Embryology · Heart and vessels
Atrial septal defect and patent foramen ovale !
- Malformation
A hole between the atria lets blood pass from left to right; it often causes no symptoms until adulthood.
Updated 2026-11-01
See it in 3D: Interatrial septum · Right atrium · Left atrium · Right ventricle · Pulmonary veins
What it is
An atrial septal defect (ASD) is missing tissue in the interatrial septum that allows a left-to-right shunt. It is acyanotic and usually silent in childhood, so many are diagnosed in adults. A patent foramen ovale is not a true ASD: the septum is complete but its two layers never fused.
Embryological cause
- Weeks 5–6: if cell death in the septum primum is excessive, the ostium secundum is too large and the septum secundum can't cover it.
- If the septum secundum is underdeveloped, it also fails to cover the ostium secundum: secundum ASD.
- If the septum primum fails to fuse with the endocardial cushions, the ostium primum persists: primum ASD.
- If the sinus venosus is poorly absorbed into the right atrium, a high sinus venosus ASD remains.
- Patent foramen ovale: both layers exist but do not seal after birth.
Markers
- TBX5
- NKX2-5
- GATA4
How it presents
- Infants and children: almost always asymptomatic; found because of a murmur.
- Adults: exercise intolerance, palpitations, atrial fibrillation.
- Large unrepaired defects: pulmonary hypertension and, rarely, shunt reversal (Eisenmenger).
- Patent foramen ovale: cryptogenic stroke in young people from paradoxical embolism.
Diagnosis
- Color Doppler echocardiography; transesophageal echo defines sinus venosus defects better.
- Agitated saline (bubble) study for a patent foramen ovale.
- ECG: incomplete right bundle branch block; left axis in primum defects.
Test yourself
Think of the answer, then tap the question to see it.
What is the embryological cause of a secundum ASD?
Excessive resorption of the septum primum or underdevelopment of the septum secundum, leaving the ostium secundum open.
Which malformation is a primum ASD related to?
The atrioventricular canal defect: the septum primum fails to fuse with the endocardial cushions.
Which auscultatory sign is typical of an ASD?
A fixed split second heart sound that does not change with breathing.
How does a patent foramen ovale differ from an ASD?
No tissue is missing: the layers exist but never fused, and the passage opens only when right-sided pressure rises.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Atrial septation: septum primum, septum secundum and foramen ovale
- Atrioventricular septal defect (endocardial cushion defect)
- Ventricular septal defect
- Fetal circulation and the changes at birth
- Ebstein anomaly
Related conditions
- Atrial septal defect
- Ischaemic stroke
- Pulmonary hypertension and cor pulmonale
- Down syndrome (trisomy 21)
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Langman's Medical Embryology, 13th ed., ch. 13: Cardiovascular System (Wolters Kluwer, 2015); Atrial Septal Defect (ASD) | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2024).
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Representative model, for educational purposes only.