Atrial septal defect

Atrial septal defect (ASD) is the second most common congenital heart defect after ventricular septal defect (VSD). Diagnosis is often delayed until adulthood when complications arise such as atrial arrhythmias or pulmonary hypertension. The radiologist may be the first to suggest the diagnosis by recognising the typical features on a chest radiograph.

Terminology

The term ASD is used loosely to describe any communication that allows blood to shunt between the atria in adults, including defects in the endocardial cushion, sinus venosus and coronary sinus.

The true atrial septum is the small area around the fossa ovalis which could be removed without creating an epicardial fistula. The small size of this area of atrial contiguity is a consequence of the method of formation and the extent of the normal septum secundum. This septum results from infolding of the atrial wall with associated epicardial tissues. The fossa ovalis is not covered by the septum secundum and the floor of the fossa is the embryologic septum primum.

Although defects in the true septum are commonly known as ‘secundum ASDs’ they could also be described as fossa ovalis defects. ‘Secundum defect’ most likely refers to the normal absence of septum secundum in this area however a pathological ASD requires that the fossa ovalis is not closed by the underlying septum primum which can be due to malformation of one or both of these structures.

Secundum ASD is often confusingly taken to refer to the ostium secundum, the normal hole that develops in the septum primum close to the atrial roof. In the foetus this is connected to the fossa ovalis forming the channel which allows right to left shunting to continue after the ostium primum closes (when the septum primum fuses with the endocardial cushions). At autopsy, in up to 34% of adults this connection persists as a probe-patent tunnel running from the fossa ovalis to the ostium secundum, and this has been labelled ‘patent foramen ovale’ 16.

Epidemiology

Atrial septal defects account for 10% of congenital heart disease and are more common in women, M:F = 2:1. Close family history of ASD was found in 2%.

Associations

Atrial septal defects are seen in association with the following:

• Down syndrome: 50% have congenital heart disease and endocardial cushion defects predominate, outnumbering secundum ASD

• other trisomies

• Holt-Oram syndrome: autosomal dominant inheritance, mutations of the T-box-5 gene (TBX5) prevent normal development of the upper limbs and heart

• Ellis-van Creveld syndrome (chondroecterdermal dysplasia): 50% have mutations in the EVC or EVC2 genes, autosomal recessive inheritance, rare but more prevalent in Old Order Pennsylvanian Amish and Western Australian indigenous peoples

• familial ASD (without skeletal manifestations), autosomal dominant due to mutations in the cardiac transcription factor NKX2.5

• GATA4 gene variants also have a role in ASD

Clinical presentation

All patients with the above syndromes are best evaluated with ultrasound. Non-syndromic ASD is typically clinically occult (in contrast with small transitory VSDs, - the high pressure difference generates a loud noise and even a palpable thrill). There are typically no symptoms and subtle flow-related signs, such as splitting of the first heart sound, tricuspid flow murmur and fixed wide splitting of the second heart sound. A pulmonary flow murmur is hard to distinguish from normal aortic flow in a child. Progress depends on shunt size and associated defects. Exercise intolerance, exertional dyspnoea and recurrent pneumonia may occur or an abnormal ECG or CXR may suggest the diagnosis.

Even with good healthcare, more than 50% do not present until adulthood and the onset of complications such as atrial fibrillation, pulmonary hypertension and right heart failure. Left ventricular compliance decreases with age, causing increased left to right shunting. By the age of 60, almost all will be symptomatic.

ECG

Uncomplicated secundum ASD:

• sinus rhythm

• right axis deviation

• rSR’ trace in V1

• right bundle branch block (indicates dilated RVOT)

Ostium primum ASD:

• prolonged PR interval (due to left atrial dilatation)

• left axis deviation

• rSR’ trace in V1

• right bundle branch block or interventricular conduction delay

Sinus venosus defect:

• negative P wave in lead III

• left axis deviation

Pulmonary hypertension affects these findings in the following ways:

• loss of rSR’ trace in V1

• tall monophasic R wave and deep inversion of T wave

Finally, a ‘crochetage’ notch in the terminal upstroke of the the R wave in the inferior limb leads has been reported as a marker of secundum ASD which correlates with the size of the shunt 10.

