Aneurysmal bone cysts (ABC) are uncommon non-neoplastic expansile lesions of the bone. They usually affect long bones and vertebrae in childhood and early adulthood. Skull involvement is quite rare. Only a few cases of orbital involvement were described in the literatures. We are presenting a case of ABC that affects the orbit causing proptosis and external ophthalmoplegia.

An 11-year-old girl known to have a chronic seizure disorder and school performance difficulties presented with a four month history of rapidly progressing left orbital swelling and redness. This was associated with double vision, occasional throbbing headache and feelings of nausea. The patient and her family denied any history of trauma. Her past medical and family histories were otherwise non-contributory. Physical examination revealed a left eye proptosis with mild conjunctivitis (Figure 1). She had complete left external ophthalmoplegia with pupil sparing. The left orbit was non-tender with no detectable bruit. Visual acuity in both eyes was 20/20 (right) and 20/50 (left). Fundoscopic examination revealed mild papilledema in the left eye. The rest of the physical examination was essentially normal. Computerized tomography scan of the brain revealed a left fronto-orbital expansile bony lesion with multiple cystic compartments, distorting the left orbital roof (Figure 2). The lesion pushed the eye globe laterally and externally. Magnetic resonance imaging of the left orbit confirmed that the lesion was extradural, displacing rather than infiltrating the left frontal dura, causing mass effect on the left frontal lobe.Within the lesion, the cystic components exhibited fluid-fluid levels (Figure 3 a,b,c).