Prevalence and Risk Factors for Refractive Errors: Korean National Health and Nutrition Examination Survey 2008-2011
Jee D (2013) Prevalence and Risk Factors for Refractive Errors: Korean National Health and Nutrition
Examination Survey 2008-2011. PLoS ONE 8(11): e80361. doi:10.1371/journal.pone.0080361
Prevalence and Risk Factors for Refractive Errors: Korean National Health and Nutrition Examination Survey 2008-2011
Eun Chul Kim 0
Ian G. Morgan 0
Hirohiko Kakizaki 0
Seungbum Kang 0
Donghyun Jee 0
Yingfeng Zheng, Zhongshan Ophthalmic Center, China
0 1 Department of Ophthalmology and Visual Science , Buchon St. Mary's Hospital, College of Medicine, Catholic University of Korea , Suwon , Korea , 2 Research School of Biology, ARC Centre of Excellence in Vision Science, Australian National University , Canberra , Australia , 3 Department of Ophthalmology, Aichi Medical University , Nagakute, Aichi , Japan , 4 Department of Ophthalmology and Visual Science , Daejon St. Mary's Hospital, College of Medicine, Catholic University of Korea , Suwon , Korea , 5 Department of Ophthalmology and Visual Science, St. Vincent's Hospital, College of Medicine, Catholic University of Korea , Suwon , Korea
Purpose: To examine the prevalence and risk factors of refractive errors in a representative Korean population aged 20 years old or older. Methods: A total of 23,392 people aged 20+ years were selected for the Korean National Health and Nutrition Survey 2008-2011, using stratified, multistage, clustered sampling. Refractive error was measured by autorefraction without cycloplegia, and interviews were performed regarding associated risk factors including gender, age, height, education level, parent's education level, economic status, light exposure time, and current smoking history. Results: Of 23,392 participants, refractive errors were examined in 22,562 persons, including 21,356 subjects with phakic eyes. The overall prevalences of myopia (< -0.5 D), high myopia (< -6.0 D), and hyperopia (> 0.5 D) were 48.1% (95% confidence interval [CI], 47.4-48.8), 4.0% (CI, 3.7-4.3), and 24.2% (CI, 23.6-24.8), respectively. The prevalence of myopia sharply decreased from 78.9% (CI, 77.4-80.4) in 20-29 year olds to 16.1% (CI, 14.9-17.3) in 60-69 year olds. In multivariable logistic regression analyses restricted to subjects aged 40+ years, myopia was associated with younger age (odds ratio [OR], 0.94; 95% Confidence Interval [CI], 0.93-0.94, p < 0.001), education level of university or higher (OR, 2.31; CI, 1.97-2.71, p < 0.001), and shorter sunlight exposure time (OR, 0.84; CI, 0.76-0.93, p = 0.002). Conclusions: This study provides the first representative population-based data on refractive error for Korean adults. The prevalence of myopia in Korean adults in 40+ years (34.7%) was comparable to that in other Asian countries. These results show that the younger generations in Korea are much more myopic than previous generations, and that important factors associated with this increase are increased education levels and reduced sunlight exposures.
Refractive errors can lead to visual impairment and
ultimately, even blindness . Although they can usually be
corrected by wearing glasses or contact lenses, and via
surgery, these solutions pose public health challenges and/or
economic burdens [2-4]. Uncorrected refractive errors are a
major cause of visual impairment , and may lead to loss of
productivity. In addition, refractive errors are risk factors for
various ocular diseases. Myopia, especially high myopia is
associated with open-angle glaucoma, rhegmatogenous retinal
detachment, cataract, and chorioretinopathy such staphyloma
glaucoma , and acute
ischemic optic neuropathy .
