The contributions of genetic and environmental factors towards the perceived “epidemic” of myopia in Asia need further evaluation.¹ In Asia, the prevalence of myopia is highest in urban Chinese populations such as Hong Kong, Taiwan, Singapore and southern China, and lowest in non-Chinese rural populations such as Nepal and India.^2–7 The differences in myopia rates may be attributed to variations in genetic susceptibility or environmental lifestyles in subpopulations. It has been purported that near work activity and a competitive education system may be the primary environmental factors related to myopia, whereas the genetic susceptibility to myopia may be polygenic in nature.^8,9

Multiethnic populations allow the evaluation of genetic effects and inter-racial differences in culture and lifestyle. Comparative studies across different populations with similar genetic pools may provide insights into the importance of environmental influences in the development of myopia. Singapore and Malaysia are neighbouring countries, separated by a narrow waterway, and are examples of multiethnic societies in East Asia. The three major ethnic groups are the same in both countries: Chinese (77% in Singapore and 26% in Malaysia), Malays (14% in Singapore and 65% in Malaysia) and Indians (8% in Singapore and 8% in Malaysia), and other minorities (1% in Singapore and 1% in Malaysia).^10,11 Chinese in Singapore and Malaysia migrated from the same localities in south China (primarily Fujian and Guangdong Provinces), and Indians in Singapore and Malaysia migrated from the same parts of India, primarily from south India and Sri Lanka. Most Chinese and Indian families have lived for decades (primarily second, third or fourth generation), and the Malays are native to Singapore and Malaysia.

We aim to evaluate differences in the prevalences of refractive errors, including myopia, in 7–9-year-old children of similar ethnicity—Malays, Chinese and Indians—from the Refractive Error Study in Children (RESC) and the Singapore COhort study of the Risk factors for Myopia (SCORM) conducted in two neighbouring countries, Malaysia and Singapore.^12–16

METHODS

Singapore population

Children from grades 1 to 3 from the Northern and Western schools and grades 1 and 2 in the Eastern school were invited to participate. Ninety four children with serious medical or ocular disorders were excluded. The participation rate was 71.8% (693/965) in the Northern school, 80.0% (956/1195) in the Western school and 49.1% (313/638) in the Eastern school. The parents completed a baseline questionnaire, and the completed educational levels of fathers and mothers were assessed. Written informed consent was obtained from the parents.

Children were examined on the school premises in 1999 in the Northern and Eastern schools, and in 2001 in the Western school by a team of ophthalmologists and optometrists who were blinded to questionnaire data. After distance logarithm of the minimum angle of resolution visual acuity measurements, cycloplegia was induced with three drops of 1% cyclopentolate solution administered at 5-min intervals. At least 30 min after the last drop, refraction was obtained with one of two autokeratorefractometers (RK5, Canon, Tochigiken, Japan). A total of 1962 children were refracted: 851 (43.4%) were 7 years of age, 630 (32.1%) were 8 years of age and 481 (24.5%) were 9 years of age. There were 992 (50.6%) boys and 970 (49.4%) girls. Ethnicity was distributed as follows: in the Northern school, there were 415 (59.9%) Chinese, 227 (32.8%) Malays, 42 (6.1%) Indians and 9 (1.3%) of other ethnicity; in the Western school, there were 743 (77.7%) Chinese, 121 (12.7%) Malays, 83 (8.7%) Indians and 9 (0.9%) of other ethnicity; and in the Eastern school, there were 309 (98.7%) Chinese, 1 (0.3%) Indian and 3 (1.0%) of other ethnicity. Further details of the SCORM protocol have been described elsewhere, along with findings from the baseline examinations.^13–16

Malaysia population

The Malaysian study population was obtained by random cluster sampling of children aged 7–15 years in Gombak District in the metropolitan Kuala Lumpur area. Clusters were defined geographically, and eligible children were enumerated by name, sex and age using house-to-house visits within randomly selected clusters. All eligible children recruited attended primary school. The ethnicity of the father and years of schooling for each parent were recorded. Years of schooling was grouped as no formal or incomplete primary education (<6 years of schooling), completed primary education (6–9 years of schooling), completed secondary education (10–11 years of schooling) or completed tertiary education (⩾12 years of schooling). Written consent for each child was obtained from a parent or guardian.

