To understand glaucoma, it is necessary to understand how the fluid inside the eye normally flows and maintains normal intraocular pressure.
The eye is filled with aqueous humor, a fluid derived from blood. Aqueous humor enters the eye through the pupil and drains from the eye at the draining angle, a sieve-like network at the junction of the clear cornea and the colored iris. Aqueous humor is made inside the eye and passes from the eye at the same rate. This results in a stable intraocular pressure of 15-25 mm of mercury (Hg).
American Cocker Spaniels, Basset Hounds, Siberian Huskies, Samoyeds, and Shar Peis are prone to glaucoma, although any breed of dog may be affected.
Glaucoma is a frequent cause of blindness in both humans and pets and results from a blockage of the outflow of aqueous humor. Continued fluid secretion into the eye with no path for the fluid to exit elevates the pressure inside the eye – much like filling up a balloon – and damages the retina and optic nerve. Pain comparable to that of a migraine headache can result from pressure higher than 30 - 40mmHg, while pressure greater than 60mmHg can result in permanent vision loss in fewer than 12 hours.
Glaucoma can be categorized in two ways. One is based on how the drainage angle appears on examination. The drainage angle may be open, narrowed, closed or abnormally developed. Breeds in which primary open-angle glaucoma is found are Beagles, Miniature Poodles, and Norwegian Elkhounds. Narrow-angle glaucoma is an abnormal narrowing of the outflow channel and is seen in American and English Cocker Spaniels. In a developmental abnormality of the drainage angle, the outflow is decreased during times of inflammation. This condition is commonly seen in basset hounds; American and English Cocker Spaniels; Samoyeds; Flat-Coated Retrievers; and Chow Chows.
Glaucoma can also be categorized by disease process as either primary or secondary. Primary glaucoma is an abnormally narrowed drainage angle that occurs without any obvious disease in the eye and is considered an inherited disorder. It commonly affects such breeds as Basset Hounds, Beagles, Miniature Poodles, Norwegian Elkhounds, Cocker Spaniels, Chow Chows, Siberian Huskies, Shar Peis, and Standard Poodles.
Secondary glaucoma is caused by changes that affect the eye and is the result of some intraocular condition that interferes with the natural flow of aqueous humor – usually ocular inflammation, lens dislocation, neoplasm, previous surgery or injury to the eye. Lens luxation is often the cause of secondary glaucoma in terrier breeds.
Glaucoma results in blindness by blocking the nerve impulse through the optic nerve. Once the optic nerve has been permanently damaged, it is impossible to restore vision. With early surgery and then medical therapy, your pet's vision may be maintained. Frequently, with extreme elevations of pressure the eye becomes permanently blind and painful very rapidly.
A glaucoma diagnosis is based on your pet’s history, clinical signs, measuring the intraocular pressure, and visually examining the drainage angle using a process known as gonioscopy. Pain as a diagnostic criteria is generally disregarded since your pet cannot tell the veterinarian that they hurt. Clinical signs of glaucoma include some or all the following: excessive tearing; green or yellow eye discharge; a reddened eye; an eye that suddenly looks blue; an eye with a large pupil that will not move when light is shined into it; a pet who sleeps a lot; a pet who hides under the bed or suddenly becomes frightened or irritable. People with glaucoma often report a constant headache medication will not help. An eye with glaucoma becomes enlarged in later stages of the disease.
In some cases, your vet may use a special lens to examine the drainage angle in a process called a gonioscopy. This involves placing a dome-shaped contact lens called a goniolens on the corneal surface after numbing the eye with topically-applied anesthetics. The lens allows your vet to directly visualize the drainage angle. Gonioscopy occasionally requires sedation, but in most pets it can be performed after the use of only a topical anesthetic. In addition to helping to determine the cause of the glaucoma, gonioscopy may also provide a prognosis for vision. The technique is essential to evaluate the glaucoma-free eye for risk of a future problem.
Whenever your pet is showing signs of a health issue, your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from glaucoma or another serious condition, a veterinary specialist is available at an ExpertVet certified or affiliated hospital.