Volume 23, Issue 7 , Pages 789-790, July 2010
Lipomatous Hypertrophy of the Interatrial Septum Revisited
Article Outline
To the Editor:
The term lipomatous hypertrophy of the interatrial septum (LHIS) was coined by Prior1 in 1964. Notwithstanding its entrenchment in the literature, pathologists find it objectionable for several reasons.2 The designation lipomatous is misleading because, unlike lipomas, which are encapsulated, the fatty lesions of LHIS are not. Furthermore, LHIS is characterized histologically by adipocyte hyperplasia, not hypertrophy. We wish to point out an additional misconception promulgated by the term LHIS, namely, that fat accumulation occurs within the interatrial septum. In point of fact, anatomic dissection studies3, 4 have revealed that such “septal” fat is actually epicardial (extracardiac).
Echocardiographically, LHIS produces a characteristic hourglass appearance (Figure 1) consisting of a larger superior fat mass and a smaller inferior fat mass that rests on the crest of the ventricular septum. These are separated by a narrow waist corresponding to the fossa ovalis, which is devoid of fat.5 Figure 1 illustrates that the walls of the left and right atria fold inward toward each other, forming a fat-filled depression between them called Waterston's groove.3 Fat contained therein corresponds to the superior echodensity seen echocardiographically. It is also apparent that the fat contained within Waterston's groove is contiguous with that overlying the epicardial surface of the heart. The echodense mass beneath the fossa ovalis represents the fat-filled inferior pyramidal space. The fat within this space, which is bordered by the left and right atrial walls and by the crest of the ventricular septum, is contiguous with epicardial fat contained within the atrioventricular groove.4 It is also worthwhile noting that computed tomographic imaging studies, which readily distinguish fat from muscle, demonstrate excessive epicardial fat in 75% of subjects with LHIS.6
Figure 1
On the left is a gross pathologic specimen of LHIS. Compare this with the transesophageal image on the right. The superior fat mass (S) corresponds to Waterston's groove (WG), and the inferior fat mass (I) corresponds to the inferior pyramidal space (IPS). FO, Fossa ovalis; LA, left atrium; RA, right atrium. Reprinted with permission from J Am Coll Cardiol7 and from Nadra I, Dawson SA, Schmitz SA, Nihoyannopoulos P. Lipomatous hypertrophy of the interatrial septum: a commonly misdiagnosed mass leading to unnecessary cardiac surgery. Heart 2004;90:e66.
Anderson et al3 defined the true atrial septum as the region confined to the fossa ovalis. It follows that if one were to pass a sharp instrument through this structure, an atrial septal defect would result. In contrast, passing a sharp instrument from either atrium into Waterston's groove or into the inferior pyramidal space would not produce an atrial septal defect but would result in exsanguination, because these are fat-filled extracardiac spaces.3
The characteristic appearance of LHIS makes echocardiographic recognition straightforward. Septal thickness > 2 cm is considered diagnostic of LHIS.6, 7 It should be noted, however, that fat is a constant feature of Waterston's groove3 and the inferior pyramidal space,4 regardless of septal thickness.
LHIS is generally a benign entity, but large collections of fat have been described obstructing superior vena caval inflow.8 A mass effect causing geometric distortion of adjacent atrial musculature may account for the intra-atrial conduction disturbances and atrial arrhythmias seen in LHIS.9 Malignant transformation is exceedingly rare, and recurrence following resection has not been reported.2
The designation LHIS inaccurately characterizes the anatomic and pathologic features of this lesion. Recognizing the role that epicardial fat plays in its pathogenesis has important implications for echocardiographers, surgeons, and electrophysiologists.
References
- . Lipomatous hypertrophy of cardiac interatrial septum. Arch Pathol. 1964;7:11–15
- . Fatty lesions of the atria and interatrial septum. Human Pathol. 2006;37:1245–1251
- . Clinical anatomy of the atrial septum with reference to its developmental components. Clin Anat. 1999;12:362–374
- . Topographic anatomy of the inferior pyramidal space: Relevance to radiofrequency catheter ablation. J Cardiovasc Electrophysiol. 2001;12:210–217
- . Usefulness of transesophageal echocardiography in diagnosing lipomatous hypertrophy of the atrial septum with comparison to transthoracic echocardiography. Am J Cardiol. 1992;70:396–398
- . Lipomatous hypertrophy of the interatrial septum: a prospective study of incidence, imaging findings, and clinical symptoms. Chest. 2003;124:2068–2073
- . Clinical, electrocardiographic and morphologic features of massive fatty deposits (“lipomatous hypertrophy”) in the atrial septum. J Am Coll Cardiol. 1993;22:226–238
- . Lipomatous hypertrophy of the interatrial septum and upper right atrial inflow obstruction. Eur J Cardiothorac Surg. 2002;22:1023–1025
- . Atrial arrhythmias and lipomatous hypertrophy of the cardiac interatrial septum. Am Heart J. 1971;82:16–21
PII: S0894-7317(10)00353-6
doi:10.1016/j.echo.2010.04.015
© 2010 American Society of Echocardiography. Published by Elsevier Inc. All rights reserved.
Volume 23, Issue 7 , Pages 789-790, July 2010
