Atrioventricular Septal Defect

Atrioventricular septal defects are congenital heart diseases characterised by a defect of the interatrial septum and interventricular septum with a common or partially separated atrioventricular valve (1). 

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Atrioventricular septal defects are common congenital heart defects, accounting for 15-20% of fetal heart anomalies and 4-5% of postnatal congenital heart diseases, with an estimated incidence ranging from 0.19 to 0.31 per 1000 live births (2,3,4).

Atrioventricular septal defects are the most frequently prenatally-diagnosed structural cardiac disease (3).

Atrioventricular septal defects occur due to faulty endocardial cushion development during the embryogenesis of the heart. This disruption leads to different degrees of atrioventricular septal defects (5). Three types can be distinguished (5,6):

1. Complete atrioventricular (AV) canal is caused by a complete lack of fusion between the superior and inferior cushions. It is characterised by a common atrioventricular valve. In addition, there is an ostium primum type atrial septal defect (ASD) and a posterior or inflow septal defect (VSD). The common AV valve has five leaflets.

2. Partial or incomplete AV canal results from incomplete fusion of the superior and inferior endocardial cushions. It is characterised by the presence of a common atrioventricular valve but with two distinct valve orifices. There is also an ostium primum ASD and a mitral cleft (cleft of the anterior mitral valve). The formation of the interventricular septum is complete, and hence there is no VSD.

3. Intermediate or transitional AV canal forms between partial and complete defects where there is usually a common valvular annulus with two distinct valvular orifices with ASD and VSD.

The aetiology of this congenital heart disease is multifactorial; there is a strong association with genetic abnormalities, especially Down syndrome (3,7,8).

The clinical manifestation is related to the type and degree of the defect and the presence of associated cardiac anomalies (2).

The common valvular annulus and function in fetal life determine the clinical picture and prognosis. Severe valvular insufficiency can lead to heart failure, hydrops and even intrauterine death (6).

After birth, with the decrease in pulmonary resistance, the clinical features of heart failure begin within the first months of life. In cases with significant atrioventricular regurgitation, unbalanced ventricles or coarctation of the aorta, symptoms may appear early (2,5,9). Symptoms of heart failure in infants include feeding difficulties, drowsiness, lethargy and failure to thrive. In children, the most common clinical presentation is dyspnoea (5).

50% of AV canals occur in isolation (10). The remainder are associated with chromosomal abnormalities, cardiosplenic syndromes and other congenital heart diseases.

- Karyotype abnormalities: Isolated AV canal (without other structural abnormalities) is much more frequently associated with karyotype abnormalities (8). Trisomy 21 is the chromosomal abnormality most commonly associated with the AV canal, present in more than 50% of cases in both the complete and partial canal. It is also less frequently associated with trisomy 18 and 13 (11).

- Cardiac anomalies: AV canal is associated with other cardiac abnormalities such as tetralogy of Fallot, other conotruncal anomalies, coarctation of the aorta, pulmonary atresia, and anomalies of pulmonary or systemic drainage (11). It may also be associated with subaortic stenosis, atrial isomerism and in cases with an unbalanced AV canal with ventricular hypoplasia (2). In addition, patients with left isomerism may develop an atrioventricular block (6).

- Extracardiac abnormalities: In one-third of cases, the unbalanced AV canal is part of a cardiosplenic syndrome. In these circumstances, there is virtually no association with chromosomal abnormalities (8). Extracardiac abnormalities have been reported in up to one-third of cases, almost always in a syndromic association. AV canals have been reported in CHARGE, Noonan, Holt Oram and VACTERL syndrome(10).

In the case of a previously affected child, the risk of recurrence is 2-3% (12). 

Prenatal diagnosis of the AV canal is made at the 4-chamber view.

- Complete AV canal:

There is a defect in the heart's centre secondary to atrial septal defect and ventricular septal defect with a common AV valve.

- The peak of the crux of the heart in continuation with the interventricular septum is not identified.

- Absence of septum primum or ASD ostium primum.

- Presence of a common atrioventricular valve. There is a central zone free of tissue when the valve is open (diastole). When the single valve is closed (systole), there is a common valve annulus with a linear insertion of the leaflets, called a "hammock" valve.

- In the balanced AV canal, there is symmetry in the size of the two ventricles. However, there is asymmetry of the two ventricles in the unbalanced AV canal and sometimes in the two atria. The asymmetry may be very marked and result in a hypoplastic ventricle.

