and Susan E. Aiello, DVM, ELS
In many ways, the eyes act like tiny television cameras.
Light images enter the eye through the clear outer surface called the cornea, are focused by the lens, and then project against the retina at the back of the eye. The retina translates the image into impulses that travel up the optic nerve to the brain, where they are interpreted.
Damage to the cornea from infection, lack of tears, or foreign bodies (e.g, splinters) cause painful irritation and inflammation. In cases of severe or chronic damage, the cornea can develop an ulcer or may scar, which decreases vision. Eyes that are infected or inflamed often appear red and may have a watery discharge, or pus may collect in the corner of the eye. Dogs often hold the eye closed and may paw or rub at the eye.
The lens of the eye sits in the space created by the pupil.
Damage to the lens blocks light and makes it hard to focus images. As dogs get older, the lens normally dries out somewhat, and the eye may appear to have a slight bluish cast. This normal aging process does not affect vision.
The most common lens problem is cataracts, in which the pupils may look cloudy or milky.
Mild cataracts usually do not create a vision problem, because dogs do not read or need to distinguish sharp images. Severe cataracts that limit vision can be surgically removed.
Problems in the retina can severely compromise your dog's vision.
Some dogs have genetic disorders in which the retina thins and breaks down over time. Breeds at greatest risk for these conditions include collies and sheep dogs, as well as certain spaniels, terriers, and retrievers. Such conditions are progressive and irreversible, eventually leading to partial or total blindness. Fortunately, dogs have well developed senses of hearing and smell, which can largely make up for decreased vision.
Other conditions that can damage the retina or optic nerve include trauma to the eye (eg, a car accident), cancer, and glaucoma.
In glaucoma, the fluid pressure in the eye increases, which crushes the retina and optic nerve, and can lead to permanent blindness. Affected eye(s) are red and painful and can become obviously swollen as the disease progresses.
Depending on your dog's history and signs of eye disease, your veterinarian may perform additional diagnostic tests on the eye(s). For example, he or she can check the eye for damage to the cornea, check the tear production of the eye, examine the retina with an ophthalmoscope, or measure the pressure in the eye. Your vet will prescribe the appropriate treatment depending on the cause of the problem.
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