What is glaucoma?
Glaucoma is the elevation of pressure inside the eye (intraocular pressure) beyond a specific point at which vision is no longer possible. Glaucoma is a frequent cause of blindness in humans and in our pets. To understand glaucoma, it is necessary to understand how the fluid inside the eye normally flows and maintains normal intraocular pressure. Fluid inside the eye (aqueous humor) is produced behind the colored area of the eye (iris) in a portion of the eye called the ciliary body. This aqueous humor is made by filtering blood. The fluid flows through the dark hole in the eye (pupil). Finally the aqueous humor drains from the eye at the junction of the clear cornea and the colored iris (drainage angle) inside the eye and then the aqueous rejoins the blood. The drainage angle is a sieve-like network. This 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 Hg. Glaucoma is the consequence of a blockage of the outflow of aqueous humor and a subsequent buildup of pressure inside the eye. The resulting high pressure compresses the optic nerve and results in pain and in blindness.
Are there types of glaucoma?
There are two categories of glaucoma. Primary glaucoma occurs without any obvious disease in the eye. Secondary glaucoma occurs when some other cause is present. Another way to categorize glaucoma is based on how the drainage angle appears on examination. The drainage angle may be open, narrowed, closed, or abnormally developed. Primary glaucoma is known to occur in certain purebred breeds of dogs and is thought to be inherited. Breeds in which we see primary open-angle glaucoma are the Beagles, Miniature Poodles and Norwegian Elkhounds. Narrow-angle glaucoma (an abnormal narrowing of the outflow channel) is seen in American and English Cocker Spaniels. In a developmental abnormality of the drainage angle (goniodysgenesis) the outflow is decreased during times of inflammation. This condition is commonly seen in the Basset Hound, American and English Cocker Spaniel, Samoyed, Flat-coated Retriever and Chow Chow. Secondary glaucoma is the result of some intraocular condition that interferes with the natural flow of aqueous humor. Conditions that commonly cause secondary glaucoma include the result of ocular inflammation (uveitis), lens dislocation (luxation), neoplasm (cancer), previous surgery and injury to the eye. Glaucoma results in blindness by blocking the nerve impulse through the optic nerve. Once the optic nerve has been permanently damaged, there can be no restoration of 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. The aim of therapy at that point is to keep your pet pain-free and maintain a cosmetic eye.
How is glaucoma diagnosed?
The diagnosis of glaucoma is based on history, clinical signs, measuring the intraocular pressure [tonometry] and visually examining the drainage angle using a process known as gonioscopy. We cannot use the signs of "pain" as a criteria as our pets cannot tell us of their pain directly. Clinical signs of glaucoma include some or all the following: excessive tearing, a green or yellow eye discharge, a reddened eye, an eye that suddenly looks blue, an eye with a pupil that is large and will not move when light is shined into it, a pet who sleeps a lot, a pet who hides under the bed or a pet who suddenly becomes frightened or irritable. People with glaucoma often report a constant headache that medication will not help. An eye with glaucoma becomes enlarged in later stages of the disease. Tonometry is the measurement of pressure within the eye. A variety of techniques can be used to estimate intraocular pressure, including SchiÖtz tonometry and applanation tonometry. In our clinic, we use the highly accurate applanation tonometer. Gonioscopy is a technique used to evaluate the drainage angle. It involves placing a dome-shaped contact lens [goniolens] on the corneal surface after numbing the eye with topically applied anesthetics. This lens allows us to directly visualize the drainage angle. Gonioscopy occasionally requires sedation but in most pets it can be performed after the use of topical anesthetic only. The technique is essential to evaluate the eye that doesn¹t have glaucoma for risk of a future problem.
How is glaucoma treated?
Many of us have friends or relatives who have glaucoma. They simply place drops in their eyes several times a day and have very few problems that result in vision loss. In some people medication will not resolve the glaucoma and surgery is necessary. This is what we face in animals all the time and this is what makes glaucoma very difficult to treat in domestic animals. After the initial diagnosis of glaucoma is made, your pet is aggressively treated with medication if there is any hope of saving vision. This will require a period of hospitalization. During periods of hospitalization, medication (mannitol and Diamox®) may be given directly into the vein to help reduce the intraocular pressure. Additional drugs are used that are aimed at increasing the outflow of aqueous humor and/or suppressing its production. These drugs may include pilocarpine, Trusopt®, timolol, Betoptic®, epinephrine, newer synthetic epinephrine-like drops and combinations of these drugs. Topically applied prostaglandins, such as lantaprost, also have been used in humans to reduce intraocular pressure. Yet more medications, known as carbonic anhydrase inhibitors, are aimed at reducing the production of aqueous humor. Examples of these medications are Daranide® and Neptazane®. Once the pressure has been controlled, surgery is essential to maintain vision. It is impossible to control glaucoma with medication alone in dogs, cats and horses! A variety of surgical techniques have been developed to aid in the control glaucoma.
At the recent A.C.V.O. (American College of Veterinary Ophthalmologists) meeting in Seattle, WA, Dr. Dennis Brooks from the University of Florida, an internationally known expert on glaucoma in animals, reported that calcium channel blocking drugs may help prevent damage to the optic nerve in patients with glaucoma. Such drugs as Norvasc® may thus help prevent damage to the retina and optic nerve and should therefor be given to pets with glaucoma.
