During a routine neonatal screening, a 3-day-old term infant is found to have a loud systolic murmur at the left sternal border. Echocardiography confirms a secundum atrial septal defect (ASD). Which of the following is the most common site of ASD formation?

## Atrial Septal Defects: Embryological Classification **Key Point:** Secundum ASD (ostium secundum defect in the region of the fossa ovalis) is the most common type, accounting for ~70% of all ASDs. ### Embryological Basis of ASD Types | ASD Type | Embryological Origin | Frequency | Location | |---|---|---|---| | **Secundum (Ostium secundum)** | Inadequate growth of septum secundum + excessive resorption of septum primum | ~70% | Fossa ovalis region (central atrial septum) | | **Primum (Ostium primum)** | Defective endocardial cushion development | ~15–20% | Lower atrial septum, near AV valves | | **Sinus venosus** | Abnormal development of sinus venosus and septum secundum | ~10% | Superior vena cava entry (most common) or inferior vena cava entry | | **Unroofed coronary sinus** | Absence of wall between coronary sinus and left atrium | <1% | Coronary sinus region | ### Why Secundum ASD Is Most Common **High-Yield:** The fossa ovalis is the thinnest part of the atrial septum and is derived from the overlapping of septum primum and septum secundum. Defects here result from: 1. Excessive resorption of the septum primum 2. Inadequate growth of the septum secundum 3. These are the most frequent developmental anomalies in atrial septation ### Clinical Features of Secundum ASD - **Soft systolic ejection murmur** at the left sternal border (due to increased pulmonary blood flow) - **Wide, fixed splitting of S~2~** (hallmark finding) - Left-to-right shunt (low-pressure defect) - Often asymptomatic in childhood; may present with atrial arrhythmias or heart failure in adulthood **Mnemonic:** **SINUS** — Secundum (most common), Inferior vena cava, Sinus venosus, Unroofed, Primum (endocardial cushion) ### Why Other ASDs Are Less Common - **Primum ASD:** Results from endocardial cushion defect; accounts for only 15–20% of ASDs. Often associated with cleft mitral valve. - **Sinus venosus defect:** Accounts for ~10% of ASDs; associated with partial anomalous pulmonary venous return. - **Unroofed coronary sinus:** Extremely rare (<1%); may be associated with left superior vena cava. **Clinical Pearl:** Secundum ASD is often missed in childhood because it is hemodynamically well-tolerated and produces subtle auscultatory findings. The key finding is **wide, fixed splitting of S~2~** (not variable with respiration, as in normal subjects).