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.). 

1. Partial AV Canal: In this type of defect, there is a partial division of the atria and ventricles, resulting in a partial common atrioventricular valve. The atrial septum is usually intact (or present a very small orifice), while the ventricular septum has a variable degree of defect, which may range from a small hole to a larger opening. Partial AV canal defects are often associated with left atrio-ventricular valve abnormalities. To be noted, in AV canal configuration, the left and right atri-ventricular valves cannot be termed "mitral" or "tricuspid". 

2. Complete AV Canal: This is a more severe form of AV canal, where there is a near-absence of the atrial and ventricular septa at its crux, resulting in a common atrioventricular valve. The left AV valve is usually "cleft" (tri-foliate configuration of the left atrio-ventricular valve, not a real "cleft" per say) and has a bridging leaflet that connects the anterior and posterior leaflets. This defect is commonly associated with Down syndrome.

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).