Examining the Equine Eye

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.
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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

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Written by:

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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