Policy and practice
Global visual impairment caused by uncorrected refractive errors
64
Serge Resnikoff et al.
Bulletin of the World Health Organization | January 2008, 86 (1)
Best-corrected vision is the visual acuity
in the better eye achieved by subjects
tested with pinhole or refraction.
Visual impairment caused by un-
corrected or inadequately corrected re-
fractive errors is defined as visual acuity
of less than 6/18 in the better eye that
could be improved to equal to or better
than 6/18 by refraction or pinhole, thus
spanning the low vision and blindness
categories as currently defined in the
ICD-10.
It should be noted that in the
revision of the ICD-10 categories of
visual impairment proposed in 2003 by
a WHO consultation on the develop-
ment of standards for characterization
of vision loss, low vision is replaced by
two categories: moderate visual impair-
ment (presenting visual acuity less than
6/18 but equal to or better than 6/60)
and severe visual impairment (presenting
visual acuity less than 6/60 but equal to
or better than 3/60).
4
Population estimates and WHO
subregions
Estimates of population size and struc-
ture were based on the latest estimates
of world population (for 2004) in the
World population prospects: the 2004
revision; estimates of demographics were
based on the World urbanization pros-
pects 2003 – both sources from the
United Nations Population Division.
5,6
For the classification of WHO
Member States into 17 epidemiological
subregions, see Murray & Lopez, 1996.
7
General inclusion criteria
The following criteria were used to select
studies.
The prevalence of best-corrected and •
presenting visual acuity of less than
6/18 had to be reported or, alterna-
tively, the distribution of causes of
presenting visual impairment.
In children, refractive diagnostics •
had to be determined by objective
refraction under cycloplegia plus sub-
jective refraction.
8
The studies had to be population-•
based, representative of the area
sampled, with definitions of visual
impairment clearly stated. Studies
with inadequate sample sizes and re-
sponse rate were not included.
Data reported only for eyes or for •
the worse eye could not be included
in the estimates calculated for people
and the better eye.
For further discussion of selection cri-
teria, see Resnikoff et al.,
2
and Pascolini
et al., 2004.
9
Sources of epidemiological data
Literature sources were searched sys-
tematically in Medline up to April
2006. Most surveys meeting the selec-
tion criteria were conducted within the
past five years; the earliest surveys date
from 1995. Unpublished data were
provided by academic institutions and
national programmes for the prevention
of blindness.
Table 1 shows the 31 countries for
which surveys that met the selection
criteria were available, the bibliography
can be found at http://ftp.who.int/nmh/
references/RE-estimates-references.pdf
and in the WHO Prevention of Blind-
ness and Deafness Programme’s global
data on visual impairment.
10
For the age group 5–15 years, 16
surveys were found to fit the selection
criteria. Of these, 10 were conducted
in different countries using a specially
designed protocol to estimate the preva-
lence of visual impairment from uncor-
rected refractive errors (the refractive
error study in children (RESC; see
Négrel et al., 2000, for the details of the
protocol).
11
The RESC studies provided
extensive information on visual acuity,
refractive errors and use of spectacles.
For the age group 50 years and
older, 38 surveys met the inclusion cri-
teria. Of these, 30 were surveys for the
Table 1. Surveys used to estimate global visual impairment from uncorrected
refractive errors by WHO subregion, 2004
WHO subregion
a,b
Number of
surveys
Countries
Afr-D 2 Mali, Mauritania
Afr-E 1 South Africa
Amr-A 3 United States of America
Amr-B 5 Argentina, Brazil, Chile, Paraguay, Venezuela
(Bolivarian Republic of)
Amr-D 2 Guatemala, Peru
Emr-B 5 Iran (Islamic Republic of), Lebanon, Oman, Qatar
Eur-A 2 Ireland, Italy
Eur-B2 2 Armenia, Turkmenistan
Sear-B 6 Malaysia, Philippines, Singapore
Sear-D 13 Bangladesh, India, Nepal, Pakistan
Wpr-A 4 Australia
Wpr-B1 7 China
Wpr-B2 16 Cambodia, Myanmar, Viet Nam
a
Afr, WHO African Region; Amr, WHO Region of the Americas; Emr, WHO Eastern Mediterranean Region;
Eur, WHO European Region; Sear, WHO South-East Asia Region; Wpr, WHO Western Pacific Region.
b
In subregions Emr-D, Eur-B1, Eur-C and Wpr-B3, no population-based surveys met the selection criteria.
rapid assessment of cataract surgical
services (RACSS), which also provide
prevalence of presenting and best-
corrected visual acuity.
12
An additional 14 surveys reported
age-specific prevalence of presenting vi-
sual impairment and its causes in other
age groups.
Estimation of prevalence of visual
impairment from uncorrected
refractive errors
For the age group from 5 to 15 years,
the prevalence is estimated by the dif-
ference between the prevalence of pre-
senting and best-corrected visual acuity
of less than 6/18 with refraction under
cycloplegia: this difference corresponds
to the prevalence of presenting visual
acuity that could be improved to equal
to or better than 6/18 by appropriate
correction. In the case of studies report-
ing only the prevalence of presenting
visual acuity, the prevalence of visual
impairment due to refractive error was
determined from the distribution of
causes determined in the surveys.
The prevalence for people aged
16–39 years was estimated to be the
same as that for those aged 5–15 years,
on the assumption that from the ages of
16 years to 39 years, the refractive status
generally does not undergo changes that
require further correction.
13
The prevalence for people aged
40–49 years was either estimated from
