Horses are very reliant on their eyes for sight and depth perception. Anything that affects their eyes has the potential to significantly impact their ability to survive in the wild. For domestic horses, eye issues can affect performance as well as day-to-day life.
Without two healthy eyes, horses’ vision – a key player in their flight or fight response – can be seriously compromised, particularly if they need to jump when running away from a predator. A horse requires two eyes to judge depth and jump effectively; with only one eye and no rider on board to help them with depth perception, there is an increased risk of injury when they jump.
Show jumping stallion Cornado NRW, ridden by Germany’s Marcus Ehning, was already competing successfully at the elite 1.60m level when he suffered an injury that ultimately resulted in the loss of an eye at the end of 2018. By March 2019, the stallion was back competing at the Dutch Masters in Sweden, with Marcus Ehning saying, “It’s like nothing changed for him.” Image by Michelle Terlato Photography.
Interestingly, when it comes to domestic life there are several examples where one-eye horses have remained reliable ridden mounts – with some even competing at the highest levels of showjumping.
The fast speed at which horses can move, the sudden turning of their heads and the prominent position of their eyes on their skull make them very susceptible to eye injury, especially when hit with debris or foreign matter, leading to ulceration.
An ulcer occurs when there is a loss of part of the cornea, the tissue that forms the clear dome-shaped outer structure over the front of the eye. The equine cornea is approximately 1mm thick and consists of three layers – an outer epithelium, a middle stroma and an inner endothelium. The outer epithelial layer is generally the layer that is affected with superficial ulceration caused by fine debris, small foreign bodies or low-grade impact events. These ulcers are very thin, typically around one tenth of a millimetre or less, and heal very quickly provided they are not contaminated with debris or become infected with bacteria or fungus.
This image shows a large ulcer in the cornea that has not been stained and is visible to the naked eye. Note the degree of tearing and the partially closed eyelid. Image by Dr Maxine Brain.
“Never ignore an ulcer,
no matter how small it
may appear….”
Ulcers that go deeper than the epithelium and enter the stroma are often more painful and take longer to heal. They are also more likely than superficial epithelial ulcers to become infected, due to the longer duration the stromal tissue is exposed compared to the epithelial ulcers.
When corneal ulcers extend down to the endothelium, the risk of eye rupture becomes incredibly close. A very thin dark membrane known as Descemet’s membrane lies immediately below the endothelium and separates the endothelium from the front, fluid-filled chamber of the eye called the anterior chamber. When the endothelium is breached, this membrane tends to plug the hole, giving the ulcer an appearance of a black spot in the middle of the defect.
If the corneal ulcer becomes full thickness and extends through into the anterior chamber, the eye effectively ruptures, allowing fluid to escape from the eye and potentially causing the loss of the eye. In these situations, the coloured part of the eye, known as the iris, can prolapse out through the hole and “plug” the defect, preventing the eye from collapsing, saving some vision but negatively affecting the ability of the pupil to function.
PARTIAL CLOSURE OF EYELIDS
The first thing most owners notice when their horse has an eye ulcer is partial closure of the eyelids, known as blepharospasm. This can vary from a mild drooping of the upper lid to full closure of the eye, depending on the degree of pain and the severity of the ulcer.
This is a deeper ulcer that has damaged the endothelial layer, causing oedema in the stromal layer. Image by Dr Maxine Brain.
Small ulcers and scratches on the surface of the cornea are more likely to cause only partial blepharospasm, which can be missed much more easily than the fully closed eye, especially if care is not taken by the owner to look closely at the horse. The best way to detect if there is blepharospasm in the eye is to assess the eyelashes and compare their orientation relative to each other. In a normal eye, the eyelashes are orientated close to horizontally, but as the eyelids start to close with pain, they become progressively more vertically aligned – so comparing one eye to the other is a good way of monitoring if there is pain in the eye.
Severe ulcers tend to cause more tearing than superficial ulcers; however, tearing can also indicate the presence of a foreign body in the eye, so a superficial ulcer with a lot of tearing should be checked carefully for a possible foreign body, especially one hidden behind the third eyelid. Infected ulcers can also present with a lot of tearing.
Uveitis, which is an inflammation of the middle vascular layer that lies between the sclera and the retina of the eye, commonly occurs secondary to corneal ulceration. Because of this uveitis, the pupil becomes very constricted, causing further pain. Accessing the interior of the eye for signs of uveits can help with the diagnosis of a suspect ulcer.
This is a smaller ulcer but has secondary uveitis; note the constricted pupil. Image by Dr Maxine Brain.