Previous epidemiological studies on refractive errors have
revealed marked differences between ethnic groups in different
parts of the world [9,10]. In particular, the rate of myopia has
increased very rapidly in East Asia [1,11-15]. We previously
reported an extremely high prevalence of myopia (96.5%) and
high myopia (20.6%) in 19-year-old males in urban areas of
Korea , and a relative high prevalence of myopia (83.3%)
and high myopia (6.8%) in a rural area . Because of the
limited age range and specific locations of these studies, the
results may not be representative of the general Korean
population. Comprehensive data on prevalence of refractive
errors in Korean adults has not previously been available, and
accurate information on the prevalence of refractive errors is
important for estimating the need for eye care in this
fastchanging health sector in Korea. It may also be useful in
elucidating the causes of the current epidemic of myopia, and
thus predicting the emergence of problems in less-developed
countries as well as potential problems in developed countries
that do not have yet an epidemic of myopia.
Although numerous studies have investigated ethnic
variations in myopia [13,16-18], most studies have used
random sampling of a specific district or city. These studies
may therefore not be representative of the population of a
given country, and may be limited through the confounding of
ethnicity with study site. To our knowledge, only the US
National Health and Nutrition Evaluation Study (NHANES) has
attempted to use stratified, multistage, clustered sampling
methods over a whole nation to describe the prevalence of
refractive error, along with many other health parameters
[9,10,19]. The Korean National Health and Nutrition
Examination Study (KNHANES) uses a similar sampling
procedure to give nationally representative data, and, as with
the more recent rounds of US NHANES, uses noncycloplegic
refraction to measure refractive error.
Risk factors for refractive errors have been extensively
investigated in previous population based studies [20-23].
Among these, education is a strong and consistent risk factor
for myopia [12,23-25]. Our previous study on Korean young
adults also showed that, of several variables, including height,
only education was associated with myopia in multivariable
analysis . However, the effect of education on myopia in a
population-based sample of Korean adults has not yet been
Recently, time spent outdoors has been identified as a
potential protective factor against myopia in children [21,26].
Moreover, time spent outdoors, rather than any specific activity
performed outside, appears to be the important factor, which
suggests that exposure to bright sunlight might play a critical
role in the prevention of myopia. Hence, there is a greater need
to evaluate outdoor activity associated factors, such as sunlight
exposure time. In addition, since most studies evaluating
outdoor activity and myopia have been carried out on children,
the effect of sunlight exposure time on myopia as adults has
only been rarely studied.
In the present study, we examined the prevalence of
refractive errors in a representative Korean population aged 20
years old or older, selected using a stratified, multistage,
clustered sampling method. We also evaluated associated risk
factors, focusing on education and sunlight exposure times.
Subjects and Methods
This study was based on data acquired in the Korean
National Health and Nutrition Examination Survey (KNHANES).
The KNHANES is an ongoing population-based
crosssectional, and nationally representative survey conducted by
the Division of Chronic Disease Surveillance, Korean Center
for Disease Control and Prevention. The survey consists of a
health interview, a nutritional survey, and a health examination
survey. The survey collected data via household interviews and
by direct standardized physical examinations conducted in a
specially equipped mobile examination center. In the
KNHANES, the sample design and size are designed so that
the survey results can be generalized to the whole Korean
Each year, 4,000 households in 200 enumeration districts
were selected by a panel to represent the civilian,
noninstitutionalized South Korean population using stratified,
multistage clustered sampling based on National Census Data.
All members of each selected household were asked to
participate in the survey, and the participation rate between
2008 and 2011 ranged from 77.8% to 82.8%.
All participants were given information about the study, and
gave written informed consent. The study design followed the
tenets of the Declaration of Helsinki for biomedical research
and was approved by the Institutional Review Board of the
Catholic University of Korea in Seoul, Korea.
Refraction without cycloplegia was measured using an
autorefractor (KR-8800; Topcon, Tokyo, Japan) by ophthalmology
residents or ophthalmologists. Refraction measurements were
converted into spherical equivalents, calculated as the
spherical value plus half of the astigmatic value. Myopia was
defined as -0.50 diopters (D) and as -1.0 D , high myopia
was defined as -6.0 D, hyperopia was defined as +0.5 D
and as +2.0 D, and emmetropia was defined as between -0.5
D and +0.5 D. Astigmatism was defined as > +1.0 D. A
slitlamp examination (BM 900; Haag-Streit AG, Koeniz,
Switzerland) was performed by ophthalmologists to identify
pheudophakic and aphakic patients, who were excluded from
the study, and to evaluate cataract status.
