The clear outer front capsule of your pet's eye, its window to the world, is called its cornea. Corneal cells are the only cells in the body that are transparent.
All other living cells in the body are nourished by tiny blood vessels. To remain clear, instead of being nourished by blood vesicles, the cornea is bathed in a nourishing protective liquid, the tears. Because of its delicate nature, and its position on the outermost edge of the eye, your pet's cornea is subject to scratches, scrapes and ulceration. That is particularly true in short-nosed dogs who have no protective nose for a bumper.
When a scrape or scratch is shallow, the cornea heal very rapidly. These shallow scrapes and scratches are most common in young active animals.
Adult dogs and cats are more subject to slower-healing erosions of the cornea called ulcers. The most common cause of these ulcers is a deficiency in protective tears - but corneal ulcers can also be due to viral, bacterial and fungal infections. In some cases, they appear to be allergic in cause. (ref) Adult cats commonly develop these ulcers due to infection with feline herpesvirus-1. Left untreated, corneal ulcers can develop serious complications that destroy vision. You can see the structures involved if you click on the image of the eye at the top of this page.
Ulcers of the cornea can be shallow or deep. They result from everything from pawing to bee stings, the scratch of a thorn or an infectious bacteria. Once the outermost layer of the cornea has been torn, the area becomes irritated and painful. This causes the pet to squint and the eye to tear. We call that reflex blepharospasm.
Corneal ulcers can be quite painful. They often itch. In response, many pets rub the affected eye which just makes matters worse. A discharge usually accumulates in the corner of the eye nearest the nose (the medial canthus) and the blood vesicles of the whites of the eye (the sclera) enlarge with blood. If the damaging object is no longer in the eye and the ulcer is not deep it generally heals rapidly.
However when an infection is present, when the ulcer or cut is deep or when the pet rubs its eye, the ulcer can progress to affect the deeper structural layers of the cornea. Once much of the cornea has been eroded away a bulge may form similar to a sidewall blowout on a tire (a descemetocele). If a descemetocele bursts, the anterior portion of the eye will collapse and the contents will spill out destroying the eye. This is why all corneal ulcers need to be observed closely and frequently by your veterinarian.
Large, corneal abrasion, and ulcers causes the entire cornea to swell with fluid (edema). A normal cornea is ~78% water. When inflamed and edematous, its water content increases and it turns a milky bluish white in color. If the eye is normal, within a few days tiny blood vessels will form in the cornea that aid in the healing process. Once the superficial layer of your pet's cornea has fully healed, those blood vessels should retreat and no more than a small, milky scar remain.
Corneal ulcers are graded according to their depth. If only the outer layer is lost the lesion is called a superficial corneal ulcer. Those ulcers generally heal without the help of new blood vessels. When more than one half the thickness of the cornea is affected it is called a deep corneal ulcer. As I mentioned earlier, superficial traumatic ulcers in health eyes heal in a matter of days. Deep ulcers can take several weeks to heal. It is the deep ulcers that require new blood vesicles to grow into the area. These are the more dangerous ulcers. An eye with this problem needs intensive care and protection. They can also leave a more substantial corneal scar. When there are underlying causes for these ulcers, some refuse to heal or heal only to re-ulcerate again. Those type of ulcers are considerably more common in pekingeses, pugs and dogs with similar face conformation.
Fluorescein dye strips are used to detect and demonstrate that there is a tear or abrasion on the surface of the cornea. First the eye is anesthetized with a drop of topical anesthetic (such as lidocaine or tetracaine). Then the strips are moistened with saline and the dye allowed to flow out over the cornea. The eye is then gently washed with saline to remove all excess dye. In a normal eye no dye remains on the cornea. Fluorescein dye attaches only to raw abraded areas where it can be seen when viewed with an ultraviolet light source through an ophthalmoscope.
A second test used to evaluate the eye is the Shirmer tear test. This test measures the amount of tears present in the eye. One cause of corneal ulcers is a lack of adequate tear flow.
If a veterinarian suspects that the pressure within the eye is too high (glaucoma) and that this increased pressure and distortion of the shape of the eye is the underlying cause of the ulceration they usually perform another test. This test I uses a tonometer , an instrument that measures pressure within the eye. Ulcers due to the increased intraoccular pressure of glaucoma require a different course of treatment.
Scraping and puncture wounds of the eye are the most common cause of corneal ulcers in dogs. Cats are less likely to poke their eyes with blunt or sharp objects and more likely to damage their corneas with claws while scratching or fighting. These ulcers are often linear or oblong in shape. If they are not deep they tend to heal quite rapidly.
Dogs and cats with bulging eyes are more susceptible to corneal ulcers. This is because bulging eyes are more likely to be scraped and scratched. Misplaced eyelashes (distichiasis) and eyelids that curl inwardly (entropion) or outwardly (ectropion) can also cause corneal ulcers. Shar pei and Chow Chows commonly suffer from entropion. Hound breeds are prone to ectropion.
film of tears over protruding eyes often does not reach the center.
