Rapid, painless unilateral vision loss in a 37-year-old healthy woman.

A 37-year-old woman experienced painless, progressive vision loss to no light perception in the left eye over the course of 3 days. The right eye was unaffected. On examination, the only other abnormal finding was a +4 left afferent pupillary defect. She was initially diagnosed with retrobulbar optic neuritis and admitted for treatment with intravenous methylprednisolone. Neuro-imaging revealed a large right anterior cerebral artery aneurysm that crossed the midline to compress the left optic nerve. The aneurysm was treated with coil embolization, which was technically successful but which did not lead to significant improvement in vision.