Demographic and socioeconomic information was obtained
from a health interview. Height was measured using a
wallmounted measuring scale, and weight was measured with the
individual wearing light clothing without shoes in kilograms
using calibrated electronic scales. Body mass index (BMI) was
calculated using the universally recognized formula: weight
(kg) / height (m)2.
The educational level of participants and their parents was
classified on a scale of 0 to 4, as follows: 0, no education; 1,
elementary education; 2, middle school graduate; 3, high
school graduate; and 4, university graduate or higher.
Economic status was classified into four quartiles, which was
based on personal annual income. Data on current sunlight
exposure time was obtained by selecting a single question from
the two questions: under 5 h, or over 5 h a day. Current
Body mass index
Education level (%)
No school 0.9 1.7 5.6
Elementary school 9.2 24.4 47.4
Middle school 7.0 13.9 16.6
High school 40.4 37.0 20.4
University 42.5 24.0 10.0
Father's education (%)
No school 8.0 15.6 26.5
Elementary school 32.9 44.3 52.0
Middle school 18.8 14.9 8.5
High school 25.7 16.9 8.8
University 14.7 8.3 4.3
Mother's education (%)
No school 16.1 29.7 50.4
Elementary school 39.8 44.0 39.6
Middle school 18.4 12.6 5.0
High school 20.6 11.3 4.1
University 5.0 2.4 1.0
1st quartile (low) 23.9 24.9 24,6
2nd quartile 24.2 26.8 25.7
3rd quartile 26.1 23.4 25.8
4th quartile 25.8 24.9 24.0
Sun exposure time (> 5 hours) (%) 15.2 23.7 30.0
Current smoker (%) 20.9 20.6 16.7
Data are expressed as means standard deviation or frequency (%).
The ANOVA was used for continuous variables, and the Chi-square test was used for categorical variables.
smoking history was also examined by questionnaire which
consisted of yes or no question.
Age- and gender-specific prevalences of myopia, hyperopia
and astigmatism were assessed. The analysis of variance or
chi-square test was used to compare the demographic
characteristics. Logistic regression models were used to
determine the risk factors for myopia, hyperopia and
astigmatism in subjects aged 40 years or older. All variables for
logistic regression analysis were examined for multicollinearity,
and only variables with a variance inflation factor less than 10
were used. Sampling was weighted by statisticians by adjusting
the oversampling and non-response rate . Analyses were
performed using the Statistical Package for the Social Sciences
(SPSS ver. 18.0; SPSS, Inc., Chicago, IL, USA).
Of 23,392 eligible subjects aged over 20 years, refractive
error was measured for 22,562 subjects (96.5%). Of these,
subjects with pseudophakic or aphakic eyes (1,206 eyes) were
excluded. Thus, 21,356 subjects were included the analysis for
prevalence of myopia. Analysis of risk factors for refractive
errors was restricted to the subjects aged 40+ years (n =
The demographic characteristics of 21,356 subjects enrolled
in the study are summarized by refractive status in Table 1.
Subjects with myopia were more likely to be younger, taller,
female, smokers and have higher education levels, higher
parental education levels, higher economic status, and shorter
sun exposure times than those without myopia (P < .001,
Distribution of refractive error in each age group is shown in
Figure 1. Kurtosis and skewness for distribution of refractive
error in each age groups was presented in Table 2. For
comparative purposes, the distributions of mean SER in 19
year-old males in Seoul and the rural area of Jeju Province are
also shown. In the older age groups, kurtosis was high and
skewness was low, with a hyperopic peak of refraction, which
is the classical picture of the distribution of refractive error .
In the younger people, there is a much less kurtosis and much
greater spread in the distribution, with a pronounced skew
towards myopia. This is seen in an exaggerated form in the
data from Seoul. Decreased kurtosis and greater skew
progressively appears in the younger age-groups.
