Myopia, lifestyle, and schooling in students of Chinese ethnicity in Singapore and Sydney.

Abstract

To compare the prevalence and risk factors for myopia in 6- and 7-year-old children of Chinese ethnicity in Sydney and Singapore.
Two cross-sectional samples of age- and ethnicity-matched primary school children participated: 124 from the Sydney Myopia Study and 628 from the Singapore Cohort Study on the Risk Factors for Myopia. Cycloplegic autorefraction was used to determine myopia prevalence (spherical equivalent < or = -0.5 diopter). Lifestyle activities were ascertained by questionnaire.
The prevalence of myopia in 6- and 7-year-old children of Chinese ethnicity was significantly lower in Sydney (3.3%) than in Singapore (29.1%) (P < .001). The prevalence of myopia in 1 or more parents was 68% in Sydney and 71% in Singapore. Children in Sydney read more books per week (P < .001) and did more total near-work activity (P = .002). Children in Sydney spent more time on outdoor activities (13.75 vs 3.05 hours per week; P < .001), which was the most significant factor associated with the differences in the prevalence of myopia between the 2 sites.
The lower prevalence of myopia in Sydney was associated with increased hours of outdoor activities. We hypothesize that another factor contributing to the differences in the prevalence of myopia may be the early educational pressures found in Singapore but not in Sydney.

Full text

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EPIDEMIOLOGY
Myopia, Lifestyle, and Schooling in Students
of Chinese Ethnicity in Singapore and Sydney
Kathryn A. Rose, PhD; Ian G. Morgan, BSc, PhD; Wayne Smith, BMed, MPH, PhD;
George Burlutsky, MStat; Paul Mitchell, PhD; Seang-Mei Saw, PhD
Objective: To compare the prevalence and risk factors
formyopiain6-and7-year-oldchildrenofChineseeth-
nicity in Sydney and Singapore.
Methods: Two cross-sectional samples of age- and eth-
nicity-matchedprimaryschoolchildrenparticipated:124
from the Sydney Myopia Study and 628 from the Sin-
gaporeCohortStudyontheRiskFactorsforMyopia.Cy-
cloplegic autorefraction was used to determine myopia
prevalence (spherical equivalent?−0.5 diopter). Life-
style activities were ascertained by questionnaire.
Results: The prevalence of myopia in 6- and 7-year-old
children of Chinese ethnicity was significantly lower in
Sydney(3.3%)thaninSingapore(29.1%)(P?.001).The
prevalence of myopia in 1 or more parents was 68% in
Sydney and 71% in Singapore. Children in Sydney read
morebooksperweek(P?.001)anddidmoretotalnear-
work activity (P=.002). Children in Sydney spent more
timeonoutdooractivities(13.75vs3.05hoursperweek;
P?.001), which was the most significant factor associ-
atedwiththedifferencesintheprevalenceofmyopiabe-
tween the 2 sites.
Conclusions: The lower prevalence of myopia in Syd-
ney was associated with increased hours of outdoor ac-
tivities. We hypothesize that another factor contribut-
ing to the differences in the prevalence of myopia may
betheearlyeducationalpressuresfoundinSingaporebut
not in Sydney.
Arch Ophthalmol. 2008;126(4):527-530
M
aprevalenceofmorethan20%intheearly
primary school years,1-3resulting in a
prevalence of myopia in young adults of
more than 80%1,3,4and an increasing pro-
portion (approximately 20%) of young
adults with high myopia.3
There has been considerable debate
about the factors that contribute to the
present epidemic of myopia in urban East
Asia.5-7Inthisarticle,wedocumentmarked
differences in the prevalence of myopia in
age-matched students of Chinese origin in
2 distinct physical, social, and educational
environments, Singapore and Sydney, and
analyzetheriskfactorexposuresthatcould
contribute to these differences.
YOPIA IS A COMMON RE-
fractive error associ-
ated with excessive
axialelongationofthe
eye. In East Asian cit-
ies,theonsetofmyopiaisnowearly,with
METHODS
STUDY POPULATION
Children of Chinese ethnicity living in Sydney
(n=124)weredrawnfromasurveyof1742year
1studentsselectedusingastratifiedrandomclus-
terdesign.8Thechild’sethnicitywasdefinedas
Chinese only if both parents self-identified this
ethnic origin. Data were also obtained for 628
children of Chinese ethnicity from 631 grade 1
students examined in 3 schools in Singapore.2
Inthisstudy,ethnicitywasdeterminedusingthe
father’sreportedethnicity,asdefinedbytheSin-
gapore Population Census 2000 (http://www
.singstat.gov.sg).Interethnicmarriageatthetime
of parental marriages was less than 10%; thus,
thedifferencesindefinitionofethnicityarenot
significant.
