The aortic arch, the arterial duct, and the right and left pulmonary arteries have close spatial relationships with the major airways and the oesophagus. 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, the bronchuses, or the oesophagus. Many of the abnormalities of position and branching, along with the so-called pulmonary arterial sling, are characterised by a complete or partial encirclement of the trachea and oesophagus, 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, and vascular sling an encirclement which is incomplete. Vascular rings and slings are the classic anomalies that cause symptoms and signs of obstruction to the airways and oesophageal compression. Not all rings and slings, however, result in clinically recognisable symptoms and signs. Conversely, there are other vascular abnormalities that do not form a ring or sling, and yet may produce significant obstruction of the airways and oesophagus. In this chapter, we discuss these various anomalies of the aortic arch and pulmonary arteries that may cause obstruction to the central airways.
ANOMALIES OF THE AORTIC ARCH
In this section, we discuss the abnormalities in position and/or branching of the aortic arch. The obstructive lesions within the arch, such as coarctation or interruption, are not generally included in this category. They are addressed in Chapter 46 . 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 which 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 making reference to the model introduced by the pioneer pathologist, Jesse E. Edwards, in 1948 ( Fig. 47-1 ). 1,2 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 will not be discussed here, in part because the precise morphogenesis of the outflow tracts has still to be clarified, but also because most of the congenital malformations involving the aortic arch can be understood without knowledge of these earlier events, which are reviewed in Chapter 3 .
In the hypothetical model, symmetrical aortic arches connect the ascending and descending aorta on each side, forming a complete vascular ring around the trachea and oesophagus. 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 oesophagus. The hypothetical model, therefore, is made up of two vascular rings joined together at the descending aorta.
With normal development, it is the left aortic arch and left-sided arterial duct which persist, while the right aortic arch distal to the origin of the right subclavian artery, along with the right-sided arterial duct, regress ( Fig. 47-2 ). As a result, the proximal part of the embryological 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. It seems particularly perverse, however, to continue to designate an artery supplying vessels to the head and arms as being unnamed! The left-sided aortic arch, in sequence, gives rise to the brachiocephalic, left common carotid, and left subclavian arteries ( Figs. 47-3 and 47-4 ). 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 still remains difficult to predict the embryological 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 vascular ring, or else a sling, around the trachea and oesophagus. 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 oesophagus, since the anomalous artery takes a retro-oesophageal course. The arterial duct, regardless of whether it is patent or ligamentous, may also contribute to encirclement of the trachea and oesophagus. Occasionally the distal aortic arch itself has a retro-oesophageal course that causes oesphageal and tracheal compression. The assessment and description of the anomalies, therefore, 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 vascular ring or sling around the trachea and oesophagus are:
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Aortic arch anomalies forming a 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 retro-oesophageal left arterial duct between right-sided descending aorta and left pulmonary artery
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Circumflex retro-oesophageal aortic arch
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Aortic arch anomalies forming a vascular sling or incomplete 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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Circumflex retro-oesophageal aortic arch
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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 and most commonly recognised form of vascular ring. 1–10 It refers to the presence of two aortic arches, one on each side of the trachea and oesophagus ( Fig. 47-5 ). Both the left and right aortic arches of the hypothetical model persist, without regression of any segment. An arterial duct, more frequently the left than the right, persists, although cases with bilateral ducts have rarely been described. 11 During fetal life, when the arterial duct is patent, the composite arrangement of the two arches and a patent arterial duct produces a figure of 9 or 6 configuration at fetal echocardiography. 12,13 Each aortic arch gives rise to common carotid and subclavian arteries. In the majority of the cases with double aortic arch, both arches are patent. Usually the right arch is larger than the left arch, or less commonly the two arches are equally sized. The left arch is dominant in less than one-fifth 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 visualised by any imaging modality. It is difficult, therefore, 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 symmetrical arrangement. 14 The patent part of the atretic left aortic arch tends to have a more posterior position than the left brachiocephalic artery arising from the right aortic arch. An inferior kink of the proximal part of the common trunk for the subclavian and common carotid arteries in the presence of a diverticular outpouching from the descending aorta is a telltale sign of the presence of an atretic segment between the kink and the apex of the diverticulum. 15 The proximal 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 (see Figs. 47-6 through 47-10 ). The distal remnant of the left aortic arch, along with the aberrant left subclavian artery, produces the retro-oesophageal component of the ring. It has previously been described that the aberrant artery may course either between the trachea and oesophagus, or in front of the aorta. 