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Coarctation and Interrupted Aortic Arch
Introduction
Coarctation derives from the Latin term coartatio , which translated literally means “a drawing together.” Aortic coarctation, therefore, indicates a narrowing at some point along the course of the aorta. When used in the context of the congenitally malformed heart, coarctation most usually described an area of narrowing of the thoracic aorta in the region of the insertion of the arterial duct, with or without additional abnormalities of the aortic arch. Obstructive lesions can be found more proximally, involving the ascending aorta. These are considered, along with lesions of the aortic valve, in Chapter 45. Those distal to the thoracic aorta, together with acquired lesions, are beyond the scope of this chapter. Within this chapter, however, interruption of the aortic arch is considered. This involves discontinuity between two adjacent segments of the aortic arch. In hemodynamic terms, it includes cases with a fibrous cord between the discontinuous segments. In this respect, interruption can be interpreted as the extreme end of the spectrum of obstruction of the aorta ( Fig. 45.1 ).
Historical Considerations
The first description of aortic coarctation is generally attributed to Johann Freidrich Meckel, the Prussian anatomist, who presented the case of an 18-year-old female to the Royal Academy of Sciences of Berlin in 1750. At postmortem, she was found to have an aorta that was “so narrow that its diameter was smaller by half than that of the pulmonary artery, which it should have exceeded or at least have equaled in caliber.” Some argue, however, that it was Morgagni who should be given priority. As pointed out by Craigie, a more recognizable description was published in Desault's Journal de Chirurgie in 1791. According to Craigie, Monsieur Paris, Prosector of the Amphitheatre at the Hotel-Dieu, described, in the winter of 1789, the postmortem of “a very emaciated woman about 50 years old.” In addition to recognizing that the thoracic arteries were thicker and more tortuous than normal, he gave the following description. “The part of the aorta which is beyond the arch, between the arterial ligament and the first inferior intercostal, was so greatly narrowed that it had at most the thickness of a goosequill. Hence, in taking apart its walls, which had not decreased in this place, there remained only a small lumen. The part of the vessel which was above the constriction was slightly dilated; the distal part was of normal calibre. The most careful dissection did not reveal either in the aorta or in its vicinity any cause to which this extraordinary condition could be attributed.”
With regard to interruption of the aortic arch, which can be considered as the severest end of the spectrum of aortic coarctation (see Fig. 45.1 ), Celoria and Patton classified this lesion into alphabetic subtypes ( Fig. 45.2 ).
Interruption at the aortic isthmus had been the first pattern described, being recognized in 1778 by Stiedele in Vienna. The more common variety, with interruption between the left common carotid and left subclavian arteries, was described some 40 years later by Siedel. The least frequent variant, with interruption between the brachiocephalic and left common carotid arteries, was not seen until 1948.
Prevalence and Etiology
Aortic coarctation accounts for 7% of liveborn children with congenitally malformed hearts, with a higher incidence in stillborn infants. The overall incidence is in the region of 1 in 12,000, with a slightly increased occurrence in males. Coarctation is generally said to show multifactorial inheritance, although genetic factors are clearly important in certain groups. The lesion was found in 10% of a large Danish series of patients with Turner syndrome, albeit with a lower incidence in patients with mosaicism or those with structural anomalies of the X chromosome. Inheritance has also been reported as an autosomal dominant trait. It is now known that cells migrating from the neural crest populate the aortic arches, and a 22q11.2 deletion is well recognized as being associated with interruption between the left common carotid and subclavian arteries. Coarctation is also found with 22q11.2 deletion, supporting an association with abnormal migration of cells from the neural crest. There is a reported seasonal incidence, with paucity of males born between April and August but without identification of an exogenous etiologic agent.
Interruption of the aortic arch accounts for just over 1% of cases of so-called critical congenital cardiac disease. As already emphasized, there is a known association between deletion of chromosome 22q11.2, specifically DiGeorge syndrome, and interruption between the left common carotid and subclavian arteries. As many as one-third of those with the DiGeorge phenotype have an interruption of this type; conversely, two-thirds of those with interruption between the left common carotid and subclavian arteries have DiGeorge syndrome. Taking those with interruption as a group, interruption between the left common carotid and subclavian arteries accounts for between 50% and 75% of such cases, interruption as the isthmus for 40%; interruption between the carotid arteries is rare. Interruption as an isolated lesion is also rare. The combination of congenital absence of the aortic isthmus, patency of the arterial duct, and ventricular septal defect, however, is very common, occurring in 75% of patients with interruption. The incidence is equal between the genders.
