Texas Equine Veterinary Association

www.texasequineva.com • 7 by Andrew Rogen, DVM ABOUT THE AUTHOR Dr. Andrew Rogen grew up on a small Angus farm in Brandon, South Dakota. He finished his undergraduate training at South Dakota State University and graduated from Iowa State University with his DVM in 2015. Andrew's internship and residency training included time in Oregon, California, and Wisconsin. Andrew finishes his residency training program in July 2020. While his average day involves treating Bulldogs and Shih Tzus he enjoys leaving the office to examine equine patients. glaucoma Focus on: Introduction: Glaucoma is often equated with increased fluid pressure of the eye (intraocular pressure or iOP); however, this belies the complexity of the disease. Our current understanding recognizes glaucoma as a collection of diseases that often culminate in blindness for which increased IOP is the most common denominator. This review will cover pertinent pathophysiology, examination techniques, diagnostics, and treatments for equine glaucoma. Anatomy and Pathophysiology of Glaucoma: The anterior chamber of the eye is filled with aqueous humor, which provides nutrition and removes toxins from anterior segment structures 1-4 . The ciliary body located behind the iris continuously produces the aqueous that moves through the pupil and exits through the iridocorneal angle 2 . From the iridocorneal angle, aqueous humor leaves the eye via either conventional (trabecular meshwork) or unconventional (uveoscleral) pathways 1-4 . The horse has a large unconventional capacity estimated up to 50% of aqueous humor compared to 3% in cats and 15% in dogs 2,4 . Intraocular pressure is a reflection of the rate of aqueous humor production and outflow 1-4 . Aqueous humor overproduction has not been described in domestic species; therefore, increased intraocular pressure occurs due to impaired aqueous outflow 2 . Elevations in intraocular pressure is thought to be the most important risk factor for glaucomatous optic nerve damage in horses 1,4 . Elevated intraocular pressure results in decreased blood flow, decreased optic nerve axoplasmic flow, retinal cell death, optic nerve atrophy, and blindness 4,5 . Glaucoma in horses is divided into congenital, primary, and secondary 4,6 . Congenital glaucoma is present at birth and poorly responsive to therapy 7 . Primary glaucoma is infrequently reported in horses 6 . Secondary glaucoma due to equine recurrent uveitis (ERU) is widely regarded as the most common cause of glaucoma in horses 1,4,6 . Other secondary causes include neoplasia, corneal perforation, or lens luxation 4,6,8 . Examination Techniques: When evaluating a horse with suspected glaucoma it is critical to examine the horse from a distance. If possible, the horse should be evaluated while navigating in the environment. An obstacle course of large objects (ex: poles, hay bales, etc.) can assist in critically assessing the horse's vision. The eyes should also be evaluated for symmetry. Eyelashes tipped downward often indicates a painful eye. Furthermore, chronically glaucomatous eyes may become enlarged (buphthalmic). Prior to sedation, a neuro-ophthalmic exam should be performed. The palpebral reflex should first be tested to confirm periocular skin sensation and the ability to blink. The menace response is used to assess vision. The menace response relies on a functional optic nerve, visual pathway, visual processing, and ability to blink. The menace response is a learned response and not a true reflex; therefore, the response is normally absent in young foals. Examiners should be careful not to contact tactile hairs or produce an air current capable of causing a false-positive menace response. Dazzle and pupillary light reflexes are best evaluated using a Finhoff transilluminator or very bright pen light. Normally, the pupil should constrict (become miotic) in the presence of light. Pupillary light reflex deficits can be variable in glaucomatous horses. End-stage glaucoma often results in a mydriatic (dilated) pupil that is unresponsive to light. An exception to this rule occurs

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