Eye examinations were conducted in 140 schools by two clinical teams, mostly between March and July 2003. After distance logarithm of the minimum angle of resolution visual acuity measurements, cycloplegia was induced with two drops of 1% cyclopentolate administered 5 min apart by ophthalmic assistants, with a third drop after 20 min, if necessary. Cycloplegia and pupil dilation were evaluated after an additional 15 min—pupillary dilation of ⩾6 mm with absence of light reflex was considered complete cycloplegia. Refraction was carried out by an optometrist with a handheld autorefractor (Retinomax K-Plus; Nikon, Tokyo, Japan).

Of the 2104 children between the ages of 7 and 9 years, 1781 (84.6%) were examined and 1752 (83.3%) had autorefraction measurements after successful cycloplegic dilation: 581 (33.2%) were 7 years old, 601 (34.3%) were 8 years old and 570 (32.5%) were 9 years old. There were 924 (52.7%) boys and 828 (47.3%) girls. The ethnic composition was 1245 (71.1%) Malays, 285 (16.3%) Chinese, 152 (8.7%) Indians and 70 (4.0%) of other ethnicity.

The complete RESC protocol has been described elsewhere, as have further details regarding the specific sampling and examination methods used in Gombak District.^12,17 Examination participation rates in both Malaysia and Singapore were reasonably similar across age groups and sex. In Malaysia, the participation rate was highest in the other ethnicity category (92.1%), followed by Malays (84.1%), Chinese (81.0%) and Indians (77.6%). In Singapore, the participation rate was highest in Indians (82.1%), followed by Malays (78.9%), others (77.1%) and Chinese (67.4%).

The SCORM study was approved by the Institutional Review Board of the Singapore Eye Research Institute. The RESC study protocol was approved by the World Health Organization Secretariat Committee on Research Involving Human Subjects, Geneva, and the Ethical Committee of the Standing Committee for Medical Research, Malaysia Ministry of Health.

Definitions and data analysis

Definitions of refraction from the RESC studies were used. Myopia was defined as spherical equivalent refractive error of at least −0.50 diopter (D) and hyperopia as ⩾+2.00 D. Astigmatism was defined as cylinder of at least 0.75 D. Clustering effects associated with the cluster sampling design in Malaysia were taken into account in all statistical analyses. Statistical analyses were carried out using Stata Statistical Software, Release 8.0 svy commands for analysing complex survey design data with clusters as primary sampling units.¹⁸ The primary sampling unit was each geographical cluster in Malaysia, and the Singapore data were considered as one cluster. Differences in the prevalences of myopia, hyperopia and astigmatism between Malaysia and Singapore were considered significant if the 95% confidence intervals of the differences in the prevalences did not cross zero and p values were <0.05. The data were pooled to evaluate the effect of living in Singapore versus living in Malaysia on myopia, astigmatism and hyperopia in each ethnic group. Age, sex, education of the father or myopia-adjusted odds ratios (ORs) were presented.¹⁸

