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 oval fossa defects. ‘Secundum defect’ most likely refers to the normal absence of septum secundum in this area however a persistent shunt requires that the fossa ovalis is not closed by the underlying septum primum which could be due to malformation of one or both of these structures.

Secundum ASD should not be confused with the ostium secundum, the hole that develops in the septum primum close to the atrial roof. 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, up to 34% of adults have a persistent 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

• 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

Clinical presentation

All patients with the above syndromes are best evaluated with ultrasound. Non-syndromic ASD is usually clinically occult, with 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% present as adults with 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

There are characteristic ECG findings in patients with an atrial septal defect.Uncomplicated secundum ASD:

• sinus rhythm

• incomplete right axis deviation

• rSR’ trace in V1

• right bundle branch block (RBBB morphology with QRS duration between 110-120 ms(indicates dilated RVOT)

  • increased specificity with crochetage sign in the inferior (II, III, aVF) leads

• right precordial (V1-3) "defective T waves"Ostium primum ASD:

  • describes biphasic morphology, initial T wave flattening or inversion and sharp, positive upstrokeprolonged PR interval (due to terminal positive deflection 9left atrial dilatation)

  • peakleft 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:

  • loss of therSR’ trace in V1

  • tall monophasic R wave and deep inversion of T wave delayed when compared to lateral precordial leads

• notching of the terminal upstroke of the R wave (crochetage sign) correlates with the size of ASD and implies a greater degree of shunting 10

• left axis deviation

  • characteristic of ostium primum defects and anatomical distortion of the left bundle branch fascicles

  • associated first degree AV block

• right axis deviation

  • suggests ostium secundum defect

• low atrial ectopic rhythms

  • negative P wave polarity in lead II

  • found in sinus venosus ASDs

Pathology

Embryology

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

The foetal circulation must bypass the lungs until birth when oxygenation relies on the lungs and the pulmonary vasculature is 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 major types of atrial septal defect, distinguished according to their location within the septum:

• secundum ASD

  • 60-90% of all ASDs

  • usually an isolated abnormality

• primum ASD

  • 5-20%

  • associated with cleft anterior mitral valve leaflet (partial atrioventricular septal defect)

• sinus venosus

  • 5%

  • associated with anomalous right pulmonary venous return to the superior vena cava or right atrium

• coronary sinus type ASD("unroofed coronary sinus")

  • <1%

A patent foramen ovale (PFO) is a form of atrial septal defect.

Radiographic features

Plain radiograph

• can be normal in early stages when the atrial septal defect is small

• signs of increased pulmonary flow (pulmonary plethora or shunt vascularity)

  • enlarged pulmonary vessels

  • upper zone vascular prominence

  • vessels visible to the periphery of the film

  • eventual signs of pulmonary arterial hypertension

• chamber enlargement 

  • right atrium

  • right ventricle

  • note: left atrium is normal in size unlike VSD or PDA

  • note: aortic arch is small to normal

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