Eye Medications for Painful Horse: Which One?

Even when cared for meticulously, horses are susceptible to eye injuries. In fact, corneal ulcers constitute the most common ophthalmic emergencies for equine veterinarians. These are often treated with antibiotic and antifungal medication, a cycloplegic agent to dilate the pupil, and an antiprotease to limit additional breakdown of the cornea. In addition, a local anesthetic block can make affected horses more comfortable. Not all local anesthetics work equally well or long, so which local anesthetic should your vet choose?

“The cornea, also referred to as the ‘window’ of the eye, is the thin, outermost layer of cells on the surface of the eye. Despite its of lack of bulk, the cornea is highly innervated, making injuries to the cornea painful,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER).

To correctly treat corneal disease, appropriate diagnostic tests must be performed, and these may cause additional discomfort. Several pain medications are used to make horses more comfortable during treatment. Bupivacaine, mepivacaine, and lidocaine are local anesthetics that can be injected into specific areas of the eye to temporarily numb the nerves of the cornea.

To determine which product most effectively blocks the nerves and lasts the longest, one research group* recruited eight healthy horses to test the three products. No trauma to the corneas was involved during this study. Instead, the researchers used a device that gently applies pressure to the surface of the eye until a blink response occurred. That test, referred to as corneal touch threshold or CTT, was used after blocking the eyes with one of the three local anesthetics or saline.

Researchers found:

• Bupivacaine and lidocaine resulted in a similarly decreased CTT, meaning the horses presumably felt less pressure on their corneas, of 105 minutes and 104 minutes, respectively;
• Mepivacaine resulted in a markedly longer decreased CTT of 139 minutes;
• Saline resulted in a decreased CTT of only 7.5 minutes; and
• No evidence of corneal toxicity was noted with any treatment.

All three products appeared to effectively and safely reduce corneal sensitivity and should be used during diagnostic and surgical procedures involving the cornea.

“Studies also show that in addition to standard medical approaches to corneal issues, omega-3 fatty acid supplementation also helps minimize pain associated with the cornea,” relayed Crandell.

*Jinks, M.R., R.L. Fontenot, R.W. Wills, et al. The effects of subconjunctival bupivacaine, lidocaine, and mepivacaine on corneal sensitivity in healthy horses. Veterinary Ophthalmology. In press.