Equine Glaucoma: High-Pressure Eyes

"Glaucoma (increased fluid pressure within the eye to a level incompatible with eye health) in horses is not well recognized, but look for it in cases of cloudy cornea, corneal edema, or severe unrelenting ocular inflammation," said Brian Gilger, DVM, MS, Dipl. ACVO, of North Carolina State University, during his presentation on diagnosing and treating glaucoma at the 2003 American Association of Equine Practitioners convention.

Aqueous humor, or the fluid that fills the eye, is constantly produced. Glaucoma is caused by decreased outflow of this fluid from the equine eye, he said. This could be due to an abnormally developed, damaged, or obstructed drain.

Gilger said because of differences in glaucoma signs between horses, dogs, or humans, and the different type of tonometer required to evaluate intraocular pressure in the horse, "Glaucoma is not commonly recognized and therefore frequently not treated appropriately in many horses."

"The most common cause of glaucoma in horses is chronic or recurrent uveitis (a type of secondary glaucoma)," he said. "These horses have multiple episodes of intraocular (within the eye) inflammation followed by a severe unrelenting bout of ocular cloudiness and discomfort (as a result of the development of glaucoma) that does not respond to traditional uveitis therapy. In general, the horse tends to lose vision much later in the disease process compared with dogs and humans."

There are two types of glaucoma--primary (caused by an abnormally developed drain) and secondary (caused by damage or obstruction from an injury or disease). Secondary glaucoma is much more common than primary forms, and it can be caused by recurrent uveitis, blunt trauma, intraocular neoplasia (a melanoma), or a chronic cataract. Each of these causes can have different outcomes, and they should be treated somewhat differently, he said.

Eye Exam

A tonopen is the proper tool for measuring glaucoma in the horse, Gilger said. He recommended auriculopalpebral nerve blocks prior to examination, since eyelid tension can artificially elevate intraocular pressure (IOP). Sedation can lower the IOP. The pressure measurement should be taken from the most normal area of the cornea, he added.

"Accurate measurement (of IOP) is essential to make the definitive diagnosis and to monitor the response to therapy," he added.

"A thorough and complete ophthalmic examination should also be done to help differentiate the cause of the glaucoma and to rule out other causes of corneal edema, such as keratitis (corneal inflammation)," he said. "The complete ophthalmic examination will also determine if the glaucoma is primary or secondary. Glaucoma secondary to intraocular disease other than uveitis is rare, but is possible with intraocular tumors and luxation of the lens.

Treating Glaucoma

"Ideally, glaucoma should be treated by increasing the outflow of fluid from the eye," Gilger stated, adding that treatment by decreasing the production of aqueous humor can also be helpful. He said many medications designed to alleviate glaucoma in humans and dogs don't work well in horses, but medications and surgical options for horses do exist.

"Systemic anti-inflammatory medications, such as flunixin meglumine (Banamine), should be used initially in all cases of glaucoma to help control intraocular inflammation," he said. "Topical therapy (with timolol alone or in conjunction with dorzolamide) to decrease aqueous humor should also be used. If there continues to be poor IOP control and there is potential for vision, then laser cycloablation (laser destruction of part of the ciliary body that produces aqueous humor) is indicated."

Gilger noted that existing uveitis/inflammation must be controlled and other intraocular disease should be ruled out before laser treatment is done. The candidate for laser surgery does not respond to antiglaucoma medication. Also, if the eye is blind and painful, he said it should be removed. A prosthesis can be added for cosmetic reasons.

The laser surgery is usually done under general anesthesia, and the amount of laser therapy applied should be adjusted for each case, he said. Antiglaucoma medications are often required indefinitely after surgery, and many eyes need to be treated again in six to 12 months.

"A recent review of efficacy of diode laser therapy in horses revealed 64% of the eyes treated with laser remained sighted, but 90% required topical antiglaucoma medication," Gilger reported.

The Other Eye

"Horses with primary glaucoma (in one eye) are unfortunately predisposed to bilateral (in both eyes) involvement," Gilger said. Thus, "It is essential that the other eye be evaluated and closely monitored. In addition to routine monitoring, the unaffected eye requires preventive therapy." This might be systemic medication for both eyes, or topical medication for the currently unaffected eye.

Gilger recommended evaluation of the unaffected eye every three to four months, or until glaucoma appears and a different evaluation/treatment schedule is put in place.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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