What's in an Equine Eye Exam?
A thorough eye examination can identify numerous potentially vision-threatening problems in horses.
Photo: Dennis E. Brooks, DVM, PhD, Dipl. ACVO
Editor's Note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C.
Sometimes a problem's obvious: Your horse's normally bright eye is swollen shut and draining profusely. Other times, trouble is more subtle: You notice your horse's pupil is constricted, despite the fact he's inside a dimly lit barn, as slow tears stain his face. Whatever the reason, once your veterinarian arrives, chances are he or she will perform an ocular exam.
At the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C., Dennis Brooks, DVM, PhD, Dipl. ACVO, professor of ophthalmology at the University of Florida's College of Veterinary Medicine, reviewed the following current standards of care for equine eye exams with veterinary attendees.
"The standards of care for doing eye exams on horses have changed," Brooks began.
First, he said, a veterinarian should "stand back and look at the horse." Signs of pain or discomfort can be identified easily during an ophthalmic exam by evaluating:
- The eye lash position ("The first sign of a painful eye often is the eyelashes of the upper eyelid pointing downward," he said, as opposed to them being perpendicular to the eye's surface as seen in healthy horses.);
- If the horse is squinting (which typically is a sign of eye pain);
- The presence of tearing;
- Ocular discharge (If present, Brooks noted it means that "either there's an increased production of tears or the tears are not draining properly.");
- The drainage angle (the gray band on the outer portion of the eye, which contain tiny holes; if these holes collapse and appear solid, or "closed," there is an increased risk of glaucoma.)
- Any color changes of the iris or cornea;
- The pupil size (constricted pupils are a sign of pain);
- The clarity of the aqueous humor (clear eye fluid that, if cloudy in appearance, could indicate uveitis);
- Eye blood vessel symmetry (Thicker spots of red mean more inflammation in one area, Brooks said.)
- Lens and cornea clarity; and
- The anatomy of the eye via an ophthalmoscope, transilluminator, or slit lamp.
Although it's not possible to test how well a horse sees, it is possible to ensure the retina is functional, Brooks said. Depending on the diagnosis, a functional or nonfunctional retina can help decide treatment protocol. Brooks recommends veterinarians use either the dazzle response or the indirect pupilary light reflex (PLR) test to find out if an eye has the potential to see.
The dazzle reflex involves shining a bright light in the horse's eye. If the retina is functional, the horse should squint quickly, or "dazzle", he said. "If they have a dazzle, then the retina is working. It doesn't tell you how well they see, but the potential for vision is there," he explained.
Similarly, the indirect PLR involves shining a bright light in the bad eye and watching for pupillary constriction in the "good" eye. "A good response is a very positive finding," he said.
Brooks also recommended examining the eyelids for potential abnormalities.
Simple Diagnostic Tests
After the initial visual exam, additional diagnostic methods might be needed to definitively identify the problem. Brooks reviewed the diagnostic options available for further investigation.
Flourescein dye--"Every horse eye exhibiting signs of pain should be stained with flourescein," Brooks stressed. Retention of this yellow-green dye detects the presence of epithelial defects or corneal ulcers, he explained. It's an important step, Brooks stressed, as the presence or lack of an ulcer can change treatment directives. To ensure an accurate result, it's important not to dilute the flourescein with water, Brooks stressed.
Flourescein can also identify perforated corneas or leaking corneal structures, Brooks said. Practitioners can identify these more serious lesions by observing whether flourescein changes color to a darker, brownish shade, which will happen if it leaks through a hole; this process is called the Seidel's test.
Rose Bengal stain--As the name suggests, this rose-colored stain is used to evaluate the eye's tear film integrity, Brooks explained. Rose bengal retention indicates tear film instability and can be associated with a number of conditions including dry eye, fungal and viral keratitis, severe edema, and granulation tissue. He noted that this stain should be diluted with water before use to prevent false positives.
Corneal scrapings--Especially when dealing with corneal ulcers, Brooks recommends veterinarians take a bacterial culture first, then evaluating cells with cytology. Once he removes superficial debris, he gathers a deeper scraping at the edge and base of an ulcer, using the handle end of a metal scalpel blade. By evaluating what, if any, bacteria or fungus is residing in the ulcer a veterinarian can select a targeted treatment method.
Intraocular pressure--Horses intraocular pressure should be tested if they are deemed at risk for glaucoma, Brooks said. He recommended veterinarians use a device called a tonopen or a tonovet to measure the pressure and determine whether glaucoma is present.
For some diagnostic testing, Brooks reminded that motor nerve blocks are useful to keep the horse from blinking or moving their eye during examination. He reviewed the administration process with the attendees.
Additionally, depending on the results of the initial diagnostic testing, more advanced diagnostics such as ocular ultrasound.
So what might a veterinarian find during an exam? Brooks reviewed several conditions that can be identified during an equine ocular exam, including:
- Hyphema (blood in the eye);
- Corneal abscess, necrosis, edema, or ulcers;
- Stromal abscesses;
- Subepithelial infiltrates;
- Immune mediated keratitis;
- Glaucoma; and
- Retinal or optic nerve disease.
These conditions, along with others, were discussed in more detail throughout the conference.
A thorough eye examination can identify numerous potentially vision-threatening problems in horses. Calling a veterinarian at the first signs of ocular distress can allow a prompt diagnosis and, further, allow treatment to begin earlier, giving the horse the best shot at a full recovery.
About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado.