Lipomatous hypertrophy of the inter-atrial septum

Lipomatous hypertrophy of the interatrial septum (LHIS) is a relatively uncommon disorder of the heart characterised by benign fatty infiltration of the interatrial septum. It is commonly found in elderly and obese patients as an asymptomatic incidentally discovered finding. 

The prevalence of LHIS is estimated to be between 1-8%. It usually occurs in older, obese people and there may be a higher incidence in women 6.

The condition is asymptomatic incidentally discovered in a majority of individuals 5.

Microscopically, LHIS is characterized by fat infiltration between the myocardial fibers of the atrial septum. LHIS also can create a masslike bulge. There is typical sparing of the fossa ovalis 9, with LHIS lying anterior to the the fossa ovalis.

Associations
CT

Diagnosis is made with CT when a smooth, non-enhancing, well-marginated fat-containing lesion in the interatrial septum is identified. It does not enhance.

The lesion characteristically takes on a dumbbell shape, with relative sparing of the fossa ovalis (this may best be seen on reconstructed short axis views).

Cardiac MRI

MRI diagnosis is straightforward in classic cases, and this entity is characterized by a bilobar interatrial septal thickening revealing homogeneous high signal intensity similar to that of subcutaneous fat tissue. 

  • T1: high signal
  • T2 non fat suppressed: high signal
  • T2 fat suppressed: low signal

The exclusively fatty nature of such masses can be seen on fat-suppressed imaging.

FDG PET/CT

Usually shows moderate degree of FDG-uptake. It may vary on serial studies and the FDG-avidity was initially thougt to be caused by metabolic activity of brown adipose tissue (BAT) 12,13. More recently this theory has been questioned by other authors, with suggestion of either inflammation or an hitherto unidentified mechanism of BAT activation as potential mechanism 14.

Be that as it may, there is consensus on the necessity of hybrid imaging and correlation with former imaging studies to avoid misclassification.

It is benign in many individuals and often does not warrant any treatment, in situations where there is severe superior vena cava obstruction or intractable rhythm disturbance, surgical excision with reconstruction of the interatrial septum is sometimes considered.

Although histologically benign, LHIS has been associated with adverse clinical sequelae including 

  • cardiac rhythm disturbances 7-8: lipomatous hypertrophy is usually situated in the area of at least two described interatrial conduction pathways (anterior and middle internodal pathways), and their interruption could be the major reason for rhythm disorders such as 
    • supraventricular arrhythmias  
    • atrial fibrillation
    • atrial premature contractions
    • atrioventricular block
  • obstructive flow symptoms including dyspnea - especially if large 6,16
  • syncope
  • sudden death

It was first described by J T Prior in 1964.

Differential diagnoses should be thought of when there is sparing of the fossa ovalis.

Fat-containing neoplasms that can arise in the atrial septum including:

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Article Information

rID: 1588
Systems: Vascular, Cardiac
Section: Pathology
Tag: cases
Synonyms or Alternate Spellings:
  • Lipomatous enlargement of the interatrial septum
  • Lipomatous hypertrophy of the interatrial septum (LHIS)
  • Lipomatous hypertrophy of the inter-atrial septum
  • Lipomatous hypertrophy of inter-atrial septum
  • Lipomatous hypertrophy of the intraatrial septum
  • Lipomatous hypertrophy of interatrial septum

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