Cataracts Health Article

Definition

A cataract is a cloudiness or opacity in the normally transparent crystalline lens of the eye. This cloudiness can cause loss of vision and may lead to eventual blindness.

Description

The human eye has several parts. The outer layer of the eyeball consists of a transparent dome-shaped cornea and an opaque white sclera, which is a fibrous membrane. 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; the pupil is the small dark round hole in the middle of the iris. The pupil and iris allow light into the eye. The ciliary body contains muscles that help the eye to focus. The lens lies behind the pupil and iris. It is covered by a cellophane-like capsule. The lens is normally transparent, elliptical in shape, and somewhat elastic. This elasticity allows the lens to 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 changing of shape is called accommodation. As people age, the lens hardens and accommodates less easily, which makes it harder for the person to see close objects. This hardening of the lens generally occurs around the age of 40 and continues until about age 65. The condition is called presbyopia. It is a normal condition of aging, generally resulting in the need for reading glasses.

The lens is made up of approximately 35% protein and 65% water. As people age, the proteins in the lens begin to degenerate. Changes in the proteins, water content, enzymes, and other chemicals are some of the causes of cataract formation.

The major parts 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. Opaque areas can develop in any part of the lens. Cataracts, then, can be classified according to location (nuclear, cortical, or posterior subcapular cataracts). The density and location of the cataract determines the extent of vision affected. If the cataract forms in the area of the lens directly behind the pupil, the person's vision may be significantly impaired. A cataract that occurs on the outer edges or side of the lens will cause less severe impairment.

Cataracts in the elderly are so common that they are considered a normal part of the aging process. People between the ages of 52–64 have a 50% chance of developing a cataract, while at least 70% of those 70 and older are affected. Cataracts associated with aging (senile or age-related cataracts) most often occur in both eyes, with each cataract progressing at a different rate. At first, these cataracts may not affect vision. If the cataract remains small or at the periphery of the lens, the visual changes may be minor.

Cataracts that occur in people other than the elderly are much less common. Congenital cataracts occur very rarely in newborns. Genetic defects or an infection or disease in the mother during pregnancy are among the causes of congenital cataracts. There is a condition called blue cataracts, which is inheritance-linked and affects primarily Tibetans and some other Asians. Traumatic cataracts may develop after an injury or foreign body damages the lens or eye. Systemic illnesses like diabetes may result in cataracts. Cataracts can also occur secondary to other eye diseases—for example, an inflammation of the inner layer of the eye (uveitis) or glaucoma. Such cataracts are called complicated cataracts. Toxic cataracts result from chemical toxicity, such as steroid use. Cataracts can also result from exposure to the sun's ultraviolet (UV) rays.