Thanks to a number of unique attributes, your cats vision enables it to thrive very well in a world inhabited by larger animals that it would like to avoid and smaller creatures that it would like to eat.
Unfortunately, the feline visual apparatus is subject to injuries and a variety of diseases that can diminish its acuity and, at worst, render a cat partially or totally blind. The alert cat owner should understand the causes of feline blindness, the signs of its onset, and the special needs of a cat that has lost its sight.
Anatomy of the Eye
In most respects, the feline eye functions just like the human eye and is made up of the same structural components, including the cornea (the transparent outer covering of the eyeball); the pupil (the slit-like opening that permits the entry of light from the environment); the iris (the muscular, pigmented membrane surrounding the pupil that changes its size to regulate the amount of incoming light); the lens (which adjusts its shape as needed to focus the light rays); and the retina (a light-sensitive membrane lining the back, inside surface of the eyeball). The retina receives the light that has been focused by the lens and transmits the resulting visual information to the brain, via the optic nerve, which is attached to the back of the retina.
A cats eyes have certain distinctive features that substantially improve its chances for survival as an animal that often finds itself, as circumstances dictate, either predator or prey.
Among these characteristics is a third eyelid, a thin, pale membrane positioned at the inner corner of the eye, between the lower eyelid and the eyeball. This extra eyelid, along with the cats ability to pull the eyeball deeper into the socket, helps protect it from injury during a scrap with another animal.
Another special feature of the feline eye enables a cat to see especially well in dim light or near-darkness – just before sunrise, for example, and at dusk, the perfect times for hunting mice, chipmunks, and other delectable little critters. This ability is due in part to the fact that the feline retina, compared to the human retina, is densely packed with rods – photoreceptor cells that respond to dim light.
In addition to a rod-packed retina, cats have a specialized reflective layer of tissue beneath the retina that reflects incoming light. This structure – the tapetum lucidum – reflects light forward through the retina, thus giving it a second chance to detect the light and transmit the nerve signals to the brain. This same structure is responsible for appearing to make a cats eyes glow in the dark. Researchers estimate that a cats sensitivity to light is about six times greater than that of a human.
As powerful as they are, cats eyes are vulnerable to traumatic and potentially blinding injuries, such as corneal scratches, lacerations (cuts on the outer surface of the eye) and punctures, which are common.
Cats are also subject to a host of diseases that can cause permanent damage to any or all of the eyes components. Many of the disorders that threaten feline vision also afflict humans. These disorders include cataracts, in which the lens gradually clouds up and prevents light from entering the eye; glaucoma, a condition marked by increased fluid pressure within the eyeball that can damage light-sensitive tissues; 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.
A host of other feline eye diseases are attributable to viruses, bacteria and fungal organisms that specifically target cats – such as feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), feline infectious peritonitis (FIP) virus, feline herpesvirus, toxoplasma (a parasitic organism) and cryptococcus (a yeastlike fungus commonly found in soil). As causes of blindness in cats, says Thomas Kern, DVM, associate professor of ophthalmology at Cornell Universitys College of Veterinary Medicine, These are the well known, well described incriminatory agents, although there are possibly many others.
Among all feline eye disorders, he says, the one most frequently treated at the Cornell University Hospital for Animals (CUHA) is conjunctivitis, an inflammation of the conjunctiva – the mucous membrane that lines the inner surface of the eyelid and part of the outer surface of the eyeball. This condition, usually a byproduct of an upper respiratory disease caused by either a bacterial or viral organism, is endemic in most cat populations, he notes. The disorders clearest sign is runny eyes, and it is readily curable if treated promptly.
Causes of Blindness
As for the diseases that frequently lead to blindness, Dr. Kern says, the most common is uveitis, or inflammation of the uvea, 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 redness and swelling of the conjunctiva, squinting, excessive tearing, discoloration of the iris and sometimes a cloudy appearance of the eye.
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 detachments are most frequently caused by high blood pressure associated with an overly active thyroid gland (hyperthyroidism) or with kidney disease. In some cases, prompt and proper veterinary treatment can restore partial vision to a cat with a retinal detachment, but in most cases, permanent blindness will result. In addition to an overnight onset of blindness, a major sign of this disorder is a grossly dilated pupil.
At CUHA, says Dr. Kern, We see cats that are blind or going blind several times a week. Most of them have eye disease as a primary problem – 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 the case with virtually all feline diseases, the earlier a cats vision problems are diagnosed, the better it may be treated – unless, of course, an animals blindness has already become irreversible by the time veterinary care is pursued.
The signs of failing – or lost – sight may be behavioral. A visually impaired cat may, for example, become less mobile, bump into familiar objects, sleep excessively, and have trouble finding its food bowl and litter box.
But the behavioral signs may be too subtle, Dr. Kern points out. So the owner should routinely check the animals eyes, noting changes in the appearance of either eye. Look for changes in the color of the iris or see if the eye seems to be cloudy or the cats two pupils dont resemble one another. Such changes can reveal problems before they reach an irreversible stage. So look for anything unusual and get it checked out by your veterinarian.
Early treatment, he says, may prevent or delay the onset of blindness. For example, he notes: If toxoplasmosis is found to be causing the vision problem, the eyes are apt to respond well to treatment. In some cases, we can treat a cats hypertension and thereby slow down the rate at which the eyesight is failing. And we can sometimes get a detached retina to reattach itself.
Unfortunately, not many cats that are clinically blind by the time we see them will regain their sight.