Morphology
The lesions considered in this chapter all occur in proximity to the junction of the aortic arch and the arterial duct. This junction is clearly of significance to their evolution and morphology, although isolated coarctation can also exist proximal to the brachiocephalic arteries or in the descending thoracic aorta. Aortic coarctation, however, is not a uniform entity. Rather, it represents a spectrum of lesions, generally encompassing variable degrees of hypoplasia within the aortic arch. The extreme end of the spectrum is interruption of the aortic arch (see Fig. 45.1 ). A fibrous cord can be found between the interrupted segments of the arch, so that there is hemodynamic interruption but anatomic continuity. This is also known as atresia of the arch ( Fig. 45.3B ). Tubular hypoplasia is present when there is a uniform narrowing of part of the arch (see Fig. 45.3A ).
Discrete coarctation is produced by a localized shelf-like lesion within the lumen of the arch, often with a degree of proximal tapering of the arch itself toward the obstructive shelf ( Fig. 45.4 ).
Obstructive Lesions
Presence of a discrete obstructive shelf within the lumen of the arch is much more common than tubular hypoplasia. The most systematic approach for categorizing the morphologic variables was suggested by Edwards. He emphasized the need to describe the site of coarctation precisely, irrespective of whether the arterial duct is patent, closed, or ligamentous and irrespective of the presence of additional anomalies. The other features, of course, also require description. Nonetheless, in terms of location, when the arterial duct is patent, the obstructing lesion can be preductal, paraductal, or postductal ( Fig. 45.5 ).
The most common site for discrete coarctation is at the junction of the aortic isthmus, the arterial duct or ligament, and the descending aorta. When the duct is open, there is usually a degree of isthmal hypoplasia, with the isthmus tapering down toward the junction with the duct, and the obstruction is preductal ( Fig. 45.6A ).
The obstructive lesion itself takes the form of a discrete waist associated with infolding of the aortic wall (see Fig. 45.6B ). In the majority of cases, the shelf is formed by ductal tissue, which completely encircles the lumen of the isthmus (see Fig. 45.4 ). The ductal shelf produces the major obstruction to flow, being the most important factor in coarctation. The “waisting” of the aortic wall usually accompanies the ductal shelf but can occur in isolation. When the arterial duct is closed, the ductal shelf becomes converted to a fibrous diaphragm, often with a pinhole meatus ( Fig. 45.7 ).
Paraductal coarctation occurs directly opposite the mouth of the duct at its insertion to the aorta and is found in 10% of cases ( Fig. 45.8 ). Postductal obstruction is seen distal to the aortic origin of the arterial duct and again accounts for about 10% of cases seen in infancy. The most important consequence of this variant is the lack of improvement of such critically ill infants despite maintenance of ductal patency with prostaglandin. Such postductal coarctation is the norm in adults, although it occurs in a postligamental rather than postductal position. Tubular hypoplasia, or the presence of a uniformly narrow segment of the aortic arch, frequently coexists with discrete coarctation, but it can exist in isolation (see Fig. 45.3A ). It is distinct from the gradual tapering of the isthmus characteristically seen with discrete coarctation (see Fig. 45.6A ). Histologically, the wall of an affected segment is normal, in contrast to the ductal and fibrous nature of discrete coarctation. The most frequent site for tubular hypoplasia is in the segment of arch between the origins of the left common carotid and the left subclavian arteries (see Fig. 45.3A ). When narrowed, the isthmus is usually the type that tapers toward the descending aorta (see Fig. 45.6A ). It is rare for the segment between the brachiocephalic and left common carotid arteries to be affected. Atresia of the aortic arch exists when the arch itself is anatomically continuous but there is no patency within one of its segments. The most common site for such atresia is the aortic isthmus (see Fig. 45.3B ), but atresia can also be found in the segment between the brachiocephalic and left common carotid arteries. Interruption is present when there is anatomic as well as hemodynamic discontinuity between segments of the aortic arch ( Fig. 45.9 ).
The most common site for interruption, accounting for about 75% of cases, is between the left common carotid and subclavian arteries, the type B pattern of Celoria and Patton (see Fig. 45.9A ). Most of the remaining cases show interruption at the isthmus, the type A pattern (see Fig. 45.9B ). Interruption between the brachiocephalic and the left common carotid arteries, the type C pattern, is extremely rare (see Fig. 45.2 ).