PROCEDURES
Refraction was measured using table-
mounted autokeratorefractors (model RK-F1
in Sydney and model RK-F5 in Singapore;
Canon, Tokyo, Japan) after cycloplegia, in-
ducedafterinstillationoflocalanestheticusing
cyclopentolate, 1% (3 drops in Singapore and
2 drops in Sydney with additional tropicam-
ide,1%9).InSydney,cycloplegiawasassessed
by pupil dilation greater than 6 mm and the
absence of pupil miosis in response to light or
an accommodative target. In Singapore, cy-
cloplegia was not formally assessed. Corneal
radius of curvature, anterior chamber depth,
and axial length were measured using an op-
tical biometer (IOLMaster; Carl Zeiss Med-
itecInc,Jena,Germany)inSydney.Autokera-
torefractionwasusedtomeasurecornealradius
of curvature in Singapore, whereas anterior
chamber depth and axial length were mea-
Author Affiliations: Discipline
of Applied Vision Sciences,
Faculty of Health Sciences,
University of Sydney, Lidcombe,
Australia (Dr Rose); ARC
[Australian Research Council]
Centre of Excellence in Vision
Science, Research School of
Biological Sciences, Australian
National University, Canberra
(Dr Morgan); Centre for
Clinical Epidemiology and
Biostatistics, University of
Newcastle, Callaghan, Australia
(Dr Smith); Centre for Vision
Research, Westmead Hospital,
Department of Ophthalmology,
University of Sydney, and the
Westmead Millennium
Institute, Westmead, Australia
(Mr Burlutsky and Dr Mitchell);
and Department of Community,
Occupational, and Family
Medicine, Yong Loo Lin School
of Medicine, National
University of Singapore,
Singapore (Dr Saw).
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sured using A-scan ultrasonography. Axial length and ante-
riorchamberdepthdimensionsareslightlygreaterwhenmea-
sured using the IOLMaster compared with ultrasonography.9
In both studies, parents completed questionnaires on their
child’s near-, middle-, and distance-vision activities. The Sin-
gapore questionnaire has been validated using 24-hour near-
work diaries,10and the Sydney data were similarly validated
against a 24-hour clock for weekdays and weekends. Parents
reportedethnicitybyself-identificationandrefractivestatusvia
questions on the use of glasses for distance vision.
These studies were approved by the University of Sydney
human ethics committee, the New South Wales Department
of Education and Training, the Catholic Education Office of
theArchdioceseofSydney,andtheethicscommitteeoftheSin-
gapore Eye Research Institute and adhered to the tenets of the
Declaration of Helsinki.
STATISTICAL ANALYSES
Analyses were conducted on the right eye only because refrac-
tive error was highly correlated between eyes in the Sydney
(r=0.95) and Singapore (r=0.92) samples. The data were en-
teredintoadatabaseprogram(MicrosoftAccess;MicrosoftCorp,
Redmond, Washington). All statistical analyses were per-
formedusingasoftwareprogram(SASV9.1;SASInstituteInc,
Cary, North Carolina). The ?2test was used to compare pro-
portions (prevalence) between the 2 cities. At test was used to
assessthesignificanceofthedifferencesbetweenthemeanval-
ues of the selected characteristics at the 2 sites. Stepwise mul-
tiplelinearregressionwasusedtoexaminetherelationshipbe-
tween refraction and baseline characteristics.
RESULTS
The prevalence of myopia (defined as a right eye spheri-
cal equivalent of at least –0.5 diopter [D]) was much
higher in children of Chinese ethnicity in Singapore
(29.1%)thaninsimilarlyagedchildreninSydney(3.3%)
(P?.001).Themeansphericalequivalentrefractionwas
considerably more myopic in Singapore than in Sydney
(−0.16 vs ?0.86 D; P?.001), which is reflected in the
rangeofsphericalequivalents(−6.70to?4.85Dvs−2.88
to?3.50D).Consistentwiththesedifferencesinrefrac-
tion,axiallengthsandanteriorchamberdepthsweresig-
nificantlygreaterinSingaporethaninSydney(Table 1).
One factor that could contribute to these large differ-
encesisparentalmyopia,whichhasbeenassociatedwith
a greater likelihood of myopia in children,11although
whetherthisrepresentstheeffectofsharedgenesorshared
environments is not clear. However, there was no dif-
ference in the proportion of children with 0, 1, or 2 my-
opicparentsinthe2cities(P=.84).IntheSydneysample,
32% of children had no parents with myopia, 43% had 1
myopic parent, and 25% had 2 myopic parents com-
pared with 29%, 43%, and 28%, respectively, in Sin-
gapore. To the extent that parental myopia is a surro-
gate measure of genetic differences, this suggests that
genetic differences related to myopia in the 2 popula-
tions are not significant.
