The cornea is the clear, glistening membrane that makes up the surface of the eyeball.
There are three layers in the cornea, all of which are highly specialized skin cells. The outermost layer is the epithelium, which is a very thin layer of cells. Below the epithelium is the stroma, which is the main supportive layer of the cornea, and the deepest layer is Descemet's membrane. Because all of these layers are clear, it is not possible to see them without special stains that colour specific cells and highlight them when the tissue is examined under a microscope.
A penetration or erosion through a few layers of the outermost corneal epithelium is called a corneal erosion or corneal abrasion. A corneal ulcer is an erosion through the entire epithelium down into the stroma. With a corneal ulcer, fluid is absorbed from the tears into the stroma, giving a cloudy appearance to the eye. If the ulcer goes through the stroma to the level of Descemet's membrane, a descemetocele is formed. A descemetocele is a very serious 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 cause is trauma. An ulcer may result from blunt trauma, such as a cat rubbing its eye against a rough surface, or due to a laceration, such as a cat-claw scratch or contact with a sharp object such as a thorn. The second most common cause is chemical burn of the cornea. This may happen when irritating shampoos, other liquids, chemicals or dust such as drywall 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.
What are the clinical signs of corneal ulcers?
A corneal ulcer is very painful. In response to this intense pain, most cats will rub the affected eye with a paw or on the carpet or furniture. To protect the eye, they will squint, blink rapidly or keep the lids tightly closed. 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. Large ulcers are easily seen, while tiny ulcers may be enhanced by the use of a special ophthalmic lights and filters. This is the most basic test performed 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 for culture and cell study prior to applying the stain or any other medication.
How is a corneal ulcer treated?
Corneal abrasions generally heal within three to five days. Medication is used to prevent bacterial infections and to relieve spasms or pain. Antibiotic drops are only effective for a short time (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.
If a deep corneal ulcer or a descemetocele is present, steps must be taken to protect the eye and to promote healing. Since cats do not wear eye patches well, surgery may be required to protect the injury! Often, the surgeon will 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, preventing normal healing; if this occurs, surgical debridement and/or a grid keratectomy (removal of the dead and dying tissue) will be necessary so that healing can occur. Your veterinarian will choose the most appropriate surgical technique or combination of techniques for your cat.
Can a corneal abrasion progress to become an ulcer or a descemetocele?
Yes. In spite of treatment, the erosion or ulceration can sometimes continue or worsen. Since this can happen and could result in permanent loss of vision.
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