Prepared by Dr. Pavni Patel and Dr. Charles S. White at the Department of Radiology, University of Maryland School of Medicine, Baltimore, MD.

CASE SUMMARY

A 46-year-old diabetic woman fell from a standing height. The next day she presented to the Emergency Department with lower back pain. On physical examination she had signs of a peripheral neuropathy. A CT scan of the lumbar spine revealed degenerative changes in the spine and excluded a fracture. A CT scan of the abdomen and pelvis was also performed to assess for any injury. CT images of the lower chest and heart are shown (A, B, C).

DIAGNOSIS

Lipomatous hypertrophy of the interatrial septum

IMAGING FINDINGS

An incidental abnormality is evident in the CT of the lower chest. CT images demonstrate a smoothly marginated, fat-containing dumbbell-shaped mass in the region of interatrial septum with relative sparing in the region of fossa ovalis (images A, B, C). There is an extension of mass along the lateral wall of the right atrium and a convex bulge of the septum into the left atrium. Superiorly, the mass appears to be in contact with the superior vena cava; however, no definite invasion is present. Inferiorly the mass extends to the level of coronary sinus.

DISCUSSION

Lipomatous hypertrophy of the interatrial septum (LHAS) is a benign entity characterized by fat accumulation in the interatrial septum of the heart. Although it is frequently asymptomatic and found incidentally on echocardiography, cases of lipomatous hypertrophy are reported to be associated with atrial arrhythmias. There may also be an association between LHAS and obesity. Excision is performed only in those cases in which superior vena caval obstruction necessitates surgical removal. ^1,2

Embryologically, LHAS is thought to be due to the mesenchymal cells becoming trapped with the interatrial septum and developing into adipocytes later in life.

CT shows a non-enhancing, homogenous, smooth, well-marginated mass of fat density confined to the atrial septum. LHAS can vary in size from a few cm to a large mass. In a study by Meaney et al, ¹ the average dimensions were 7 cm for cranio-caudal extent, 4.5 cm along the interatrial septum, and 2.7 cm perpendicular to the septum. CT is also useful in determining the extent of the infiltration of the mass into the right atrium and differentiating it from other cardiac lesions. MR imaging is an excellent noninvasive method, since it can distinguish tissue composition and document any caval obstruction. On T1-weighted MR images, LHAS is seen as a high signal intensity structure in the interatrial septum projecting into the right atrium. On fat-suppressed MR images, LHAS exhibits signal suppression similar to the subcutaneous fat. ^3,4

Cardiac lipoma is an important differential of LHAS; however, it is typically an encapsulated fat density in a subendocardial and subpericardial location and occurs in a younger age group. Cardiac myxoma, the most common primary cardiac tumor, has the same signal intensity as adjacent myocardium on both T1- and T2-weighted images and should not be mistaken for LHAS. Liposarcoma is a rare malignant cardiac tumor, which shows fat density sometimes admixed with soft tissue. However, no cases of liposarcoma have been reported in the region of interatrial septum.

Angiosarcoma, the most common primary malignant cardiac tumor in adults, usually arises from the right atrial free wall and often spares the atrial septum. Rhabdomyosarcoma is the most common cardiac malignancy in infants and children. The tumor is often multi-focal, arises from the myocardium, and may involve cardiac valves and invade the pericardium. MR imaging is useful in differentiating this entity from LHAS because rhabdomyosarcoma appears isointense compared to the myocardium. Leiomyosarcomas are masses that arise from the wall of atrium but they have a more irregular and lobulated appearance than LHAS. They are also more invasive and may be seen as multiple filling defects in the pulmonary veins suggesting invasion. ⁴