Contributed by Abdul-Haseeb Naeem, Matthew Kubica, Amirpouyan Namavarian, Sachin Pandey, Qi ZhangFINAL DIAGNOSIS
Spinal hibernoma (brown adipocytes in an otherwise unremarkable bone marrow)
DISCUSSION
Hibernomas were initially documented by Merkel in 1904 (1). The name reflects the resemblance of the tumors to the brown fat seen in hibernating animals. The tumors are benign, slow-growing and well demarcated, with a predilection for subcutaneous tissue and skeletal muscle (2). The peak incidence is in the fourth decade of life without gender differences (3). The typical histopathological appearance is of eosinophilic, multi-vacuolated cells with granular cytoplasm and central nuclei (3). The molecular events underlying the development of hibernomas are unknown but may involve a failure of expression of retinoblastoma protein (pRb) (4).
Though soft tissue hibernoma is well-documented in the medical literature, hibernomas with intra-osseous involvement in adult humans are exceedingly rare. Even rarer are hibernomas involving the spine. In our patient, the hibernoma was an incidental finding during the workup for a cauda equina syndrome. The imaging features of a spinal hibernoma, when intraosseous, mimic metastatic disease, benign notochordal cell tumors and hemangiomas (1). In our case the initial working diagnosis was a spinal metastasis, resulting in a futile search for a primary source. For this reason and the nonspecificity of the imaging findings a diagnostic biopsy of the lesion was clearly essential.
The microscopic analysis showed numerous brown adipocytes characterized by cells containing abundant foamy cytoplasm. Brown adipocytes stain positively for S100 and should be differentiated from morphologically similar cells (i.e., foamy histiocytes and metastatic adenocarcinoma) by the lack of immunolabelling for CD68 (a histiocyte marker) and broad-spectrum cytokeratins (markers for epithelial cells).
We could find only three case reports of thoracic spinal hibernomas (1, 3) in the medical literature. Two were outside the dura and in the bone (1) and one was in the thoracic epidural space (3). The tumor was an incidental finding in two (1) of the three thoracic hibernoma cases and in the remaining one case (3), neurological symptoms and signs drew attention to local compressive effects of the tumor on adjacent neural structures. The symptomatic hibernoma patient (3) underwent surgical resection with good post-operative outcome and improvement in their pre-operative neurological symptoms. On the other hand, in both asymptomatic thoracic hibernoma cases (1), an image guided biopsy was carried out for a definitive diagnosis and more importantly these patients had no disease progression on imaging follow-up.
In summary, hibernomas are usually an incidental finding. They arise from vestigial brown adipose tissue. They are benign, slow growing tumors, they have no malignant potential and, if asymptomatic, they can be monitored by surveillance imaging (1-4). If they are symptomatic surgical decompression and excision of the tumor is indicated. Over the last two years our patient's tumor has been asymptomatic and stable on surveillance imaging.
Acknowledgement: We thank Dr. David Ramsay for reviewing this case and providing comments and suggestions in the manuscript.
REFERENCES
Contributed by Abdul-Haseeb Naeem, Matthew Kubica, Amirpouyan Namavarian, Sachin Pandey, Qi Zhang
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