Coarctation of the Aorta

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CHAPTER 38 Coarctation of the Aorta

Coarctation of the aorta is a narrowing of the proximal descending thoracic aorta. It can occur as an isolated lesion or in the presence of other congenital lesions, most commonly a bicuspid valve, patent ductus arteriosus, ventricular septal defects, or hypoplastic left heart. The severity of the coarctation and associated lesions determines the pathophysiology and clinical presentation. The clinical presentation varies from congestive heart failure in the newborn to asymptomatic hypertension or a murmur in older patients. Treatment options are surgical or interventional.

Definition

Coarctation refers to a stenosis of the proximal descending thoracic aorta that is almost always opposite the insertion of the ductus arteriosus—that is, at the junction of the distal aortic arch and the descending aorta just below the origin of the left subclavian artery (Fig. 38-1).¹^–⁴

FIGURE 38-1 Diagram of aortic coarctation showing narrowing of the proximal descending aorta (arrow) just below the level of the left subclavian artery.

Prevalence and Epidemiology

Coarctation of the aorta occurs in 3.2 of 10,000 births and accounts for 5% to 10% of all cases of congenital heart disease.¹^–⁴ Coarctation is more common in white males than in white females, with a male-to-female ratio of 1.3 to 2.0 : 1.³ Most cases are sporadic, but there may be a genetic inheritance.

Coarctation is thought to be the result of a malformation of the aortic media, leading to a posterior infolding or shelf.^1,^5,⁶ Most often, the shelf is discrete and opposite the ductus arteriosus. However, the malformation may be a long segment and circumferentially surround the aorta. In the latter form, there is typically diffuse tubular hypoplasia of the transverse arch and isthmus.^1,^7,⁸

Histologic examination shows thickened intima and media that protrude posteriorly and laterally into the aortic lumen. The ductus arteriosus or ligamentum arteriosus inserts anteromedially at the same level.¹

The physiologic changes in cardiovascular function vary with the severity of the coarctation (severe or mild) and the presence of associated anomalies. In critical or severe coarctation, impedance to left ventricular outflow increases. The resultant hemodynamic changes include diminished stroke volume, increased left ventricular end-diastolic pressures, and elevated left atrial pressures. If there is associated aortic stenosis or a large ventricular septal defect, left ventricular systolic pressure and end-diastolic volume will further increase. The end result is heart failure and pulmonary edema. In addition to cardiac abnormalities, there are changes in vascular physiology. The diminished perfusion of the distal aorta and lower body leads to renal failure and bowel ischemia.

In milder forms of coarctation, the left ventricular myocardium hypertrophies to normalize myocardial wall stress and maintain normal systolic ventricular function. Left ventricular ejection fraction is often normal to increased. Collateral vessels also develop, which act as a source of lower body perfusion and help maintain normal flow to abdominal viscera.¹

MANIFESTATIONS

Clinical Presentation

Critical or severe coarctation manifests in the first few days to first week of life. Symptoms include cyanosis (especially of the lower body), heart failure, shock, multiorgan failure and necrotizing enterocolitis.¹^–⁵

Milder forms of coarctation manifest later in life. Because left ventricular function is maintained and cardiac outflow is good, patients come to clinical attention because of hypertension or a heart murmur related to a bicuspid aortic valve.¹^–⁴ Blood pressure in the upper extremities is typically higher than that in the lower extremities. However, if there is an aberrant right subclavian artery, the pressure in the right upper arm may be equal to or lower than that in the left upper arm.

Imaging Studies

Indications and Algorithm

Imaging is performed to confirm the site, morphology and severity of the coarctation, extent of associated collateral circulation, any associated valvular lesions, and degree of left ventricular hypertrophy. The initial imaging examination is usually chest radiography and echocardiography. In uncomplicated cases, computed tomography (CT) or magnetic resonance imaging (MRI) is the study used to follow chest radiography to confirm the diagnosis of coarctation.

The identification of collateral flow is important in surgical planning. The aorta is cross-clamped during surgical repair if collateral flow is sufficient for lower body perfusion. If collateral formation is inadequate, the use of cardiopulmonary bypass for perfusion of the lower body may be indicated.

Techniques and Findings

CT angiography is performed with a pulmonary embolism protocol using thin collimation (<1 mm), pitch lower than 1.5,⁹^,¹⁰ fast scan time, and a single breath-hold, when possible. In adolescents and adults, contrast medium is injected via a power injector at a high flow rate of 3 to 4 mL/sec using a contrast volume of 100 to 150 mL (280 to 320 mg I/mL). Scan delay time can be determined with an automatic bolus tracking system with the cursor over the aortic isthmus or empiric timing. Empiric timing, using a delay of 20 to 25 seconds after the start of the injection, is a default method if the bolus tracking fails to trigger. Electrocardiographic (ECG) gating is not needed for the evaluation of coarctation.

The volumetric data are reconstructed at 3- to 5-mm slice thickness for routine viewing and at 1- to 2-mm slice thickness for multiplanar reformatting and three-dimensional reconstructions. A standard reconstruction algorithm is used for reconstruction.

