Understanding Equine Vision and Eye Disease
By Lynne Sandmeyer DVM, DVSc, DACVO
Professor of Ophthalmology
Department of Small Animal Clinical Sciences
WCVM, University of Saskatchewan
The equine eye is a complex and elegantly designed organ that functions to allow capture of light and conversion of light into an electrical stimulus, which is then transmitted to the brain and interpreted into vision. This function is reflected in the structure of the eye.
Anatomy of the Equine Eye
- Photo Courtesy of Lynne Sandmeyer, DVM
The globe (eyeball) has three layers or ‘tunics’: an outer fibrous tissue layer which is composed of the sclera (the whites of the eye) and the cornea (the clear dome in the front); a vascular tissue layer (called the uvea) which is made up of the iris (the coloured part of the eye), the ciliary body (behind the iris), and the choroid (which sits underneath the retina); and an inner layer of nervous tissue which makes up the retina.
The lens is a disc-like structure suspended by ligaments attached to the ciliary body situated behind the iris. The pupil is the hole in the centre of the iris through which light passes. In front of the lens there is a space filled with clear water-like fluid called aqueous humor. This fluid bathes the lens and the inside of the cornea, bringing oxygen and nutrients to these structures and removing their wastes. Behind the lens there is a large space filled with a clear gel-like fluid called vitreous humor which helps hold the retina in position and acts as a shock absorber for the inside of the eye.
On the outside of the globe, there are various protective structures. The conjunctiva is a layer of tissue that covers the sclera and lines the inside of the eyelids and the third eyelid. The eyelids cover the eye in front and the third eyelid is present in the ventral, nasal region.
The orbit encasing the globe is made up mostly of bone. The eye is held in the bony orbit by muscles that attach to the sclera. These muscles are responsible for moving the eye. There is a large lacrimal gland in the upper temporal region of the orbit and a smaller one on the inside of the third eyelid which produce watery tears to keep the surface of the eye moist.
Light enters the eye by passing though the clear structures of the cornea, aqueous humor, lens, and vitreous humor. These structures are refractive, meaning they bend the light such that it is focused onto the retina. Light is passed through the retina where it is converted to an electrical impulse. This electrical impulse leaves the eye in the optic nerve which extends from the retina, through an opening in the skull, and into the brain, where it is interpreted into vision.
Lateral placement of the eyes results in 65 degrees of binocular overlap and a monocular field of 146 degrees. Blind spots exist directly behind the head, in the forehead region, and directly under the body to about four feet in front of the legs.
Photo courtesy of Lynne Sandmeyer, DVM
Horse vision is particularly adapted for peripheral motion detection and low light conditions. This is likely due to the evolution of the horse as a prey species and the need for constant monitoring of the environment while grazing. The horse has very large eyes and a horizontally elongated pupil which allows for maximum light capture.
I’m keeping my eye on you, baby! The horse’s blind spots exist directly behind the head, between the eyes, and directly under the body to about four feet in front of the legs.
Photo: Robin Duncan Photography
The lateral position of the eyes allows for a large visual field of about 325º, this reduces the binocular visual field to about 65º. Because binocular overlap is required for depth perception, the latter ability is also restricted somewhat in the horse. By comparison, the human binocular visual field is about 120º. Binocular overlap in the horse is oriented down the nose, so horses are known to rotate their noses upward to better view distant objects. Blind spots exist directly behind the head, between the eyes, and directly under the body to about four feet in front of the legs.
Visual acuity in the horse is difficult to determine but is estimated to be about 20/30 on the Snellen Chart, or 0.6 times that of humans. This is actually better than many common domestic species. Vision in the dark is enhanced by a reflective layer called the tapetum and a pupil which dilates widely to allow maximum light entry. The horse retina has a 20:1 ratio of rod photoreceptors (responsible for vision in the dark) to cone photoreceptors (responsible for colour vision and visual acuity). Therefore, colour vision is poorly developed in the horse. While people are trichromats (seeing three basic colours), horses are dichromats (seeing two basic colours). The horse sees green/yellow and blue/gray but does not see red. It is also thought that the colours seen by the horse may be “washed out” compared to the vibrancy that humans would see.
