The endocardial cushions are embryological precursors of the lower portion of the interatrial septum, the upper portion of the superior end of the interventricular septum, and portions of the mitral and tricuspid valves.
Structurally the epicentre of the endocardial cushions is at the crux of the heart, so that many structures can be affected when there is a disorder of the embryological tissue.
In the extreme of disease, known as a complete A-V canal defect, all the chambers of the heart are potentially connected and blood from any one of the chambers can flow to any one of the others. In addition the conduction system is close by and so it is often affected by endocardial cushion defects. The atrial septum can be affected in both the complete and the partial defects. An isolated primum ASD affects the lowest portion of the atrial septum and usually affects the conduction system and left axis deviation ofn the EKG is characteristic. Sometimes a cleft mitral valve is associated with the primum ASD.
The diagnosis is made clinically with ejection flow murmur, loud fixed P2, and occasionally a murmur of mitral regurgitation. Echocardiography is the study of choice.