Developmental Disabilities |
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?
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What lighting is best for the person to do different
tasks?
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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:
Batshaw ML. Children with disabilities
(4th edition). Baltimore, MD: Paul H. Brookes Publishing Co.; 1997.
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.
Date: October 29, 2004
Content source: National Center on Birth Defects and Developmental
Disabilities
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