Glaucoma in dogs and cats results from a variety of abnormal conditions of their eyes which cause a rise in the pressure of the fluids within the globe.
Normal intraocular pressure (IOP), the pressure of fluids inside the eye, is controlled by a delicate balance between fluid production and fluid removal. Specifically this fluid is aqueous humor, produced in the posterior chamber of the eye (behind the iris) by the ciliary body. The iris is the portion of the eye that provides “eye color” and which opens and closes to make the pupil larger or smaller. Aqueous humor flows between the iris and the lens to the anterior chamber (in front of the iris) where it exits the eye via the trabeculae in the drainage angle, a hidden part of the eye just beyond the cornea, the clear portion of the front of the eye. The iris also plays a part in removal of aqueous humor.
Pressure measurement is generally performed using a tonometer in a procedure called tonometry. There are three types in common use, the Tonopen®, Tonovet® (manufactured by a competing company), and the Schiotz. Tonopen and Tonovet are fairly automated devices that are considered to give easy, accurate readings. Some longtime users of the Tonopen® are switching to the TonoVet®, while some doctors don’t care for it at all. Like many things in practice, it’s a personal preference. The Schiotz tonometer is a mechanical device that gives a reading, which is then converted to intraocular pressure. IOP is usually read in millimeters of mercury (mm/Hg) by both devices. Normal is considered to be between 10 and 20 for the Tonopen and TonoVet, and 15 and 30 (some veterinary ophthalmologists use 25 as the upper limit) for the Schiotz.
When pressure begins to increase, the eye responds with redness, “steamy” (cloudy) cornea, dilated and unresponsive pupil, blindness, bulging of the eyeball. These are the outwardly visible changes and not necessarily all of them are present in every eye. Ophthalmoscopic examination by your pet’s doctor can reveal additional changes inside the eye.
Time is of the essence in diagnosis and treatment of glaucoma. In short order the high pressure inside the eye will damage the retina, leading to permanent blindness. Indeed, approximately half of the patients in whom glaucoma is diagnosed have already lost vision in one eye.
The good news is that vision may be spared for a time in the other half of affected eyes and unaffected eyes (on the other side) can be treated prophylactically, perhaps permanently saving vision in that eye.
In 1980, when I graduated from Auburn University’s College of Veterinary Medicine, a glaucoma diagnosis came with good and bad news. The good news was the medicine to treat glaucoma was cheap. The bad news was that it didn’t work very well, and your pet would soon go blind.
Today there is still good and bad news. The good news is that the medicines we use to both control and prevent glaucoma work great, and vision can be preserved for approximately one to one and one half years. The bad news is that it’s pretty expensive.
Modern medication for glaucoma treatment is typically used twice daily in affected eyes and once daily in unaffected eyes predisposed to glaucoma. In the initial stages of emergency, high-pressure glaucoma medications might be used much more often until the problem is controlled.
If one eye becomes glaucomatous, it is just a matter of time before the other one is also affected. Dr. Laurence Galle, ACVO, calls them “yet-to-be-affected eyes.”
Some glaucomatous eyes that fail to respond to topical medications will improve with surgery performed by a board-certified veterinary ophthalmologist. Your pet’s doctor can refer you if that becomes necessary.
Untreated or uncontrolled glaucoma is a very painful condition. Imagine lying on your back with a brick balanced standing on end on one of your eyes. Now balance five more bricks on top of that one. The pressure is unbearable and the pain and headache soon wear down the happiness, immune system and overall health of the affected dog or cat. When your veterinarian tells you that your pet has glaucoma you must either begin treatment or, if you cannot afford it, allow the doctor to remove the eye (especially if it is no longer visual).
Likewise, if treatment has failed and IOP cannot be controlled, please allow your veterinarian to surgically remove the eye(s).
In summary, we are blessed to have medications that can almost always control canine and feline glaucoma with ease, at least for a time. It no longer carries a prognosis of “go home and go immediately blind.” There may be sacrifices to make to afford your pet’s glaucoma medication, but the reward of a visual pet is more than worthwhile.