Intramuscular lipoma of the superior oblique muscle.

We describe a rare intramuscular infiltrating lipomatous tumor affecting the superior oblique muscle. A 37-year-old female presented with eyelid swelling, proptosis and motility disturbance. Imaging studies revealed enlargement of the superior oblique muscle with a fat-density infiltrating lesion within its belly. The clinical and radiologic findings suggested a cystic or necrotic lesion. Biopsy showed an infiltrative tumor consisting of adipocytes with included degenerated muscle fibers. Histologic features were consistent with a benign intramuscular lipoma, rather than the more aggressive well-differentiated liposarcoma. A review of the literature found three previously described periorbital cases, two in the eyelid, and one in the anterior orbit. Lipomas are common tumors that usually do not cause functional impairment but are very rare in the orbit. Intramuscular lipoma is a rare subtype that can result in significant morbidity. Enlargement of an extraocular or periorbital muscle with a low-density infiltrating lesion should raise suspicion for this unusual tumor.