Look Into His Eyes
It is important to approach each eye problem in the horse in an ordered and systematic manner, and also as a medical emergency. Painful eye conditions in horses need thorough evaluation for corneal ulcers, corneal abscesses, and uveal inflammatory diseases (such as uveitis or moon blindness). The majority of cases can be diagnosed by using standard ophthalmic clinical examination techniques.
Obtaining a complete history on a horse is important before performing the ophthalmic (eye) examination. Your veterinarian will want to know the environment of the horse, whether there has been intermittent or constant ocular pain, the length of time clinical signs were noted, and any history of previous systemic or ophthalmic disease. If some form of visual disability is suspected, it helps the veterinarian to know the rate of progression of such signs, and how the horse performs under different lighting conditions (is he worse at dusk or in bright light?). Knowledge of prior therapy is important to successful veterinary treatment.
To be able to perform a proper ophthalmic examination in a horse, it is necessary for your veterinarian to have a bright focal light source such as a transilluminator or a direct ophthalmoscope. Other equipment he/she might use is listed in "Diagnostic Equipment and Drugs" on page 62.
The head is examined for eye symmetry, globe (eyeball) size, globe movement and position, presence and quality of ocular (eye) discharge, and blepharospasm (squinting or closing of the eyelid in response to eye pain). The general appearance of the eyes and related structures are noted.
It can be useful to examine the angle created by the eyelashes of the upper lid and the cornea. In normal horses, the eyelashes usually are almost perpendicular to the corneal surface. Droopiness of the lashes of the upper lid such that the lashes point down might indicate the first sign of a painful eye in a horse (see image below right).
Making a sudden, threatening hand motion toward the eye to cause a blink response and/or a movement of the head tests the menace response. This is a crude test of vision. Care must be taken not to create air currents toward the eye when performing this test. Adult horses and older foals have a very sensitive menace response.
The horse should also quickly squint or "dazzle" when a bright light is abruptly shown close to the eye.
Eyelid reflex is tested by gently touching the eyelids and observing the blink response.
Vision can be further assessed with maze testing using blinkers alternatively covering each eye. The maze test should be done under dim light conditions, and consists of watching the horse walk around a course of barriers while on a lead or in a large stall.
The pupillary light reflex (direct and indirect) evaluates the integrity of the retina, optic nerve, midbrain, oculomotor nerve, and iris sphincter muscle. The normal equine pupil responds somewhat sluggishly and incompletely unless the stimulating light is particularly bright. In the horse, stimulation of one eye results in the constriction of both pupils. The pupillary light reflex is valuable in testing potential retinal function in eyes with severe corneal opacities.
Intravenous sedation, a nose or ear twitch, and nerve blocks might be necessary to facilitate the eye examination by your veterinarian. There are several nerve blocks that can be used by your veterinarian to help in the examination. Topical anesthetics also are used to reduce corneal and conjunctival sensitivity.
In his armament of eye testing devices, your veterinarian has a simple tool to determine if your horse is producing enough tears. The Schirmer tear test is used for chronic ulcers and eyes in which the cornea appears dry. This test must be done prior to use of any medications in the eye. The test strip is folded and inserted under the lower eyelid. The strip is removed after one minute and the length of the moist end measured. Strips are frequently saturated in horses after one minute as values up to 30 mm of wetting/minute would be considered normal. Values less than 10 mm of wetting/minute are diagnostic for a tear deficiency state.
Cultures of the cornea also can be used to diagnose the causative problem in an eye infection. Corneal cultures should be obtained prior to placing any topical medications in the eye. For this test, a swab is gently touched to the corneal ulcer for sampling.
Sometimes veterinarians use corneal scrapings to obtain cytology (cell) specimens for detecting bacteria and fungal elements. These can be obtained at the edge and base of a corneal lesion using topical anesthesia on the eye and the handle end of a sterile scalpel blade (not the sharp end) to touch the lesion. Superficial swabbing cannot be expected to yield the organisms in a high percentage of cases.
The cornea should be clear, smooth and shiny. Placing fluorescein dye in the eye to identify corneal ulcers should be routine in every eye examination of the horse. An ulcer stains bright green with fluorescein (see photo below) and rinses off the cornea if there is no ulcer. This might help your veterinarian detect small corneal ulcers that might otherwise go unnoticed. Rose bengal dye should be used after fluorescein dye in some cases to identify the integrity of the precorneal tear film. Rose bengal stains a dark, bright magenta if the tear film is abnormal.
