Glaucoma is caused by increased pressure build up within the eye. Cells inside the eye produce a clear fluid (“aqueous humor”) that maintains the shape and architecture of the eye, while nourishing the tissues inside the eye. The balance of fluid production with subsequent drainage is responsible for maintaining a normal pressure homeostasis within the eye. With glaucoma, the drain becomes clogged or stymied, but the eye keeps producing fluid. As a result, the pressure in the eye increases. The increased pressure in the eye actually can cause the eye to stretch and enlarged, while destroying delicate strcutures within the eye.
Glaucoma is classified as either primary or secondary in dogs and cats. Primary Glaucoma is a genetically inherited condition. Primary glaucoma is prevalent in many breeds, most notably Labrador Retrievers, Basset Hounds, American Cocker Spaniels, Chow Chows, Shar Peis, and Spitz type breeds
Glaucoma is uncommon in cats.
Primary Glaucoma usually presents with just one affected eye, but in the majority of cases will affect the other eye leading to complete blindness. Secondary Glaucoma occurs when other eye diseases lead to decreased intraoccular fluid drainage. Common causes of secondary glaucoma are inflammation within the eye (uveitis), cataracts, cancer of the eye, chronic retinal detachment, luxation of the lens. Glaucoma in cats is usually secondary to uveitis.
It is very important to determine if the patient is affected by primary vs secondary glaucoma, as the treatment needed and the prognosis for vision is different for each type. Diagnostic techniques and instruments used by veterinary ophthalmologists include: slit lamp biomicroscopy, indirect ophthalmoscopy, and gonioscopy to determine the type and cause of glaucoma in your dog or cat. Gonioscopy specifically determines how predisposed the remaining normal eye is to develop glaucoma when primary glaucoma is suspected. This test involves placing a special contact lens on the eye which allows examination of the drainage angle. Gonioscopy is usually requires sedation or general anesthesia.
Pressure damage to the optic nerve and decreased blood flow to the retina, results in vision loss. However, if the pressure in the eye remains chronically elecated, the retina degenerates and vision is permanently lost. Permanent blindness can occur within only several hours if the pressure is severely elevated and the glaucoma develops rapidly.
Unfortunately, the first eye to develop primary glaucoma in dogs is often already without vision by the time the disease is recognized, and the dog brought to the vet. Treatment in these cases is therefore focused on relieving pain in the blind eye and preventing or delaying glaucoma development in the other eye. Gonioscopy of the remaining visual eye helps determine the appropriate treatment for this eye.
Increased intraocular pressure is quite painful. Dogs and cats, as well as humans have normal intraocular pressures between 10 and 20 mmHg. Glaucoma commonly results in pressures of 20-28 mmHg in humans, but pressures of 45-65 mmHg are common in dogs and cats. For this reason, it is logical to assume that glaucoma in pets is more painful than glaucoma in humans. The pain is perceived as constant headache or migraine. This discomfort can result in lethargy, less desire to play, irritability, or decreased appetite. It is important foir the owner to recognize these signs, as dogs and lack the ability to verbalize their pain.
Since glaucoma occurs because intraoccular fluid is not draining from the eye efficiently enough, the logical treatment is to open up the drain. Unfortunately, opening the drain and keeping it open is challenging. Therefore, many glaucoma therapies are also aimed at inhibiting intraoccular fluid production by the eye.
There are several different types of eye drops and pills that help to decrease fluid production or increase fluid drainage from the eye. While these medications are helpful in animals, they tend to be expensive, and usually do not control glaucoma longterm. Consequently, they are used mostly to help prevent or delay the onset of glaucoma in the remaining normal eye, and as temporary treatment until surgical correction can be performed in the affected eye.
The type of surgical procedures available for glaucoma depends upon whether vision can potentially be restored. For visual eyes, intraocular pressure can be reduced by performing a cycloablation procedure and a drainage implant procedure. For permanently blind eyes, the eye can be removed (enucleated) with the option of placing a sterile prosthetic ball implant in the eye for asthetic purposes. Another option is to administer an injection of a drug calle Gentamycin into the eye that kills the fluid-producing cells and reduces the pressure for comfort. Gentamycin injection is not recommended for cats.
Which procedure is the most appropriate for your dog or cat affected with glaucoma depends on the type of glaucoma, the potential for restoration of vision, and your preference for the cosmetic appearance of your pet. Glaucoma is a frustrating disease because it entails constant monitoring, may require several different therapies, usually involves a substantial financial commitment, and despite excellent care often still results in permanent loss of vision. The key to preserving vision
early detection and regular follow up ophthalmic care.
It is important to realize that glaucoma may still lead to blindness in spite of our best efforts. A high level of owner comliance and commitment to treatment with regular ophthalmic examinations, is required for the best chance of preserving a pet’s vision. If your dog or cat is diagnosed with primary glaucoma, please notify the dog’s breeder, as this is a genetic disease.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 10/27/2012