The eyes have it

All about canine eyes



Introduction

On a broad and arbitrary scale, mammals can be divided into two categories: some are predators and some are prey. Prey animals, like deer, horses, squirrels, and goats, need one or more defense mechanisms to avoid becoming lunch for a predator animal.

Predators, like wolves, cats, coyotes, and dogs, need equipment for the chase and kill.

Prey animals are sensitive to movement so they can flee danger. They have eyes on the sides of their heads to increase field of vision and bodies built for speed or climbing.

Predator animals are also sensitive to movement, but they have eyes in the front of their heads to increase the ability to focus on the prey and judge distances.

Eyes on the sides of the head work independently and provide monocular vision; eyes in front of the head work together to provide binocular vision.


A dog’s eye view

Sitting in a bed of fat that cushions the bony socket, the dog’s eyeball is attached to its skull by a set of muscles that provide stability and allow movement.

Most of the dog’s eye is the pigmented iris; the white of the eye is far less obvious than in the human orb. The pupil is in the center of the iris; the cornea covers both. The anterior chamber lies between the cornea and the iris and pupil; then comes the lens, the posterior chamber, and the retina. The posterior chamber contains a clear jelly.

Light enters the eye through the cornea and travels through the anterior chamber, the pupil, the lens, and the posterior chamber to reach the retina. The pupil regulates the light by contracting if the light is bright and expanding if it is dim. The retina transfers the image to the vision centers in the brain via the optic nerve.

The dog’s eye is more discerning of dim light than man’s and is less able to sharply focus on detail; thus he sees better in the dark than we do but surrenders some visual acuity. It has long been thought that dogs do not discern color, but that supposition has been debunked by recent research. Current wisdom holds that dogs see some colors but do not have the detailed color vision of primates, including man.

Dog eyes are protected by upper and lower eyelids, by a third eyelid or haw, and by tears. The eyelids are lubricated by a thin layer of mucous to avoid irritating the surface of the cornea.

The haw generally sits all but unnoticed at the inner corner of the eye but can be seen in some dogs, particularly St. Bernards and Basset Hounds. The haw, also known as the nictitating membrane, may become obvious when the dog is stressed or ill. Its job is to keep the eye lubricated and protected.

Dogs have lashes on the upper eyelid to help keep debris out of the eye.


Dog eye troubles

Dog eyes are subject to a variety of troubles, some serious, some not. Any eye condition that varies from the norm needs attention.

Entropian or ectropion can occur when the eyelid does not properly fit the eye. Entropian is the turning in of the eyelid that can cause corneal abrasions or irritate the conjunctiva, the mucous membrane that covers the eyeball. Ectropion is the turning out of the eyelid and can cause exposure problems. Both can be surgically corrected.

Breeds that are susceptible to entropion include Chinese Shar Pei, Chow Chow, St. Bernard, Basset Hound, Bernese Mountain Dog, Bloodhound, Bulldog, and Chesapeake Bay Retriever.

Eyelashes that grow in such a fashion as to irritate the eye can cause problems and may require surgery. Some dogs are prone to excessive tearing caused by tiny hairs near the inside of the eye that act like a wick for tears and cause staining of the face.

Lagophthalmos or rabbit eye can occur in breeds and individual dogs with an impaired ability to blink. Blinking distributes tears over the surface of the eye to keep it moist; inability to blink properly can cause corneal ulcers. Surgery can alleviate the problem if it is detected early.

Eyelid tumors are usually benign but should be attended to.

Cherry eye may occur if the third eyelid becomes more prominent as a result of dehydration, severe dental disease, atrophy of the head muscles, or disease that causes the eye to lose moisture. When this happens, the tear duct may enlarge and appear as a round, red mass. The condition can lead to irritation of the cornea or conjunctiva because it interferes with tear production. It can sometimes be dealt with by application of topical medication but surgery is often necessary.

Uveitis or inflammation of the eye can be caused by foreign bodies or by systemic disease. It can cause symptoms such as blinking, squinting, or reluctance to enter brightly-lighted areas and can produce a watery discharge. The appearance of the eye may change depending on where the inflammation is centered. Some eyes become dull or bluish; in some cases the white becomes red or swollen or the iris changes color.

Diagnosis of uveitis is done with special instruments that illuminate the interior of the eye. Unless the cause is obvious, blood tests and other lab work are necessary to determine the cause. Diseases that can cause uveitis include ehrlichiosis, Rocky Mountain Spotted fever, Lyme disease and brucellosis. Other causes may be a foreign body in the eye, a bacterial infection, an autoimmune disease, a scratch, a cataract, or cancer or arasitic disease.

