Lipomatous atrial septal hypertrophy (LASH) is a histologically benign cardiac lesion characterized by excessive fat deposition in the region of the interatrial septum that spares the fossa ovalis. The etiology of LASH remains unclear, though it may be associated with advanced age and obesity. Because of the sparing of the fossa ovalis, LASH has a pathognomonic dumbbell shape. LASH may be mistaken for various tumors of the interatrial septum. Histologically, LASH is composed of both mature and brown (fetal) adipose tissue, but the role of brown adipose tissue remains unclear. In interventional procedures requiring access to the left atrium, LASH may interfere with transseptal puncture, as traversing the thickened area can reduce the maneuverability of catheters and devices. This may cause the needle to enter the epicardial space, causing dangerous pericardial effusions. LASH was once considered a contraindication to percutaneous device closure of atrial septal defects because of an associated increased risk for incorrect device deployment. However, careful attention to preprocedural imaging and procedural intracardiac echocardiography enable interventional cardiologists to perform procedures in patients with LASH without serious complications. In this review article, the authors describe anatomic and functional aspects of LASH, with emphasis on their roles in percutaneous interventions.
Highlights
- •
LASH is characterized by >2-cm fat deposition in the area of the interatrial septum.
- •
LASH is composed of mature adipose tissue and BAT.
- •
LASH may be associated with advanced age and obesity as well as atrial arrhythmias.
- •
Echocardiography, CT, and MRI are used to diagnose LASH, which has a bilobed appearance.
- •
LASH may not be a contraindication to percutaneous interventional procedures.
Attention ASE Members:
The ASE has gone green! Visit www.aseuniversity.org to earn free continuing medical education credit through an online activity related to this article. Certificates are available for immediate access upon successful completion of the activity. Nonmembers will need to join the ASE to access this great member benefit!
Anatomy: Where Is LASH Located?
Although LASH is technically referred to as lipomatous hypertrophy of the interatrial septum, this is a misnomer. Anatomically, the true interatrial septum is confined to the region of the fossa ovalis and the area just below the fossa near the orifice of the tricuspid valve (the septum primum). The muscular rim surrounding the fossa in the superior, anterior, and posterior margins (the embryologic septum secundum) is in fact an infolding of the atrial wall extending into the right atrial cavity with epicardial fat on the outside. These infoldings are sometimes referred to as Søndergaard’s groove or Waterston’s groove. The fat accumulation of so-called LASH does not actually occur within the true septal tissue but rather in infoldings of the atrial wall adjacent to the true interatrial septum.
LASH has a pathognomonic “dumbbell” shape, as fat accumulation is cephalad and caudad to the fossa ovalis, with sparing of the fossa itself. The cephalad thickening is usually more extensive than the caudad, and both masses project into the right atrial cavity. Furthermore, the cephalad component is contiguous with the rest of the subepicardial fat, which is generally markedly increased in patients with LASH.
Although LASH is typically confined to the atrial infoldings, fat accumulation can be so extensive that it emulates a neoplasm. On very rare occasions, it may extend toward the superior vena cava, causing obstruction of right atrial inflow. Patients with vena cava obstruction may present with symptoms of congestive heart failure.
Anatomy: Where Is LASH Located?
Although LASH is technically referred to as lipomatous hypertrophy of the interatrial septum, this is a misnomer. Anatomically, the true interatrial septum is confined to the region of the fossa ovalis and the area just below the fossa near the orifice of the tricuspid valve (the septum primum). The muscular rim surrounding the fossa in the superior, anterior, and posterior margins (the embryologic septum secundum) is in fact an infolding of the atrial wall extending into the right atrial cavity with epicardial fat on the outside. These infoldings are sometimes referred to as Søndergaard’s groove or Waterston’s groove. The fat accumulation of so-called LASH does not actually occur within the true septal tissue but rather in infoldings of the atrial wall adjacent to the true interatrial septum.
LASH has a pathognomonic “dumbbell” shape, as fat accumulation is cephalad and caudad to the fossa ovalis, with sparing of the fossa itself. The cephalad thickening is usually more extensive than the caudad, and both masses project into the right atrial cavity. Furthermore, the cephalad component is contiguous with the rest of the subepicardial fat, which is generally markedly increased in patients with LASH.
Although LASH is typically confined to the atrial infoldings, fat accumulation can be so extensive that it emulates a neoplasm. On very rare occasions, it may extend toward the superior vena cava, causing obstruction of right atrial inflow. Patients with vena cava obstruction may present with symptoms of congestive heart failure.
Embryology
The exact etiology of LASH is unclear. However, it has been postulated that fat accumulation is due to embryologic development of the interatrial septum. In utero, the septum secundum and septum primum fuse, leaving the foramen ovale as the residual communication. Outgrowths of tissue from the walls of the primitive atria create infoldings that fuse with the region of fossa ovalis to form the apparent edges of the interatrial septum. During this fusion, mesenchymal cells become trapped within the atrial wall and later develop into mature adipocytes with appropriate stimuli.
