Purpose
To assess the influence of factors related to migration and acculturation on myopia in migrant Indians in Singapore.
Design
Population-based cross-sectional study.
Methods
A total of 3400 Singaporean Indians (75.6% response rate) aged over 40 years participated in this study. Information regarding country of birth, migration age, and language of interview were collected from interviews. Indians born outside of Singapore were defined as “first-generation” immigrants, while Indians born in Singapore were defined as “second-generation (or higher)” immigrants. Refraction was determined by autorefraction and refined by subjective refraction. Ocular biometry including axial length (AL), anterior chamber depth (ACD), and corneal radius (CR) were measured by partial coherence interferometry. Myopia and high myopia were defined as spherical equivalents (SE) of less than −0.5 diopter (D) for myopia, and < −5 D for high myopia, respectively.
Results
The prevalence of myopia (30.2% vs 23.4 %) and high myopia (4.8% vs 2.5%) were higher in second-generation immigrants compared with first-generation immigrants. Second-generation immigrants had longer AL (23.50 mm vs 23.37 mm, P = .004) than first-generation immigrants after multivariate adjustment. The excess prevalence of myopia was reduced by 37.5% but remained statistically significant ( P = .02) after further controlling for educational level. Among first-generation immigrants, those migrating to Singapore before the age of 21 had significantly higher prevalence of myopia (odds ratio [OR]: 1.85; 95% confidence interval [CI]: 1.32, 2.59) and longer AL (regression coefficient: 0.27; 95% CI: 0.11, 0.43) than those migrating after 21 years of age. Also, first-generation immigrants interviewed in English had higher prevalence of myopia (OR: 1.46; 95% CI: 1.00, 2.17) than their non-English-interviewed counterparts.
Conclusions
The prevalence of myopia among second-generation (or higher) Indian immigrants in Singapore is higher than first-generation immigrants. Country-specific environmental factors may be important for the increasing prevalence of myopia in Asia.
It is well known that both genetic and environmental factors contribute to the etiology of myopia. Because the prevalence of myopia has increased significantly in many urban Asian cities, it has been suggested that this reflects major shifts in environmental factors such as increasing education pressure and urbanization. Migrant studies may provide further clues to the role of environmental effects on myopia. In migrant studies, people moving from one country to another are compared with people born in the new country of the same genetic heritage and thus help to tease the effects of environmental exposures from genetics. Such information is also important from a public health perspective, considering that there are more than 200 million people traveling internationally and another 750 million people migrating within their own country around the world.
Few myopia migrant studies have been reported previously. Studies on the Inuit populations showed that the prevalence of myopia increased among generations as people moved into new settlements. The Los Angeles Latino Eye Study (LALES) reported that US-born Latino immigrants had higher prevalence of myopia than those born outside the United States (22.66% vs 13.99%). In addition to the effect of migration, the degree to which people acculturate to the main culture may also influence the prevalence of myopia. The LALES found that higher acculturation level as measured by a 9-item questionnaire increased the risk of myopia.
To the best of our knowledge, there have been few migration studies on myopia in urbanized Asian countries. Singapore is a highly urbanized city-state located in southeast Asia, consisting of immigrants of Chinese, Malaysian, and Indian ancestries. Indians account for about 9.2% of the whole Singapore population. Singaporean Indians who originated from the Indian subcontinent including India, Pakistan, Bangladesh, and Sri Lanka migrated to Singapore, mostly in the early part of the 20th century. This population structure provides us a unique opportunity to explore the variation of myopia prevalence between different generations of immigrants.
In this report, we compared the prevalence of myopia and ocular biometric parameters between “second-generation (or higher)” and “first-generation” Indian immigrants living in Singapore. We also assessed the influence of factors related to migration and acculturation on myopia.
Methods
Study Cohorts
The Singapore Indian Eye Study is a population-based, cross-sectional study of 3400 Indian adults aged over 40 years living in Singapore. The detailed study protocol has been described elsewhere. Briefly, an age-stratified random sampling strategy was conducted to select 6350 names of Indian ethnicity in southwest Singapore. Of these, 4497 individuals were deemed eligible to participate, and 3400 participants took part in the study, giving a 75.6% response rate. In general, participants on average were younger than nonparticipants ( P < .001), but there was no sex difference ( P = .28).
