Dogs and cataract.
The normal lens of the eye is a focusing device.
It is completely clear and suspended by tissue fibers (called zonules) inside the pupil. The pupil opens and closes to control the light entering the eye to project an image onto the retina in the back of the eye, the way a projector projects an image onto a movie screen. The lens focuses the projected image in a process called accommodation. The focusing power of the dog’s lens is at least three times weaker than that of a human lens, while a cat’s lens is at best, half the power of a human.
(Dogs and cats have a sense of smell at least 1,000 times more accurate than ours, which is their primary means of perceiving the world.
Despite its clarity, the lens is made of tissue fibers. As the animal ages, the lens cannot change its size and grow larger; instead, it becomes more compact with fibers. The older lens, being denser, appears cloudy. This condition is called nuclear sclerosis and is responsible for the cloudy-eyed appearance of older dogs. The lenses with nuclear sclerosis may look cloudy, but they are still clear, and the dog can still see through them; these are not cataracts.
The fibers making up the lens come together in the center, forming a “Y” shape that is sometimes visible when one looks into the eye. These Y-shaped lines are often called the sutures of the lens.
The lens is enclosed in a capsule that, if disrupted, allows the immune system to see the lens proteins for the first time, recognize them as foreign, and attack. The resulting inflammation (uveitis) is painful and can damage the eye. A cataract can result from this inflammation or any other reasons listed below.
Cataracts can be congenital (born with it), age-related; of genetic origin (the most common cause); caused by trauma; dietary deficiency (some kitten milk replacement formulas have been implicated); electric shock; or toxins. The patient with a cataract is not able to see through the opacity. If the entire lens is involved, the eye will be blind.
Many things can cause the lens to develop a cataract.
One cause is diabetes mellitus. In this condition, the blood sugar soars as does the sugar level of the eye fluids. The fluid of the eye’s anterior chamber is the fluid that normally nurtures the lens, but there is only so much glucose that the lens can consume. The excess sugar is absorbed by the lens and transformed into sorbitol. Sorbitol within the lens, unfortunately draws water into the lens, causing an irreversible cataract in each eye. Cataracts are virtually unavoidable in diabetic dogs no matter how good the insulin regulation is; diabetic cats have alternative sugar metabolism in the eye and do not get cataracts from diabetes.
Many owners cannot tell which portion of the eye looks cloudy. As caused by other eye diseases, cloudiness on the cornea can be mistaken for a cataract by an inexperienced owner.
Also, in dogs, the lens will become cloudy with age as more and more fibers are laid down, as described above. Nuclear sclerosis, as described, can mimic the appearance of a cataract, yet the eye with this condition can see and is not diseased. It is a good idea to have your veterinarian examine your pet if you think there is a cataract, as you could be mistaken.
The area of the lens the cataract involves amounts to a spot that the patient cannot see through. If the cataract involves too much of the lens, the animal may be blind in that eye, and there could be cataracts in both eyes, which means the pet could be rendered completely blind.
A cataract can luxate, meaning it can slip from the tissue strands that hold it in place. The cataractous lens can thus end up floating around in the eye, where it can cause damage. If it settles to block the eye’s natural fluid drainage, glaucoma (a buildup in eye pressure) can lead to pain and permanent blindness. A cataract can also cause glaucoma when it partially absorbs fluid and swells to obstruct drainage.
Cataracts can begin to dissolve after they have been there long enough. While this sounds like it could be a good thing it is a highly inflammatory process. The deep inflammation in the eye creates a condition called uveitis, which is painful and can lead to glaucoma. If there is any sign of this type of inflammation in the eye, it must be controlled before any cataract surgery.
A small cataract that does not restrict vision is probably not significant. A complete cataract may warrant treatment. Cataracts have different behavior depending on their origin. Suppose a cataract is a type that can be expected to progress rapidly (such as the hereditary cataracts of young cocker spaniels). In that case, it may be beneficial to pursue treatment (i.e. surgical removal) when the cataract is smaller and softer, as surgery will be easier.
Cataract treatment generally involves surgical removal or physical dissolution of the cataract under anesthesia. This is invasive, expensive, and not considered unless it can restore vision or resolve pain. Pets with one normal eye and the other with a cataract can still see with their good eye and may not need surgery depending on circumstances.
The patient must be in good general health to undergo surgery; diabetic dogs must be well-regulated before cataract surgery. Also, it should be obvious that for a patient to be a good candidate for surgery, the patient must have a temperament conducive to getting eye drops at home. Pre-anesthetic lab work can be done with the patient’s regular veterinarian. Some ophthalmologists prefer that patients have their teeth cleaned before surgery to minimize sources of infection in the eye.
Products containing N-acetylcarnosine have been marketed to dissolve cataracts, leading to a great deal of false hope. N-acetylcarnosine is an antioxidant eye drop that may have beneficial effects on the eye but they do not include any dissolution of a mature cataract. For smaller cataracts, it may be possible to dilate the pupil so that the pet can see around the cataract. Still, there is some controversy about doing so as these medications have other effects on the eye. Ask more detailed questions of your veterinarian.
Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.
