Are You Confident of the Diagnosis?
What you should be alert for in the history
Hibernoma is typically seen in young adults.
Characteristic findings on physical examination
Characteristic findings include 5- to 10-cm slow-growing, painless nodules in subcutaneous tissue of the scapular/ interscapular regions, mediastinum, and upper thorax.
Expected results of diagnostic studies
Histopathology demonstrates lesions composed of embryonic brown fat cells or lipoblasts/mulberry cells (Figure 1). Four histological variants have been described including the lipoma like, myxoid, classic/typical type, and the spindle cell type. The cells, like other adipose tissue, stain with the Oil red O stain.
Computed tomography (CT) without contrast reveals low-density areas reflecting adipose tissue content and a more diffuse enhancement in hibernomatous areas; CT with contrast displays enhancement due to increase vasculature. MRI yields better tissue deliniation and extent of tumor; these findings include homogenous low intensity areas with no contrast enhancement and heterogenous areas with extremely high contrast enhancement, depending on weighting of imaging, the mix of adipose to hibernoma tissue, and vascularity.
Lipomas are histologically distinct with only normal-appearing adipocytes seen. Epidermal inclusion cysts demonstrate an overlying punctum, are less stable in size, and are histologically distinct. Pilar cysts are usually on the scalp, presenting as marble-like nodules and are histologically distinct, as are angioleiomyoma, subcutaneous neurofibromas, subcutaneous dermatofibromas, neurilemomas, rheumatoid nodules (associated with rheumatologic disease), pseudorheumatoid nodules/ deep granulare annulare, subcutaneous sarcoidosis, abscesses, and hematomas (usually associated with trauma).
Who is at Risk for Developing this Disease?
Peak incidence is observed in the third decade. Most are seen between 20-50 years old.
What is the Cause of the Disease?
The etiology involves clinically significant persistence of remnants of or accumulation of brown fat in regions where small amounts of brown fat normally persist as vestigial remnants.
Pathophysiology involves cytogenic abnormalities of chromosomes 11q13-21 and 10q22, which can cause a deletion in the MEN-1 gene. Structural rearrangements involving the long arm of chromosome 11 are characteristic. In particular this involves the 11q13 region which can result in MEN1 tumor suppressor gene and PPP1CA deletions. Although lipomas (or hibernomas) have been seen with MEN1 syndrome, this particular cytogenetic abnormality has not been regularly associated with lipomas other than hibernomas.
Systemic Implications and Complications
If large enough, hibernomas can cause compression of underlying or adjacent structures, and for this reason require surgical removal.
Surgical excision is associated with a high rate of recurrence if incompletely excised; lesions may bleed more than traditional lipomas due to increased vasculature.
Optimal Therapeutic Approach for this Disease
If treatment is desired, surgical excision is optimal.
Patients can be reassured that these lesions are benign. There is controversy as to whether true malignant hibernomas exist, and reports of such cases, which may represent liposarcomas, are very rare.
Unusual Clinical Scenarios to Consider in Patient Management
-The intramuscular type of hibernoma is rare.
-Size greater than 20cm has been reported.
-About 1.6% of benign lipomatous tumors have a hibernomatous component
-A rare case of a large hibernoma with endocrine hormome secretory activity (cortisol and testesterone) has been reported.
What is the Evidence?
Pandya, KA, Radke, F. “Benign skin lesions: lipomas, epidermal inclusion cysts, muscle and nerve biopsies”. Surg Clin N Am. vol. 89. 2009. pp. 677-87. (Review of lipoma and epidermal inclusion cysts including workup and treatment.)
Bancroft, LW, Kransdorf, MJ, Peterson, JJ, O’Conner, MI. “Benign fatty tumors: classification, clinical course, imaging appearance, and treatment”. Skeletal Radiol. vol. 35. 2006 Oct. pp. 719-33. (Review of different fatty tumors with special discussion of imaging findings.)
Allegra, SR, Gmuer, C, O’Leary, GP. “Endocrine activity in a large hibernoma”. Hum Path. vol. 14. 1983. pp. 1044-52. (Case report of a large hibernoma that was found to possess endocrine secretory activity.)
Nishio, J. “Contributions of cytogenetics and molecular cytogenetics to the diagnosis of adipocytic tumors”. J Biomed Biotechnol. 2011. (Discussion of various tumors derived from lipocytes.)
Hameed, M. “Pathology and genetics of adipocytic tumors”. Cytogenet Genome Res. vol. 118. 2007. pp. 138-47. (Discusses and reviews the pathological and genetic findings in tumors derived from adipocytes.)
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