A cataract is defined as any opacity on the lens that interferes with transmission of light to the retina. A spot on the lens that blocks out light, regardless of size, technically is a cataract.
Cataracts of all types are rare in cats. Most cataracts are caused by eye injuries and infections. Inherited cataracts can be accompanied by other eye birth defects, such as microphthalmia (an abnormally small eye) or persistent pupillary membrane (which is a tissue tag across the iris or from the iris to the cornea). Cataracts can develop in diabetic cats, but this is not common.
As a cat gets older, there is normal aging of the eye. New fibers, continually forming on the lens surface throughout the cat's life, push toward the center. The lens also loses water as it ages. These changes lead to the formation of a bluish haze seen on the lens behind the cornea in older cats. Usually this does not interfere with vision and does not need to be treated. This condition, called nuclear sclerosis, should be distinguished from a cataract.
Treatment: A cataract is significant only when it impairs vision. Blindness can be corrected by removing the lens (cataract extraction) and replacing it with an artificial one. There are three general techniques for cataract extraction: extracapsular lens extraction (ECLE), intracapsular lens extraction (ICLE), and phacofragmentation (also called phacoemulsification or “phaco” for short). ECLE is rarely done, and only if the lens is too hard for phacofragmentation. ICLE is mainly done for lenses that have slipped from their normal location. Phacofragmentation is the preferred technique of most veterinary ophthalmologists for cataract removal. This technique uses ultrasonic waves to liquefy the lens, to suck out lens fragments, and to irrigate the eye. An artificial lens can then be put in the eye to restore semi-normal vision. If the lens is not replaced, there is a loss of visual acuity because the lens is not present to focus light on the retina.
Cataract surgery tends to be reserved for cats with cataracts in both eyes who are having problems getting around. Before this surgery is done, the cat needs to have a thorough eye exam, including an electroretinogram (ERG) to verify that the retina and the rest of the eye are normal, so that removing the damaged lens will actually restore vision. If the retina is damaged, it makes no sense to put the cat through this surgery.