Dennis Brooks, DVM, PhD, Dipl. ACVO, of the University of Florida veterinary school, led the panel at the in-depth session on ophthalmology at the 2007 AAEP convention in Orlando, Fla., held Dec. 1-5. He was excited to share many new ideas that have developed in the last six months to allow veterinarians to save eyes and sight. He stressed that in most cases an eye exam does not require special equipment, but it does require a "lack of fear" in addition to a bright light, a direct ophthalmoscope or otoscope (an instrument for examining ears), and skills to perform a thorough exam.

Brooks pointed out that human physicians have a number of vision tests, including asking a patient to count fingers, but a veterinarian is limited by the nature of the beast. Assessment of a menace reflex from hand motion is a crude measure, making it hard to determine if the horse is reacting to the feel of air or to hand motion. The best equine test for vision is the "dazzle reflex," a sensitive test for retinal function. An observer watches to see if the horse squints in response to shining a bright light into the injured eye. Although there might be a variable pupillary response to the light, squinting denotes the perception of light. If the horse's cornea is opaque, yet he squints, then the retina is still working.

Another useful test is the "flashlight test" or indirect papillary reflex that stimulates the bad eye with light while observing for pupillary reaction in the other eye. Often inflammation is confined to the front of an eye, and a positive indirect pupillary reflex indicates that there is a chance to save sight and the damaged eye.

Brooks also pointed out that, in general, the upper eyelashes of a painful eye tend to point down. The most subtle sign of eye discomfort is a droopiness of the lashes of the upper lid, and this is one of the last things to return to a normal position. It might be necessary to view the eyes from a distance, even with binoculars, for a horse that is protective of examination of the eye and tends to close when a person approaches. Brooks described helpful regional nerve blocks to anesthetize branches of cranial nerves to curtail blinking or sensation of pain and to facilitate careful examination of eye structures.


There are really only two eye diseases: corneal ulcers, and everything else. �Dr. Dennis Brooks

With wry humor, he noted there "are really only two eye diseases: corneal ulcers, and everything else." Staining the eye with fluorescein dye indentifies defects in the corneal epithelium (the outer layer of the cornea) and the extent of an ulcer. The corneal epithelium is eight layers thick, but the dye will be picked up with loss of only one to two layers. The Seidel's test detects a corneal perforation as the fluorescein changes color as it leaks into a hole. Normal passage of fluorescein through the nasolacrimal duct (the duct that connects the eye to the nasal passages) can take up to 10-15 minutes.

Use of Rose Bengal stain evaluates stability and integrity of the tear film. Normally, the tear film takes 10-15 seconds to break up, but rapid dissipation indicates roughening on the corneal surface that won't allow the tear film to hold together. Dry eye is one cause of a roughened corneal surface, and this can occur with viral or fungal diseases.


Dr. Dennis Brooks examines an equine eye

Any horse with eye pain, squinting, sensitivity to light, and/or excess tearing should be examined by a veterinarian and possibly by a veterinary ophthalmologist to determine the exact cause.

An observant veterinarian can tell a lot from color changes within the cornea: white might indicate an abscess, blue discoloration is consistent with edema expected with an ulcer, red indicates blood vessel entry for healing, while a dark color is of concern for imminent rupture. A pigmented iris will change color, becoming dark with inflammation from uveitis, melanoma, or hemorrhage. The size of the pupil also gives specific information--a large pupil might indicate glaucoma or retinal or optic nerve disease; small pupil size is symptomatic of uveitis. Aqueous flare is visible as little white spots seen with a slit lamp; this results from protein leaking from blood vessels of the iris and is another sign of uveitis.

For a corneal ulcer, Brooks recommends obtaining a bacterial culture first, then evaluating cells with cytology. Once he removes superficial debris, he gathers a deep scraping is gathered at the edge and base of an ulcer, using the handle end of a metal scalpel blade. Brooks stressed that even after treatment kills bacteria, white blood cells (neutrophils) die and dump enzymes (proteases) into the tear film that continue to worsen an ulcer. One objective in treatment is to return the tear film proteases back to normal. Blood vessels grow from the periphery at 1 mm/day, while white blood cells move in at a rate of 8 mm/day.

The horse has the highest intraocular pressure (IOP) of any land mammal, and the horse's head must be up when measuring it; IOP increases 87% with the head in the down position. Looking at the drainage angle (the gray band on the lateral, or outer, portion of the eye), one should see minute holes. If these holes collapse and close and appear solid, there is an increased risk of glaucoma.

Regarding the lens, horses around 8-10 years of age can develop nuclear sclerosis, which is a gray appearance to the lens that does not affect sight. The examiner can see through nuclear sclerosis of the lens, hence a horse can also see out. If a horse has a cataract, the opacity of the lens will block the horse's visual image, but it will not interfere with a normal pupillary light reflex. Brooks also recommends the use of ultrasound to examine the back of the lens and look for indications of retinal detachment.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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