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Authors: Riyad NH Seervai and Claire Jordan Wiggins, Medical Students, Baylor College of Medicine, Houston, Texas, USA. DermNet NZ Editor in Chief: Adjunct A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy edited by Gus Mitchell. June 2020.
Hibernoma, also known as fetal lipoma or lipoma of embryonic fat, is a rare subtype of benign lipomatous tumour derived from brown fat. It was first reported as a ‘pseudolipoma’ (a ‘peculiar fat tumour’) by Merkel in 1906  and was given its name by Gery in 1914 for its resemblance to brown fat found in hibernating animals .
The main function of brown adipose tissue is non-shivering thermogenesis (‘burning’ fat), which it accomplishes through fatty-acid dependent uncoupling of adenosine triphosphate (ATP) production in the mitochondria via the uncoupling protein UCP1 . Brown fat deposits are typically found in infants [4,5], and were believed to disappear during adulthood [6,7]. However, several reports indicate the existence of brown fat in adults [8–10], with debate over whether these deposits constitute classical brown fat or are ‘beige’ fat [11,12]. Continuous growth of these vestiges of brown fat lead to the formation of a hibernoma.
Hibernoma is associated with chromosomal rearrangements in the long arm of chromosome 11 (11q13-21), which includes the multiple endocrine neoplasia type 1 (MEN1) region [13–15].
Loss of AIP and MEN1 genes are considered to be essential for hibernoma development . There are reports of hibernoma in patients with multiple endocrine neoplasia type 1 (MEN1) syndrome . However, the cytogenetic pattern seen in hibernoma differs from those seen in MEN1-associated tumours and it is believed they progress via different mechanisms .
Two isolated cases of hibernoma have been reported with high expression of TP53, with the authors speculating that inactivation of the protein may be important for their development .
Hibernomas are rare, comprising ~1% of all adipocytic tumours . They occur in the third to fourth decade of life (the mean age of onset is 38) and they are uncommon in children. They are more common in men .
Hibernoma presents as a progressive, slow-growing, painless subcutaneous mass . The most common locations for hibernoma include the thigh, shoulder, back, neck, chest, arm, and retroperitoneum . Other locations include the breast, [21,22], larynx , pleura , pelvis , vulva , and scrotum [27,28]. Approximately 10% of hibernomas are intramuscular .
Complications of hibernoma are related to the rapid tumour growth and infiltration of neighbouring structures. Pressure on nerves can result in neuropathic pain.
Radiographic imaging is used when considering a diagnosis of hibernoma [19,35].
The diagnosis of hibernoma is confirmed by histopathology on biopsy or excision.
There are four hibernoma variants .
The lobular hibernoma is the most common variant (82%), which appears as a mixture of pale cells, hibernoma cells, and eosinophilic cells.
A myxoid hibernoma contains a loose basophilic matrix and foamy histiocytes. It is more common in men and in the head and neck region (including the scalp and shoulder).
Lipoma-like hibernoma has univacuolated white adipocytes with scattered hibernoma cells. It arises most commonly in the thigh.
Spindle cell hibernoma is the least common variant, accounting for 2% of hibernomas. It has features of spindle cell lipoma and hibernoma. Histology shows hibernoma cells, CD34+ spindle cells, thick collagen bundles, mast cells, myxoid stroma, and mature adipocytes. It is found on the scalp and posterior neck.
The differential diagnosis for hibernoma includes benign and malignant tumours [19,35].
Benign soft tissue tumours that could be interpreted as hibernoma include:
Malignant soft tissue tumours that could be interpreted as hibernoma include:
The definitive treatment for hibernoma is complete surgical resection . Since hibernomas are almost never malignant , no treatment is required if the patient is asymptomatic.
Hibernomas are typically benign, and recurrence is rare after complete excision .
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