Special Considerations for Pediatric Patients

The contrast volume is 2 mL/kg (not to exceed 125 mL). Intravenous contrast medium can be administered with a power injector or manual (hand) injection. A power injector is used when a 22-gauge or larger cannula can be placed in an antecubital vein. For a 22-gauge catheter, flow rates are 1.5 to 2.5 mL/sec.¹⁰^,¹¹ Flow rates for larger gauge catheters are similar to those described above. With a manual injection, the contrast is pushed as quickly as possible. Determination of the scan initiation time can be made by an empiric or bolus tracking method. In pediatric patients weighing less than 10 kg, an empiric scan delay of 12 to 15 seconds after the start of the intravenous contrast injection suffices.¹⁰ In larger patients, the delay time is 15 to 25 seconds.

Magnetic Resonance Imaging

MRI, like CT, can accurately define the location, extent, and degree of aortic narrowing, collateral vessel formation, and valvular and ventricular morphology. In addition, MRI can provide an assessment of coarctation physiology, including information on blood flow and the pressure gradient across the stenosis.¹^,³ Because MRI yields functional information and does not rely on ionizing radiation, many centers prefer this modality over CT, especially for children.

Noncontrast-enhanced methods, both black blood and bright blood techniques, and three-dimensional contrast-enhanced MR angiography (CEMRA) are routinely acquired. CEMRA is performed by acquiring a three-dimensional gradient-echo sequence following a rapid bolus of gadolinium. There are several ways to time contrast delivery. The best guess method is the simplest, the automated detection method is the most operator-independent,¹² and the timing run is the most reliable method.¹³

Velocity-encoded (VEC) phase contrast techniques are used to determine flow, peak flow rates, and the volume of flow per unit time (see Chapter 17, Fig. 17-8).¹⁴^,¹⁵ The pressure gradient across the obstruction is calculated with a modified Bernoulli equation (product of the cube of the peak velocity as measured in meters per second).

Angiography

Angiography is no longer needed for the diagnosis of uncomplicated coarctation if conventional noninvasive imaging can clearly delineate the anatomy and physiology.¹^,⁴ If conventional studies are equivocal or indeterminate, or if there are possible associated intracardiac anomalies, angiography is performed.

Classic Signs

Chest Radiographs

In infants with critical coarctation, the chest radiograph demonstrates cardiomegaly and pulmonary edema (Fig. 38-2). In older children, the heart size is normal or slightly enlarged. There may be dilatation of the ascending aorta and a reverse 3 sign caused by aortic dilatation proximal and distal to the coarctation. Rib notching between the third and eighth ribs may be seen in older children, but rarely in children younger than 5 years (Fig. 38-3).⁴

FIGURE 38-2 Anteroposterior radiograph of a 3-day-old infant shows moderate cardiomegaly and increased vascularity consistent with mild pulmonary edema.

FIGURE 38-3 Chest radiograph shows rib notching (arrows) caused by erosion of the undersurface of the ribs by dilated intercostal vessels.

Computed Tomography and Magnetic Resonance Imaging

The classic CT and MRI findings of coarctation are discrete short-segment aortic narrowing with an infolding or shelf-like appearance of the posterior wall (Fig. 38-4).^9,¹⁶^–¹⁸ The isthmus and possibly the transverse arch will also be narrow in the diffuse form of coarctation (Fig. 38-5). In older individuals, other CT and MRI findings include dilatation of the aorta proximal to the coarcted segment, collateral vessel formation, and rib notching (Fig. 38-6). The common collateral pathways are the intercostal arteries (usually third through eighth) and internal thoracic arteries (Fig. 38-5; see Fig. 38-4B).³ Other cardiac lesions, such as ventricular septal defect, bicuspid valve, and patent ductus arteriosus, may be seen.

FIGURE 38-4 Focal coarctation. A, Axial CT image shows intercostal vertebral collateral vessels (arrows). The caliber of the ascending (A) and descending aorta (D) is equal. B, Sagittal volume rendered CT reconstruction demonstrates the characteristic narrowing (small arrow) of the aortic lumen just below the origin of the left subclavian artery (S) and aortic dilatation proximal and distal to the obstruction. Note also enlarged posterior intercostal (black open arrow) and internal mammary arteries (white open arrow). Of note, location and severity of the coarctation are best seen on reconstructed images.

FIGURE 38-5 Diffuse coarctation. This sagittal view from a contrast-enhanced three-dimensional MR shows diffuse narrowing of the aortic isthmus (arrows) and enlarged internal thoracic artery and multiple enlarged intervertebral arteries.

FIGURE 38-6 Rib notching. This posterior coronal multiplanar CT reformation shows erosions of the undersurfaces of several ribs (arrows) and collateral vessels.

Reconstructed images in the sagittal and parasagittal planes are best to show the location and extent of coarctation (see Fig. 38-4).¹⁹ Coronal and sagittal reconstructions may help delineate rib notching and/or collateral vessel formation (see Fig. 38-6).

