Disease and surgery of the lens (Proceedings)
The lens is composed of crystalline fibers specifically arranged to allow light rays to transilluminate through the lens to the retina. The lens focuses light rays on the retina. The lens is enclosed in a capsule composed of a basement membrane, epithelium, and differentiated lens fibers. The lens is avascular and is suspended in the globe by zonular fibers that insert in the ciliary body processes. Specific biochemical processes must be maintained in the lens to maintain clarity; most diseases that affect the lens will lead to opacity. Many different processes may occur in the eye that require surgery.
The most common disease that affects the lens is opacification of the lens or cataract formation. Any opacity no matter how small is technically termed a cataract. There are multiple classification schemes for cataract description including degree of opacification, location of the opacity in the lens, appearance of the cataract, etiology of the cataract, and age of onset of the opacity. The stage or degree of opacity provides a visual picture of the progression and may be most helpful in communication with the owner. Cataract formation must be differentiated from nuclear or lenticular sclerosis, the normal aging change in the lens. The easiest way to do this is to dilate the pupil and then transilluminate the lens. If a fundic reflex is present throughout the whole lens then the lens change, no matter how cloudy, is sclerosis.
Cats have few reports of primary cataract formation and the majority of cataracts in cats develop secondary to persistent uveitis. Uveitic cataracts usually develop slowly with opacification beginning in the cortex. Posterior synechiae, rubeosis iridis, and preiridial and pupillary inflammatory membranes may be present.Cataract formation may be considered a developmental abnormality when opacification is present at a very young age, however most cataracts would be classified as acquired. Aphakia, microphakia, and lens colobomas are true congenital abnormalities that are vision impairing; other ocular abnormalities may be present and cataract formation may be present as well. Other congenital abnormalities are lenticonus and lentiglobus; lenticonus leads to protrusion of the front or back of the lens and lentiglobus results in a more spherical lens than normal. Visual acuity will be affected and other ocular abnormalities may be present as well. Vascular abnormalities of embryonic origin may also lead to visual defects and other complications. Persistent hyperplastic primary vitreous (PHPV) develops when the embryonic blood supply to the lens during development does not recede and atrophy normally. The pupillary membrane, extending from the iris, and the intravitreal hyaloid vascular system begin to atrophy by day 45 of gestation in the dog. If they do not completely regress then the remnants lead to persistent pupillary membranes (PPM) anteriorly and PHPV posteriorly. Six grades of PHPV have been described consisting of 1) Grade 1: retrolental fibrovascular dots, 2) Grade 2: dots and proliferation of retrolental tissue on the posterior lens capsule, 3) Grade 3: retrolental plaque and persistent parts of the hyaloid vascular system, 4) Grade 4: plaque and posterior lenticonus, 5) Grade 5: combination of grades 3 and 4, 6) Grade 6: combinations of above grades and lens coloboma, microphakia, and retrolental clots of pigment or free blood. The Doberman Pinscher, Staffordshire Bull Terrier, and Bouvier des Flandres are predisposed breeds. Anomalies in the Doberman originate from the tunica vasculosa lentis (TVL) and posterior lens capsule; microphakia, cataract formation, and PPM's are associated lesions. In the Staffordshire Bull Terrier the disease does not usually involve PPM's or PTVL and secondary cataracts are uncommon. An autosomal recessive inheritance is most likely. Cataract formation may be part of other multifactorial ocular abnormalities such as merle ocular dysgenesis. The Australian Shepherd as well as any dog with the merle gene is predisposed. Double merle offspring are at higher risk. Cataract formation may also be present with retinal dysplasia. The Labrador Retriever and the Samoyed have a syndrome of dwarfism with retinal dysplasia; these dogs are predisposed.