AQP4 and diabetes mellitus: As autoantibodies, including aquaporin-4 (AQP4) antibodies, were confirmed to be negative, the patient was diagnosed with suspected idiopathic optic neuritis and was discharged after starting follow-up oral steroids at 30 mg/day. To prevent the worsening of concomitant diabetes, the steroid dose was tapered relatively rapidly by 10 mg/day every week, and within one month of starting oral administration, it was tapered to 5 mg/day.