Embryology of Anomalous Venous Return
There are two types of anomalous venous return, partial (PAVR) and total (TAPVR). These defects are further sub categorized into supracardiac (i.e. The pulmonary veins drain into the superior vena cava or the brachiocephalic veins), intracardiac (the pulmonary veins drain directly into the right atrium via the coronary sinus or the roof of the right atrium) and infracardiac (the pulmonary veins drain typically into the ductus venosus below the heart).
PAVR is characterized by one or more of the pulmonary veins attaching to a morphologic left atrium while the rest are aberrantly attached to one of the systemic veins that eventually drain into the right atrium.
TAPVR occurs when all four pulmonary veins aberrantly drain into the systemic venous circulation and eventually into the right-sided atrium.
Please see my blog on morphology of the atria for more information about atrial development. www.HeartDefectsSimplified.com
After a few weeks of fetal development, the venous circulatory system is beginning to be fairly complete. It is a series of paired vessels with several lateral channels that interconnect. On each side of the developing heart there is a vertical or â€cardinalâ€ vein, one on the right (anterior cardinal vein or right caval vein) and one on the left (posterior cardinal vein or left caval vein).
Superiorly, these veins attach to the right and left brachiocephalic veins respectively. Midline is the common cardinal vein (or confluence) that drains each vertical vein into the sinus venosus, later to become the coronary sinus, attached to the posterior right atrial wall. Inferiorly, is the ductus venosus. There are numerous other lateral veins that are attached to the cardinal veins e.g. the azygos vein which drains the thorax into the SVC, renal veins, mesenteric veins etc.
Meanwhile, the lungs are just buds that begin to develop a little later in fetal life. The pulmonary veins emerge from the lung buds and migrate medially until they reach the common cardinal vein. Eventually, the common cardinal vein becomes the coronary sinus and the pulmonary veins will migrate and attach appropriately to a morphologic left atrium.
The umbilical cord contains three vessels; two umbilical arteries that are attached to the fetal iliac arteries and carry deoxygenated blood from the fetus to the placenta. The umbilical vein carries oxygenated blood from the placenta to the fetus, splits into two vessels, one of which passes through the liver and the other attaches to the ductus venosus through the developing right vena cava and on to the right atrium. This sounds counterintuitive, but this is how it works.
The ductus venosus, like the ductus arteriosis closes shortly after birth.
In time, the left cardinal vein (left caval vein) will disintegrate and merge with other developing vessels leaving in place the right superior and inferior vena cava.
TAPVR is highly associated with (but not always) malformations of the atria and their attachments. The right atrium is morphologically different than the left atrium. Most significant are the appendages. The right atrium has a large triangular appendage with heavy pectinate musculature, while the left atrium has a small tubular appendage. The coronary sinus and vena cava are equipped to drain into the right atrium, while the pulmonary veins are meant to attach to a morphologic left atrium.
Three things can happen to the atria during embryologic development; they may be morphologically normal and attached to their appropriate ventricles i.e. A morphologic right atrium is attached to a morphologically normal right ventricle, and a morphologically normal left atrium it is attached to a morphologically normal left ventricle.
Isomeric atria means that both atria are either morphologically right atria or morphologically left atria; i.e. isomeric right atria means that both atria are morphologically right atria, one right atrium is attached to the appropriate right ventricle and is concordant, and the other right atrium is attached to the left ventricle and his discordant.
Vice versa, if both atria are morphologic left atria, then one morphologic left atrium will attach to the right ventricle and is discordant, and the other morphologic left atrium will be attached to the left ventricle and is concordant.
Mirror image atria: the atria are switched. The morphologic left atrium is it attached to a morphologically normal right ventricle, and a morphologically normal right atrium is it attached to a morphologically normal left ventricle.
This is important because the pulmonary veins want to attach to a morphologic left atrium. If the morphologic left atrium is right-sided, the pulmonary veins will attach there, creating a cyanotic defect. If both atrium are morphologically right, this means that the large appendage and pectinate muscles of both right atria will crowd out the coronary sinus and prevent the pulmonary veins from connecting to a left atrium.
This can result in the confluence as well as the left sided cardinal vein (left vena cava), to persist. and the pulmonary veins will flow through the confluence and into the right side of the heart.
Typically, pulmonary venous return is supracardiac or intra-cardiac; If either of these paths are obstructed, then pulmonary venous return will find its way through the infra cardiac destination back to the heart through the ductus venosus (which empties into the inferior vena cava).
The etiology of PAPVR is not that well understood. Why do one or more pulmonary veins deviate and attach to another central venous structure? They may attach to any of the systemic venous vessels. It is very important to image the left atrium in all views to document that there are four pulmonary veins draining into the left atrium. If not, try to find the deviant vein and figure out where it attaches. This defect may go unnoticed for entire lifetime, but it makes up 70% of all anomalous venous drainage.
TAPVR on the other hand is a cyanotic defect that must be dealt with immediately. Remember that as long as the fetus is in the womb, total SAO2 levels are about 75%, which means that the fetus can survive almost any defect in utero. Once born, the baby is on itâ€™s own, and pulmonary defects are very dire. What does not come out of the right side does not make it through the lungs and in the left side. This can be fatal.
Fortunately, TAPVR almost always drains into one structure, as opposed to PAPVR, which can be very hard map. It may involve a CT scan or an MRI.
Unless you do a TEE, it is very difficult to determine which atrium is which. Most importantly, determine the morphology of all of the chambers, determine arterial flow as well as venous flow, and rule out other defects. These infants tend to be severely cyanotic and you do not want to confuse this defect with univentricular defects.
Ken Heiden RDCS
I would love to hear from others who have experience with this defect!