Glaucoma in Cats
Glaucoma is caused by an increase in fluid pressure within the eyeball. Normally, there is a continuous (although very slow) exchange of fluid between the eyeball and the venous circulation. Anything that upsets this delicate balance can cause a buildup of pressure and produce a hard, enlarging eye. When pressure within the eye becomes greater than the arterial blood pressure, arterial blood cannot enter the eye to nourish the retina.
Inflammations and infections within the eye are the most common causes of acquired or secondary glaucoma in cats (see Uveitis). Other causes are cataracts, eye injuries, and cancers within the eye. A lens that is out of alignment may block the outflow of aqueous fluid. Primary (congenital) glaucoma is rare but has been observed in Persians, Siamese, and domestic shorthairs.
A cat suffering from acute glaucoma exhibits mild to moderate tearing and squinting and there is a slight redness to the white of the eye. The affected pupil is slightly larger than the opposite pupil. The eye is painful when gently pressed and feels harder than the other eye. As fluid pressure increases to greater than 30 to 50 mmHg, the eye becomes noticeably larger and the surface begins to bulge. (Normal pressure is 10 to 20 mmHg.) In time, the retina is damaged. The lens may be completely or partially pushed out of alignment. This entire sequence can occur suddenly or over a matter of weeks.
To diagnose glaucoma, intraocular pressure is measured with a technique called tonometry, which uses an instrument placed on the surface of the eye. The interior of the eye must also be examined, and a procedure called gonioscopy checks the flow of fluid out of the eye. Ultrasound may also be used to evaluate the eye.
Every effort should be made to distinguish glaucoma from conjunctivitis and uveitis, both of which produce similar signs. It is critical to begin treatment of glaucoma before irreversible injury occurs to the retina. Some permanent vision may be lost before the disease is discovered.
Treatment: Acute glaucoma may require emergency hospitalization. Veterinarians use various topical and oral drugs to lower intraocular pressure. Mannitol may be used in the short term to lower pressure.
Maintenance drugs are used for chronic glaucoma. These might include carbonic anhydrase inhibitors topically or orally and, possibly, pilocarpine. Any underlying eye disorder should be treated. Treatment is for the life of the cat.
Failure to respond to medical management may suggest surgery is needed, if there is a potential to retain some vision. Surgery may try to decrease the fluid production or increase the rate of fluid escape from the eye, this reducing pressure within the eye. For an eye that is blind and painful, the best approach is to remove the entire eye. A prosthesis can be inserted for appearance.
It is felt that some glaucoma damage results from secondary nerve damage due to the cellular chemical glutamate. Glutamate is an amino acid and is extremely toxic to the retinal ganglion cells; basically, it overstimulates them. Drugs that block glutamate receptors, and calcium channel blockers-used to protect the retina and optic nerve-are being studied for possible therapy.
This article is excerpted from “Cat Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.
Copyright © 2008 by Delbert Carlson, DVM, and James M. Giffin, MD. All rights reserved.