Need-to-know anatomy

Ordinarily, light rays reflected from an object enter the eye through the cornea and lens, which together focus the light onto the retina (the innermost layer of the eye, which consists of light-sensitive nerve tissue) 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.

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The lens is made of protein fibers arranged in a specialized way so that the lens is transparent. The lens is composed of four distinct structures: At the center is the nucleus, which is surrounded by the cortex, then the lens epithelium, and finally the lens capsule. 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). Nuclear cataracts are the most common, and their incidence increases with age. Cortical cataracts also become more common with age and are related to lifetime exposure to ultraviolet light. Posterior subcapsular cataracts are most likely to occur in younger people and are often the result of prolonged use of corticosteroids (such as prednisone), inflammation, trauma, or diabetes. People often have more than one type of cataract in the same eye.