A dog’s eyes are in some ways like complex, inflated balloons. They maintain their globoid shape due to the pressure that is generated inside. The portion of the eye that is located in front of the iris (the pigmented tissue that surrounds the pupil) is called the anterior chamber. The anterior chamber is filled with aqueous humor, a fluid continually produced by cells within the eye.
"Glaucoma is extremely painful and can rapidly cause permanent blindness."
Aqueous humor must drain from the eye at the rate it is made to maintain a steady pressure within the eye. Aqueous humor drains through the filtration angle located at the base of the iris and is subsequently absorbed into the blood stream.
What is Glaucoma?
Glaucoma is defined as higher than normal pressure within the eye. Most often, glaucoma develops because aqueous humor is not flowing out normally through the filtration angle. The fluid is still being produced however, so pressure within the eye increases.
Glaucoma can be either a primary disorder or develop as a result of another disease. Primary glaucoma is caused by abnormal development that impairs the ability of the filtration angle to function. Some breeds of dogs are genetically predisposed to primary glaucoma, including:
- Cocker Spaniels
- Basset Hounds
- Chow Chows
- Siberian Huskies
- Boston Terriers
- Shih Tzus
Secondary glaucoma develops as a result of injury or disease that adversely affects the eye’s filtration angle. Infection and inflammation, either within the eye itself or elsewhere in the body, can produce cells, proteins, and other materials that clog the holes through which aqueous humor should flow. Glaucoma can also be caused by trauma, tumors within the eye, lens luxation (when the eye’s lens is in an abnormal position), cataracts, and as a complication to surgery to the eye.
Symptoms of Glaucoma in Dogs
The symptoms of glaucoma can develop gradually when there is just a slight disparity between the production and drainage of aqueous humor or within a few hours when the filtration angle is suddenly and completely blocked. Glaucoma is extremely painful and can rapidly cause permanent blindness. Dogs with some or all of the following symptoms should receive veterinary attention immediately.
- rubbing at the eye or other evidence of pain
- third eyelid elevation
- the whites of the eye appear red
- increased tear production
- the pupil is dilated in bright light
- the surface of the eye becomes cloudy
- enlargement of the eye
Diagnosing Glaucoma in Dogs
Many diseases and injuries can cause dogs to develop a red, painful eye. Tonometry, measurement of a patient’s eye pressure, is a quick way to determine if glaucoma is to blame. Drops are applied to numb the surface of the eye and then the eye pressure measured with one of several different devices. Higher than normal readings in the absence of another explanation are diagnostic for glaucoma.
If a dog’s eye pressure is extremely high, the veterinarian will immediately begin emergency treatment. Otherwise, he or she will search for an underlying cause. This will include a complete history and physical exam and sometimes laboratory tests to determine if a dog is suffering from a systemic illness that has resulted in glaucoma. The veterinarian will also examine both eyes using an ophthalmoscope looking for evidence of infection, inflammation, anatomic abnormalities, or other problems. If necessary, a primary care veterinarian may refer a dog with glaucoma to a veterinary ophthalmologist for advanced diagnostics and treatment.
Treating and Preventing Glaucoma in Dogs
Severe glaucoma can result in blindness within only 12 to 24 hours. In these cases, eye drops, oral medications, and intravenous infusions can all be used to bring a dog’s eye pressure down to a safe level as fast as possible. Once the patient’s condition is stable, the veterinarian will design a treatment protocol aimed at keeping the dog’s eye pressure in the low-normal range. When dogs have secondary glaucoma, appropriate treatment of the primary problem will often resolve the glaucoma as well, meaning that glaucoma treatment can eventually be discontinued.
On the other hand, primary glaucoma or secondary glaucoma that is caused by a condition that cannot be cured or fully managed usually requires life-long treatment. This may include topical and/or oral medications that reduce the production of aqueous humor, increase the flow of fluid through the filtration angle, and decrease inflammation. Oftentimes, dogs require combination therapy with several different medications to maintain a healthy eye pressure. Medications commonly used in the long-term treatment of glaucoma include acetazolamide, dorzolamide, methazolamide, pilocarpine, demecarium bromide, and latanoprost.
Dogs with primary glaucoma often first develop symptoms in only one eye. However, the other is also at high risk for glaucoma, and veterinarians will often prescribe medications for both eyes and/or a systemic medication to try to prevent a sudden increase in eye pressure in the dog’s “good” eye.
If eye drops and oral medications can’t control a dog’s glaucoma, surgery is an option. A veterinary ophthalmologist can destroy some of the cells that produce aqueous humor using a laser or other implements or implant tubes that drain fluid from the eye. If a dog’s eye is permanently blind but still causes pain, it can be surgically removed or an injection given into the back of the eye to destroy all the cells that produce aqueous humor.
Dogs with glaucoma need to be closely monitored to ensure that their eye pressures remain within an appropriate range, and treatment protocols may need to be modified as the patient’s condition changes. However with appropriate care, dogs with glaucoma, even those that have gone blind, can enjoy an excellent quality of life.
Epidemiology of canine glaucoma presented to University of Zurich from 1995 to 2009. Part 1: Congenital and primary glaucoma (4 and 123 cases). Strom AR, Hässig M, Iburg TM, Spiess BM. Vet Ophthalmol. 2011 Mar;14(2):121-6.
Epidemiology of canine glaucoma presented to University of Zurich from 1995 to 2009. Part 2: secondary glaucoma (217 cases). Strom AR, Hässig M, Iburg TM, Spiess BM. Vet Ophthalmol. 2011 Mar;14(2):127-32
The efficacy of topical prophylactic antiglaucoma therapy in primary closed angle glaucoma in dogs: a multicenter clinical trial. Miller PE, Schmidt GM, Vainisi SJ, Swanson JF, Herrmann MK. J Am Anim Hosp Assoc. 2000 Sep-Oct;36(5):431-8.