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Vision Impairment

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. [Read more about developmental disabilities]

References:

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Day S. Normal and abnormal visual development. In: Taylor D, editor. Paediatric ophthalmology (2nd edition). Malden, MA: Blackwell Science; 1997, p 13-28.

Holbrook MC (Editor). Children with visual impairments: a parents' guide. Bethesda, MD: Woodbine House; 1996.

Kaminer RK, McMahon E. Blindness and visual impairment. Pediatrics in Review 1995;16:77-8.

Puckett CD.  The educational annotation of ICD-9-CM (4th edition).  Reno, NV: Channel Publishing, Ltd., 2001, p. 641.

Sonksen PM, Petrie A, Drew KJ. Promotion of visual development of severely visually impaired babies: evaluation of a developmentally based programme. Developmental Medicine and Child Neurology 1991;33:320-35.