A cataract is an opacity in the lens. Cataracts can be congenital, which means born with it, age-related, of genetic origin, caused by trauma, dietary deficiency, 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.
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.
The normal lens of the eye is a focusing device. It is completely clear and suspended by tissue fibers called zonules just 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, in fact, 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 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 result is painful and can be damaging to the eye. A cataract can result from this inflammation.
A small cataract that does not restrict vision is probably not significant. A more complete cataract may warrant treatment. Cataracts have different behavior depending on their origin. If a cataract is a type that can be expected to progress rapidly (such as the hereditary cataracts of young cocker spaniels), 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 and expensive and is 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.
Obviously, 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 in order 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 infection sources in the eye.
A complete examination of the eye should be performed. If your veterinarian is not comfortable treating cataracts or does not have the appropriate equipment, your pet may be referred to a veterinary ophthalmologist.
It is impossible to see the retina through a cataract; an electroretinogram test can determine if the eye has a functional retina and can benefit from cataract surgery. Ultrasound of the eye can be used to look for retinal detachments. If the eye has a blinded retina, there is no point in subjecting the patient to surgery. Inflammation in the eye will require treatment before surgery. Sometimes, other eye drops are prescribed for a period before surgery, depending on the veterinarian’s preference.
Historically, removing the cataract meant surgically cutting into the eye and physically removing the lens. This is still done for older patients whose lenses are compact. For younger patients in whom the lens is soft, a technique called phacoemulsification is preferred.
This technique has become the most common method of removing cataracts in dogs. Here, the lens is broken apart by sound waves and removed with an instrument similar to a small vacuum cleaner.
After surgery, the pet must wear an Elizabethan collar for a good three weeks, and eye drops to reduce inflammation will be needed for several months. A harness may be recommended for walks instead of a collar to reduce pressure on the head and eye from pulling. There will be a schedule of recheck appointments.
Bleeding after surgery can be an enormous complication and can easily be caused by excess barking or activity after surgery. Small bleeds are of little consequence, but a large bleed could ruin vision.
Glaucoma can develop at any time after cataract surgery. This complication is not only blinding but painful as well. The risk of this complication has been decreased by placing a prosthetic lens
A cataract by itself does not necessarily require treatment. If there is no associated inflammation or glaucoma and the only problem is blindness, it is perfectly reasonable to have a blind pet.
Blind animals have a good quality of life and do well, though it is important not to move furniture around or leave any hazardous clutter in the home. Some dogs, however, become anxious or even aggressive when they lose their vision. Restoring vision for the pet is weighed against risk and expense and is a decision for each owner to make individually. Many cataracts will progress to a premature state where they will begin to dissolve, and anti-inflammatory eye drops are needed, as mentioned.
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 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.
Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.
