What is a corneal ulcer?
First, a little information about the cornea. The cornea is the clear, glistening dome at the front of the eye. It is like a pane of glass, being a barrier to the outside while letting light into the eye. Although it is clear, the cornea consists of many layers of cells. The layers are divided into three regions:
- Epithelium - the thin membrane on the outer surface of the cornea
- Stroma - the layers of cells between the inner and outer membranes
- Descemet's membrane - the thin membrane on the inner surface of the cornea
Injury to the cornea that damages only the epithelium is called a corneal abrasion. Injury that extends through the epithelium and into the stroma is called a corneal ulcer. A deep ulcer that extends down to Descemet's membrane can result in a descemetocele [des-met-o-seal]. This structure develops when the fluid in the eye causes the exposed part of Descemet's membrane to bulge through the ulcer, forming a little bubble. If the descemetocele ruptures, or if the corneal injury was deep enough to damage Descemet's membrane, the fluid inside the eye leaks out and the eyeball may collapse.
What causes corneal ulcers?
The most common cause of a corneal ulcer is trauma. Common examples include a blow to the eye, a cat scratch, and rubbing at a painful or itchy eye. Irritating chemicals, such as dips and shampoos, can also damage the cornea. Less commonly, bacterial or viral infections can cause corneal damage. (In addition, bacterial invasion of an already damaged cornea can worsen a corneal ulcer or slow its healing.)
What are the signs of a corneal ulcer?
Corneal injuries are very painful, so the cat may keep its eye closed and rub the eye with its paw or on the carpet. The eye may produce excessive tears that spill over the eyelid and wet the face below the eye. The cat will resist if you try to look at the eye, so these signs may be all you see. If you get a good look at the eye, you may see that the normally clear cornea is cloudy or milky. This indicates corneal edema, or swelling of the cornea with fluid as a result of inflammation.
How is a corneal ulcer diagnosed?
Large or deep ulcers may be visible just by looking at the eye. In other cases it is necessary for the veterinarian to use a special stain (fluorescein dye) to detect corneal damage. The dye does not stain the undamaged epithelium. When the epithelium is damaged, the fluorescein dye stains the exposed stroma an iridescent green, which highlights the ulcer.
If the ulcer has been present for some days, or if it is very deep, the veterinarian may sample the damaged cornea to check for bacteria or fungi before staining the eye. Sampling the damaged cornea involves gently swabbing the ulcer or gently scraping some of the damaged tissue with a tiny blade. The swab may be sent to a laboratory for bacterial culture and identification. (This procedure can be important in selecting the appropriate antibiotic.) The tissue scraping may be examined under a microscope in the veterinarian's office.
Note: If there is a chance that the ulcer is infected, it is important that you do not put any drops or ointment into the cat's eye until the veterinarian has examined it. If the medication contains antibiotics, the culture results may be misleading.
How are corneal ulcers treated?
Treatment depends on the degree of corneal damage, and on how long it has been since the injury.
Corneal abrasions require little treatment and generally heal within 3-5 days. Your veterinarian will probably prescribe some antibiotic eye drops or ointment to prevent infection. The medication must be placed in the eye 4 to 6 times a day. Eye drops are often easier to get in the eye than ointments, but they are quickly diluted and washed from the eye by normal tear production. To be effective, antibiotic eye drops must be put in the eye every hour or two, which is impractical for most owners.
If the eye seems quite painful, your veterinarian may also prescribe atropine eye drops or ointment. This medication dilates the cat's pupil. Constriction of the pupil is a common reaction by an injured eye, and it can add to the pain of a damaged cornea. The effects of atropine last for several hours, so this medication is given only twice a day, whether as drops or ointment.
If the cat's eye is still quite painful after 2-3 days of medication, call your veterinarian. The corneal injury may be more serious than it first appeared, and may need more intensive treatment.
Superficial ulcers may be treated as described above for corneal abrasions. Deeper ulcers require more intensive treatment. In addition to antibiotics and atropine, your veterinarian may decide that the cornea needs to be protected while the ulcer heals, to prevent rupture of the eyeball. If there is already a descemetocele, there is no question; if the eye is to be saved, it must be protected.
