What Is A Cataract?

Tina D. Turner, M.D. Ophthalmologist

Contributing Editor, Dr. Tina D. Turner received a B.A. in chemistry from the University of Tennessee and her M.D. from Baylor College of Medicine in Houston. She completed her ophthalmology residency at the University of Michigan's Kellogg Eye Center. She is currently a staff comprehensive ophthalmologist at Henry Ford Health System's Grosse Pointe Ophthalmology.

A cataract is a progressive cloudiness (also called opacity or opacification), hardening, and yellowing of the normally transparent lens of the eye. According to the National Eye Institute, approximately 50% of all Americans will either have a cataract or will have had cataract surgery by age 80.

To talk about cataracts, it's helpful to understand the parts of the eye, including the location and function of the lens, as shown in this diagram of the eye:

The lens is composed of transparent, flexible tissue and is located directly behind the iris and the pupil. Like the lens in a camera, the lens in the eye helps to focus light and images on the retina, which is the light-sensitive membrane that lines the inside surface of the eye. Nerve cells in the retina convert incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (which is like a television cable) to the brain, which finally interprets them as visual images.

At birth, the natural lens is clear, colorless, and very flexible Because it is flexible, it is able to change shape, without the help of bifocals or reading glasses, to focus on objects and people that are either nearby or at a distance.

The lens becomes more rounded to focus on near objects (see Fig 1) and more elongated (or stretched) to focus on objects that are far away (see Fig 2).

Fig 1: Lens is more rounded to focus on near objects

Fig 2: Lens is more elongated to focus on near objects

Over time, however, two primary changes begin to occur in the lens, usually after age 40:

• The lens becomes less flexible, begins to harden, and loses its ability to become more curved. As a result, it becomes difficult to focus on near images (especially print) without the help of bifocals or reading glasses.
• The lens gradually changes color, becoming yellowish or brownish, and is no longer transparent. As a result, vision acquires a "brownish" tint, making it difficult to tell the difference between certain colors, such as navy blue, brown, and black, or blue, green and purple.

This hardening and yellowing of the lens over time also causes the most common type of cataract, called a nuclear sclerotic cataract. "Nuclear" refers to the gradual clouding of the central portion of the lens, called the nucleus; "sclerotic" refers to the hardening, or sclerosis, of the lens nucleus.

How might cataracts affect everyday activities?

Cataracts cause overall general blurring — people, objects, and colors look hazy, cloudy, and "washed out." This lack of detail makes it difficult to tell time, read, watch television, see food on a plate, and walk safely indoors and outdoors. Surgery can usually be effective in removing cataracts. The National Eye Institute [all links on this page open in a new window] has provided the following photos.

Here is what a person with normal vision sees:

Here is what a person with cataracts sees: