The human eye is like a camera that collects, focuses, and transmits light through a lens to create an image of its surroundings. In a camera, the image is created on film or an image sensor. In the eye, the image is created on the retina, a thin layer of light-sensitive tissue at the back of the eye.

Like a camera, the human eye controls the amount of light that enters the eye. The iris (the colored circular part of the eye) controls the amount of light passing through the pupil. It closes up the pupil in bright light and opens it wider in dim light. The cornea is the transparent, protective surface of the eye. It helps focus light, as does the lens, which sits just behind the iris.

When light enters the eye, the retina changes the light into nerve signals. The retina then sends these signals along the optic nerve (a cable of more than 1,000,000 nerve fibers) to the brain. Without a retina or optic nerve, the eye can't communicate with the brain, making vision impossible.

What Is Visual Impairment?

Many people have some type of visual problem at some point in their lives. Some can no longer see objects far away. Others have problems reading small print. These types of conditions are often easily treated with eyeglasses or contact lenses.

But when one or more parts of the eye or brain that are needed to process images become diseased or damaged, severe or total loss of vision can occur. In these cases, vision can't be fully restored with medical treatment, surgery, or corrective lenses like glasses or contacts.

The American Foundation for the Blind estimates that 10 million people in the United States are visually impaired. Visual impairment is a term experts use to describe any kind of vision loss, whether it's someone who cannot see at all or someone who has partial vision loss.

Some people are completely blind, but many others have what's called legal blindness. They haven't lost their sight completely but have lost enough vision that they'd have to stand 20 feet from an object to see it as well as someone with perfect vision could from 200 feet away.

What Causes Visual Impairment?

People rarely lose their eyesight during their teen years. When they do, it's usually caused by an injury like getting hit in the eye or head with a baseball or having an automobile or motorcycle accident.

Some babies have congenital blindness, which means they are visually impaired at birth. Congenital blindness can be caused by a number of things — it can be inherited, for instance, or caused by an infection (like German measles) that's transmitted from the mother to the developing fetus during pregnancy.

Conditions that may cause vision loss after birth include:

• Amblyopia (pronounced: am-blee-oh-pee-uh) is reduced vision in an eye caused by lack of use of that eye in early childhood. Some conditions cause a child's eyes to send different messages to the brain (for example, one eye might focus better than the other). The brain may then turn off or suppress images from the weaker eye and vision from that eye then stops developing normally. This is also known as a "lazy eye." Strabismus (misaligned or crossed eyes) is a common cause of amblyopia, since the brain will start to ignore messages sent by one of the misaligned eyes.
• Cataracts are cloudy areas in part or all of the lens of the eye. In people without cataracts, the lens is crystal clear and allows light to pass through and focus on the retina. Cataracts prevent light from easily passing through the lens, and this causes loss of vision. Cataracts often form slowly and usually affect people in their 60s and 70s, but sometimes babies are born with congenital cataracts. Symptoms include double vision, cloudy or blurry vision, difficulty seeing in poorly lit spaces, and colors that seem faded.
• Diabetic retinopathy (pronounced: reh-ton-ah-pa-thee) occurs when the tiny blood vessels in the retina are damaged due to diabetes. People with retinopathy may not have any problems seeing at first. But if the condition gets worse, they can become blind. Teens who have diabetes should be sure to get regular eye exams because there are no early warning signs for this condition. To help prevent retinopathy, people with diabetes should also avoid smoking, keep their blood pressure under control, and keep their blood sugar at an even level.
• Glaucoma is an increase in pressure inside the eye. The increased pressure impairs vision by damaging the optic nerve. Glaucoma is mostly seen in older adults, although babies may be born with the condition and children and teens can sometimes develop it as well.
• Macular degeneration (pronounced: mah-kyoo-lur dih-jeh-nuh-ray-shun) is a gradual and progressive deterioration of the macula, the most sensitive region of the retina. The condition leads to progressive loss of central vision (the ability to see fine details directly in front). Macular degeneration is often age related (it occurs in older people, especially older than 60), but sometimes it can occur in younger people. Excessive exposure to sunlight and smoking can increase the risk for age-related macular degeneration. Symptoms may include increased difficulty reading or watching TV, or distorted vision in which straight lines appear wavy or objects look larger or smaller than normal.
• Trachoma (pronounced: truh-ko-muh) occurs when a very contagious microorganism called Chlamydia trachomatis causes inflammation in the eye. It's often found in poor rural countries that have overcrowded living conditions and limited access to water and sanitation. Blindness due to trachoma has been virtually eliminated from the USA.

What Do Doctors Do?

If you, your parent, or your doctor suspects a visual problem, you'll probably pay a visit to an ophthalmologist (pronounced: af-thal-mah-luh-jist), a medical doctor who specializes in examining, diagnosing, and treating eyes and eye diseases. When someone goes for an examination, the ophthalmologist will look at the structure of that person's eye.

Other simple tests an ophthalmologist may perform include:

• Visual acuity test. A person reads an eye chart to measure how well he or she sees at various distances.
• Visual field test. Ophthalmologists use this test to measure side, or peripheral, vision.
• Tonometry test. This test determines the fluid pressure inside the eye to evaluate for glaucoma.

If your doctor determines that you have an eye condition that is likely to cause visual impairment, many treatments are available. Options may include eyeglasses, contact lenses, and eye drops or other medicines.

In some cases, surgery may be required. For instance, cataracts are often treated by removing the clouded lens and replacing it with an intraocular lens (an artificial plastic lens that requires no special care and restores vision).

Other methods can compensate for vision loss. Guide dogs can help people get from place to place independently. Braille allows those with visual impairment to read and write. Special equipment such as microscopic and telescopic glasses and voice-recognition software can make school and homework easier.

What's It Like to Be Visually Impaired?

Just as you don't think about your eye color every day, people with visual impairment don't always think about their condition every day either.

Someone with sight problems can become isolated from others more easily, though. If a visually impaired person asks for assistance, don't hesitate to help. But someone who uses a cane or a guide dog is probably self-sufficient and may not need help.

Reviewed by: Jonathan H. Salvin, MD
Date reviewed: November 2010