Atrial septal defect is characterized by incomplete closure between the two upper chambers of the heart, resulting in blood flow through a hole in the wall (septum) between the upper chambers of the heart (atria) from one side to the other, causing certain heart chambers to have to pump extra blood. The heart can dilate, the muscle can become weak, and the pressures in the pulmonary arteries can increase (pulmonary hypertension) due to the increase in blood flow. It may occur in various positions in the atrial septum: the lower part, ostium primum, 15% of cases; in the area of fossa ovalis, ostium secundum, in 75% of cases; and in the upper atrial septum, sinus venosus, in 10% of cases. Uncorrected atrial septal defect can cause pulmonary overcirculation, right heart volume overload, and premature death.
By genome-wide linkage studies, the localization of the gene causing familial atrial septal defect could be identified to a locus on chromosome 5p. This disorder is genetically heterogeneous with incomplete penetrance and variable expressivity, and can also be caused by defects in other undefined genes.