It is important to understand that high elevation in the IOP causes irreversible damage to the retina and optic nerve in a very short period of time (24-48 hours). As a result, glaucoma is considered an emergency and requires immediate treatment if vision is to be maintained. Glaucoma that is severe and lasts several days often causes an enlargement of the eye with no hope for vision return.
Canine glaucoma is divided into two major groups: primary and secondary. A normal eye constantly produces a nourishing fluid inside the eye that drains out through a structure called the drainage angle. With primary glaucoma, the drainage angle is abnormal and unable to drain, causing the fluid to “back up” inside the eye and raise the IOP. This is most commonly a breed-related, inherited condition. With secondary glaucoma, an inciting cause such as inflammation in the eye, lens luxation, cancer of the eye, or trauma physically blocks the drainage angle, leading to an elevated IOP.
What happens when the IOP is elevated?
A higher than normal pressure in the eye can affect virtually every tissue in the eye, therefore a number of different clinical signs may be seen. Dogs with early glaucoma may only have a mild redness to the eye. Moderate forms of glaucoma cause a bluish-white discoloration of the cornea, and may cause blindness in the affected eye. The retina and optic nerve (the tissues responsible for vision) are the most sensitive to the effects of high IOP. As a result, loss of vision is a common complication. In fact, glaucoma is one of the most frequent causes of blindness in the adult dog. The effects of elevated IOP in the dog vary with the age of the animal, duration, and levels of IOP. Primary glaucoma often occurs in one eye initially, but develops in the other eye within weeks to months.
What breeds are predisposed to glaucoma?
Primary glaucoma most commonly afflicts dogs at 3-7 years of age, but can occur at any age. The disease is most frequently seen in Cocker Spaniels, Poodles, Beagles, Chow-Chows, Bassett Hounds, Dalmatians, and many of the terrier breeds. However, primary glaucoma has been identified in almost every breed of dog.
How is glaucoma diagnosed?
There are three methods that are particularly useful in the diagnosis of glaucoma: (1) tonometry, (2) gonioscopy, and (3) ophthalmoscopy. Tonometry involves measuring the IOP with a special instrument. Normal IOP in dogs and cats can range between 12 and 25 mmHg, and the two eyes should be similar in pressure. Gonioscopy is a diagnostic procedure to examine the drainage angle. This is done by placing a gonioscopic lens on the cornea and using a hand-held slit lamp to permit magnification of the angle. Dogs who are predisposed to glaucoma will have an abnormally small or narrowed drainage angle in both eyes. Direct and indirect ophthalmoscopy is also necessary to evaluate the retina and the optic nerve to look for signs of pressure damage.
How is glaucoma treated?
The goal of treating early forms of glaucoma is to maintain a normal IOP and to preserve vision. Medical treatment often involves long-term administration of topical medications to decrease IOP.
In an eye that has a chance to remain visual but is not well controlled with medications, laser therapy or filtering procedures are an option. In laser therapy, the laser probe is applied directly to the eye to destroy the ciliary body (the portion of the eye that makes the fluid inside the eye). Filtering procedures, such as gonioimplants or aqueous humor shunts, have also been performed to facilitate outflow, but are frequently subject to clogging and early failure.
If vision has been lost due to chronic glaucoma, the goals of therapy are generally to make the patient comfortable. Chronic glaucoma is treated with surgery, such as enucleation (removal of the eye and closure of the skin) or evisceration (placement of a prosthetic sphere inside the eye) to eliminate the pain associated with glaucoma. Unfortunately, many forms of canine glaucoma are difficult to treat and have a guarded long-term prognosis for vision and comfort.