CATARACTS
Most cataracts develop when aging or injury changes the tissue that makes up your eye's lens. Some cataracts are caused by inherited genetic disorders that cause other health problems and increase your risk of cataracts.
How a cataract forms...
The lens, where cataracts form, is positioned behind the colored part of your eye (iris). The lens focuses light that passes into your eye, producing clear, sharp images on the retina — the light-sensitive membrane on the back inside wall of your eyeball that functions like the film of a camera. A cataract scatters the light as it passes through the lens, preventing a sharply defined image from reaching your retina. As a result, your vision becomes blurred.
As you age, the lenses in your eyes become less flexible, less transparent and thicker. Aging-related changes to the lens cause tissues to break down and to clump together, clouding small areas of the lens. As the cataract continues to develop, the clouding becomes denser and involves a greater part of the lens.
Types of cataract :
Nuclear Cataract Formation
A nuclear cataract may at first cause you to become more nearsighted or even experience a temporary improvement in your reading vision. But with time, the lens gradually turns more densely yellow and further clouds your vision. Nuclear cataracts sometimes cause you to see double or multiple images. As the cataract progresses, the lens may even turn brown. Advanced yellowing or browning of the lens can lead to difficulty distinguishing between shades of color.
Cataract formation, especially in nuclear cataracts, is caused by oxidative stress that occurs in all biological systems and particularly the lens. Oxidative stress and generation of free radicals results from normal activity of mitochondria and other metabolic processes. Oxidation is controlled by an environment of reducing agents. Reducing agents produced in the mitochondria neutralize free radicals.
Production of reducing agents requires energy output, a challenge for the deeper lens fiber cells that lack mitochondria. The enzyme systems in deeper cells are less active because they were synthesized decades earlier. These central lens fiber cells are delicate balanced between being damaged by oxidation of membrane lipids and cytoplasmic protein, and being protected by reducing agents transported from epithelial cells and immature lens fiber cells near the surface. Transport of reducing agents is difficult because there is little space between lens fiber cells. Movement is by diffusion.50
Another challenge is maintenance of protein stability for many decades. Once a lens is formed, proteins are synthesized in outer fiber cells close to the surface. Proteins deeper in the lens generated during embryogenesis have to last a hundred years or more. Accumulated damage to these proteins reduces enzymatic activity and increases protein aggregation, a component of cataract formation.
Cortical Cataract Formation
A cortical cataract begins as whitish, wedge-shaped opacities or streaks on the outer edge of the lens cortex. As it slowly progresses, the streaks extend to the center and interfere with light passing through the center of the lens. Problems with glare are common for people with this type of cataract.
Unlike nuclear cataracts, cortical cataracts show disorganization of fiber cell structure. Causes of cortical cataracts include loss of calcium balance, protein breakdown and aggregation, and diminished antioxidant protection (from glutathione). There is evidence for a genetic cause of cataract formation. There is no overall explanation why initial damage is restricted to the center of affected cells or why the preferred location of cortical cataracts is the lower half of the lens.
Posterior Subcapsular Cataract Formation
A posterior subcapsular cataract starts as a small, opaque area that usually forms near the back of the lens, right in the path of light on its way to the retina. A subcapsular cataract often interferes with your reading vision, reduces your vision in bright light and causes glare or halos around lights at night.
Posterior subcapsular cataracts are less common and occur with the other two types. A “pure” posterior subcapsular cataract is uncommon, occurring in only 10% of cases.
An important risk factor in posterior subcapsular cataract development (and cortical cataracts) is exposure to excessive X-ray or gamma-radiation. Mechanisms that initiate cellular or molecular dysfunction are poorly understood.
Congenital Cataract Formation
Cataracts you're born with (congenital cataracts). Some people are born with cataracts or develop them during childhood. Such cataracts may be the result of the mother having contracted an infection during pregnancy. They may also be due to certain inherited syndromes, such as Alport's syndrome, Fabry's disease and galactosemia. Congenital cataracts, as they're called, don't always affect vision, but if they do they're usually removed soon after detection.
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