What is the cornea?
The cornea is the transparent, glistening membrane that makes up the surface of the eyeball. One must first know how the cornea is constructed to understand a corneal ulcer.
The cornea has three layers, all of which are highly specialized skin cells. The epithelium is the outermost layer, a very thin layer of cells (usually 5-6 rows thick). Below the epithelium is the stroma, the main supportive layer of the cornea, and the deepest layer is Descemet's membrane. Because all these layers are transparent, it is impossible to see them without special stains that color specific cells and highlight them when the tissue is examined under a microscope.
"Because all these layers are transparent, it is impossible to see them without special stains that color specific cells and highlight them..."
Penetration or erosion through a few layers of the outermost corneal epithelium is called a simple corneal ulcer, abrasion, or erosion. A deep or complex corneal ulcer is a deeper erosion through the entire epithelium into the stroma. With a corneal ulcer, fluid is absorbed from the tears into the stroma, giving a cloudy appearance to the eye. A descemetocele is formed if the ulcer goes through the stroma to the level of Descemet's membrane. A descemetocele is a severe condition. If Descemet's membrane ruptures, the liquid inside the eyeball leaks out, the eye collapses, and irreparable damage occurs.
What causes corneal ulcers?
There are several causes of corneal ulcers in cats, the most common being trauma. An ulcer may result from blunt trauma, such as a cat rubbing his eye against a rough surface, or a laceration, such as a cat-claw scratch, or contact with a sharp object, such as a thorn. The second most common cause is a chemical burn to the cornea. This may happen when irritating shampoos, other liquids, chemicals, or dust gets in the eye.
Other causes of corneal ulcers include bacterial infections, viral infections, and other diseases. These conditions may originate in the eye or develop secondary to disease elsewhere in the body. Examples of other diseases include:
- Epithelial dystrophy (rare in cats)
- Manx stromal dystrophy (corneal edema and serious eye abnormalities result in this disease, which is believed to be inherited in Manx cats)
- Drying of the cornea due to decreased tear production, called keratoconjunctivitis sicca, KCS, or 'dry eye' (rare in cats)
- Endocrine diseases (e.g., diabetes mellitus and Cushing's disease)
What are the clinical signs of a corneal ulcer?
A corneal ulcer is very painful. In response to this intense pain, most cats rub the affected eye with a paw or on carpet or furniture. They will squint, blink rapidly, or keep the lids tightly closed to protect the eye. Occasionally, a discharge will collect in the corner of the eye or run down the face.
How is a corneal ulcer diagnosed?
Superficial corneal abrasions are generally not visible without the aid of special equipment and stains. Corneal ulcers are usually detected with the use of fluorescein stain. A drop of this orange-colored stain is placed on the cornea. The dye will adhere to ulcerated areas and turn green.
Once stained, large ulcers are easily seen, while tiny ulcers may be observed using special ophthalmic lights and filters. This is the most basic test and may be the only test needed if the ulcer is acute and very superficial. If the ulcer is chronic or is very deep, samples will be taken to see what bacterial infection is present (culture) and to look at cells under a microscope (histology) before applying the stain or any other medication.
How is a corneal ulcer treated?
Treatment depends on whether there is a corneal abrasion, corneal ulcer, or descemetocele.
Simple corneal ulcers or abrasions generally heal within three to five days. Medication is used to prevent bacterial infections (antibiotic ophthalmic drops or ointment) and to relieve spasms or pain (typically atropine ophthalmic drops or ointment). Antibiotic drops are only effective for a few hours, so they must be applied frequently; ointments last a bit longer but still require application every few hours. For best results, the antibiotic preparation should be applied every four to six hours. On the other hand, the effects of atropine last many hours, so this drug is used every 12 to 48 hours as needed to keep the pupil dilated.
"Since cats do not wear eye patches well, surgery may be required to protect the injury and allow for adequate healing."
If a deep corneal ulcer or a descemetocele is present, steps must be taken to protect the eye and promote healing. Since cats do not wear eye patches well, surgery may be required to protect the injury and allow for adequate healing. A surgeon will often suture the third eyelid over the ulcer or suture the eyelids together for a few days. In some cases, dead or dying cells may build up along the edges of the ulcer and prevent healing; if this occurs, surgical debridement called a keratectomy (removal of the dead and dying corneal tissue) would be necessary for healing to occur. Your veterinarian will choose the most appropriate surgical technique or combination of techniques for your cat.
Can corneal abrasion progress to an ulcer or a descemetocele?
Yes. Veterinarians often prescribe broad-spectrum topical antibiotics for simple corneal abrasions and superficial ulcers. Despite this treatment, erosion or ulceration can sometimes continue or worsen. Since this can happen and could result in blindness, your cat should be re-examined after two to three days of treatment to be sure that healing is progressing well. If healing is not progressing normally or a complication develops, your veterinarian will recommend additional treatments or surgery.
Do any of the eye medications have side effects?
A cat will occasionally be sensitive to ophthalmic medication. If your cat seems to be in more pain after administering the medication, discontinue it and contact your veterinarian.
A cat with a corneal ulcer typically has significant eye discomfort, so he keeps the eyelids tightly closed. Atropine relieves pain but also dilates the pupil widely. This means the cat is very sensitive to light in that eye and may squint or hold his eye tightly closed when exposed to bright light. The dilation of the eyes may last for several days after the medication is stopped.
Why does my cat drool excessively and paw at his mouth after administering eye medication?
Eye medications can drain through the tear ducts and end up in the mouth. Some eye medications have an unpleasant taste; atropine is particularly very bitter. Excessive and dramatic drooling may occur if the medication gets in your cat's mouth, either from running down the tear ducts or being licked off the paws. This is not a drug reaction but a reaction to the terrible taste. The drooling will subside in a short time. However, once this happens, your cat may be more challenging to medicate in the future if he anticipates tasting it again.
"Excessive and dramatic drooling may occur if the medication gets in your cat's mouth, either from running down the tear ducts or being licked off the paws."
What else can I use to control the pain?
A topical anesthetic is often used to numb the cornea so diagnostic tests can be performed. However, these drugs are toxic to the corneal epithelium and prevent healing. They are safe for one-time use when used with caution. Atropine is commonly prescribed to treat pain from a corneal ulcer. Some cats will benefit from using non-steroidal anti-inflammatory drugs (NSAIDs) or other pain-relieving medications like gabapentin, codeine, or buprenorphine. Your veterinarian will discuss this with you and determine the appropriate treatment for your pet's condition.
How do I know when the treatment is completed?
The best way to tell that the cornea has healed is to repeat the fluorescein stain test. Your veterinarian will perform this test at intervals between one and seven days, depending on the condition's severity and the healing progress. NEVER discontinue treatment on your own.
What does it mean if there are red streaks near the ulcer?
There are no blood vessels in a normal cornea. However, when a corneal ulcer or descemetocele occurs, the body will promote healing by forming a new blood supply across the surface of the cornea in a process called neovascularization. The new vessels begin at the sclera (the white part of the eye) and grow toward the ulcer.
Neovascularization will speed up the healing process. After the ulcer is healed, these vessels may remain in the cornea; if they are extensive, they obstruct vision. The size of the blood vessels may be reduced by applying a topical corticosteroid for a few days to several weeks after the ulcer has fully healed. Corticosteroid-containing eye medications are NEVER used until an ulcer has completely healed because they will slow or stop the healing and may cause the ulcer to worsen. Your veterinarian will repeat the fluorescein dye test BEFORE prescribing any corticosteroid preparation.
If corticosteroids are used, and pain occurs in the eye again, discontinue the steroids and have the eye rechecked immediately.