Dryness of the eye is sufficient to cause ulceration. Other pets
are born with deficient tear production. Tear flow over the eye
can be increased using cyclosporin ophthalmic drops or corrective
surgery. The use of antibiotics of the sulfa class has also been
associated with dry eye syndrome. The medical term for dry eye is keratoconjunctivitis sicca (KCS).
The rhinotracheitis or herpes-1 virus can form a carrier state in the eyes of cats causing periodic corneal ulcers. These ulcers come and go with stress. When inactive, the cornea is left with pinpoint, milky white, rounded scars. When this viral disease is suspected and the signs are severe or persistent, many cats receive topical antiviral drugs such as diluted trifluridine (Viroptic,trifluorothymidine) and oral famciclovire. Topical antibiotic drops or ointments might be given as well to prevent secondary bacterial infections.
The amino acid, l-lysine, has been found to help many cases of rhinotracheitis conjunctivitis resolve. This amino acid reduces the amount of another amino acid, arginine, that is present in the cat's body. Arginine is thought to be necessary for herpesvirus to reproduce. Many cat owners continue the supplement indefinitely. Lysine can be purchased at health food stores. Pick a brand that is propylene glycol-free. You can read more about this viral infection and the many ways it affects cats here, here, here here and here.
Boston Terriers and Boxers have corneas that just do not seem to heal well. When these breeds develop corneal ulcers vitamin E solution might hasten healing.
Certain bacteria have an affinity for the eye causing redness and itching. Self-trauma in these cases can cause corneal ulcers. These cases need to be placed on antibiotic or antifungal eye drops.
Indolent ulcers are ulcers that refuse or are slow to heal. These can occur when a ledge of dead corneal tissue surrounds the ulcer. These ulcers can be encouraged to heal if this dead tissue is scraped away. Other causes are persistent dry eye, infection, or concurrent disease in your pet that slows the healing process. These long-standing ulcers also respond when the third eyelid is sutured across the defect in a procedure called tarsorrhapy.
Some corneal ulcers become so deep that the inner lining, Descemet’s membrane, balloons out forming a descemetocele, which is similar to sidewall damage on a tire. If this structure ruptures, fluid from the anterior chamber of the eye leaks out over the cornea and the eye collapses. The iris in these eyes becomes sticky and will often stick to or protrude through the defect. When these eyes heal the normal anatomy of the eye does not recover. Distortions of the iris in these eyes often leads to glaucoma and loss of vision or the eye when the iridocorneal angle is changed (see diagram at the top of this page).
The treatment prescribed for dogs and cats with corneal ulcers depends on the severity of the ulcer, how long it has been present and what is suspected to be the underlying cause of the problem.
Treatment also focuses on preventing secondary infection, controlling inflammation and pain , preventing additional corneal damage. Effective treatment also minimizes residual damage to the cornea by limiting scar formation.
Many pets with corneal ulcers are in considerable discomfort and pain. These animals squint or keep their eye tightly shut. They may be sensitive to bright light as well. Atropine drops, which dilate the pupil, can help relieve this pain. This is the same medicine that ophthalmologists use to dilate your eyes before an exam. Topical anesthetic eye drops are good during initial examinations, but with persistent use they can retard healing. Therefore they are unlikely to be dispensed for long term use.
Antibiotic ointment or drops are frequently applied to affected eyes. Corticosteroid-containing medicines should not be used with corneal ulcers because they also retard the healing process. If a pet is pawing at its eye it may need to be placed in an Elizabethan collar or have its paws bandaged.
Eyes need to be recheck by your veterinarian frequently. Fluorescein dye tests can gauge the healing process.
Deep corneal ulcers require more complicated treatment. These pets often need to be hospitalize. If Descemet’s membrane is bulging through the ulcer it is really an emergency situation - nothing that can wait for the following morning. Treatment, in those situations, is best performed by a veterinary ophthalmologist. When I did them. I sometimes reinforced the area of the ulcer with very fine suture material and then sewed a patch composed of an attached slice (flap) of the third eyelid over the ulcer to shield it and bring it additional blood supply. When Descemet’s membrane has burst and the eye has already leaked its contents, the outcome of this surgery is less successful. Some of these eyes cannot be saved.
Superficial ulcers that are slow to heal (indolent) sometimes need to be scraped clean of dead tissue, especially around the margins of the ulcer, in order for new tissue to spread over the defect. This is carefully done under general anesthesia with a small scalpel blade. Tissue adhesives, can be used to provide protection while these ulcers heal.
Ointments and drops need to be applied very frequently to affected eyes. Often every two to four hours in newly treated eyes. During this time the pet must be observed closely so that it does not rub or scratch the eye.
Pets that develop corneal ulcers because of insufficient tears (keratoconjunctivitis sicca) or a bulging eye are often place on cyclosporine eye drops supplemented with artificial tears. An alternative to this is surgical transplantation of the duct of the salivary gland.
It is important that your veterinarian check the eye after four or five days of treatment. Most ulcers will have healed in this time but some will require additional time. If the ulcer has not healed in two weeks, treatment needs to be reevaluated and other procedures such as scraping (debridment) or a corneal patch might be required.