Tears drain inside the horse's head into the horse's nasal cavity, and sometimes there is a problem with that drainage system. Your veterinarian might need to determine if there is a blockage or problem with this nasolacrimal system, using irrigation or fluorescein dye.
The anterior chamber of the eye is best examined with a handheld or transilluminator mounted slitlamp. The anterior chamber contains optically clear aqueous humor. Increased protein levels in the anterior chamber can be noted clinically as cloudiness or aqueous flare. White blood cells in the anterior chamber are called hypopyon, and red blood cells in the anterior chamber are called hyphema. Aqueous flare, hypopyon, and hyphema all indicate uveitis (inflammation in the eye).
The intraocular pressure can be measured in horses just as it can in humans. Normal pressure in a horse is 16-30 mm Hg (millimeters of mercury), measured with a Tonopen applanation tonometer. (Normal human pressure is under 20 mm Hg.) High pressure means glaucoma, while low pressure means uveitis.
The lens of the eye should be checked for position, any opacities, or a cataract. There are a number of lens opacities that are regarded as normal in the horse, including prominent lens sutures (the area where the thousands of lens fibers join), the point of attachment of the hyaloid vessel, refractive concentric rings, fine "dustlike" opacities, and sparse "vacuoles" within the lens substance.
Normal aging will result in cloudiness of the lens nucleus (nuclear sclerosis) beginning at seven to eight years of age, but this is not a true cataract. The suture lines and the lens capsule can also become slightly opaque as a normal feature of aging.
Cataracts are lens opacities associated with varying degrees of blindness. They can be congenital or secondary to a previous case of uveitis, and are labeled as progressive or non-progressive. In some horse breeds, cataracts are thought to be hereditary.
The vitreous (jellylike substance behind the lens) in the adult horse eye should be free of obvious opacities. Vitreal floaters can develop with age or be sequelae (results of) equine recurrent uveitis (ERU). They are generally benign in nature.
The retina and optic nerve are examined with a direct or indirect ophthalmoscope. Your veterinarian will set the rotary lens of the direct ophthalmoscope to 0 to examine the retina and optic nerve (further away), and to a "green" number 20 to focus on the lids and cornea.
The back of the eyeball (retina and optic nerve) should be examined for any signs of ERU, such as a loss of pigment from retinal inflammation. The retina and optic nerve will be carefully examined by your veterinarian with a direct ophthalmoscope.
The retina might also become loose or detached from its normal position from congenital or traumatic problems, or secondary to ERU. Retinal detachment is a major problem as it is associated with blindness. There is no therapy for retinal detachment in horses.
An electroretinogram might be useful in evaluating retinal function in horses and usually is only performed at referral centers.
B-scan ultrasound, computerized tomography (CT scanning), and magnetic resonance imaging (MRI) are important for evaluating intraocular and orbital lesions in the horse.
As you can tell from this brief description, there are many things your veterinarian can do toward diagnosing a horse's eye problem. Remember, any eye problem is considered an emergency, and you should contact your practitioner as soon as you notice any problems related to your horse's eye, or his vision.
TOOLS NEEDED FOR AN EXAM
Your veterinarian might use the following when examining your horse's eyes:
- Tropicamide 1% for dilation of the pupil
- Topical local anesthetic, scalpel blade, and glass slides for cytology
- Culture swabs
- Lidocaine for nerve blocks
- Fluorescein dye
- Rose bengal dye
- Schirmer tear strips*
- Curved tip (#412) multipurpose naso- lacrimal flush syringe
- Direct ophthalmoscope with cobalt blue filter
- Finhoff transilluminator with 14 D lens*
- Handheld slitlamp*
- Tonopen tonometer*
- Nasolacrimal lavage installation kit*
Brooks, D.E. "Equine Ophthalmology." Veterinary Ophthalmology, 3rd ed., Gelatt, K.N. (ed), Lippincott Williams and Wilkins: Philadelphia, 1053-1116, 1999.
Veterinary Ophthalmology 3(2/3): Equine Special Issue, 2000.
Veterinary Clinics North American: Equine Practice 8(3), 451-457, 1992.
Editor's note: This is the second article in a series of eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at www.TheHorse.com for more information on eye anatomy.
About the Author
Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.
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