Lipomatous Hypertrophy of the Interatrial Septum: A Case Report and Insights from the Literature.

Lipomatous hypertrophy of the interatrial septum (LHIS) is a histologically benign cardiac lesion that is defined by excessive fat accumulation in the area of the interatrial septum (IAS) that does not include the fossa ovalis. Another unusual illness is lipomatosis, which is defined as a broad overgrowth of mature adipose tissue that involves a large portion of an extremity or trunk. We describe a rare case with significant LHIS accompanied by subcutaneous lipomatosis. Echocardiography revealed a mass in the right atrium in this patient. Magnetic resonance imaging revealed that this mass was composed of the adipose tissue and was an extension of a huge thickened IAS. Furthermore, this significant hypertrophy of the IAS was in direct continuation with the excessive mediastinal and epicardial fat. Copyright:

INTRODUCTION

Lipomatous hypertrophy of the interatrial septum (LHIS) is a relatively uncommon benign cardiac disease defined by the presence of an unencapsulated mass of fatty tissue in the interatrial septum (IAS).[1] Conventionally, fat deposition in the septum was described as being >2 cm thick.[2] A definite diagnosis is extremely rare during one's lifetime. Lipomatosis, another uncommon disorder, is characterized as a widespread overgrowth of mature adipose tissue that affects a significant amount of an extremity or trunk. We report a rare case with large LHIS that extended into the right atrium and was associated with subcutaneous lipomatosis.

CASE REPORT

A 55-year-old male with multiple subcutaneous lipomas was referred to our cardiology unit during preoperative investigations to assess cardiac function. He was obese (body mass index of 32 kg/m2) and reported a history of hypertension in drug therapy (angiotensin-converting enzyme inhibitor). The clinical examination revealed a mild murmur of mitral regurgitation, posteroanterior chest radiography showed a mild dilated cardiac silhouette, and electrocardiography revealed that the patient was in sinus rhythm with no other significant signs. At transthoracic echocardiography (TTE), a very hypertrophic IAS associated with a mass of not clear definition on the right atrial side was observed [Figure 1a]. The next step performed in the diagnostic process was transesophageal echocardiography (TEE). TEE showed a massive “hypertrophy” of the IAS, exceeding 23 mm in midesophageal bicaval view, with a dumbbell shape sparing the fossa ovalis (FO) [Figure 1b]. Moreover, a mobile iso-hyperechogenic round mass (20 mm × 18 mm) in the right atrium near the inferior vena cava and just below the IAS, but with a not clear implant base, was observed [Figure 1c and Video 1]. In light of these findings, we performed cardiac magnetic resonance (CMR), which showed, on cine-balanced steady-state free precession (bSSFP) sequences, a hyperintense, thickened IAS with typical “India ink” artifact at the interface with cavities,[3] sparing the FO [Figure 2a]. On T1-weighted sequences, the aforementioned thickening had increased signal intensity [Figure 2b], similar to subcutaneous and pericardial fat, whereas on short-tau inversion recovery T2-weighted sequences, low-intensity signal was seen [Figure 2c]. In first-pass perfusion images, there were no signs of increased signal, and in late gadolinium enhancement images, there was a mild inhomogeneous enhancement. These findings were pathognomonic of an extreme form of LHIS. Moreover, LHIS was extending in the right atrium near the ostium of the two vena cava, accounting for the mass seen in the right atrium on both TTE and TEE. The huge fatty “infiltration” of the IAS was accompanied by a massive mediastinal lipomatosis, resulting in an overrepresented epicardial and pericardial adipose tissue, not infiltrating the myocardium. The patient was reassured as to the benign nature of the lipomatous lesion and discharged with a recommendation for regular follow-up.

Figure 1

(a) Transthoracic echocardiography apical four-chamber view showing a hypertrophic IAS, with a mass extending in the right atrium (arrow), (b) transesophageal echocardiography midesophageal-modified bicaval view showing a massive “hypertrophy” of the IAS with a “dumbbell” shape (arrow), and (c) midesophageal bicaval view showing hypertrophic IAS and a mass in the right atrium (arrow). IAS = Interatrial septum, IVC = Inferior vena cava, LA = Left atrium, LV = Left ventricle, RA = Right atrium, RV = Right ventricle, SVC = Superior vena cava

Figure 2

(a) Cine-bSSFP image showing overrepresented mediastinal and epicardial fat and a hypertrophic, with high signal surrounded by India-ink artifact, IAS (arrow), (b) TSE T1-weighted image, showing a thickened IAS (arrow), with the same signal intensity of the subcutaneous and epicardial fat, extending in RA, and (c) TSE STIR T2-weighted image showing signal suppression of the IAS (arrow), like the subcutaneous and epicardial fat tissue. IAS = Interatrial septum, LA = Left atrium, LV = Left ventricle, RA = Right atrium, RV = Right ventricle, bSSFP: Balanced steady-state free precession, STIR: Short-tau inversion recovery, TSE = Turbo spin echo

