Fetal atrioventricular septal defect

Atrio-ventricular septal defect (AVSD) is characterized by an incomplete division of the atria and ventricles, resulting in a single common valve between the atria and ventricles. This condition is commonly seen in children with Down syndrome. Typically, there are 2 big categories of AV canal (AVSD): partial AV canal and complete AV canal. Complete AV canal can further be subdivided regarding balanced and unbalanced. There may also be associated anomalies, as more complex anatomical variants (such as those with heterotaxy, total abnormal pulmonary venous connection/return, interruption of inferior vena cava with with azygos continuation, etc.). Unbalanced AV canal may present complex variations, with: outflow tract obstruction (pulmonary or systemic), the inflow(s) committed to one ventricle, the hypoplasia of one of the ventricular chambers, etc.). 

Both types of AV canal defects can result in varying degrees of blood flow mixing between the left and right sides of the heart, leading to pulmonary overcirculation (often less marked in infants with T21, as they have higher PVR), eventual signs of heart failure, and other potential complications in the long-term if not corrected (failure to thrive, pulmonary hypertension, etc.). Treatment include surgical repair of the defect, which often involves closing the septal defect and separating the common valve into two distinct valves. The repair of unbalanced AV complete is more complex and depends on the underlying anatomy.

More on post-natal atrio-ventricular septal defect here (see presentation).

Case of balanced atrio-ventricular septal defect

Fetal case of complete atrioventricular septal defect. The atrial and ventricular components with common atrioventricular orifice. There is balanced ventricles with no left or right-sided atrioventricular valvular regurgitation. No evidence of LVOT obstruction or antenatal suspicion of coarctation. Both atrioventricular valves are at the same level (in normal anatomy: tricuspid valve implanted lower than the mitral valve).

Cases by Dr Wadi Mawad - pediatric cardiologist at the Montreal Children's Hospital: Balanced, unbalanced, single-ventricle AVSD with TAPVR:

Common AV valve opening equally over 2 well developed ventricules

Unbalanced AVSD

AVSD with abnormal total pulmonary venous connection. A hint is the large separation between the descending aorta and the posterior aspect of the LA. These structures are usually in very close proximity. When they are not, TAPVC should be suspected and excluded.

AVSD with abnormal total pulmonary venous connection demonstrated by colour flow

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