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Does Education Explain Ethnic Differences in Myopia Prevalence? A Population-Based Study of Young Adult Males in Singapore

Wu, Hui-Min MBBS, MSc; Seet, Benjamin FRCS(Edin), MPH; Yap, Eric Peng-Huat MBBS, PhD; Saw, Seang-Mei MPH, PhD; Lim, Tock-Han MMed, FRCS(Edin); Chia, and Kee-Seng MMed, MD

Optometry and Vision Science: April 2001 - Volume 78 - Issue 4 - p 234-239
Original Article

Purpose. To study interethnic variation in myopia prevalence and severity in young adult males in Singapore and to determine whether these variations are related to differences in education level.

Methods. A population-based survey of refractive errors in a cohort of 15,095 military conscripts between July 1996 and June 1997 using noncycloplegic autorefraction and a standard questionnaire. Prevalence rates of myopia (<−0.5D) and severe myopia (<−6.0 D) were determined for Chinese, Malay, and Indian men; prevalence rate ratios were compared after adjusting for education level.

Results. Singapore has one of the highest prevalences of myopia (79.3%) and severe myopia (13.1%), with Chinese having higher rates (82.2%, 95% confidence interval 81.5, 82.9) compared with Indians (68.7%, 95% confidence interval 65.1, 67.1) and Malays (65.0%, 95% confidence interval 62.9, 67.1). Education was strongly associated with prevalence and severity of myopia. However, significant interethnic variation persisted after adjusting for education.

Conclusions. There is a high prevalence of myopia in Singapore. Although prevalence and severity of myopia were strongly associated with education, interethnic variation observed was not fully explained by differences in education level.

Defence Medical Research Institute, Singapore (H-MW, BS, EP-HY), Military Medicine Institute, Singapore Armed Forces, Singapore (BS), Department of Community, Occupation and Family Medicine, National University of Singapore (S-MS, K-SC), Department of Ophthalmology, Tan Tock Seng Hospital, Singapore (T-HL)

Hui-Min Wu

Defence Medical Research Institute

#02-04, Clinical Research Center

10 Medical Drive

Singapore 119206

e-mail: nmiv14@nus.edu.sg

Urban populations from the newly industrializing countries in East Asia have some of the highest prevalence rates of myopia worldwide. 1–7 There is high risk of developing myopia in Singapore, with a trend of increasing prevalence of the condition in young adults from 26% in the late 1970s, to 43% in the late 1980s and 66% in the mid-1990s. 7,8 A similar trend has been reported among Singapore school children. 9 There is also increasing severity of myopia, with data from the Singapore Armed Forces showing a rise in proportion of highly myopic recruits (>−8.00 D) from 1.7% in 1993 to 3.5% in 1997 (unpublished data, Medical Classification Center, 1998).

Variation in myopia prevalence and severity between different races and ethnic groups has been reported. 10–15 The National Health and Nutrition Examination Survey conducted in the United States in the 1970s identified racial differences in myopia prevalence, with higher rates in white persons compared with black persons. 14 Similar findings were reported in the Baltimore Eye Survey, which compared myopia prevalence between different races after adjusting for age, gender, and education. 15 Previous studies from Japan, Taiwan, and Hong Kong reported high myopia prevalence but looked primarily at subjects of Oriental descent, 1–6 whereas earlier studies from Singapore compared myopia in different ethnic groups, although these were based on visual acuity data and did not involve actual refraction. 7,8 A recent Singapore survey studied refractive errors in an older adult Chinese population. 16

There is strong association between myopia and education, 11,12,17,18 and it is hypothesized that the rapid rise in myopia prevalence in Singapore is related to the increase in education demands. It has been postulated that higher education has exposed successive cohorts of Singaporeans to increased duration and intensity of near-work activity, which contribute to myopia development. 17,19,20 Because different ethnic groups within the population may have different levels of education, this becomes an important variable in the analysis of interethnic variation in myopia trends.

