One of the most devastating side effects of diabetes mellitus in dogs is blindness from cataracts. In dogs with normal sugar metabolism, glucose in the lens is metabolized by an enzyme called hexokinase into small molecules that diffuse out of the lens. When the glucose levels are high, hexokinase cannot keep up and the excess glucose is metabolized by aldose reductase into sorbitol. Sorbitol is a large molecule that cannot diffuse out of the lens. It causes water to migrate into the lens leading to swelling and cloudiness. Within a short time, there is so much sorbitol in the lens that light cannot penetrate through the lens to the retina in the back of the eye and the dog becomes blind. The only treatment available at this point is surgical removal of the lens.
Until recently, good glycemic control was the only method of preventing sugar cataract formation. Now there is a new drug called Kinostat ™ to help out these dogs. Kinostat™ blocks aldose reductase which prevents the formation of sorbitol. The drug is currently in phase 3 clinical trials at several veterinary ophthalmology clinics across the United States. Hopefully, it will be available soon.
Source:
Kadon P.F., et al., “Topical KINOSTAT™ ameliorates the clinical progression of cataracts in dogs with diabetes mellitus” Vet Ophthalmol Nov. 2010;13(6):363-8.