Lynne Sandmeyer DVM, DVSc, DACVO
Associate Professor Ophthalmology
University of Saskatchewan

Anatomy of the Eye
- Extraocular structures
• Orbit
• Eyelids
• Third eyelid
• Lacrimal gland
- Globe – 3 layers
• Fibrous outer layer: cornea (anterior clear portion) and sclera (posterior white portion)
• Vascular middle layer: uvea (iris, ciliary body, choroid)
• Sensory inner layer: retina, optic nerve
- Intraocular structures
• Chambers
* Anterior chamber – filled with watery aqueous humor
* Vitreous chamber – filled with gel-like vitreous
• Lens

Lynne Sandmeyer

Lynne Sandmeyer grew up in Calgary, Alberta and is an Associate Professor of Ophthalmology at WCVM in Saskatoon. She completed a Residency and Doctor of Veterinary Science in Ophthalmology at the Ontario Veterinary College and is board certified in Ophthalmology. Lynne's main research interests involve the investigation of inherited retinal diseases including Congenital Stationary Night Blindness in the Appaloosa horse.

The Basics of Vision
- Light enters and travels through the clear cornea, anterior chamber, lens, and vitreous.
• The cornea and lens function to bend (refract) the light so it is focused on the retina.
- Light strikes the retina where it is converted into an electrical signal by photoreceptors (rods and cones) – this is called transduction.
• Rods – specialize in night vision
• Cones – specialize in day and color vision
- The signal passes through other cells in the retina which process and modify it.
- The signal is then transmitted to the brain via the optic nerve.
- The brain processes the signal into a visual image.

What Does the Horse See?
- Horses have large eyes and a wide horizontal pupil which allows large amounts of light to enter.
- Horizontal pupil allows wider view of the horizon.
- Visual perspective varies with head position and breed (e.g.. Miniature horse versus Clydesdale).
- Lateral position of the eyes gives a panoramic view.
• Each eye can see about 180 degrees horizontally by itself.
• Binocular overlap of 65 degrees (directed down the nose and toward the ground, i.e.. where it would feed).
• Total visual field of about 325 degrees when head is pointed forward.
• Small blind spot directly behind the head and in forehead region.

- Peripheral vision is adapted to detect motion.
- The nose prevents the horse from seeing directly under the body and forward for about 4 feet in front of the legs.
- Visual acuity: 20/30 vision (Snellen Chart): horse has 0.6X the acuity of humans, 1.5x acuity of dogs, 3x acuity of cats .
- Better able to discriminate between objects on the ground versus 70 cm above ground.
- Accommodation (ability to focus on near objects by changing the shape of lens) is limited – horse changes head position to bring near objects into focus.
- Have stereopsis or depth perception.
- Good vision in dark - 9:1 ratio of rods (photoreceptors specializing in night vision) to cones (photoreceptors specializing in day and color vision).
- Reflective tapetum increases light capture ability in the dark.
- Color vision is poorly developed – can discriminate red and blue from gray, ability to discriminate green and yellow is variable.

Causes of Visual Disturbances
- Inability of light to enter the eye and strike the retina
• Opacification of clear structures
* Corneal infection, edema, scarring
* Cloudiness of the aqueous humor
* Lens opacities (cataracts)
* Cloudiness in the vitreous
- Inability to convert light into an electrical signal
• Disease of the retina, e.g. degeneration or detachment
- Inability to transmit signal to the brain
• Disease of optic nerve, chiasm, or radiations, e.g. inflammation degeneration, tumors
- Inability of brain to process the signal into a visual image
• Cerebral cortex inflammation, degeneration, tumors

Common Causes of Visual Disturbance in the Horse
Congenital Disease
Congenital cataracts
- The most common congenital eye disease in the horse
- Breed-related in: Appaloosa, Arabian, Morgan, Quarter Horse, American Saddlebred, Miniature horse, Shetland pony
- Known inherited in: Belgians and Thoroughbreds (dominant), in Rocky Mountain Horse (as part of the anterior segment dysgenesis syndrome)
- Clinical signs: white or cloudy pupil
- Treatment: Cataract surgery (phacoemulsification): 50-80% success rate. No prosthetic lenses commercially available yet for the horse.

Congenital stationary night blindness
- Appaloosa, (Maybe also in Quarter Horse, Standardbred, Paso Fino)
- Congenital and non-progressive
- Cannot see in the dark due to an abnormality in signal transmission from the rod photoreceptor.
- Foals may appear disoriented, stare off into space, seek light, have reluctance to enter darkened areas or difficulty riding in the evening.
- Normal on ocular examination
- Diagnosis is by electroretinography (ERG)
- No treatment

Acquired Disease

Equine recurrent uveitis
- The most common cause of blindness
- AKA: moon blindness, iridocyclitis, periodic ophthalmia
- Episodes of intraocular inflammation that occur periodically, interspersed with periods of quiescence
- Clinical signs of inflammation: redness, cloudiness, tearing, squinting, small pupil size
- Immune-mediated: immune system triggers inflammation
- Blindness results from: cataract development, adhesions of iris to lens, retinal detachment, retinal degeneration, optic nerve degeneration
- Often becomes bilateral
- Treatment: medical management of inflammatory episodes

Corneal disease
- Multiple causes: Corneal laceration, ulceration, abscessation
- Large prominent cornea prone to injury
- Treatment: medical and surgical therapies depending on cause and severity
- Blindness or visual disturbance occurs due to opacification and scarring of the cornea as well as secondary intraocular inflammation
- Size and position of the lesion is important: large central lesions worse than small peripheral ones