To study interethnic variation in myopia prevalence and severity in a high-risk age group within an East Asian population, we report our findings in Chinese, Indians, and Malays from an entire age-cohort of young adult males in Singapore. We further determined whether any observed variation in myopia rates are related to differences in education level.

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METHODS

The Singapore Armed Forces Myopia Study was a population-based survey of refractive errors in an entire age cohort of military conscripts presenting for medical screening and categorization between July 1996 and June 1997. In Singapore, all male citizens and permanent residents are required to enlist for military national service at 17 years of age and undergo prior medical examination at the Medical Classification Center, Singapore Armed Forces. A total of 15,095 conscripts were examined over this period, representing close to the entire male population in this age cohort, the exceptions being individuals with severe disabilities (e.g., legal blindness and chronic psychiatric disease) and individuals absent from the country throughout this period (<5%).

As part of a standardized ophthalmic examination, all subjects underwent noncycloplegic autorefraction of both eyes using a nonaccommodative picture target with standard background illumination on the Nikon NRK-8000 (Nikon, Japan) with standard room lighting conditions. The mean of three readings was obtained. A short interview was conducted at the eye clinic by a trained technician to obtain basic demographic information as well as a medical and refraction history. Ethnicity and highest level of education attained were self-reported. All subjects with previous history of ocular pathology, except refractive errors, were examined using slitlamp biomicroscopy and indirect ophthalmoscopy following a standard protocol. Patients with pathology affecting clarity of the ocular media or with previous lens or refractive surgery were excluded from the study.

Validation of noncycloplegic autorefraction was performed in our study population using a randomized stratified sample of 670 subjects with spherical equivalent ranging from +2.0 to −16.0 D. These subjects received one drop of 1% cyclopentolate (Cyclogyl) and one drop of 1% tropicamide (Mydracil) three times at 5-min intervals and had autorefraction repeated after 20 min. The intraclass correlation coefficient was 0.99, 0.94, and 0.99 for spherical power, cylinder power, and spherical equivalent, respectively, which established noncycloplegic autorefraction as a valid tool for measuring refractive errors in our study population.

We elected to use only right eye data to study prevalence and distribution of refractive errors in our population. We defined myopia as >−0.5 D spherical equivalent and severe myopia as >−6.0 D spherical equivalent. Hyperopia was defined as >+0.5 D spherical equivalent, astigmatism as >0.5 D of cylinder, and anisometropia as >1.0 D spherical equivalent difference between the right and left eyes. Subjects were stratified according to the highest level of education attained. We have categorized primary education as subjects who have no formal education, only primary school education (1 to 7 years) or who have failed to complete secondary school education (7 to 9 years); secondary education as subjects who have completed the Cambridge General Certificate in Education “Ordinary” and “Normal” level examinations (10 to 12 years); and tertiary education as subjects who have completed the Cambridge General Certificate in Education “Advanced” level examinations, polytechnic, or university education (≥12 years of schooling).

All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS version 9.01, Cary, NC). Prevalence rate ratios for ethnic groups were determined using Malay ethnicity as the reference group and are presented at the 95% confidence interval (CI). Multivariate analysis (modified Cox’s proportional hazards model) was carried out to calculate the prevalence rate ratios for ethnicity adjusted by education. 21 Age was not included in the model because it is not a confounder. A χ2 test was applied to test the statistical difference between categorical variables in ≥2 groups. This study was approved by the ethics committee of the Defense Medical Research Institute and complied with the Singapore guidelines for good clinical practice.

