The lens of the eye is normally transparent. A cataract is a condition in which the lens of the eye becomes cloudy or opaque. This cloudiness can impair vision and may lead to eventual blindness.
The human eye has several parts. The outer layer of the eyeball consists of a transparent dome-shaped cornea and an opaque, white sclera. The cornea and sclera help protect the eye. The next layer includes the iris, pupil, and ciliary body. The iris is the colored part of the eye and the pupil is the small, dark, round hole in the center of the iris. The pupil is primarily responsible for allowing light into the eye. The ciliary body contains muscles that help the eye focus. The lens, which lies behind the pupil and iris, is covered by a cellophane-like capsule. It is normally transparent, elliptical in shape, and somewhat elastic. Due to this elasticity, the lens can focus on both near and far objects. The lens is attached to the ciliary body by fibers (zonules of Zinn). Muscles in the ciliary body act on the zonules, which then change the shape of the lens. This process is called accommodation-the lens focuses images to help make vision clear. As people age, the lens hardens and changes shape less easily. As a result, accommodation becomes more difficult, making it harder to see things up close. This normal aging condition, called presbyopia, generally occurs around age 40 and continues until about age 65. Individuals with this condition generally need reading glasses.
The lens is made up of approximately 35% protein and 65% water. As people age, degenerative changes in the lenses’ proteins occur. Changes in the proteins, water content, enzymes, and other chemicals are some of the reasons for the formation of a cataract.
The major areas of the lens are the nucleus, the cortex, and the capsule. The nucleus is in the center of the lens, the cortex surrounds the nucleus, and the capsule is the outer layer. Opacities can occur in any area of the lens, and cataracts can be classified according to their location (nuclear, cortical, or posterior subcapular cataracts). The density and location of the cataract determines the amount of vision affected. If the cataract forms in the area of the lens directly behind the pupil, vision may be significantly impaired. A cataract that occurs on the outer edges or side of the lens causes less visual impairment.
Cataracts in the elderly are so common that they are thought to be a normal part of aging. Cataracts affect about 50% of individuals between the ages of 52-64, while at least 70% of those 70 and older are affected. Cataracts associated with aging (senile or age-related cataracts) are usually bilateral (occur in both eyes) with asymmetric progression (different rates of progression). Initially, cataracts may not affect vision. If the cataract remains small or at the periphery of the lens, the visual changes may be minor.
Cataracts are much less common in younger people. Congenital cataracts are rare in newborns. When they do occur, they may be due to genetic defects or an infection or disease in the mother during pregnancy. Traumatic cataracts may develop after a foreign body or trauma injures the lens or the eye. Systemic illnesses, such as diabetes, also may result in cataracts. Cataracts can occur secondary to other eye diseases-for example, uveitis or glaucoma. Such cataracts are called complicated cataracts. Toxic cataracts result from chemical toxicity, such as steroid use. Cataracts also can result from exposure to the sun’s ultraviolet (UV) rays.
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