Equine Ophthalmology

Equine Recurrent uveitis is a leading cause of vision loss in horses and often results in blindness. This is a frustrating disease to treat as recurrence can be frequent, long term medication is often required, and a cure is rarely achieved.

Clinical signs include blepharospasm (indicating pain), conjunctivitis, corneal edema (causing a blue-white color to the cornea), aqueous flare (protein and cells in the anterior chamber), and miosis. A detailed ophthalmic examination may reveal rubeosis iridis, cataract, subluxated or luxated lens, vitreous degeneration, chorioretinitis, retinal degeneration or detachment, or glaucoma (see companion article).

The underlying initial insult in any case of uveitis is tissue damage and breakdown of the blood-aqueous barrier. Trauma, infection, inflammation or neoplasia can initiate uveitis via these mechanisms. Although most cases are idiopathic, Leptospirosis, Brucellosis, Toxoplasmosis, Onchocerciasis, (which is uncommon due to frequent use of ivermectin for deworming) and sepsis are some of the known causes of recurrent uveitis. Idiopathic cases probably have a primary immune mediated etiology.

As a definitive diagnosis is often elusive, treatment is nearly always symptomatic. Usually steroids are used topically and subconjunctivally, and non-steroidal anti-inflammatory medications are given systemically. Subconjunctival and systemic therapy may be advantageous when blepharospasm makes topical medication difficult. Horses who experience frequent recurrence may benefit from long term low dose prophylactic therapy, such as oral aspirin or phenylbutazone. Long term use of topical steroids for prophylaxis may predispose to corneal infection, and is therefore not advised.

Atropine has been used commonly as an adjunct for treatment of uveitis. The mydriatic effect of atropine may persist for a week or longer, even after it is discontinued. Mydriasis can increase intraocular pressure by partially closing the iridocorneal angle. Because of this, tropicamide may be safer than atropine for use in uveitis. Tropicamide has the same beneficial effects as atropine, yet its duration of action is much less. If intraocular pressure rises after mydriasis, the effects of tropicamide will diminish within a day.

The horses cornea must be examined carefully before use of topical and particularly subconjunctival steroids. Corneal ulcers can occur with uveitis, and management of the two conditions simultaneously can be challenging.

Equine Glaucoma

Chronic glaucoma with globe enlargement, dense corneal edema, vascularization and striae.

Although primary glaucoma exists in the horse, the secondary form appears to be most common. Trauma, lens luxation, and particularly uveitis are known causes of secondary glaucoma in the horse.

The clinical signs of glaucoma in the horse can be confusing. The most common signs include an opaque cornea (caused by edema) and resultant vision impairment. These signs could be confused with recurrent uveitis. In addition to the corneal changes, an eye with uveitis usually shows conjunctival hyperemia, epiphora, aqueous flare, and blepharospasm. By comparison, a glaucomatous eye in a horse is usually quite comfortable with the conjunctiva being relatively unaffected.

Intraocular pressure is measured most easily in the horse using an applanation tonometer, such as a Tonopen. Proper use of a Schiotz tonometer requires that the cornea be directed upwards; moving the head to accomplish this is not feasible in horses.

Treating glaucoma in the horse, as in other species, is challenging. Because of the propensity for uveitis to be a predisposing factor, oral and topical anti-inflammatory therapy is often indicated. There are some data which suggest that inducing mydriasis could be beneficial in increasing aqueous outflow, thereby lowering intraocular pressure in some cases. However, these data are far from conclusive, and it is known that dilating the pupil can increase intraocular pressure by closing the iridocorneal angle. Our opinion is that these medications should not be used for treatment of glaucoma. There are several topical anti-glaucoma medications available, though these are not effective enough to be used alone. The most effective long term therapy appears to be cyclophotoablation and hot tip sclerostomy using a diode laser. General anesthesia is necessary, and the procedure takes about 30 minutes.

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