The basic defect is a deficiency of the AV septum which separates the left ventricular inlet from the right atrium. It causes anomalies that range in severity from a small ostium primum atrial septal defect to a complete AV septum, which also involves defects in the interventricular septum and the mitral and tricuspid valves.
The atrial septum forms during the embryonic development of the heart. Defects may develop at a number of locations in the wall.
Most commonly the defect involves the foramen ovale, an oval-shaped hole in the wall that is present in all children during the fetal period but closes spontaneously soon after birth. If the foramen ovale is larger than it should be or if the flap of tissue that usually closes it is displaced or deficient, the hole may remain after birth.
Perhaps 25 percent or more of atrial septal defects diagnosed incidentally, because a murmur is heard, or through echocardiography done during infancy, will close spontaneously.
When the child reaches 2 or 3 years of age, most atrial defects that are producing clinical signs (enlarged right heart, substantially increased pulmonary blood flow), although not necessarily symptoms, should be repaired.
An atrioventricular septal defect (canal defect) is usually quite complex, and can be partial or complete.
The complete defect, which is more comon, involves the portion of the heart where that atrial septum (the wall vertically dividing the heart's upper chambers) meets the ventricular septum (separating the heart's lower chambers), as well as the valves - mitral and tricuspid - that divide these chambers horizontally.
Partial defects involve only the lower portion of the atrial septum (called ostium primum atrial septal defect and usually associated with a mitral valve defect) or, rarely, may involve only the ventricular septum, with or without a mitral valve abnormality.
The effect of the complete canal defect is a large hole spanning both the upper and lower parts of the septum and the presence, in place of two discrete mitral and tricuspid valves, of one large valve that spans both sides of the defect.
The defect is compounded by the fact that this rudimentary valve does not always close properly, so that some of the blood flows back, or regurgitates, into one of the upper chambers.
These effects result in an excessive amount of blood flowing to the lungs (a large left-to-right shunt) early in life, which produces severe symptoms of congestion and pulmonary hypertension.
Infants with this congenital defect are often emaciated because of the hard work required to breathe and consequent inability to take adequate nourishment.
Atrial septal defect is often associated with down syndrome. Approximately 25 percent of children born with this syndrome have the defect, while approximately 50 percent of children with the defect have down syndrome.
The evidence suggests that one or more genes responsible for heart development may be found on chromosome 21. Several candidate genes are being currently investigated.
On the other hand, ASD can occur in children with no other genetic abnormality.