Abstract
purpose. To determine the epidemiology of refractive errors in an adult Chinese
population in Singapore.
methods. A disproportionate, stratified, clustered, random-sampling procedure
was used to select names of 2000 Chinese people aged 40 to 79 years
from the 1996 Singapore electoral register in the Tanjong Pagar
district in Singapore. These people were invited to a centralized
clinic for a comprehensive eye examination, including refraction.
Refraction was also performed on nonrespondents in their homes. Myopia,
high myopia, and hyperopia were defined as a spherical equivalent (SE)
in the right eye of less than −0.5 D, less than −5.0 D, and more than+
0.5 D, respectively. Astigmatism was defined as less than −0.5 D of
cylinder. Anisometropia was defined as a difference in SE of more than
1.0 D between the two eyes. Only phakic eyes were analyzed.
results. From 1717 eligible people, 1232 (71.8%) were examined. Adjusted to the
1997 Singapore population, the overall prevalence of myopia, hyperopia,
astigmatism, and anisometropia was 38.7% (95% confidence interval[
CI]: 35.5, 42.1), 28.4% (95% CI: 25.3, 31.3), 37.8% (95% CI:
34.6, 41.1), and 15.9% (95% CI: 13.5, 18.4), respectively. The
prevalence of high myopia was 9.1% (95% CI: 7.2, 11.2), with women
having significantly higher rates than men. The age pattern of myopia
was bimodal, with higher prevalence in the 40 to 49 and 70 to 81 age
groups and lower prevalence between those age ranges. Prevalence was
reversed in hyperopia, with a higher prevalence in subjects aged 50 to
69. There was a monotonic increase in prevalence with age for both
astigmatism and anisometropia. Increasing educational levels, higher
individual income, professional or office-related occupations, better
housing, and greater severity of nuclear opacity were all significantly
associated with higher rates of myopia, after adjustment for age and
sex.
conclusions. The results indicate that whereas myopia is 1.5 to 2.5 times more
prevalent in adult Chinese residing in Singapore than in similarly aged
European-derived populations in the United States and Australia, the
sociodemographic associations are similar.
The few available population-based studies on myopia and other
refractive errors have been conducted almost solely in white
European-derived populations. In the 1970s, a prevalence of 25% was
reported in a United States study among persons aged 12 to 54
years.
1 In persons more than 40 years of age, prevalence
observed in recent surveys in the United States was 22.7% in the
Baltimore Eye Survey
2 and 26.2% in the Beaver Dam
Study.
3 In Australia, the overall prevalence of myopia was
17% in the Visual Impairment Project
4 and 15% in the
Blue Mountains Eye Study,
5 and in Israel, a prevalence of
18.4% was reported.
6
For other racial groups, population-based data are sparse. In the
Baltimore Eye Survey, the prevalence of myopia in blacks (19.4%) was
lower than in whites (28.1%).
2 In the Barbados Eye Study
the prevalence of myopia in black persons more than 40 years of age was
21.9%.
7
In East Asia, although the prevalence of myopia is often presumed to be
much higher, precise population-based data are not
available.
8 At present, the available information is based
on studies conducted in selected subgroups of the population. In
Taiwan, two studies covering school children aged 6 to 18 showed a
prevalence of more than 80% by age 18 years.
9 10 Another
study in a Japanese student population showed an overall prevalence of
approximately 50%,
11 and a pilot study in Hong Kong in
355 adult Chinese aged 40 or more years showed a prevalence of
approximately 40%.
12 In Singapore, several studies in
students and army recruits have indicated that the prevalence may be
among the highest in the world.
13 14 However, the
prevalence of myopia in the adult population of Singapore or other East
Asian countries is not known.
For other refractive errors such as hyperopia, astigmatism, and
anisometropia, the prevalences have been estimated in the adult white
and black populations,
1 2 3 4 5 6 7 but information in adult East
Asian populations is limited.
Although the cause of myopia is debatable,
15 16 risk
factors in the European-derived populations are well documented.
