetical model, symmetric aortic arches connect the ascending and descending aorta on each side, forming a complete vascular ring around the trachea and esophagus. Each aortic arch gives rise superiorly to a common carotid artery and a subclavian artery. On each side, right-sided and left-sided arterial ducts pass between the pulmonary arteries and the distal part of the aortic arches, forming an additional vascular ring around the trachea and esophagus. Therefore the hypothetical model is made up of two vascular rings joined together at the descending aorta.
With normal development, the left aortic arch and left-sided arterial duct persist, while the right aortic arch distal to the origin of the right subclavian artery, along with the right-sided arterial duct, regress ( Fig. 47.2A ). As a result, the proximal part of the embryologic right aortic arch remains as the brachiocephalic artery, which bifurcates into the right common carotid and right subclavian arteries. The brachiocephalic artery, of course, is also known as the innominate artery. The left-sided aortic arch, in sequence, gives rise to the brachiocephalic, left common carotid, and left subclavian arteries ( Fig. 47.2B–C ). Anomalies can be positional or reflect abnormal branching due to persistence of part or parts of the double arch that normally should have regressed. In exceptional cases, nonetheless, it remains difficult to predict the embryologic mechanism, even using the concept of the double arch.
Classification
Anomalies can be classified into four groups, depending on the position of the aortic arch relative to the trachea and the pattern of branching of the brachiocephalic arteries:
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Left aortic arch with aberrant right subclavian or brachiocephalic artery
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Right aortic arch with aberrant left subclavian or brachiocephalic artery
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Right aortic arch with mirror-image branching
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Double aortic arch
Most of the major anomalies produce a complete or partial vascular ring around the trachea and esophagus. The only exception is the classic form of right aortic arch with mirror-image branching. Aberrant origin of a subclavian or brachiocephalic branch of the aortic arch produces encirclement of the trachea and esophagus because the anomalous artery takes a retroesophageal course. The arterial duct, regardless of whether it is patent or ligamentous, may also contribute to encirclement of the trachea and esophagus. Occasionally the distal aortic arch itself has a retroesophageal course that causes esophageal and tracheal compression. Therefore the assessment and description of the anomalies should include description of:
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The position of the aortic arch relative to the trachea
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The location of the most proximal part of the descending aorta in relation to the spine
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The presence or absence of an aberrant branch
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The origin and insertion of the patent or ligamentous arterial duct or, rarely, ducts
The anomalies producing a complete or partial vascular ring around the trachea and esophagus are:
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Aortic arch anomalies forming a complete vascular ring:
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Double aortic arch
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Right aortic arch with aberrant left subclavian or brachiocephalic artery and left-sided arterial duct
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Left aortic arch with aberrant right subclavian or brachiocephalic artery and right-sided arterial duct
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Right aortic arch with mirror-image branching and retroesophageal left arterial duct between right-sided descending aorta and left pulmonary artery
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Circumflex retroesophageal aortic arch
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Aortic arch anomalies forming a partial vascular ring:
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Left aortic arch with aberrant right subclavian or brachiocephalic artery and left-sided arterial duct
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Right aortic arch with aberrant left subclavian or brachiocephalic artery and right-sided arterial duct
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Other anomalies that may have clinical significance include the cervical aortic arch, isolated origin of the left or right subclavian artery from a pulmonary artery, and double-barreled, or double lumen, aortic arch.
Morphology and Morphogenesis of Individual Anomalies
Double aortic arch is the tightest form of vascular ring. 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. 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. 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. 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. The descending aorta is left sided in just over two-thirds of patients with double aortic arch, being right sided in almost all the rest and only rarely occupying a neutral midline position.
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. It is currently usually believed that arteries that do not take a retroesophageal course are collateral arteries. The persistent arterial duct is usually left sided, connecting the left pulmonary artery to the distal remnant of the left aortic arch. 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. 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 U-shaped 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. 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. 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. 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.
