Your dog’s eyes are not only among its most expressive anatomical features, they are also a complex sensory organ vulnerable to injury and disease. It is important to be alert to eye ailments if they arise and prevent them whenever possible.

The way dogs see is similar to the way we see. First, light passes through the transparent cornea. It then reaches the colored iris (the pupil), which expands or contracts depending on the amount of light in the environment. The pupil controls the size of the pupil, thus regulating how much light reaches the lens. The lens bends light rays and focuses them on the back of the eye, the retina. The retina, then, changes light into nerve impulses through a complex process. These impulses are transmitted along the optic nerve to the brain, which interprets the impulses as images.

Despite basic structural and functional similarities, however, canine and human eyes have significant differences. Taking these differences into account, it is probably safe to say dogs see the world in a different light than people.

The delicate structure of your dog’s eyes makes them vulnerable to numerous diseases and injuries. Owners should be on the lookout for discharge, squinting, redness, and cloudiness – the most common warning signs of eye disease. And, of course, if your dog starts bumping into things, you should naturally suspect vision loss.

Diseases of the eyelid:

Conjunctivitis is an inflammation of the membrane that lines your dog’s eyelids and the front of the sclera (the white part of the eye) – is one of the most common canine eye ailments. Causes include infection, scratches, and eye irritants like smoke or dust. Typical signs are redness of the eyelid lining, squinting, and discharge. To treat conjunctivitis, veterinarians usually prescribe medicated ointment or drops, depending on the cause.

Entropion – is a congenital defect common in dogs with deeply wrinkled facial features. Dogs with entropion have eyelids that turn inward, causing the eyelashes to scratch the cornea. Conversely, in entropion – another congenital condition – the eyelid rolls outward, creating a pouch where tears pool, which deprives the eye’s surface of necessary lubrication. Surgery can correct these conditions.

“Cherry eye” -A dog’s eye has a third eyelid (nictitating membrane) that helps cleanse and lubricate the eye like a windshield wiper. But sometimes the third eyelid’s tear gland protrudes outward, appearing as a bright red bulge in the inside corner of the dog’s eye (nicknamed “cherry eye”). Surgery for cherry eye usually involves repositioning the gland. Veterinarians are now reluctant to remove the tear gland because doing so increases the risk of decreased tear production. This is a syndrome that makes the cornea susceptible to damage.

Diseases of the Cornea:

Scratches or abrasions of a dog’s cornea are also quite common, especially in breeds that have bulging eyes.

Corneal damage often causes eye redness. And dogs may also paw at their eyes because of discomfort. Any sign of corneal irritation will require veterinary attention. Some of the other common ailments to the cornea include foreign bodies. If you observe a foreign body, attempt to take a gauze pad soaked in warm water over the open eye and attempt to flush the eye. Even if you succeed in flushing out the object, take your dog to the veterinarian. Once at your veterinarians, the eye exam will include a check for corneal damage by applying a special stain to your dog’s eye.

Also, your pet will have a minor procedure to measure the tear output from your dog’s lacrimal (tear) glands using a filter-paper strip. Any delay in treating corneal lesions could lead to the development of a painful corneal ulcer or an inflammatory condition known as keratitis (a cloudiness of the cornea leading to loss of vision). Many breeds are prone to a decreased tear duct production and this test should be performed with the yearly exam.

Diseases of the Lens:

The lens of the eye is also subject to disease. A cataract is an opacity of the lens. Cataracts are usually inherited, and genetically predisposed breeds include cocker spaniels, poodles, huskies, Labrador retrievers, and schnauzers. Surprisingly, inherited cataracts usually occur in your animals, although some dogs suffer from late-onset inherited cataracts. Another common cause of cataracts is diabetes mellitus (sugar diabetes).

Large cataracts can reduce lens functions to the point of blindness, but surgical removal of cataractous lenses has restored sight in many dogs. The practitioner must check the retina to see if it is functional before deciding whether to surgically remove a cataract. But cataracts block a clear view of the retina and make it challenging for veterinary ophthalmologists to examine the retina with a hand-held ophthalmoscope. Therefore, practitioners often measure the retina’s electrical response to light with an electroretinogram prior to cataract surgery.

