Abnormal atrial septum morphology Atrial septal dilatation 2012-07-19T11:35:07Z UMLS:C0521533 SNOMEDCT_US:95440004 A bulging of the interatrial septum towards one side. In adults, atrial septal aneurysm can be defined as a protrusion of the aneurysm of >10 mm beyond the plane of the atrial septum as measured by transesophageal echocardiography. Atrial septal aneurysm HP:0011995 Atrial septal aneurysm formation may be secondary to raised interatrial pressure gradients, producing a bulging septal shift toward the low-pressure side. However, it has been also found in patients with normal atrial pressures, suggesting a primary (congenital) malformation.