Introduction
Coarctation of the aorta is defined as a congenital narrowing of the upper descending aorta/distal aortic arch opposite the arterial duct. This accounts for 5 to 8 per cent of all congenital heart defects. It is an isolated finding in 25 per cent of cases but is associated with other cardiac lesions in 75 per cent, most commonly ventricular septal defect (VSD). There is a strong association with bicuspid aortic valve (25–40 per cent). It is the most common cardiac defect in Turner syndrome.
Morphology and Histology
The constriction consists of localized medial thickening with infolding of medial and neointimal layers. It may form a shelf within the lumen of the aorta or may be concentric narrowing. The constriction may be discrete or, more rarely, a long tubular segment. All congenital coarctations are related to the site of duct insertion (‘juxtaductal’) with a ‘ductal sling’ of tissue encircling the aorta; however, coarctations that are inflammatory or autoimmune in aetiology may develop in the descending thoracic aorta or abdominal aorta, although they are extremely rare. There is commonly a degree of hypoplasia of the isthmus of the aorta, just proximal to the coarctation, and the transverse arch may be hypoplastic. Untreated coarctation can develop significant collateral vessels from proximal to the coarctation to the lower part of the body.
Coarctation may be a feature of more complex cardiac defects, typically those with a left-right shunt, such as VSD, atrio-ventricular septal defect (AVSD) transposition of the great arteries (TGA)/VSD and Taussig-Bing syndrome. It is also commonly seen as part of hypoplastic left heart syndrome, where there tends to be more extensive arch hypoplasia, and also as part of the constellation of left-sided obstructive lesions known as the ‘Shone complex’.
Pathophysiology and Natural History
The severity of the coarctation dictates the age of presentation. Severe cases present in neonates, usually as the ductus closes in the first few days of life. The haemodynamic consequences are a rapidly increasing afterload on the left ventricle leading to acute left ventricular failure and under-perfusion of the body beyond the coarctation and circulatory collapse.
In less severe coarctation, left ventricular (LV) failure is less likely, but LV hypertrophy and systemic hypertension gradually evolve. There is development of collateral circulation bypassing the coarcted segment. In infancy, congestive heart failure rarely occurs, and these children are usually asymptomatic and diagnosed only on chance clinical findings. Hypertension develops due to the mechanical obstruction and the activation of renin-angiotensin-mediated pathways.
Uncorrected coarctation leads to a curtailed life expectancy of 30 to 40 years, with causes of death including aortic rupture, intra-cranial haemorrhage, cardiac failure, accelerated coronary artery disease and bacterial endocarditis. Beyond infancy, even after correction, there is a lifetime risk of hypertension and its sequelae. After correction, freedom from complications or hypertension is only 20 per cent at 25 years.
Presentation
Neonates present with collapse, acidosis, hypotension, heart failure and absent femoral pulses. During infancy, there may be a degree of congestive heart failure causing dyspnoea and failure to thrive; femoral pulses may be weak.
Children/Young Adults.
Most commonly diagnosed on chance finding of abnormal CXR, ECG or hypertension. May present with headaches, lower extremity weakness, exertional dyspnoea and, rarely, stroke or a transient ischaemic attack (TIA). Examination reveals radio-femoral delay or weak femoral pulses, with differential blood pressure between upper and lower limbs.
CXR may show the classic ‘3’, or hourglass, sign of a coarctation along the contour of the upper mediastinum and rib notching on the underside of the ribs (caused by the large intercostal collateral vessels) The heart shadow may be enlarged if there is a degree of heart failure. The ECG shows left ventricular hypertrophy (LVH).
Echocardiography.
In neonates, infants and young children, trans-thoracic echo is diagnostic. Doppler measures the velocity across the coarctation, enabling estimation of the pressure gradient. Colour flow mapping demonstrates the diastolic ‘tail’ of persistently raised velocity in the descending aorta. The aortic valve should be carefully assessed to examine for bicuspid morphology.
In older children and young adults, MRI or CT scan becomes the imaging modality of choice, giving excellent delineation of the aortic anatomy and extent and size of collateral vessels. These modalities are not usually necessary in younger children if echo has provided clear definition of the anatomy.
Management of the Collapsed Neonate
Neonates may require resuscitation with intubation, ventilation and inotropic support if necessary. Prostaglandin E2 is essential to attempt to reopen the duct (usually successful because the duct has only just closed) and will usually stabilize the circulation. Surgery usually can be delayed until the patient is stable but is generally performed within the next 24 hours.
In an older child, hypertension should be treated, usually with beta-blockers, but surgery should not be delayed while waiting for normotension.
Surgery
Choice of technique and surgical approach depends on the age of the patient, the extent of the lesion and the presence of any associated cardiac defects. Neonates and infants with isolated coarctation are treated in one of two ways, usually via left posterolateral thoracotomy.
Resection and Extended End-to-End Anastomosis.
The site of the coarctation, the descending aorta, and the arch as far forward as the ascending aorta can all be mobilized and controlled. Clamps are placed on the descending aorta and on the arch as far forward as the left carotid artery. The coarctation segment and the isthmus are resected. The opening into the distal arch is extended forwards, and the two open ends are then anastomosed together (Figure 6.1). In extreme cases, the isthmus can be ligated and an incision made more anteriorly into the underside of the arch – the descending aorta is then brought up into this opening (an ‘end-to-side’ anastomosis). Care must be taken not to create too much tension in the arch, which can trap the left main bronchus.
Subclavian Flap Angioplasty.
(Figure 6.2) The aorta is mobilized and clamps placed across the descending aorta and transverse arch. The left subclavian artery is ligated distally, transected and then laid open along its length, extending the incision across the site of the coarctation. The opened subclavian artery is then folded down and used as a flap to enlarge the narrowed segment. This technique has the disadvantage of leaving the coarctation ridge in situ, which can lead to a higher recurrence rate and sacrifices the subclavian artery (which is well tolerated in neonates but can lead to slight hemi-smallness of the left arm).
In older infants and children, balloon angioplasty is the first choice for localized coarctation, but if ballooning is unsuccessful or if there is associated arch hypoplasia, then surgery may be required. Direct anastomosis remains the technique of choice but becomes more difficult in older children, in whom the vessels are less elastic. In older children, an interposition graft can be placed as long as an adult-sized graft can be accommodated, but this is becoming increasingly rare as balloon angioplasty and the use of intravascular stents become more feasible.
Children of any age with associated arch hypoplasia may be best treated via median sternotomy, mobilizing the arch and descending aorta on bypass. The coarctation segment is resected and the transverse arch laid open, coming as anteriorly as necessary. The posterior wall of the aorta is reconstructed end to end, and the opening in the concavity of the arch is then augmented with a patch (typically pulmonary homograft or xenograft pericardium). This is a reliable technique and is ideal if there are concomitant intra-cardiac lesions that require attention (e.g. VSD).
An alternative approach for discrete coarctation with VSD is to repair the aorta via a left thoracotomy and place a pulmonary artery band. However, there is an increasing trend towards single-stage repair for combined lesions such as these, repairing both the arch and VSD via median sternotomy on bypass.
Simple patch repair of the coarctation segment in older children is generally not recommended due to the high incidence of recurrent coarctation and of late false aneurysm at the suture lines. Adults presenting with re-coarctation usually can be successfully treated with balloon angioplasty and stent placement. If stenting is not feasible, or if there is associated transverse arch hypoplasia, then surgery can be considered – using an interposition graft via sternotomy or left thoracotomy with left heart bypass. Extra-anatomic grafts from the ascending aorta to the descending aorta (approached through the posterior pericardium) can avoid having to access the site of complex re-coarctation.
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