Left aortic arch with aberrant right subclavian artery is the most common anomaly involving the aortic arch but is usually asymptomatic. It results from abnormal regression of the right arch between the origins of the right common carotid and right subclavian arteries, leaving the right subclavian artery attached to the distal remnant of the right-sided aortic arch ( Fig. 47.6A and B ). As a consequence, the distal remnant of the right aortic arch and the right subclavian artery together constitute the aberrant segments. In most cases, it is the left arterial duct that persists. This combination forms a partial vascular ring around the left side of the trachea and esophagus. Typically, the aberrant right subclavian artery courses behind the esophagus but has been described to course between the trachea and esophagus, although again these vessels may have been collateral arteries. When there is a right-sided arterial duct between the aberrant artery and the right pulmonary artery, there is a complete vascular ring ( Fig. 47.7 ). The ring consists of the ascending aorta, left aortic arch, descending aorta, distal remnant of the right aortic arch, right arterial duct, right pulmonary artery, and pulmonary trunk, with the heart itself completing the ring. In fetal life, when the arterial duct is a wide channel that connects the pulmonary artery to the descending aorta, an L-shaped loop is formed around the trachea and esophagus. With closure of the right-sided arterial duct after birth, the distal remnant of the right aortic arch persists as the diverticulum of Kommerell. It is theoretically possible for a left-sided aortic arch to be associated with aberrant origin of the right brachiocephalic artery. As far as we are aware, this has not been reported. Aberrant subclavian or brachiocephalic arteries coexisting with either right-sided or left-sided aortic arches are often associated with other anomalies, including a common carotid arterial trunk, anomalous origin of the vertebral artery from the common carotid artery on the same side, anomalous point of entrance of the vertebral artery into the cervical spine, and an abnormal drainage site of the thoracic duct. Although these anomalies are clinically silent, they may be of practical importance to the surgeon.
A right-sided aortic arch with a mirror-image branching results from abnormal regression of the left aortic arch distal to the origin of the left subclavian artery ( Fig. 47.8A and B ). This anomaly of the aortic arch only rarely forms a vascular ring regardless of the sidedness of the arterial duct. In this pattern, the persisting arterial duct is usually on the left, connecting the base of the left brachiocephalic artery to the left pulmonary artery. Less commonly, the arterial duct is either on the right or bilateral. Rarely, the arterial duct arises from the descending aorta on the right side and takes a retroesophageal course to connect to the left pulmonary artery. This is the only combination that constitutes a complete vascular ring in the presence of mirror-image branching. ( Fig. 47.9 ) The anomaly results from abnormal regression of the left aortic arch distal to the origin of the left subclavian artery and proximal to the insertion of the persisting left arterial duct, with regression of the right arterial duct. The distal left aortic arch remnant persists as a diverticulum of Kommerell.
Circumflex retroesophageal aortic arch is a rare form of aortic arch anomaly in which the aortic arch and the proximal descending aorta are placed on opposite sides of the spine ( Fig. 47.10A and B ). This combination requires the aortic arch to make an additional arc to the other side behind the trachea and esophagus, thus reaching the descending aorta on the opposite side. The patterns of branching of the brachiocephalic arteries are variable. It is hard to explain this rare malformation. It occurs much more frequently with a right-sided than with a left aortic arch. When it occurs with a right aortic arch, the arch gives rise to the left common carotid, right common carotid, and right subclavian artery from its segment on the right side of the trachea. Then the aortic arch makes a sharp oblique leftward and usually downward turn to connect to the left-sided descending aorta. The left subclavian artery arises from the transitional point of the retroesophageal part of the arch to the descending aorta. It can be named as an aberrant artery in the sense that it is the last, instead of the first, branch of the right aortic arch. It is not retroesophageal in location, but the aortic arch itself is behind the esophagus. In most cases the left subclavian artery arises from the aorta through a diverticulum of Kommerell. The apex of the diverticulum connects to the left pulmonary artery through a left arterial ligament, thus forming a complete vascular ring around the trachea and esophagus. A circumflex retroesophageal aortic arch is rarely seen without aberrant origin of a subclavian artery but does exist. Hypoplasia of the retroesophageal segment of the aortic arch is common.
The aortic arch is described as being cervical when its apex reaches the upper mediastinum above the level of the clavicles ( Fig. 47.11 ). It may be recognized as a pulsatile mass in the supraclavicular fossa or lower neck. A cervical arch is slightly more common on the right, often taking a circumflex retroesophageal course to form a vascular ring. A double aortic arch can also adopt a cervical position. The branching of the brachiocephalic arteries is abnormal in the majority of cases. In addition, it is common to find unusual tortuosity, obstruction and aneurysm of the aortic arch, and obstruction of a brachiocephalic branch or branches ( Fig. 47.12 ). A cervical aortic arch is often associated with tracheal obstruction because of crowding of vascular structures and airway in a confined small space of the upper mediastinum, especially when the aortic arch is right sided and takes a hairpin turn.
Isolated origin of the subclavian artery from the pulmonary artery through the arterial duct is a rare type of anomaly in which the subclavian artery is disconnected from the aorta, instead taking its origin from the pulmonary artery on that side through the persistently patent arterial duct ( Fig. 47.13A–B ). It is explained on the basis of abnormal regression at two locations in the hypothetical double arch, one proximal and the other distal to the origin of the affected subclavian artery. Such isolation occurs more commonly when the aortic arch is right sided, with the left subclavian artery being the isolated artery in the majority of cases. Flow to the isolated artery varies according to the size of the persistently patent arterial duct and the patency of the pulmonary outflow tract. When the arterial duct is wide open and there is no obstruction at the infundibular level, the left subclavian artery is supplied through the pulmonary arteries. If associated with significant pulmonary obstruction, the flow through the arterial duct may be reversed. Postnatally, when the arterial duct closes, the anomalous artery may lose its primary supply of blood and can result in vertebral steal on the side of the isolated artery. Brachiocephalic or carotid arteries can also be isolated in comparable fashion.