DISCUSSION

In 1964, Prior described for the first time a nonencapsulated LHIS in a postmortem examination.[4] In 1982, the first case of LHIS diagnosed by cardiac tomography was reported. For a variety of reasons, several investigators, including the current authors, consider the expression “lipomatous hypertrophy of the interatrial septum” as inaccurate. To begin, the word “hypertrophy” is erroneous, as the LHIS is histologically defined by an increased number of adipocytes; hence, hyperplasia appears to be a more accurate definition. Then, the definition of IAS may be confusing and requires more clarification. The region immediately around the FO and referred as the septum secundum (SS) is really an infolding of the atrial roof situated between the superior vena cava orifice and the right pulmonary vein; thus, this infolding is not a true “septum,” and it was correctly described as the superior interatrial groove. The SS includes adipose tissue and tiny vessels originating from the epicardial region.[5] CMR is the best imaging technique for illustrating the IAS's complex architecture. When a cine-bSSFP sequence is employed, CMR enables a clear differentiation between muscle and adipose tissue. The intensity of the signal of distinct organs is dependent on the T1/T2 ratio in this sequence. Because both blood and adipose tissue have a high T1/T2 ratio, both tissues provide a strong signal. Muscular tissue, on the other hand, has a low T1/T2 ratio, which results in a low signal. In addition, when the fat–water interface is included in the same voxel for partial volume effect in cine-bSSFP images with a repetition time/echo time (TR/TE) around 2, the resultant signal is nulled, resulting in the so-called “India ink” artifact.[3] As a result, the sequence is effective for clearly identifying adipose tissue. In our patient, the expansion of overrepresented epicardial adipose tissue in the IAS was readily seen on cine-bSSFP CMR images. This fat does not infiltrate any cardiac region, as the SS is not a cardiac structure in the conventional sense. In addition, fat accumulation occurs cephalad and caudal to the FO, sparing the FO itself and leading to the shape of a “dumbbell.” However, because words are made to communicate, it is impossible to replace the name LHIS in the modern era.

LHIS' occurrence ranges from 2% to 8%. It was associated with obesity and advanced age, risk factors for atrial fibrillation. While the majority of patients with LHIS are asymptomatic, it is rarely associated with atrial arrhythmias and less commonly with malignant arrhythmias and sudden cardiac death.[2] The specific mechanism through which malignant arrhythmias manifest themselves in people with LHIS is uncertain. These arrhythmias may be caused by the involvement of IAS and the right atrial wall interfering with the architecture of atrial myocytes, hence, impairing conducting pathways.[6] In addition, the extension of fat in the right atrium can obstruct superior or inferior vena cava, resulting in congestive heart failure symptoms.[7]

LHIS should be differentiated from benign and malignant cardiac tumors affecting the IAS and the right atrium, such as metastases, myxomas, rhabdomyomas, fibromas, and fibroelastoma.[8]

Several imaging modalities, including echocardiography, computed tomography (CT), and CMR, have been used to diagnose LHIS.[9] Two-dimensional TTE and TEE are the first diagnostic modalities due to their widespread availability, low cost per test, and lack of side effects. The distinctive hypertrophy of the proximal and distal atrial septum, sparing the FO (dumbbell shaped), allows to establish the diagnosis of LHIS by echocardiography in most cases. Three-dimensional TEE, thanks to its ability to show the surfaces of the IAS from both left-side and right-side perspectives with an accuracy comparable to anatomic specimens, could provide more information on pathological findings of this area and their anatomical relationships. However, in less conventional cases, like ours, CMR or CT could help in the differential diagnosis. In our patient, in fact, CMR confirmed the diagnosis of LHIS and allowed to identify the round mass observed at echocardiography as part of the hypertrophic IAS and not a distinct mass. As aforementioned, CMR has a crucial role in determining the morphology of the IAS, helping in identifying the borders of LHIS and the extension into the right atrium.[10] Furthermore, it can provide valuable tissue information (fat, solid, cystic, or fibrous tissue). In addition, performing a perfusion study aids in the confirmation of a malignancy diagnosis and a clearer characterization of its type.[11]

CT can facilitate the differentiation of LHIS from malignancies due to their dissimilar relative densities. LHIS, in particular, has the attenuation coefficient of adipose tissue, which is missing in malignancies.[12]

The optimum care for these patients is a prompt diagnosis, patient reassurance, and continuous follow-up. However, surgical resection and septal reconstruction are possible treatment options in extremely rare cases of lesions causing circulatory obstruction or malignant arrhythmias.

CONCLUSIONS

LHIS is a rare benign deposition of fatty tissue within the SS, mostly diagnosed incidentally. Even though echocardiography allows a prompt diagnosis in most conventional cases, a multimodality imaging approach involving CMR or CT is crucial in less typical cases and in differential diagnoses with intracardiac masses.

With the general population's life expectancy growing, the progress of noninvasive imaging technology, and the increased incidence of obesity, LHIS diagnosis can become more likely.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.