foramen secundum

**Normal embryology (weeks 4-6):** 1. Septum primum grows down from the roof of the common atrium toward the endocardial cushions. The opening beneath it is the **foramen primum**. 2. Before the septum primum fuses with the cushions (closing the foramen primum), apoptosis creates multiple perforations in its upper portion. They coalesce into the **foramen secundum**, so right-to-left flow continues. 3. The **septum secundum** then grows down to the right of the septum primum, leaving an incomplete opening, the **foramen ovale**. The septum primum acts as the one-way valve. 4. After birth, rising left atrial pressure (increased pulmonary venous return) and falling right atrial pressure push the septum primum against the septum secundum, functionally closing the foramen ovale. Anatomic fusion follows, leaving the **fossa ovalis**.

**Clinical significance:** - **Secundum ASD** (the most common ASD, ~75%) arises from excessive resorption of the septum primum or an inadequate septum secundum. The defect lies in the central fossa ovalis. - **Patent foramen ovale (PFO)** is failure of fusion (present in ~25% of adults). It is usually asymptomatic but can allow paradoxical embolism (DVT → stroke), with a right-to-left shunt when right pressures rise (e.g., Valsalva, PE). - **Primum ASD** is a separate defect from failure of the septum primum to fuse with the endocardial cushions. It is associated with Down syndrome (AV septal defects) and often has a cleft mitral valve. - **Sinus venosus ASD** is high in the atrial septum near the SVC and is associated with anomalous pulmonary venous return.

**Secundum ASD presentation and findings:** - Often asymptomatic in childhood. Adults may develop exertional dyspnea, fatigue, palpitations (atrial fibrillation/flutter), and paradoxical emboli. - Wide, fixed splitting of S2 (the left-to-right shunt equalizes right-sided volume across the respiratory cycle). - Systolic ejection murmur at the left upper sternal border (increased flow across the pulmonic valve). A mid-diastolic tricuspid flow rumble may occur with a large shunt. - ECG: right axis deviation or RBBB (rsR' in V1). Chest x-ray: enlarged right atrium/ventricle, prominent pulmonary arteries. - Echo with bubble study is diagnostic. - Chronic left-to-right shunt causes right heart volume overload and pulmonary hypertension. This can lead to **Eisenmenger syndrome** (shunt reversal to right-to-left, with late cyanosis, clubbing, and polycythemia).

**Associations:** Holt-Oram syndrome (TBX5; upper limb/radial anomalies, secundum ASD), Down syndrome, fetal alcohol syndrome, and lithium exposure (Ebstein anomaly, with ASD/PFO).

**Management:** Observation for small defects. Percutaneous device or surgical closure is used for hemodynamically significant shunts (RV enlargement, Qp:Qs >1.5). Closure is contraindicated once Eisenmenger physiology develops.

**High-yield pearl:** Do not confuse the embryonic *foramen secundum* (a normal perforation in the septum primum) with the *septum secundum* (the muscular-ish septum that grows to the right of it) and with the *foramen ovale* (the final opening between the two septa).