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?
-
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:
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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