Lipomatous Hypertrophy of the Atrial Septum

To be distinguished from a lipoma, a benign tumor of the heart, is this non-neoplastic lesion caused by hyperplastic fat in continuity with epicardial fat. The lesion can be associated with arrhythmias.

Hyperplastic fat surrounds residual bundles of atrial muscle. In contrast, a lipoma usually has a discrete border with a capsule.

Sometimes, especially in the elderly, the atrial septum is widened by the deposition of nonencapsulated brown fat with entrapped myocytes. This is referred to as lipomatous hypertrophy of the atrial septum or lipomatous hamartoma. This tumor is more frequent than a true lipoma which is rare in the heart. Cardiac lipomas are encapsulated, are composed of mature adipocytes and myocytes are not seen within the tumor. Patients with lipomatous hypertrophy of the atrial septum are subject to arrhythmias, sometimes lethal. Rarely, extension into the superior vena cava with obstruction of right atrial inflow can occur, Lesions can be demonstrated by echocardiography, magnetic resonance imaging, or computed tomography. This non-neoplastic lesion consists of hyperplastic fat in continuity with epicardial fat. Increase in subepicardial fat is a normal phenomenon in the elderly and the distinction between that normal phenomenon and this entity is sometimes subject to controversy. The appearance of brown fat with centrally placed nuclei and vacuolated cytoplasm may be helpful.