foramen primum

**Embryology (weeks 4-5):** - The septum primum grows downward from the roof of the primitive common atrium toward the endocardial cushions (AV cushions). - The gap that remains between the free (inferior) edge of the septum primum and the endocardial cushions is the **foramen (ostium) primum**. - It allows blood to flow from the right atrium to the left atrium during early development. - The septum primum fuses with the endocardial cushions, closing the foramen primum. - Before it closes, apoptosis in the upper septum primum creates the **foramen secundum**, so right-to-left shunting is maintained. - The septum secundum then grows down on the right side of the septum primum and incompletely covers the foramen secundum. The remaining opening is the **foramen ovale**. - The septum primum becomes the **valve of the foramen ovale**.

**Sequence summary:** 1. Septum primum grows down, leaving the foramen primum. 2. The foramen primum closes by fusion with the endocardial cushions. 3. The foramen secundum forms by perforation of the septum primum. 4. The septum secundum grows, leaving the foramen ovale. 5. At birth, increased left atrial pressure functionally closes the foramen ovale, and it later fuses to form the **fossa ovalis**.

**Clinical significance:** - **Ostium primum ASD** results from failure of the septum primum to fuse with the endocardial cushions. - It is a type of **endocardial cushion defect (AV septal defect)**. - It is located in the **lower** part of the atrial septum, adjacent to the AV valves. - It is often associated with a **cleft anterior mitral leaflet** (mitral regurgitation), and may coexist with a VSD (partial vs complete AV canal). - It is strongly associated with **Down syndrome (trisomy 21)**. - Contrast with the **ostium secundum ASD**, the most common ASD type. It is caused by excessive resorption of the septum primum or an inadequate septum secundum, and it is located in the mid-atrial septum (the fossa ovalis). - Findings include a wide, fixed split S2 and a systolic ejection murmur at the left upper sternal border from increased pulmonary flow. Left-to-right shunting can cause right heart volume overload and pulmonary hypertension. Eisenmenger syndrome is a late complication. - ECG classically shows **left axis deviation** in ostium primum ASD, unlike the right axis deviation seen in secundum ASD. - Diagnosis is by echocardiography. Treatment is surgical repair, including mitral cleft repair if needed.

**High-yield pearls:** - Primum = low in the septum, endocardial cushion defect, Down syndrome, left axis deviation. - Secundum = most common ASD, fossa ovalis, right axis deviation. - Sinus venosus ASD = high in the septum near the SVC, often with anomalous pulmonary venous return. - Paradoxical embolism can occur through any ASD or PFO.