A cataract is an opacity, or clouding, of the lens inside the eye. The lens is found behind the pupil and is normally transparent. The role of the lens is to focus light rays into images on the retina (the tissue at the back of the eye that converts light into impulses to send back to the brain).
In younger people, the lens is elastic and changes shape easily, allowing the eyes to focus clearly on both near and distant objects. As people reach their mid-forties, the lens starts to show signs of hardening and loses its elasticity. The initial result of this hardening is presbyopia, or the need for reading glasses in almost everyone as they age.
As time goes on, proteins in the lens clump together, forming cloudy ( opaque) areas in the lens called cataracts. These cloudy areas block or scatter light entering the eye, thus causing blurry vision, glare, and light sensitivity. In most cases cataracts progress slowly, taking years to "mature" to the point that they must be removed.
Cataracts can form in any of three parts of the lens and are termed by their location. The lens is structures similar to an onion - an inner core (nucleus), outer layers (cortex), all of which is surrounded by a casing (capsule).
- Nuclear sclerotic cataracts: The nucleus (the inner core) of the lens becomes hard, and turns a slight brown color. This is the most common variety of cataract and is part of the normal aging process.
- Cortical cataracts: These form in the cortex (the outer layers of the lens).
- Posterior sub-capsular cataracts: Found on the back surface of the lens, posterior sub-capsular cataracts are found commonly in people with diabetes and can be induced by various steroids treatments.
Although older age is the primary risk factor for cataracts, experts are still not certain about the exact biologic mechanisms that tie cataracts to aging.
- Oxygen-Free Radicals (Oxidants) and Glutathione - created by toxins, smoking, ultraviolet radiation, infections, and many other factors can create reactions that produce excessive amounts of these oxygen free radicals. One theory poses that as the eye ages barriers develop preventing glutathione and other protective antioxidants from reaching the nucleus in the lens, thus inhibiting the protection against oxidation.
- Radiation and Electromagnetic Waves - The destructive effects of sunlight and ultraviolet radiation. penetrates the layers of the skin as well as the eye. Long-term exposure to low levels of UVB radiation can cause changes in the lens that contribute to cataract development.
- Radiation Treatments. Cataracts are common side effects radiation treatments given for certain cancers.
- Smoking - Cataracts are one of the many negative effects caused by smoking. Many studies have implicated smoking in the development of nuclear cataracts.
- Genetic Factors - Hereditary factors are often involved in the development of cataracts in children and may play a role some adult cataract formation.
- Systemic Diseases - Diabetes, hypertension, arthritis and other connective tissue disorders have a strong correlation with cataract development.
- Medications - Long-term use of oral steroids is a well-known cause of cataracts.
- Alcohol - Alcohol increase the risk for cataracts in the cortex. It is not clear whether alcohol works directly on the proteins in the lens itself, or indirectly by affecting absorption of nutrients into the lens.
Effects on Vision
- Cloudy vision, glare, light sensitivity may be the first signs
- Images may take on a yellowish tint due to the "browning" of the nucleus
- Reading may become difficult as cataracts progress
- Sensitivity to bright lights may make it difficult or impossible to drive at night. Light from the headlights of oncoming cars hits the cataract which scatters the light inside the eye.
- If left untreated, cataracts can become so dense that profound vision loss occurs.
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