RESULTS

The Singapore population had a greater proportion of Chinese, 7-year olds and fathers with tertiary educational level than the Malaysian population (table 1). Similarly, the Singapore site (10.7%) had a greater proportion of mothers with tertiary education than the Malaysian site (5.8%; p<0.001). The mean (standard deviation (SD)) spherical equivalent refractive errors of the right eye in Malaysian Malay (+0.65 D (0.82)), Chinese (−0.14 D (1.74)) and Indian children (+0.57 D (1.10)) indicated less myopia when compared with those of Singapore Malay (+0.15 D (1.42); p<0.001), Chinese (−0.52 D (1.69); p<0.001) and Indian (−0.22 D (1.74); p<0.001) children (fig 1). Similarly, the mean (SD) refractive errors of both Malaysian boys and girls indicated less myopia compared with Singapore boys (+0.50 D (1.03) v −0.49 D (1.76); p<0.001) and girls (+0.51 D (1.17) v −0.25 D (1.56); p<0.001). The mean (SD) of refractive errors indicated less myopia in Malaysian children compared with Singapore children for 7 year olds (+0.64 D (0.92) v −0.03 D (1.38); p<0.001), 8 year olds (+0.51 D (0.96) v −0.29 D (1.57); p<0.001) and 9 year olds (+0.35 D (1.35) v −1.08 D (2.01); p<0.001).

Overall, the prevalence of myopia was higher in Singapore (36.3%) than in Malaysia (13.4%; 95% CI of the difference in rates = 20.3 to 25.4; p<0.001; table 2). The prevalence of high myopia (spherical equivalent in the worse eye of at least −6 D) and moderate myopia (spherical equivalent in the worse eye <−6 D but at least −2 D) were 1.4% (95% CI 1.0 to 2.1) and 15.0% (95% CI 13.4 to 16.6), respectively, in Singapore, versus 0.5% (95% CI 0.1 to 0.8; p = 0.004) and 3.0% (95% CI 1.8 to 4.3; p<0.001), respectively, in Malaysia. Myopia prevalences were significantly higher in Singapore than in Malaysia within specific strata defined by ethnicity alone (Chinese, Malays, Indians), age alone, sex alone, paternal education alone and maternal education alone.

The mean (SD) of astigmatic cylinder in the right eye for Malays in Malaysia versus Singapore was (−0.34 D (0.56)) v (−0.83 D (0.88); p<0.001), that for Chinese (−0.51 D (0.68) v −0.71 D (0.67); p<0.001) and that for Indians (−0.46 D (0.72) v −0.94 D (1.03); p<0.001).

Overall, the prevalence of astigmatism was higher in Singapore (42.6%) than in Malaysia (22.2%; 95% CI 17.3 to 23.5; p<0.001; table 3). The rates of astigmatism were higher in Singapore than in Malaysia within specific strata defined by ethnicity, age, sex, paternal and maternal educational levels.

The overall prevalence of hyperopia was lower in Singapore children (1.7%) than in Malaysian children (2.9%; 95% CI −2.1 to −0.2; p = 0.005; table 4). The hyperopia rates were lower in Singapore than in Malaysia within the strata for 7-year-old children only (p<0.001) and the strata of males only (p<0.001), but similar within other strata defined by ethnicity, age, sex, paternal and maternal education levels.

The multivariate analyses show that the rates of myopia are higher in Singapore than in Malaysia for Malays only, Chinese only and Indians only (table 5). Singapore Malays and Indians had significantly different astigmatism rates compared with Malaysian Malays (p<0.001) and Indians (p = 0.011) after adjusting for other factors including myopia. After adjusting for mother’s education instead of father’s education, the multivariate ORs for myopia in Singapore versus Malaysia for Malays only, Chinese only and Indians only were 3.57, 1.58 and 3.82, respectively. The multivariate ORs for astigmatism after controlling for mother’s education instead of father’s education for Malays only, Chinese only and Indians only were 3.30, 1.33 and 1.82, respectively.

DISCUSSION

The ethnicity-specific prevalences of myopia are higher in Singapore than in Malaysia. Malays are native to both Malaysia and Singapore, the Chinese to both countries migrated from the same parts of southern China, and the Indians to both countries migrated from similar localities in India (southern India) and Sri Lanka several decades ago. Thus, the intercountry ethnicity-specific differences in myopia prevalences are unlikely to be due to genetic dissimilarities, and may be primarily environmental in nature. For example, most children in Singapore attend preschool (kindergarten or a childcare centre), and the syllabus maybe more structured and vigorous, with a greater use of information technology compared with the Malaysian preschool system, although the evidence is scarce and primarily anecdotal.¹⁹ Differences in urban population density may also be relevant. Singapore is a small, urban city centre (population density of 6026/km²) compared with the Gombak District, with a population density of 851/km², based on the 2000 census.¹¹ In Singapore, 82% of the people live in housing development board apartments, whereas most Malaysians live in private houses.¹⁰ The per capita gross domestic product of Singapore is $24 040 compared with $9120 in Malaysia; hence, people with certain characteristics may tend to migrate to a relatively more prosperous Singapore.

