Lens Anatomy and Physiology

Cataracts are opacities within the lens that may affect a small portion of the lens or the entire lens. Cataracts are a leading cause of blindness in dogs. They may be due to hereditary factors, metabolic diseases, inflammation, or other causes. A thorough ophthalmic and physical examination, complete blood cell count, biochemical evaluation, electroretinography, and ocular ultrasonography should be used to determine whether a patient is a candidate for cataract surgery (phacoemulsification). Several medications are prescribed during the preoperative and postoperative periods. Complications that can occur after phacoemulsification include anterior uveitis, ocular hypertension, glaucoma, corneal ulcers, and retinal detachment. Cataracts are the most common cause of treatable visual deficits and blindness in dogs. Proper selection of patients is vital to a successful visual outcome after cataract surgery (phacoemulsification). The ideal surgical candidate is a middle-aged patient with an immature cataract and no other ocular or systemic abnormalities. In reality, the perfect situation rarely exists. The purpose of this article is to familiarize the practicing veterinarian with the causes and appearances of cataracts; the ocular abnormalities, revealed by an examination, that may influence the surgical outcome; and the surgical procedure for cataract removal. LENS ANATOMY AND PHYSIOLOGY Knowledge of the unique anatomy and physiology of the lens helps one understand cataractogenesis and the treatment of cataracts. In dogs, the lens is a biconvex structure suspended between the iris anteriorly and the vitreous posteriorly by zonules that originate from ciliary body processes. The lens measures 7 mm anterior to posterior and 10 mm in diameter. It consists of two parts: the crystalline lens and the lens capsule. The lens capsule, which envelops the crystalline lens, is a basement membrane secreted by the lens epithelial cells. The anterior capsule thickens throughout life, whereas the posterior capsule remains about 2 to 4 μm thick, approximately twice the thickness of an erythrocyte. The lens epithelial cells, the only metabolically active cells in the lens, occur only subjacent to the anterior lens capsule in the postnatal animal. These epithelial cells undergo differentiation at the lens equator; after the cells lose their nuclei, they become lens fibers. The center of the lens is the nucleus and contains the oldest lens fibers. The tips of the fibers come together to make a Y-shaped pattern in the center of www.VetLearn.com n Results of a complete ophthalmic examination of a dog with cataracts may reveal subtle abnormalities that may postpone or preclude surgery. n Proper selection of patients is key to a better prognosis for long-term vision after surgery. n Any systemic disease should be detected and managed before cataract surgery. 812 Vol. 25, No. 11 November 2003 Article #1 (1.5 contact hours) Refereed Peer Review CE