Ridges and Pouches: A Case Series of Anomalous Atrial Septal Fusion

Anomalies in the fusion of the interatrial septum may result in several anatomic variants, including atrial septal pouch, left atrial septal ridge, and a mixed anatomy that includes both a pouch and a ridge.1 Left atrial pouch is a kangaroo pocket–like structure in the interatrial septum, without an interatrial shunt, occurring because of incomplete fusion between the septum primum and septum secundum; the pocket opens into either the left atrium or, less commonly, into the right atrium.2 A left atrial ridge is a localized thickened area of tissue thought to be due to irregular fusion of the septa, typically running along the region of the fossa ovalis on the left atrial side of the septum.3 As seen in this case series, some individuals may have a combination of an atrial septal pouch with an interatrial septal ridge (mixed anatomy). The presence of a left atrial ridge may have significant clinical implications for patients undergoing catheter-based procedures that require puncture of the interatrial septum, such as percutaneous mitral valve repair or replacement, atrial fibrillation ablation, or left atrial appendage closure. Additionally, presence of a left atrial septal pouch may have clinical significance as a nidus for thrombus formation and source of intracardiac source of embolization. In this case series, we report nine cases that highlight these anatomic variants of atrial septal fusion.