Treating glaucoma in your equine veterinary patients
At that time there were no measuring devices to determine intraocular pressure (IOP), no tonometers. "Tonometry became available during the early 1980s so that we could measure the IOP on horses," says Brooks. "We found out that horses did get glaucoma and that it's treatable, so we can at least stabilize their remaining vision for a relatively long period of time."
Going back as far as the 1860s, literature suggests that army veterinarians used their fingertips to detect higher pressure in the diseased eyes of cavalry horses with uveitis.1 "At the time they called the condition hypertensive uveitis," says Brooks, "but they could not take measurements to determine it quantitatively. They knew that horses with 'moon blindness' had abnormal IOP, but they didn't understand the disease glaucoma in those early days of the practice."Causes and physiology
Equine glaucoma is most often a result of intraocular inflammation from equine recurrent uveitis. Similar to glaucoma in humans, the disease in horses is sometimes painful and eventually results in vision loss. In the early stages of glaucoma, affected horses may only show subtle cloudiness of the cornea. An obstruction of aqueous humor outflow inside the front of the eye, or anterior chamber, causes the elevation in IOP.2 The increased IOP causes reduced blood flow to the retina and obstruction of optic nerve axoplasmic flow, eventually resulting in retinal cell death, compression of the optic nerve and blindness.2
Aqueous humor is produced by the ciliary body processes and provides metabolic support to the cornea and internal structures of the eye. It flows from the posterior ocular chamber through the pupil to the anterior ocular chamber and drains from the eye via the iridocorneal or uveoscleral outflow pathways.2 Horses are unique in that up to 50 percent of aqueous humor drainage is via the uveoscleral outflow pathway.
Glaucoma in horses is seen as primary (i.e. resulting from an abnormally developed aqueous humor outflow pathway) and secondary (i.e. blocked outflow pathways in response to equine recurrent uveitis, intraocular tumors, corneal perforation or lens luxation). For some foals, it can also be a congenital condition present at birth.
Appaloosas are more susceptible to uveitis and glaucoma. And while the condition is thought to be genetic, no one really knows why these horses are affected. The bottom line? All horses can get glaucoma.3
Clinical signs of glaucoma vary. They may be subtle, or one may observe a red, cloudy, painful or squinty eye, says Erica Tolar, DVM, DACVO, with Blue Pearl Veterinary Hospital in Louisville, Ky. Many horses with acute and chronic glaucoma don't exhibit the classic signs of ocular pain, which include eye closure (blepharospasm) and excess production of tears (epiphora).2
"Eyes can only do so many things when they're 'angry,'" Tolar says. "A practitioner may see a horse with a 'cloudy' eye that stains negative. The next step is to check the IOP, which will give us one more piece of the puzzle."
Horses with glaucoma show variable pupillary light reflex deficits, but the disease often results in dilation of the pupil (mydriasis) due to damage to the retina, optic nerve and iris sphincter muscle. Horses with concurrent intraocular inflammation may have miosis (constriction) of the pupil. Dyscoria, or an abnormally shaped pupil, may result from adhesions between the iris and the lens capsule.2