Identifying corneal disease and first aid

Introduction

Infectious keratitis is the most common ocular disease in horses, often leading to blindness. The equine cornea is relatively slow to heal, prone to infection, and receives much trauma (because of the prominent location of the eyes on a horse and because of the propensity for horses to throw their heads).

Ulcerative Corneal Disease

Corneal ulceration is usually the result of trauma, but secondary infection is common.¹ A corneal ulcer is present when there is a break in the corneal epithelium (Figure 1). Clinically, this results in lacrimation, blepharospasm, photophobia, conjunctival hyperemia, corneal edema, and possibly miosis and aqueous flare. The diagnosis of a corneal ulcer is made based on these clinical signs and fluorescein staining of the cornea. Bacterial and fungal cultures should be submitted on any ulcer that has not healed in 3-7 days. Mixed bacterial and fungal infections are common. It is essential with all corneal ulcers to find the cause of the ulceration and eliminate it. Topical corticosteroids must not be administered in the presence of a corneal ulcer, and a history of previous topical corticosteroid therapy increases the likelihood of infectious, especially fungal, keratitis.¹

Simple uncomplicated corneal ulcers have characteristics of corneal epithelial cell loss with exposed corneal stroma, acute onset, absence of signs associated with infection (stromal malacia, cellular infiltrate, stromal defects). Treatment should consist of a topical broad-spectrum antibiotic every 6 hours (e.g., oxytetracycline; neomycin, bacitracin, gramicidin; ofloxacin); topical 1% atropine once daily; and treatment of any secondary uveitis, if present (e.g., systemic non-steroidal anti-inflammatory medications [NSAIDS]). Topical corticosteroids are contraindicated in equine ulcerative keratitis, and topical NSAIDS may delay re-epithelialization of the cornea and therefore are also contraindicated.

Complicated corneal ulcers

Complicated corneal ulcers are those that: do NOT heal within 72 hours, have a collagenase component (i.e., melting corneal ulcers or stromal loss), have a mechanical obstruction to healing (i.e., foreign body, indolent), are infected (either with bacteria or fungus), and or are in danger of perforation.

Indolent corneal ulcers in horses are similar to small animal indolent ulcers. They are chronic, superficial corneal ulcers where the corneal epithelium will not adhere to the underlying corneal stroma. The characteristic appearance is a superficial ulcer with a redundant epithelial border. Other signs include minimal corneal neovascularization, focal edema, and moderate discomfort. Indolent ulcer treatment is similar to that for small animals (debridement, diamond burr keratotomy, soft contact lenses, topical broad-spectrum antibiotics [esp. tetracycline class], serum).²

Bacterial keratitis generally has stromal involvement that produces edema, cellular infiltration, and stromal defects, potentially accompanied by keratomalacia, or “melting”. Secondary anterior uveitis may be severe, resulting in miosis, aqueous flare, hypopyon, and hypotony. Culture and cytology should be collected in cases of stromal ulcers. Treatment should include frequent use of broad-spectrum topical antibiotics (e.g., moxifloxacin every 1-2 hours), topical atropine (q12 hours), systemic NSAIDs, and an anti-collagenase medication (autogenous serum (q1-2 hours) or EDTA). Rapidly progressing lesions should be managed surgically with the use of a keratectomy followed by a conjunctival graft.

Deep corneal ulceration in the horse is almost always the result of infections. A descemetocele is a severe deep corneal ulcer where the overlying corneal stroma has sloughed, exposing Descemet’s membrane (DM). DM is easy to identify because it is usually transparent (i.e., does not become edematous) and does not retain fluorescein dye. DM is surgical emergency in most horses and a corneal graft, either fresh or synthetic, with an overlying conjunctival graft gives the best chance for the successful healing. ^3,4

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