Cystic optic nerve sheath meningioma.

A 58-year-old woman with a 35-year history of left proptosis underwent neuroimaging that revealed a large cystic lesion. Surgery revealed an optic nerve sheath meningioma associated with cyst formation. The cyst was part of the tumor, a phenomenon that is well described in intracranial meningiomas but not in optic nerve sheath meningiomas.