Glaucoma
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.
Diagnosing Glaucoma
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.
Treatment
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.
