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Glaucoma in Veterinary Patients

 

Glaucoma is the elevation of pressure inside the eye, known as intraocular pressure (IOP) beyond a specific point at which vision is compromised or is no longer possible. Glaucoma is a frequent cause of blindness in humans and animals.

To understand glaucoma, it is necessary to understand how the normal flow of intraocular fluid maintains normal intraocular pressure. The fluid inside the eye is called the aqueous humor which is produced in the ciliary body which is located behind the iris. This fluid flows through the pupil and drains from the eye through a sieve-like network located at the junction of the cornea and the iris called the iridocorneal cleft or drainage angle. The aqueous humor is produced and drains from the eye at approximately the same rate, resulting in a stable pressure inside the eye of 15 to 20 mmHg (millimetres of mercury). Glaucoma occurs as a consequence of inadequate outflow of aqueous humor and a subsequent buildup of pressure inside the eye. The resulting high pressure damages the optic nerve and results in blindness.

There are two categories of glaucoma. Primary glaucoma occurs without any other ocular cause. Secondary glaucoma occurs when some other inciting cause is present.

Primary glaucoma is known to occur in certain purebred breeds of dogs and is thought to be inherited. Breeds in which we see open-angle glaucoma are the Beagles and Norwegian Elkhounds. Narrow-angle glaucoma (an abnormal narrowing of the outflow channel) is seen in American and English Cocker Spaniels. A developmental abnormality of the drainage angle called goniodysgenesis which results in decreased outflow during times of inflammation is seen in the Basset Hound, American and English Cocker Spaniel, Samoyed, and Chow Chow.

Secondary glaucoma is the result of some intraocular condition that interferes with the natural flow of ocular fluid. Diseases that commonly cause secondary glaucoma include ocular inflammation (uveitis), lens dislocation, intraocular tumours 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 aggressive and appropriate surgical intervention and then medical therapy, your pet's vision can at times be maintained. Frequently with extreme elevations of pressure, the eye becomes permanently blind and painful. The aim of therapy at that point is to keep your pet pain-free and maintain a cosmetic appearance to the eye.

Diagnosis of Glaucoma

The diagnosis of glaucoma is based on history, clinical signs, tonometry and 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 of 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. In later stages of glaucoma, the eye becomes enlarged. Tonometry is the measurement of pressure within the eye. A variety of techniques can be used to estimate intraocular pressure, including Schiotz tonometry and Applanation tonometry. Most veterinary ophthalmologists use the highly accurate applanation tonometer. Gonioscopy is a technique used to evaluate the drainage angle. It involves placing a goniolens - a dome-shaped contact lens - on the corneal surface after freezing the cornea 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 with the use of topical anesthetics only. The technique is essential to evaluate the non-glaucomatous eye for risk of a future attack of glaucoma.

Glaucoma
Schiotz Tonometer

Glaucoma
Tonopen

Glaucoma
Goniolens

Glaucoma
Normal Drainage Angle

Glaucoma
Abnormal Drainage Angle
Pictures courtesy of Dr. J. Schoster, University of Wisconsin, School of Veterinary Medicine

Treatment of Glaucoma

Glaucoma is very difficult to treat in domestic animals. Many of us have friends or relatives who have glaucoma and they simply place drops in their eyes several times daily and have very little problems resulting in vision loss. Our pets are not like this. In some people medication will not resolve the glaucoma and surgery is necessary. This is what we face in animals all the time.

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 may be given directly into the vein to help reduce the intraocular pressure. Additional drugs commonly used include those that are aimed at increasing the outflow of aqueous humor and/or suppressing its production. These drugs include pilocarpine, timolol, epinephrine, some newer synthetic epinephrine-like drops and combinations of these drugs. Yet more medications, known as carbonic anhydrase inhibitors, are aimed at reducing the production of aqueous humor. Examples of these medications are Daranide(R) and Neptazane(R).

Once the pressure has been controlled, surgery is essential to maintain vision. It is impossible to control glaucoma with medication alone. A variety of surgical techniques have been developed which aid in the control of intraocular pressure.

Treatment for an Eye with Glaucoma Where Vision is Still Present

Laser Cyclophotocoagulation

Glaucoma

Laser surgery is the treatment of choice in pets with primary glaucoma who can still see. Like the freezing procedure listed below, no cutting is required. Your pet has to be anesthetized so he or she won't move. The laser burns completely through the white outer layer of the eye (without damaging it except for some redness and swelling) and selectively kills small areas of the ciliary body in an effort to reduce the production of aqueous fluid to create a balance with the poor drainage. Occasionally the ciliary body will not be damaged enough and a second procedure is needed to restore normal intraocular pressure. This procedure may also be recommended as a preventative in the second eye of pets who have glaucoma and are blind in one eye and remain visual in the second eye.

Cyclocryothermy

Glaucoma

Cyclocryothermy 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 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 which 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. Finally, the glaucoma may return at a later date requiring a second surgery.

Anterior Chamber Shunts

Some veterinary ophthalmologists recommend a surgical procedure where a small valve like device is implanted just under the surface of the white of the eye. This device has a small tube which enters the eye through a tiny incision and this tube provides an alternate drainage pathway for the aqueous fluid to leave the eye. While some ophthalmologists report frustration with this technique since the little tube may become blocked with fibrin, or the functioning of the valve may be compromized by scarring, other ophthalmologists report considerable success with the procedure. In certain situations, a laser procedure AND the implantation of a glaucoma valve is indicated.

Treatment for an Eye with Glaucoma Where Vision is Lost

Evisceration and Implantation of an Intrascleral Silicon Prosthesis

One technique employed to result in a cosmetic, pain-free eye for your pet is the implantation of a silicone implant within the eye. This is called an intraocular prosthesis. The technique involves surgically removing of the contents of the eye, leaving the outer shell or sclera, and implanting a silicone implant within the walls of the eye. The shape of the eye is maintained and the eye moves normally. Following surgery, minimal care is needed and the eye is maintained in a relatively normal cosmetic appearance while being free of pain. Complications of this technique are that corneal ulceration may occasionally occur following surgery. In some cases scarring of the cornea results in a gray appearance.

Ciliary Ablation by Intravitreal Injection of Gentamycin

Another technique used to control glaucoma is the injection of gentamycin (an antibiotic) into the inside of the eye. This drug 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 injection 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. A brief anesthetic is required and the antibiotic is injected into the eye through the white of the eye or sclera. Complications of this technique are generalized shrinking of the eye, return of the glaucoma at a later time and occasionally chronic pain. This technique is generally only recommended in quite elderly pets where the other choices are not acceptable to the client.

Enucleation

Finally, the blind, painful eye may be surgically removed or enucleated. After enucleation, the skin is stitched shut and the hair will soon re-grow over the surgery site. This surgery again requires that your pet be anesthetized. Rarely are there any complications to this technique except possible infection. One main advantage of enucleation is that it gives the opportunity for the veterinary pathologist to examine the eye to determine the cause of the glaucoma if there was any uncertainty over this point. This knowledge may help in assessing the risk of the development of glaucoma in the opposite eye.

Prognosis

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 usually 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. Your veterinary ophthalmologist teamed with your regular veterinarian will recommend appropriate therapies suitable for your pet and your circumstances.