Possible lifestyle factors that contribute to the differ-
ences are outlined in Table 2. The children of Chinese
origin living in Sydney read slightly more books; spent
more time reading, writing, or using computers outside
of school; and watched slightly less television than did
the Chinese children living in Singapore. They also had
fewerhoursperweekofcoaching(additionallessonsout-
side school hours) than did the children from Sin-
gapore. All these differences were statistically signifi-
cantbutweresmallinmagnitude.However,acumulative
measure of near-work activity was significantly differ-
ent between the 2 cities, with more total near-work ac-
tivitybeingperformedbythechildreninSydney.Thelarg-
est difference observed was that children of Chinese
ethnicity spent nearly 14 hours per week in outdoor ac-
tivitiesinSydneycomparedwithjustmorethan3hours
per week in Singapore (P?.001). In Sydney, near-work
activity was highly correlated with outdoor activity
(r=0.55); however, in Singapore it was not (r=0.08).
Linear regression of the pooled data, with spherical
equivalent refraction as the dependent variable, identi-
fied 4 significant independent variables: location, out-
door activities, parental myopia, and number of books
read. A model using location alone gave an R2of 0.146,
and the sequential addition of the other identified fac-
tors increased the R2only marginally to 0.148. Outdoor
activities on their own gave an R2of 0.080, suggesting
that all the individual factors were subsumed within lo-
cation and that the differences in outdoor activity ac-
Table 1. Distribution of Refractive Error and Ocular Biometry Values in the Right Eyes of Children of Chinese Origin Living
in Singapore and Sydney
SydneySingapore
P
Value
.60
?.001
?.001
?.001
?.001
?.001
?.001
?.001
Children, No.
(n=124) Mean (SD)a
53.2
3.3
6.41 (0.35)
0.86 (0.78)
22.60 (0.67)
3.27 (0.22)
7.87 (0.25)
2.87 (0.07)
Children, No.
(n=628)Mean (SD)a
50.3
29.1
7.16 (0.39)
−0.16 (1.43)
23.13 (0.90)
3.58 (0.27)
7.73 (0.25)
2.99 (0.10)
Female sex, %
Myopia (spherical equivalent of ?−0.5 D), %
Age, y
Spherical equivalent refraction, D
Axial length, mm
Anterior chamber depth, mm
Corneal radius of curvature, mm
Axial length to corneal radius ratio
124
124
123
124
124
123
628
628
613
613
626
611
Abbreviation: D, diopter.
aExcept where noted otherwise.
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counted for a significant part, but not all, of the location
effect.
COMMENT
These data demonstrate major differences in the preva-
lenceofmyopiainchildrenofChineseoriginaged6and
7 years growing up in Sydney and Singapore, with ma-
jorassociationswithlocation-specificengagementinout-
door activities. The ocular biometry profile in Sin-
gapore(longeraxiallength,greaterdepthoftheanterior
chamber, and a higher axial length to corneal radius ra-
tio) is consistent with the higher prevalence of myopia.
The strong association between increased time spent
outdoors and decreased myopia is consistent with other
studies11,12of such associations with sports and outdoor
activities. Data from the Sydney Myopia Study13suggest
thatthecriticalfactoristimespentoutdoorsratherthan
engagementinsportsbecauseindoorsportingactivityhas
noeffect,whereasoutdoorsportsandoutdoorleisureac-
tivities show the association. It is not clear whether the
effectoftimespentoutdoorsisaresultofthegreaterview-
ingdistancesorthebrighterlighttypicalofoutdoorday-
light hours. Brighter light could reduce the develop-
ment of myopia through pupil constriction, resulting in
less visual blur, or through stimulation of the release of
dopamine from the retina, which is known to act as an
eye growth inhibitor.14However, the differences in time
spent outdoors do not explain all the between-site
differences.
The Singaporean children are a mean of 10 months
olderthanthechildreninSydney.However,thisagedif-
ferencecannotaccountforthehigherprevalenceofmyo-
pia in Singapore, even given the high incidence rates in
Singapore.15The greater number of books read in Syd-
ney was surprising given the reported association be-
tween more books read per week and higher myopia in
Singapore in 6- and 7-year-olds,16but subsequent stud-
ies in Singapore suggest that this measure is not associ-
ated with later incident myopia.15
The 2 locations have a variety of intrinsic differ-
ences. For any difference to provide an explanation for
the rapid increase in the prevalence of myopia in Sin-
gapore during the past few decades,4,17-19it would have
had to emerge at an appropriate time and at a relatively
rapid rate. It would need to have a plausible biological
mechanism for affecting the development of myopia.
