Glaucoma is a relatively rare find in horses (occurring in less than 1 in a 1,000 horses) however, it is a severe problem for affected horses. Glaucoma is due to a rise in the pressure inside the eye, caused by problems with drainage of the aqueous humor in the eye. The aqueous humor is the clear fluid in the eye between the lens and the cornea. This fluid is produced by the ciliary body in the eye and should drain at a similar rate to its production. If untreated, glaucoma can result in pain and blindness from optic nerve damage. At the end stages, removal of the eye is often recommended to remove the source of pain.
Equine glaucoma usually develops in horses that also have chronic recurrent uveitis (moon blindness). The inflammation associated with uveitis clogs the drainage portals in the eye. Clinical signs of glaucoma include a cloudy blue cornea (sometimes with white lines crossing it), redness in the white tissues around the cornea, signs of pain such as squinting and tearing, big pupils that don't shrink in bright light, an enlarged eye, and blindness.
An ophthalmic examination by a veterinarian is needed to diagnose glaucoma. Local anesthetic blocks will likely be done to facilitate the examination. Sedation may also be needed depending on the temperament of the horse. A diagnosis of glaucoma is made by measuring the pressure in the eye using a handheld tonometry instrument.
Therapy is directed at lowering the intraocular pressure. Topical therapy such as eyedrops or eye ointments should be tried first, although some horses may not tolerate them. Horses often do not respond to the medications used in other species. Dorzolamine hydrochloride is a topical medication that is given 3 times a day. This drug reduces the production of aqueous humor in the eye. Timolol, a topical medication given every 12 hours, can mildly decrease the intraocular pressure by decreasing fluid production and is often given along with the dorzolamine. If uveitis is present, systemic non-steroidal anti-inflammatories (such as banamine) and topical corticosteroid ointment are used to reduce inflammation.
If topical therapy does not control the glaucoma, or topical therapy is difficult or impossible, there are several surgical options. One option is using a laser to perform "transscleral cyclophotocoagulation". In this procedure, a laser is used to destroy parts of the ciliary body which produce the fluid. This procedure is done under general anesthesia or standing sedation with local anesthesia. The surgery can reduce the intraocular pressure, decrease or eliminate the need for topical medication, and maintain comfort and vision.
Gonioimplantation can also be performed. A tiny drain is surgically placed in the eye to increase drainage of the aqueous humor; however, this procedure has a low success rate and is uncommonly done. Treatment of anterior uveitis is crucial to minimize the development of glaucoma in horses.
In summary, glaucoma is a relatively uncommon but serious ocular condition in the horse. It can lead to pain, blindness and eventual removal of the eye (enucleation) if not recognized and treated. Contact your veterinarian if you suspect your horse may have glaucoma.