Clouding of the cornea, or corneal oedema, is caused by damage to either the cells of the epithelium or endothelium that are responsible for regulating the fluid balance in the cornea by pumping out excess fluid into the anterior chamber. When the endothelium is damaged, excess fluid remains within the cornea, causing a blue effect on the eye. Normally, the epithelium forms a barrier to prevent the stroma from absorbing fluid from the liquid tear film over the front of the eye. When the epithelium is damaged, there can often be a small area of oedema around the ulcer, but the intensity and size of the oedema is usually much smaller than seen with endothelium damage.
INTOLERANCE TO BRIGHT LIGHT
Intolerance to bright light is another clinical sign that can be seen with corneal ulceration, although normally not the primary sign. It is evident when a bright light – generally a torch or phone light – is shone into the eye to assess whether an injury is visible. When the light source is shone directly into the affected eye, the horse typically squints and pulls its head back to get away from the stimulus. This squinting can also be seen when the horse is housed outside in the sunlight, which is often the reason why horses suffering from a corneal ulcer usually wear an eye patch or fly mask and are moved to a stable.
Diagnosing an eye ulcer is mostly a routine procedure with some ulcers able to be seen clearly with the naked eye and others requiring a fluorescein dye applied to the eye to highlight the defect. In some circumstances, particularly when dealing with difficult or very painful patients, sedation is required and the top eyelid is nerve-blocked.
This means local anaesthesia is applied to the nerve responsible for movement of the upper lid to prevent the horse closing the eye and allows the vet to fully examine the cornea and look for the presence of any foreign body or structure that has contributed to the formation of an ulcer. Topical anaesthesia may be required if more invasive procedures need to be performed on the eye, such as looking behind the third eyelid or collecting a sample for cytology.
Fluorescein dye is used to highlight a defect in the cornea as the dye adheres to the underlying stroma. This is especially helpful when the ulcer is as small as a pin prick, as these become very easy to see once the dye adheres. The dye can help in identifying other aspects of the ulcer such as the presence of an exposed Descemet’s membrane (the dye forms a halo around a black area) and the presence of an ulcer where the endothelium has lifted off the stroma, forming what is called an indolent ulcer.
Fluorescein dye is used to diagnose a corneal ulcer. Image by Dr Maxine Brain.
For many ulcers, this is as far as the diagnostic procedure goes before treatment is commenced. However, when ulcers are not responding to treatment or have been referred to a specialist, the ulcer may be scraped to identify the cells and the presence of bacteria or fungi hyphae. This is done with the intention of tailoring the medication to a potential infection as opposed to using a broad-spectrum medication to cover a wide range of infectious agents which commonly occur initially.
TREATMENT ON THREE FRONTS
Treatment for ulcers is usually directed at three key issues: reducing pain, healing the ulcer and preventing the ulcer becoming infected; or if already infected, treating the infection. To reduce pain, the horse is often given a non-steroidal anti-inflammatory medication such as finadyne or phenylbutazone. The more painful ulcers require twice daily dosing of anti-inflammatory medication.
Part of the pain associated with eye ulcers is due to secondary uveitis and the tightly constricted pupil, so treatment often includes the frequent topical use of atropine eye medication applied directly onto the eye. This acts to dilate the pupil and therefore relieve some of the pain. In very severe cases of uveitis, atropine is required every few hours to achieve and maintain full dilation, whereas in normal eyes, a single dose of atropine in the eye can cause a pupil to dilate for a week or more.
Monitoring pupil dilation between doses of atropine can be useful at allowing the veterinarian to see how the inflammation within the eye is improving, with further doses only administered when the pupil is noted to be constricting.
Small ulcers can repair very quickly with the epithelium of the cornea able to slide and adhere over a day or so, healing the ulcer and protecting the underlying stroma. For larger ulcers, healing based solely on medical treatment is not always possible, and surgery is required.
There are several types of surgical grafts that can be performed to treat ulcers. One treatment is to take part of the mucosa from the inside conjunctiva (soft tissue under the eyelid) and sew it over the defect, still leaving one end attached to the tissue around the eye in a procedure known as a conjunctival graft. After several weeks, the graft is removed, leaving a small part of the mucosal tissue on the corneal.
This forms a scar on the cornea but preserves the function of the eye, with the main impediment being the area of scar tissue as the horse will not see through the scar, instead seeing a dark spot in its vision corresponding to the size of the scar.
