Cataract is defined as “an opacity of the crystalline lens of the eye” (credit Dorland’s Illustrated Medical Dictionary, 25th Edition). This definition is often expanded to include the capsule, or outer covering of the lens.
Throughout life the canine lens produces new fibers. These fibers are contained inside the capsule, preventing the lens from enlarging. Therefore, older fibers are compressed toward the center, or nucleus, of the lens. Compression of these fibers results in lenticular sclerosis. While lenticular sclerosis (also called nuclear sclerosis) can result in opacity of the lens, it is not a cataract.
Age-related cataracts occur when the lens of the eye undergoes chemical and physical changes, and/or absorption of water. The resulting changes leave the formerly-clear structure cloudy.
Certain breeds are known to be more likely to develop cataracts as they age and there may be a familial (genetic) cause. Those breeds include Boston terriers, miniature poodles, Cocker spaniels, minature schnauzers and Labrador retrievers. Less-frequently-affected breeds include golden retrievers, old English sheepdogs, beagles, Afghan hounds and standard poodles.
Cataracts can occur at any age. Developmental cataracts, for example, may occur in the fetus and be noted on a puppy or kitten’s first examination. When present at birth, these are called congenital cataracts. Persistent pupillary membrane can also result in cataract formation where the remnant attaches to the anterior (front) portion of the lens.
Juvenile cataracts occur in the young, and are considered developmental because they occur while the puppy or kitten is still maturing. Certain breeds are predisposed to these early cataracts.
Degenerative cataracts comprise a group with a wide range of causes.
Diabetic cataract is the most commonly-seen degenerative cataract in veterinary medicine. While the average age of onset of diabetes mellitus in dogs is 6-9 years, sudden-onset cataract formation is more likely to occur in the young diabetic dog. When a young dog is presented with a history of “I woke up this morning and his eye was white,” we want to test for diabetes immediately.
The vast majority of canine diabetics of all ages will experience cataract formation leading to blindness, even if their diabetes is well-controlled. Cataract formation is slower in more-mature diabetics.
Traumatic cataracts usually occur from a penetrating foreign body. Perforating the lens capsule allows fluid from the aqueous humor to enter the lens and the resulting hydration of the lens fibers results in irreversible opacity.
Radiation cataracts might be seen in dogs affected by release of radiation in the Japanese tsunami-related nuclear power plant meltdown. Dogs undergoing cancer radiation in the vicinity of the head may also be affected.
Toxic cataracts may occur in certain diseases, including kidney failure, as well as from poisons, such as naphthalene.
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Breeds predisposed to juvenile cataracts include, but are not limited to poodle, cocker spaniel, wirehaired terriers, schnauzers, Boston terriers, Afghan hounds and Sealyham terriers.