Glaucoma Discussed at Equine Ophthalmology Meeting

Glaucoma, a group of diseases resulting from alterations in the formation and drainage of aqueous humor (clear eye fluid), which causes an increase in intraocular pressure above what's compatible with normal function of the retina and optic nerve, was another topic covered by Dennis Brooks, DVM, PhD, Dipl. ACVO, professor of Ophthalmology at University of Florida College of Veterinary Medicine, at the recent AAEP Focus on Ophthalmology meeting.

Brooks told veterinarians who are seeing a lot of uveitis in their equine practice that they are also likely seeing horses with glaucoma. The condition is often noticed by the owner as corneal edema, which appears as a bluish tinting to the cornea, along with dilation of the affected eye. Unlike in humans and small animals, glaucoma in horses does not seem to cause pain, but blindness stemming from atrophy (partial or complete wasting away) of the optic nerve can often develop as a secondary consequence of glaucoma.

Glaucoma in the horse is frequently classified as either primary, secondary, or congenital:

  • In primary glaucoma both eyes are usually affected and intraocular pressures are elevated with no obvious ocular abnormality to account for increased pressures. The condition might be heritable.
  • Secondary glaucoma typically occurs due to an identifiable reason, such as lens luxation (movement away from normal position), neoplasia within eye, iridocyclitis (inflammation of the anterior chamber of eye in front of lens and behind cornea), and/or chronic uveitis.
  • Congenital glaucoma is a condition the animal is born with; it is caused by developmental abnormalities.

Glaucoma in the horse is often difficult to distinguish early in the course of the condition due to the mildness of the clinical signs presented. The discomfort often obvious in small animals and humans with glaucoma is not apparent in the horse.

Keeping intraocular pressure at a level that is compatible with maintaining retinal and optic nerve health is often the goal of treatment.

The pupils of a horse with glaucoma often present as fixed and dilated. The lenses of glaucomic horses might either subluxate or fully luxate. A veterinarian might also see retinal degeneration. Diagnostically, intraocular pressure (within the eye) greater than 30 could indicate a problem; however, positioning of the head during assessment can have a major impact on the pressure of the eye.

The goals of treating the animal with glaucoma are often focused on keeping intraocular pressure at a level that is compatible with maintaining retinal and optic nerve health.

Veterinarians often try to control the production of aqueous humor and increase the outflow of aqueous humor. A target intraocular pressure (IOP) for horses diagnosed with glaucoma is less than 20mmHg.

Surgical options, including laser surgery, implants, and removal of the ciliary body (the part of the eye responsible for production of aqueous humor), might be viable when medical therapy is not enough to control elevated IOP.

No matter the cause, glaucoma, although rare in the horse, should be evaluated by a veterinarian and treatment options discussed.

About the Author

Kristen Slater, DVM

Kristen Slater, DVM, practices with Kasper & Rigby Veterinary Associates in Magnolia, Texas. Her practice interests include preventive medicine, reproduction, sports rehabilitation, and conditioning.

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