There are other types of grafts that can be used such as Acell, and these products have their advantages and disadvantages over using the horse’s own tissue. The type of graft used is determined by the size and location of the ulcer.
Sometimes the eyelids are sutured together in a procedure known as a temporary tarsorrhaphy. This gives the eye some protection and allows growth factors and natural defence proteins in the horse’s body to bathe the eye and help facilitate healing. This type of surgery is usually used for horses when the owners are under financial constraints and can’t afford to seek specialist ophthalmic treatment.
Perhaps the most vital aspect of veterinary intervention is the prevention or treatment of infection in the eye. Eye ulcers can become infected with bacteria or fungi, with fungal infections occurring more commonly in chronic ulcers (ulcers that persist for a couple of weeks).
In the initial days of an ulcer, veterinarians usually use a broad-spectrum antibiotic ointment to cover against a variety of bacteria. The specific cream can vary and will often be determined by the veterinarian’s previous experiences and preferences. More complicated ulcers generally require more specifically targeted antibiotics, with some antibiotics better able to penetrate through the cornea and others specifically aimed at a certain bacterium. As stated earlier, in some cases of ulceration a scraping of the eye ulcer is taken and examined and/or cultured to identify the offending bacterium.
A large corneal ulcer, with fluorescein dye highlighting the size. Image by Dr Maxine Brain.
FUNGAL INFECTIONS
The longer an ulcer persists, there is an increased likelihood of a fungal infection being involved. Fungal infections are particularly likely to occur in cases where antibacterial eye creams have been used for an extended time because the normal eye flora, that would generally repel fungal elements in normal healthy eyes, is severely compromised and, in some cases, eliminated, leaving fungi available to flourish. Fungal hyphae require specific antifungal preparations that need to be incorporated into an ointment that allows the medication to penetrate through the corneal to kill the hyphae. Treatment normally needs to continue for several weeks before the fungi are eliminated, and the eye can be repaired.
An ulcer that fails to heal or continues to recur should be carefully assessed for signs of a foreign body or a persistent cause that has not been identified or eliminated in the early stages of the injury. This commonly requires a veterinary ophthalmologist to intervene, as they have access to more specialised equipment able to magnify the eye and lids to a size that greatly aids in identifying sources of irritation.
One example of this is the presence of an ectopic cilia or eyelash that grows in an unusual position inside the eye lid and rubs on the cornea. Initially, the cornea may only be irritated but as the cilia grows and persistently rubs the cornea, an ulcer forms. These ulcers look simple and uncomplicated at first but either fail to heal or continue to recur. The specialist detects these cilia, which are very difficult to see with the human eye alone, using powerful magnification and removes them surgically to allow the ulcer to heal.
Due to the pain elicited with corneal ulceration, many horses become intolerant of frequent application of medication into their eye, and it becomes necessary to surgically implant a tube that enables medication to be topically administered. Referred to as a subpalpebral lavage tube, this is a long-specialised silicone tube that has a disc (foot plate) at one end and a port at the other end that allows eye medications to be administered.
Usually under sedation and local anaesthesia, the tube is inserted through the conjunctiva of the eye, leaving the foot plate to sit deep in the conjunctival fornix or in front of the third eyelid so that no contact between the foot plate and the cornea can occur. The long part of the tube exits through the skin and is anchored through the mane so that the end port is fixed closer towards the wither. Medication is administered through the port near the wither and exits into the space around the eye, allowing frequent dosing to occur daily without anyone touching the eye. These tubes are removed once the eye has recovered.
POSSIBLE COMPLICATIONS
Prognosis for most eye ulcers is good but there are several complications that can occur. Sometimes a specific bacterium called pseudomonas infects the ulcer and causes the eye to “melt”. This becomes a medical emergency, as it is possible for the eye to rupture in a very short time if not treated promptly. Stromal abscesses can form in some cases because the epithelial surface closes and traps either fungi or bacteria under the surface, and they continue to grow. These require specific eye medications that are capable of being absorbed through the cornea, or surgery to scrape off the top layer of the cornea to enable the medication to reach its target.
Indolent ulcers occur when the ulcer doesn’t heal properly and the epithelium doesn’t adhere onto the underlying stromal layer. These normally present as an ulcer that might improve initially but continue to smolder away without getting worse or better and veterinary intervention is required to remove the loose epithelium, enabling healing to occur.
Ulcers can be simple or life-changing to the horse, but the most important take-home message here should be never ignore an ulcer, no matter how small it may appear. Speedy intervention and close monitoring can be the difference between a few days of treatment or an unfortunate loss of sight. EQ
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