Description
An atrial septal defect (ASD) is a congenital heart defect characterised by a hole in the septum that separates the heart’s two upper chambers (atria). This abnormality allows oxygen-rich blood from the left atrium to mix with oxygen-poor blood from the right atrium, causing an increase in the total amount of blood that flows towards the lungs.
Pathogenesis
ASDs occur during foetal development when the septum does not form correctly, leaving an opening. The exact cause of ASDs is unknown, although they appear to be both hereditary and linked with environmental factors. For example, certain genetic disorders, such as Down syndrome, are associated with an increased risk of ASDs.
Epidemiology, Risk Factors & Associations
ASD is the second most common congenital heart disease, accounting for about 10% of all congenital heart defects (ventricular heart defects are the most common). Females are twice as likely to have an ASD compared to males.
Risk factors and associations include:
- Genetic syndromes:
- Anomalous pulmonary venous return (especially with sinus venosus defects): Total anomalous pulmonary venous return (TAPVR) and partial anomalous pulmonary venous return (PAPVR)
- Down syndrome (trisomy 21) – approximately 40-50% of children with Down syndrome have a congenital heart defect, and of those, about 8% have an atrial septal defect (ostium primum ASD).
- Holt-Oram syndrome: Rare genetic disorder characterised by skeletal abnormalities of the hands and arms, as well as heart defects. Up to 75% of individuals with Holt-Oram syndrome have a cardiac defect, and atrial septal defects are among the most common.
- Maternal risk factors: Maternal diabetes, alcohol consumption, smoking, and use of certain medications during pregnancy.
- Family history: Siblings of a child with an ASD have a significantly increased risk.
- Turner syndrome: Affects females. Caused by a completely or partially missing X chromosome. Congenital heart defects are found in up to 50% of individuals with Turner syndrome, with ASDs being a common type (also bicuspid aortic valve and coarctation of the aorta)
- Ellis-van Creveld syndrome: Rare genetic disorder characterised by short stature, polydactyly, nail and tooth abnormalities, and congenital heart defects. Among those with heart defects, about 15-20% have ASDs.
- Maternal alcohol consumption: Infants born to mothers who consumed alcohol during pregnancy have an increased risk of congenital heart defects, including ASDs. The exact risk increase is challenging to quantify due to variables in alcohol consumption levels, timing, and individual genetic susceptibility.
Clinical Features
Many individuals with ASD are asymptomatic, particularly if the defect is small. However, larger defects can cause:
- Shortness of breath
- Fatigue
- Swelling of the legs, feet, or abdomen
- Heart palpitations or skipped beats
- Frequent lung infections
- Stroke
Complications
If not treated, a large ASD can lead to complications such as right heart failure, atrial arrhythmias, pulmonary hypertension, or rarely, Eisenmenger syndrome.
Subtypes
ASDs are classified into four types:
- Ostium secundum: The most common type of ASD (75%), located in the middle of the atrial septum. Well tolerated, and symptoms and complications usually only present in the third decade or later
- Ostium primum: Located in the lower atrial septum, often associated with other heart defects. 2nd most common (15-20%). Most children with small defects are asymptomatic. Those with larger defects are predisposed to recurrent chest infection and heart failure.
- Sinus venosus: Located near the entrance of the superior vena cava (5-10%).
- Coronary sinus: The rarest type, located near the coronary sinus (<1%).
A patent foramen ovale is also considered a form of atrial septal defect.
Pathological Features
Morphology
- Ostium secundum – Incomplete adhesion of septum secundum to flap valve of foramen ovale
- Ostium primum – Incomplete fusion of septum primum with endocardial cushion
- Sinus venosus – Abnormal fusion of sinus venosus and right atrium
- Coronary sinus – Unroofing of coronary sinus
Histopathology
There are typically no significant histopathological findings associated with an ASD itself.
Genetics
Genetic syndromes, such as Down syndrome or Holt-Oram syndrome, are associated with an increased risk of ASDs. Genetic testing may identify these underlying conditions.
Radiological Features
CXR
A chest X-ray may show cardiomegaly due to right atrial and ventricular enlargement and increased pulmonary vascular markings due to increased pulmonary blood flow.
Differential Diagnosis
- Ventricular septal defect: Most common congenital heart defect. Characterised by a defect in the ventricular septum, diagnosed by a harsh holosystolic murmur.
- Patent ductus arteriosus: Persistence of the fetal connection between the aorta and pulmonary artery, resulting in a continuous machinery murmur.
- Atrioventricular septal defect: Associated with Down syndrome, results in shunting at both atrial and ventricular levels.
Management
The treatment of ASDs is typically overseen by a cardiologist. Small ASDs may close on their own during infancy or early childhood. For larger defects or those that do not close on their own, treatment options include cardiac catheterisation or surgical repair. After repair, long-term follow-up with a cardiologist is recommended to monitor for potential complications.