Acute inflammation of the optic nerve behind the eyeball in the eye socket, usually affecting only one eye, causing loss of vision, and sometimes complete blindness. Another symptom is headache, which is worsened by eyeball movement. The pupil reacts less well to light. The condition is usually caused by multiple sclerosis (in 60 to 70 per cent of cases), of which it is sometimes the first symptom. Another possible cause is an infectious disease elsewhere in the body, or sinusitis which has penetrated the eye socket; other less common sources are meningitis or an abscess in the vicinity of the optic nerve. Examination often reveals nothing to explain the symptoms. If the condition is an early symptom of multiple sclerosis there is a good chance that vision will not be permanently affected; it is often restored after 2 to 3 weeks. The condition can recur, however. Treatment is sometimes by corticosteroids, sometimes no therapy is possible. If the neuritis is caused by an abscess or inflammation, this condition must be treated first.