Vision Testing In Horses
- Apr 1, 1997
Have you noticed any changes in your vision? Yes, in fact I'm starting to have trouble reading the street signs while driving. Well then, sit back in the chair and relax. I'm going to dim the lights and have you read the letters while looking through the machine--which letter is sharper? This one or this one? This one or this one? This one or this one...Sound familiar? For those of us who wear glasses, this is a routine visit to the optometrist.
Have you noticed any changes in your vision? Yes, in fact I'm starting to have trouble seeing the jumps--especially at dusk. I also have been bumping into the pasture fence more that usual. Well then, stand there and relax. I'm going to dim the lights and have you read the images while looking through the machine--which carrot is sharper? This one or this one? This one or this one? This one or this one? Unfortunately this conversation will never happen.
We have worked with a horse which has just completely freaked out over some really stupid thing on the ground or in the barn that has been there for years. What is it about today that caused the tire on the ground or brush on the ledge of a grooming stall to trigger the release of every inner demon of your horse's mind, simultaneously? Could it be the beginning of a vision problem? Most likely not, but it never hurts to rule it out.
Theories and controversy regarding just exactly how a horse visually perceives its world have existed for more than a century. The main questions asked and remaining ones to be definitively answered are:
- Does the horse bring a visual image into focus using the "focusing" system of the lens, as people do, or via movement of the head?
- Does a horse see in color?
- Do horses suffer from visual defects such as nearsightedness (myopia) or farsightedness (hyperopia) that might affect performance?
Whatever the answers to these questions, normal vision is extremely important to the horse's daily existence. As stated by the late James Law, Professor of Veterinary Science, Cornell University, in a 1923 edition of a U.S. Dept. of Agriculture special report on Diseases of the Horse: "We can scarcely overestimate the value of sound eyes in the horse, and hence all the diseases and injuries which seriously interfere with vision are matters of extreme gravity and apprehension...as a mere matter of beauty, a sound, full, clear, intelligent eye is something which must always add a high value to our equine friends and servants."
The structure and function of the eye has been described in a previous article ("The Equine Eye," The Horse of July 1996). To briefly review here, the essence of the visual system is to collect light and focus it onto the retina in the back of the eye, where the image is then transmitted to the brain via the nervous system. To the point where the light, and hence the image, is focused on the retina, the eye functions very much like a camera lens. In people and domestic animals, the lens (the soft structure that sits behind the pupil) can have its shape changed by small muscles; it is this change in shape that brings an image into focus on the retina. If the light image is not in focus, then the image perceived by the brain is not in focus--time for glasses or contacts!
The controversy of the "ramped retina" started in 1930. It is known that, relative to other species, the horse's lens does not have a great ability to change shape and therefore might not be capable of being the sole mechanism of focusing light on the retina. It was proposed in 1930, 1942, and 1960 by independent researchers that the retina and hence the back of the horse's eye is not perfectly round, but in fact is sloped or "ramped." In short, this means that the horse might be able to "find" a location on the retina where the light is in focus by raising or lowering its head.
In 1975, one study failed to support the previous findings, but a study in 1977 supported the idea of the horse having to move its head or eye, or both, for optimal visual acuity. It is generally believed that the horse must elevate its head to "focus" on objects close to him, and conversely lower his head to focus on objects
Currently, vision testing in horses is not an exact science. The main goal of the ophthalmic examination is to identify abnormalities of the eye and speculate on how they could affect the vision based on known structure and function of the eye. Obviously, if there is a large, dense scar on the cornea from a prior corneal infection or a very dense cataract (opacity of the lens), vision will be affected. The difficulty is trying to ascertain to what degree vision is affected when smaller lesions are found.
The Eye Exam
The examination starts with historical information. Is the horse head-shy in absence of behavior modification techniques, i.e., getting smacked on the muzzle for biting frequently? Does the horse have a "sidedness" to the head-shyness, i.e., does he only react when someone or something is on a particular side? Does the horse have similar behavior when turned out in pasture, free in the stall, or just when being worked? Has there ever been any head trauma--hit his head on the trailer; kicked in the head; hit his head in the starting gait; flipped over on the cross-ties?
The next step is to stand back and look at the horse. Is there anything abnormal about the appearance of the eyes--are they symmetrical? Does the horse have any scars or fresh wounds only on one side of the head that might indicate frequently bumping into things on that side? If there are positive historical or general examination findings, an index of suspicion for a visual deficit might be raised.