1 It is now usually believed that arteries that do not take a retro-oesophageal course are collateral arteries. 2 The persistent arterial duct is usually left-sided, connecting the left pulmonary artery to the distal remnant of the left aortic arch ( Fig. 47-6 ). 1–7,10,16 This combination is the second commonest type of ring reported in most series. During fetal life, when the arterial duct is widely patent, this combination is characterised by a U -shaped vascular loop that encircles the trachea and oesophagus from behind ( Figs. 47-6 and 47-7 ). 12,17–20 This U -shaped loop consists of the ascending aorta, the right aortic arch, the distal remnant of the left aortic arch, the left-sided arterial duct, and the 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 ( Figs. 47-7 and 47-8 ). 21–24 Flow through this distal remnant, is from the left-sided arterial duct into the descending aorta in the fetal circulation, but switches its direction with ductal closure so that the aberrant left subclavian artery is supplied from the descending aorta in postnatal circulation (see Fig. 47-6 , right panel). Postnatally, therefore, the presence of a diverticulum of Kommerell is indicative of 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 oesophageal and, to a certain extent, the 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 ( Figs. 47-9 and 47-10 ). This is because the flow of blood through the left arterial duct was reduced, or even reversed, during fetal life. The distal remnant of the left aortic arch, therefore, does not persist as a diverticular outpouching after ductal closure. 12 Postnatally, an arterial ligament is suspected when the proximal left subclavian artery is tethered inferiorly toward the left pulmonary artery. The right-sided aortic arch with aberrant origin of the left subclavian artery is occasionally associated with persistence of the right arterial duct, or even absence of arterial ducts bilaterally (see Fig. 47-9 ). The latter combination is typically seen in tetralogy of Fallot with pulmonary atresia and pulmonary arterial supply via major aortopulmonary collateral arteries (see Fig. 47-10 ). This combination forms an incomplete encirclement or a vascular sling around the right side of the trachea and oesophagus. The right aortic arch with aberrant origin of the left brachiocephalic artery is rare. 25,26 It results from abnormal regression of the left aortic arch proximal to the origin of the left common carotid artery. The persisting arterial duct is usually left-sided, completing a vascular ring.
Left aortic arch with aberrant right subclavian artery is the most common anomaly involving the aortic arch, but is usually asymptomatic. 1,2,24,27,28 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 ( Figs. 47-11 to 47-13 ). 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 which persists. This combination forms a vascular sling around the left side of the trachea and oesophagus (see Fig. 47-12 ). Typically the aberrant left subclavian artery courses behind the oesophagus, but has been described to course between the trachea and oesophagus, although again these vessels may have been collateral arteries. 1,2 When there is a right-sided arterial duct between the aberrant artery and the right pulmonary artery, there is a complete vascular ring (see Fig. 47-13 ). The ring consists of the ascending aorta, the left aortic arch, the descending aorta, the distal remnant of the right aortic arch, the right arterial duct, the right pulmonary artery, and the 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 (see Fig. 47-13 , middle panel), an l -shaped loop is formed around the trachea and oesophagus. With closure of the right-sided arterial duct after birth, the distal remnant of the right aortic arch persists as the diverticulum of Kommerell (see Fig. 47-13 , right panel). It is theoretically possible for a left-sided aortic arch to be associated with aberrant origin of the right brachiocephalic artery, but thus far, as far as we are aware, this has not been reported. Aberrant subclavian or brachiocephalic arteries co-existing 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. 16 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 ( Figs. 47-14 and 47-15 ). This is the only anomaly of the aortic arch that does not constitute a vascular ring or sling, regardless of the presence of a left-sided or right-sided 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. 29 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 retro-oesophageal course to connect to the left pulmonary artery ( Fig. 47-16 ). 3,6,29–32 This is the only combination that constitutes a complete vascular ring in the presence of mirror-image branching. 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 retro-oesophageal 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 ( Figs. 47-17 to 47-19 ). 33,34 This combination requires the aortic arch to make an additional arc to the other side behind the trachea and oesophagus, 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 (see Fig. 47-17 , left panel). 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 retro-oesophageal 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 retro-oesophageal in location, but the aortic arch itself is behind the oesophagus. 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 oesophagus. A circumflex retro-oesophageal aortic arch is rarely seen without aberrant origin of a subclavian artery, but does exist (see Fig. 47-19 ). 29 Hypoplasia of the retro-oesophageal segment of the aortic arch is common. 34
The aortic arch is described as being cervical when its apex reaches the upper mediastinum above the level of the clavicles ( Figs. 47-20 and 47-21 ). 35–37 It may be recognised 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 retro-oesophageal 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 (see Figs. 47-20 and 47-21 ). 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. 38
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 ( Figs. 47-22 and 47-23 ). 39–41 It is explained on the basis of abnormal regression at two locations in the hypothetical double arch ( Fig. 47-22 , left panel), 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 pulmonary obstruction, 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. 42
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