Atrial septal defect Abnormal coronary sinus morphology Coronary sinus atrial septal defect 2012-04-09T09:07:42Z ICD-10:Q21.1 Coronary sinus atrial septal defect is the rarest type of ASD and is the result of a partially or completely unroofed coronary sinus. Blood from the left atrium enters in the coronary sinus, through a route independent of the atrial septum, and then drains into the right atrium. A persistent left superior caval vein draining into the coronary sinus is usually present. The morphological types have been classified into 4 groups: Type I, completely unroofed with persistent LSVC; type II, completely unroofed without persistent LSVC; type III, partially unroofed mid portion; and type IV, partially unroofed terminal portion. 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. There is partial (either focal or fenestrated) or complete absence of the roof of the CS, which results in a communication between the CS and the LA. Unroofed CS is the rarest type of atrial septal defect. It is often associated with persistent left superior vena cava (LSVC) and other forms of complex congenital heart disease, usually heterotaxia syndromes. UMLS:C2063331 Unroofed coronary sinus HP:0031297 HP:0011643