Associated Malformations
The presence of associated cardiovascular lesions was one of the criteria commonly used to differentiate coarctation found in patients presenting in infancy from those presenting, often with lesions in isolation, in later childhood and adulthood. The typical associated anomalies are those that tend preferentially to potentiate flow to the pulmonary rather than the systemic arterial pathway. Such lesions lead to reduced flow through the aortic isthmus in fetal life. This can be produced by defects that produce left-to-right flow at either the level of the ventricles or the great arteries. The most common of these lesions is a ventricular septal defect. The archetypical defect is seen in hearts from patients with interruption of the aortic arch with posterior deviation of the muscular outlet septum or its fibrous remnant into the subaortic area, the deviation leading to subaortic obstruction ( Fig. 45.10 ).
Such defects with deviation of the outlet septum or its fibrous remnant can also be found in the setting of coarctation. In the latter situation, however, the defects are more frequently perimembranous and associated with posteroinferior alignment of the aortic valve. Interruption of the aortic arch can be found with an aortopulmonary window rather than a ventricular septal defect (see Fig. 45.9B ). It, and coarctation, of course, can be found in the setting of discordant ventriculoarterial connections, double-outlet ventricle, or common arterial trunk. The coarctation or interruption is considered the associated lesion. These associations are dealt with in the appropriate chapters of this text. Very rarely, interruption can be found in the absence of associated lesions and with a closed arterial duct. The supply to the lower half of the body is dependent on collateral circulation. Associated lesions can also be found when they produce obstruction to the outflow from the left ventricle both pre- and postnatally. Lesions falling into this second category include valvar and subvalvar aortic stenosis along with the aortic valve with two leaflets. Congenital stenotic lesions of the mitral valve will also lead to decreased flow through the aortic arch, as will a supravalvar mitral shelf, a stenosing left atrial ring, or divided left atrium. The presence of several such lesions in combination presents a particularly poor prognosis, with the heart itself in such settings overlapping with the hypoplastic left heart syndrome. The best-known combination, described by Shone and now referred to as Shone syndrome, involves the coexistence of a parachute mitral valve, supravalvar left atrial ring, subaortic stenosis, and aortic coarctation.
Although the aortic valve with two leaflets is often found in patients having coarctation, the lesion is usually hemodynamically insignificant in early postnatal life. In later life, the abnormality of the aortic valve predisposes to calcific stenosis, regurgitation, and infective endocarditis. When found in association with coarctation, however, the morphology of the bifoliate valve is significantly different from that seen as an isolated lesion. When found in patients with coarctation, the valve typically has two equally sized leaflets, whereas bifoliate aortic valves seen in isolation usually have leaflets of unequal size. Irrespective of the valvar morphology, there is a known association between the bifoliate valve and weakness in the aortic wall, this accounting for the long-term incidence of dilation and subsequent dissection of the aortic root and ascending aorta. Whether this association will produce problems in the long-term follow-up of patients with coarctation has still to be established.
Anomalies of the subclavian arteries can accompany either discrete coarctation or interruption of the aortic arch. They are important both clinically and surgically, being seen more frequently in association with interruption. The most common anomaly is origin of the right subclavian from the aorta distal to the site of the ductal insertion (see Fig. 45.9A ). The anomalous artery pursues a retroesophageal course, often arising from the expanded segment of aorta called the diverticulum of Kommerell. The left subclavian artery can also be anomalous, arising distal to the junction between the duct and the descending aorta (see Fig. 45.3B ). In this setting, the isthmus itself is nonexistent. The mouth of the subclavian artery can be incorporated into the area of ductal tissue and has a tendency to be stenosed at its origin.
As might be anticipated, either coarctation, atresia, or interruption can also occur when the aortic arch is right sided. Coarctation can also be found in the setting of a double aortic arch, whereas interruption of part or parts of the hypothetical double aortic arch is an essential part of the understanding of vascular rings (see Chapter 48 ). Coarctation or interruption is also a frequent associated lesion when the ventriculoarterial connections are abnormal, including discordant ones, double-outlet right ventricle with subpulmonary defect, or the Taussig-Bing malformation, aortopulmonary window with intact septum, and congenitally corrected transposition.
