When I had Optic Neuritis, the MRI never was able to see a corresponding lesion. They just diagnosed it based on symptoms. Sometimes the MRI can't see the lesion, whether it's just not big enough to be seen on the MRI, or the MRI machine just isn't strong enough to detect it.

My ophthalmologist just had me wait out the symptoms. My vision got better. But I've had optic neuritis more than once. I've done the IV steroids twice for vision problems because it was bad enough those times to cause me some pretty serious problems. The Optic Neuritis Treatment Trial recommends that you do IV steroids for optic neuritis, because oral steroids for some reason can make you more susceptible to recurring episodes of optic neuritis.