To the Editor:
The term lipomatous hypertrophy of the interatrial septum (LHIS) was coined by Prior in 1964. Notwithstanding its entrenchment in the literature, pathologists find it objectionable for several reasons. 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 studies 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. 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. 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. 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.
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