It is late here but I was doing a little looking around and see that anesthesia can cause blindness, nitrous oxide was mentioned. Lack of oxygen or an underlying disorder
Oh my. I have never heard of that happening. Do you by chance have a Vet school/college near you? Like UC Davis is closest to me. If so, I think I would contacting them for a consult. Do you have a canine Opthomologist near you?
I would suspect maybe an anesthesia complication but I really have no idea.
Hope you can get to the bottom of this. So sorry and please keep us posted