Indications that your cat has become visually impaired may be strikingly obvious. The animal may, for example, start bumping into furniture, trip as it climbs up or down a flight of stairs, or appear to have trouble finding its litter box or food bowl. In some cases, however, the behavioral signs that something is going wrong with your cat’s vision may be too subtle, says Dr. Thomas Kern. Therefore, he advises, you should have your cat’s eyes thoroughly examined periodically by your veterinarian. In addition, you should routinely monitor the health of your animal’s eyes.
“Look for changes in the color of the iris, for example, or see if the eye seems to be cloudy or if the cat’s two pupils appear to differ from one another,” he advises. “Such changes can reveal a problem before it progresses to an irreversible stage.”
Healthy feline eyes will be bright and clear, the pupils will be of equal size, and the cat will not be squinting with either eye. There will be little or no tearing in the corners of the eye; the tissue lining the eyelid will be a healthy pink; and the membrane of the third eyelid will not protrude.
“If you spot anything unusual,” Dr. Kern advises, “get it checked out promptly by your veterinarian.” Early treatment, he points out, may prevent or delay the onset of blindness.
Cornell Feline Health Center
Cornell University College of Veterinary Medicine
Ithaca, New York 14853
In most cases, early diagnosis and treatment can keep a cat from losing its eyesight.
Owing to several unique features developed over centuries in the wild, the modern cat typically possesses powerful visual acuity that enables it to function effectively both indoors and beyond the confines of its home. Unfortunately, the feline ocular apparatus is vulnerable to injuries and a wide variety of diseases that can dramatically impair a cat’s eyesight or, worst case, render it partially or totally blind.
"We see cats that are either blind or going blind several times a week," says Thomas Kern, DVM, associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine. "Most of these animals have eye disease as a primary disorder—they have no other health problems. And most of them are middle-aged or elderly cats whose vision loss has been progressing for years."
As is true for virtually all threats to feline health, the earlier a cat’s vision problem is diagnosed, the more effectively it can be treated—unless, of course, the animal’s blindness has progressed irreversibly by the time it is noticed. Therefore, Dr. Kern urges owners to keep an eye out for any behavioral or physical indications that a cat is having a vision problem and to report any such signs to a veterinarian without delay.
Overall, feline eyes function in the same way that human eyes function and are made up of the same components. The major structural and functional components include:
While structurally similar in most respects to human eyes, feline eyes have acquired over thousands of years a number of distinctive features that improve its chances for survival as both predator and prey. Among these features is a third eyelid—or “haw”—a thin, pale membrane positioned at the inner corner of the eye, between the lower eyelid and the eyeball. This extra eyelid helps keep the surface of a cat’s eyeball moist, protects it from being scratched by erratically growing hairs, and can help shield it during a scrap with a rival cat or other animal.
Cats also have a specialized layer of tissue beneath the retina that reflects incoming light. This structure—the tapetum lucidum—reflects light not absorbed by the retina during its first passage through the eye, thus giving the light a second chance to be absorbed and transmitted to the brain. Thanks to the tapetum lucidum, a cat’s sensitivity to light is thought to be about six times greater than that of a human’s.
Cats are subject to a host of diseases that can cause permanent damage to any or all of the eye’s components. These disorders include cataracts, in which the lens gradually clouds up—often impenetrably—and prevents light from entering the eye; glaucoma, a condition marked by excessive fluid pressure within the eyeball that can cause it to harden; progressive retinal atrophy, in which the retinal tissue degenerates and loses its ability to function properly; and a variety of tumors—either malignant or benign—that develop within the eye or adjacent to it.
Many other feline eye diseases are attributable to viruses, bacteria and fungal organisms that specifically target cats—such as the feline immunodeficiency virus (FIV), the feline leukemia virus (FeLV), the feline infectious peritonitis virus (FIP), feline herpesvirus (FHP), toxoplasma (a parasitic organism) and cryptococcus (a yeastlike fungus commonly found in soil).
The most frequently diagnosed feline eye disorder, Dr. Kern notes, is conjunctivitis, an inflammation of the conjunctiva—the mucous membrane that lines the inner surface of the eyelid and the outer surface of the eyeball. This highly contagious condition, usually a byproduct of an upper respiratory disease caused by either a bacterial or viral organism, is often observed in densely populated cat shelters, he notes. The disorder’s clearest symptom is runny eyes, and it is readily curable if treated promptly.
Among the diseases that frequently lead to feline blindness, Dr. Kern says, the most common is inflammation of the uvea (uveitis), the middle area of the eye that is made up of the iris, the ciliary body (which produces the fluid inside the eye), and the choroid (which supplies nutrients to the retina). This disease, which is most often associated with FIV, FeLV, FIP and other infectious organisms, is usually chronic and is likely to result in gradual blindness. Among its signs are inflammation of the eyeball, squinting, swollen third eyelids and noticeably enlarged eyes.
The second most common blindness-causing disorder in cats, he says, is retinal detachment, a condition in which the retina becomes separated from its underlying tissue, typically resulting from a leakage or oversupply of fluid between the two layers. Retinal detachment is most frequently associated with high blood pressure, an overly active thyroid gland, or kidney disease. In some instances, prompt and proper veterinary treatment can restore partial vision to a cat with a retinal detachment, but in most cases, permanent blindness will result.
Cats’ eyes are also vulnerable to traumatic and potentially blinding injuries, notes Dr. Kern, such as corneal lacerations (cuts on the outer surface of the eye), which are common. As is the case with virtually all types of feline health problems, the earlier a cat’s vision impairment is diagnosed—whether the result of disease or injury—the better it may be treated. In some cases, unfortunately, an animal’s failing vision or blindness may have become irreversible by the time veterinary care is pursued.
By Tom Ewing
December 20, 2010