The basic defect in this cardiac anomaly is a failure of fusion of the superior and inferior endocardial cushions. This malformation is seen in about 20% of the cases of Down syndrome; otherwise it is relatively uncommon cardiac anomaly. The failure of fusion of the endocardial cushions leads to a variety of secondary cardiac defects which include: (i) ASD consisting of osteum primum defect, (ii) membranous variety of VSD and (iii) malformations of the atrioventricular valves.