Treatment is critical to relieve pain and to prevent scarring, glaucoma, or blindness. Aspirin, corticosteroids, or other anti-inflammatory drugs may be prescribed to minimize soreness. Oral or topical antibiotics are useful in bacterial infections. Dilating drops or ointments may be used to relax the muscles in the eye and thus help reduce pain. Corticosteroid drops are not used in cases of corneal ulceration because they could slow healing or actually worsen the ulcer.

Although early treatment is often successful, some dogs have chronic uveitis and in some, the eye has to be removed.

Keratoconjunctivitis sicca or dry eye is caused by a breakdown in the tear film that protects the cornea. When this film breaks down, the cornea is deprived of the nourishment it needs to remain healthy and deterioration begins. Brown coloration, scar tissue, ulcers, and blood vessel growth can then develop and interfere with eyesight.

A dog with dry eye is a dog whose eyes sting constantly. The lack of tears also contributes to the increase in mucous in the eye, resulting in a greenish discharge that adheres to the hairs near the eye. The discharge can be cleared up with medication, but if dry eye is the cause, it returns when the dosing ceases.

A breakdown in the tear film and a loss of the aqueous layer causes dry eye. This loss results in dryness to areas of the corneal surface or in more advanced cases, drying to the entire corneal surface. When the cornea is deprived of oxygen and nutrients through the tear film, it rapidly undergoes destructive changes. These changes result in brown pigmentation, scar tissue growth, ulcer development, and blood vessel growth across the cornea leading to partial vision loss.

Dry eye can be triggered by hypothyroidism, tear gland infections, and the toxic effects of some drugs that are necessary to treat other conditions. In a few cases, chronic ear infections and nerve disorders can also cause dry eye to flare up. Treatment includes lubrication of the eye, reduction of bacteria and inflammation, and stimulation of natural tear production.

Glaucoma is the increase of pressure within the eye. It can destroy the retina and damage the optic nerve, thus causing blindness, if not treated. Glaucoma occurs when the eye’s natural ability to maintain intraocular pressure by dispersing fluid is impaired . It can occur as a result of structural changes such as lens displacement in older dogs or from causes such as uveitis or injury.

Some breeds of dogs are susceptible to glaucoma that does not arise from a secondary cause such as age-related changes, inflammation, or injury. These breeds include the Beagle, Norwegian Elkhound, American and English Cocker Spaniels, Basset Hound, Samoyed, and Chow Chow.

Cataracts are spots on the dog’s eye lens that interfere with vision. The normal lens is clear; a cataract is a cloud on the lens that does not allow light to reach the retina.

Cataracts can be slow-growing or may appear to get worse overnight. They first appear as small dots or blisters and may eventually grow to cover the entire lens. They act by preventing light form reaching the retina.

If a cataract is suspected, the dog should be examined by a veterinary ophthalmologist to determine if the opacity is actually a cataract, how far it has progressed, and, because some cataracts do not cause significant impairment to vision, whether surgery will benefit the dog.

Some dogs inherit cataracts. Others may develop them as a result of uveitis, eye injury, or diabetes, or as part of the aging process.

Blindness can be caused by any of a number of inherited or acquired eye diseases. Gradual vision loss may not be apparent to the owner of a dog that stays at home until the furniture is moved, a new fence is installed, or the family moves to new digs. If the environment changes and the dog starts bumping into things, if he can’t find a toy tossed for him to retrieve, and if he suddenly fears being left alone, vision loss should be suspected.

Inherited eye problems such as progressive retinal atrophy, retinal dysplasia, or corneal dystrophy can be diagnosed by eye examination and dogs with these diseases can be removed from the breeding pool. Unfortunately, the diseases are only obvious when symptoms begin, so the test must be done every year to keep track.

PRA causes changes in the retina that begin with night blindness and progress to total blindness. There is no effective treatment.

Retinal dysplasia – the abnormal development of the retina – can be genetic or congenital. If the latter, it can be caused by Vitamin A deficiency, a virus, certain drugs, or uterine trauma in the pregnant bitch.

Corneal dystrophy is the presence of opaque bodies in or near the middle of the cornea. Some corneal dystrophies are superficial; others infect the deepest layer of the structure and can cause blindness.

There are many other eye diseases that can affect dogs. If any eye problem is suspected, a trip to the veterinary clinic is in order. Although a dog can function adequately if he loses his eyesight, every step possible should be taken to assure that that loss does not occur.


Sources

Information for this article came from the following sources:

Norma Bennett Woolf

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