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RESULTS

A total of 15,095 subjects were examined, with 12 eyes from nine subjects excluded from analysis because of congenital cataracts (six eyes), previous refractive surgery (four eyes), and ocular trauma (two eyes). The demographic profile of the study population is summarized in Table 1. All subjects were male, with a median age of 19 years (mean 19.5 ± 1.4, range 16 to 25 years) and with no significant age differences between ethnic groups. Of the subjects, 82% were Chinese, 12.8% were Malays, 4.3% were Indians, and <1.0% were from other ethnic groups. This is comparable to the national distribution for different ethnic groups in 1995 (all ages), which was 77.3% Chinese, 14.1% Malays, 7.3% Indians, and 1.3% other ethnic groups. 22 Data from “other ethnic groups” have been excluded from further analysis because they comprise a small and heterogeneous group of the Singapore population. Among the 14,247 subjects for whom education status was obtained (missing data for 839), most (63.6%) of the study population received a tertiary level of education. More Chinese received tertiary education compared with Indians or Malays.

Table 1

Table 1

The distribution of refractive errors in spherical equivalent by ethnic group is shown in Fig. 1. There were distinct differences in refractive error distribution between different races, with a greater shift toward myopia and severe myopia in the Chinese population compared with Indians and Malays. The median refractive error was −2.75 D, −1.13 D, and −0.88 D for the Chinese, Indians, and Malays, respectively. The unadjusted prevalence rate of different refractive errors by ethnic group is shown in Table 2. The overall prevalence of myopia was 79.3% (95%CI 78.6, 80.0), whereas the prevalence of severe myopia was 13.1% (95%CI 12.6, 13.6). Prevalence of myopia and severe myopia were highest in Chinese, followed by Indians and Malays (χ2 = 13.25, p = 0.01). Similar interethnic differences in the prevalence rates of astigmatism and anisometropia were observed, with higher rates observed among the Chinese. Overall prevalence of hyperopia was low, with no significant difference noted between the three ethnic groups.

FIGURE 1

FIGURE 1

Table 2

Table 2

The unadjusted prevalence rate of refractive errors in different education groups is shown in Table 3. Education was strongly associated with the prevalence of myopia (χ2 = 20.79, p < 0.0001). Myopia prevalence increased from 64.1% in those with primary education to 85.0% in those with tertiary education. Severe myopia was almost five times more prevalent in subjects with tertiary compared with primary education. The highest rates of astigmatism and anisometropia and the lowest rate of hyperopia were observed among those with tertiary education.

Table 3

Table 3

The relationship between the prevalence of myopia and ethnicity for different education levels is illustrated in Fig. 2, and the adjusted prevalence rate ratios between different ethnic groups is summarized in Table 4. The Chinese are consistently more myopic than Indians or Malays within each education group. A similar finding is noted for severe myopia. After adjusting for education, ethnic differences in myopia outcome persisted. The adjusted prevalence rate for myopia remained highest in Chinese, whereas no significant difference was noted between Indians and Malays. The adjusted prevalence rate ratio for severe myopia was about 1.5 times greater between the Chinese and other ethnic groups.

FIGURE 2

FIGURE 2

Table 4

Table 4

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DISCUSSION

Our data provide the first population-based comparison of myopia between different ethnic groups in a high-risk East Asian country. This confirms that the prevalence of myopia in young adults in Singapore is one of the highest worldwide (79.3%), supporting findings from earlier studies that estimated myopia prevalence in this population using visual acuity data. 7,8 Prevalence of myopia was highest in the Chinese ethnic group (82.3%), with rates similar to those reported in other Oriental populations from Taiwan and Hong Kong. 1–4 An unexpected finding was the high prevalence of myopia in Indians (68.7%) and Malays (65.0%), which, to our knowledge, represented some of the highest rates of myopia reported in non-Oriental populations. Earlier surveys of young adults from urban parts of India found a myopia prevalence of only 16.6% to 32.3% in the 1980s 23,24 and 19.4% in 1999, 25 whereas a study conducted in Singapore in the early 1990s estimated a myopia prevalence of 30% in Indians and 24% in Malays. 7

Our finding of high prevalence of myopia in all three ethnic groups is of national concern. Our study population comprised almost all Singapore males in an entire age cohort who had been exposed to the same urban environment. Ninety percent of Singaporeans live in high-rise buildings, and 85% live in public housing apartments. 22 The high prevalence of myopia in our population, along with Hong Kong, Taiwan, and Japan, could be attributed to the urbanization of the areas. All school children in Singapore, regardless of race, are exposed to a similar competitive school system from a young age. The exposure to a common environmental factor such as reading, stress, or personality may contribute to high myopia prevalence rates for all three races in Singapore.