Vision Problems in Horses
Vision problems are common in horses and have many causes. Vision disturbances may be more easily detected if both eyes are involved than if only one eye is affected. Ocular diseases often have an impact on the performance of a horse. Signs of a vision problem include clumsy behaviour and self-trauma; horses may be resistant to move from one place to another, and this may be especially prominent when moving from a lighted area to a dark one; they may shy away or spook easily and occasionally have head shaking behaviours. There may also be a change in herd behaviour; horses are herd animals that use visual cues to establish dominance order within a group. Animals with vision problems may be bullied by more dominant individuals.
Horses rotate their noses upward to better view distant objects.
Photo: Robin Duncan Photography
Causes of visual problems are many and can be separated into those that impair the ability of light to enter the eye (opacity of the cornea, the fluid in the anterior chamber or vitreous chamber, or cataracts in the lens), inability to convert the light impulse into an electrical signal (diseases of the retina), inability to transmit that signal to the brain (optic nerve disease), and inability of the brain to process the signal (diseases of the visual cortex).
Diagnosis of a vision problem begins with a complete eye examination. In some cases referral to a veterinary eye specialist (ophthalmologist) is necessary. Ophthalmologists have expertise in the diagnosis and treatment of eye diseases. If the cause of the visual disturbance is not evident on examination alone, an electrical test of retinal function (electroretinogram or ERG) may be required. Refraction to look for near-or far-sightedness is also done if necessary.
Management of a visually impaired horse requires special care. The horse must be provided with a safe environment. This requires removal of any hazards, such as sharp objects, low-hanging tree limbs, debris, and holes in the ground. Barbed wire and electric fences should be avoided. When placed in new enclosures, demonstration of boundaries is important. Visually impaired horses can be dangerous as they may be fearful and easily spooked, especially early on. Signs should be posted so that people are aware of their condition. Handlers should be cautious to avoid sudden movements or noises and speak to them calmly when near them. Consistency in care and handling the animal is important, for example, keeping food and water in a consistent location and using consistent verbal commands in training. Choosing appropriate turn-out companions is also important. A calm, sighted horse can be good company and even act as a guide-horse for the one visually impaired. Horses will adapt to visual impairment with time and patience. Some excellent resources for information on management of visually impaired horses include websites such as www.BlindHorses.org and a textbook chapter by A. Dwyer; Practical management of blind horses. In: Gilger B. (ed.) Equine Ophthalmology.
Common Ocular Diseases in the Horse
Ocular disease is common in the horse and may manifest as a vision problem or with redness, tearing, squinting, or cloudiness of the eye. These may be signs of serious eye disease and should never be ignored. Some of the more common conditions diagnosed in the horse are described below.
Corneal ulcers: Corneal ulcers are one of the most common acquired ocular diseases in the horse. A corneal ulcer is a break in the surface layer of the cornea. Ulcers usually develop secondary to trauma, often from plant material; for example, tree branches or straw scratching the eye. Signs of a corneal ulcer include redness of the eye, tearing, squinting, opacities in the cornea, and roughened or irregular areas on the corneal surface. Veterinarians diagnose corneal ulcers using fluorescein staining.
Corneal ulcers may be simple or complex. Simple ulcers are acute, superficial breaks in the surface layer of the cornea. These are treated with antibiotic drops to prevent infection and often heal uneventfully. However, an ulcer can become infected rather rapidly by the bacteria and fungi that normally live around the eye and infection can cause the ulcer to enlarge, deepen, and even perforate through the cornea. Infected ulcers require more specific and frequent antibiotic/antifungal medications as well as anti-inflammatory treatment. Many require surgery to repair the corneal defect such as conjunctival or corneal grafting procedures. In some cases, infection or perforation of the cornea may even result in loss of the eye. Early detection and appropriate therapy of a corneal ulcer will reduce the chances of a serious complication such as loss of the eye.
