Massive lipomatous hypertrophy of the right atria.

A case of a 70-year-old female with a history of hypertension, atrial fibrillation, pacer implantation for symptomatic bradycardia, and a prior cerebrovascular accident, and had developed persistent methicillin-sensitive Staphylococcus aureus bacteremia is reported here. As part of her evaluation, a transesophageal echocardiogram was performed, and even though no vegetations were seen on either pacer wires or cardiac valves, a massive homogeneous thickening of the superior portion of the interatrial septum extending to the posterior and roof portions of the right atrial wall as well as to the superior vena cava causing proximal compression of this vessel was noted. Computed tomographic examination of the chest helped to determine that this mass density was not a tumor but in fact intrapericardial fat. Imaging findings and existing literature on this topic are reviewed.

INTRODUCTION

Lipomatous hypertrophy of the interatrial septum (LHIS) is an exaggerated growth of normal fat existing within the septum and is not a true tumor. The septal hypertrophy may be as much as 2 cm in thickness and is seen primarily in older patients and in those who are obese. It has been suggested that this disorder is associated with the presence of coronary artery disease in proportion to the degree of atrial septal thickness.

Lipomatous hypertrophy of the interatrial septum is indistinguishable from lipoma except that the former occurs in the atrial septum with a typical distribution (generally sparing the fossa ovalis). In the absence of symptoms of atrial arrhythmias, heart block or rare vena caval obstruction, they do not require resection.

Nowadays, with advanced imaging techniques, LHIS are picked up incidentally in the course of work-up for other conditions. Such a case is presented here and imaging findings and literature on this lesion are reviewed.

CASE REPORT

A 70-year-old female with a history of hypertension, atrial fibrillation, pacer implantation for symptomatic bradycardia, and a prior cerebrovascular accident was transferred to our institution for further evaluation and management of a recently identified persistent methicillin-sensitive Staphylococcus aureus bacteremia.

During workup of her bacteremia, a transesophageal echocardiogram was requested to rule out the presence of endocarditis. Although no vegetations were seen on either pacer wires or cardiac valves, a massive homogeneous thickening of the superior portion of the interatrial septum was noted [Figure 1a and b]. This mass effect extended to the posterior and roof portions of the right atrial wall as well as to the superior vena cava, causing proximal compression of this vessel [Figure 1c and d]. In view of these findings, a chest computed tomographic examination was requested for better definition of this mass density. Following contrast administration, the mass effect noted by transesophageal echocardiography was identified as intrapericardial fat density that extended toward the interatrial septum without evidence of infiltration of the interatrial septum directly [Figure 2]. Additionally, a nodular opacity was also found in the lateral left upper lobe with a greatest diameter of 1 cm that was thought to be a potential septic embolus given the recent history of bacteremia (not shown).

Figure 1

(a) Standard mid-esophageal four-chamber transesophageal view showing right atrium (RA), right ventricle (RV), left atrium (LA), and left ventricle (LV), as well as the pacer wire, (b) Pulling up the transesophageal probe to interrogate posterior aspects of both right and left atria demonstrated the presence of a diffusely homogeneous mass effect noted in the posterior aspect of the right atrium as demarcated by the arrow heads, (c) At a 90° angle, bi-atrial mid-esophageal view, both left (LA) and right (RA) atria are seen. The homogeneous mass is also seen compressing the superior vena cava (SVC) with the pacer wires, (d) Another view of the mass, demarcated by arrows, from the superior aspect of the right atria compressing the superior vena cava (SVC) shown by the hatched arrow

Figure 2

Transverse chest computed tomographic view showing the fat density in relation to the pacer wires, left atrium (LA), right coronary artery (RCA), and aortic valve (AV)

The initial continuous infusion of Nafcillin was later changed to Oxacillin, as surveillance blood cultures were all negative. The plan was to remove her transvenous pacer and replace it once she had completed her antibiotic therapy and follow-up blood cultures off antibiotics remained negative.

DISCUSSION

Lipomatous hypertrophy of the interatrial septum (LHIS) is a rare, but increasingly recognized benign cardiac lesion characterized by excessive deposition of fat in the interatrial septum.[1] While the true incidence of LHIS has not been determined; the expanding use of non-invasive imaging techniques in recent years has quoted an incidence up to 8%, compared to an incidence of 1% based on autopsy reports.[1]

Although the exact etiology of LHIS remains unclear, some theories have suggested the existence of embryonal mesenchymal cells within the primitive atria that can develop into adipocytes with an appropriate stimulus, particularly obesity and advanced age.[1] The resultant effect is adipocyte hyperplasia and fat accumulation occurring in the epicardium, that is an extracardiac deposition fat, rather than within the interatrial septum as recently illustrated by Silbiger, et al. In this report, the authors demonstrated how the walls of the left and right atria fold inward toward each other, forming a fat-filled depression between them called Waterston's groove.[1] Fat contained in this region is not a true interatrial septal density, but in reality is fat that overlies the epicardial surface of the heart.

Transesophageal echocardiography is one of the most commonly used imaging modalities to interrogate cardiac structures, in which routine examination of the interatrial septum is performed. The characteristic echocardiographic appearance of LHIS involves an interatrial septal thickness >2 cm usually represented by an hourglass appearance denoted by fat mass superior as well as inferior, sparing the fossa ovalis in between.[2]

Most patients with LHIS are usually asymptomatic; however, in some cases, this fatty infiltration can either cause intraatrial conduction disturbances and atrial arrhythmias or when this fatty infiltration extends beyond the atrial septum may cause right atrial inflow obstructive symptoms.[34]

However, there are times that echocardiography alone is not capable of characterizing myocardial tissue, particularly when the extent of the process as described in our case is so massive. In such cases, the spatial resolution of cardiac MRI is ideal in providing detailed information about the size and location of such masses. Furthermore, with its ability to evaluate T1 and T2 characteristics of the mass, as well as the ability to use fat-suppression techniques in addition to gadolinium, the histopathologic characteristics of a mass can be clearly evaluated by cardiac MRI.[5] Alternatively, chest computed tomography can be also used to help make the diagnosis.[16] In this case, chest computed tomography was preferred over cardiac MRI due to the patient's pacemaker.

CONCLUSION

LHIS might mimic a large atrial mass. However, careful recognition of some anatomical findings might be useful in distinguishing this rare, but increasingly recognized benign cardiac lesion with other cardiac pathology.