ATRIAL SEPTAL DEFECTS (ASD)

The atrial septal defect (ASD) is an abnormal communication between the two atria due to a structural alteration of the atrial septum. It is the second most common congenital heart disease after ventricular septal defect, with an incidence of approximately 1 case per 1,500 live births.

Classification of ASD

Based on the location of the atrial communication, atrial septal defects are classified as follows:

• Ostium Secundum ASD: This occurs due to a developmental defect of the septum secundum or excessive resorption of the septum primum. It is located in the central portion of the atrial septum at the level of the fossa ovalis. Not to be confused with a patent foramen ovale (PFO), which is not a true ASD but rather a failure of the postnatal anatomical fusion of the foramen ovale. This type accounts for approximately 70% of all ASDs.
• Ostium Primum ASD: This results from an anomaly of the endocardial cushions that prevents the closure of the ostium primum. It is located in the lower part of the atrial septum, near the atrioventricular valves, and is often associated with defects of the anterior leaflet of the mitral valve, potentially leading to mitral regurgitation. It is considered a variant of partial atrioventricular canal defects and constitutes about 15% of ASDs.
• Sinus Venosus ASD: This is located near the junction of the superior vena cava in the posterosuperior part of the atrial septum. It is frequently associated with anomalies of pulmonary venous return, such as partial anomalous pulmonary venous drainage. This type represents approximately 10% of ASDs.
• Coronary Sinus ASD: This is a rare type, characterized by an atrial communication located in the inferoposterior part of the septum, near the coronary sinus, sometimes associated with the absence of its roof. It accounts for less than 5% of ASDs.

Pathophysiology

The presence of an ASD allows blood to pass from the left atrium, which has higher pressure, to the right atrium, which has lower pressure, creating a left-to-right shunt. The extent of the shunt depends on the size of the defect and pulmonary vascular resistance. Small defects may remain asymptomatic and go undiagnosed until adulthood. When the shunt is significant, it leads to right heart overload, causing dilation of the right atrium and ventricle and increased pulmonary blood flow. Over time, this overload can result in pulmonary hypertension, particularly in large untreated defects. If right atrial pressure increases beyond left atrial pressure, the shunt may reverse (from left-to-right to right-to-left), causing cyanosis and Eisenmenger syndrome, a rare complication in ostium secundum ASDs but more common in large, untreated defects.

Clinical Manifestations and Diagnosis

Many patients remain asymptomatic until adulthood. When symptoms occur, they may include reduced exercise tolerance, fatigue, palpitations due to arrhythmias such as atrial fibrillation, exertional dyspnea, and in advanced cases, cyanosis. In children and young adults, the first sign of the condition may be the detection of a heart murmur during a routine examination. The physical examination may reveal an ejection systolic murmur (grade 2-3/6) at the pulmonary valve area due to increased flow through the pulmonary valve, a fixed splitting of the second heart sound due to delayed right ventricular emptying, and in larger defects, a tricuspid diastolic rumble. The ECG may show right axis deviation, right atrial enlargement, and right bundle branch block. Chest X-ray may demonstrate an enlarged cardiac silhouette due to right heart chamber dilation, along with pulmonary artery enlargement and increased pulmonary vascular markings. Echocardiography with Doppler is the first-choice examination to locate the defect, quantify the shunt, and evaluate right heart dilation. In doubtful cases, cardiac MRI or cardiac catheterization may be helpful for more detailed assessment, while exercise testing can evaluate functional capacity in adults.

Treatment and Prognosis

Percutaneous closure with a device is the preferred treatment for ostium secundum ASDs with suitable margins. The most commonly used device is the Amplatzer Septal Occluder. Surgical closure is indicated for large ASDs, ostium primum defects, or defects unsuitable for percutaneous closure. Surgical repair typically involves direct suturing or the use of an autologous pericardial or synthetic patch. Patients with untreated ASDs or signs of cardiac overload require regular cardiologic follow-up. If the defect is corrected early, cardiac function normalizes completely. In adults with uncorrected ASDs, defect closure may still be indicated to prevent heart failure or atrial arrhythmias. The prognosis is excellent in patients treated early. In cases of late diagnosis or untreated ASDs, complications may include severe pulmonary hypertension, atrial arrhythmias, and heart failure. Long-term mortality is increased in untreated patients, with a significant risk of atrial fibrillation and embolic stroke.

References
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