The overall prevalence of myopia and high myopia was
48.1% (CI, 47.448.8) and 4.0% (CI, 3.74.3), respectively.
There is no significant difference in prevalence of myopia
between males and females (p = 0.142). The prevalence of
myopia was markedly higher at 78.9% (CI, 77.480.4) in 2029
year olds compared to 16.1% (CI, 14.917.3) in 6069 year
olds (Table 3). Prevalences of myopia and high myopia
according to age were shown in Figure 2. Once again, data
from our previous studies in Seoul and Jeju Province are
19 yrs 19 yrs 20-29 30-39 40-49 50-59 60-69 > 70
(Seou) (Jeju) yrs yrs yrs yrs yrs yrs
Kurtosis 0.43 0.51 1.08 5.87 9.54 19.84 25.68 17.18
Skewness -0.32 -0.60 -1.04 -1.88 -2.40 -3.10 -3.39 -2.74
Kurtosis and skewness were presented according to the age groups, and
compared with those of previous studies for Korean 19 years old population in
Seoul and Jeju.
included for comparative purposes. The prevalence of myopia
in Korean adults showed a biphasic pattern, declining with age
and then increasing in the higher aged group after 60-69 years
old. However, when analysis was done after excluding subjects
with cataract, this biphasic pattern was not shown, and
Age Number Myopia (<-0.5 D) Myopia (<-1.0 D)
20-29 2690 2123 (78.9, 77.4-80.4) 1731 (64.4, 62.6-66.2)
30-39 4381 3189 (72.8, 71.5-74.1) 2385 (54.3, 52.8-55.8)
40-49 4318 2623 (60.7, 59.2-62.2) 1746 (40.4, 38.9-41.9)
50-59 4056 1322 (32.6, 31.2-34.0) 769 (18.9, 17.7-20.1)
60-69 3438 555 (16.1, 14.9-17.3) 335 (9.7, 8.7-10.7)
70+ 2473 450 (18.2, 16.7-19.7) 311 (12.6, 11.3-13.9)
> 20 21356 10262 (48.1, 47.4-48.8) 7277 (34.0, 33.4-34.6)
> 40 14285 4950 (34.7, 33.9-35.5) 3161 (22.1, 21.4-22.8)
P for trend P < .001 P < .001
20-29 1148 894 (77.9, 75.5-80.3) 734 (64.0, 61.2-66.8)
30-39 1828 1312 (71.8, 69.7-73.9) 985 (53.7, 51.4-56.0)
40-49 1883 1157 (61.4, 59.2-63.6) 778 (41.3, 39.1-43.5)
50-59 1713 591 (34.5, 32.2-36.8) 367 (21.4, 19.5-23.3)
60-69 1545 248 (16.1, 14.3-17.9) 146 (9.4, 7.9-10.9)
70+ 1082 179 (16.5, 14.3-18.7) 119 (11.0, 9.1-12.9)
> 20 9199 4381 (47.6,46.6-48.6) 3129 (34.0, 33.0-35.0)
> 40 6223 2175 (35.0, 33.8-36.2 ) 1410 (22.6, 21.6-23.6)
P for trend P < .001 P < .001
20-29 1542 1229 (79.7, 77.7-81.7) 997 (64.7, 62.3-67.1)
30-39 2553 1877 (73.5, 71.8-75.2) 1400 (54.7, 52.8-56.6)
40-49 2435 1466 (60.2, 58.3-62.1) 968 (39.7, 37.8-41.6)
50-59 2343 731 (31.2, 29.3-33.1) 402 (17.1, 15.6-18.6)
60-69 1893 307 (16.2, 14.5-17.9) 189 (10.0, 8.6-11.4)
70+ 1391 271 (19.5, 17.4-21.6) 192 (13.9, 12.1-15.7)
> 20 12157 5881 (48.4, 47.5-49.3) 4148 (34.0,33.2-34.8)
> 40 8062 2775 (34.4, 33.4-35.4) 1751 (21.7, 20.8-22.6)
P for trend P < .001 P < .001
Prevalence is expressed as percentage and 95% confidence interval.
