An echinococcal cyst or rupture of a Sinus of Valsalva aneurysm into the interventricular septum.

The parasite echinococcus (aka hydatid disease) is a tapeworm rarely seen in the United States. Infection with this parasite can result in large cysts in organs such as the heart, where it is often localized to the interventricular septum.

When a Sinus of Valsalva aneurysm ruptures, the aortic root can communicate with essentially any cardiac chamber. On occasion, the rupture can be contained dissecting into the interventricular septum where small pockets of blood accumulate and can thrombose. This gives a cyst-like appearance on echocardiography.