Glaucoma is an elevation of the pressure within the eye that is incompatible with normal function of the eye. It is a disorder of the outflow of fluid (aqueous humor) from the eye and not a disease of overproduction of fluid within the eye. Sudden, high elevations of pressure within the eye are common in the dog and can occur without warning over several hours. These acute elevations in pressure can cause devastating and irreparable damage to the retina (which acts like the film a the camera) and the optic nerve (which sends information from the eye to the brain).
The causes of glaucoma are both primary (spontaneous, probably inherited) and secondary (arise in association with other diseases within the eye). Primary Glaucoma The list of predisposed breeds is long, but some of the most commonly affected dogs are the American and English cocker spaniel, basset hound, beagle, chow chow, Chinese shar-pei, Siberian husky, Dalmatians, Great Danes, Bouvier des Flandres, Boston terrier, Norwegian elkhound, Samoyed, English and Welsh springer spaniel, Alaskan malamute, poodle, and shih tzu.
The fluid outflow pathway in the eye is commonly known as the drainage angle. For proper outflow, the angle needs to be open and functioning properly. The drainage area looks and acts almost like the sieve in a kitchen drain. In many breeds of dog, this drainage angle can be abnormal. Either the access to the sieve is narrow, there are too few drainage holes, or the drain behind the sieve does not work properly. Even though the fluid passes through the sieve it does not drain into the veins around the eye well and backs up, causing the pressure in the eye to increase.
Inflammation within the eye that gives rise to glaucoma has many different causes. In the dog, chronic anterior uveitis (an inflammation of the iris and surrounding tissues) is a common cause of secondary glaucoma. Chronic uveitis may arise with certain infectious diseases such as Lyme disease or fungal infections, or as a side effect of cataract development (lens-induced uveitis). In some cases of chronic inflammation, the cause is never determined. One or both eyes may be affected by the inflammation, and one or both eyes may develop glaucoma.
Tumors within the eye generally occur in older pets. Only one eye is usually affected and the progression to glaucoma may be relatively slow.
Trauma can happen to any unrestrained dog, at any age. Trauma might include being hit by car, being hit in the head, falling, or being attacked by another animal.
Primary lens luxations (dislocations) are much more common in the terrier breeds. These breeds include the Parson Russell terrier, Tibetan terrier, Sealyham terrier, smooth and wirehaired fox terrier. The Border collie, Australian blue heeler, and individuals in some other dog breeds may also develop primary lens luxations. It is believed that these dogs have an inherited premature degeneration of the fibers that hold the lens in place. Once the fibers holding the lens break down, then it is free to move around within the eye. When the lens moves in front of the pupil, severe acute glaucoma can develop within hours.
Secondary lens luxations are more likely to cause slow-developing or chronic glaucoma. The lens may dislocate in association with cataract development, the growth of tumors within the eye, and progressive detachment of the retina.
What to Watch For
Many diseases can cause a red, squinty, teary and cloudy eye, but glaucoma should always be considered as a cause. Frequently when the pressure is high, the dog is quite painful and holds the eye closed or keeps the third eyelid (a pink/red membrane) up over the eye.
If the pressure inside the eye remains elevated for sometime, the eyeball may actually stretch and enlarge. As the eye enlarges or becomes buphthalmic, the eyelids may no longer be able to protect the surface of the eye, and the eye may be traumatized easier.
Assessing vision in the dog can be difficult. Your pet should blink and try to close the eye when a very bright light is shown into the eye. Without creating airflow, see if your dog blinks when you wave your hand in front of the eye (known as a "menace test" because your pet is responding to a menacing gesture). You can also try to throw objects like cotton balls (anything that won't make a sound when it lands) in front of your pet to see if he follows the object. Remember that a dog with good vision in one eye will act normally, so it may not be possible to tell if vision is altered in the glaucomatous eye.