Lisa McIntosh, age 15, of Albuquerque, N.M., for her question:
WHAT CAUSES A CATARACT?
A cataract is the clouding of the lens of the eye. Many cataracts start as small spots in the lenses of the eye that interfere slightly with vision. They may cause blindness by spreading until the entire lens becomes milky white and nontransparent.
Cataracts happen for a variety of reasons. Senile cataract, which results from aging, happens more frequently than any other form of the condition. It often produces complete opaqueness of the lens.
Some cataracts result from such eye inflammations as iritis or from injuries to the eye. Diabetes can also cause cataracts.
Sometimes cataracts may develop if the parathyroid glands, which controls the amount of calcium in the body, does not work properly.
Some babies are born with cataracts. Such cataracts may be caused by an infection before birth or by abnormal chemical processes in the body.
The lens helps the eye focus. Light rays from an object first strike the cornea, the transparent part of the outside of the eyeball. The cornea bends the light rays toward each other, but not enough to focus them into an image.
The light rays then pass though the lens, which bends them further and causes them to focus an image on the retina, or back layer of the eyeball.
Because the lens is flexible, it can change shape to help a person focus on objects at different distances. Thus, clear vision depends on light passing through the cornea and lens easily and on the lens focusing correctly.
Doctors do not know how to prevent or cure most kinds of cataracts, but sight can be restored to most cataract patients. The lens must sometimes be removed by surgery. Light can then reach the retina, but it cannot be focused properly. Special glasses or contact lenses can then provide the patient with good focusing.
In some cases, eye surgeons implant an artificial plastic lens in the eye. Most people who have had cataract surgery can see well enough to carry on their normal activities.
Doctors and patients have found that even with corrective steps, surgery, glasses, contact lenses and implanted lenses are not as flexible as the natural lens was, so the patient cannot see objects equally well at all distances.
Doctors use an instrument called an ophthalmoscope to examine the inside of the eye. The ophthalmoscope shows conditions within the eye as clearly as if they were on the surface. It also allows the doctor to examine the blood vessels in the eye.
The condition of the blood vessels in the eye reveals certain general conditions of the body's circulatory system. For this reason, doctors examine the inside of the eye during a routine physical examination.
Doctors recommend that children receive regular examinations of the eyes at least once each year. Adults over the age of 40 need a thorough eye examination at least every other year.