This simple, superficial corneal ulcer has been stained with fluorescein dye to highlight its location. Courtesy of Lynne Sandmeyer, DVM
A complex corneal ulcer that is deep and infected with bacteria. Note the crater-like appearance of the ulcer, which is discoloured white by infiltrating inflammatory cells. The cornea has developed ingrowth of blood vessels due to the ulcer being present for several days. Courtesy of Lynne Sandmeyer, DVM
A complex corneal ulcer that is infected with fungus. Note the white discharge around the eye and the white corneal discolouration present due to infiltration of inflammatory cells. The eye is also very painful. Courtesy of Lynne Sandmeyer, DVM
A healing conjunctival pedical graft surgically placed three weeks previously to heal a deep and infected corneal ulcer. The graft is incorporating into the cornea and the animal is comfortable. The long pedicle portion (arrow) is not attached but serves as a blood supply to the healing tissue. It can be trimmed off after two months of healing time. Courtesy of Lynne Sandmeyer, DVM
Uveitis: Uveitis is inflammation of the uveal tissue inside the eye. The clinical signs of uveitis include redness of the eye, squinting, and tearing, as well as cloudiness of the cornea or ocular fluid, a small or constricted pupil, and iris colour changes (e.g. yellowing of a normally blue iris or darkening of a brown iris). Uveitis is treated with topical and oral anti-inflammatory medications and drops to dilate the pupil.
There are many causes of uveitis. Equine recurrent uveitis (ERU) is uveitis that reappears over and over with new episodes triggered by the immune system. In some horses, rather than recurring episodes of overt inflammation, there is ongoing, low-grade inflammation that is never resolved. With this ongoing or recurring inflammation, permanent damage is done to structures inside the eye. This includes the iris becoming stuck down to the lens, cataract formation in the lens, corneal scarring, retinal detachments, retinal scarring, and retinal and optic nerve degeneration. Eventually, these changes lead to permanent blindness.
ERU is the most common cause of blindness in the horse and it is a painful condition. There is no cure for ERU but an attempt is made to minimize damage to the inside of the eye with prompt and aggressive treatment to control the inflammation. It is important that episodes of inflammation be detected early so appropriate treatment can be applied and the consequences of inflammation may be reduced. Because uveitis is a painful condition, any permanently blind and inflamed eyes should be removed surgically. Alternatively, a specialized surgery to remove the ocular contents and replace them with a silicone ball (intrascleral prosthesis), can be completed by an ophthalmologist.
Horse eye with early uveitis. The iris of this horse is normally blue. Inflammation has caused a temporary colour change to yellow. The conjunctiva is red, the pupil is small and irregular as adhesions of the pupil are starting to form to the lens. Blood vessels have grown into the cornea in response to the intraocular inflammation. The eye is very uncomfortable. Courtesy of Lynne Sandmeyer, DVM
The results of chronic or recurrent uveitis in a horse eye. A cataract has formed in the lens, giving the pupil a white discolouration. The iris has become adherent to the lens in several places, giving the normally horizontal pupil an irregular appearance. Courtesy of Lynne Sandmeyer, DVM
After removal of her left eye, this mare is faithfully negotiating an eventing course. Photo: Robin Duncan Photography
Glaucoma: This is a condition in which the fluid in the eye cannot drain properly and builds up causing increased pressure inside the eye. This leads to permanent damage to the retina and optic nerve and eventual blindness. Glaucoma may occur on its own, but is most commonly a secondary effect of chronic or recurrent uveitis in the horse. Some of the most common clinical signs of glaucoma include redness, tearing, a cloudy cornea, a dilated or enlarged pupil, an enlarged globe, and vision loss.
Glaucoma is treated with pressure lowering eye drops, as well as anti-inflammatory medications. Laser surgery to destroy the cells that make the fluid inside the eye may also be effective in controlling the pressure, but may only work temporarily. Eventually, most eyes with glaucoma become blind. Because glaucoma is a painful condition, a blind glaucomatous eye should be removed surgically an intrascleral prosthesis placement can be completed by an ophthalmologist.
