Children with this defect usually have a heart murmur and may have difficulty feeding and breathing. However, most grow well.

The atrial septal defect allows the mixing of oxygen-rich blood from the left atrium with oxygen-poor blood in the right atrium. This is known as a left to right shunt. The mixed blood in the right atrium is pumped to the lungs via the right ventricle, reducing the efficiency of the circulatory system. This may lead to heart failure with congestion of the lungs.

Eventually, the atrial septal defect will cause the enlargement (dilatation) of the right atrium and right ventricle, which may lead to irregular atrial pumping (arrhythmia) and/or right ventricular dysfunction.

The malformed mitral valve allows the leakage of blood between the left atrium and the left ventricle. The amount of leakage depends on the degree of deformation of the mitral valve. With time, a cleft mitral valve can result in the enlargement (dilatation) of the left atrium.