ABSTRACT
During the 4th to 6th week of gestation the common atrial chamber is divided into two by the growth of endocardial cushions and two septae, the septum primum and the septum secundum. A primum defect constitutes 20% of ASDs and results from failure of fusion of the endocardial cushions with the septum primum. The complex defect involves the inferior aspect of interatrial septum. The mitral valve is often clefted and regurgitant. The posteromedial papillary muscle is more anterior. The abnormal left ventricular cavity has a broad inlet and narrow outlet, producing a characteristic goose-neck appearance on angiography. Other associated anomalies include mitral stenosis, hypoplastic left heart and aortic coarctation. Failure of the septum secundum to fuse with the septum primum results in a patent foramen ovale and if the defect is large a secundum ASD. Seventy percent of ASDs are of the secundum variety. This defect lies at the center of the fossa ovalis but may extend towards the inferior vena cava (IVC) or posteriorly. A multiple defect in the central portion of the fossa ovalis is termed a chiari network. Sinus venosus defects form 10% of ASDs and appear in the interatrial septum above but separate from the fossa ovalis with the right superior pulmonary vein draining into the superior vena cava (SVC). Unroofed coronary sinus defect is a rare anomaly associated with left-sided SVC producing a left-to-right shunt from the left atrium to coronary sinus.