Non–cataract-related myopia rates have been reported to be associated
with higher education,
1 2 3 4 5 17 certain occupations
associated with near work
4 7 and higher family
income.
1 Whether the same risk factors for myopia apply in
East Asian countries is less clear.
The purpose of this study was to estimate the prevalence of different
refractive errors in an adult Chinese population aged 40 years and more
in Singapore. In particular, we were interested in evaluating risk
factors associated with myopia in Chinese. Singapore is an urban
city-state with a population of 3.2 million, 78% of whom are ethnic
Chinese. Because a large number of Chinese residents in our study age
group were first-generation immigrants from the southern provinces of
Fujian and Guandong in China, some generalizability of our data to the
Chinese population is possible.
The study was part of a population-based survey of ocular
disorders among adult Chinese living in Singapore. The 1996 Singapore
electoral register was used as the sampling frame in this study.
Because electoral registration is a legal requirement in Singapore, the
register provides a complete record of all Singapore citizens aged 21
years and older. Tanjong Pagar is located in the center of Singapore
and was chosen because the population demographics of the Chinese
residents are representative of the remainder of Singapore. The
electoral register listed names of 15,082 Chinese people aged between
40 and 79 years and residing in the district. Two thousand (13.3%)
names were selected by a disproportionate (with more weights given to
the older age groups), stratified, clustered, random-sampling method.
These persons were invited for a comprehensive eye examination at
the study center. After this, domiciliary examinations of nonresponding
persons were performed by a field team. The study was performed between
October 10, 1997, and August 14, 1998.
Before each examination, identity was verified from subjects’
national identity cards, after which written, informed consent was
obtained after an explanation of the study in Chinese, one of the
Chinese dialects, or English. Consent was signed in Chinese in 46% of
the participants, in English in 44%, and a cross in the remaining
10%.
Visual acuity was assessed in the following sequence as part of a
standardized clinical examination. Habitual visual acuity was first
determined with the initial distance spectacle correction (if any)
using the logMAR chart (The Lighthouse, Long Island City, NY) under
standard lighting conditions at 4 m.
18 The visual
acuity testing was performed as follows: Subjects began reading at the
6/12 equivalent line (0.3). If they were able to read more than three
letters on that line, they proceeded to the next line below, until two
mistakes were made on one line (i.e., only three letters correctly
identified on that line). This line would then be recorded as the
habitual visual acuity. If they were unable to read at least three
letters, they would begin to read the line above and continued this
process until at least three letters on one line had been read. If
unable to read the chart at 4 m, the subject was moved to 2 m
and then to 1 m from the chart. If subjects were unable to achieve
minimum recordable logMAR acuity, the vision was recorded as perception
or nonperception of light, as appropriate.
Refraction data on all subjects were recorded as follows. An initial
objective refraction result was recorded by using an autorefractor
(Retinomax K-plus; Nikon, Tokyo, Japan). Manual subjective refraction
was then attempted to refine vision, using the results of the objective
refraction as the starting point. The best corrected visual acuity was
found, and both the derived refraction data and the visual acuity were
recorded. Because of the age of our study population, no cycloplegia
was used.
To maximize response rates, those who did not attend the clinic visit
were offered an abbreviated examination in their homes. For this
domiciliary examination, habitual visual acuity was assessed using a
3-m Snellen chart, with distance spectacle correction if needed. If the
visual acuity was less than 6/12, the acuity was remeasured using a
pinhole (best corrected visual acuity). Refraction was assessed using a
handheld autorefractor (Retinomax K-plus; Nikon, Toyko, Japan). No
manual subjective refraction was conducted on these subjects.
For this study, refraction data were based on subjective
refraction when participants had both subjective and objective
refraction and on objective refraction when only this information was
available. Preliminary analysis of data from participants with phakic
eyes who had both types of refraction in the right eye (n = 981)
indicated that the overall correlation between objective and subjective
refraction in spherical equivalents (SEs) was high (ρ = 0.98,
P < 0.001). Other reports will examine in detail the
variations in correlation between the two methods of refraction among
different age groups and refractive status. For our definitions of
emmetropia, myopia, hyperopia, and anisometropia, the refractive data
were converted to SE, which is derived by adding the spherical
component of the refraction to half the cylindrical component. The data
on the right and left eyes were initially analyzed separately. However,
because the correlation between the right and left eyes for SE was high
(ρ = 0.83,
P < 0.001), for the analysis of all
refractive errors other than anisometropia, we present data on only the
right eye. Emmetropia was defined as an SE of between −0.5 and 0.5 D.
Myopia was defined and analyzed on two levels separately: an SE of less
than −0.5 D and less than −1.0 D. High myopia was defined as SE of
less than −5.0 D. Hyperopia was defined an SE of more than +0.5D,
whereas anisometropia was defined as an SE difference between the right
and left eyes of more than 1.0 D. Astigmatism was analyzed in minus
cylinders and was defined as a less than −0.5 D of cylinder, without
reference to the axis. These definitions were chosen to enable direct
comparison between our data and those in other
studies.
1 2 3 4 5
Lens nuclear opacity was graded by slit lamp examination using the
modified Lens Opacity Classification System (LOCS) III
score.
19 Under this scoring system, nuclear opacity was
graded in increasing severity from grade 1 to grade 6.
Because our study population was selected based on a
disproportionate sampling technique (with more weights given to the
older age groups), to provide a more accurate estimate of the actual
prevalence of refractive errors in the population, age-adjusted
rates were calculated based on the appropriate 1997 Singapore
Chinese population (men, women, and both men and women).
20 The associations between gender, age, and other socioeconomic
variables, as well as lens nuclear opacity and the risk of myopia and
high myopia were estimated by the odds ratio (OR) and its 95%
confidence interval (CI). Multiple logistic regression was used to
assess the mutually confounding effect of these variables on the risk
of myopia and high myopia and to obtain adjusted estimates of the
individual OR. Statistical analyses of the data were performed by a
computer program (SPSS, Chicago, IL).
This study was approved by the ethics committee of Singapore National
Eye Center and performed in accordance with the tenets of the World
Medical Association’s Declaration of Helsinki.
Among the 2000 subjects selected, 46 died and 235 moved to
addresses outside Tanjong Pagar district before or during the study
period. Two persons were considered ineligible for the survey (one
being near death and the other severely mentally disabled). At the
initial research clinic, 1072 were examined. An additional 160 subjects
were examined in their homes. Of these, 18 were then referred to the
research clinic for definite diagnostic examination. The total number
of subjects examined in either setting was 1232. Therefore, the overall
response rate was 71.8% (1232 of 1717 subjects), whereas the response
rate based only on the clinic examination was 63.5% (1090 of 1717
subjects).
Of the 1232 subjects examined, 103 (8.4%) had had cataract extraction
in their right eyes and were excluded. Sixteen (1.3%) had no
refraction data for their right eyes, including four subjects with
corneal opacities, dense cataracts, and other media opacities in which
accurate refraction could not be performed.
Further analyses on myopia, hyperopia, and astigmatism were based on
refraction data from the 1113 subjects with phakic eyes with refraction
in the right eye.
Table 1 shows the characteristics of subjects included in and excluded
from the analysis. In general, subjects included in the analysis were
younger; more likely to be professionals, office workers, and
salespeople; lived in better housing; and had higher education levels
and individual income. There was little difference between the two
groups in gender and smoking status.
Of the 1113 subjects, 500 (44.9%) were men, and 613 (55.1%) were
women. The mean age of these subjects was 58.8 ± 11.0 (SD) years.
There was no significant difference in mean ages between men (59.2 ± 11.0 years) and women (58.5 ± 11.0 years; t-test, P = 0.28).
