Cataracts in Dogs
The canine eye has a clear lens behind the pupil, which focuses light on the retina at the back of the eye. A cataract is defined as any opacity of this lens or its capsule. Cataracts affect light transmission through the eye by reducing lens transparency, and therefore cause visual deficits. Depending on the size of the opacity, visual effects can be insignificant or severe, causing blindness. Cataracts also cause inflammation of the interior of the eye- uveitis, and can lead to painful and blinding complications such as lens luxation and glaucoma.
Cataracts can be present at birth (congenital) or acquired through life. They can result from metabolic disease, such as diabetes mellitus-when they develop so quickly they can cause vision loss overnight. Nutritional cataracts are caused by inappropriate diets, traumatic cataracts by an injury such as a thorn penetrating the eye, while toxic cataracts can be drug-induced. They can also be inherited.
Hereditary cataracts can be primary –where they are not linked to any other ocular disease-or secondary, where they are associated with other ocular diseases such as progressive retinal atrophy (PRA), persistent embryonic/foetal vasculature diseases or retinal dysplasia. Primary hereditary cataracts are common in dog breeds, especially in certain pedigree breeds-which are listed on the BVA Eye scheme site. Samoyeds do not yet appear on this list of certified or under investigation cataracts, but investigations are underway by the Animal Health Trust and Michigan State for a genetic cause of hereditary cataracts in Samoyeds and other Northern breeds.
The only means of treatment of cataracts is surgical removal by phacoemulsification, where ultra-sonic waves are used within the eye to breakdown the lens, allowing it to be removed through a small incision in the cornea. A folded artificial lens can be inserted through this same incision, which then unfolds inside the lens capsule to allow normal vision.
Various drops have been marketed claiming to reverse the lens changes and dissolve the cataracts. It is worth remembering that the proteins of the lens are not unlike those of the egg. Once an egg has been fried, there is no process to make the egg-white transparent again. Some drops have been developed which stop diabetic cataract formation, but these have yet to be released on the market, and do not reverse existing cataracts.
It was previously considered best to wait until the cataract had fully developed, when the patient was completely blind, before surgery was considered. Advances in surgical technique and in our understanding of the pathology of cataract formation and its consequences have meant that early referral for assessment and treatment lead to far better visual outcomes for canine patients. The inflammation which accompanies all cataracts becomes worse with time, and has other consequences within the eye. Early surgery in a less inflamed eye has a much better success rate than late surgery in chronically inflamed eyes.
BVMS CertVOphthal MRCVS