Cocker Spaniel glaucoma
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.
Normal Drainage Angle
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 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...