Definition of Immigrant Status
Participants were categorized as 2 cohorts based on the country of birth: Singaporean Indian residents born outside of Singapore were defined as “first-generation” immigrants, while Singaporean Indian residents born in Singapore were defined as “second-generation (or higher)” immigrants. Among the 3400 Indian participants, 2024 (59.5%) were born in Singapore, 813 (23.9%) were born in India, 495 (14.6%) were born in Malaysia, and the other 68 (2.0%) were born in other southeast Asian countries such as Pakistan, Bangladesh, Brunei, and Sri Lanka; thus, 1376 (40.5%) were classified as first-generation immigrants and 2024 (59.5%) were classified as second-generation (or higher) immigrants.
Refraction and Ocular Biometry Assessment
Noncycloplegic autorefraction was performed using an autorefractor (Canon RK-5 Autorefractor Keratometer; Canon Inc, Tokyo, Japan). Refraction was then subjectively refined by study optometrists until the best visual acuity was obtained. These subjective refraction results were used in analysis. If the subjective refraction was not available, results of autorefraction were used instead. Myopia and high myopia were defined as spherical equivalent (SE) of less than −0.5 diopter (D) for myopia, and < −5 D for high myopia, respectively.
Ocular biometric parameters including axial length (AL), anterior chamber depth (ACD), and corneal radius of curvature (CR) were measured by noncontact partial coherence interferometry (IOL Master V3.01, Carl Zeiss Meditec AG, Jena, Germany).
Measurement and Definitions of Risk Factors
All participants underwent a detailed interview using standardized questionnaires. Information on country of birth, migration age to Singapore, socioeconomic status (eg, education, income, house type), and lifestyle risk factors (smoking and time spent reading per day) were collected. The questionnaires were administered in 3 languages, including English, Tamil, and Malay, based on the participant’s preference. English questionnaires were translated into the other 2 languages using a “forward-backward” translation procedure.
Lens nuclear opacity was graded under the slit lamp using Lens Opacities Classification System III (LOCS III) scores.
Statistical Analyses
Statistical analyses were performed using statistical software (Statistical Package for Social Science, SPSS V16.0; SPSS Inc, Chicago, Illinois, USA). Odds ratios (OR) and 95% confidence intervals (95% CI) were shown. P values less than .05 were taken to indicate statistical significance.
Since both SE and ocular biometric parameters were highly correlated in the left and right eye, only right eye data were used for analyses. The age- and sex-standardized prevalence was calculated by direct standardization of the study samples to the Singapore ethnic Indian population, using the 2000 Singapore census data. We also calculated the mean refraction, AL, ACD, and CR in both first- and second-generation immigrants, using analysis of covariance to adjust first for age and sex and then further for educational level, height, and lens nuclear opacity. Multivariate regression models were fitted to estimate the associations of acculturation factors (age at migration and preferred language for interview) with the prevalence of myopia, SE, and AL adjusting for age, sex, educational level, lens nuclear opacity score, and height.
To evaluate the extent that education level and other risk factors may explain the excess prevalence of myopia and high myopia in second-generation immigrants compared with first-generation immigrants, we estimated the percentage reduction in odds associated with adjustment for these factors according to the following formula: (Ra−Rb)/(Ra−1) × 100, where Ra is the odds ratio of myopia in second-generation immigrants compared with first-generation immigrants, adjusted for age and sex only (reference model), and Rb is the odds ratio in models after additional adjustment.
Results
After excluding participants with previous cataract surgery, 1109 first-generation and 1877 second-generation Asian Indian immigrants contributed to this analysis. A total of 685 first-generation immigrants (61.8%) and 1418 second-generation immigrants (75.5%) completed the interview in English, respectively. Among first-generation immigrants, the average migration age to Singapore was 20.0 years (standard deviation [SD] = 12.7). Compared with second-generation Indian immigrants, first-generation immigrants were older ( P < .001), shorter ( P = .03), and less educated ( P < .001). They had lower body mass index ( P < .001), lower monthly income ( P < .001), smaller houses ( P = .002), and higher lens nuclear opacity score ( P < .001) ( Table 1 ).
| First Generation (N = 1109) | Second (or Higher) Generation (N = 1877) | P Value b | |
|---|---|---|---|
| Age | 59.1 (10.1) | 54.2 (7.8) | <.001 |
| Female sex | 525 (47.3) | 959 (51.1) | .05 |
| Height (cm) | 161.9 (9.4) | 162.7 (9.1) | .03 |
| BMI (kg/m 2 ) | 25.8 (4.2) | 26.5 (4.9) | <.001 |
| Education (no formal education) | 109 (9.8) | 107 (5.7) | <.001 |
| Monthly income (<SGD$1000) | 394 (36.5) | 439 (24.0) | <.001 |
| Housing type (1- to 2-room flat) | 60 (5.4) | 63 (3.4) | .002 |
| Time spent reading and writing per day (h) | 1.8 (1.4) | 1.9 (1.5) | .05 |
| Lens nuclear opacity (LOCS III) | 2.4 (1.4) | 2.0 (1.1) | <.001 |
a Data presented are means (standard deviations) or number (%), as appropriate for variable.
b Comparing the differences between the 2 generations of immigrants, based on χ 2 test or t test, as appropriate.
