Anja Welihozkiy, DVM, DACVO
The crystalline lens is an avascular, transparent and highly structured “fine-tuning” refractive tissue that focuses light rays on the retina, which is the beginning of image transmission to the brain. The lens is composed of mostly water and protein arranged in a manner that keeps it transparent, thereby enabling light to pass through it.
Any opacity within the lens or its associated lens capsule that is causing loss of transparency is termed a cataract. Cataracts are among the most common intraocular lesions in dogs and are the leading cause of vision loss in this species.
Cataract formation results from denaturation and clumping of lens proteins causing an opacity within the lens that obstructs light from passing and being focused on the retina. They are frequently progressive. As the opacity progresses, a loss of visual acuity occurs. People with cataracts report a loss of sharpness to the image they are seeing, less contrast between objects, and less vivid colors. A glare is sometimes described and probably occurs due to the scattering of light in different directions by the abnormal lens. Ultimately, vision will be lost.
Most cataracts in dogs are inherited and can occur at any age. The cataract may develop rapidly over weeks, or slowly over years, and occur in one or both eyes. Different breeds of dogs have different characteristics of cataract development.
The second most common cause of cataracts in dogs is diabetes mellitus. Over 75% of diabetic dogs will develop blinding cataracts within the first year of diagnosis; 60% of dogs have cataracts at initial diagnosis of diabetes mellitus. Diabetic cataracts can develop VERY fast – sometimes within a few days – and may be considered medical and surgical emergencies in certain situations.
The third most common cause of cataracts in dogs is a toxic reaction in the lens secondary to an underlying primary ocular disease or (much less commonly) due to a drug reaction. These are called “toxic cataracts.” Toxic cataracts caused by ocular disease are quite common in dogs and can be caused by
- Retinal degeneration, especially progressive retinal atrophy (PRA)
- Uveitis (intraocular inflammation) of any cause, including trauma; and
- Glaucoma (increased intraocular pressure) of any cause.
Cataracts can also develop due to penetrating trauma (cat scratches to the lens), nutritional deficiencies (puppies on an artificial milk-replacer diet), age-related onset of cataract formation (senile cataract) and many other potential causes, such as birth defects, infection and radiation therapy to the head. Discussion of these causes is beyond the scope of this article.
Once a lens has developed a cataract, there is no known medical method to increase transparency in the lens. Immature, mature and hypermature cataracts can be removed surgically via phacoemulsification. The sooner a cataract is removed the better the postoperative outcome. A cataract becomes surgical if it is deemed progressive and is starting to cause visual behavior changes (vision loss).
A small incision is made in the cornea, and an opening is created in the anterior lens capsule. Phacoemulsification is then performed during which a special probe ultrasonically emulsifies and removes the cataract (all of the lens contents inside the capsule). After the cloudy lens is removed, the empty lens capsule remains and is called the capsular bag. An artificial replacement lens, called an intraocular lens, or IOL, is placed in the bag to refocus the light onto the retina. The eye is closed with small absorbable sutures.
The success rate of uncomplicated cataract surgery with intraocular lens implantation in dogs is approximately 90-95%. Short and long-term complications include glaucoma, uncontrolled uveitis and retinal detachment, with a prevalence from 5-10%. Not having a cataract removed can also cause the same complications as described above, even to a higher percentage rate. Regardless of whether or not a patient receives cataract surgery, long-term topical anti-inflammatory treatment is necessary to control intraocular inflammation.
What is lenticular sclerosis (nuclear sclerosis)?
Lenticular sclerosis is a normal, age-related clouding of the lens of the eye, which is not a cataract. Lens epithelial cells located on the anterior lens capsule are continually producing new protein fibers compressing older fibers in the center of the lens. The denser lens center causes light to scatter giving the lens a hazy grayish-blue appearance to the observer. Unlike cataracts, light can still penetrate the lens to reach the retina producing a visible tapetal reflection. Lenticular sclerosis is not believed to significantly affect vision, and therefore treatment is not recommended.