I would ask your vet for a referral to an ophthalmologist. I had to take Lady to one this summer when she started having eye problems that her regular vet couldn't clear up. They have much more sophisticated equipment and the expertise to deal with more difficult eye problems.
I was also pleasantly surprised that they really weren't much more expensive than my regular vet. The office visit with exam was $10 more, but the prescriptions were cheaper as were a couple of the tests they repeated.