As animals age, the lens may also develop a cloudiness. This can be differentiated from cataracts and may not require surgical intervention. Although aging is not a disease, it often brings changes in your dog’s vision. In general, older dogs are not as responsive to visual stimuli as younger dogs. Older dogs also often show signs of nuclear sclerosis also called lenticular sclerosis, a nonpathological hardening of the lens that causes lens cloudiness. Owners sometimes mistake this sign of aging for a cataract.

Other common diseases of the eye:

Glaucoma – increased eye pressure caused by fluid buildup in the eye – is a serious disease that can lead to partial or total blindness. Signs include redness, cloudiness, excessive tearing, eyeball swelling, pain, and sensitivity to light. After diagnosing glaucoma by measuring eye pressure with a tonometer, veterinary ophthalmologists treat the disease with pressure- relieving medication or laser surgery In some cases, surgical removal of the eye is necessary.

Progressive retinal atrophy (PRA) passes a group of vision-destroying diseases caused by deterioration of the retina’s image-processing cells. An inherited ailment with no known treatment, PRA often develops so gradually owners may not notice the signs until a dog experiences loss of night vision or total blindness. Veterinary geneticists and ophthalmologists are intensively studying PRA, but at present, the only known preventive measure is to not breed a dog that has PRA or has produced puppies that later developed the condition.

Preventative care:

Common sense is the soundest strategy for preventing eye problems. Always keep your dog head inside moving vehicles, check your dog’s eyes on a regular basis, and make sure its annual veterinary checkup includes a thorough eye exam. Seek veterinary help for eye problems as soon as you notice them.

If you’re considering adding a purebred puppy to your family, ask your vet if inherited eye problems are common in the breed you’re interested in. Select a breeder who routinely has a veterinary ophthalmologist test parents and puppies and submits the data to the Canine Eye Registry Foundation (CERF). Located in West Lafayette, Indiana, CERF collects and disseminates information about breed-specific eye disease in an effort to curb genetically transmitted ocular conditions. (Keep in mind, however, that a parent’s or puppy’s CERF certification is no guarantee of lifelong ocular health.)

Despite an owner’s best efforts to head off eye problems, some dogs do go blind – often very gradually. However, vision is not as overwhelmingly important for dogs as it is for people. In fact, vision-impaired dogs in familiar surroundings can usually move around comfortably, relying their powerful senses of smell and hearing. Several years ago in our practice, we had a canine patient develop irreversible glaucoma in both eyes. This lovely pet underwent surgery to remove both eyes and prosthetic devices replaced the globes of the eyes for cosmetic appearance. One day the owner brought her pet into the office, I was amazed how responsive this cute little animal was to her surroundings.

What blind dogs need most is a caring owner – a “seeing eye person” – who can keep the blind dog’s home environment as stable and predictable as possible. If your dog is blind, always supervise it when outdoors and don’t rearrange the furniture. And if you take your dog off its home turf, use a short lead and gentle voice commands to direct its movements.

What does your pet see:

Dogs appear to be better than people with peripheral vision, but their close-up vision does not seem as sharp as a human’s. This is due in part to the placement of the eyes in the dog’s skull. The dog’s lateral eye placement allows better wide-angle vision but hinders depth perception and close-up viewing because there is minimal visual overlap (binocular convergence) between the two eyes. Hence, your dog can easily snag a ball moving sideways but may have trouble catching a ball tossed right at his nose.

The structure of the dog’s retina helps explain other idiosyncrasies of canine vision. Abundant light-sensitive cells called rods in your dog’s retina help it detect motion and see well in dim light – clear advantages for canines stalking prey at dusk or dawn or guarding territory against intruders. In contrast, the human retina is made up predominantly of cones – receptors that are better at detecting color and processing bright light. Contrary to popular belief, dogs are not completely colorblind for they do have cones in their retinas. But dogs can’t distinguish the full spectrum of colors.

The tapetum lucidum is another ocular structure that gives canines enhanced ability to see in dim light. This thin, reflective tissue is part of the choroid layer, which lies behind the retina. The tapetum reflects light back through the retina, giving the retina a second opportunity to absorb light. You may be more familiar with the function of the tapetum lucidum than you think: it’s what causes the bright reflection from a dog’s eyes when car headlight shine on them at night.