High myopia (<-6.0 D) Hyperopia (> 0.5 D)
Hyperopia (> 2.0 D)
Astigmatism (> 1.0 D)
prevalence of myopia declined even in the oldest age group
(15.4% in 60-69 years old and 10.5% in 70+ years old).
Although the overall prevalence of hyperopia (>0.5D) was
24.2% (CI, 23.624.8), the age-specific prevalence was lowest
at 2.8% (CI, 2.23.4) in the 2029 year olds and much higher
at 58.7% (CI, 57.160.3) in the 6069 year olds (Table 3). The
overall prevalence of astigmatism was 34.0% (CI, 33.434.6),
increasing from 22.1% (CI, 20.923.3) in the 3039 year olds
to 47.9% (CI, 46.249.6) in 6069 year olds (Table 3).
Education levels by age and gender in the representative
Korean population are shown in Table 4. The proportion of
those with high education levels ( high school graduate) was
very much higher in 30-39 year olds (96.9%) than in 70+ year
olds (16.2%). Younger age groups were not considered, since
many were still completing their education. In addition, the
proportion of those with low education levels ( elementary
school graduate) was very much lower in 30-39 year olds
(0.8%) than in 70+ year olds (74.7%). Height was taller in
20-29 years olds than in 70+ year olds (P for trend <0.001;
Table 5), whereas the proportion of those with current longer
sunlight exposure times (> 5 h) was lower in 20-29 year olds
than in 70+ year olds (P for trend <0.001; Table 5).
Table 6 shows the risk factors for myopia, hyperopia and
astigmatism. In a multivariable regression analysis, the
prevalence of myopia was inversely associated with the age
(OR, 0.93; CI, 0.93-0.94), and longer sun exposure time > 5
hours a day (OR, 0.84; CI, 0.76-0.93) in Korean adults.
Education levels of university or higher (OR, 2.31; CI, 1.97
2.71) and high school (OR, 1.36; CI, 1.191.56) were
significantly associated with myopia compared to elementary
school education or less. Education levels of parents or height
were not significantly associated with myopia.
Hyperopia was associated with the age (OR, 1.10; CI,
1.10-1.10), and inversely associated with high education level
of high school (OR, 0.79; CI, 0.69-0.91), and university or
higher (OR, 0.55 CI, 0.46-0.66). Astigmatism was associated
with age (OR, 1.06; CI, 1.05-1. 06), and inversely associated
with height (OR, 0.99; CI, 0.98-1.00) and higher levels of
economic status (OR, 0.80; CI, 0.71-0.90) compared to lower
20-29 years 30-39 years
Both gender (n = 2690) (n = 4457)
No Education (%) 0.0 0.0
Elementary School (%) 0.5 (0.2-0.8) 0.8 (0.5-1.1)
Middle School (%) 1.7 (1.2-2.2) 2.2 (1.8-2.6)
High School (%) 52.1 (50.2-54.0) 41.4 (40.0-42.8)
University (%) 45.7 (43.8-47.6) 55.5 (54.0-57.0)
Male (n = 1166) (n = 1845)
No Education (%) 0.0 0.0
Elementary School (%) 0.3 (0.0-0.6) 0.8 (0.4-1.2)
Middle School (%) 1.6 (0.9-2.3) 2.4 (1.7-3.1)
High School (%) 62.8 (60.0-65.6) 37.0 (34.8-39.2)
University (%) 35.3 (32.6-38.0) 59.9 (57.7-62.1)
Female (n = 1560) (n = 2612)
No Education (%) 0.0 0.0
Elementary School (%) 0.7 (0.3-1.1) 0.9 (0.5-1.3)
Middle School (%) 1.7 (1.1-2.3) 2.1 (1.6-2.6)
High School (%) 44.2 (41.7-46.7) 44.8 (42.9-46.7)
University (%) 53.5 (51.0-56.0) 52.2 (50.3-54.1)
Prevalence is expressed as percentage and 95% confidence interval.
