Atrial septal defect Primum atrial septal defect 2009-09-14T09:59:38Z With this type of ASD, communication persists when the septum primum fails to fuse with the endocardial cushions, thus leaving an opening between both atria near the atrial floor, being a mid/late event in atrial septation. Due to the association with endocardial cushion abnormalities, this defect may be associated with either atrioventricular valve anomalies, such as mitral valve cleft, or a complete atrioventricular septal defect. Septum primum defect Ostium primum atrial septal defect HP:0010445 Primum atrioventricular canal defect UMLS:C0344735 UMLS:C0741296 SNOMEDCT_US:253373002 An ostium primum atrial septal defect is located in the most anterior and inferior aspect of the atrial septum. The ostium primum refers to an anterior and inferior opening (ostium) within the septum primum, which divides the rudimentary atrium during fetal development. The ostium primum is normally sealed by fusion of the superior and inferior endocardial cushions around 5 weeks' gestation. Ostium primum defects result from a failure of the fusion of the embryologic endocardial cushion and septum primum. Fyler:1110 ostium primum ASD Atrial septal defect, primum type