Hi

My dog has been dignoised with optic neuritis and is having tests for cancer of the lymph nodes

I can not believe this is happening

Very sorry, a very worrying time for you.

Sorry to hear of your dog being poorly. My husband has optic neuritis, he's had an attack in both eyes and has lost some vision but can still see to drive. Not sure if its the same in animals as humans though.
Fingers crossed for a positive outcome for your dog.