Download What is Wrong With My Horse`s Eye?

What is Wrong With My Horse’s Eye?
Bruce P. Whittle, DVM
Eye problems are common in horses. Because a horse only has two eyes,
damage to one of them can be very serious. Although there are many conditions
that can affect the equine eye, the two most common disorders are corneal
ulceration and uveitis. Other disorders such as lacerations of the eyelids and
neoplasia can also have negative effects on the eye or vision and require prompt
treatment. In order to better understand the seriousness of these conditions, it is
important to have a basic knowledge of the anatomy of the eye.
The globe of the eye (aka eyeball) is generally spherical in shape. It is
bordered by the front transparent portion called the cornea and the rear whitecolored portion called the sclera. The cornea is
clear because it is dehydrated, despite the fact that
it is surrounded on the outside by the tear film and
on the inside by the aqueous fluid in the front
chamber of the eye. It becomes cloudy if damage
occurs either on its outer or inner surfaces that
allows fluid to enter the dehydrated interior of the
cornea. Also of interest is the fact that the cornea
is less than 1 millimeter thick! Our goal when
treating the eye is to keep the cornea clear and
intact if at all possible. Behind the cornea is the
Normal horse's eye
anterior chamber of the eye. It is filled with
aqueous fluid that helps maintain the shape of the globe as long as the pressure of
the fluid is within a normal
range. The back “wall” of the
anterior chamber is the iris, the
typically
brown
colored
structure with a black hole in it.
The black hole is the pupil. It
lets light into the back portion of
the eye. The amount of light
allowed to pass is regulated by
the diameter of the pupil that is
in turn regulated by contraction
or relaxation of the iris. Just
behind the pupil is the lens. The
lens helps to focus the light
entering the eye. This focused
light then interacts with the retina. The retina, which lines the back interior portion
of the eye, converts the light signals received by the eye into electrical signals that
are then transmitted up the optic nerve to be interpreted by the brain. The eye is
undoubtedly an incredible structure that we often take for granted.
Horses’ eyes can have a multitude of problems. They can become infected,
inflamed, neoplastic, or traumatized. It is important that you understand what your
horse’s eyes normally look like. This makes it is easier to recognize when something
is wrong. The cornea should be clear, the eyelids should conform nicely to the globe,
and the area beneath the eye should be dry. You should be able to readily see the
entire iris and pupil and the iris should typically be the same color throughout
although some horses such as Paints can have a multicolor iris that is normal. The
eyelids should also be symmetrically open on both eyes – squinting can be a sign of
ocular pain. Owners will often notice a brown blob hanging down from the top of the
pupil. This is a normal structure called the corpora nigrans. While we cannot
visually examine the back portion of the eye that consists of the vitreous and retina,
we can suspect eye problems in an otherwise normal appearing eye if the horse
suddenly begins to stumble or appears blind. Any appearance of the eye that is
abnormal should prompt an examination by your veterinarian. The longer an eye
problem is allowed to go untreated, the less likely it can be treated successfully.
While there are numerous things that can happen with the horse’s eyes,
some conditions are much more common than others. One of the most common
problems of the equine eye is cloudiness of the
cornea. This usually indicates that there has
been damage to either the inside or outside
layer of the cornea, allowing fluid to enter the
normally dehydrated middle section of the
cornea called the stroma. Your veterinarian
will apply a stain called fluorescein to the eye
to determine if the damage to the cornea is
external or internal. Fluorescein is a special
stain that sticks to the corneal stroma but not
Flourescein staining ulcer (note green
to the corneal epithelium (outer layer of the
area just below pupil)
cornea). If the stain does stick to the cornea as
in the picture to the right, an ulceration of the cornea is present. This can lead to
rupture of the eye and possibly blindness if not treated appropriately. Corneal
ulcerations are typically caused by trauma, bacteria, fungi, or a combination of these
factors. If disease of the surface of the cornea is suspected, your veterinarian may
collect samples for either cytology (examination of the cells present under a
microscope)
or
culture
(laboratory
examination looking for bacteria or fungi).
Treatment with a proper medication is very
important and is best based upon laboratory
testing to determine the most likely cause of
the disease.
If, on the other hand, the corneal
epithelium is still intact, stain will not stick to
it. In this case, the cloudiness of the cornea is
typically due to disruption of the inner layer of
the eye, the corneal endothelium, indicating inflammation within the eye. This is a
very serious condition called uveitis that can lead to blindness if not properly
treated. This condition is sometimes referred to as “moonblindness”. Your
veterinarian will look at the eye carefully with a bright light during the examination
to look for any abnormalities and to make sure the iris is constricting (closing) and
dilating (opening) in response to varying intensity of light. Sometimes, especially if
the eye is cloudy and the interior of the eye cannot be visually examined, your
veterinarian will use ultrasound to evaluate the internal structures of the eye.
Differentiating uveitis from corneal ulceration is important because the medications
used to treat one of these conditions can make the other condition worse.
Unfortunately, sometimes a horse can concurrently have both uveitis AND corneal
ulceration making treatment decisions more complicated. Uveitis is generally
treated with some type of anti-inflammatory agent, an agent such as atropine that
dilates the pupil, and topical antibiotics.
Lacerations to the eyelid, if not treated
appropriately, can lead to long term eye
problems because the eyelids play an
important role in holding the tear film
against the cornea.
Lacerations cannot
always be sutured but, as with any wound,
the best chance of success is within the first 8
hours after injury. Permanent disruption of
the eyelids can lead to a problem called “dry
eye” due to constant leakage of the tear film.
The other fairly common cause of eye
problems is neoplasia or cancer. Early treatment
of neoplasia of or around the eye is critical
because many cancers of the eye of the horse
spread readily.
There are many possible
treatments for neoplasia of the eye or eyelids
including surgical removal of the mass, topical
chemotherapeutic agents, freezing or cryotherapy
and laser excision. Sometimes, due to extensive
spread of cancer cells, the only treatment is an enucleation or surgical removal of
the affected eyeball and surrounding tissues.
Eye problems in horses can be very debilitating. At the first sign of eye
problems, you should call your veterinarian because an early diagnosis and an
appropriate treatment can be critical. You should not try treating an eye on your
own with ocular medications you might have around from another case without a
diagnosis by a veterinarian as some medications can make certain conditions worse.
Damage to the eye, particularly in the case of uveitis, can be cumulative and lead to
blindness. Blindness in the horse can be job-ending as many equestrian disciplines
do not allow blind horses to compete. Because of the complexity of ophthalmic
problems, your veterinarian may recommend referral to a veterinarian who
specializes in ophthalmology and has advanced diagnostic equipment. You can help
your horse have a long career by keeping a close watch for eye problems and calling
your veterinarian as soon as a problem is identified.