A cataract is an opacity or cloudiness of the lens. The lens is found inside the eye behind the iris and in front of the retina. It is ovoid in cross section and was developed in evolution as a means of bringing into focus on the retina, structures close to the eye. In younger animals the lens has the ability to change shape to accommodate so that the eye can focus on objects at near or far distance.
Many causes of cataracts have been identified. These include genetic, nutritional, metabolic, traumatic and post-inflammatory. Drug induced cataracts have also been seen in laboratory animals and are suspected to occur in other species as well. Steroid (“cortisone”) induced cataracts are seen in man but have not been suspected in dogs.
Not all cataracts are complete and in many cases only part of the lens is involved. In many of these cases the change may not be progressive over time, however if the cataract matures the eye will become blind without cataract surgery to restore vision.
Juvenile cataracts are first seen while the dog is young and are seen in a number of breeds. These cataracts usually mature early and cause blindness. This type of cataract is strongly suspected to be heritable.
Another type of cataract seen in young dogs is the nuclear cataract: this type of lens is seen in the nucleus or the centre of the lens which is the earliest part of the lens to form embryonically. These cataracts usually reflect an insult during embryonic development and may not necessarily be heritable. They are often accompanied by other ocular abnormalities such as microphthalmos (undersized eye) and retinal dysplasia (abnormal structural development of the retina).