The American Association of Equine Practitioners’ guidelines for prepurchase examinations currently lack standardized practices for the ophthalmic component of the exam, stated Kelly Knickelbein, VMD, DACVO, Assistant Professor in the Section of Ophthalmology at the Cornell University College of Veterinary Medicine, during a Burst presentation at the 2024 AAEP Convention. She called the ocular exam, “a critical step in prepurchase examinations.” Knickelbein assured practitioners this examination does not require a lot of expensive equipment but only a few key items to improve their ability to recognize subtle lesions and see to the back of the eye.
Equine Ophthalmic Examination
“You need some sort of magnification (such as high magnification readers), a light source (e.g., a pen light or Finoff transilluminator), tropicamide ophthalmic solution to dilate the pupil (avoid atropine due to its long duration), and either a direct ophthalmoscope or an indirect condensing lens to perform a fundic exam,” said Knickelbein.
Begin the exam by observing the horse’s visual behavior and facial symmetry. Then, perform a complete neuro-ophthalmic examination, including a menace response, palpebral reflex, dazzle reflex, pupillary light reflexes, as well as testing for ocular motility.
“Know what is normal, and appreciate that there is a lot of normal variation, often due to differences in pigmentation of various ocular tissues,” said Knickelbein.
Dilating the pupil is essential to a complete examination of the lens and fundus. Use tropicamide instead of atropine, because the former has a short onset (~15 mins) and duration (hours). Atropine can dilate the pupil of a normal eye for a week or more.
“It is important to be familiar with withdrawal times for tropicamide if performing prepurchase examinations on the show ground,” advised Knickelbein.
Common Lens Abnormalities
The most common lens abnormalities identified during a prepurchase exam are cataracts, which are opacities of the lens.
“Cataracts can be easily missed without dilation of the pupil,” said Knickelbein. “Not all cataracts are significant, but some impact vision or progress to impact vision, so having a complete exam to allow for a buyer to make a fully informed decision is important.”
The fundic exam is probably the most challenging part of the ophthalmic exam. Dilating the pupil makes the examination easier and allows for a complete view of the optic nerve, retina, and underlying choroid. Retinal lesions are common, and their significance depends on how extensive they are and if other ocular abnormalities are present.
“Bullet hole lesions,” which are punctate chorioretinal scars, are generally of no consequence when few in number. “Butterfly lesions,” or peripapillary chorioretinal scars, might be incidental or concurrent with other signs of chronic uveitis. While senile retinopathy is common in aged horses and doesn’t appear to significantly impact vision, diffuse retinal degeneration would cause significant vision impairment.
Related Reading
- Researchers Develop Eye Drop to Treat Uveitis in Horses
- Disease Du Jour: Equine Recurrent Uveitis
- AAEP Health Coverage: Novel Drug (SOCS1-KIR) to Treat Uveitis in Horses
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