Lens disorders: Diagnosis and treatment (Proceedings)
Lens opacities and position will be discussed via lots of photos. Treatment of the associated abnormalities will be discussed. However there are three main points to take home from this discussion:
1. Lens induced uveitis is greatly under-diagnosed and under-treated. We will evaluate what to look for and how to manage it.
2. Even if your clients do not want to pursue lens surgery, an evaluation by an ophthalmologist may save their pet significant problems in the future and them significant cost.3. Cataract surgery success has continued to improve year by year with newer technologies and is now 80-90% successful. If your clients may want to consider lens surgery, refer them ASAP before complications occur which diminish the percentage of success.
Lens opacities can be classified in many ways such as by size, location, and maturity. Although some classify lens opacities by age, this is typically irrelevant with respect to how they are managed other than young dogs tend to sustain more lens induced uveitis. Congenital development will be discussed so that abnormalities can be recognized and treated appropriately.
A brief description of lens anatomy is important to recognize the origination of new lens fibers as well as the formation of lens suture cataracts. Lens fiber nuclei are positioned at the lens equator at the far periphery behind the base of the iris. If iris dilation is not performed for lens evaluation, newly forming cataracts in this position will be missed. Cataracts in this peripheral location also tend to be rapidly forming and as such provoke a more significant lens induced uveitis. Conversely, as the lens fibers grow and join each other near the mid- lens forming Y sutures, cataracts in this position progress less frequently and in most breeds, may not progress at all. The dog and cat lenses are much larger than humans at 0.5ml and 0.6ml respectively compared to 0.3ml in humans. The lens is predominantly protein. The fibers are arranged in a symmetrical pattern with minimal intracellular structures allowing for maximal light transmission. An extremely thin basement membrane [2-50um] surrounding the lens serves as the lens capsule.
Congenital defects in lens development may lead to abnormally small lenses which fall forward, backwards, or are associated with small globes. These lenses may need to be removed to prevent chronic damage to the cornea and pain from glaucoma but the other intraocular structures need to be evaluated as they may also suffer from maldevelopment. Persistent Pupillary Membrane or PPM may attach strands of the iris to the lens capsule causing focal cataract progression. In rare cases this may be progressive. In some breeds this is considered a reason to not breed and they will fail a CERF, Canine Eye Registry Foundation. In the Besenji, Mastiff, and Corgi the intraocular damage from extensive PPM attachment may be so severe as to blind puppies at birth. Pupil dilation of these pups with 1% Tropicamide at initial examination is important to evaluate the entire lens.
Congenital cataracts are due to many causes such as random abnormalities in development, genetics, toxic or infectious events. Merle Ocular Dysgenesis refers to the significant multiple congenital abnormalities that may occur with breeding merle to merle. The retinal pigment epithelium abnormality leads to retinal dysplasia, detachments, colobomas, iridal dysplasia, glaucoma, and cataracts all to varying degrees. As with many juvenile cataracts these can progress very rapidly with significant associated inflammation. Evaluation of these cases before cataract development is extremely important if owners wish to pursue cataract surgery in that critical evaluation of the retina is important prior to lens opacification. Colobomas, defects in the sclera structure, may warrant a recommendation of preoperative LASER retinopexy. These dogs may also be deaf and preservation of vision is often of great importance to owners.
The Labrador Retriever and Samoyed may suffer from a combination of cataracts and chrondrodysplasia. Their limbs have varying degrees of valgus and varus deformities although the cataracts almost always progress within the first two years of life. These puppies should be monitored over time as glaucoma and retinal detachment may ensue. Young dogs tend to sustain scleral stretching at much lower pressures than adult dogs. If pressures elevate over about 25mmHg, the sclera will stretch and lagophthalmos, inability to close the eyelids, may occur resulting in corneal erosions. Enucleation or intraocular prosthesis should be recommended at this stage. Due to the combination of both recessive and dominant factors of inheritance in these dogs, future breeding of both parents is discouraged.