Atrial Septal Defect

Introduction

Atrial septal defect (ASD) is the second most common type of congenital heart disease (CHD). ASD occurs when the septum between your left and right atrium is not formed completely. Due to the pressure in left atrium (LA) being greater, ASD is classified as an acyanotic CHD, as oxygenated blood from the LA is being forced through the ASD into the right atrium (RA) (see figure 1).

If the ASD is small enough, it may close without treatment after birth. However, larger defects may require treatment as it can lead to complications.

Epidemiology

ASD is diagnosed in 1.6/1000 live births³. Isolated atrial septal defect accounts for around 5 – 10 % of all CHD and co-exists in around 30 – 50 % of all CHD types¹. ASD incidences appear to be on the rise due to the ability of echocardiography in detecting its presence. Females are however more prone to ostium secundum when compared to males, 2:1 ratio.

Pathophysiology

The atrial septum is formed from two separate endocardial cushions, beginning during the 4^th week of gestation⁴. The primary atrial septum is the septum primum, which grows from cranial to caudal; so from the roof of the atrium down towards the atrioventricular endocardial cushions⁵. This closes off the ostium primum. The ostium secundum develops again from the atrial roof and grows downwards towards the septum primum. The space formed between septum primum and secundum is known as the foramen ovale (FO). FO closes shortly after birth when vascular resistance changes: systemic BP increases with decreasing pulmonary pressure, with a decrease in right atrium pressure⁶. Figure 1 demonstrates an Atrial Septal Defect.

By TeachMeSeries Ltd (2024)

Fig 1. Atrial Septal Defect showing direction of blood flow in normal pressure

There are five types of ASD², from commonest to least:

• Patent foramen ovale
• Ostium secundum defect
• Ostium primum defect
• Sinus venosus defect
• Coronary sinus defect

Ostium secundum:

Occurs when there is incomplete occlusion of ostium secundum by septum secundum, or too much reabsorption of septum primum from atrium roof (during apoptosis in forming foramen secundum⁷).

Ostium primum:

Occurs when septum primum fails to fuse with endocardial cushions, allowing blood to travel from left to right atrium. This is the 3^rd most common type of ASD⁵.

There are 2 types of ostium primum defects⁸:

• Complete AVSD:

A common atrioventricular (AV) valve, but defect spanning from atrium to ventricles. (1 common AV orifice). Check out our atrioventricular septal defect article for more information.

• Partial AVSD:

A partial AVSD is a defect of just the ostium primum, with an intact ventricular septum. (2 separate AV valves and orifices)

Sinus venosus defect:

This can be split into superior and inferior defects:

• Superior defect:
  When superior vena cava (SVC) opening runs on top of oval fossa (foramen ovale remnant) of atrial septum. This renders SVC draining blood from both LA and RA. Usually co-exists with abnormal communication between SVC and right superior pulmonary vein⁵.
• Inferior defect:
  Less common than superior defect, but occurs when IVC orifice overrides LA & RA. Can co-exist with abnormal communication between IVC and right inferior pulmonary vein⁵.

Coronary sinus defect⁹:

Unroofed coronary sinus defect is an absence in the roof of the coronary sinus. This can be partial or focal, allowing transmission between coronary sinus and left atrium.

Risk factors

Autosomal dominance inheritance has been found to have a  link with ostium secundum ASD clusters. Risk also increases in children with a family history of ASD¹⁰. Certain maternal risk factors identified have been found to be associated with ASDs¹¹:

• Maternal smoking in 1^st trimester
• Maternal diabetes
• Maternal rubella
• Maternal drug use e.g. cocaine & alcohol

CHDs are often linked to congenital syndromes, and ASD is no exception. Ostium secundum ASD is commonly associated with syndromes³:

• Treacher-Collins syndrome
• Thrombocytopenia-absent radii syndrome (TAR syndrome)

Although ASD itself is not usually life-threatening, co-existing CHD may increase mortality.

Clinical features

From history

It is important to remember that vast majority of ASD are asymptomatic. Most ASD and PFO are diagnosed following stroke / TIA assessments in adulthood.

