Defining the morphologic phenotypes of atrial septal defects and interatrial communications

True atrial septal defects are limited to deficiencies totally within the confines of the oval fossa and its antero-inferior rim. Other communications between the two atriums, such as the superior or inferior sinus venosus defects, coronary sinus defect, and the ostium primum defect, are less frequently seen. They represent interatrial communications rather than septal defects, since the communication between the atrial chambers is outside the confines of the true atrial septum. The ostium primum defect, for example, has all the characteristics of an atrioventricular septal defect, existing only in the setting of a common atrioventricular junction. The unifying physiological feature of all these variants, whether their morphology is that of a defect within the area of the oval fossa, or an opening elsewhere within the atrium, is that mixing of the systemic and pulmonary blood occurs at atrial level. In this review, we emphasise the distinction between true atrial septal defects and defects which result in communications between the atriums, underlining the phenotypic characteristics, along with the notable morphological features that are of significance in current strategies for clinical management.