Cor triatriatum causes


 * Associate Editor(s)-In-Chief: Priyamvada Singh, M.B.B.S. [mailto:psingh@perfuse.org]; ; Keri Shafer, M.D. [mailto:kshafer@bidmc.harvard.edu]; Assistant Editor(s)-In-Chief: Kristin Feeney, B.S. [mailto:kfeeney@perfuse.org]

Overview
As with many congenital heart diseases, the causation of cor triatriatum has not been definitively proven. A number of theories exist to explain how this defect occurs including: Malincorporation theory, Malseptation theory, and the Entrapment theory. During pregnancy, both genetic disease and environmental factors such as viral infection and poor nutrition may play role to develop Cor triatriatum in fetus.

Malincorporation Theory
It has been hypothesized that cor triatriatum sinister occurs when the common pulmonary vein fails to incorporate the pulmonary circulation into the left atrium. As a result, the common pulmonary venous ostium remains narrow. The anatomic result is a septumlike structure that divides the left atrium into two compartments. However, this theory fails to explain the following:
 * 1) The presence of the fossa ovalis and atrial muscle fibers within the walls of the proximal chamber where only a venous wall should be present.
 * 2) Furthermore, several cases have been reported in which 1 or 2 pulmonary veins drain into the proximal (or accessory) chamber and the others drain directly into the true left atrium.

Malseptation Theory
A second alternate hypothesis is that the membrane dividing the left atrium is an abnormal growth of the septum primum.

Entrapment Theory
A third hypothesis asserts that the right horn of the embryonic sinus venosus entraps the common pulmonary vein and thereby prevents its incorporation into the left atrium.

The significance of a prominent or persistent left superior vena cava in the pathogenesis of cor triatriatum is unclear.

Cor triatriatum dextrum
In normal development, the right horn of the sinus venosus is gradually incorporated into the right atrium. This forms the smooth posterior portion of the right atrium. The original embryologic right atrium ultimately becomes the trabeculated anterior portion of the right atrium.

The embryologic right atrium and the right horn of the sinus venosus are then connected through the sinoatrial orifice. The sinoatrial orifice is surrounded on either side by the two valvular folds (the right and left venous valves). The right atrium is divided in two by the right horn of the sinus venosus. This right valve forms a sheet that serves to direct the oxygenated venous return from the inferior vena cava across the foramen ovale to the left side of the heart during fetal life as the Chiari network.

Normally, this network regresses and leaves behind the crista terminalis superiorly and the eustachian valve of the inferior vena cava and the thebesian valve of the coronary sinus inferiorly.

Complete persistence of the right sinus valve of embryonic life results in separation of the smooth and trabeculated portions of the right atrium and constitutes cor triatriatum dextrum. If this membrane is extensively fenestrated and weblike in appearance, then it is referred to as the Chiari network.