Imaging description

Aneurysm of the membranous ventricular septum appears as a thin-walled, round or lobulated outpouching of the basilar ventricular septum just inferior to the aortic valve annulus (Figure 17.1). It bulges into the right ventricle because of higher left ventricular pressure; however, it may change in size dynamically during the cardiac cycle. Signal characteristics on imaging match the left ventricular blood pool (Figure 17.2), unless complicated by thrombus. Membranous septal aneurysms (MSA) are wide-necked and typically measure 10–15 mm in size; however, much larger aneurysms have been reported.

Importance

Membranous ventricular septal aneurysms may be incorrectly diagnosed as sinus of Valsalva aneurysms or cardiac masses, potentially leading to inappropriate surgery and patient anxiety. Membranous septal aneurysms generally have a benign course; however, thrombus formation, right ventricular outflow tract (RVOT) obstruction, rupture, and infection have been reported.

Typical clinical scenario

Membranous septal aneurysms are typically incidentally diagnosed at echocardiography, cardiac CT, or cardiac MRI. They are rare congenital malformations and when found in children, are often associated with other types of congenital heart disease with an estimated prevalence of 0.3% in children with congenital heart disease. When diagnosed in children, membranous septal aneurysms are associated with a ventricular septal defect (VSD) and left-to-right shunt in approximately 20% of cases and some experts believe they may play a role in spontaneous closure of these shunts. Given this association, identification of a membranous septal aneurysm should prompt evaluation for membranous defect and left-to-right shunt. There is an increased incidence of atrioventricular block in patients with membranous septal aneurysms.

Differential diagnosis

The primary differential diagnosis is a sinus of Valsalva aneurysm. These originate above the level of the aortic valve annulus; however, similar to MSA they can protrude into the right ventricular outflow tract. Periaortic pseudoaneurysm due to endocarditis may also have a similar appearance; however, patients will have an abnormal aortic valve and will be quite ill.