Cataracts: Cataracts are opacities within the lens. They may be small and cause very little visual disturbance, or they may involve more of the lens and cause blindness. The clinical signs of cataracts include a white lens or white discoloration in the pupil opening. Cataracts may be inherited or occur secondary to trauma, or chronic inflammation (such as a consequence of ERU). Some foals are born with cataracts (congenital cataracts) and these may be inherited but may also occur related to maternal or environmental influences (maternal fever, poor nutrition, toxin exposure during pregnancy etc.).
When they are large, cataracts will also cause inflammation inside the eye (a form of uveitis called lens-induced uveitis). Inflammation associated with cataracts should be treated with anti-inflammatory medications. Cataract surgery can be performed on horses and there are specialized prosthetic lens implants that can be placed at the time of surgery to help restore vision. This surgery is complex and only performed by veterinary ophthalmologists. There are also many potential complications of the surgery which should be discussed with the ophthalmologist, but it can be very rewarding when successful.
Squamous cell carcinoma: This is the most common cancer of the equine eye region. It occurs commonly on the third eyelid, conjunctiva, cornea, eyelids and may spread to involve the orbit. Older horses, those with white skin around the eyelids, and those exposed to ultraviolet light are most at risk. The tumour may appear as a pink, raised, roughened mass. It may also appear as erosive sores when involving the eyelids. It is common for this tumour to spread into local tissues, and it may spread to distant sites as well. Treatment depends on the location and size of the tumour. Small tumours involving the third eyelid, conjunctiva and cornea are the most easy to treat successfully with local surgical removal. The eye can usually be spared if these are treated early on. The eyelid and orbital squamous cell carcinomas have a lower success rate, and often require removal of the eye and surrounding tissue.
Corneal squamous cell carcinoma in a horse. The tumour is pink, cobble-stone like in texture. Tumours such as this one can be successfully removed surgically saving the eye and vision.
Photos courtesy of Lynne Sandmeyer, DVM
Below: A horse following enucleation surgery (surgery to remove the eye). A silicone ball orbital prosthesis was placed to reduce sinking in of the tissues during healing.
Glaucoma in a horse eye resulting in generalized corneal cloudiness due to edema. The eye is red and painful. Glaucoma is the result of increased pressure within the eye.
Cataract in the lens of a horse. This cataract is generalized and causing vision loss.
Husbandry and Management to Improve Ocular Health
Most blinding and painful eye diseases in horses are acquired and there are some practical measures that can be taken which may reduce the chances of eye injury or minimize eye inflammation. In general, reduction of ocular irritation from dust, insects, and U.V. light, as well as, decreased ocular trauma are extremely important. Maintenance of general health is also essential.
Trauma is one of the most common causes of serious eye disease in the horse, therefore, the most important recommendation is to decrease the chances of ocular trauma. This includes eliminating sharp objects from the environment whenever possible. Removal of low tree branches, exposed nails and hooks, and thorny plants is recommended. Use of a fly mask may also be beneficial to reduce trauma, as well as exposure to dust and insects which may insight inflammation around the eye. Reduction in sun exposure by providing adequate shelter during peak U.V. times and through a protective fly mask may reduce sunburn, and will help protect against periocular squamous cell carcinoma development. Optimum general health maintenance is also very important for the best possible healing capabilities and immune-system function; proper vaccination and deworming programs, proper nutrition, hoof and dental care are essential.
In addition to vision loss, it is very common for serious ocular disease to manifest with some tearing, squinting, and redness of the eye. Cloudiness of the ocular surface or inside the eye also indicates a problem. Lumps or bumps around or on the eye may also be a sign of serious ocular disease. Signs of ocular pain may include squinting, rubbing the eye, becoming head-shy, and decreased activity. Many eye conditions can be successfully managed if treatment is started early. Delaying treatment may lead to permanent vision loss or loss of the eye.
It is important to seek veterinary attention if you suspect anything is wrong with the eye. Veterinary ophthalmologists are specialists in diagnosing and treating ocular disease. There are several ophthalmologists in Canada and in the USA. You can locate an ophthalmologist by visiting the website of the American College of Veterinary Ophthalmologists www.acvo.org.
This article originally appeared in the October 2013 issue of Canadian Horse Journal.