Pathology

Embryology

(For a full description see development of the heart.)

The foetal circulation must bypass the developing lungs until birth when they take on their role in respiration. The pulmonary vasculature is now sufficiently developed to function in series with the systemic circulation.

Division of the atrial chambers begins as the thin septum primum grows from the atrial roof towards the endocardial cushions at the atrioventicular junction. The ostium primum is the gap between the septum and the cushions and before this closes the ostium secundum must open. This lies near the atrial roof.

The septum secundum forms by incomplete invagination of the atrial wall along the right atrial aspect of the septum primum leaving a bare area of septum primum at the fossa ovalis. Blood continues to shunt from right to left through the patent foramen ovale, exiting through the ostium secundum. After birth and the onset of respiration, the left atrial pressure rises. This pushes the septum primum against the fossa ovalis (like a flap valve) causing functional closure. In about 2 out of 3 people adherence closes the tunnel over the ensuing months.

Classification

There are four main types of defect commonly included, however only the first two and PFO are true septal defects:

• secundum ASD (fossa ovalis)

  • 75% of all ASDs, 7% of congenital heart defects rising to 40% after 40 years of age

  • usually an isolated abnormality

• primum ASD (endocardial cushion defect) 

  • 15 - 20% of ASDs

  • a spectrum from primum ASD with cleft mitral valve to atrioventricular septal defect with common AV valve (AV canal)

  • strong association with trisomy 21

  • 50% mortality within 1 year if untreated

• sinus venosus defects

  • < 10%

  • superior is much more common and is typically associated with anomalous drainage of the right superior pulmonary vein into the SVC

  • inferior defect is typically associated with anomalous drainage of the right inferior pulmonary vein into the IVC

• unroofed coronary sinus

  • <1%

  • coronary sinus drains to left atrium

  • associated with persistent left SVC

A patent foramen ovale (PFO) is a persistent small tunnel from the fossa ovalis to the foetal ostium secundum. They can be a source of paradoxical emboli when right atrial pressure is elevated (e.g. Valsalva, pulmonary hypertension). In one autopsy series, 34% probe patency was established.

Radiographic features

Plain radiographChest X-ray appearances depend on the shunt size, the presence or absence of mitral incompetence and the age of the patient:

• can be normal in early stages when the atrial septal defect is small and there are no complications

• signs of increased pulmonary flowplaeonaemia (pulmonary plethora or shunt vascularity)) due to increased pulmonary blood flow

  • enlarged pulmonary vessels

  • upper zone vascular prominence throughout the lungs

  • vessels visible to the periphery of the film

  • eventual signs of pulmonary arterial hypertensionlung

• cardiomegaly due to right atrial and right ventricular dilatation

• chamber enlargement pulmonary hypertension

  • right atriumdilated proximal pulmonary arteries

  • right ventricleperipheral (arterial) pruning

  • note: left atrium is normal in size unlike VSD or PDAright ventricular hypertrophy (upturned apex, broad area of sternal contact on lateral view)

  • note: small normal aortic arch is small to normalknuckle

  • left atrial dilatation only occurs if there is mitral incompetence

Treatment and prognosis

Atrial septal defects do not cause any impairment in cardiac function in utero and even most neonates are asymptomatic. The defect can be closed surgically or percutaneously (e.g. atrial septal occlusion device). However, careful evaluation has to be made to ensure lack of development of elevated right heart pressures or a right to left shunt before any intervention.  

Complications

In approximately 10% of untreated patients, pulmonary hypertension develops. In this situation, flow through the shunt eventually reverses and becomes right-to-left leading to cyanosis, known as Eisenmenger syndrome.

Other complications include:

• paradoxical emboli

• cardiac conduction defects, e.g. atrial fibrillation, atrial flutter