The present study showed that the prevalence of myopia
was high, particularly in the younger age groups, in Korea, in
adults aged 20 years or older. In a comparison with other Asian
studies with similar ages, the prevalence of myopia in 40 years
or older was 34.7%, which is comparable to that in China (26.7
- 32.3%) [16,17], Singapore (Chinese (38.7%) , Malays
(30.7%)  and Indians (28.0%)), and in India (34.6%)
, but somewhat higher than in Mongolia (17.2%) . When
compared with that of Western countries, the prevalence of
myopia is higher than in Latinos (16.8%) and those of
European origin in Australia (15.0 - 16.9%) [35,36], comparable
to that for African-Americans in the United States (36.5%) ,
but lower than that for whites in the United States (42.6%) .
Prevalences of myopia in major population based studies are
summarized in Table 7.
Age-specific prevalence data are in many ways more
meaningful. The prevalence of myopia in 20-39 year-old
Korean (75.1%) is higher than in age-matched black (49.0%)
and white people (51.3%) in the U.S. NHANES, which used
same examination methods, and a similar sampling frame
(Figure 3). The prevalence of myopia in 40-49 year-old
Koreans (60.7%) is much higher than in similar age groups in
other Asian countries. Our data is much higher than Chinese in
rural areas (22.0%) , Malays in Singapore (31.0) ,
Indians in Singapore (33.3%) , Chinese in Singapore
(48.9%) and in Indians in India (19.2%) . However, that
of Koreans 60 years or older (16.1-18.2%) is comparable or
lower than that in other Asian countries (14.4 - 40.0% in
Chinese , 16.4 - 36.8% in Singapore[29-31], and 54.1
56.0% in India ). After compared with the other Asian
studies without cataract, the prevalence of myopia in those
without cataract in 60-69 years (15.4%) and 70+ years (10.7%)
is similar to that of Indians and Malays in Singapore [30,31].
Cataract has been found to cause myopic shifts in older adults,
which reflects an increased power of the lens rather than
increased axial length [30-32]. The difference in the prevalence
of myopia between 40-49 and 60-69 years aged groups in
Korea (44.0%) was much higher than in Chinese (7.6%) , in
Singapore (13.0 - 20.4%) [29-31], and Western countries (11.2
- 28.1%)[18,35,36]. This finding suggests that the prevalence of
myopia in Korean adults increased more sharply than in other
The reason for rapid increase of myopia in Korean adults is
not known. But the history of Korea provides a plausible
explanation. After a period of Japanese colonization and the
Second World War, Korea was impoverished, but, despite the
Korean War, the Republic of Korea has undergone rapid
economic development for the last 60 years, documented in a
rapid increase in per capita GDP from US$ 67 in 1953 to US$
20,050 in 2007 . The increased height seen over this period
is further evidence of this economic development. Part of this
process of development has been the development of a mass
education system. As the questionnaire results show, the
proportion of high levels of education (high school or university
graduate) in Korean adults increased sharply from 16.2% to
96.9% over 40 years, while those with low education levels (no
education or elementary school graduate) decreased from
74.7% to 0.8% over the same period (Table 4).
The hypothesis that education has played a crucial role is
further supported by an Organization for Economic
Cooperation and Development (OECD) report concerning
change in education over fifty years (accessible at http://
report shows that rates of educational expansion have varied
greatly among countries over recent decades. Korea has made
higher education dramatically more accessible, and Korea has
been transformed from a country where only a minority of
students graduated from secondary school to one in which
virtually all students graduate and a high proportion go on to
higher education. Thus, Korea has moved from the 21st to the
first rank among 25 OECD countries.
