The term blindness implies total or
partial loss of vision involving both eyes. The exact
level of vision defined as blindness, however,
varies in different countries because of differing
legal or social requirements. In the United States,
blindness is defined as unimprovable vision of
20/200 (6/60) or worse. This means that an
individual is generally considered blind who, even
with the use of ordinary eyeglasses, can see no
better at 20 ft (6 m) than a person with normal
vision can see at 200 ft (60 m). On the other hand,
the World Health Organization (WHO) Program
Advisory Group on the Prevention of Blindness
lists the vision level suggested as blindness as a
visual acuity of less than 10/200 (3/60), which is
twice as low as the U.S. definition. The WHO
level of visual acuity is also described as the
inability to count fingers in daylight at a distance of
10 ft (3 m), because in many regions a great
number of people cannot receive formal eye
examinations but may be tested by unspecialized
personnel. From data available in the mid-1980s,
the number of persons worldwide who have a
visual acuity of less than 10/200 is estimated as 28
million. This level of handicap precludes an
individual from functioning effectively in the
community without special assistance and
rehabilitation (see BLIND, EDUCATION OF
THE). Were the definition of blindness instead
taken as 20/200, as is done in a number of
industrialized countries besides the United States,
the total number of blind persons in the world
would be about 42 million. The prevalence and
causes of blindness vary according to the
geographical location and economic status of a
region. Thus in developing countries in tropical
areas, the number of blind persons may reach at
least 21 million (using the WHO definition). This
high figure is due to the massive prevalence of
such eye diseases as TRACHOMA,
onchocerciasis (RIVER BLINDNESS), and the
keratomalacia and xerophthalmia. In addition, few
eye doctors are present in such regions to treat
CATARACT or accidental eye injuries. In other
developing countries where eye care is not easily
available and the presence of untreated cataract
and undetected GLAUCOMA is also critical, the
number of blind persons may reach 5.5 million. In
developed countries with advanced medical
services, on the other hand, most curable
blindness is treated. Thus although many persons
may have vision problems, the exact number of
blind individuals may be less than 2 million. The
main causes of blindness in such countries are
age-related cataract, glaucoma, DIABETES, and
macular degeneration (see EYE DISEASES). In
the United States about 1 million cataracts are
removed each year and vision improved with
either an intraocular plastic lens, a contact corneal
lens, or a cataract glass. Several million Americans
are also being treated for glaucoma, which if
caught in its early stages can usually be dealt with
by medication or surgery. PAUL HENKIND,
M.D. Bibliography: Dobree, J. H., and Boulter,
Eric, Blindness and Visual Handicap: The Facts
(1982); Faye, E. E., Clinical Low Vision (1976);
Henkind, Paul, Priest, R. S., and Schiller, G.,
Compendium of Ophthalmology (1983); Kirchner,
Corinne, Data on Blindness and Visual Impairment
in the U. S. A. (1985); Koestler, Frances, The
Unseen Minority: A Social History of Blindness in
the United States (1976); Lewis, Vicky,
Development and Handicap, 3d ed. (1987);
Monbeck, Michael E., The Meaning of Blindness
(1973); Naumann, G. O., and Apple, D. J.,
Pathology of the Eye (1986); Sommer, Alfred,
Nutritional Blindness (1982); Warren, David,
Blindness and Early Development (1977); Wilson,
John, World Blindness and Its Prevention, 2 vols.

Category: Science