Glaucoma is a serious eye disease that often leads to blindness. There is a continuous (although very slow) exchange of fluid between the chambers of the eye and the systemic venous circulation. Fluid in the eye is produced by the ciliary body and leaves the eye at the angle formed by the iris and the cornea. Glaucoma occurs when fluid in the eye is produced faster than it can be removed. This leads to a sustained increase in intraocular pressure. High intraocular pressure causes degenerative changes to the optic nerve and the retina. Glaucoma is described as primary or secondary.
Primary glaucoma is a hereditary disease that affects Beagles, Cocker Spaniels, Basset Hounds, Samoyeds, and other breeds. In 50 percent of cases the second eye is involved within two years of the first.
Secondary glaucoma is a complication of another eye disease such as uveitis, displacement of the lens, or trauma to the eye. Treatment of secondary glaucoma is directed toward the underlying eye problem.
Glaucoma may also be acute or chronic, depending on how quickly the signs develop and how long the glaucoma has been present. An eye with acute glaucoma is exquisitely painful, with tearing and squinting. The affected eye feels harder than the normal eye and has a fixed, blank look due to the hazy and steamy appearance of the cornea and enlarged pupil.
Glaucoma in the chronic stage is associated with enlargement of the globe and protrusion of the eyeball. The eye may be tender to pressure and feel harder than the unaffected eye. In nearly all cases the affected eye is blind.
The diagnosis of glaucoma can be made only by a veterinary eye examination and measurement of intraocular pressure.
Treatment: Acute glaucoma is a veterinary emergency that can produce blindness in a matter of hours. This is one reason why it is so important to take your dog to a veterinary hospital immediately on suspicion of a painful eye. Medical treatment involves the use of drugs to rapidly lower intraocular pressure.
The initial drug of choice has been intravenous mannitol. Other veterinarians will start with prostaglandins, such as Xalatan, and give carbonic anhydrase inhibitors orally. Mannitol increases serum osmotic pressure and draws fluid out of the anterior chamber into the circulatory system. Other drugs used in treating glaucoma include carbonic anhydrase inhibitors that block the enzyme that produces the intraocular fluid. Topical medications increase outflow of fluid by constricting the pupil. This widens the angle between the iris and the cornea.
If medical treatment is not effective, a surgical procedure such as cyclodestructive surgery or filtering surgery may be done. These reduce fluid production in the eye. Some veterinarians use cryosurgery, which involves freezing and destroying a portion of the ciliary body to reduce the production of intraocular fluid. The operation can also be done with a laser, but this requires referral to a special canine eye center.
In chronic glaucoma the affected eye is blind and thus susceptible to corneal injuries and other problems, including intense pain. If these develop, the eye should be removed. If desired, a prosthesis can be inserted for cosmetic reasons.
Prevention: Eye examinations (such as the CERF exam discussed in Retinal Diseases, on this page)will detect small increases in intraocular pressure, thereby allowing sufficient time to start preventive treatment before glaucoma develops. Annual eye examinations should be performed on all dogs with a hereditary predisposition to primary glaucoma.
A dog with glaucoma in one eye must be watched carefully for signs of glaucoma in the other eye. Intraocular pressure should be measured every four months in these high-risk individuals. Dogs with primary glaucoma should not be used for breeding. There is now evidence that pulling on a neck collar increases intraocular pressure. Dogs with increased intraocular pressure, weak or thin corneas, or full-blown glaucoma should therefore be walked with a harness rather than a collar.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.