Corneal Ulcers in Horses
Dr. Melissa Mazan
The cornea is composed of multiple layers of cells that have the important quality of being transparent. The outermost layer of cells is called the epithelium, the inner layers are called the stroma, and the innermost layers are called Descemet's membrane and the endothelium. Tearing is the eye's way of ridding the eye of any foreign body – in a way, it flushes out the eye. It is due to stimulation and inflammation of the tear ducts.
Although the cornea has no blood vessels, it has many nerves – as anyone who has had a speck of dust in his eye can attest. This very extensive innervation is important – it reminds us to blink our eyes to prevent injury and to keep the tear layer over the cornea – but it causes intense pain with even the most minor of eye injuries.
The endothelium has no ability to heal, so it is very important to prevent ulcers from penetrating through that layer. Most ulcers are the result of trauma – most frequently due to a foreign body, such as a piece of hay or other small debris. However, many ulcers can quickly become seeded with bacteria or fungi, which makes treatment more difficult.
Bacteria that produce destructive enzymes, such as Pseudomonas (a gram negative bacterium) and Streptococcal species are most likely to cause melting ulcers. The cornea will actually start to look as though it is melting – it will have the appearance of wax dripping down the side of a candle. The destructive enzymes are usually collagenases, or enzymes that destroy collagen connective tissue.
Corneal ulcers are most difficult to treat when they are down to Descemet's membrane, have a fungal component (mycotic ulceration), are indolent, or are in an immune-compromised horse or a horse that has been treated with corticosteroids.
A descemetocele refers to a corneal ulcer that is so deep that it extends to Descemet's membrane – the single layer of cells that forms the very innermost layer of the cornea. This single layer is much thinner than the thinnest hair – and is all that lies between the anterior chamber of the eye and the outside world. Once that inner membrane – Descemet's membrane – is ruptured, the contents of the inner eye will begin to leak out, and the inside of the eye will rapidly become infected. At this point, it is very difficult to repair the damaged, delicate cornea, and a surgical repair will be necessary. Whereas simple, uncomplicated corneal ulcers usually heal well with a minimum of treatment, some ulcers become deep, chronic, or simply non-responsive.
Corneal ulcers most frequently become infected with bacteria in warm, wet conditions or in eyes that have been treated with corticosteroids, which decreases the eye's ability to fight off infection, or with long-term or multiple antibiotics. Antibiotic treatment can lead to fungal infection because it may kill off all the bacteria that naturally compete with the fungi.
Fungal infections are usually seen several days to weeks after initial ulceration, and may appear as a sudden worsening of an ulcer that had apparently been getting better. They often appear to be very deep, and often have a whitish to yellowish plaque of accompanying material – this is actually cellular debris and the fungi themselves. The fungi have a preference for Descemet's membrane, which contributes to the ulcers becoming very deep and extensive.
Fungal ulcerations must be treated with anti-fungal agent – most of which are not approved for eyes, but are useful nonetheless. Anti-fungal treatment can be very expensive. When ulcers have a fungal infection they can take months of treatment in order to save the eye.
Surgical treatment is often necessary with mycotic ulcers.
An indolent ulcer is one that does not heal properly despite appropriate treatment. Indolent ulcers are more common in older horses, horses with equine Cushing's disease (ECD), and horses that have been on corticosteroids treatment.
Equine Cushing's disease makes indolent ulceration more likely because it increases the body's own production of corticosteroids, thus causing immunosuppression and poor wound healing. Indolent ulcers often do not have the acute, painful appearance of an infected ulcer. In fact, the vessels that are so critical for ulcer healing often fail to appear.
With indolent ulcers, the cornea produces the cells that are necessary to heal the ulcer, but these ulcers lack the ability to stick to the underlying membrane, and merely slide off. It is often necessary to perform a grid keratectomy or a conjunctival flap in order to encourage healing.
What to Watch For
The typically cloudy eye is actually corneal edema. This refers to fluid being retained between the layers of cells in the cornea, and tells us that the cornea is not functioning properly.
The swollen eyelid is often the result of the horse rubbing his eye in an attempt to relieve himself of the pain. Unfortunately, the result is often that the horse does even further damage to his eye.
The reddened eye reflects both inflammation and formation of new blood vessels. With deep, extensive, infected ulcers, the eye will not be able to heal until blood vessels grow into the cornea and can help to heal the cornea as well as helping to deliver antibiotics and anti-inflammatory drugs. These vessels only grow one millimeter each day in the best of situations – consequently the healing process is limited by the growth of the blood vessels. The blood vessels are a double-edged sword, however – although they help to heal the eye, they also interfere with vision. In treating corneal ulcers, we are often torn between welcoming the appearance of blood vessels and wanting to limit the excessive growth of blood vessels so that minimal disruption to sight occurs.