Figure 1 shows the distribution of the refraction in SEs in the study
population. A total of 324 (29.1%) subjects were classified as
emmetropic, 389 (35.0%) were classified as myopic (worse than −0.5
D), and 400 (35.9%) were hyperopic. Seventy-seven subjects (6.9%) had
high myopia. This formed nearly 20% of the myopes in the study
population. In addition, astigmatism was present in 489 subjects
(43.9%).
The crude and age-adjusted prevalence of emmetropia and different
refractive errors are shown in
Table 2 . The overall age-adjusted prevalence of myopia (worse than −0.5 D) was
38.7% (95% CI: 35.5, 42.1), with higher rates in women (40.5%, 95%
CI: 35.3, 44.5) than in men (36.5%, 95% CI: 31.8, 41.0). There was no
statistical difference between the two sexes in age-adjusted rates of
all refractive errors, except for high myopia, in which women had
significantly higher prevalence (χ
2 test,
P = 0.04).
For the analysis of anisometropia, subjects who had had cataract
surgery in either eyes were excluded (
n = 131). Also
excluded were subjects without refractive data in either eye (
n= 25). Of the remaining 1076 subjects with phakic eyes with
refractive data in both eyes, 215 (20.0%) had anisometropia (SE
difference between the right and left eyes >1.0 D). The age-adjusted
prevalence of anisometropia is shown in
Table 2 .
There were distinctive age and sex patterns for different refractive
errors
(Table 3) . For example, for myopia
(Fig. 2) , the highest prevalence was in the 40 to 49 age group, and the second
highest was in the 70 to 81 age group. Lower prevalence was seen
between these two age groups. The highest prevalence in our population
occurred in women aged 40 to 49 (51.7%, 95% CI: 43.7, 59.6) and the
lowest in men aged 50 to 59 (25.2%, 95% CI: 17.3, 33.2). In contrast,
the age pattern was reversed for hyperopia, with highest prevalence in
the 60 to 69 age group
(Fig. 2) . For both astigmatism and
anisometropia, there was a monotonic increase in prevalence with age in
both sexes
(Fig. 3) .
We further examined the prevalence of different refractive errors in
subgroups of the population
(Table 4) . Increasing levels of education, better housing, and higher individual
monthly income were significantly associated with increasing prevalence
of myopia and high myopia. Professionals and office workers had the
highest prevalence of myopia (52.4%) in comparison with other
occupations, whereas laborers had the lowest prevalence (25.9%).
Subjects with nuclear cataract who had LOCS III scores of 5 or 6 had
the highest levels of myopia, followed by subjects with LOCS III scores
of 1 or 2. The lowest rates of myopia were seen in subjects with
moderate nuclear cataract (LOCS scores of 3 or 4). This pattern was
probably related to the confounding effects of age on the prevalence of
nuclear cataract and myopia, which was explored further with
multivariate analysis (see
Table 5 and the Discussion section). Smokers had a lower prevalence of myopia
and high myopia, but the relationship was again likely to be confounded
by age.
The risk of different associations with myopia and high myopia were
analyzed in various multivariate models and are selectively presented
in
Table 5 . When adjusted for age, there was no difference in risk of
myopia between men and women. However, women had a 69% greater risk of
high myopia (age-adjusted odds ratio [OR], 1.69, 95% CI: 1.03,
2.77). The lowest risk of myopia was seen in both genders aged 50 to 59
years. Men and women aged 40 to 49 had an approximately one and a half
times higher risk of myopia than those aged 50 to 59. The age pattern
was exaggerated for high myopia in both genders.
Increasing education, better housing, and higher individual income were
associated with higher odds of myopia and high myopia, after adjusting
for age and sex. Professionals and office workers had higher odds of
myopia compared with those in other occupations, after adjusting for
age and sex. Finally, nuclear opacity was associated independently with
myopia and high myopia, after adjusting for age, sex, and individual
income (as an indicator for socioeconomic status). Similar results for
the association with nuclear opacity were obtained when adjustment for
socioeconomic status was made with other proxy variables, such as
housing types and education, evidenced by the high correlation of these
variables with one another.
