Equine Glaucoma

The glaucomas are a group of diseases resulting from alterations in the formation and drainage of aqueous humor (clear eye fluid), which causes an increase in intraocular (within the eye) pressure (IOP) above that compatible with normal function of the retina and optic nerve. Blindness is the final outcome due to permanent damage to the optic nerve. Glaucoma in the horse is being recognized with increased frequency, but the prevalence of glaucoma in the horse is surprisingly low (e.g., 0.07% in the United States) given the horse's propensity for ocular injury and marked intraocular inflammatory responses.

Aqueous humor is a clear eye fluid that is important for corneal and lens nutrition, and critical for the precise optical arrangement of the cornea, lens, vitreous (transparent gel filling the eyeball), and retina. The enzyme carbonic anhydrase plays an important role in aqueous humor production in the ciliary body; aqueous humor then passes into the posterior chamber through the pupil, and into the anterior chamber. It then exits either through the iridocorneal angle (angle at the anterior edge of the eye; conventional outflow pathway) or is absorbed through the iris, ciliary body, sclera, and cornea (membrane covering the anterior eyeball; unconventional outflow pathway). Studies indicate potentially extensive use of the conventional and unconventional aqueous humor outflow pathways in the horse, suggesting that obstruction of aqueous humor movement at the pupil, iris face, iridocorneal angle, or any other part of the aqueous outflow system must be quite extensive and pronounced before increased intraocular pressure occurs in the horse.

The Glaucomas

The glaucomas are frequently categorized as primary, secondary, and congenital types. All types of glaucoma have some type of cause or inciting event, but primary glaucoma appears to be due to biochemical alterations of the internal structures of the eye, since in these cases horses have no obvious eye problems to account for the increase in intraocular pressure. Primary bilateral (in both eyes) glaucoma has never been reported in the horse.

Secondary glaucomas are common in horses, however, and have identifiable causes such as anterior uveitis, iris and ciliary body tumors, and lens luxation (dislocation). Sometimes aqueous humor drainage pathways get clogged with inflammatory debris, tumor cells, or lens material, resulting in an increase in intraocular pressure. Congenital glaucoma is reported in foals and is associated with developmental anomalies of the iridocorneal angle.

Anterior uveitis often leads to the formation of vascular membranes on the iris surface that can limit aqueous absorption by the iris, leading to an inability to drain excess fluid from the eye, and it can also lead to obstruction of the iridocorneal angles with inflammatory cells and debris, impairing fluid drainage.

Clinical Signs and Risk Factors

Clinical signs of glaucoma include buphthalmos or enlarged eye. Equine glaucoma might not be easily recognized in the early stages because of the subtle clinical signs. There is usually a low index of suspicion for glaucoma in the horse, although horses with recurrent uveitis, those older than 15 years, and Appaloosas are thought to be at increased risk for glaucoma.

The pupils are often only slightly dilated, and overt eye pain is uncommon. Dilated pupils, corneal striae (stretch marks), corneal edema (fluid swelling), decreased vision, lens luxations, mild uveitis, and optic nerve atrophy can also be found in horses with glaucoma (see Figures 1 and 2 at right).

The presence of corneal striae or corneal endothelial "band opacities" in normal-sized equine eyes warrants a high degree of suspicion for glaucoma, but corneal striae also can be found in horse eyes that have normal intraocular pressures. Corneal striae are linear, often interconnecting, white opacities found deep in the cornea and are commonly seen in grossly enlarged or buphthalmic eyes. (Buphthalmos can also be associated with corneal ulcers.)

Elevated intraocular pressure can lead to rapid progression of optic nerve damage and blindness in the horse, although slight vision might be retained for extended periods despite dramatically high intraocular pressure. Blindness occurs in nearly all horses with glaucoma.

Diagnosis: Tonometry

Accurate diagnosis of glaucoma in the horse requires applanation tonometry (see Figure 3), which measures elevated intraocular pressure by applying a small flat disk to the cornea (the same method used to diagnose glaucoma in humans). Early referral for applanation tonometry should be considered to institute therapy and maintain vision. The mean equine intraocular pressure ranges from 17 to 28 mm Hg. Auriculopalpebral nerve blocks (which temporarily prevent eyelid movement and blinking) should be used during tonometry or the result can be an overestimate of intraocular pressure. Horses which require sedation for ocular examination might show dramatic decreases in intraocular pressure, as illustrated by a study in which xylazine (a common tranquilizer) decreased intraocular pressure by 23%.

Intraocular pressure in glaucomatous horses does not remain consistently elevated, but varies throughout the day. Frequent intraocular pressure measurements at different times of the day might be necessary to detect these transient intraocular pressure spikes. These widely fluctuating measurements not only interfere with diagnosing glaucoma, but they also complicate monitoring the response to therapy.


Therapy for equine glaucoma is hard to understand when we don't yet understand the events initiating the obstruction to aqueous humor outflow, the mechanisms by which these events lead to aqueous outflow obstruction, or the nature of the obstruction itself. Various combinations of drugs and surgery might be necessary to reduce the intraocular pressure to levels compatible with preservation of vision. Glaucoma is particularly aggressive and difficult to control in the Appaloosa; however, the reason why is unknown.

Several treatments have been used to lower intraocular pressure, including systemically administered carbonic anhydrase inhibitors acetazolamide and dichlorphenamide, topical miotics (medications that make the pupil small) demecarium bromide and pilocarpine, and the beta-blocker timolol maleate. Topical carbonic anhydrase inhibitors such as dorzolamide can also be beneficial in horses with glaucoma.

Because miotics can activate the clinical signs of uveitis, miotic therapy is generally contraindicated in horses with glaucoma secondary to uveitis; it should be used cautiously, with careful intraocular pressure monitoring, in horses with mild or inactive recurrent uveitis. Topical prostaglandins such as latanaprost are used effectively in humans with glaucoma, but they appear to exacerbate intraocular pressure elevations in horses with glaucoma.

Glaucoma in some horses might, paradoxically, respond to topical corticosteroid and atropine therapy even if active uveitis is not clinically evident. Anti-inflammatory therapy, consisting of topically and systemically administered corticosteroids with or without topically and systemically administered non-steroidal anti-inflammatory drugs such as phenylbutazone or flunixin meglumine also appear to be beneficial in controlling intraocular pressure. Atropine therapy for equine glaucoma should be used cautiously, and only in conjunction with careful intraocular pressure monitoring since it could increase intraocular pressure in some horses with uveitis.

When medical therapy is inadequate, Nd:YAG or diode laser cyclophotoablation (which is a partial laser destruction of the ciliary body) might be a viable alternative for long-term intraocular pressure control. I recommend that laser therapy not be performed until any corneal edema present is controlled. The corneal edema of the horse with glaucoma appears to result more from uveitis than from increased intraocular pressure, and this corneal edema can become permanent after laser cyclophotoablation because of further damage to the corneal endothelium. Filtration gonioimplant surgeries use small tubes placed in the eye to drain off excessive levels of aqueous humor. These surgeries are experimental in the horse, but such procedures have been successful in humans and dogs. Chronically painful and blind buphthalmic eyes should be removed or have an intrascleral prosthesis (false eye) implanted.

Editor's note: This is the eleventh in a series of eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at www.TheHorse.com, for more information on normal eye anatomy.

About the Author

Dennis E. Brooks, DVM, PhD, Dipl. ACVO

Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More

Free Newsletters

Sign up for the latest in:

From our partners