The atrial septation starts with the development of septum primum, which is followed by the formation of septum secundum. Subsequently, both septae fuse and thus form the interatrial septum. This process is complex, and its disruption can lead to different types of atrial septal defects.
Patent foramen ovale is a common finding in young children, and its overall prevalence is estimated at approximately 20% in the general population. It is a small interatrial communication, where the septum primum and secundum overlap but fail to fuse after birth, allowing shunting. Ostium secundum atrial septal defect is a frequently encountered anomaly characterized by an incomplete cover of the ostium secundum by the septum secundum. This is due to either excessive absorption of the septum primum or insufficient growth of the septum secundum.
Ostium primum atrial septal defect is the result of incomplete fusion between the septum primum and the atrio-ventricular endocardial cushions. This malformation falls into the spectrum of the atrio-ventricular septal defects, but is intentionally briefly mentioned in this chapter. Sinus venosus superior or inferior defects are characterized by the superior or inferior vena cava overriding the interatrial septum without its involvement. These defects are typically associated with a partial anomalous pulmonary venous connection of the right-sided pulmonary veins.
Coronary sinus defect is an abnormal communication between the coronary sinus and left atrium that is functionally equivalent to an interatrial communication. Treatment of atrial septal defects consists either in surgical, or in some patients, in transcatheter closure.