This first test performed to ascertain if the horse can see is called the "menace." This test is just that. A menacing gesture is made toward the horse's eye with the hand to see if the horse will blink. This must be done carefully so that: 1) the other eye cannot see the movement as both eyelids will blink; and 2) the motion of the hand does not touch the lashes or create a strong enough current of air to be felt--both of which will make the horse blink. Obviously if the horse is totally blind or has significant visual impairment, he will not blink. If there is a partial "blind-spot," it cannot be well assessed in this manner.
Before getting into the specific ocular examination, it is important to understand the anatomy of the eye. The outer surface of the eye is called the cornea. The cornea is very thin (approximately a half-millimeter thick) and should be clear. Behind the cornea is the space known as the anterior (front) chamber. This is the space that exists between the inner surface of the cornea and surface of the iris and lens and is filled with fluid called the aqueous humor. The iris is typically brown tissue encircling the black pupil. This is the tissue that constricts or dilates the pupil depending on the light intensity.
The lens is a soft structure that sits in the pupil. It is normally clear--the blackness of the pupil is caused by the dark inner eye. Unless, of course, it is red on a photograph from the light of a camera flash (people) or bright yellow-green from a car's headlights (many animals) illuminating the inner eye.
From the posterior (back) surface of the lens comes a clear, jelly-like substance called the vitreus, then the retina on the posterior part of the eye.
The retina is a thin, translucent layer that transmits light information to the brain via the optic nerve that enters the back of the eye--this can be seen in the back of the eye as a small white dot (the optic disk).
The two common instruments used in assessing the horse's eye are a penlight or other strong focal light source and an ophthalmoscope. The eye is evaluated using a systematic approach looking for corneal scars or abnormalities in the anterior chamber, iris, or lens.
Corneal scars are the result of some prior corneal infection or trauma. Depending on the cause, they can vary in size from pinhead to the majority of the cornea. If they are small, it is likely that they don't affect vision significantly and, as one would assume, the larger they are the greater the chance of having a negative effect on vision.
On rare occasions, a cyst on the iris can be observed hanging down over the pupil like a small punching bag. If large enough, these iris cysts can obstruct the pupil and actually move in the aqueous humor, which could create a moving "shadow" or visual deficit to which the horse might adversely react. After suffering from inflammation in the anterior chamber (infection, uveitis), the iris in some horses can form an adhesion to the surface of the lens. These horses would be unable to dilate their pupil in low light environments, or the adhesion could be significant enough to obstruct the pupil. These horses can suffer a significant visual deficit if the adhesions are severe enough.
Cataracts are rare in adult horses and usually are secondary to a prior ocular trauma or chronic uveitis, but can obstruct the flow of light through the eye and affect vision. The larger and more dense the cataract, the greater chance it will negatively affect vision.
The ophthalmoscope is used to evaluate the fundus (back of the eye). It provides a way to look back there and 15 times magnification to help with the evaluation.
The health of the optic nerve can be evaluated and the remainder of the fundus evaluated for scars. Eyes that have suffered from uveitis or severe ocular inflammation from other causes can develop scar tissue and therefore dysfunctional areas of retina in the fundus. These scarred areas can be related to "blind-spots" in vision, and the larger the scars, the greater the chance of a significant effect on vision.
Another more unusual and infrequent finding in the fundus is a "vitreal floater." Sometimes after serious inflammation in the back of the eye, the vitreous humor becomes more liquid and small collections of inflammatory debris can actually "float" around. It has been hypothesized that these, if large enough, could float by, disrupting the visual field, when a horse moves its head, thus causing him to spook. It has been observed, but the true relationship between the floater and the behavior is unknown.
What if the eye inspects normal, but is blind? This is where history might be helpful. In some cases of head trauma, the eye is not overtly damaged, but the optic nerve might have been damaged. It can take several weeks or longer for the damage to become apparent during a fundic examination; it is often said that this is a disease where the patient sees nothing and the doctor sees nothing. Other non-ocular causes for blindness would also be evaluated.
Could a horse need glasses?
There was a report from the American Museum of Natural History in 1961 indicating that some degree of domesticated horses are nearsighted (myopic). Myopia is when the light and therefore the image comes into focus before the central part of the retina; the image the brain perceives as determined by the retina is blurred. But, if the horse does have a "ramped-retina," he might be able to just raise or lower his head until the image is in focus. This, of course, is speculation, and how the image is truly perceived by the horse's brain as he gazes out on the horizon of a summer pasture will most likely remain a mystery to humans for eternity.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
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