Any condition that blocks light from getting to the retina impairs a dog’s vision. Corneal diseases and cataracts fall into this category. Glaucoma, uveitis, and retinal diseases are other important causes of blindness in dogs.
Most causes of blindness will not be evident on general observation of the eye itself. But there are some signs that suggest a dog may not be seeing as well as before. For example, a visually impaired dog may step high or with great caution, tread on objects that normally are avoided, bump into furniture, and carry his nose close to the ground. Dogs who normally catch well may suddenly start to miss objects thrown to them. The inactivity of older dogs is often attributed simply to old age, but failing eyesight may also be a cause.
Submissive urination is normal canine communication. Dogs do it to show social appeasement. When a dog submissively urinates, he’s trying to convey that he’s not a threat. Not all dogs submissively urinate. However, some will urinate when they’re exceptionally excited or feeling submissive or intimidated. Dogs who submissively urinate usually do so when greeting people or animals (especially unfamiliar ones), during exciting events, while playing, during physical contact, such as petting, or...
Shining a bright light into the dog’s eye to test for pupil constriction is not an accurate test for blindness, because the pupil can become smaller from a light reflex alone. This doesn’t tell you whether the dog is able to form a visual image.
One way to test eyesight is to observe the dog in a dark room in which the furniture has been rearranged. As the dog begins to walk about, see if he moves with confidence or hesitates and collides with the furniture. Turn on the lights and repeat the test. A completely blind dog will perform the same way on both tests. A dog with some sight will show more confidence when the lights are on. Performance tests such as these give qualitative information about eyesight, but the degree of impairment can only be determined by veterinary examination.
A diagnosis of blindness or irreversible vision loss is not a catastrophe. The fact is that most dogs, even those with normal eyesight, do not really see very well. They rely to a greater extent on their keen senses of hearing and smell. These senses take over and actually become more acute as eyesight fails. This makes it relatively easy for visually impaired dogs to get around in areas they know. However, a blind dog should not be turned loose in unfamiliar surroundings or he could be injured. In the house, try to avoid moving furniture, because your dog will have a “mental map” of where things are. When left outdoors, confine a visually impaired dog to a fenced yard or run. Walking on a leash is safe exercise. The dog learns to rely on his owner as a “seeing-eye person.”
It is important to be aware of impending blindness while the dog is still able to see. This allows time for retraining in basic commands such as “stop,” “stay,” and “come.” When the dog actually does go blind, obedience training can be a lifesaver.
WebMD Veterinary Reference from "Dog Owner's Home Veterinary Handbook"