A cataract is a clouding of the eye's lens that causes decreased vision. The lens of the eye focuses light rays onto the retina (the light-sensitive tissue at the back of the eye) where an image is recorded. This allows us to see things clearly. The lens of the eye comprises mostly water and protein. The protein is arranged in a way that keeps the lens clear and lets light pass through it. A cataract develops when some of the protein clumps together and starts to cloud an area of the lens. A cataract won't spread from one eye to the other, although many people develop cataracts in both eyes.
As the cataract matures and gets cloudier, it may become difficult to read and do other normal tasks. Some people with “ripe” cataracts describe their vision as “trying to see through a waterfall.”
Many people are not aware that their vision is blurry as cataracts usually progress slowly. That is just one reason why regular comprehensive eye examinations are important.
The exact cause of this clouding is not known. However, a number of factors are known to contribute to the formation of cataracts, including, but not limited to:
Mayo Foundation for Medical Education and Research
website. Available at:
The Merck Manual of Medical Information.
17th ed. Simon and Schuster, Inc; 2000.
National Eye Institute website. Available at:
Last reviewed December 2014 by Michael Woods, MD
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