An atrial septal defect characterized by a deficiency in the tissue separating the coronary sinus from the left atrium (LA). This results in partial or complete unroofing of the coronary sinus leading to a predominantly left-to-right shunt through the coronary sinus (LA to coronary sinus to right atrium [RA]). The orifice of the ostium is frequently large because of the increased flow. From the RA side, the defect is located at the level of the coronary sinus ostium and may also include some deficiency in atrial tissue around the ostium. From the LA side, the size can be variable depending on the degree of unroofing of the coronary sinus. [from
HPO]