Lipomatous Hypertrophy of the Interatrial Septum: An Overview

Archives of Pathology & Laboratory Medicine, Mar 2006 by O'Connor, Siobhan, Recavarren, Rosemary, Nichols, Lawrence C, Parwani, Anil V

Lipomatous hypertrophy of the interatrial septum is a rare, but increasingly recognized, anomalous developmental or neoplastic lesion of the heart. This entity was first described in 1964 at autopsy and is identified before death based on its distinctive characteristics on echocardiography, computed tomography, and magnetic resonance imaging. Although it is often asymptomatic, the mass has been associated with supraventricular arrhythmias and sudden death. In rare patients who experience intractable symptoms, surgical excision of the lesion may provide relief. Therefore, lipomatous hypertrophy of the interatrial septum is of interest to the pathologist when a cardiac mass is received for evaluation or at autopsy when a patient has experienced sudden death from an unknown cause.

(Arch Pathol Lab Med. 2006;130:397-399)

Lipomatous hypertrophy of the interatrial septum is a rare, but increasingly recognized, cardiac lesion that can cause right atrial obstruction, intractable supraventricular arrhythmias, or sudden unexpected cardiac death. It was first described by Prior1 in 1964 in 5 patients at postmortem examination. Since then, approximately 200 cases have been reported, with 1% being diagnosed at autopsy2 and 8% being diagnosed by transthoracic echocardiography.3 In a prospective study of 1292 consecutive patients who underwent multislice computed tomography, Heyer et al4 found 28 cases (2.2%) of lipomatous hypertrophy. Although first diagnosed at autopsy, antemortem detection has become more common because of advancements in imaging techniques. Several studies have suggested an association between lipomatous hypertrophy of the interatrial septum and older age (mean age, 70-71 years) and obesity.2,4,5

GROSS AND MICROSCOPIC FINDINGS

Lipomatous hypertrophy consists of an unencapsulated accumulation of mature adipose tissue with cells resembling brown fat, which demonstrate vacuolated cytoplasm and more centrally placed nuclei. Because the lesion is due to a proliferation of fat cells rather than hypertrophy of the cells, alternative names have been proposed,5,6 but the designation lipomatous hypertrophy of the interatrial septum persists. The cephalad portion of the mass is usually thicker than the caudal portion, and the fossa ovalis is generally spared, giving it a characteristic "dumbbell" shape.1 The Figure shows a gross image of lipomatous hypertrophy of the interatrial septum discovered at autopsy in a patient with hypertension, hyperlipidemia, and chronic obstructive pulmonary disease, who experienced sudden cardiac arrest after presenting to the emergency department complaining of cough and dysphagia. The lesion is often contiguous with the subepicardial adipose tissue (Figure, A). The amount of subepicardial adipose tissue tends to increase with age and with accumulated fat elsewhere in the body7 but is abnormally pronounced in patients with lipomatous hypertrophy of the interatrial septum." In one study, 55 hearts retrieved at autopsy that had enough subepicardial fat to float in water showed greater than 20-mm thickness of the cephalad portion of the interatrial septum.7 No absolute diagnostic criteria have been established, but a septal thickness of 20 mm is often quoted.5,9,10 In a review of 38 cases, the thickness of the mass ranged from 15 to 50 mm, with the largest mass measuring 7 mm,2 and another study included cases with a septal thickness of 10 mm or greater." Generally, the lesion is

confined to the atrium, and in a retrospective evaluation of 24 hearts, the masses projected only into the right side.3 However, extension into the free wall of the left atrium has been documented.12

Histologically, hypertrophied cardiac myocytes with nuclei exhibiting variation in shape and size are dispersed within the fat (Figure, B through D). Although the myocytes may have a bizarre appearance, mitoses are absent, distinguishing this lesion from a malignancy.1,13 Bands of collagen and focal infiltrates of chronic inflammatory cells may be present." The pathologic differential diagnosis includes cardiac myxoma, lipoma, and liposarcoma. Myxomas can be distinguished from lipomatous hypertrophy of the interatrial septum based on the presence of typical myxoma (lepidic) cells and myxoid areas within the tumor and on the absence of fat and hypertrophied myocytes.5 Lipomas, more rare in the heart, are encapsulated and are composed of only mature adipocytes and few, if any, muscle fibers.11 A well-differentiated liposarcoma may be suspected when the multivacuolated fat cells are mistaken for lipoblasts. In contrast to the fat cells in lipomatous hypertrophy of the interatrial septum, lipoblasts contain large clear vacuoles and hyperchromatic indented nuclei.5

IMAGING AND DIAGNOSIS

Lipomatous hypertrophy of the interatrial septum was first diagnosed in a living patient in 1982, by computed tomography." Since then, transthoracic echocardiography or transesophageal echocardiography, sometimes combined with computed tomography or magnetic resonance imaging, has been used in an attempt to establish the best modality for noninvasive diagnosis.14MS The standard initial imaging tool to evaluate primary cardiac masses is echocardiography,19 in which lipomatous hypertrophy of the interatrial septum appears as a bilobar echogenic enlargement of the interatrial septum sparing the fossa ovalis.20 However, accurate tissue characterization may not be possible using echocardiography,11' and computed tomography or magnetic resonance imaging is superior in localizing extension of the disease process.17 Computed tomography shows a nonenhancing, homogeneous, dumbbell-shaped mass of fat attenuation with smooth margins, confined to the interatrial septum and absent from the fossa ovalis.21,22 According to Gaerte et al,22 myxoma, rhabdomyoma, fibroma, fibroelastoma, and mesothelioma can be excluded if computed tomography reveals fat attenuation. Magnetic resonance imaging shows the characteristic bilobar interatrial septal thickening. The mass displays homogeneous high signal intensity similar to that of subcutaneous fat.12 Fat-suppressed imaging can confirm the fatty nature of the thickening.12 Although a benign process is usually diagnosed by its homogeneous appearance on magnetic resonance imaging, Aziz and Julsrud23 demonstrated that certain cases of lipomatous hypertrophy of the interatrial septum may appear inhomogeneous, depending on the number and distribution of myocytes within the lesion. They caution that, in these cases, magnetic resonance imaging may not reliably differentiate a benign from a neoplastic process. More recently, Fan et al24 found that lipomatous hypertrophy of the interatrial septum can display increased fluorodeoxyglucose uptake on positron emission tomography, possibly related to the presence of brown fat. They recommend that these findings be correlated with results of computed tomography or magnetic resonance imaging to avoid a false diagnosis of malignancy. In some cases, histopathologic evaluation may be necessary for definitive diagnosis. Percutaneous transvenous biopsy was performed to obtain tissue in one such case.25 The lesion has been diagnosed during surgery, as well.26