During a neonatal screening echocardiogram, a defect in the atrial septum is identified in an asymptomatic newborn. The defect is located in the region of the fossa ovalis. Which is the most common site of atrial septal defect?

Explanation

## Most Common Site of Atrial Septal Defect **Key Point:** Ostium secundum defect is the most common type of atrial septal defect, accounting for approximately 70–75% of all ASD cases. ### Classification and Epidemiology of ASD | Type | Location | Frequency (%) | Associated Features | |------|----------|---------------|---------------------| | **Ostium Secundum** | **Fossa ovalis region** | **70–75** | **Most common; may have associated cleft mitral valve** | | Ostium Primum | Lower part of atrial septum | 15–20 | Part of endocardial cushion defects; associated with Down syndrome | | Sinus Venosus | Near entry of SVC or IVC | 10–15 | Often associated with partial anomalous pulmonary venous return | | Coronary Sinus | Rare | <1 | Very uncommon; may be associated with unroofed coronary sinus | **High-Yield:** The question stem specifies "fossa ovalis" — this is the anatomical location of ostium secundum defect. Ostium secundum defects are the most common type of ASD and are located in the region of the fossa ovalis, which is the normal site of the foramen ovale. ### Embryological Basis ```mermaid flowchart TD A[Atrial Septation<br/>Weeks 4-7]:::action --> B{Which septum<br/>fails to form?}:::decision B -->|Septum Secundum<br/>defect| C[Ostium Secundum ASD<br/>70-75% of cases]:::outcome B -->|Endocardial Cushion<br/>defect| D[Ostium Primum ASD<br/>15-20% of cases]:::outcome B -->|Sinus Venosus<br/>defect| E[Sinus Venosus ASD<br/>10-15% of cases]:::outcome B -->|Unroofed Coronary<br/>Sinus| F[Coronary Sinus ASD<br/><1% of cases]:::outcome ``` ### Clinical Pearl Ostium secundum defects are often discovered incidentally in asymptomatic children or even adults during routine screening or imaging for other reasons. They may remain hemodynamically insignificant for years. The defect allows left-to-right shunting at the atrial level, leading to increased pulmonary blood flow and eventual right heart volume overload if the defect is large. **Mnemonic: OSPC** — Ostium Secundum (70%), Ostium Primum (15%), Sinus venosus (10%), Coronary sinus (<1%) — in order of decreasing frequency. [cite:Park 26e Ch 3]