Collateral Circulation
Although rarely present in infants, collateral circulation gradually develops throughout childhood in those with subcritical coarctation. Such collateral arteries bypass the obstruction and augment perfusion to the lower body. The most common pattern involves a large aberrant artery arising from the right subclavian artery, supplying the aorta below the coarctation, together with various branches of the left subclavian artery, including the thyrocervical trunk, and the left intercostal arteries via the left internal thoracic artery. This leads to notching of the ribs and the anterior spinal artery through the left vertebral artery. One particular vessel in this circulation has achieved recognition as the artery of Abbott. This anomalous vessel ( Fig. 45.11 ) arises from the posterior aspect of the isthmus, passing medially behind the carotid artery and transverse arch.
Secondary Pathology
Secondary pathology can be considered in terms of local effects, effects on the myocardium, and distant effects, with the last in general caused by hypertension. The local changes are seen in older children and adults when fibrous intimal thickening is superimposed on the site of coarctation. The thickened layer is composed of concentric layers of collagen with varying degrees of elastin and smooth muscle cells. Depletion and disarray of elastic tissues seen with cystic medial necrosis has also been observed. The intimal proliferation together with superimposed thrombus can lead to near or complete obliteration of the lumen. In such instances all distal perfusion becomes dependent on the collateral circulation. The distal aortic wall often shows poststenotic dilation and is somewhat thinner than normal. The abdominal aorta may be somewhat hypoplastic owing to diminished flow. The combination of these local changes accounts for the occasional development of aortic dissection in unoperated patients with advanced disease. The physiologic changes induced by pregnancy, particularly in the last trimester and peripartum, impose an increased hemodynamic strain on the aortic wall. In this setting aortic dissection and even rupture can initially be misdiagnosed as preeclampsia. Such complications can also follow an apparently successful repair. Whether earlier surgical intervention decreases the occurrence of the changes in the aortic wall is not yet known.
The direct effect on the myocardium of obstruction to left ventricular ejection depends on the rapidity of the onset as well as the degree of the increase in afterload, the left ventricle having numerous compensatory mechanisms. In the neonate undergoing rapid decompensation subsequent to ductal closure, left ventricular systolic and diastolic dysfunction rapidly leads to congestive heart failure. Diastolic flow in the coronary arteries decreases as left ventricular wall stress increases. This leads to ischemia, especially of the subendocardium. The resultant decrease in cardiac output causes and perpetuates a metabolic acidosis, which further depresses left ventricular contractility. In part, this is a consequence in infancy of the inability of the myocardium to mount the usual adaptive responses to increased impedance to outflow. These features are discussed further on under Pathophysiology. Unless intervention is performed, death can quickly ensue. Of those who survive the initial insult, some develop marked subendocardial fibrosis. If the onset of obstruction is less abrupt, compensatory adaptations can occur, primarily in the form of left ventricular hypertrophy. Ischemic heart disease eventually occurs in many patients, even in the absence of proximal coronary arterial occlusion. Distant complications include the well-recognized and classic “berry” or saccular aneurysm of the circle of Willis. All the organs in the upper body can sustain pathology secondary to hypertension. These changes are not entirely ameliorated by the initial relief of obstruction. This is also discussed at greater length in the following sections.
Morphogenesis
Three main aberrations in embryologic development have been suggested to explain abnormalities of the aortic arch. The first, abnormal embryogenesis of the vessels of the arch, and the second, abnormal development of the arterial duct, are closely interlinked. The third implicates changes in the ratio of flow between the pulmonary and systemic arterial pathways. In the usual situation with a left-sided arch, it is the artery of the left fourth pharyngeal arch that becomes the definitive aortic arch. The arterial duct is the persisting artery of the left sixth pharyngeal arch, which connects to the dorsal aorta. The left subclavian artery, in contrast, forms from the seventh segmental artery. This must undergo a cephalad migration through differential growth before it assumes its definitive position proximal to the aortic isthmus. It must cross many structures in its migration. Derangements in this process are suggested to be of importance in the pathogenesis of coarctation. The hypothesis implicating the arterial duct is based on the presence of the ductal sling around the entire circumference of the aortic isthmus in the setting of coarctation. Unequivocal evidence of such a ductal sling ( Fig. 45.12 ) was provided initially by Wielenga and Dankmeijer and subsequently confirmed by others.
The third proposal is that the patterns of flow of blood in the fetal circulation influence embryogenesis, specifically that a reduction in the volume of blood passing through the ascending aorta in fetal life leads postnatally to the development of coarctation. Such a hypothesis is strongly supported by the common association of coarctation with other obstructive lesions in the left side of the heart along with those malformations that result in decreased flow in the fetal ascending aorta.
