What is Hibernoma?
Hibernoma is a type of rare, benign tumour derived from brown fat. It arises from the remnants of the fetal brown adipose tissues. Brown adipose tissue or Brown fat is one of the two types of adipose tissue present in the body. Brown adipose tissues have dark red to tan colour because they have a large number of iron-rich mitochondria. The brown lipocytes have many minute lipid droplets whereas white fat has only one single large droplet.
The brown adipose tissues are present in the region that has more capillaries because of their high oxygen demand and are mostly located in the neck, thigh, shoulders, and back. The primary function of the brown adipose tissue is to produce heat and keep the body warm. The brown adipose tissues get activated in the cold and generate heat by burning calories. The level of brown adipose tissue is higher in infants than in humans. The level of brown adipose tissue gradually decreases with age. Hibernoma is the accumulation of the fats as vestigial remnants in the region where brown adipose tissues are present.
Merkel initially reported the Hibernoma in 1906 as “Pseudolipoma”. Merkel had described it as a peculiar fat tumour that is composed of brown adipose tissues. Later, in 1914, Grey had given it the name Hibernoma based on its resemblance to the brown fat present in hibernating animals. The Hibernoma is also known as lipoma of embryonic fat or fetal lipoma. Hibernoma is a non-tender, small benign tumour that has lobules.
It is painless and grows slowly. Hibernoma is more common in females than in males. It usually occurs in adults between the age of 20 to 40 years. Hibernomas ranges in colour due to lipid concentration. The colour may vary from light brown to Grey. They are mostly present in the regions where there are brown adipose tissues.
When seen in a microscope, Hibernomas appears as a large vascularized cell with multiple vacuoles and a large number of adipose cells. The diagnosis of the Hibernoma is made through Radiographic imaging which includes Computed tomography (CT) and Magnetic Resonance Imaging (MRI) and Fluorodeoxyglucose (FDG)-positron emission tomography (PET). Histopathology on biopsy and excision is used to confirm the diagnosis.
The location of Hibernoma is usually associated with the areas that have a high concentration of the brown adipose tissues. The Hibernoma, slowly growing benign tumour, is very rare and mostly occurs in the thigh region. Other regions include the neck, check, between shoulder blades, arms, back, abdominal wall and pericardium. Most of the Hibernomas are subcutaneous, only 10 % comprises intramuscular Hibernoma. According to one of the published reports, 30% of the patients had Hibernomas on the thigh, whereas only 10 cases had it on the neck.
There is not any specific symptom of the Hibernoma. Most of the symptoms are associated with the tumour mass and its impact on other structures. The patient often comes with a slow and progressively growing, painless subcutaneous mass. It is rarely seen in the intramuscular tissues. It might remain asymptomatic for years if the Hibernoma is completely removed then the chances of recurrences decreases. Hibernoma is a benign tumour with least chances of metastasis.
Hibernoma is a slow-growing, benign tumour that arises from the brown adipose tissues. The primary function of the brown adipose tissue is to burn the calories and provide heat. The brown adipose tissues achieve this function by the generation of adenosine triphosphate (ATP) in the mitochondria through fatty acid-dependent uncoupling by the use of uncoupling protein (Thermogenin) UCP 1. The level of brown adipose tissue is higher in infants than in humans. Its level decreases with age.
But in Hibernoma, the vestiges of brown fat continue to grow, which leads to Hibernoma. Another theory related to the pathophysiology of Hibernoma associates it with the cytogenetic abnormalities in chromosome 11. The chromosomal rearrangement of the chromosome 11q13-21 in Hibernoma is associated with the Multiple Endocrine Neoplasia Type 1 regions, commonly known as MEN1. The Hibernoma develops due to the loss of MEN1. The patients who have Multiple Endocrine Neoplasia Type 1 Syndrome have a higher chance of Hibernoma.
The hibernomas are not metastatic, which means that they are not transferred to another body part. If the patient is asymptomatic, then Hibernoma does not require any treatment because it is not malignant. The only treatment for the Hibernoma is the surgical excision of the tumour. Hibernoma should be entirely excised; otherwise, it will lead to the recurrence.