I think you need to get a diagnosis from the vet. Nystagmus is a neurological sign, not a diagnosis unto itself. Without proper dx., how can we say if there is treatment or not?
I realize it's a diagnosis, but she is also very old and very sick. she was diagnosed with lymphoma a month ago so who knows if this is a result of that. Either way, it is what it is. Don't think my brother is going to put her through chemotherapy. It makes no sense.
A human being on the other hand, who has a lymphoma, it makes sense to prolong his/her life with chemotherapy.