[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Heart and vessels](https://durgical.com/en/embryology/#a-heart) › Atrioventricular septal defect (endocardial cushion defect) [Embryology](https://durgical.com/en/embryology/) · [Heart and vessels](https://durgical.com/en/embryology/#a-heart) # 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](https://durgical.com/en/anatomy/interatrial-septum/) · [Interventricular septum](https://durgical.com/en/anatomy/interventricular-septum/) · [Mitral anterior leaflet](https://durgical.com/en/anatomy/anterior-leaflet-of-left-atrioventricular-valve/) · [Mitral posterior leaflet](https://durgical.com/en/anatomy/posterior-leaflet-of-left-atrioventricular-valve/) · [Tricuspid septal leaflet](https://durgical.com/en/anatomy/septal-leaflet-of-right-atrioventricular-valve/) · [Right atrium](https://durgical.com/en/anatomy/right-atrium/) · [Left atrium](https://durgical.com/en/anatomy/left-atrium/) · [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/) · [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) ## Key points - Cause: failed fusion of the dorsal and ventral endocardial cushions (week 5). - Complete: primum ASD + inlet VSD + common AV valve. - Partial: primum ASD with a cleft anterior mitral leaflet. - Typical defect of Down syndrome: every newborn with trisomy 21 warrants an echocardiogram. - Acyanotic at first; early heart failure and fast-developing pulmonary hypertension if unrepaired. - ECG: left axis deviation (abnormal position of the conduction system). ## 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. ## Memory aid On the four-chamber view a normal heart draws a cross; in an AV canal the cross vanishes because the center was never stitched. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/atrioventricular-septal-defect/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Interatrial septum](https://durgical.com/en/anatomy/interatrial-septum/) - [Interventricular septum](https://durgical.com/en/anatomy/interventricular-septum/) - [Mitral anterior leaflet](https://durgical.com/en/anatomy/anterior-leaflet-of-left-atrioventricular-valve/) - [Mitral posterior leaflet](https://durgical.com/en/anatomy/posterior-leaflet-of-left-atrioventricular-valve/) - [Tricuspid septal leaflet](https://durgical.com/en/anatomy/septal-leaflet-of-right-atrioventricular-valve/) - [Right atrium](https://durgical.com/en/anatomy/right-atrium/) - [Left atrium](https://durgical.com/en/anatomy/left-atrium/) - [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/) - [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) ## Related - [Ventricular septation and the endocardial cushions](https://durgical.com/en/embryology/ventricular-septation-and-the-endocardial-cushions/) - [Atrial septation: septum primum, septum secundum and foramen ovale](https://durgical.com/en/embryology/atrial-septation-septum-primum-septum-secundum-and-foramen-ovale/) - [Atrial septal defect and patent foramen ovale](https://durgical.com/en/embryology/atrial-septal-defect-and-patent-foramen-ovale/) - [Ventricular septal defect](https://durgical.com/en/embryology/ventricular-septal-defect/) - [Meiosis and nondisjunction: where trisomies come from](https://durgical.com/en/embryology/meiosis-and-nondisjunction-where-trisomies-come-from/) ## Related conditions - [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/) - [Atrial septal defect](https://durgical.com/en/conditions/atrial-septal-defect/) - [Ventricular septal defect](https://durgical.com/en/conditions/ventricular-septal-defect/) - [Heart failure](https://durgical.com/en/conditions/heart-failure/) - [Pulmonary hypertension and cor pulmonale](https://durgical.com/en/conditions/pulmonary-hypertension-and-cor-pulmonale/) [← All of embryology](https://durgical.com/en/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)](https://www.cdc.gov/heart-defects/about/atrioventricular-septal-defect.html) (Centers for Disease Control and Prevention, 2024). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/atrioventricular-septal-defect/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/atrioventricular-septal-defect/