We have shown a strong association between education and the prevalence of myopia (Table 3), which is well established from other studies. 11,12,17,18 We have also demonstrated an association between education and the severity of myopia, with an almost five times higher prevalence of severe myopia (>−6.0 D) in subjects with tertiary education compared with primary education. This education–myopia association remains for three major ethnic groups in the country. Educational level is related to levels of near work such as reading, writing, and computer work. It has been documented that more educated people who have had greater exposure to reading and other near work related to the process of education have a greater prevalence and severity of myopia than people less educated. 11 However, the association with education does not in itself prove that studying and reading cause myopia. There may be a common factor that influences both myopia and academic ability. For example, it has been showed that myopes in general achieve higher intelligence test scores than nonmyopes, and people with higher IQ scores are more likely to achieve higher educational levels. 26

Ethnicity is important in the etiology of disease because it is one of the expressions of genetic constitution, and certain ethnic groups show strong susceptibility to certain diseases. 27 Singapore has a multiracial population residing in the relatively homogenous environment and offers an excellent opportunity for risk factor studies on racial difference of myopia. Cultural difference among different races in Singapore, such as reading and writing habits and education attainment may explain the interethnic difference in myopia. However, differences in the prevalence of severe myopia continue to persist between the Chinese and other ethnic groups after adjusting for education (Table 4). This would suggest that interethnic variation in severe myopia outcome could be attributed to genetic factors or to differences in exposure to environmental variables. This, in turn, indicates that genetic (racial) constitution, environmental factors (including culture), or a combination of both plays an important role in the myopia development.

Another concern is the high proportion of subjects with severe myopia. In our young adult population, 13.1% (95%CI 12.6, 13.6) had myopia >−6.0 D. In individuals who received tertiary education, this proportion increased to 17.4% (95%CI 16.6,18.2). With an aging population and rising education levels in successive generations of Singaporeans, it is anticipated that the number and proportion of severely myopic individuals will increase. Although ocular morbidity is currently uncommon in our young population, severe myopia is known to be associated with important complications like posterior vitreous detachment, macular atrophy, peripheral retinal degeneration, retinal breaks, and retinal detachment 28–32 and has been found to be associated with problems like decreased contrast sensitivity and impaired dark adaptation function. 33,34 There are insufficient data to predict the future impact of myopia on ocular morbidity in Singapore, but it is reasonable to hypothesize that this would become substantial when these younger cohorts reach middle and elderly ages, with myopia-related complications becoming an important cause of visual disability.

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CONCLUSIONS

The prevalence of myopia is extremely high in Singapore. Although ethnicity remains an important risk factor, our findings show a high prevalence of myopia in all three major ethnic groups in the country. This suggests an underlying environmental basis such as education, reading, stress, and urbanization for myopia development in Singapore. Ethnic differences in the prevalence of severe myopia persisted after adjusting for education, which implied that this outcome could not be fully explained by differences in education “exposure.” It could be attributed to differences in genetic predisposition to the condition or to the exposure to other environmental risk factors.

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ACKNOWLEDGMENTS

The Singapore Armed Forces Myopia Study was supported by a grant from the Defense Medical Research Institute, Singapore. The authors thank Cho-Eing Tay and Alan Chua for their help with data collection and management, Siew-Pang Chan for his assistance with statistical analysis, and Tien Yin Wong for his valuable comments.

Received October 19, 2000; revision received January 16, 2001.

Hui-Min Wu

Defence Medical Research Institute

#02-04, Clinical Research Center

10 Medical Drive

Singapore 119206

e-mail: nmiv14@nus.edu.sg

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Keywords:

myopia,; prevalence,; education,; ethnicity,; Singapore

© 2001 American Academy of Optometry