Equine Corneal Ulcers Discussed In-Depth at AAEP Convention

At the 2007 AAEP convention's In-Depth Ophthalmology session, Caryn Plummer, DVM, Dipl. ACVO, assistant professor at of the University of Florida veterinary school, described the outer corneal layer (epithelium) as the windshield of the eye, and explained that it is protective and supportive to underlying corneal tissues. Beneath the epithelium is the stroma, the thickest layer of the cornea, comprised of collagen and fibroblasts. Beneath that is the thin basement membrane (Descemet's membrane), and at the bottom of the cornea is the single-cell endothelial layer that serves as a pump to move accumulated fluid from the interior of the cornea.

Plummer noted that ulcers are classified according to their depth, cause (etiology), and response to treatment. She noted that following an injury, mitosis (cell division) stops, and the cells at the edge of the wound enlarge and lose their attachment, allowing them to slide over the defect. Initially, this forms a single layer of cells, but eventually a multilayered sheet of cells will cover the corneal wound. The epithelial cells migrate at a rate of 0.6 mm/day if there is no concurrent infection; it takes at least six weeks for the epithelium to securely attach once it has crossed the defect. An indolent ulcer is one that won't heal either due to an abnormality of the basement membrane or a problem of adherence.

A defect in the stroma requires a balance of resorption and remodeling, which takes longer than healing of more superficial ulcers. Any imbalance in this restorative process can lead to "melting" of the cornea. This can occur due to upregulation of enzymes of bacteria or white blood cells that cause "housekeeping cleanup" to exceed the rate of repair. Normal tear fluid contains soluble proteases essential to corneal health. Disproportionate amounts of these, as produced by inflammatory cells, lead to pathologic degradation. Normal repair also requires corneal blood vessels, which move at the rate of 1 mm/day, yet they are slowed by therapy with non-steroidal anti-inflammatory medications (NSAIDs). White blood cells invade the cornea at 8.6 mm/day, so they are the first to respond to injury. Epithelial cells move at 0.6-1.2 mm/day.

A melting ulcer is not always due to bacterial influences, and this is a true emergency due to the possibility of prolapse of internal eye tissues within hours or days. It is always prudent to do a bacterial culture and sensitivity and cell cytology, in addition to implementing aggressive treatment with topical antibiotics and possibly antifungal agents. Removal of necrotic debris surrounding the margins of an ulcer allows epithelium to cover the defect. Plummer said it is important to keep in mind that "sterility does not equate with healing." Dilation of the pupil deters internal damage, and NSAIDs quiet the uveitis and lessen pain.

Enzymes are responsible for collagen destruction, so medications that inhibit destructive enzymes are critical for success. Equine serum reduces enzymes by 90%, while 0.2% EDTA (a chemical used as an anticoagulant) slows proteolytic activity (cleavage of proteins by enzymes) by 99.4%. Plummer noted there might be advantages in obtaining serum from another horse on the property. Other enzyme-inhibiting products are available with similar effects. The objective is to stop corneal melting, restore drainage, sterilize the wound, provide structural support for ulcer repair, decrease pain and inflammation, and prevent secondary damage to the interior of the eye. Treatment might be required every two to six hours. Surgery and placement of a subpalpebral (beneath the eyelid) lavage tube might be necessary to facilitate healing.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at Shop.TheHorse.com or by calling 800/582-5604). 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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