Large anomalous fibrous sac in the right side of the heart

In an adult with atrial septal defect, an unusual conical-shaped fibrous sac resembling a closed windsock occupied the right ventricle and main pulmonary artery. The base of the sac being attached by fibrous strands to the right atrium suggests that the structure is derived from the right valve of the sinus venosus. The sac is considered to have filled with blood, thereby reducing potential right ventricular diastolic volume. It is probable that the sac emptied into the right atrium during right ventricular systole, thereby creating a functional state comparable to that of tricuspid insufficiency. Occupation of right ventricular space by the sac established a state somewhat comparable to that in Ebstein's anomaly of the tricuspid valve.