Glaucoma
- Elevation of pressure inside the eye due to reduce outflow of fluid
- Primary: heritable, bilateral potential, no overt ocular abnormality to account for the elevation in pressure
- Secondary: intraocular disease such as uveitis, lens luxation, neoplasia
- Reported in many breeds, Appaloosas have increased risk
- Clinical signs: dilated pupil, cloudy cornea, streaks/lines in cornea, enlarged eye
- Gradual onset of blindness due to optic nerve and retinal degeneration
- Treatment in sighted eyes: medical management, surgical – laser cyclophotocoagulation or filtration implant surgery
- Treatment in blind eyes: surgical removal of the eye or placement of an intrascleral prosthesis

Head trauma and optic nerve degeneration
- Rearing, falling backward; striking the poll
- May cause fracture of bones in skull and compression or hemorrhage around the optic nerve
- Motion of the brain can cause tension and shearing of the optic nerves
- Sudden blindness results
- Clinical signs: fixed dilated pupil, edema and hemorrhage around optic nerve
- Over time optic nerve degenerates
- Treatment: usually of no benefit although immediate use of anti-inflammatory agents is advised
- Prognosis: if vision has not returned in 2 weeks – grave

Management of Blind Horses

General Notes
- Horses have natural history as a prey species, wary temperament, when cornered may kick, strike or run
- Herd animals, visual cues important in establishing dominance order in a group Individuals that ignore the visual cues of herd mates are often bitten, shoved, or kicked by dominant individuals
- Horses that become trapped in fences or other hazards tend to panic causing themselves injury

Signs Suggestive of Blindness
- Gradual onset in many cases – progressive uncertainty, especially in low-light
- May bump into walls or fences, show reluctance to walk over unfamiliar terrain
- Herd behavior may change
- May shy frequently when ridden
- Fear or anxious behavior (e.g.. circling, calling to herd mates, crashing into walls, ignoring restraints)
- Traumatize themselves by running into unfamiliar obstacles
- Balance and posture may be altered initially
- Behavior becomes more settled as they adapt over weeks to months

Adaptation to Blindness
- Temperament will affect how horses adapt to blindness
- Horses showing excess fear are dangerous – can injure themselves or handlers
- Animals with calmer temperaments may adapt well
- Blind horses can develop a “mental map” of their environment (know perimeter of different paddocks or pastures, run, play within the boundaries)
- Increased use of other senses: hearing, smell, touch (horses nose and lip has similar concentration of sensory nerves as the human hand)

Social Interactions
- Dominant sighted horses may “bully” a blind horse
• Blind horse may keep to itself and hang back from the herd and may lose weight because of poor access to food
- Occasionally one or more protective horses will act as a companion to the blind individual
• Guide the blind horse around obstacles
• If guide horse wears a bell on the halter the sound clues can provide guidance
- Blind mares with foals need to know the foal is nearby
• Foal wearing a bell on the halter may make mare more relaxed
• When restraining, hold the foal near the mare’s front end so she can touch, smell and hear the foal

How to Assist a Horse in Adaptation

1. Provide a safe environment
- Remove hazards:
• Tape up bucket handle hooks
• Cover sharp nails
• Take down pieces of wire in stalls and paddocks
• Cut down low hanging tree branches
• Consider board fencing for paddocks
- Initially: a treeless paddock with a board fence and a stall or run-in shed with smooth, solid walls
- Later: remove hazards from pastures, use board or smooth fencing, maintain a safe stall or shed
- Demonstrate the boundaries of any new enclosure
- Encourage the horse to touch fences, gates and water sources with muzzle
- Signs should be posted on stalls or paddocks to alert visitors that the horse is blind

2. Behavioural Considerations
- Identify stressors causing anxiety (primarily confinement and separation from other horses)
- Be aware: regardless of temperament, any horse that has lost vision can change its behavior quickly and spook if outside stimulus suddenly scares it

3. Companions
- Initially house blind horses with or without presence of a calm, sighted companion in a paddock or barn
- Later choose turnout companions that are non-threatening
- If a guide horse emerges, consider putting a bell on its halter

4. Horsemanship
- Steady handling, regular grooming
- Predictable schedules for meals and turnout
- Respond to voices, smell and touch of people they knew and trusted when visual so speak to horse when approaching it and working with it
- Use consistent phrases and inflections when teaching voice commands: “whoa”, “step up”, “step down”, “stand”
- Approach the shoulder area first and stay near it when working around the horse as this is the safest position
- Do not clip whiskers on muzzle
- Keep feed and water in same location
- Handle bilaterally blind horses from both sides of the body
- Avoid initiating alarming sounds or sensations on the blind side of unilaterally blind horses
- Practice loading on and off trailer
- Be patient

Use of Blind Horses
- Riding? – Cannot be recommended, however, each horse and its circumstances are unique (Safety is a top priority)
- Pasture pets – Treasured family members
- Trail horses – Strong bond with the rider
- Broodmares
- Athletes – e.g.. Dressage: requires precise communication between horse and rider
- Unilaterally blind – use as a sport horse with caution (have half the visual field and little to no depth perception)

Sources
Miller PE, Murphy CJ. Equine Vision: Normal and Abnormal. In: Equine Ophthalmology (ed. Gilger BC), Elsevier Saunders, St. Louis, 2005:371-408

Timney B, Macuda T. Vision and hearing in horses. J Am Vet Med Assoc 2001;218:1567-1574

Brooks DE. Equine Ophthalmology. In: Veterinary Ophthalmology 3rd edition. (ed. Gelatt KN), Lippincott/Williams & Wilkins, New York, 1999:1053-1116

Dwyer A. Practical Management of Blind Horses. In: Equine Ophthalmology (ed. Gilger BC), Elsevier Saunders, St. Louis, 2005:449-456

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