Table 2 compares the prevalence of myopia (SE <−0.5 D), high myopia (SE <−5.0 D), and mean ocular biometric parameters between first- and second-generation immigrants. Second-generation immigrants had higher prevalence of myopia (30.2% vs 23.4%) and high myopia (4.8% vs 2.5%) than first-generation immigrants. They also had longer AL (23.50 mm vs 23.37 mm; P = .004) after adjusting for age, sex, educational level, height, and lens nuclear opacity score. ACD and CR were not significantly different between the 2 groups.
| First-Generation Immigrants a (N = 1109) | Second-Generation (or Higher) Immigrants a (N = 1877) | P Value | |
|---|---|---|---|
| Prevalence of myopia (SE <−0.5 D) (%) | |||
| Age and sex standardized | 23.4; 20.6, 26.1 | 30.2; 28.1, 33.0 | |
| Prevalence of high myopia (SE <−5.0 D) (%) | |||
| Age and sex standardized | 2.5; 1.3, 3.7 | 4.8; 4.0, 5.7 | |
| Spherical equivalent (D) | |||
| Age and sex adjusted | −0.05; −0.19, 0.10 | −0.37; −0.49, −0.24 | <.001 |
| Multivariate adjusted b | 0.01; −0.12, 0.15 | −0.13; −0.23, −0.02 | .11 |
| Axial length (mm) | |||
| Age and sex adjusted | 23.40; 23.33, 23.46 | 23.59; 23.53, 23.65 | <.001 |
| Multivariate adjusted b | 23.37; 23.31, 23.44 | 23.50; 23.45, 23.55 | .004 |
| Anterior chamber depth (mm) | |||
| Age and sex adjusted | 3.15; 3.12, 3.17 | 3.15; 3.14, 3.17 | .64 |
| Multivariate adjusted b | 3.15; 3.12, 3.17 | 3.15; 3.13, 3.17 | .53 |
| Corneal radius of curvature (mm) | |||
| Age and sex adjusted | 7.61; 7.59, 7.63 | 7.61; 7.60, 7.62 | .75 |
| Multivariate adjusted b | 7.61; 7.60, 7.63 | 7.61; 7.60, 7.63 | .94 |
a Data are presented as value and 95% confidence interval.
b Adjusted for age, sex, educational level, height, and lens nuclear opacity score.
We estimated the reduction in odds of myopia and high myopia associated with second-generation immigrants with adjustment of myopia-related factors. Adjustment for height or educational level led to reduction in the excess prevalence of myopia in second-generation immigrants by 7.5% or 37.5%, respectively. On the contrary, adjustment for lens nuclear opacity score increased the excess prevalence of myopia in second-generation immigrants by 5.0%. The prevalence of high myopia in second-generation immigrants was reduced by 33.1% when educational level was adjusted ( Table 3 ).
| Model | Myopia (SE <−0.5 D) | High Myopia (SE <−5.0 D) | ||||||
|---|---|---|---|---|---|---|---|---|
| OR a | 95% CI | P Value | % Reduction Excess Prevalence b | OR a | 95% CI | P Value | % Reduction Excess Prevalence b | |
| 1 | 1.40 | 1.14, 1.71 | .001 | Reference | 2.54 | 1.56, 4.15 | <.001 | Reference |
| 2 | 1.37 | 1.13, 1.67 | .002 | 7.5 | 2.57 | 1.58, 4.19 | <.001 | −1.0 |
| 3 | 1.25 | 1.05, 1.49 | .02 | 37.5 | 1.70 | 1.08, 2.66 | .02 | 33.1 |
| 4 | 1.42 | 1.16, 1.73 | .001 | −5.0 | 2.46 | 1.51, 4.01 | <.001 | 3.1 |
| 5 | 1.25 | 1.04, 1.50 | .01 | 37.5 | 1.70 | 1.09, 2.66 | .02 | 33.1 |
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