Yesterday, your cat seemed to be perfectly normal. Today, something has clearly gone awry. The animal starts bumping into familiar furniture and has trouble finding its food bowl. It seems to be uncharacteristically withdrawn and reclusive, as if fearful of dangers that might be lurking in its environment. These bizarre and puzzling behavioral changes are characteristic of a condition called “sudden blindness”—a loss of vision that, while possibly developing over weeks or months, can manifest over the course of a week or so and perhaps just overnight.

A wide variety of conditions can precipitate sudden blindness in a cat, including a burst of bleeding into the eye’s interior or a traumatic blow to the head. In rare cases, a cat’s ingestion of an antibacterial medication called enrofloxacin has also been shown to damage the feline retina and cause sudden blindness.

An important distinction must be made, however, between “sudden blindness,” which is relatively rare, and “suddenly noticed blindness,” which is far more common, according to Thomas Kern, DVM, associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine. “Truly sudden blindness,” he explains, “usually occurs overnight or over the course of a few days.” Suddenly noticed blindness, on the other hand, tends to be a progressive condition that has occurred gradually over an extended period of time—perhaps years—before it becomes evident to a cat’s owner.

“The ones with so-called sudden blindness that we see tend to be elderly cats with chronic hypertension,” says Dr. Kern, “which can cause eventual detachment of the retina and bleeding in the back of the eye. The changes may not be acute at first, but they progress over time to the point when the last bit of vision goes and the animal seems to have suddenly gone blind. In retrospect, it will occur to the owner that the cat hasn’t been doing this or that for quite a while.” In other words, the cat hasn’t gone blind suddenly. What’s been sudden is the owner’s awareness of the condition.

So-called sudden blindness is relatively rare in a younger cat, he explains, unless it has been injured or has developed a systemic condition, such as lymphosarcoma, the ocular manifestations of which can be “very aggressive” and become evident within a matter of days.

In some respects, says Dr. Kern, “The term sudden can be defined by the behavior of the animal. Cats that have recently or quickly become blind—over a period of days or weeks, usually appear very confused, bewildered, and fearful,” he notes. “They recognize that something is different, that something has gone wrong. If alarmed, they won’t run or shy away—they’ll simply freeze in place. They’ll have a fearful look on their face and be clearly intimidated by what’s going on. On the other hand, cats that have lost their eyesight gradually seem to realize that something has been going on for quite a while and that what they are experiencing is just the end of it. They’re more relaxed, albeit blind.”

As is true for virtually all feline disease, the earlier a cat’s vision problems are diagnosed, the better they may be treated—unless, of course, an animal’s blindness has already become irreversible by the time veterinary care is pursued. Dr. Kern advises: “An owner should routinely check a cat’s eyes, noting changes in the appearance of either eye. Look for changes in the color of the iris and look to see whether either eye seems to be cloudy or if the two pupils don’t resemble each other. Such changes can reveal problems before they reach an irreversible stage.”

In many cases, he points out, early treatment can prevent blindness. For example, he says, “We can treat a cat’s high blood pressure with a drug called amlodipine, which may allow a retina to reattach itself, and the cat can get some of its vision back.” However, he adds, even prompt care by a veterinary ophthalmologist may fail to reverse a case of feline blindness, whether or not its onset has been sudden. “Unfortunately,” says Dr. Kern, “not many cats that are clinically blind by the time we see them will regain their sight.”