The environmental hypothesis is also supported by comparisons of myopia prevalences among 7–9-year-old Chinese children in the two mainland China RESC studies: 18.4% in urban Guangzhou in southern China and 8.7% in semirural Shunyi District in northern China.^5,20 Prevalences of myopia in the RESC studies of 7–9-year-old Indian children in both urban and rural India were relatively low: 4.6% and 3.9%, respectively.^7,21

Astigmatism is more common in Singapore Malays compared with Malaysian Malays, even after adjusting for age, sex, completed education level of fathers and myopia status. Evidence gathered from the RESC studies suggest that country-specific differences in astigmatism are not accounted for solely by differences in the prevalence of myopia. Other factors must come into play, although little is known about specific environmental risk factors for astigmatism. The prevalence of astigmatism in 7–9-year-old children in the RESC studies is as follows: Guangzhou, China (41.6%), Shunyi District, China (13.9%), urban India (14.6%) and rural India (8.4%).^5,7,20,21

The prevalences of hyperopia are low in both Malaysia (2.9%) and Singapore (1.7%). Comparatively, the prevalence of hyperopia in 7–9-year olds is higher in Guangzhou, China (5.1%), Shunyi District, China (4.2%) and urban India (8.0%).^5,20,21 The prevalence of hyperopia is lowest in rural India (0.3%).⁷

Two features were particularly advantageous in this direct comparative evaluation of refractive errors from two separate studies in Malaysia and Singapore: similar school-going populations and essentially identical measurement methods. Firstly, the multi-ethnic similarity between neighbouring Malaysia and Singapore (Malays, Chinese and Indians), each with a common genetic origin, allowed us to better understand the relative influences of genes and environment. Secondly, the use of autorefraction for measurement of refractive error under cycloplegia induced with 1% cyclopentolate in both studies reduced the possibility of differences introduced by systematic measurement bias. Although a handheld autorefractor was used for measurement in Malaysia and a table-mounted autorefractor in Singapore, the spherical equivalent readings using the handheld autorefractor were more negative compared with those using the table-mounted autorefractor.^22,23 In our study, because the handheld autorefractor readings in Malaysia may tend to deviate towards more negative readings, measurement biases may contribute to smaller perceived differences in the prevalences of myopia across countries.

It is recognised, however, that our comparisons may not necessarily apply to all children in Singapore or Malaysia. The three schools in Singapore may not be entirely representative of Singapore school children, nor is there evidence that children in Gombak District are representative of Malaysian children. Selection bias may occur because the response rate in the Eastern school (among the top 20 schools) in Singapore was only 49.1%: the overall rates of myopia in the Singapore sample could be underestimated. Perhaps parents of myopic children in Singapore may perceive that their child will not benefit from the study and may not enrol their children. The multivariate regression models show that the differences in myopia rates in Malaysia and Singapore cannot be completely explained by differences in parental educational level, and other environmental factors may contribute to the observed differences. The lack of near work activity, preschool activity, height or parental myopia from both countries may preclude definitive conclusions about the nature of country-specific differences. Future studies could investigate specific risk factors for differences in myopia rates across countries.

Nonetheless, we conclude that the prevalences of myopia in 7–9-year-old children are higher in Singapore Chinese than in Malaysian Chinese, higher in Singapore Indians than in Malaysian Indians and higher in Singapore Malays than in Malaysian Malays.