Bilateral optic neuritis; This is a literature case, initially received on 10-Sep-2019, concerning a 6-year-old, child, female patient.   The patient had no previous medical problems. There was no family history of autoimmune, ocular, or neurologic disorders.  On an unknown date, the patient was administered with unspecified Flu vaccine [quadrivalent inactivated flu vaccine, route of administration: intramuscular, dose, batch number and anatomical location: not provided] for an unknown indication.   On an unspecified date, one week after receiving a flu vaccine the patient developed bilateral vision loss and was diagnosed with bilateral optic neuritis.  The patient's identical twin received the same vaccination without subsequent changes in vision.  At presentation to the emergency department, her visual acuities were hand motion in the right eye and counting fingers in the left with no afferent pupillary defect (APD). Examination revealed severe optic disc swelling with vessel obscuration bilaterally and peripapillary hemorrhages on the left. There were no anterior chamber cells, vitreous cells, or macular exudates in either eye. Magnetic resonance imaging (MRI) of the brain, orbits, and spinal cord with and without intravenous (IV) contrast showed increased T2/FLAIR signal and abnormal enhancement in the retrobulbar optic nerves bilaterally, but no white matter lesions in the brain or spinal cord. Cerebrospinal fluid (CSF) analysis revealed 1,000 red blood cells/mL, 5 nucleated cells (WBC)/mL, protein level of 23 mg/dL, and glucose level of 58 mg/dL. There were no oligoclonal bands, positive cultures, Lyme antibodies, aquaporin-4 antibodies, or evidence of any of the following by polymerase chain reaction in the CSF: EpsteinBarr virus, herpes simplex virus, and varicella-zoster virus. Serologic testing was also negative for culture growth, Lyme antibodies, and aquaporin-4 antibodies; white blood cell count, erythrocyte sedimentation rate, C-reactive protein, and angiotensin-converting enzyme levels were normal.  The patient was treated with intravenous methylprednisolone.   After 5 days of IV methylprednisolone, visual acuities improved to 20/20, and she recognized 10/10 Ishihara color plates with each eye. The patient was discharged to home without a steroid taper.  Six days after her last dose of IV steroids, the patient experienced decreased vision in the left eye. Examination showed visual acuities of 20/25 in the right eye and no light perception in the left eye. There was a left APD.   Ophthalmoscopy showed mild atrophy of the right optic disc from her initial inflammation and swelling of the left optic disc. Optical coherence tomography (OCT) of the peripapillary retinal nerve fiber layer showed average thicknesses of 144 µm in the right eye and 203 µm in the left eye, consistent bilateral disc edema.   The patient was started oral prednisolone 2 mg/kg/day. Two weeks later, her visual acuities returned to 20/20 with normal color vision (14/14 Ishihara plates) in each eye. Optic disc swelling in the left eye had resolved, leaving mild pallor of both optic discs. The oral steroid dose was tapered over the subsequent 4 weeks without recurrence of vision loss. At her last follow-up evaluation (11 weeks after the influenza vaccination and 10 weeks after initial vision loss), neuro-ophthalmic examination was stable.  The patient has remained without recurrence off steroids over the past 2 years.  Per reporter, the divergent reactions to the influenza vaccine in their patient and patient's identical twin suggest that the interplay between environmental exposures and genetic predisposition in postinfluenza vaccination optic neuritis may be more complicated and warrants further research.  Company Comment: The patient experienced optic neuritis one week after vaccination with unspecified Flu vaccine. Chronology is plausible. It was reported that the patient had no previous medical problems. Examination revealed severe optic disc swelling with vessel obscuration bilaterally and peripapillary haemorrhages. No concomitant medications were reported. Considering all the above mentioned, causal relationship between the suspect product and the event is assessed as related.; Reporter's Comments: The patient experienced optic neuritis one week after vaccination with unspecified Flu vaccine. Chronology is plausible. It was reported that the patient had no previous medical problems. Examination revealed severe optic disc swelling with vessel obscuration bilaterally and peripapillary haemorrhages. No concomitant medications were reported. Considering all the above mentioned, causal relationship between the suspect product and the event is assessed as related.