To report a rare case of optic neuropathy caused by a fungal ball in an Onodi cell.

A 63-year-old female was referred to our clinic with relapsed visual loss and ocular pain in the right eye. She had been diagnosed as optic neuritis 14 days before and given pulse steroid therapy. She recovered to normal but relapsed 7 days before. In medical referral there was no suspected multiple sclerosis but only a few inflammation in the paranasal sinuses. On our initial examination, best corrected visual acuity was counting finger at 10 cm in the right eye, and 1.0 in the left eye, along with relative afferent pupillary defect in the right eye. The fundoscopic examinations disclosed disc swelling; nearly total visual field defect was observed on visual field examination and visual evoked potential test revealed decreased amplitude at P100 wave in the right eye. Clinical impression was relapsed optic neuritis. After the administration of pulse steroid therapy, her disc swelling was decreased and visual acuity was recovered to 0.6, however, visual acuity was exacerbated to 0.4 in 2 weeks. We checked outside brain magnetic resonance imaging (MRI) and the result showed optic neuropathy caused by a fungal ball in an Onodi cell. The patient was referred to otorhinolaryngologist and fungal ball was removed by endoscopic sinus surgery. 3 weeks after surgery the patient's visual acuity was 0.9, no disc swelling was found and visual evoked potential was recovered to normal.

An Onodi cell lesion should be considered in the differential diagnosis of optic neuritis, identified by imaging studies and promptly removed by surgery for visual recovery.