Eyes and Your Dog
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. We, therefore, must 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.
Courtesy: Columbia Animal Hospital @ Hickory Ridge
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Through the Eyes of Your Canine
By: Sarah Probst, Information Specialist
University of Illinois, College of Veterinary Medicine
Owners who want to better understand their canine companions must recognize that dogs
see the world from a different visual perspective. The differences begin with the structure of
the eye. "We have a good idea what canines see because we know the make-up of the
retina of a dog's eye," says Dr. Ralph Hamor, a veterinarian and specialist in ophthalmology
at the University of Illinois College of Veterinary Medicine Teaching Hospital.
The retina, which covers the back of the inside of the eyeball, contains cones and rods-two
types of light-sensitive cells. Cones provide color perception and detailed sight, while rods
detect motion and vision in dim light. Dogs, which have rod-dominated retinas, see better in
the dark than humans do and have motion-oriented vision. However, because they have
only about one-tenth the concentration of cones that humans have, dogs do not see colors
as humans do.
"I generally explain that dogs see like a color-blind human," says Dr. Hamor. "Many people
think that a person who is red/green color blind cannot see any color, but there are
variations of being color blind. Most people have vision that is trichromatic (three color
variations). People who are red/green color blind are dichromatic (two color variations).
Dogs can pick out two colors-blue-violet and yellow-and they can differentiate among
shades of gray." Dogs are unable to distinguish among green, yellow, orange, and red. They
also have difficulty differentiating greens and grays.
Dogs use other cues (such as smell, texture, brightness, and position) rather than rely on
color. Seeing-eye dogs, for example, may not distinguish whether a stoplight is green or
red; they look at the brightness and position of the light. This and the flow and noise of
traffic will tell the dog that it is the right time to cross the street.
The set of dog's eyes determines the amount of field of view and depth perception. Prey
species tend to have eyes set on the sides of their head because the increased field of view
allows them to see approaching predators. Predator species, like humans and dogs, have
eyes set closer together. "Human eyes are set straight forward while dog eyes, depending
on the breed, are usually set at a 20 degree angle. This angle increases the field of view and
therefore the peripheral vision of the dog."
However, this increased peripheral vision compromises the amount of binocular vision.
Where the field of view of each eye overlaps, we have binocular vision, which gives us
depth perception. The wider-set eyes of dogs have less overlap and less binocular vision.
Dogs' depth perception is best when they look straight ahead, but is blocked by their noses
at certain angles. "Predators need binocular vision as a survival tool," Dr. Hamor says.
Binocular vision aids in jumping, leaping, catching, and many other activities fundamental to
predators.
In addition to having less binocular vision than humans, dogs also have less visual acuity.
Humans with perfect eyesight are said to have 20/20 vision-we can distinguish letters or
objects at a distance of 20 feet. Dogs typically have 20/75 vision-they must be 20 feet from
an object to see it as well as a human standing 75 feet away. Certain breeds have better
acuity. Labradors, commonly used as seeing-eye dogs, have been bred for better eyesight
and may have closer to 20/20 vision.
Don't expect your dog to recognize you across the field by sight. He'll recognize you when
you do some sort of motion particular to yourself or by smell or hearing. Because of the
number of rods in the retina, dogs see moving objects much better than they do stationary
objects. Motion sensitivity has been noted as the critical aspect of canine vision. "So much
of dog behavior deals with posture and appropriateness. Small changes in your body
posture mean a lot to your dog," Dr. Hamor adds. Dog owners need to modify training
based on this fact. If you want your dog to perform an action based on a silent cue from
you, Dr. Hamor suggests using a wide sweeping motion to cue your dog.
When dogs go blind, owners often wonder if the dogs' quality of life has diminished to the
point where they are no longer happy. "We know that humans deal well with being blind,
and humans are much more dependent on their eyes than are dogs," Dr. Hamor says. "Blind
dogs lead happy lives if they are comfortable." The owner may need to make some
adjustments in the pet's environment, such as having a fenced yard, taking leashed walks,
and not leaving unusual objects in normal pathways. "When blind dogs are in their normal
environment, most people don't know they are blind." When clients visit Dr. Hamor asking
about quality of life for their newly blind dog, Dr. Hamor suggests that they take a month to
see if they and their dog are happy. In the majority of cases, the owners never come back.
For further information on dog vision and problems with your dog's eyes, contact your local
veterinarian.
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