Managing diseases of the lens—clarification of a cloudy type (Proceedings)
The crystalline lens is a remarkable structure. As with the cornea, nature found a way to arrange living cells such into an optically transparent structure. The purpose of the lens is to serve as a final focus of light entering the eye onto the retina and to accommodate focus for near and far images. This discussion will focus on the normal anatomy and development of the canine lens as well as the most common clinical conditions affecting the lens.
The lens develops from surface ectoderm cells that grow inward to form the lens placode. The orientation of the lens depends on the proximity to the developing retina; cells nearest the retina become the primary lens fibers whereas the opposite cells form the mitotically active lens epithelial cells. The primary lens fibers become the embryonic nucleus, the most central cells in the lens. Lens epithelial cells produce new lens cells at the equator which become the lens cortex. These new cortex cells become very elongated and lose their nucleus and most of their organelles. They extend to join other lens fiber cells, forming the lens sutures where they meet. The sutures are visible on ophthalmic exam as a "Y" anteriorly and an inverted "Y" posteriorly. The fully developed lens contains several distinct layers. Starting from the outside, they are the lens capsule, the anterior lens epithelium, the cortex, and the nucleus. The nucleus can be subdivided into an adult, fetal, and embryonic nucleus. The ability to localize abnormalities to a specific layer of the lens can be helpful in determining cause and the potential for progression. The lens is suspended in place by tiny strands extending from the ciliary processes and attaching to the equatorial lens capsule. These strands, known as zonules, can degenerate as a hereditary condition in some breeds, most notably terriers, and result in a lens luxation.
As stated earlier, the function of the lens is to focus incoming light on to the retina. Despite being significantly thicker than the cornea, it is not as effective at bending light rays. Light rays change direction when they enter a medium with a different refractive index. The greater the difference in the refractive index between two media, the more light will bend as it passes through them. The refractive index difference between air and the pre-corneal tear film is significantly different, while the difference between lens cells and the surrounding aqueous and vitreous is relatively small. Thus, the lens exists to provide fine focus. It is also plays a critical role in changing the depth of focus, a process called accommodation. In many species, such as humans, primates, and birds, the lens is quite soft and changes shape easily to change the degree to which the light is bent. The light from near objects must be bent more than that from distant objects to focus on the retina, so the lens becomes more spherical. It then becomes more disc shaped for distance viewing. Dogs and cats have a much reduced ability to alter their lens curvature. It is thought that they accommodate primarily through anterior and posterior translocation of the lens.Cataracts are easily the most common clinical problem seen with the lens. Simply, a cataract is any opacity of the lens or lens capsule that can block or distort light. There are multiple ways to classify cataracts: density, location, shape, age of onset, and etiology. Some cataracts, especially those confined to the nucleus or which have a classic hereditary shape, do not progress to cause vision defects. In these cases, no treatment or surgery is needed. Most cataracts, however, will progress at some point. Genetic cataracts are commonly seen. Although many breeds commonly develop hereditary cataracts, among the more common are Boston Terriers, Pugs, Cocker Spaniels, Shih Tzus, miniature Poodles, and Siberian Huskies.