Equine Corneal Ulcers are a relatively common eye condition and are usually treatable rapidly and successfully. They do, however, vary greatly in severity and due to their potential severity, and the difficulty in distinguishing them from other eye conditions without veterinary attention, should never be neglected or underestimated.
What Is A Corneal Ulcer?
The horse’s cornea consists of 4 main layers; from front to back these are:
- Corneal Epithelium – this is the outer surface, consisting of several layers of flattened cells
- Corneal Stroma – this is the bulk of the cornea, consisting mainly of collagen fibres. The fibres are all aligned in the same direction, which is what creates the optical clarity
- Descemet’s Membrane – This is a thin layer of protein that attaches the stroma to the endothelium
- Endothelium – A single layer of cells that forms the inside of the cornea. Importantly, they pump water out of the cornea to keep it dehydrated, as any water within the stroma would render it opaque.
A corneal ulcer occurs when the corneal epithelium is disrupted to the extent that corneal stroma is exposed. A shallow ulcer involves just disruption of epithelial cells, while a deep ulcer would involve significant loss of stroma as well. Extremely deep ulcers can entirely remove corneal epithelium and corneal stroma, exposing Descemet’s Membrane. This is known as a descemetocoele, and is an extremely serious condition.
What Causes Corneal Ulcers?
In the majority of cases there is no obvious cause of the ulceration, and it is assumed that some form of trauma initiated it. Less commonly ulcers can be caused by a foreign body stuck to the inside of either the eyelids or the nictitating membrane (3rd eyelid). These cases often have a characteristic linear ulcer where the foreign body is continuously swept across the cornea. Corneal ulcers can also be seen secondary to a problem with blinking, most often due to facial nerve paralysis, although this is extremely uncommon.
What Signs Will I See?
The signs of a corneal ulcer are very similar to the signs seen with any other form of ocular pain, namely:
- Blepharospasm – a closed eyelid
- Photophobia – avoiding light
- Increased Lacrimation – more tears being produced
- Miosis – a closed pupil
If your horse’s eye is open enough, you may also be able to see a small cloudy area on the cornea.
Diagnosing Corneal Ulcers
Your vet will carry out a comprehensive examination of the eye (click the link to read more). The vital stage is application of fluoroscein dye. This dye does not stain a corneal epithelium, but will stain corneal stroma. If the epithelium has been damaged and an ulcer created, the stroma will be exposed and become stained by the dye, as in the picture.
The large green stained area is obvious to see in this horse’s eye, which has an extensive but relatively shallow ulcer. Around the rim of the ulcer we can see a slightly paler staining area. This is where epithelium is still intact, but the dye has diffused through the stroma underneath the epithelium.
In the case of a descemetocoele, described above, then a similar staining pattern will be seen. However, there will be a central dark area that does not pick up dye. This is because Descemet’s membrane will not stain.
What Are The Consequences?
The surface of the horse’s eye normally has a population of “friendly” bacteria that live within the conjunctival sacs. They cause no problems here, and the corneal epithelium prevents them from establishing infection. If the protective barrier of the epithelium is breached, as with an ulcer, then infection can be set up rapidly with serious consequences. These bacteria release, and induce the horse to release, certain enzymes that break down protein (Matrix MetalloProteinases or MMPs) into its constituent amino acids. The corneal stroma is made up primarily of collagen protein, and these enzymes can rapidly destroy the integrity of the stroma, and the cornea itself, eventually leading to rupture of the eye. This is known as a melting corneal ulcer. The activation of these MMPs, and the effects on the corneal stroma must be stopped before this occurs.
The formation of a corneal ulcer also creates significant inflammation. Inflammation is a primarily vascular process, and due to the lack of blood vessels in the normal cornea does not occur here, but “spills over” into nearby structures. Toward the outside of the eye, this is seen as conjunctivitis, with reddening and swelling of the conjunctiva. This can be seen in the above photo. Unfortunately, this inflammation also travels into the eye, and is seen as inflammation of the iris, ciliary body and choroid, together known as uveitis. This is an extremely painful process, and is usually seen as a constricted pupil. In severe cases inflammatory cells and proteins can be seen floating in the front of the eye. Not only is this extremely painful, but unless treated can cause permanent iris adhesions, with permanent visual problems.
