Ophthalmology - Special Services, Technology, & Information
Glaucoma is a common eye disease in dogs, cats, and horses in which the intraocular pressure (IOP) is abnormally high. It often results in irreversible blindness and is usually painful for the animal. Glaucoma can occur spontaneously in certain breeds of dogs (primary glaucoma) or may be caused by cataracts, lens displacement, inflammation, trauma or certain forms of cancer of the eye (secondary glaucoma). Canine glaucoma is divided into 2 major groups: primary and secondary. With primary glaucoma, the drainage angle in the eye is too small, causing fluid to “back up” in the eye and raise the IOP. This occurs primarily in purebred dogs.
Breeds 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, many of the terrier breeds, Poodles, Beagles, Chow-Chows, Bassett Hounds and Dalmatians. However, primary glaucoma has been identified in almost every breed of dog.
Effects of Elevated IOP
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. 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 effected eye. 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 more than 10-14 days often causes an enlargement of the eye.
There are 3 methods that are particularly useful in the diagnosis of glaucoma: (1) tonometry, (2) gonioscopy, and (3) ophthalmoscopy. Tonometry involves measuring the IOP with an instrument, most commonly a Schiotz Tonometer or a TonoPen. Normal IOP in dogs can range between 12 and 25 mm Hg and the two eyes should be similar in pressure. Gonioscopy is a diagnostic procedure to examine the angle of the anterior chamber. 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 angle in both eyes. Direct and indirect ophthalmoscopy is necessary to evaluate the retina and particularly the optic nerve.
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. A laser surgery is often also necessary to control the IOP. If the 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 to relieve the pain associated with high IOP. Unfortunately, many forms of canine glaucoma are difficult to treat and have a somewhat guarded long-term prognosis for vision and comfort.