If my pet has glaucoma and can still see, what can be done?
Laser surgery is the treatment of choice in pets with primary glaucoma who can still see. As with the freezing procedure listed below, no cutting is required. Your pet does have to be anesthetized so he or she won¹t move. The laser burns completely through the white outer layer of the eye without damaging it except for redness and swelling. The laser selectively kills small areas of the ciliary body and this reduces fluid production. The main complication of this technique is that it is like the Marshall of an old west movie who is shooting through the wall of a building to try to kill the villain. He can't see the villain and similarly the Doctor cannot see the ciliary body. Occasionally the ciliary body will not be damaged enough and a second procedure is needed to lower fluid production and restore normal intraocular pressure. Another complication is that the intraocular pressure may actually increase in about 5-10% of patients due to an initial increase in fluid production. This complication usually occurs in patients who have had previous glaucoma "episodes." The elevation of intraocular pressure will require a filtering "shunt" be placed to draw off excessive fluid. If a pet has glaucoma secondary to the loosening and dislocation of the lens (luxation), the lens must be removed to resolve the glaucoma. Laser cycloablation surgery may be needed at the time of the original surgery or during a second surgery to permanently control glaucoma. This procedure is also recommended as a preventative in the second eye of pets who have glaucoma and are blind in one eye and are currently visual in the second eye.
Cyclocryothermy (cryosurgery) is a freezing procedure that was developed a number of years ago to decrease the production of intraocular fluid in the eyes of pets who can still see. The technique involves freezing the ciliary body with a small probe placed on the outside of the eye. No cutting is required. Again, your pet will have to be anesthetized to prevent them from moving. The freezing kills the cells in the ciliary body that produce the aqueous humor. A number of sites are frozen depending on how elevated the pressure is. After surgery, there is considerable swelling and redness to the white of the eye that is to be expected. Complications of this technique include retinal detachments, severe intraocular inflammation, high intraocular pressure immediately following the freezing that may lead to permanent blindness, shrinkage of the eye or cataract formation. As with the laser surgery, if cryosurgery is performed and the intraocular pressure increases, a filtering "shunt" may be needed to reduce the pressure. Finally, the glaucoma may return at a later date requiring a second surgery.
What if my pet cannot see any longer?
The goal of surgery in this situation is to help make your pet pain free. One technique employed to result in a cosmetic, pain-free eye for your pet is the placing of a silicone implant inside the eye [intraocular prosthesis]. The technique involves removal of the contents from the inside of the eye--leaving the outer shell of the eye--and implanting a silicone implant within the wall of the eye. The shape of the eye is maintained and the eye moves normally. If you think of making a 'forever' grape by scooping out the insides and placing a marble within the grape skin and finally sewing the skin of the grape closed, you have some idea of how the surgery works. Following the initial postoperative treatment, minimal care is needed and the eye is maintained in a relatively normal cosmetic appearance while being pain-free. Complications of this technique are that corneal ulceration occasionally occurs following surgery (due to drying) and scarring of the cornea (resulting in a gray appearance.)
Another technique used to control glaucoma is the injection of antibiotic compounds into the inside of the eye. These antibiotics in high concentrations result in a killing effect on the ciliary body resulting in the reduction or cessation of the aqueous humor production. If the eye was visual the antibiotic would also kill the retina resulting in permanent blindness. Therefore, this technique can be used only on eyes that are definitely blind due to chronic pressure elevations. Your pet will have to be sedated and an anesthetic is applied to the eye and injected immediately beneath the white conjunctiva. Complications of this technique are generalized shrinking of the eye, return of the glaucoma at a later time, blindness if the eye was visual and occasionally chronic pain. This technique is only of value in quite elderly pets or pets who cannot safely undergo anesthesia for medical reasons such as kidney failure.
Finally, the blind, painful eye may be removed (enucleated). After the eye is removed, the skin is stitched shut and the hair will re-grow over the surgery site and a slight amount of pigment will be left. This surgery again requires that your pet be anesthetized. The possible complication to this technique is possible infection.
What is the long term chances for my pet?
Glaucoma is seldom diagnosed early enough to restore vision in the first eye affected. Therefore, during the initial examination time will be spent to evaluate the "good" eye. Eventual outcome depends upon early accurate diagnosis, possible laser preventative surgery, appropriate medical therapy, and regular and consistent reevaluations to save the vision of the remaining eye.
CONCLUSION
Glaucoma remains a leading cause of blindness in veterinary patients. Because of the nature of the disease, many pets are presented at a time when it is not possible to restore vision to the first eye affected. Glaucoma is very difficult to treat in our pets. Unlike humans where medication resolves over 80% of the cases of glaucoma, surgery is almost always required in veterinary patients. The goal of the Veterinary Ophthalmologist in treating a pet with glaucoma is to restore vision when possible and, if vision is not possible, to help your pet remain pain-free. Additionally, we want to maintain vision in the second eye as long as possible. Your Veterinary Ophthalmologist teamed with your regular Veterinarian will recommend appropriate therapies suitable for your pet and your circumstances.
Have any questions on this subject? Contact
Dennis Hacker via E-mail:
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