DIFFERENTIAL DIAGNOSIS

From Clinical Presentation

The differential diagnosis of coarctation in the neonate includes other lesions that cause critical obstruction of the left ventricle outflow tract, including the following: (1) interrupted aortic arch; (2) hypoplastic left heart; and (3) critical aortic stenosis.⁵ Most neonates with critical left ventricular outflow obstruction present in the first few hours to first week of life with signs of shock and cardiac failure and radiographic findings of cardiomegaly and pulmonary edema.⁵

Interruption of the Aortic Arch

This is a rare anomaly, accounting for 1% to 1.5% of all critically ill infants with congenital heart disease.⁴^,²⁰ This is a marked form of coarctation in which a segment of the aortic arch is completely absent. There are three types of interrupted arch based on the site of interruption:

Type A—distal to the left subclavian artery

Type B—between the left common carotid artery and the left subclavian artery

Type C—between the brachiocephalic trunk and the left carotid artery

A dilated patent ductus arteriosus usually supplies the descending aorta beyond the interruption (Fig. 38-7).¹⁹^,²¹ Associated cardiac defects include ventricular septal defect, bicuspid aortic valve, aortopulmonary window, truncus arteriosus, single ventricle subaortic stenosis.³^,⁴ CT and/or MRI can have a role in diagnosis.

FIGURE 38-7 Interrupted aortic arch, type B. This three-dimensional volume rendered sagittal CT reconstruction shows a normal-caliber ascending aorta (A), hypoplastic transverse arch (arrow), and absence (i.e., interruption) of the aorta proximal to the left subclavian artery (S). A large patent ductus arteriosus (PDA) supplies the descending aorta (DA).

Hypoplastic Left Heart Syndrome

This is a spectrum of left heart obstructive lesions characterized by underdevelopment of the mitral valve, left ventricle, aortic valve, and aorta. The right heart and main pulmonary artery are enlarged. Atrial and ventricular septal defects and patent ductus arteriosus are common associated defects.²²^,²³ Diagnosis is confirmed by echocardiography and cardiac angiography. CT and MRI usually have no role in preoperative assessment.

Critical Aortic Stenosis

This is a condition caused by fusion of the valve leaflets, resulting in a slit-like aortic orifice, severe outflow tract obstruction, left ventricular hypertrophy, and ultimately cardiac failure and pulmonary edema.⁵ Diagnosis is confirmed by echocardiography and cardiac catheterization. CT and MRI have no role in preoperative assessment.

There is almost no differential diagnosis of coarctation in older children and adults if there is a classic clinical history of upper extremity hypertension.

From Imaging Findings

The plain radiographic differential diagnosis of coarctation in the neonate includes any of the lesions discussed that are associated with cardiomegaly, heart failure, and pulmonary edema.

Pseudocoarctation is in the CT-MR differential diagnosis of coarctation in older patients. It is associated with an elongated, redundant aortic arch and appears as a buckle in the arch at the insertion of the site of the ligamentum arteriosum (Fig. 38-8). Luminal obstruction is absent. The blood pressures in the upper and lower extremities are almost always similar, although minor differences have been reported. There is no collateral circulation, which helps distinguish between pseudocoarctation and coarctation.

FIGURE 38-8 Pseudocoarctation. This sagittal CT reconstruction shows a tortuous proximal descending aorta with a focal kink below the origin of the left subclavian artery. Note the absence of collateral vessel formation.

SYNOPSIS OF TREATMENT OPTIONS

Medical

In neonates with heart failure, prostaglandin E₁ is administered to reopen the ductus arteriosus and establish perfusion to the distal aorta and abdominal viscera. Treatment of congestive heart failure with digoxin and diuretics is also instituted. As soon as the patient condition improves, primary surgical repair is performed.^1,^2,⁴

Surgical and Interventional

Surgical management of the patient depends on the age of the patient, type of associated malformations, and morphology of the coarctation itself. Surgical procedures include the following:

1 Primary surgical repair, involving resection of the narrowed segment and an end-to-end anastomosis. This is the procedure of choice in neonates and young infants.

2 Subclavian flap aortoplasty, consisting of dividing the subclavian artery and inserting a flap of the vessel longitudinally into the aorta at the site of coarctation. It is no longer commonly performed because it requires sacrifice of the left subclavian artery, which may lead to claudication with exercise and diminished growth of the left arm.²⁴

3 Patch aortoplasty in which the aorta is opened longitudinally, the fibrous shelf is excised, and a prosthetic patch is inserted to widen the aortic lumen. It is seldom used now because of the high incidence of late aneurysm formation.

4 Percutaneous balloon angioplasty with or without stent placement. It is used as treatment for residual stenosis or recoarctation after previous surgery ²⁵^,²⁶ and for native coarctation in patients older than 12 months.²⁷^,²⁸ It has limited use in younger patients because restenosis is common.

The surgical mortality rate from a primary repair is less than 5% in neonates and less than 1% in older patients.⁴ Late complications of surgical and interventional treatment include recoarctation or residual coarctation and aneurysm formation (Fig. 38-9).²⁹ The incidence of aneurysm formation is higher after angioplasty and patch graft repair than after primary surgical repair.³