Understanding Vision Impairment

by CDC

What is vision impairment?

Vision impairment means that a person's eyesight cannot be corrected to a "normal" level. Vision impairment may be caused by a loss of visual acuity, where the eye does not see objects as clearly as usual. It may also be caused by a loss of visual field, where the eye cannot see as wide an area as usual without moving the eyes or turning the head.

There are different ways of describing how severe a person's vision loss is. The World Health Organization defines "low vision" as visual acuity between 20/70 and 20/400, with the best possible correction, or a visual field of 20 degrees or less. "Blindness" is defined as a visual acuity worse than 20/400, with the best possible correction, or a visual field of 10 degrees or less. Someone with a visual acuity of 20/70 can see at 20 feet what someone with normal sight can see at 70 feet. Someone with a visual acuity of 20/400 can see at 20 feet what someone with normal sight can see at 400 feet. A normal visual field is about 160-170 degrees horizontally.

Vision impairment severity may be categorized differently for certain purposes. In the United States, for example, we use the term "legal blindness" to indicate that a person is eligible for certain education or federal programs. Legal blindness is defined as a visual acuity of 20/200 or worse, with the best possible correction, or a visual field of 20 degrees or less.

Visual acuity alone cannot tell you how much a person's life will be affected by their vision loss. It is important to also assess how well a person uses the vision they have. Two people may have the same visual acuity, but one may be able to use his or her vision better to do everyday tasks. Most people who are "blind" have at least some usable vision that can help them move around in their environment and do things in their daily lives. A person's functional vision can be evaluated by observing them in different settings to see how they use their vision. A functional vision evaluation can answer questions such as these:

• Can the person scan a room to find someone or something?
• What lighting is best for the person to do different tasks?
• How does the person use his or her vision to move around in a room or outside?

Vision impairment changes how a child understands and functions in the world. Impaired vision can affect a child's cognitive, emotional, neurological, and physical development by possibly limiting the range of experiences and the kinds of information a child is exposed to.

Nearly two-thirds of children with vision impairment also have one or more other developmental disabilities, such as mental retardation, cerebral palsy, hearing loss, or epilepsy. Children with more severe vision impairment are more likely to have additional disabilities than are children with milder vision impairment.

How common is vision impairment?

Vision impairment is not very common among children. To learn just how common it is, CDC is tracking the number of children with vision impairment in a five-county area in metropolitan Atlanta, Georgia. This activity is part of the Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP). For 1996 we found that about 1.4 in 1,000 8 year olds had low vision or blindness. For 2000, we found this estimate to be slightly lower at 1.2 per 1,000 8 year olds. We also found that vision impairment was more common in older children (ages 6 to 10 years) than in younger children (ages 3 to 5 years). Two-thirds of the children had one or more other disabilities in addition to their vision impairment.

CDC also studied how many children in metropolitan Atlanta were legally blind in the mid-1980s. This project was done as part of the Metropolitan Atlanta Developmental Disabilities Study (MADDS), which studied how common certain disabilities were in 10-year-old children. We found that nearly 7 of every 10,000 children 10 years of age had legal blindness. Two-thirds of the children also had another disability, such as mental retardation, cerebral palsy, or epilepsy.

Vision impairment is more common in older people than in children.. A 2002 report by the National Eye Institute and Prevent Blindness America estimates that more than 1 million people ages 40 years or older in the United States are blind (best corrected visual acuity of 20/200 or worse or a visual field of less than 20 degrees). Another 2.4 million are visually impaired (best corrected visual acuity of 20/40 or worse). The report states that the number of adults with vision impairment likely will double over the next 30 years.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.