How would a doctor treat himself if he had optic neuritis?

He or She wouldn't. Doctors have to learn to be patients and listen to their doctors. Having said that, optic neuritis is either treated with IV steroids or not at all. Vision recovers the same regardless, but steroids can speed recovery if given within 72 hours. An MRI of the brain should also be done to make sure further evaluation for diseases like multiple sclerosis is not needed.
See another doctor. Most doctors would do the same thing that anyone else would do when they are ill - seek medical treatment. Since optic neuritis can be blinding and studies have shown that treatment initiated early can be protective against other conditions, everyone - regardless of profession - should seek the help od a physician to treat optic neuritis (and most other diseases as well, for that matter).
Curious question. I doubt a doctor would treat themselves if they lost vision in one eye. If they had no access to medical consults, they would probably want to use IV steroids, such as solumedrol, (methylprednisolone) once a day for three days. They would probably want to be a patient in the hospital for this, and see a neurologist and an ophthalmologist, and have an MRI of the brain. This is the standard treatment for this case,
Optic neuritis. The cause of the optic neuritis needs to be determined and will influence the type of treatment.