DISCUSSION
Primary cardiac tumours are uncommon and their incidence is around
0.02% as reported in autopsy studies (1). Hibernoma is a very rare type
of benign lipomatous tumour. In 1906, it was first described by Merkel,
as being composed of brown adipose tissue (2). In 1914, Gery described
the similarity of morphological features between hibernoma and
hibernating glands of animals (3). Only two previous cases of cardiac
hibernoma are reported in the literature.
Hibernoma generally occurs in adults with a peak incidence in the third
decades of life. Macroscopically, they are well-defined, encapsulated or
circumscribed mass and are usually mobile. Depending on lipid
concentration, their colour may range from light brown to gray. Furlong
et al identifies four main histological subtypes of hibernoma: typical
(82%), myxoid (9%), lipoma-like (7%) and spindle (2%) with typical
hibernoma cells, either pale or glandular (4). Typical and lipoma-like
variants have slight male predominance, while other have predominance in
female. Heaton reports that highest amount of brown fat is found in
interscapular, mediastinal, perinephric and neck sites in human and
these deposits decrease with age and peripheral ones are lost first.
Thus, it is hypothesised that incidence of hibernoma is more frequent in
the mediastinum or neck of young people (5). Furlong et al partially
agrees with the hypothesis that 15% of their cases involved chest or
neck, but their most common anatomical location was thigh (30%) (4).
Cardiac lipoma are often asymptomatic and therefore remain undetected or
found incidentally. When symptomatic, they vary and depend on location
of heart involved. They can create a mass effect on nearby structures
and may lead to obstruction of blood flow and congestive heart failure
(6,7). Embolisation is a rare phenomenon, as they are typically
encapsulated.
Definite diagnosis requires tissue sampling, but echocardiogram remains
as first line imaging modality for cardiac tumours, which is a simple
and non-invasive approach. It however cannot visualise smaller tumours
and additional imaging, such as CT or magnetic resonance imaging (MRI)
can be used. MRI yields a large differential diagnosis for lipomatous
tumours. Imaging can vary in relation to proportional components of
white and brown fat and hibernoma classically have increased signal in
both T1 and T2 weighted images (8). MRI therefore helps to distinguish
hibernoma from simple lipoma, but may not rule out from
well-differentiated liposarcoma. Due to high metabolic activity of brown
adipose tissue, positron emission tomography (PET) has been used to show
increased uptake of hibernomas. However, some studies demonstrated that
amount of brown adipose tissue may be inversely proportional to body
mass index (BMI) (9).
Pre-operative biopsy may be appropriate in an asymptomatic lesion, but
surgical excision remains as curative treatment with good long-term
prognosis.