No single hypothesis can explain the morphogenesis of all obstructive lesions in the aortic arch. It is most likely that there is interplay between the various mechanisms. It is highly likely, for example, that decreased flow to the aorta in some way influences the distribution of ductal tissue in the aortic arch. These basic mechanisms certainly help in clarifying the clinical presentation, early management, and even successful treatment of the various obstructive lesions to be described in the following text.
Presentation and Clinical Symptomatology
Neonates and Infants
Most infants with coarctation or interruption present with varying degrees of heart failure. If coarctation presents itself immediately, this is typically manifested by hemodynamic collapse with the closing of the ductus arteriosus. After the first weeks, poor feeding, sweating, breathlessness, and failure to thrive are the prevailing symptoms. The onset of cardiac failure is commonly seen within the first 3 months of life, but a significant number of patients present within the first week of life; they uniformly will have critical narrowing of the aorta. They present when the supplementary effect of blood flow through the duct from the right ventricle to the descending aorta is interrupted by its narrowing or closure, critically limiting the blood flow to the lower body. This situation, therefore, is often described as a duct-dependent systemic circulation. This process causes the neonate to become acutely unwell with metabolic acidosis, shock, renal failure, and necrotizing enterocolitis.
The secondary effects of acidosis on the myocardium lead to a global reduction in ventricular function and cardiac output. The coarctation itself may be recognized only as the infant is being resuscitated. With the introduction of prostaglandin E 1 in the 1970s by Elliott, it was possible to temporarily maintain ductal patency; this revolutionized the management of these infants.
Similarly, interrupted aortic arch tends to present with cardiac heart failure of acute onset occurring simultaneously with closure of the arterial duct within the first few days of life. In the most common situation, where interruption is associated with a patent arterial duct and ventricular septal defect, the infant will initially be well because pulmonary vascular resistance is high and blood will therefore pass through the arterial duct to the systemic circulation preferentially. One of two events will precipitate collapse in these infants. First, ductal constriction and closure will lead to a critical reduction in lower body perfusion and the rapid development of acidosis and shock. Second, the presence of a nonrestrictive duct combined with falling pulmonary vascular resistance will lead to preferential flow of blood to the pulmonary circulation to the detriment of the systemic circulation. Progressive tissue acidosis leading to hemodynamic collapse may also occur. In those few infants with interruption occurring in isolation, there must be a collateral circulation, usually via the head and neck vessels, which can develop very rapidly.
Physical Findings
The signs on presentation in infancy include tachypnea with intercostal retractions. If markedly low cardiac output is present, these patients will often show profound skin mottling, slow capillary refill, and peripheral cyanosis. Central cyanosis will occur only in the presence of an associated cyanotic congenital cardiac lesion or when there is persistence of the fetal circulation. The presence of palpable femoral pulses in the first day or two of life does not exclude the diagnosis of coarctation or interruption, since flow of blood to the lower body may be maintained through the persistently patent arterial duct. Once symptoms occur, the femoral pulses are usually weaker or absent, with the maintenance of radial pulses. This is not always the case if there is an aberrant right subclavian artery distal to the stenosis ( ) or the ostium of the left subclavian artery is involved in the ductal tissue. If the patient has severe low output and no pulses are felt, resuscitation usually results in the return of pulses at least in the right arm. The precordium is often active unless myocardial function is depressed. On auscultation there is usually a summation gallop rhythm. There is often a systolic murmur along the left sternal edge from the site of coarctation, and this may also be audible posteriorly. Associated cardiac or central vascular defects, such as the persistently patent arterial duct, can produce additional murmurs. An ejection systolic murmur may indicate an associated bifoliate aortic valve. Signs of congestive heart failure, such as hepatomegaly and crepitations on auscultation, are commonly found. The finding of pulsus paradoxus can also be found in neonates with interrupted aortic arch.
Auscultation in patients with interrupted aortic arch is usually unhelpful. Often a gallop rhythm is present, and the heart sounds are usually easily audible, the second being split. An ejection click may indicate the presence of associated bifoliate aortic valve, but this is nonspecific. If a murmur is present, it is often pan- or midsystolic and of low intensity, indicating the nonrestrictive nature of the ventricular septal defect.
