An increase in eye pressure can lead to blindness if not treated early. | Photo: iStock

Editor’s Note: This article was revised by the author to reflect new and updated information in November 2017.

An increase in eye pressure can lean to blindness if not treated early

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