An atrial septal defect (ASD) is a congenital malformation resulting from incomplete development of the atrial septum, leading to an abnormal communication between the left and right atria. ASDs can be classified into primary (primum) and secondary (secundum) types. According to the latest classification, primum ASDs are categorized under atrioventricular septal defects (endocardial cushion defect). Primum ASDs are located anteroinferiorly near the coronary sinus and are often associated with a cleft in the anterior mitral valve leaflet. Secundum ASDs can be further subdivided into central (fossa ovalis type), superior (sinus venosus type), inferior, and mixed types based on anatomical location. Most defects are single and oval-shaped; however, some may present as fenestrated or multiple defects. Smaller central-type ASDs can be easily confused with a patent foramen ovale (PFO).
Pathophysiology
Normally, the left atrial pressure (8–10 mmHg) is slightly higher than the right atrial pressure (3–5 mmHg). Blood flows from the left atrium to the right atrium through the atrial septal defect. The volume of shunting depends on factors such as the size of the defect, pressure differences between the atria, ventricular filling pressures, and pulmonary vascular resistance. In primum ASDs, the degree of left-to-right shunting is also influenced by the severity of mitral regurgitation. The increased volume load caused by the shunting results in dilation of the right atrium, right ventricle, and pulmonary arteries. In the early stages, pulmonary arterioles undergo spasm, and over time, the vessel walls undergo cellular hyperplasia and thickening, leading to pulmonary arterial hypertension (PAH). When the pressure in the right atrium exceeds that in the left atrium, the direction of blood flow reverses, resulting in a right-to-left shunt and cyanosis, a condition known as Eisenmenger syndrome.
Clinical Presentation
Secundum ASDs in children are often asymptomatic, but in cases with larger shunting, symptoms such as growth retardation and reduced exercise tolerance may occur. In adolescence or young adulthood, symptoms such as fatigue and shortness of breath with exertion may progressively develop. Primum ASDs typically present earlier and progress more rapidly.
Physical Examination
Increased pulmonary circulation and relative pulmonary valve narrowing may produce a grade II–III blowing systolic murmur at the second or third intercostal space along the left sternal border. The pulmonary valve's second heart sound is accentuated and exhibits fixed splitting. Primum ASDs associated with a cleft mitral valve may produce a grade II–III systolic murmur at the cardiac apex. In advanced stages, there may be manifestations of atrial fibrillation as well as signs of right-sided heart failure, such as hepatomegaly, ascites, and lower limb edema.
Auxiliary Investigations
Electrocardiography (ECG)
Secundum ASDs may show right-axis deviation, incomplete or complete right bundle-branch block, and right ventricular hypertrophy. Primum ASDs may demonstrate left-axis deviation, prolonged PR intervals, and left ventricular hypertrophy. Atrial fibrillation or flutter is a common finding in the advanced stages of ASDs.
Chest X-ray
Findings typically include enlargement of the right atrium and right ventricle, prominence of the pulmonary artery segment, and a hypoplastic aortic knob. The heart may assume a "pear-shaped" appearance on imaging. Increased pulmonary vascular markings and a "pulmonary dance sign" (enhanced pulsation of the hilar vessels) may be seen under fluoroscopy. Primum ASDs may also show left ventricular enlargement.
Ultrasonography
Echocardiography is highly accurate in identifying the location and size of the defect, the presence of atrial-level shunting, and the relationship between the defect and adjacent structures such as the superior vena cava, inferior vena cava, mitral valve, and tricuspid valve. Primum ASDs may reveal enlargement of both the right and left heart chambers, as well as findings of a cleft mitral valve or mitral regurgitation.
Right Heart Catheterization
This may be performed to measure pulmonary artery pressures and calculate pulmonary vascular resistance. The presence of an atrial septal defect is indicated by an increase in oxygen saturation in the right atrium compared to the superior and inferior vena cavae (difference in oxygen saturation >8%) or by the passage of the catheter into the left atrium.
Diagnosis
Diagnosis is typically established based on clinical symptoms, physical examination findings, and echocardiography, with additional support from ECG and chest X-ray findings if needed.
Treatment
Indications for Surgery
Asymptomatic patients with significant enlargement of the right atrium and right ventricle are candidates for surgical treatment. Age is not a primary determinant for surgery, and early surgical intervention is recommended for patients with pulmonary hypertension. Adults over 50 years of age, those with atrial fibrillation, or those with medically controlled heart failure should also be considered for surgery. Eisenmenger syndrome is a contraindication for surgical correction.
Surgical Methods
Minimally invasive approaches are commonly employed, including totally thoracoscopic surgery or thoracoscopic-assisted small incisions via the right axillary or right anterolateral thoracic routes. Cardiopulmonary bypass is established, and the right atrium is opened for repair. Defects may be closed directly or with patch materials, depending on the size of the defect. Partial anomalous pulmonary venous drainage, if present, is corrected by redirecting the anomalous veins to the left atrium using patch materials. In primum ASDs, mitral valve clefts are repaired first, followed by closure of the septal defect using a patch. Common surgical complications include air embolism and complete atrioventricular block.
Percutaneous and Transcatheter Device Closure
For appropriately positioned secundum ASDs, closure devices may be implanted under fluoroscopic or transesophageal echocardiographic guidance. This method avoids the need for cardiopulmonary bypass and offers reduced trauma, making it suitable for patients with favorable defect size and location. PFO closure may be considered in adult patients with cryptogenic stroke, transient ischemic attacks (TIA), or medically refractory migraine, particularly if significant right-to-left shunting is demonstrated on right heart contrast imaging with a Valsalva maneuver. The procedure is especially important in patients with concurrent lower extremity venous thrombosis.