Protection of the damaged cornea involves surgically covering the area with a piece of the eyelid lining (the conjunctiva) or suturing the eyelids closed. After several days, the veterinarian reverses the procedure and examines the ulcer. Depending on how well the ulcer is healing, the veterinarian may do one of several things: "freshen up" the edges of a slow-healing ulcer to stimulate healing; cover the ulcer again for a few more days; or leave the eye open and the healing/healed ulcer uncovered.
The best way to be sure the cornea has healed is to restain the eye with fluorescein. For deeper ulcers that were covered surgically, your veterinarian will probably perform this test when the cat returns for reexamination. For superficial ulcers that did not require surgery, your veterinarian may recommend that you make an appointment for 5-7 days' time so that the eye can be reexamined and stained.
Another type of corneal injury is a corneal laceration. Like a cut elsewhere on the body, corneal lacerations can be superficial or deep. Superficial corneal lacerations are treated like corneal abrasions. Deeper lacerations are treated surgically.
Are there any side effects of the medications?
Eye drops and ointments that are specially prepared for use in the eye (ophthalmic preparations) are very safe and cause few problems. If your cat's eye seems more painful after you have put the medication in, discontinue treatment and contact your veterinarian.
Overuse of atropine may slow bowel activity and cause constipation. So use the atropine only as advised by your veterinarian. Because atropine keeps the cat's pupil dilated, the cat may be sensitive to light and may keep the eye closed in bright sunlight. The pupil may remain dilated for days after treatment has ended and the cornea has healed, but this is not a problem. Atropine has a very unpleasant taste. If any gets into the cat's mouth, it may cause drooling for a few minutes. But this, too, is not a problem.
Are there any long-term consequences of a corneal ulcer?
You may see some red streaks tracking across the cornea toward the ulcer after a few days. These are tiny blood vessels that grow in from the outer edges of the cornea to supply the damaged area with white blood cells and substances that promote healing. This process is called neovascularization (which literally means production of new blood vessels). It is most likely to develop with deep ulcers, especially if the ulcer is infected.
Neovascularization can speed healing of an ulcer. It may leave dense pink or white areas in the cornea that persist long after the ulcer has healed. These areas do not cause discomfort, but they can obstruction the cat's vision if they are in the center of the cornea. To limit this process, your veterinarian may prescribe eye drops or an ointment that contains cortisone. Cortisone is a powerful anti-inflammatory drug. However, it can delay healing of a corneal ulcer, so cortisone is not used until a fluorescein stain test indicates that the surface of the cornea is completely healed. If, at any time during treatment with cortisone, you notice that the cat's eye seems painful again, discontinue treatment and contact your veterinarian.
Deep corneal ulcers may leave a white scar in the cornea once they have healed. As with neovascularization, the scar may impair the cat's vision in that eye. Unlike neovascularization, scarring cannot be resolved with cortisone. It is permanent.
If the ulcer is so deep that the eyeball ruptures, or if the pain is severe and cannot be controlled, your veterinarian may recommend removing the cat's eye. Fortunately, this is a very uncommon necessity. The good news is that most cats function very well with one eye, and when the procedure is performed because the eye is so painful, the relief is instant and dramatic. Cats that were not eating because of the pain begin eating immediately after the surgery and quickly return to normal activities and behavior.
The keys to successfully treating an eye injury and preserving vision are these:
- early assessment
- appropriate treatment
- monitoring of healing
Any cat with a painful eye should be examined by a veterinarian immediately. If you are undecided about whether to call your veterinarian, just consider what you would do if it were your child with a painful eye, and act accordingly.
It can be difficult to put drops or ointment into a cat's eye, especially if you have no-one to help you. Also, in working households it may not be possible to medicate the cat's eye during the day. It is important that the cat receive the medications as prescribed. If you are having trouble treating your cat's eye, ask your veterinarian for some suggestions. The veterinarian may recommend hospitalizing the cat for a few days so that the hospital staff can medicate the eye several times a day.