What is the cornea?
The cornea is the transparent, shiny membrane that makes up the front of the eyeball. Think of it as a clear windowpane. To understand a corneal ulcer, you must first know how the cornea is constructed.
There are three layers in the cornea, all of which are composed of 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 tissue of the cornea. The deepest layer is Descemet’s membrane. All these layers are clear, so it is impossible to see them without special stains that color specific cells and highlight them when the tissue is examined under a microscope.
What is a corneal ulcer?
Erosion of a few layers of the epithelium is called corneal erosion or corneal abrasion. A corneal ulcer is deeper erosion through the entire epithelium and into the stroma. With a corneal ulcer, fluid accumulates in the stroma, giving a cloudy appearance to the eye as the cornea is no longer transparent. If the erosion goes through the epithelium and stroma to the deepest level of Descemet’s membrane, a descemetocele is formed — a serious condition. If Descemet’s membrane ruptures, the liquid inside the eyeball leaks out, the eye collapses, and irreparable damage occurs.
How does a corneal ulcer occur?
There are several causes of corneal ulcers in dogs. The most common cause is trauma. An ulcer may result from blunt trauma, such as a dog rubbing its eye on the carpet, or from a laceration, such as a cat scratch or contact with a sharp object like a stick.
Another common cause is chemical burn of the cornea. Such burns may happen when an irritating chemical or substance such as shampoo or drywall dust gets into the eye. Because of their anatomy, brachycephalic dogs such as pugs and bulldogs can be more prone to corneal ulcers.
Less common causes of corneal ulcers include bacterial infections, viral infections, and other diseases. These conditions may originate in the eye or may develop secondary to a disease elsewhere in the body. Examples of other diseases that may predispose a pet to corneal ulcers include:
- Epithelial dystrophy, which is a weakening of the cornea that can be inherited in some breeds, such as boxers
- Keratoconjunctivitis sicca (KCS or “dye eye”) in which decreased tear production leads to drying of the cornea
- Eyelid abnormalities such as entropion (inward curling of eyelid skin) or distichiasis (abnormal eyelashes rubbing on the cornea), which causes excessive irritation to the cornea
- Endocrine diseases, such as diabetes mellitus, Cushing’s disease (hyperadrenocorticism), and hypothyroidism
What are the clinical signs of a corneal ulcer?
A corneal ulcer is very painful. Most dogs rub the affected eye with a paw or on the carpet in an attempt to relieve this intense pain. To protect the eye, they 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?
Your veterinarian will look for clinical signs, which include squinting and rubbing the affected eye. The cornea may lose its transparency and appear cloudy, or tiny blood vessels may be seen crossing the surface of the cornea. The conjunctiva may appear red and swollen. Superficial corneal abrasions are generally only visible with special tests and equipment. Corneal ulcers are detected with the use of special stains such as fluorescein. A drop of this orange-colored stain is placed on the cornea. The dye will adhere to ulcerated areas and turn green.
"Corneal ulcers are detected with the use of special stains such as fluorescein."
Once stained, large ulcers are easily seen, while tiny ulcers may be observed using special ophthalmic lights and filters. A fluorescein stain test is the most common eye test performed and may be the only test needed if the ulcer is acute and very superficial. If the ulcer is chronic or very deep, samples may 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 present.
Corneal abrasions and superficial ulcers generally heal within three to five days. Medication is used to prevent bacterial infections (antibiotic eye drops or ointment) and to relieve pain (oral pain medications and/or atropine drops to relax the muscles inside the eye). Your dog must wear an Elizabethan collar (E-collar) to prevent them from rubbing the eye.
Antibiotic drops are only effective for a short time, 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, depending on your pet’s condition and acceptance of the medication.
Occasionally, superficial ulcers can be indolent, meaning slow to heal. This slow healing can be more prevalent in certain breeds, such as boxers and golden retrievers. Treatment involves removing any loose epithelium that is not sticking down, and possibly a minor surgical procedure called a keratotomy, where the surface of the ulcer bed is lightly scored to promote healing and adherence of the epithelium.
If a deeper corneal ulcer or a descemetocele is present, steps must be taken to protect the eye and to promote healing. Medical management includes frequent use of potent topical antibiotics and possibly oral antibiotics, pain medications, and medications to slow the destruction of the corneal stroma by bacteria.
One such treatment is serum; this is the liquid portion of blood that can be obtained from your own dog or another dog and administered as an eye drop. Serum contains healing nutrients and proteins to stop the breakdown of the stroma. Since dogs do not wear eye patches well, surgery may be required to protect the injury and allow for normal healing. In certain cases, it may be necessary to perform surgery to remove dead or poorly healing layers of corneal tissue (grid keratectomy), perform a corneal graft, or third eyelid flap. Your veterinarian will choose the best surgery to optimize healing for your dog.
Can a corneal abrasion turn into a corneal ulcer or descemetocele?
Yes, corneal abrasions and superficial ulcers can sometimes continue or worsen, despite treatment with broad-spectrum topical antibiotics. This progression can result in blindness, so your dog should be re-examined after two to three days of treatment to ensure that healing is progressing well. If healing is not progressing normally, or if a complication develops, your veterinarian will recommend additional medical treatment or surgery.
Are there any side effects from eye medications?
Occasionally, a dog will be sensitive to eye medication. If your dog seems to be in more pain after the medication, discontinue it and contact your veterinarian immediately.
A corneal ulcer is extremely painful, so the dog usually keeps the eye tightly shut. Atropine relieves the pain but also dilates the pupil widely, which means the eye is very sensitive to light and many dogs will squint or close the eye when exposed to bright light. The dilation of the eyes may last for several days after the drug is discontinued.
Eye medications can drain through the tear ducts and eventually get to the throat, where the dog can taste them. Atropine and many other eye medications have a very bitter taste, which may cause them to drool and paw at the mouth. You are seeing your dog’s response to a bad taste, not a drug reaction.
What else can I use to control the pain?
A topical anesthetic is often used to numb the cornea, so diagnostic tests may be performed. However, these drugs are only safe for one-time use, when used with caution, as they are toxic to the corneal tissues and may delay healing.
Dogs will benefit from the use of non-steroidal anti-inflammatory drugs (NSAIDs) and other oral pain medications. Atropine may be prescribed to relax the muscles inside the eye and dilate the pupil. Your veterinarian will discuss which medications are safe and appropriate for your pet.
How do I know when the treatment is completed?
The best way to determine whether the cornea has healed is to repeat the fluorescein stain test. Your veterinarian will perform this test every one to seven days after treatment has begun, depending on the severity of the ulcer and the progress of healing. Never discontinue treatment without your veterinarian’s approval.
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 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 of the ulcer. The new vessels remain on the cornea once the ulcer has healed; if they are extensive, they will obstruct vision, so it is best to minimize their size. After the ulcer has completely healed, your veterinarian may prescribe drops or ointment containing a corticosteroid to reduce the size of the blood vessels. These drops will be used for a few days to several weeks, depending on how much neovascularization is present.
It is important that corticosteroids (steroids) are not used in the eye too soon because they will slow or stop the healing process and may cause serious complications. Your veterinarian will repeat the fluorescein dye test before beginning this type of medication. If steroids are used and the eye becomes painful again, immediately discontinue the steroids and have your veterinarian check the eye again.