Corneal ulcers can progress rapidly, so early and aggressive treatment is always indicated even for seemingly minor ulcers. The primary targets for our treatment are:
- Reduce the amount of ocular pain
- Prevent and/or treat infection of the ulcer
- Prevent and/or reverse proteolytic enzyme activation
- Reduce inflammation both internally and externally
The treatments commonly used are:
- Systemic anti-inflammatory and painkiller
- This has beneficial effects for aims 1 and 4
- Flunixin (Finadyne) has the greatest effect on the eye, so is most commonly used
- Topical Antibiotic
- Achieving aim 2
- Topical antibiotic cream or drops may be used. Creams have poorer penetration but do not need to be applied so often.
- Oral or injectable antibiotics are rarely used as they will not reach sufficient concentration at the target site
- Topical Atropine
- Helps with aim 4
- Atropine is a drug known as a parasympatholytic, meaning it has effects we would typically associate with a “fight or flight” response. In a fight/flight situation the pupil would normally dilate, to allow for greatest vision, and so atropine dilates the pupil.
- Pupil dilation is important to relieve pain, and to prevent iris adhesions forming
- Overuse can be associated with impaction colic, so it is used only as needed to keep the pupil dilated. Faecal output should be closely monitored while treatment is ongoing.
- Topical Anti-Protease Treatment
- Aim 3
- There are several options to achieve this, including plasma extracted from your horse’s blood.
- In severe cases, several different anti-protease treatments may need to be used.
Medical treatment may need to be extremely frequent initially, in the most severe cases on an hourly basis. When treating a horse’s eye this frequently they can soon become resentful and head shy, so the placement of a Sub-Palpebral-Lavage-System (SPLS) may be needed. This allows reliable delivery of treatments into the horse’s eye from a remote site, usually a small tube attached to the mane. These catheters are inserted through the eyelid under standing sedation, after desensitisation of the eyelid with local anaesthetic.
In the most severe cases of ulceration, and especially with melting ulcers and descemetocoeles, then surgical treatment may be necessary. The latest surgical treatments include conjunctival pedicle grafts, where a piece of conjunctiva is used to cover the ulcer, and repair of the defect using small pieces of amniotic membrane. These are specialist procedures carried out at very few practices in the UK, and we are fortunate to have one of these centres close by at Liverpool University’s Leahurst Campus. Previously 3rd eyelid flaps have been performed, where the 3rd eyelid is sutured across the ulcer to protect it, and to provide blood supply. These are rarely performed now, as it does not allow the cornea to be examined, and newer techniques are far more successful.
In the initial stages of managing a corneal ulcer, your vet will want to re-examine your horse’s eye regularly to ensure that treatment is working and no complications are occuring. This is vital, as any complications must be dealt with swiftly to ensure a positive outcome.
Uncomplicated corneal ulcers should heal quickly, and leave minor scarring at worst. New corneal epithelium will migrate inwards from the edge of the ulcer at a rate of approximately 1mm/day. A 10mm diameter ulcer should therefore heal in 5 days (as migration will occur from all edges). If healing has not occurred within the expected time frame, then there may be a problem. Potential reasons for slow healing are numerous, and include old age, immunosuppression (e.g. with Cushing’s Disease) and infection.
This pony was treated with topical Chloramphenicol ointment (an antibiotic), topical EDTA (an anti-protease), topical atropine (to dilate the pupil) and systemic Finadyne paste (an anti-inflammatory and painkiller). Just 36 hours later the size of the ulcer has reduced drastically. We can also see how widely dilated the pupil is in the right photo. This pony made a full recovery with no corneal scarring.