Symptoms of large ASD in paediatrics:

• Tachypnoea
• Poor weight gain
• Recurrent chest infections

Symptoms of untreated large ASDs in adults¹³:

• Exercise intolerance
• Palpitations
• Recurrent chest infections
• Fatigue
• Syncope

From examination

On auscultation¹⁴:

• Murmur: soft, systolic ejection murmur, best heard over pulmonary valve region (2^nd ICS, figure 2).
• Wide, fixed split S2
• Diastolic rumble in lower left sternal edge in patients with large ASD

By TeachMeSeries Ltd (2024)

Figure 2: Diagram illustrating the ideal position for listening to specific valves. ASD best heard over upper left sternal edge (pulmonary area)

Differential diagnosis

• Atrioventricular septal defect
• Ventricular septal defect (VSD)
• Innocent murmur: usually soft and musical
  • ASD murmurs maximum grade 2/6 with wide, fixed, split S2 (from increasing flow through pulmonary artery)
• Pulmonary stenosis murmur: more turbulent

Investigations

Bedside test

ECG: usually normal in children with small ASD

Possible ECG findings in large ASD:

• Tall P wave (right atrial enlargement)
• Right bundle branch block (incomplete)
• Right axis deviation

In adults, atrial fibrillation (AF) or atrial flutter may be seen.

Imaging or invasive tests^3,11

• Transthoracic echocardiogram is gold standard in diagnosing ASD, as it provides information regarding both the size of ASD, and direction of blood flowing through the defect (using Doppler). It is also able to approximate the pulmonary artery pressure.
• Cardiac MRI and CT can also assess the structures around the heart.
  • Cardiac MRI, usually carried out in CATH lab measures RV volume, showing RV overload in large ASD.
  • CT isn’t imaging modality of choice due to radiation risk
  • Cardiac MRI can measure pulmonary vs systemic blood flow ratio (Qp/Qs). The ratio of pulmonary to aortic flow informs us how significant the ASD shunt is.
• CXR identifies cardiomegaly:
  • CXR usually normal in children with small shunts

Management

Initial management

Conservative:

If ASD < 5mm, spontaneous closure should occur within 12 months of birth.

In adults, if patient is presenting with no signs of right heart failure and a small defect, then monitor every 2 – 3 years with echocardiogram³.

If presenting with arrhythmia, control rhythm with drugs & anticoagulated before definitive surgical treatment¹⁸.

Medical¹¹:

In children with heart failure, diuretics might be needed. Endocarditis prophylaxis is not currently required.

Definitive management¹⁹

Surgical closure is the definitive treatment, usually in patients with ASD > 1 cm. Surgery can be carried out percutaneously (transcatheter) or open chest²⁰ (central stenotomy) using cardiopulmonary bypass. Surgical closure is not recommended in patients where pulmonary hypertension is present (mean pulmonary pressure of 30mmHg), as this can induce RV failure if the ASD is closed up.

Percutaneous closure is carried out in cath lab and chosen method dependent on age of child. Percutaneous closure requires adequate septum to be present in order for the devise to hold in place.

Complications of percutaneous closure:

• Arrhythmias
• Atrioventricular block
• Thromboembolism (VTE aspirin)

Indications for surgical closure:

• TIA / stroke
• Ostium primum defects
• Sinus venous defects
• Coronary sinus defects

Complications

Consequences of untreated large ASDs¹³:

• Arrhythmias (caused by atrial stretch leading to abnormal foci development)
• Pulmonary hypertension
• Eisenmenger syndrome (presenting with: chronic cyanosis, exertional dyspnoea, syncope, increased risk of infections, increased pulmonary vascular resistance)¹⁵
• Cyanosis (only if Eisenmenger)
• Peripheral oedema (if eventually leading to heart failure)

• TIA / stroke

Prognosis²¹

Children born with isolated ASD are expected to have the same life expectancy as general population. If isolated ASD isn’t diagnosed until later life and symptoms develop as a result of ASD, then mortality rate increases. Surgical repair of ASD helps improve symptoms of co-morbidities, but risk of atrial flutter and AF still remain high.

References