How does the ophthalmologist know that I have optic neuritis?

A few patients with optic neuritis have swelling of the optic disc (the beginning of the optic nerve) at the back of the eye. This is referred to as papillitis. One sign usually detected by your eye doctor is the presence of an afferent pupillary defect. This indicates that there is less light being sensed by the affected eye than the opposite eye. This is found by swinging a bright light back and forth between your two eyes while observing how your pupil reacts.