How to Diagnose and Treat Glaucoma in the Horse (21-Nov-2003)

1. Introduction Aqueous humor, which acts to supply metabolic needs of intraocular structures, is produced constantly by the ciliary body of the equine eye. The fluid must also drain constantly from the eye through the iridocorneal and uveoscleral outflow pathways. Obstruction of this outflow of fluid can be the result of an abnormally developed drain (i.e., primary glaucoma) or through damage to the drain from scarring, vascularization, or accumulation of debris (i.e., secondary glaucoma). The result of this obstruction is retention of aqueous humor and subsequent increase in the pressure within the eye. The pathologic disease state associated with elevated intraocular pressure is called glaucoma. Horses have a much greater percentage of aqueous humor outflow through the uveoscleral outflow compared with dogs and humans [1] (the species in which most glaucoma research has centered), where the outflow is primarily through the iridocorneal angle. Therefore, the causes, clinical findings, and treatments are different for horses with glaucoma. Because of these differences and the fact that few veterinarians have a suitable tonometer (e.g., instrument to measure intraocular pressure) for horses, glaucoma is not commonly recognized and therefore frequently not treated appropriately in many horses. The purpose of this paper is to describe the clinical features, diagnosis, and appropriate therapy for glaucoma in horses.