The lens of the eye is made of protein fibers arranged in a specialized way that results in its transparency. The lens has four layers: At the center is the nucleus, which is surrounded by the next layer, the cortex. Surrounding the cortex is the lens epithelium. The layer at the surface is the lens capsule.
In healthy eyes, light rays reflected from an object enter the eye through the cornea and lens, which together focus the light onto the retina to produce a sharp image. When a cataract develops, however, light rays are no longer precisely focused. Instead, the rays are scattered before reaching the retina.
The three common types of cataracts are defined by where they occur in the lens: nuclear, cortical, and posterior subcapsular (in the rear of the lens capsule). It is possible for a person to have more than one type of cataract in the same eye.
Nuclear cataracts are the most common type. The incidence of nuclear cataracts increases with age and cigarette smoking.
Cortical cataracts also become more common with age. The development of this type of cataract is related to lifetime exposure to ultraviolet light.
Posterior subcapsular cataracts are most likely to occur in younger people. This type of cataract is often the result of prolonged use of corticosteroids (such as prednisone), inflammation, trauma, or diabetes.
The extent and rapidity of vision damage depend not only on the size and density of the cataract, but also on its location in the lens. For example, a cataract on the outside edge of the cortex has little effect on vision because it does not interfere with the passage of light through the center of the lens. But a dense nuclear cataract causes severe blurring of vision.