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A
cataract is a cloudy or opaque area in the normally transparent lens of
the eye. As the opacity thickens, it prevents light rays from passing
through the lens and focusing on the retina, the light sensitive tissue
lining the back of the eye. Early lens changes or opacities may not
disturb vision. But as the lens continues to change, several specific
symptoms including blurred vision; sensitivity to light and glare;
increased nearsightedness; or distorted images in either eye, may develop.
 The
lens is located behind the iris, the colored
portion of the eye, and the pupil, the dark center of the eye. Tiny
ligaments, called zonules, support the lens capsule within the eye.
The lens has three parts, the capsule, the nucleus,
and the cortex. The outer membrane, or capsule,
surrounds the cortex which in turn surrounds the center or nucleus of the
lens. If you imagine the lens as a piece of fruit, the capsule is the
skin, the cortex is the fleshy fruit, and the nucleus is the pit.
Types of Cataracts
There are three types of cataracts. Each is described by its location
on the lens. The most common type of cataract and the one associated with
aging is called a nuclear cataract.
 A
nuclear cataract occurs in the center of the lens. Common
symptoms include blurring or dimming of your vision, glare and visual
distortion. A nuclear cataract can induce myopia, or nearsightedness, a
temporary improvement in your reading vision sometimes referred to as
"second sight." Unfortunately "second sight"
disappears as the cataract gets worse.
 The
cortical cataract begins as wedge-shaped spokes in the
cortex of the lens. The spokes extend from the outside of the lens to the
center. When the spokes reach the center, they interfere with the
transmission of light and cause glare and loss of contrast. Many people
with diabetes develop this type of cataract. Although a cortical cataract
usually develops slowly, it may impair both distance and near vision so
significantly that surgery may be suggested at a relatively early stage.
 A
subcapsular cataract develops slowly and starts as a small
opacity under the capsule, usually at the back of the lens. Significant
visual symptoms may not appear until the cataract is well developed.
Typical symptoms are glare and blur. A subcapsular cataract is often found
in people with diabetes or high myopia, adults with retinitis pigmentosa,
and in people taking steroids.
Photos courtesy of The Lighthouse Inc., (Lighthouse National Center
for Vision and Aging) New York, NY, 800-334-5497.
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