The soft, transparent tissue that is located directly behind the iris is called the lens. Its job is to help focus light coming into the eye onto the retina. The most common disorders of the lens in dogs usually affect its transparency, such as cataracts, or its position.
When eye lens become cloudy or opaque a cataract occurs. This will effectively block light from reaching the retina, leading to a myriad of vision problems that range from mild vision impairment to partial blindness. It is important not to confuse the common and perfectly normal minor lens imperfections that dogs have with cataracts. Additionally, there is a normal increase in the hardening and thickening of the central lens tissue of a dog’s eye as he or she ages. This, too, is not the same thing as a cataract. For dogs who do have cataracts, they are usually inherited. However, outside causes can also be present, such as exposure to radiation, inflammation, trauma, diabetes, and malnutrition. If cataracts disappear on their own in young animals, it is possible for sight to be regained.
Cataracts that are congenital, meaning they are present at birth, may reduce in size as the animal (and the lens of its eye) grows. This allows for some measure of vision restoration during the animal’s maturation process. Topical medication in the form of eye drops given two to three times per week may help animals who have immature or incomplete cataracts. Pet owners usually have a difficult time seeing signs of cataracts in dogs because they often use their senses of smell and hearing, which are very strong, to compensate for any vision loss. The result is that they don’t appear to behave any differently or have any trouble seeing, even if they are losing their vision. However, when it is detectable, some owners have said their dogs either react with increased or decreased sensitivity to bright light. Dogs who seem to be more cautious with movement and stay closer to their owners than usual may be experiencing vision impairments.
Generally speaking, surgery is necessary to remove and treat cataracts. During this surgery the affected lenses or lens must be removed. Dogs whose cataracts are removed before the cataract fully matures usually benefit the most, and their surgeries are typically more successful. It is also beneficial to remove the cataracts before inflammation occurs in the front chamber of the eye, as this can cause leakage of the lens material, which further complicates the situation. While advancements have been made to perfect the procedure, complications are still possible. If surgery is not performed it is still important for owners to keep careful watch over their pets, and to follow all advice given by the dog’s veterinarian regarding treatment, time between check-ups, and so on.
While lens displacement can and does occur in all breeds of dog, it is most commonly seen in various Terrier breeds as a primary inherited defect. Trauma, eyeball enlargement due to glaucoma, degenerative changes, and trauma can also be causal factors in lens displacement. If the lens becomes completely displaced and pushed into the front chamber of the eye there are usually very obvious symptoms that are accompanied by swelling of the cornea and glaucoma. Surgical removal of the lens is the only effective treatment in these situations.
If the lens is displaced backwards into the main eye chamber, which is called the vitreous cavity, there may be no obvious symptoms, and inflammation of the eye and/or glaucoma may never occur. Dogs who have only partially dislocated lenses usually present with a trembling iris and lens. The severity of the symptoms will be a determining factor in a veterinarian’s decision as to whether or not to remove a partially located lens.
If the lens has to be removed surgically due to displacement it is likely that postoperative complications such as retinal detachment and glaucoma will occur, since these effects are seen in higher concentrations under these circumstances. Veterinarians will evaluate a pet’s condition carefully and choose a course of action that will result in the best possible outcome for the pet, and owners are well advised to follow any home care instructions given by their pet’s veterinarian.