Ophthalmology

Tips for Acing the Prepurchase Ophthalmic Exam

By Paul Basilio

A prepurchase examination is an excellent way to mitigate risk when buying a horse, but sometimes the eyes can take a backseat.

“We don’t have standardized guidelines on how to perform and record the ophthalmic component of the prepurchase exam, which is a critical component,” said Kelly Knickelbein, VMD, DACVO, an assistant clinical professor in the Section of Ophthalmology at the Cornell University College of Veterinary Medicine, in New York.

Dr. Knickelbein presented some practical prepurchase ophthalmic exam tips during a Burst session at the 70th Annual AAEP Convention in Orlando, Fla.

For much of the examination, no specialized equipment is necessary. For the anterior portion of the eye, including the eyelids, cornea, anterior chamber and the iris, a light source and some sort of magnification are all that is needed. A simple pen light and a pair of high-magnification reading glasses are sufficient to see small lesions of these structures.

“For the posterior segment examination, which is the limiting factor in a lot of these examinations, some specialized equipment is needed,” Dr. Knickelbein said.

A finoff transilluminator, an indirect condensing lens (20 or 14 diopter), and tropicamide ophthalmic solution will provide the gateway to the examination of the fundus.

Neuro-ophthalmic Examination
The neuro-ophthalmic examination typically comes first. By simply observing the horse and watching how it interacts with its environment or handler, you may be able to quickly identify vision disturbances that can aid in identifying serious issues in the horse. Note the horse’s facial symmetry as well, including the position of the eyes and the symmetry of the ears, muzzle and bones of the skull.

“And then you can start your cranial nerve exam,” she explained. “For the dazzle reflex, a bright light should induce a quick blink, and touching the medial and lateral canthus will assess the palpebral reflex.”

For the pupillary light reflexes, shining a light should induce ipsilateral and contralateral pupillary constriction. To assess ocular motility, use the halter to move the horse’s head from side to side and up and down, and observe the movement of the globes to make sure they redirect toward the direction of their gaze.

“Our true assessment of vision in horses—and all animals—is very crude,” Dr. Knickelbein said. “We rely on the menace response, which involves directing your hand toward the eye to see if the horse blinks in response. They don’t need to have much vision to register an intact menace response.”

Dilate, But Keep it Brief
“You cannot diagnose what you don’t look for,” she explained. “If you’re not dilating the pupil, then you’re not seeing the entire lens, and you’re not going to see the entire retina.”

Dr. Knickelbein emphasized that for routine dilation, tropicamide is vastly superior to atropine in clinical practice. Tropicamide has a short onset of about 15 to 20 minutes, and a short duration of a few hours.

“That puts the horse out of commission for the afternoon,” she said. “If you use atropine, you’re likely to have some unhappy clients, because atropine can dilate the pupil in a normal eye for several weeks.”

Dilation is essential for identifying abnormalities that would not be visible otherwise. Dr. Knickelbein described an eye that appeared to have a small cataract at first, but after dilation she could see that the cataract was quite extensive. The abnormality appeared to be congenital and the potential for progression or impacted vision was uncertain, but she said that it was something that the buyer would want to know before the sale.

For competition horses, the United States Equestrian Federation states that a horse must have a 24-hour withdrawal from tropicamide and will require a medication report form (MRF) if the drug was used within the prior 7 days; the Fédération Equestre Internationale requires a 7-day withdrawal.

Fundus Time
The fundic exam is usually the part of the ophthalmic examination that can be challenging for those who don’t examine eyes every day.

“Dilating the pupil is going to make this a whole lot easier for you,” Dr. Knickelbein said.

Once again, the challenging part is knowing what’s normal. The equine fundus can vary in pigment within the retinal pigmented epithelium and the choroid. The choroidal vessels are visible if there is little or no pigment overlying them in the retinal pigmented epithelium.

Pathologies can be on the benign side, such as “bullet hole lesions,” which are common enough to be considered an incidental finding, and senile retinopathy—a degenerative change often seen in older horses. “Butterfly” lesions in horses with peripapillary chorioretinal scars can be an incidental finding in otherwise normal eyes, or an indication of an eye that is prone to uveitis.

Signs of retinal degeneration, however, could mean severe visual impairment, so it’s worth your time to take a closer look and report your findings. Referral for examination by a board-certified veterinary ophthalmologist with equine experience is indicated if abnormalities with questionable significance are identified during the initial prepurchase examination. MeV

Know What’s Normal
Normal horse eyes come in a wide variety of patterns and colors. The eyelids, conjunctiva and iris can each have varying pigmentation from horse to horse, and some horses have no or minimal pigment in these structures.

“Just make a note of that, and know what’s normal,” Dr. Kelly Knickelbein said.