Measurements of the blood pressure in all four limbs reveal a gradient between the upper and lower limbs irrespective of the method used to measure it, although Doppler appears preferable. It should be remembered that differences in pressure of up to 20 mm Hg may be revealed by Doppler interrogation in the normal neonate, presumably owing to the isthmal narrowing that is normal at this stage. It is sometimes necessary to measure blood pressures serially if the diagnosis remains unclear. Paradoxically, the diagnosis of coarctation can be made more difficult by the administration of prostaglandin. Although greatly improving the physical condition, this maneuver leads to significant diminution of the pressure difference between the arms and legs, making the clinical diagnostic process less clear. In this situation, there may be appreciable delay in femoral pulses compared with the radial pulses, which is known as as brachiofemoral delay. The combination of weak or absent femoral pulses together with a gradient in pressure between the limbs is virtually pathognomonic of aortic coarctation.
Older Children and Adults
It is not uncommon for patients with coarctation to go beyond infancy without detection, either because initially the coarctation was not severe enough to become critical following closure of the arterial duct or because of a significant early collateral circulation. The diagnosis usually follows a routine medical examination, where systemic hypertension is found, the murmur is discovered, or femoral pulses are found to be weak. The difference in pressure between the upper and lower limbs is frequently greater than 20 mm Hg. Headaches, nosebleeds, cold feet, or calf pain on exercise are often experienced. Rarely, patients present with end-stage systemic hypertensive disease, such as subarachnoid hemorrhage or hypertensive retinopathy. Detection during investigations for coronary arterial disease later in life has been reported.
With respect to interrupted aortic arch, there are currently over 40 reported cases of interrupted aortic arch presenting in adults. Presentation is predominantly due to hypertension and rarely due to congestive heart failure. These patients invariably have large, well-developed collateral systems with a closed ductus. Interestingly, type A, representing almost 80%, is far more common than type B interruption in this population.
Physical Findings
The physical findings in older patients usually rest on the appreciation of diminished or delayed femoral pulses compared with the pulse in either arm. More reliable is measurement of the blood pressures in all limbs. Indirect signs of left ventricular hypertrophy, such as a displaced apical beat and heave, are often found on palpation of the precordium. On auscultation, the first and second heart sounds are usually normal but may be accompanied by an apical fourth heart sound if the left ventricle is becoming noncompliant. The murmur of coarctation is best heard in the left infraclavicular fossa and radiates to the back over the left scapula. It is continuous, peaks late in systole, and continues into early diastole, corresponding with the diastolic tail seen on Doppler echocardiography. Additional continuous murmurs may be generated by larger collateral arteries, which can restore an adequate flow of blood to the lower body, resulting in palpable femoral pulses, albeit usually reduced and delayed. If surgery is considered, this feature will be crucial, as it determines whether or not partial cardiopulmonary bypass is required to maintain adequate perfusion of the lower body and spinal cord while the coarcted segment is excluded with clamps for the repair. A unique corkscrew appearance of the retinal arteries has been described on funduscopic exam that differs from the usual hypertensive change.
Investigations
Chest Radiography
In infants, cardiomegaly and increased pulmonary vascular markings can be seen on the radiograph. In older children with coarctation of the aorta, the heart size is often normal, but if cardiomegaly is present, it is usually caused by left ventricular enlargement secondary to hypertension. The two characteristic signs of coarctation in older children are the “figure 3” sign, which appears on the left border of the mediastinum and is caused by prestenotic and poststenotic dilation of the aorta ( Fig. 45.13 ), and rib notching, which is usually not seen until 4 to 5 years of age, although its appearance in the first year has been described.
By adulthood, around 75% of untreated patients have rib notching. It is best seen posteriorly in the medial third of the lower borders of the fourth to eighth ribs (see Fig. 45.13 ). The notching in coarctation is secondary to dilation of the collateralized intercostal arteries branching from the internal mammary. Rib notching is classically bilateral, to be differentiated from the unilateral notching seen after a classical Blalock-Taussig shunt, although unilateral notching can also occur with coarctation when a subclavian artery arises aberrantly distal to the site of obstruction.
In patients with interruption, the heart is usually left-sided with a normal abdominal and bronchial arrangement. Cardiomegaly, particularly enlargement of the left atrium, is present in 90% of neonates. Increased pulmonary vascular markings with pulmonary edema are also the norm. In the rare patients who survive infancy untreated, more specific signs can be seen, including absence of the aortic knob, a midline trachea, absence of an aortic impression on the barium swallow, and termination of the proximal descending aorta at the level of the pulmonary trunk. Rib notching can also be seen on the side of the subclavian arteries arising from the ascending aorta in the presence of a restrictive or closed arterial duct. A narrow mediastinum may suggest absence of the thymus gland, a feature of DiGeorge syndrome.
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