Histology
As previously described, LASH is located outside the true interatrial septum (the fossa ovalis). Microscopically, the myocardial fibers of the septum secundum are infiltrated with mature adipose cells interspersed with a variable number of vacuolated fetal fat cells. Myocardial cells are seen entrapped in the mass and show hypertrophic, degenerative, or atrophic changes.
Many reports suggest that LASH consists predominantly of mature adipocytes (also known as white fat) and that there is a positive correlation between LASH and generalized obesity. It is important to note that histologically, LASH does not represent a malignancy, because mitoses are absent. Furthermore, because there is no capsule around the fat deposits, LASH is a distinct entity from lipoma.
Interestingly, studies have also suggested the presence of brown fat (also known as fetal fat) in LASH. These cells are distinguishable from surrounding mature adipose tissue by their vacuolated cytoplasm and more centrally placed nuclei. The exact role of this brown adipose tissue (BAT) is a subject of debate. Some investigators have shown that focally increased fluorodeoxyglucose (FDG) uptake occurs in LASH and postulated that the metabolic activity of the brown tissue is the reason for this finding. However, more recent studies have questioned this theory, arguing that the mechanism of FDG uptake in LASH differs from that of brown fat elsewhere in the body. These researchers instead suggest that the increased uptake may be secondary to an inflammatory process in LASH.
A recent case report of a patient with LASH and isolated mediastinal FDG uptake in the interatrial septum (mimicking metastasis) suggests the possibility that neither brown fat nor inflammation is the reason for uptake. The investigators pointed out that activation of BAT is usually due to cold temperature, and external warming has been shown to deactivate BAT and reduce FDG uptake. They went on to explain that although inflammatory uptake in lesions is usually confirmed by washout of FDG on delayed imaging, there was no decrease in uptake on external warming or on delayed time point imaging in their patient. This suggests that uptake may not be due to metabolic activity of BAT or inflammation and thus that there must be an alternative explanation for FDG accumulation in this condition.
Clinical Significance
Although LASH is considered a histologically benign entity and is usually clinically silent, it has on rare occasion been associated with atrial arrhythmias and even more rarely with sudden death. As previously noted, the prevalence of LASH increases with age and obesity. Given that older age and obesity are risk factors for atrial fibrillation, more analyses are needed to determine if an independent relationship between LASH and arrhythmias exists. Nonetheless, in an elderly and/or obese patient with an atrial arrhythmia in the absence of other causative factors (such as congenital or valvular heart disease, atriomegaly, or sick sinus syndrome), the arrhythmia may be due to LASH.
A possible explanation for this association is the progressive disruption of myocardial fibers that result from fat accumulation and infiltration into pathways that ordinarily facilitate orderly atrial depolarization. This disorganization of fibers ultimately leads to fibrosis, with consequent impairment of contractility and electrical conduction.
Case reports have described the association of LASH with serious cardiac arrhythmias resulting in sudden death. In these patients, postmortem examination revealed LASH and no other pathologic findings. The authors of these reports have suggested that the location of LASH (i.e., in the area of the sinoatrial node) and its anatomic distribution within the septum may have contributed to the incidence of malignant arrhythmias. They also observed that the total volume of fat tissue may be a major factor in the development of arrhythmias, especially given that the incidence of atrial arrhythmia appears to be related to septal thickness.
Imaging of LASH
Multiple imaging modalities, including echocardiography, CT, and magnetic resonance imaging (MRI), can be used to diagnose LASH. Irrespective of the imaging modality, LASH has a characteristic dumbbell appearance of the interatrial septum, with sparing of the fossa ovalis. This typical appearance is often sufficient to establish the diagnosis of LASH by echocardiography. In less typical cases, confirmation of its fatty composition may be obtained by either CT or MRI.
On standard and contrast-enhanced CT ( Figure 1 ), LASH appears as a homogenous, dumbbell-shaped mass of fat attenuation that is confined to the region of the interatrial septum; it has smooth margins and is nonenhancing. Because CT identifies relative densities, it aids in differentiating LASH, which has the attenuation coefficient of adipose tissue, from neoplasms and other entities.
MRI can provide additional information ( Figure 2 ) about the extension of the process into the interventricular septum and ventricular free wall and is useful in distinguishing fat from solid, fibrous, or cystic tissue.
Two-dimensional transthoracic echocardiography and TEE are often the diagnostic modalities of choice for identifying LASH ( Figure 3 and Video 1 , available at www.onlinejase.com ; Figures 4 A and 4B and Videos 2 and 3 , available at www.onlinejase.com ).