To our knowledge, this study is the first to provide
population-based data on the prevalence of refractive errors in adult
East Asians. We observed an overall prevalence of myopia of 38.7% in
our Chinese residents, which is between one and a half to two and a
half times the rate seen in similarly aged populations in
whites
2 3 4 5 and blacks.
2 7 This confirms a
widely held view that myopia is more common in East Asia, based on data
in previous studies in selected populations.
8 9 10 11 12 13 14 In
addition, our data show that the same sociodemographic (higher
education, higher income, better housing, and occupations associated
with near work) and ocular (increasing lens opacity) risk factors for
myopia exist between Chinese in Singapore and the European-derived
populations.
There are difficulties in direct comparison of rates of myopia with
data in other studies, because of differing ages of participants,
definitions of myopia, inclusion criteria, and research methodology. In
the Beaver Dam study (age range: 43–84 years), an overall prevalence
of myopia of 26.2% was found in persons with phakic eyes who had
better than 20/40 visual acuity in at least one eye.
3 In
the Visual Impairment Project in Victoria, Australia, a prevalence of
17% was observed in the population (40–98 years), after exclusion of
participants with aphakia or pseudophakia, corneal diseases, and media
opacities associated with worse than 20/60 visual
acuities.
4 In the Blue Mountains Eye Study in Australia
(age range: 49–97 years), the 15% myopia prevalence reported could
not be directly compared with that in the current study, because their
study population was a decade older.
5 Similarly, the
National Health and Nutrition Examination Survey (NHANES) in 1971
estimated that 25% of the population was myopic, but the subjects were
much younger than ours (12–54 years).
1 In addition, the
investigators in that study defined myopia as any negative SE, based on
refraction in participants whose visual acuity was worse than 20/40 and
lensometry in those whose visual acuity was 20/40 or better. In the
Baltimore Eye Survey (age range: 40–89 years), a prevalence of 22.7%
was noted in white and black subjects aged 40 years and
more.
2 In that study as well as in ours, participants were
older than 40 years, myopia was defined as SE of worse than −0.5 D,
and all had noncycloplegic refraction, regardless of their visual
acuity or whether they wore spectacles.
The racial and ethnic variation of myopia has been reported in
population-based studies in European-derived societies. For other
populations, limited data are available. Racial variation was shown in
the United States, where the NHANES report indicated that the
prevalence of myopia was significantly lower among blacks than
whites.
1 Using the same definition of myopia as in the
Beaver Dam study and after adjustments for age, gender, and education,
the Baltimore Eye Survey reported a 28.1% prevalence in whites
(similar to the Beaver Dam prevalence of 26.2%) but only 19.4% in
blacks.
2 The prevalence of myopia in the black population
in Barbados (age range: 40–84 years) was recently shown to be
21.9%.
7 In contrast, the prevalence of myopia has been
reported to be lower than 5% in Australian Aborigines
21 and in the populations in the Solomon Islands
22 and
Malawi.
23
In East Asia, the apparent high prevalence of myopia was first observed
in the 1930s in China.
24 Since then, several studies have
confirmed that the prevalence may be above 50% in selected high-risk
populations.
9 10 11 13 14 Wensor et al.
4 noted
that residents in Australia who had been born in Southeast Asia had a
higher prevalence of myopia than those born elsewhere, even after
controlling for education. Our study adds support to the suggestion
that the prevalence of myopia in East Asians is higher. There are no
data that adequately explain the high prevalence of myopia in East
Asians. Both environmental and genetic factors appear to play important
roles.
8 15 16 Possible environmental reasons suggested
include an apparent increase in formal education and more time spent on
near work tasks by East Asians.
8 Genetic variations
between East Asians and European-derived populations have also been
suggested but not substantiated.
8 15 16
One particular concern is that almost 7% of our population could be
classified as having high myopia (worse than −5.0 D). Adjusted to the
adult Chinese population aged 40 to 79 years in Singapore, the
prevalence of high myopia was nearly 10%. This figure is higher than
data in reports from European-derived populations. In the Visual
Impairment Project, high myopia of similar definition was found in less
than 2% of their subjects with myopia.