- Perimembranous VSD

- Colour Doppler facilitates confirmation of the diagnosis. A common diastolic filling is seen through the single AV valve that divides at the remaining interventricular septum. The filling adopts a characteristic "H" shape. The degree of ventricular hypoplasia in an unbalanced AV canal can also be assessed with colour Doppler. In most cases, there is regurgitation of the common AV valve, evaluated with colour Doppler and pulsed Doppler. In these cases, the regurgitant jet usually originates from the centre of the valve.

- Partial or incomplete AV canal:

Atrial septal defect and a single valvular annulus with two valvular orifices are present.

- Absence of septum primum or ASD ostium primum.

- Common AV valve with two valve rings, right and left, which insert at the same level into the intraventricular septum with a line visible during systole.

- The interventricular septum is intact.

- Colour Doppler shows two independent ventricular diastolic fillings. Occasionally, some degree of valvular insufficiency is identified, usually affecting the left valve.

- Intermediate or transitional AV canal:  

An atrial septal defect, a single valvular annulus with two valvular orifices and a ventricular septal defect are seen. It has the same characteristics as a partial AV canal but with the presence of the ventricular septal defect.

Three pathologies can give similar images to an AV canal and should be considered in the differential diagnosis (6,11):

1.         Dilatation of the coronary sinus secondary to a persistent left superior vena cava. The coronary sinus is visualised in the posterior four-chamber view and, when dilated, may mimic the insertion of the AV valves at the same level. A complete scan in the transverse four-chamber view can rule out the presence of an atrioventricular septal defect.

2.         Single ventricle type univentricular heart, hypoplastic left ventricle syndrome or tricuspid atresia. These disorders may have an AV canal-like presentation. Identifying the septum primum in the four-chambers view allows differentiation of these entities.

3.         Wide perimembranous VSD. Identifying the linear insertion of the AV valves into the AV canal may help differentiate these diseases.

Prenatal diagnosis of an AV canal involves a detailed fetal ultrasound to rule out the presence of other associated cardiac or extracardiac anomalies.

Parents should be offered an invasive technique to rule out associated chromosomal or genetic abnormalities, which are present in up to half of the cases (6,8,11).

The AV canal is a heart defect diagnosed in the four-chamber view, the primary view performed in routine fetal morphology scan (3,10). The main diagnostic clue is the absence of crux cordis and linear insertion of the atrioventricular valves.

The prognosis of fetuses with atrioventricular septal defects will depend on the type of defect and associated cardiac and extracardiac abnormalities.

The prognosis of the AV canal itself is determined by the type of canal, the symmetry of the ventricles and the presence/severity of common atrioventricular valve insufficiency (10). An unbalanced AV canal has a worse prognosis, depending mainly on the degree of ventricular asymmetry. In addition, fetuses with significant common atrioventricular valve regurgitation are at increased risk of developing fetal hydrops (6). 

The long-term prognosis of the isolated AV canal is good, with overall survival at 20 years of about 95% and low intraoperative mortality (13,14). However, postoperative complications and the need for re-interventions are not uncommon (up to 25% of patients) (14,15).

Upon prenatal diagnosis of an AV canal, parents should be counselled on fetal prognosis, considering the type of canal, associated cardiac or extracardiac anomalies and chromosomal abnormalities.

Monthly ultrasound monitoring is recommended to rule out the development of fetal hydrops (if valvular insufficiency is present) or the development of an AV block in cases with left isomerism. 

This defect does not modify the standard obstetric management and does not contraindicate vaginal delivery.

Although most neonates are stable in the first weeks of life, a tertiary centre delivery is recommended.

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11. Abuhamad A., Chaoui A. A practical guide to fetal echocardiography. Normal and abnormal hearts. Second edition. Editorial Lippincott Williams&Wilki.
12. Freeman SB, Taft LF, Dooley KJ, et al. Population-based study of congenital heart defects in Down syndrome. Am J Med Genet. 1998;80(3):213-217.
13. Studer M, Blackstone EH, Kirklin JW, et al. Determinants of early and late results of repair of atrioventricular septal (canal) defects. J Thorac Cardiovasc Surg. 1982;84(4):523-542.
14. Fong LS, Betts K, Ayer J, et al. Predictors of reoperation and mortality after complete atrioventricular septal defect repair. Eur J Cardiothorac Surg. 2021;61(1):45-53. doi:10.1093/ejcts/ezab221
15. McGrath LB, Gonzalez-Lavin L. Actuarial survival, freedom from reoperation, and other events after repair of atrioventricular septal defects. J Thorac Cardiovasc Surg. 1987;94(4):582-590.

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