Atrioventricular septal defect (endocardial cushion defect) !

Malformation

The center of the heart never closes: a hole links all four chambers and one common valve replaces the mitral and tricuspid.

Updated 2026-11-01

See it in 3D: Interatrial septum · Interventricular septum · Mitral anterior leaflet · Mitral posterior leaflet · Tricuspid septal leaflet · Right atrium · Left atrium · Right ventricle · Left ventricle

What it is

In an atrioventricular septal defect the endocardial cushions fail to fuse. A primum ASD, an inlet VSD and, in the complete form, a single common atrioventricular valve remain. It is acyanotic at first with a large left-to-right shunt, and it is the signature heart defect of Down syndrome.

Embryological cause

Week 5: the dorsal and ventral cushions fail to fuse in the center of the AV canal.
The septum primum has no cushion tissue to fuse with: the ostium primum persists.
The inlet part of the ventricular septum stays open.
Without the central division the leaflets form one common five-leaflet valve, or a cleft mitral valve.
Trisomy 21 alters the extracellular matrix of the cushions (chromosome 21 genes).

Markers

Trisomy 21
Endocardial cushions
GATA4

How it presents

Complete form: fast breathing, sweating with feeds and poor weight gain in the first weeks.
Partial form: may go unnoticed until childhood or adulthood.
Murmurs of AV valve regurgitation and increased flow.
In Down syndrome pulmonary hypertension appears earlier; late cyanosis means Eisenmenger.

Diagnosis

Fetal echocardiography: loss of the heart's central 'crux' on the four-chamber view.
Postnatal echocardiography: common valve and septal defects.
ECG: left axis. Karyotype if trisomy is not known.

Test yourself

Think of the answer, then tap the question to see it.

What is the embryological cause of an atrioventricular septal defect?

Failure of the dorsal and ventral endocardial cushions to fuse in the AV canal.

Which three defects make up a complete AV canal?

Primum ASD, inlet VSD and a common atrioventricular valve.

Which syndrome is most strongly associated with AV canal defects?

Down syndrome (trisomy 21).

Which fetal ultrasound sign suggests an AV canal defect?

Loss of the central 'crux' on the four-chamber view.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

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); Atrioventricular Septal Defect (AVSD) (Centers for Disease Control and Prevention, 2024).

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Representative model, for educational purposes only.