4 The prevalence of
high myopes in the Baltimore Eye Survey, in which a slightly different
definition was used for high myopia (worse than −6.0 D), was
approximately 1.4%.
2 There are two important ophthalmic
implications in the rate of high myopia. First, high myopia has a
higher risk of cataract, glaucoma, myopic macular degeneration, and
retinal detachment.
15 Second, the results of refractive
surgery are less predictable in subjects with high
myopia.
25
Other demographic patterns have been noted in European-derived
populations with refractive errors. Age has been highly correlated with
the prevalence of different refractive errors. For both astigmatism and
anisometropia, there was an age-dependent increase in prevalence in our
study that was similar to that reported in white
populations.
2 5 For myopia, both population and
non–population-based studies have shown that the prevalence generally
follows a bimodal pattern in adults, initially declining with age and
then increasing in the upper age groups.
2 3 4 Because the
age pattern of myopia was described half a century ago,
26 the exact rationale for this observation is still controversial. One
theory involves changes in the refractive index gradient of the lens
with age.
27 In the Beaver Dam Study, increasing level of
nuclear opacity was an important determinant of the age-related
increase in myopia after 70 years.
28 An alternative
explanation to age-related variations is that a real increase in the
prevalence of myopia has occurred between the younger population
compared with older people.
29 It seems likely that both
mechanisms are operating in our population as well.
The association of gender with refractive errors has not been well
established. In the NHANES report, the prevalence of myopia was
significantly lower in men than women.
1 In the Beaver Dam
study, a small gender difference was seen in the rates of
myopia
3 but in the Baltimore Eye Survey, no gender
difference was found.
2 Similarly, our study does not
indicate significant gender differences. Although women were more
likely to have high myopia
(Table 5) , the overall rates of myopia and
other refractive errors were similar in both genders across the entire
age range
(Table 3) .
Educational status, occupation, and income have been among the most
frequently noted socioeconomic associations of myopia and hyperopia.
These associations may be an indicator of near work and support the
use–abuse theory for myopia.
15 16 In the NHANES study,
the prevalence of myopia increased with family income and educational
level.
1 Both the Baltimore and Beaver Dam studies showed a
monotonic relationship between education and myopia.
2 3 Our study found a direct relationship between increasing education and
myopia and an inverse relationship with hyperopia. The Visual
Impairment Project in Melbourne found that professionals and clerks had
higher risks of myopia.
4 One possible explanation was the
association of these occupations with greater amounts of near work. We
found similar higher odds of myopia in people who were professionals
and office workers. Income and better housing are indicators of
socioeconomic status and have been reported to be related to myopia in
white populations,
1 as was similarly seen in ours. As
expected, we observed no distinctive trends of sociodemographic factors
with either astigmatism or anisometropia.
Lens opacity appears to be an important ocular determinant of
refractive error. There was a dose–response pattern between level of
lens opacity and rate of astigmatism. For myopia, persons with the
highest levels of nuclear opacity in our study (LOCS III grade 5 or 6)
had an OR of 16 compared with the lowest levels (LOCS III grade 1 or
2). This was apparent also in the Visual Impairment Project in
Australia.
4
In conclusion, our study provides further epidemiologic data on the
prevalence of refractive errors in an adult Chinese population in Asia.
First, we showed that the prevalence of myopia was one and a half to
two and a half times higher than in similarly aged European derived
populations in the United States and Australia. Second, up to 10% of
our population had high myopia, with potentially serious ophthalmic
implications. Third, although the rate of myopia was different from
that in whites and blacks, the sociodemographic and ocular risk factors
were similar, indicating a common pathophysiological basis for myopia.
The authors thank Judy Hall, for training technical staff
and providing quality assurance services; the Clinical Audit Department
of the Singapore National Eye Center for data management; and Rachel Ng
and Bernie Poh for coordination of community volunteer participation.