Interventricular septal aneurysm is different from ventricular aneurysm which usually occurs in the cardiac apex. It is defined as a bowing of the interventricular septum of more than 15 mm on either side in adults and 5 mm in children during normal cardiac motion. It may involve either the membranous (most frequently) or muscular portion of the interventricular septum. The etiology is usually congenital.