If you have ever scratched the surface of your own eye, you know how excruciatingly painful it can be. If your dog is squinting, rubbing its eye, or has a tearing ocular discharge, please call our doctors at the Animal Hospital at Thorndale so we can quickly determine the cause of the problem and begin treatment immediately. Corneal scratches and ulcers hurt!
What is a Corneal Ulcer?
The cornea is the clear, shiny membrane that makes up the surface of the eyeball. It is much like a clear window. To understand a corneal ulcer, you must first understand how the cornea is constructed.
The cornea is comprised of three 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, 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 and a microscope.
An erosion or scratch thorough a few layers of the outer surface epithelium is called a corneal erosion or a corneal abrasion. This is very superficial. A corneal ulcer is an erosion through the entire epithelium and into the stroma, or second deepest layer. If the erosion goes through the epithelium and stroma to the level of Descemet’s membrane, a descemetocele exists. If Descemet’s membrane ruptures, the liquid inside the eyeball leaks out and the eye collapses.
Causes
There are several causes for corneal ulcers in dogs. The most common is trauma. An ulcer may result from blunt trauma, such as a dog rubbing its eye on carpet, or due to a laceration, such as a cat scratch. The second most common cause is chemical burn of the cornea. This may happen when irritating shampoo or dip gets in the eye.
Less common causes of corneal ulcers include bacterial infections, viral infections, and other diseases. These may originate in the eye or develop secondary to disease elsewhere in the body. Examples of other diseases include Epithelial Dystrophy (a softening of the cornea which is inherited in breeds such as the Boxer), Keratoconjunctivitis Sicca (drying of the cornea due to abnormal tear formation), and diseases of the endocrine system (diabetes mellitus, hyperadrenocorticism, and hypothyroidism).
Clinical Signs
A corneal ulcer is very painful. In response to pain, most dogs rub the affected eye with a foot or on the carpet. To protect the eye, they keep the lids tightly closed. The third eyelid, or nictitans, will move upward from the inner corner of the eye, usually seen as a pink or brown membrane moving across the eye. Occasionally, there will be a discharge that collects in the corner of the eye or runs down the face.
Diagnosis
Superficial corneal abrasions are usually not visible. They can be visualized with the use of fluorescein stain. A drop of this 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. This is the most basic test performed and may be the only test needed if the ulcer is acute 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
Treatment depends on whether there is a corneal abrasion, corneal ulcer, or descemetocele 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. An Elizabethan collar is often required to keep the dog from further rubbing or scratching the eye.
If a corneal ulcer or descemetocele is present, measures must be taken to protect the eye and to promote healing. Corneal abrasions and ulcers that do not respond within a week of medical management may require debridement and a grid keratectomy. Ulcers that do not heal well often have a buildup of dead cells at the ulcer edge. These dead cells prevent normal cells from the corneal surface from sliding over the ulcer edge and filling in the defect. If this appears to be part of the healing problem, the dead cells are removed (debrided) from the edges of the ulcer. In some cases, removing the dead cells may be all that is needed to start the healing process. A grid keratectomy is a procedure by which a new healing bed on the cornea can be stimulated to grow. Since dogs do not wear eye patches well, a contact lens or surgical techniques used to close the eyelid may be needed to cover the ulcer or descemetocele. These measures protect the eye for 10-14 days, then are reversed so the dog can use the eye again. If your dog’s corneal ulcer is not healing as expected, we are fortunate to have board certified veterinary ophthalmologists a short drive away from our Downingtown, PA, hospital.
Side-effects of Eye Medications
Rarely, a dog will be allergic to an antibiotic that is instilled in the eye. If your dog seems more painful after the medication is used, discontinue it and contact the veterinarian.
A dog with a corneal ulcer has quite a bit of pain in 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 dog 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 eye, the same prolonged pupillary dilation will occur.
Drooling After Treatment
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 drooling and pawing at the mouth. You are seeing your dog’s response to a bad taste, not a drug reaction.
Use of Topical or Oral Anesthetics to Control Pain
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. Oral pain relievers, such as Tramadol, are often prescribed to help relieve the pain and are very effective in making your dog more comfortable.
Conclusion of Treatment
The best way to tell that the cornea has healed is to repeat the fluorescein stain test. This should be done after about 5-7 days of treatment and again in 1 week if needed.
Red Streaks Near the Ulcer
The normal cornea has no blood vessels going through it. However, when a corneal ulcer or descemetocele 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.
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.
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.
Other Causes of Eye Pain
Some corneal ulcers and descemetoceles can coexist with an infection inside the eye. Eyes can be painful from causes other than abrasions and corneal ulcers. Glaucoma and infection in the chamber of the eye in front of the lens(anterior uveitis) are fairly common. Tumors and abscesses inside and behind the eye are less common. Tooth root abscesses can cause draining tracts just below the eye. Inability to produce tears properly (Keratitis sicca) sets eye up for drying out, corneal erosions and eye infections. Some systemic diseases, such a Lyme disease, can first appear as an eye problem. Allergic reactions can result in a sudden swelling of the lids. All eye problems should be addressed by a veterinarian as soon as possible so that permanent damage to vision does not occur.