Intraspinal Osteolipoma: A Rare Case

INTRODUCTION

Lipoma is the most common benign soft tissue tumor in the adults.^[1] They are mostly located in the superficial subcutaneous tissue, however, may also be found in deeper tissues with the presence of mesenchymal components such as fibrous connective tissue (fibrolipoma), smooth muscle (myolipoma), blood vessels (angiolipoma), chondroid (chondrolipoma), and osseous tissue (osteolipoma).^[2,3] Although lipomas originating in the bony tissues are common, isolated adipocytes with osteoid and chondroid matrix and no connection to the bone are very rare. The reported incidence of osteolipoma in literature is <1%.^[4] Most of them arise from the buccal mucosa, palate, neck, brain, thigh, and knee.^[5-9] A thorough review of literature showed only four cases of intraspinal osteolipomas – two in cervical region, one in the thoracic, and one in the lumbar region.^[10-13] We present a case of extradural intraspinal fat-containing osseous lesion in the lumbar spine and discuss the clinical presentation, differential diagnosis, and a brief review of literature.

CASE REPORT

A 67-year-old obese female presented with complains of low backache for 8–10 years. The pain was sharp and radiating to the bilateral lower limbs (L>R). There was no associated sensory or bladder/bowel complains. The pain had gradually worsened in the past 2 years, more on getting up from lying position, bending forward, and currently, she required assistance during walking.

Examination revealed positive straight-leg-raising test. The motor and sensory examinations were essentially normal. A provisional diagnosis of radiculopathy was made and a contrast enhanced-magnetic resonance imaging (MRI) of spine was done subsequently.

MRI lumbosacra spine (LS) [Figure 1a-d] revealed a 10 mm [CC] × 7.5 mm [TR] × 5.8 mm [AP] extradural lesion at L2/L3 level compressing the cauda equina and the exiting left side nerve roots. The lesion was T2/T1 hyperintense and suppressed on short inversion time inversion recovery sequence. No flow voids were seen within. Well-defined T1/ T2 hypointense rim was seen around the lesion showing peripheral enhancement on post-contrast images. No communication with the facet joint was seen. CT scan of LS spine [Figure 2a-c] was subsequently done which confirmed well-defined fat-attenuating osseous lesion in the extradural location at L2/L3 level adjacent to the left facet joint showing peripheral cortical rim. The patient was not taken up for surgery due to old age and associated cardiovascular disease.

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Figure 1:

Sagittal T2W (a), sagittal T1W (b) and axial T1W (c) images showing a hyperintense well defined lesion with hypointense rim. On post contrast fat saturated sagittal (d) images there is suppression of central fat signal with peripheral rim enhancement.

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Figure 2:

(a) Sagittal, (b) coronal, (c) axial non-contrasted cranial computed tomography sections showing a well-defined extra axial fat attenuating lesion at L2/L3 level on left side showing peripheral cortical rim. Associated degenerative changes are also noted.

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DISCUSSION

Osteolipomas have been described as metaplastic bone within a lipoma.^[3] They can be interosseous or parosteal in position. When a lipoma undergoes ossification independent of bone, it is called ossifying lipoma.^[14] They are always mostly accompanied with spinal dysraphism.

To the best of our knowledge, only four case reports of isolated intraspinal osteolipoma have been described in literature so far. Yin et al. described the first case of intraspinal extradural osteolipoma in cervical region causing nerve compression.^[10] Aiyer et al. reported another osteolipoma causing dorsal column dysfunction and present in cervical region which was reported.^[11] Kojima et al. reported isolated intramedullary osteolipoma in the upper thoracic cord.^[12] However, there has been only one other case in literature of a lumbar intraspinal osteolipoma which presented with cauda equina syndrome.^[13] Our index case resembles this case in terms of imaging characteristics both on MRI and CT.

Most of these patients present with features of compressive myelopathy over the years and diagnosed after MR evaluation. Heterogeneous T1/T2 hyperintense lesion on MRI should raise the suspicion of both fat and calcium in the lesion and a CT evaluation should be done for further confirmation.^[13]

The differential diagnosis of spinal osteolipoma includes bone containing lesions such as calcified synovial cyst, dermoid, teratomas, tumoral calcinosis, extraosseous osteochondroma, myositis ossificans, and ossifying fibromas.

A calcified synovial cyst is usually associated with adjacent facet joint, degenerative spondylosis, and vacuum phenomenon and shows T1/T2W hypointense signal when completely calcified.

Ossified ligamentum flavum and ossified posterior longitudinal ligament can mimic the appearance of an osteolipoma on imaging and present with compressive myelopathy.^[15] They can be differentiated by the presence of hypointense T1/T2W signal and location along the ligament.

Pathogenesis of osteolipomas is believed to be secondary ossification due to repetitive insults which may lead to development into osteoblasts.^[8,14,16] This has been supported by the cytogenetic analysis of three cases of osteolipoma.^[17]

CONCLUSION

Both MRI with fat suppression and CT are required for the evaluation of extradural heterogeneous hyperintense fat lesions in the spinal canal. At the same time, the interpreting radiologist should be aware of the differential diagnosis of this type of lesion in the epidural space.