Corneal Ulcers in Cats
The cornea is the clear, shiny membrane that makes up the surface of the eyeball. It is much like a clear window. To understand the significance of a corneal ulcer, you must first understand how the cornea is constructed.
The cornea is comprised of four layers. The most superficial layer is the epithelium. Actually, this layer is comprised of many, very thin layers of cells. Below the epithelium is the stroma. The next deeper layer is called Descemet's membrane. The deepest layer is the endothelium; it is composed of a single layer of cells. Because all of these layers are clear, it is not possible to see them without special stains and equipment. The cornea must maintain its transparency to ensure clear vision.
Corneal Abrasion vs. Corneal Ulcer
An erosion thorough a few layers of the epithelium is called a corneal erosion or a corneal abrasion. A corneal ulcer is an erosion through the entire epithelium into the stroma. If the erosion goes through the epithelium and stroma to the level of Descemet's membrane, a descemetocoele exists. If Descemet's membrane ruptures, the liquid inside the eyeball leaks out and the eye collapses. At times, differentiation of the two may not be straightforward. For this reason, cats with corneal disease should be rechecked frequently, usually every few days.
Because a corneal ulcer is quite painful, most cats rub the affected eye with a foot or on the carpet. To protect the eye, they keep the eyelid tightly closed or will frequently squint in an attempt to the eye closed. Some cats will seek dark areas because light is painful; they may hide under the bed or in a closet. Occasionally, there will be a thick white or yellow discharge that collects in the corner of the eye or runs down the face.
A number of different disorders may lead to corneal ulceration. Some of these include trauma, foreign bodies, abnormalities of the eyelid, decreased tear production, chemical irritation to the cornea, and certain infectious diseases. In cats, viral respiratory disease is associated with a variety of corneal problems. Fortunately, ulcers rarely develop with these common respiratory viruses. Corneal ulcers can be seen more frequently in animals that have a lot of facial folds; when these folds occur close to the eye, they allow hair to constantly rub the cornea.
Superficial corneal abrasions are usually not visible without special stains. They can be seen when a drop of fluorescein stain is placed on the cornea. The dye will adhere to an area of ulceration and is easily visualized with a special black light called a Wood's light or Wood’s lamp. This is the most basic test performed and may be the only test needed if the ulcer is acute (sudden) and very superficial. If the ulcerated area is chronic or very deep, samples are taken for culture and cell study prior to applying the stain or any other medication.
Treatment depends on whether there is a corneal abrasion, corneal ulcer, or descemetocoele present.
Corneal abrasions generally heal within 3-5 days. Medication is used to prevent bacterial infections (antibiotic ophthalmic drops or ointment) and to relieve pain (atropine ophthalmic drops or ointment). Antibiotic drops are only effective for a few minutes so they must be applied frequently; ointments last a bit longer but still require application every few hours. It is suggested that an antibiotic preparation be instilled in the eye 4 to 6 times per day. On the other hand, the effects of atropine last many hours so this drug is only used twice daily. If the corneal ulcer does not heal within these first few days, complicating factors must be identified, if possible.
If a deep corneal ulcer or descemetocoele is present, measures must be taken to protect the eye and to promote healing. Since cats do not wear eye patches well, surgical techniques are often used to close the eyelid and cover the ulcer or descemetocoele. These measures protect the eye for several days, then are reversed so the cat can use the eye again. This approach is also taken if a superficial ulcer is very slow in healing.
Ulcers that do not heal well often have a buildup of dead cells on the edge of the ulcer. These dead cells prevent the migration of new, normal cells over to the ulcer defect. If this appears to be part of the healing problem, the dead cells are removed from the edges of the ulcer before the eyelids are surgically closed. In some cases, removing the dead cells may be all that is needed to start the healing process, so surgical closing of the eyelids may not be necessary.
A cat with a corneal ulcer has quite a bit of pain inside the eye, so it keeps it tightly shut. Atropine is used to relieve that pain. However, atropine also dilates the pupil widely. This means that the cat is very sensitive to light in that eye. Because of the light sensitivity, the eye will be held closed in bright light. Atropine's effects may last for several days after the drug is discontinued. Do not be alarmed if the pupil stays dilated for several days. Should you accidentally get atropine in your own eye, the same prolonged pupillary dilation will occur.
Antibiotic drops or ointments are commonly used on ulcers. They do not cause the ulcer to heal better, but they treat or prevent bacterial infections from occurring within the ulcer. Bacterial infections greatly slow or even stop healing of a corneal ulcer. Antibiotic drops and ointments remain on the cornea for a short period of time. Therefore, they must be used several times each day. Ointments remain in contact with the cornea longer than drops, but many cats will object to the gooey medication in their eyes.
A topical anesthetic is often used to numb the cornea so the diagnostic tests may be performed. However, these drugs are toxic to the corneal epithelium; they prevent proper healing. They are safe for one time use, but they should not be used as part of treatment.
The Danger of Topical Steroids
It is important that steroids not be used in the eye too soon because they will stop healing of a corneal ulcer and may worsen it. Therefore, the fluorescein dye test should be performed before beginning this type of medication. If steroids are used and the eye becomes painful again, discontinue the steroids and have the eye rechecked. Steroids can cause the corneal ulcer to eventually perforate.
Side Effects of Treatment
Rarely, a cat will be allergic to an antibiotic that is instilled in the eye. If your cat seems more painful after the medication is used, discontinue it and contact the veterinarian.
If you see your cat drool or paw at the mouth after instillation of eye medications, do not be alarmed. The tear ducts carry tears from the eyes to the back of the nose. The eye medications may go through the tear ducts and eventually get to the throat where they are tasted. Atropine has a very bitter taste that may cause this response. Again, you are seeing your cat's response to a bad taste, not a drug reaction.
Discontinuation of Medication
The best way to tell that the cornea has healed is to repeat the fluorescein stain test. This should be done after about 2-3 days of treatment. If healing is progressing well, the eye should be checked again before treatment is discontinued to be sure healing is complete. Please consult with us before discontinuing medication unless you have been instructed otherwise.
The normal cornea has no blood vessels going through it. However, when a corneal ulcer or descemetocoele occurs, the body senses a need to increase its healing capabilities. New blood vessels are created by a process called neovascularization. The new vessels begin at the sclera (the white part of the eye) and course their way to the ulcer. You may be able to see these new vessels as a small red spot adjacent to the ulcer.
Neovascularization is a good response because it hastens healing. However, after the ulcer is healed, these vessels remain in the cornea. They are not painful, but they do obstruct vision. Therefore, it is desirable to remove them. This is done with steroid (cortisone) ophthalmic drops or ointment. Cortisone is used for a few days to several weeks, depending on how many vessels exist.
The prognosis is usually good for uncomplicated corneal ulcers that receive prompt treatment. The prognosis is less certain when the ulcer is deep, the ulcer does not respond to medication, or a complication of the ulcer cannot be successfully treated.