In a normal eye the lens is clear and translucent, the better to perform its job of transmitting and focusing light onto the retina in the back of the eye. When disease, aging, or injury clouds and/or damages this lens, a variety of vision-related problems can occur.
One commonly recognized form of lens opacity is a cataract, which causes a cloudy film to gradually grow over the lens, obscuring vision. Most cataracts in dogs are genetically related. How old the dog is when cataracts appear and how severe they become depends largely on what type of breed is affected. Cataracts also can develop after an injury to the eye, as the result of poor nutrition when the dog is young, or as a complication from diabetes and/or other diseases. In rare cases, cataracts develop spontaneously in older dogs, or in dogs of any age, as a response to medications, toxins, or environmental factors like radiation exposure, concentrated microwaves or electrical shock. Most cataracts can be successfully treated with surgery, depending upon the animal's age and physical condition. If not removed surgically, however, the dog's vision will gradually decrease, leaving the animal permanently blind.
Another condition that can cause vision-reducing opacity in the lens is known as a persistent pupillary membrane. This is a defect that begins before the dog is born. During the animal's fetal development there are blood vessels within the eye that normally lose their function and deteriorate before birth. In some cases, however, they do not deteriorate entirely, and the puppy is born with small strands of abnormal tissue within the eye chamber. These strands are fibrous and usually crisscross the eye, attaching to the lens and/or cornea. Often the dog's cornea will appear opaque, and the vision is at least partially obscured. Persistent pupillary membrane is an untreatable condition and appears to be inherited in the Basenji breed.
In older dogs, another age-related change can occur in vision, called nuclear sclerosis or lenticular sclerosis. With this condition the nucleus of the lens becomes denser, as the aging fibers in the nucleus of the lens are compressed by newer fibers. This causes light to be scattered across the eye, producing a bluish-grey haze over the lens. In all but the worst cases, however, the dog's vision is not impaired.
In normal, healthy dogs, the lens of the eye is clear, with a greenish shine that comes out when the eye is illuminated for examination. If cataracts or nuclear sclerosis are present, the eye will appear cloudy. Cataracts appear as whitish chunks of crushed ice on the lens surface, and may affect one or both eyes. With nuclear sclerosis, however, the lens will have an even, pearly/cloudy appearance, with a grayish or bluish tinge.
If your dog's eyes appear cloudy, or you have concerns about your dog's vision, a trip to the veterinarian or a veterinary ophthalmologist is in order. These professionals can use an ERG (electroretinogram) to measure the electrical activity of the retina in response to flashes of light. This test operates similarly to the way that an electrocardiogram (ECG or EKG) measures electrical activity within the heart. Once a specific diagnosis is made, then your vet can advise you on appropriate medical or surgical treatment.