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.