Double-barreled, or double lumen, aorta is a rare anomaly in which the ascending and descending components of the aorta are connected by two aortic arches on the same side of the trachea ( Fig. 47.14 ). It should not be confused with the double aortic arch, in which each arch is to the opposite sides of the trachea. In the past, this anomaly has been interpreted as persistence of the embryologic fifth aortic arch. However, the existence of the fifth aortic arch in human and mammalian embryos remains controversial, never having been encountered during normal development.
Incidence, Genetics, and Association With Anomalies
The overall incidence of anomalies involving the aortic arch is difficult to estimate because an unknown but significant number of individuals having such anomalies do not present with symptoms and thus remain undiagnosed. Therefore the reported incidences of individual anomalies vary significantly according to the population studied. A left aortic arch with aberrant origin of the right subclavian artery, which is usually found incidentally and as an isolated defect, has been reported as the most common aortic arch anomaly. It has been found in 0.5% of a large autopsy series. In patients with other cardiac defects, a right aortic arch with mirror-image branching is the most common malformation. Association with congenitally malformed hearts varies according to the individual lesions. A right aortic arch with mirror-image branching is almost always associated with congenital cardiac disease. A right aortic arch is common in tetralogy of Fallot and common arterial trunk, with the incidence ranging from 20% to more than 30% for both lesions. When tetralogy is associated with pulmonary atresia, the incidence is high at 30%. A mirror-image right aortic arch is less frequently seen in transposition, tricuspid atresia, and isolated ventricular septal defect. When not associated with intracardiac anomalies, the mirror-imaged right aortic arch is commonly associated with absence or stenosis of the proximal left pulmonary artery. The rare mirror-imaged right aortic arch with retroesophageal left-sided arterial duct is uncommonly associated with other cardiovascular anomalies. Right aortic arch with aberrant left subclavian artery is also less commonly associated with congenital cardiac disease, with the reported incidence ranging from less than 20% to more than 60%. The most commonly associated malformations are ventricular and atrial septal defects, tetralogy of Fallot, and transposition. Left aortic arch with aberrant right subclavian artery is much less commonly associated with other malformations and therefore is usually found as an incidental finding. Double aortic arch is associated with other cardiovascular anomalies in up to 20% of cases, the most common being atrial and ventricular septal defects, tetralogy of Fallot, and patency of the arterial duct. Circumflex retroesophageal aortic arch is associated with congenitally malformed hearts in up to half of cases. Although uncommon, a right aortic arch with or without an aberrant left subclavian or brachiocephalic artery may be associated with an obstructive lesion of the aortic arch, especially when the arch forms a circumflex retroesophageal or high cervical course or both. Rarely, interruption can be found in a right aortic arch.
Noncardiac anomalies are infrequently associated with anomalies of the aortic arch. The most important noncardiac anomaly is the esophageal atresia with or without the vertebral, anal, cardiac, tracheal, esophageal, renal, and limb anomalies association. It has been reported that esophageal atresia seen in association with such anomalies of the aortic arch tends to have a long gap.
Chromosome 22q11 deletion syndrome is common. This deletion is considered to affect migration of cells from the neural crest that contribute to development of the ventricular outflow tracts and pharyngeal arches in the embryo. However, the precise mechanisms of abnormal development of the aortic arches remain speculative. A fetal series showed an 8% incidence of 22q11 deletion in fetuses with a right aortic arch as an isolated abnormality, and 46% in those with right aortic arch and intracardiac abnormality. Postnatal series showed higher incidences of 22q11 deletion, this being found in up to one-quarter of cases with an isolated anomaly of the aortic arch. The higher incidence of 22q11 deletion in postnatal series is explained by selection bias because these patients are more likely to be referred to a cardiologist. More than half of the patients with an intracardiac anomaly and 22q11 deletion have an anomaly of the aortic arch. In patients with anomalies of the ventricular outflow tracts, anomalies of the subclavian arteries function as an important anatomic marker for chromosome 22q11 deletion, independent of the laterality of the aortic arch. The subclavian arterial anomalies encompass aberrant origin from the descending aorta, isolated origin, distal ductal origin from the pulmonary artery, and cervical origin. Chromosome 22q11 deletion was present in greater than 75% of such patients with abnormalities of the ventricular outflow tracts and abnormal subclavian arteries, whereas it was present in less than 30% when there was no subclavian arterial anomaly. The patients with tetralogy of Fallot with chromosome 22q11 deletion show higher incidence of cervical aortic arch. Down syndrome has also been shown to be associated with an increased risk for aberrant right subclavian artery. Therefore, in the presence of an anomaly of the aortic arch, fetal karyotyping is recommended, especially when it is associated with intracardiac anomalies, extracardiac malformations, or an increased nuchal translucency.