Current sun exposure time in Korean adults was inversely
associated with the myopia in our study. Although higher light
intensity outdoors reduces the prevalence of myopia in children
through the inhibition of eye growth which is caused by the
stimulated release of dopamine from the retina or increased
focal depth of field by reducing image blur through pupil
constriction [21,26], this effect is unlikely to applicable to adults
as well, because eyeball elongation tends to decrease after the
cessation of body growth. The most likely explanation of this
result is that this reflects some continuity in behavior or
occupation. The adults who spent more time outside during
their childhood, and hence would have been protected from
myopia , are possibly more likely to spend more time
outside as adults, because of their lesser education and the
likelihood that they followed occupational paths characteristic
for those with less education. A similar inverse relationship
between current light exposure and the prevalence of myopia
has been reported in Norfolk Island, where those who reported
less current time outdoors were less myopic and had higher
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Cycloplegia Definition Myopia
Overall 40-49 50-59 60-69 70+
34.7 60.7 32.6 16.1 18.2
26.7 22.0 13.8 14.4 35.1
32.3 NA 31.7 31.1 40.0
41.8 67.8-70.3 42.4-49.6 20.8-22.1 13.5-24.6
38.7 48.9 26.4 28.5 36.8
30.7 31.0 19.6 16.4 33.6
28.0 33.3 23.8 20.3 26.9
levels of conjunctival UV autofluorescence, which is a possible
semi-cumulative measure of life-time UV exposure .
The major strength of the present study is the large number
of participants (21,356) and the study design using systemic
stratified, multistage, clustered random sampling methods. Our
study has several limitations. First, the refractive error was
measured without cycloplegia, which may lead to the
overestimation of the prevalence of myopia, but this is a
limitation that it shares with all but one of the major studies on
refractive errors in adults, including the US NHANES. Because
of this concern, we did not use the data to provide prevalence
estimates of refractive error for those aged 5 to 19 years (6,206
subjects). However, as shown by the comparisons to the
prevalence of myopia and distribution of refractive error in 19
year-old males in Seoul and Jeju Province measured by
cycloplegic autorefraction (Figure 2), there is clear continuity
between the results obtained with cycloplegia and those
without, suggesting that the patterns are close to those that
would be obtained if cycloplegia has been used, although some
over-estimation without cycloplegia is likely. This level of
agreement is not surprising because, while lack of cycloplegia
leads to an over-estimation of myopia, the biggest problems
with noncycloplegia refractions concern the under-estimation of
hyperopia and the resulting errors in estimation of mean SER
. Figure 3 directly compares the results from this study with
those from the US NHANES, which has used similar
noncycloplegic measurements of refraction. The transition from
a lower prevalence of myopia in the older age group in Korea
to a much higher prevalence in the youngest age group
compared is very clear. This differential pattern of change is
unlikely to be due to the lack of cyclopegia. Second limitation is
the cross-sectional design, which makes inferring causality
difficult. However, in this case we have documented secular
changes in education and sunlight exposures which, on the
basis of existing evidence on their effects on the development
of myopia, could explain the marked differences between age
groups. Third, sunlight exposure time was obtained as
categorical variable of under 5 hour or over 5 hours a day, not
as continuous variables of hours per day. This may limit the
evaluation of association of sunlight exposure time with
myopia. Finally, smoking variable is a complex variable with
many different aspects and just stating it by one yes or no
question may not evaluate the enough aspect of this variable
such as level of exposure, type of smoking, qualification of
smoking, and beginning period of smoking. Unfortunately,
these aspects were not fully included in the analysis due to
insufficient information about the smoking.
In conclusion, the present study provides the first
populationbased representative data on refractive errors in the Korean
adult population. The prevalence of myopia in 40-49 year old
Koreans (60.7%) was very much higher than in other countries,
whereas that in 60-69 year olds (16.1%) was comparable to
those in many others. This major change may be associated
with the rapid economic development of Korea, and its
transformation from a poorly educated country to one with
amongst the highest educational standards in the world
(OECD/PISA). Parallel social changes appear to have reduced
the amount of time people are exposed to sunlight outdoors,
and myopia in Korea is associated with higher educational level
and a lower proportion of those with longer sunlight exposure
times (over 5 h per day). In parallel with demonstrated secular
changes in these parameters, the prevalence of myopia has
increased dramatically in Korea.
Conceived and designed the experiments: ECK IGM DJ.
Performed the experiments: ECK IGM DJ. Analyzed the data:
IGM DJ. Contributed reagents/materials/analysis tools: IGM
SK. Wrote the manuscript: IGM HK DJ.
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