Cor triatriatum is a rare congenital heart defect where a fibromuscular septum subdivides the left atrium into two chambers. Associated cardiovascular malformations are common and include atrial septal defect,patent ductus arteriosus and common atrioventricular canal. The proximal or accessory chamber lies posteriorly, and usually receives blood from each of the pulmonary veins. The distal or true left atrial chamber lies anterior, connects to the left atrial appendage,empties into the left ventricle through the mitral valve, and lodges true atrial septum containing fossa ovalis . The two atrial chambers communicate through an opening in the membrane which divides the two chambers. This opening may be single and stenotic, fenestrated, and either small or large. ^[1]

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