Years ago a friend moved out of my city and I took over care of his two red tick hound puppies. Remember they were only couple or three months old. The male very soon developed eye problems and it turned out he had a condition (don't remember the medical term) where his eyelid did not develop correctly, and for lack of better words, was turned "inside out" and the lashes were irritating his eyes. A simple surgical procedure and he was fine. You don't say if your dog is a pup or fully grown, and couldn't imagine this would be the cause if he's adult dog. Maybe allergies to something in your yard or surrounding yards, pesticides, etc. I'd be getting him to the vet as soon as possible.
i recommend calling your vet as soon as possible...she might be losing her sight but there might besomething the vet can do to prevent or even stop that process for now...