These criteria rule out some obvious differences, such
as those in climate.
The standard of living has changed during the past
decades in both locations, but more rapidly in Sin-
gapore. The reported current difference in gross domes-
tic product per capita between Sydney and Singapore
(2005 data: Australia, US $31900; and Singapore, US
$28100) is small,20and higher incomes are associated
with less myopia, whereas the reported relationship is
normally the reverse.21Population density is higher in
Singapore,20with patterns of housing that have changed
rapidly. People in Sydney predominantly live in single
houses, whereas Singaporeans predominantly live in
high-rise housing complexes. However, myopia in Sin-
gapore is associated with more spacious living condi-
tions,21possibly because of independent associations of
socioeconomic status with housing and education. A
role for diet in the etiology of myopia has also been pro-
posed.22Diet was not measured in the Sydney sample,
but migrant Chinese parents in Sydney retain a large
component of the traditional Chinese diet in their
Western environment, whereas in Singapore, traditional
diets are also retained but with an increasing popularity
of components of Western diets, suggesting that their
diets may not be markedly different.
One factor that is different and has changed rapidly is
education.23,24InSingapore,moststudentsareenrolledin
astructured3-yearpreschoolprogramwiththeaimofen-
suringthatchildrenarereadingwhentheystartschool.In
Sydney,mostchildrenhaveatleast1yearofpart-timepre-
school, which is largely concerned with social develop-
ment.Thisisfollowedbyenrollmentinafull-timekinder-
gartenyearbeforeyear1.Differencesineducationalintensity
at this early stage may have a considerable impact given
the very early appearance of myopia in Singapore.2
A link to education is plausible. Adult myopia is
strongly associated with years of schooling,17and myo-
piaduringtheschoolyearsisassociatedwithhigherschool
performance.25,26We,therefore,suggestthatanotherma-
jor factor that contributes to the high levels of myopia
in Singapore may be Singapore’s competitive, academi-
cally oriented schooling and, in particular, the empha-
sis on very early educational achievements. Associated
Table 2. Myopia Risk Factors in Children of Chinese Origin Living in Singapore and Sydney
Activity Outside School
Books read, No./wk
Reading and writing, h/wk
Computer use, including computer games, h/wk
Total near-work activity, h/wka
Coaching classes, h/wk
Television viewing, h/wk
Outdoor activities and sports, h/wk
Sydney
Singapore
P
Value
?.001
.03
.10
.002
.007
.07
?.001
Children, No.
(n=124)
119
109
108
106
118
113
102
Mean (SD)
4.44 (2.46)
20.81 (13.88)
4.65 (6.62)
29.93 (20.09)
1.21 (1.75)
11.32 (6.47)
13.75 (1.02)
Children, No.
(n=628)
628
611
625
608
622
627
586
Mean (SD)
2.39 (2.27)
17.76 (8.78)
3.55 (4.48)
23.54 (11.84)
1.74 (2.02)
12.65 (7.37)
3.05 (0.12)
aIncludes reading, writing, computer use, crafts, and playing musical instruments.
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increasesinacademicactivity,combinedwithdeficitsin
outdooractivity,maystartchildreninSingaporeonatra-
jectorytowardmyopiafromaveryearlyage.Overall,these
comparative results suggest that promotion of outdoor
activitiesmayhelpreducetheepidemicofmyopiainSin-
gapore and that reform of school structures also has the
potential to contribute to this goal.
Submitted for Publication: May 18, 2007; final revision
received September 14, 2007; accepted September 18,
2007.
Correspondence:KathrynA.Rose,PhD,DisciplineofAp-
plied Vision Sciences, Faculty of Health Sciences, Uni-
versity of Sydney (Cumberland Campus), PO Box 170,
Lidcombe, New South Wales, Australia 1825 (k.rose
@usyd.edu.au).
Author Contributions: Drs Rose and Saw had full access
to the data in the study and take responsibility for the in-
tegrity of the data and the accuracy of the data analysis.
Financial Disclosure: None reported.
Funding/Support: The Sydney Myopia Study was sup-
portedbytheAustralianNationalMedicalandHealthRe-
searchCouncilandtheVisionCo-operativeResearchCen-
tre.ThisstudywassupportedbygrantCOE561903from
the Australian Research Council (Dr Morgan).
Role of the Sponsors: The sponsors did not participate
in the design or conduct of the study; in the collection,
analysis, or interpretation of the data; or in the prepara-
tion, review, or approval of the manuscript.
AdditionalContributions:Wethanktheparentsandchil-
dren who participated in these studies and the princi-
pals and teachers who facilitated it. We also thank those
who assisted in data collection and entry.
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