The term "cataract" only means that there is an opacity to the lens of the eye, writes Dr. Michael A. Ball in the American Association of Equine Practitioners' "Ask the Vet" column in the January edition of The Horse. That opacity might be a very small spot on the lens or encompass the entire lens.
The lens of the eye is normally clear and acts like a camera lens, focusing the beam of light entering the eye on the retina in the back for transmission to the brain via the optic nerve. The lens is like a Zip-Lock® bag filled with clear Jell-O®. There is a firm capsule on the front and back surface and soft, clear tissue in the middle. Cataracts develop when some process occurs that causes any part of the clear capsule and/or center to become opaque and therefore affect the beam of light entering the back of the eye.
Diffuse cataracts affecting the entire lens occur in foals and are considered a form of congenital eye problem. The most common cause of cataract formation in the adult horse is inflammation resulting from the advanced stages of anterior uveitis or moonblindness. There are other causes in the adult horse, but they usually only affect one eye.
The first step in evaluation of a cataract is to determine if it involves the anterior (front) capsule, the posterior (back) capsule, the central part of the lens, or any combination of these structures. It then is noted how much of the lens is affected. Where is the cataract within the lens? How dense is the cataract (how much light is making it through)? After that exam, your veterinarian will attempt to determine how much the cataract affects vision -- a task easier said than done.
In most horses, you can have a surprisingly large degree of vision loss and not really notice any abnormalities in the way a horse interacts with the environment. Horses have an amazing ability to compensate for vision loss, especially if one eye is primarily affected and the loss was gradual in development.
If the cataracts are small and not severely affecting vision, the next question typically is: What will happen over the next few years? It is completely unpredictable.
Several horses under his care, Ball writes, have small, central cataracts that do not appear to affect vision greatly, and they have stayed exactly the same for five years. Others have progressed from small, relatively insignificant cataracts to causing complete blindness within a few years. Only frequent examination will tell the horse owner and veterinarian how small cataracts will change over time.
Can something be done? In foals, cataracts can be removed surgically with relative success. The adult horse situation generally is more complicated.