The lens in a feline eye—like the lens in a human or canine eye—is a small, translucent structure that adjusts its shape as needed to focus incoming light rays on the retina, a light-sensitive tissue that lines the interior surface of the eyeball. When the retina receives light impulses that have passed through the lens, the impulses are instantaneously transmitted to the brain as visual information via the optic nerve, which is attached to the back of the eye. A cataract is a condition in which the lens becomes cloudy or totally opaque. When this happens, incoming light is impeded, if not totally prevented, from passing through the eye to the retina.

In some cases, the affected area of the lens may be tiny, and the resulting impairment in vision will be inconsequential. In other cases, however, the entire lens may be opaque, in which case total blindness will result in an affected eye. Cataracts, moreover, can be either unilateral or bilateral—affecting either one or both eyes.

According to Thomas Kern, DVM, associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine, some feline cataracts develop as the result of an animal’s inability to metabolize proteins and other body chemicals, or they may in rare cases be a byproduct of such conditions as diabetes or hypertension. And older cats often get them as a natural consequence of the aging process. Other potential causes include traumatic injury that results in a perforated lens and exposure to certain drugs or toxic substances, radiation, or electric shock. In many cases, however, the cause of a cataract is unknown.

The signs of cataract-related failing vision or total loss of vision may be behavioral. A visually impaired cat may, for example, become less agile, bump into familiar furniture, or appear to have difficulty finding its food bowl or litterbox. The cat will seem reluctant to move about in unfamiliar places. And it will be tentative and noticeably cautious about going up and down stairs. “You can make life easier for the cat,” says Dr. Kern, “if you make sure to keep its food bowl and litter box in precisely the same spot at all times.”

But, he adds, the behavioral signs may be too subtle to notice. Thus, he advises, “The owner should routinely check the cat’s eyes. Look for changes in the color of the iris, for example, or see if the eye seems to be cloudy. If you see anything unusual, have the animal examined by a veterinarian.” Early treatment with a variety of medications, he notes, may prevent or delay the onset of cataract-related blindness. In some cases, for example, treatment for high blood pressure or diabetes will be effective in slowing the rate at which a cataract progresses.

In some cases, surgery may be necessary. In such a procedure, the surgeon will use an operating microscope to make small incisions, first in the cornea and then in the lens capsule before inserting an instrument that uses high-frequency sound to disintegrate and remove an affected lens. Following this procedure, an artificial lens is inserted and the incision is sutured shut.

The delicate surgery typically takes about an hour, says Dr. Kern, noting that the procedure is successful in most kittens and mature cats that have qualified as good candidates for lens implantation.