TOPIC: Cardiovascular Disease TYPE: Medical Student/Resident Case Reports INTRODUCTION: Lipomatous hypertrophy of the atrial septum (LHAS) is a rare, non-neoplastic fatty infiltration of the inter-atrial septum that spares the fossa ovalis. It is best differentiated from other cardiac tumors with transesophageal echocardiogram (TEE) and cardiac computed tomographic (CT) or magnetic resonance (MR) imaging. CASE PRESENTATION: A 52-year-old woman with diabetes, hypertension and a chronic tobacco smoker, presented with complaints of shortness of breath. An electrocardiogram showed non-specific ST-T wave changes. A TTE revealed an ejection fraction of 60% and a well-encapsulated mass in the right atrium; the differential diagnosis of the mass included entities such as atrial myxoma. Further evaluation with a TEE demonstrated a large, sessile, immobile mass in the right atrium with a characteristic dumbbell shaped thickening of the atrial septum, thus raising the possibility of lipomatous hypertrophy of the atrial septum (LHAS). Cardiac MRI confirmed the distinctive hour-glass shaped mass in the intra-atrial septum, with characteristic sparing of the fossa ovalis. There was no contrast enhancement and lacked signal on fat suppression sequences confirming benign LHAS. DISCUSSION: LHAS is a rare, non-neoplastic abnormality of the heart. Unlike other cardiac tumors such as myxomas and papillary fibroelastomas which require surgical intervention, LHAS is a benign finding. It is thought to be an infiltration of brown adipose tissue into the myocytes of the interatrial septum. Most cases remain clinically silent and are detected incidentally during echocardiography, surgery or autopsy. Echocardiography, where a TEE is more sensitive than a TTE, and cardiac MRIs demonstrate a characteristic dumbbell or hour-glass shaped appearance. Tailored MRI sequences with lack of gadolinium enhancement and fat suppression imaging, differentiate it from other intracardiac neoplasms, including myxomas. Moreover, positron emission tomography (PET) scans will show high fluorodeoxyglucose (FDG) uptake by the brown adipose tissue that may further aid in diagnosis. Rare complications of LHAS include SVC syndrome, sick-sinus syndrome and other life-threatening arrhythmias that may warrant surgical intervention. CONCLUSIONS: LHAS is an uncommon cardiac anomaly. The utility of TEE and cardiac MRI, in this case, aids in defining the benign nature of LHAS. REFERENCE #1: Nadra I. Lipomatous hypertrophy of the ineratrial septum; a commonly misdiagnosed mass often leading to unnecessary cardiac surgery. Heart. 2004;90(12):66. REFERENCE #2: Heyer C, Kagel T, Lemburg S, Bauer T, Nicolas V. Lipomatous hypertrophy of the interatrial septum. Chest. 2003;124(6):2068–2073. doi: 10.1378/chest.124.6.2068. DISCLOSURES: No relevant relationships by Adhiraj Bhattacharya, source=Web Response No relevant relationships by Rizwan Khan, source=Web Response No relevant relationships by Michael Maysky, source=Web Response