CATARACTS - A Common Ocular Disease In Dogs
Sheryl Krohne, DVM, MS Diplomate ACVOACVO Genetics Committee/CERF Liaison
Cataract is a common term used to describe changes in the lens of the eye that we usually attribute to older age, and call an "aging change." Many people have surgery to remove cataracts and we all know someone who has had cataract surgery, if we haven't had to undergo the procedure ourselves. It has a very high success rate in people, has few complications and is even an outpatient procedure performed under local anesthesia. This disease also occurs as an aging change in the eyes of dogs. Cataracts diagnosed in younger dogs are from genetic causes. This means that dogs can inherit cataracts as a "disease" from their parents. We will discuss the different types of cataracts later in this article. First, we want to explain where the lens is, what it does, and what a cataract looks like when it forms in the lens.
The lens in located inside the eye and is a soft, transparent structure without blood vessels (see picture below). It changes shape when small muscles pull on the lens and thus allows the eye to focus on views both near and far away. A capsule surrounds the lens and is necessary to supply shape and nutrition for the lens, as well as providing an anchor for the small muscles. A typical change that occurs in the lenses of dogs and people when they are older is called nuclear or lenticular sclerosis. This change occurs before cataracts form and typically is seen in dogs after they are 8 to 10 years old. The eyes will look gray, silver or bluish to the owner. The silver appearing color should come from the "inside" of the eye, not the surface. The surface or cornea should still appear clear and the iris or colored part of the eye should still be clearly visible (see picture).
A cataract is defined in the CERF book* as "a partial or complete opacity of the lens and/or its capsule. In cases where cataracts are complete and affect both eyes, blindness results." Cataracts are among the most common intraocular lesions and a leading cause of vision loss in the dog. Cataracts may be caused by genetics, trauma, ocular inflammation, diabetes mellitus, genetic retinal atrophy, persistent pupillary membranes, persistent hyaloid remnants, specific nutritional deficiencies, congenital abnormalities and uncommonly by other specific metabolic diseases. The size of cataracts is also highly variable. They may be very small and not affect vision, or complete and cause blindness. If they are incomplete, they may only be present in the cortex or outside layers of the lens, or in the nucleus or center of the lens. Some types of cataracts only affect the capsule, which covers the lens. In every case, however, the cataract is an opaque place in the lens. It doesn't affect vision when it is small because the dog can see around it. Recommendations published in the CERF book include, "breeding is not recommended for any dog demonstrating partial or complete opacity of the lens or its capsule unless the examiner has also checked the space for significance of the above punctate cataract unknown. The prudent approach is to assume cataracts to be hereditary except in unusual cases specifically known to be associated with other causes."
Some of the cataracts that are small at the time of diagnosis will progress until they cause complete blindness. This prediction of the behavior or progression of a disease is termed "prognosis." It is important to discuss the prognosis for the cataract diagnosed in your dog with the veterinary ophthalmologist so that you know what to expect. Some cataracts that are genetic will progress and some will not. This is dependent on the breed and where the cataract is located within the lens.
Treatment for cataracts is recommended for one of two reasons. Some cataracts will cause inflammation in the eye. This type of inflammation will cause squinting, tearing or watering of the eye, increased redness of the white of the eye, and sometimes increased squinting in the light. Your veterinarian or veterinary ophthalmologist should examine your dog as soon as possible because inflammation caused by cataracts can damage the eye. In some cases the damage will lead to glaucoma or retinal detachment, especially if not treated correctly. The other treatment is for blindness from cataracts. The surgery to remove cataracts is called phacoemulsification. An instrument inserted into the eye during surgery produces ultrasonic waves. The cataract is emulsified by the ultrasound and removed by aspiration. Many times a plastic lens can be placed into the eye after the cataractous lens has been removed. This replacement lens improves near vision for the dog. Surgery for cataracts can only be performed if the rest of the eye is healthy and if the dog can undergo general anesthesia. Unlike the same surgery for people, dogs must have general anesthesia for this procedure. Success rates quoted in the literature range from 90-95% restoration of functional vision 6 months after surgery.
Genetic cataracts are diagnosed in many breeds of dogs and are initially diagnosed from 2 months up to 7 years of age. The size of the cataract, whether blindness results from the cataract and the age of first diagnosis is breed dependent. Genetic cataracts will be discussed in the next newsletter.
*Ocular Disorders Presumed to be Inherited in Purebred Dogs, written and published by the American College of Veterinary Ophthalmologists and distributed by CERF.