CausesBy Mayo Clinic staff
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|How cataracts obscure vision|
Most cataracts develop when aging or injury changes the tissue that makes up your eye's lens.
Some cataracts are related to inherited genetic disorders that cause other health problems and increase your risk of cataracts. Cataracts can also be caused by other eye conditions, medical conditions such as diabetes, trauma or past eye surgery. Long-term use of steroid medications, too, can cause cataracts to develop.
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. Age-related changes cause tissues within the lens to break down and clump together, clouding small areas within the lens. As the cataract continues to develop, the clouding becomes denser and involves a greater part of the lens.
Cataracts may develop in only one eye, but they usually develop in both of your eyes. However, the cataracts usually aren't totally symmetrical, and the cataract in one eye may be more advanced than the other.
Types of cataracts
Cataract types include:
Cataracts that affect the center of the lens (nuclear cataracts). 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.
As the cataract slowly progresses, the lens may even turn brown. Advanced yellowing or browning of the lens can lead to difficulty distinguishing between shades of color.
Cataracts that affect the edges of the lens (cortical cataracts). 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. People with cortical cataracts often experience problems with glare.
Cataracts that affect the back of the lens (posterior subcapsular cataracts). 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 posterior subcapsular cataract often interferes with your reading vision, reduces your vision in bright light, and causes glare or halos around lights at night.
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.
These cataracts also may be due to certain conditions, such as myotonic dystrophy, galactosemia, Lowe's syndrome or rubella. Congenital cataracts don't always affect vision, but if they do they're usually removed soon after detection.
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