By Dr. Becker
All other parts of the eye, including the iris and pupil, lay behind it.
The cornea is transparent and admits light into the eye.
It also serves as a barrier to protect the inside of the eye from bacteria, chemicals, foreign objects – anything that could cause damage and interfere with vision.
The cornea is actually composed of four layers.
The outermost layer is the epithelium.
Below the epithelium lays the basement membrane.
Next is the stroma, which comprises most of the thickness of the cornea.
The innermost layer is called Descemet's membrane.
Causes of Corneal Ulcers
Corneal ulcers are wounds to the cornea usually caused by an abrasion, scratch, puncture or other trauma to the eye.
Other causes can include a foreign body in the eye, a chemical burn, infection, lack of adequate tears, inability to completely close the eyelids, entropion (where the eyelid folds inward), disease, and facial nerve paralysis.
These ulcers, sometimes called ulcerative keratitis, are a common eye injury in dogs and cats.
They can cause a great deal of irritation and discomfort for your pet.
A corneal ulcer can involve one layer up to all four layers. A mild or superficial ulcer involves just the epithelium. Ulcers are considered severe or deep when they also impact the middle and innermost layers of the cornea.
Superficial ulcers result in loss of a part of the epithelium. Deeper ulcers involving the stroma can cause significant scarring and perforation of the cornea.
When an ulcer extends through the stroma to the Descemet's membrane, a condition known as Descemetoceles, the problem is quite serious and can result in perforation.
The location of a corneal ulcer depends to some extent on what is causing it. An ulcer caused by trauma to the eye, dry eyes, bulging eyes or a paralyzed facial nerve are most often located in the center of the cornea.
If there is foreign matter trapped beneath the third eyelid, the corneal ulcer is usually seen toward the inside of the eye near the nose.
If there is entropion or a wayward eyelash rubbing against the eye, the ulcer will often be seen in the peripheral cornea.
An immune-mediated disease of the eye can cause ulcers at the borders of the cornea.
Because sensitive nerves are exposed when the cornea is injured, you may notice your pet's eye running or tearing more than normal, and she may also squint or paw at her eye.
There can be sensitivity to light, a noticeable film over the eye, discharge, or a red, inflamed, painful appearance. Your pet may also try to keep the painful eye closed.
Corneal ulcers are more common in dogs, but cats develop them as well.
Certain pets – those with flat faces (brachycephalic breeds) – are more prone to corneal ulcers. These breeds include the Boxer, Boston terrier, Bulldog, Pekingese, Pug, Shih tzu, and any breed of dog or cat with a short, flat muzzle and prominent eyes.
Diagnosis involves a thorough examination of the eye and cornea, and direct observation of the ulcer using a diagnostic stain. The stain or dye will allow visualization of erosions, ulcers or other injuries to the cornea. It will also indicate how deep the ulcer is into the layers of the cornea.
Staining the eye also helps rule out other eye conditions with similar symptoms to corneal ulcers.
Other tests can include a dry eye test, analysis of facial nerve function, cultures to look for bacteria or fungi, and blood tests to check for the presence of viral infection.
Unless your pet's corneal ulcer is mild (involving only the outer layer) and not caused by an underlying condition – or unless your veterinarian is very skilled in treating eye conditions – I recommend you ask for a referral to a veterinary ophthalmologist3 or other eye specialist.
Treatment of a corneal ulcer will depend to some extent on what caused the injury. Underlying conditions like dry eye, and infection or disease must be resolved or treated along with the corneal injury to prevent recurrence of the ulcer.
Treatment can include topical antibiotic therapy, pain medication and drugs to control eye muscle spasms.
Superficial ulcers can usually be healed in a week or less. During treatment, your pet may be fitted with one of those annoying plastic neck collars to keep his paws away from his eyes.
Deeper, more serious ulcers may require sutures, conjunctival grafts, conjunctival flaps, the insertion of soft contact lenses, or even a corneal transplant.
Ineffective, counterproductive treatments include oral antibiotics. They can't be absorbed in sufficient concentration to effectively treat or prevent infections of the cornea.
Topical corticosteroids and anesthetics should also be avoided because they can prevent healing and often make the ulcer worse.
Melting ulcers are a type of corneal ulcer in which the stroma layer progressively dissolves.
This type of ulcer is most often seen in pets with an infection, either bacterial or fungal. The infection produces enzymes that break down the corneal stroma.
Complete loss of the stroma can occur in as little as 24 hours in these cases.
Treatment includes antibiotics, drugs to inhibit the action of destructive enzymes, and sometimes surgery.
Refractory Corneal Ulcers
Also known as indolent ulcers or Boxer ulcers, these are a type of superficial ulcer that doesn't heal well and tends to recur.
Refractory ulcers are thought to be caused by an abnormality in the basement membrane layer of the cornea. They weaken the epithelium surrounding them, which can be easily peeled back.
Refractory corneal ulcers are usually seen in middle aged or older dogs and tend to develop in both eyes at different times. They are seen in a wide variety of breeds.
These ulcers can take months to heal. Topical antibiotics and pain medications are given on an as-needed basis. Other topical treatments found to be useful include glycosaminoglycans, chondroitin sulfate, N-acetylcysteine, and Aminocaproic acid solutions that are made specifically to be put in the eye.
Loose epithelium must be removed (under topical anesthesia) to allow healing to progress.
Sometimes more invasive treatment is required, including a keratotomy, which involves cutting or piercing the cornea to promote attachment of new epithelium.