Clinical Findings
Patients with a vascular ring may develop symptoms and signs of airway obstruction and/or esophageal compression. Clinical manifestations vary with the severity of encroachment on the trachea, bronchus, or esophagus by the abnormal artery or arteries. The severity of compression of the trachea and esophagus not only depends on the type of anomaly but is also affected by the size and shape of the thoracic cage enclosing the vascular structures and airway. For instance, a chest cage with diminished anteroposterior diameter, as in stove chest or straight-back syndrome, is associated with more severe compression than a normally shaped chest cage. In addition, dilation of a vascular component of the anomalous aorta or its neighboring vessel and hyperinflation of the lungs may further compromise the patency of the airway and esophagus.
Tighter vascular rings usually present early in life with respiratory symptoms, whereas partial rings may present later with symptoms of esophageal compression. Patients with a double aortic arch tend to have the earliest onset of symptoms. Common presenting symptoms are respiratory, including stridor, wheezing, and cough. A study of a large cohort showed that symptomatic patients have significantly altered tracheal geometry, with smaller diameters and cross-sectional areas compared with asymptomatic patients. The characteristic stridor is inspiratory , but it may be both inspiratory and expiratory. Stridor may not be obvious during sleep or quiet play and is exacerbated by exertion or crying. The severity and pattern of stridor or noisy breathing can change with position. There may be a history of recurrent respiratory infections requiring medical attention, and some patients have been referred for an evaluation of suspected asthma or bronchiolitis. Infants with a tight vascular ring may show life-threatening reflex apnea with feeding. Some infants show opisthotonic posture, with hyperextension of the neck to relieve tracheal compression. Respiratory symptoms seen in infancy or early childhood may disappear with conservative medical treatment as the thoracic cage becomes more spacious as the patient grows.
Symptoms of esophageal compression, including dysphagia or choking, usually develop later when the patient commences to take solid foods. Dysphagia is often the first symptom in older patients. This late onset of dysphagia may be related to the elongation of the aorta or be secondary to aneurysmal changes of an existing diverticulum of Kommerell or the adjacent descending aorta. Dysphagia may be associated with frequent aspiration. A vascular ring is occasionally diagnosed at the time of removal of foreign bodies, such as chicken bones and coins. When a diverticulum of Kommerell develops aneurysmal change, it may be complicated by dissection and frank rupture.
The majority of patients with isolated abnormalities grow adequately. Uncommonly, there may be failure to thrive and poor physical development due to frequent pulmonary infections and/or difficulties with feeding. Rarely, the patient may present with the symptoms of compression of a nerve plexus by an abnormally positioned aorta or its branch. Incidental discovery of an anomaly is not uncommon following imaging for other medical problems.
Diagnostic Investigations
The chest radiograph is a simple and logical starting point for the imaging algorithm ( Fig. 47.15 ). It provides information regarding not only the position of the aortic arch but also the pulmonary complications of the anomaly, if present. The laterality of the aortic arch relative to the trachea is usually readily apparent. The trachea is bent to the other side of the aortic arch. The tracheal air column usually shows a subtle indentation caused by the aortic arch. In most cases the position of the arch can be traced back from the descending aorta, which can be identified as a vertical linear stripe along the bony spinal column. When the aortic arch and the vertical linear stripe of the proximal descending aorta are seen on the opposite sides, a double aortic arch, or a circumflex retroesophageal aortic arch with the descending aorta on the other side, should be suspected. A double aortic arch may cause a concentric narrowing, with bilateral indentation in the lower trachea, but more frequently, the narrowing is asymmetric and the indentation is unilateral. Bilateral indentation by other types of vascular ring is rare. The lateral view may show anterior bowing of the distal trachea when there is a large retroesophageal component ( Fig. 47.16 ). A dilated proximal esophagus containing air may suggest the presence of a retroesophageal component. The superior mediastinum is wide when there is a cervical aortic arch, and a mediastinal mass can be mistakenly suspected. However, the superior mediastinum in young children is normally wide because of a large thymus. Atelectasis or pneumonic consolidation involving various lung regions may be present and mask the underlying malformation. In some cases, hyperinflation of the lungs or parts of the lungs may be the predominant radiographic feature.
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