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The Age-Specific Prevalence of Myopia in Asia: A Meta-analysis

Pan, Chen-Wei*; Dirani, Mohamed; Cheng, Ching-Yu*; Wong, Tien-Yin*; Saw, Seang-Mei

doi: 10.1097/OPX.0000000000000516
ORIGINAL ARTICLES

Purpose To estimate the age-specific prevalence of myopia in Asia.

Methods We searched PubMed, Embase, and Web of Science from their inception through September 2013 for population-based surveys reporting the prevalence of myopia in adults or children in Asia. We pooled the prevalence estimates for myopia by age groups and by year of birth using a random-effects model.

Results We identified 50 eligible population-based studies including 215,672 subjects aged 0 to 96 years reporting the prevalence of myopia from 16 Asian countries or regions. Myopia was found to be most prevalent (96.5%; 95% confidence interval, 96.3 to 96.8) in Koreans aged 19 years. There was no significant linear age group effect on the prevalence of myopia in the whole Asian population but there was a U-shaped relationship between both age and year of birth and the prevalence of myopia. The prevalence of myopia was also higher in those older than 70 years (36.3%; 95% confidence interval, 27.6 to 45.0) compared with other age groups, which revealed nuclear cataract-myopia shifts in refraction.

Conclusions There is a large variation in the age-specific prevalence of myopia in Asia. A U-shaped relationship between age and the prevalence of myopia was found in the whole Asian population. The analysis is essential to guide future eye health care, intervention, and clinical management in Asia.

*MD, PhD

PhD

MPH, PhD

Jiangsu Key Laboratory of Preventive and Translational Medicine for Geriatric Diseases, School of Public Health, Medical College of Soochow University, Suzhou, China (C-WP); Singapore Eye Research Institute, Singapore, Singapore (C-WP, C-YC, T-YW, S-MS); Centre for Eye Research Australia, University of Melbourne, Melbourne, Australia (MD); and Saw Swee Hock School of Public Health, National University of Singapore, Singapore, Singapore (C-YC, T-YW, S-MS).

Seang-Mei Saw Singapore Eye Research Institute 16 Medical Drive (MD 3), 117597 Singapore e-mail: seang_mei_saw@nuhs.edu.sg

Myopia is a complex eye condition that poses major public health and socioeconomic problems globally.1–4 Of the leading causes of vision impairment and blindness in the world, uncorrected myopia has been grouped with cataract, macular degeneration, infectious disease, and vitamin A deficiency.5 Most cases of myopia can be treated with conservative treatments; however, more severe stages of myopia may be associated with a range of vision-threatening ocular complications including age-related cataract and open-angle glaucoma in older adults that may require surgical interventions.6–10 It has been estimated that globally 153 million people over 5 years are visually impaired because of uncorrected myopia and other refractive errors, and of these, 8 million are blind.11 The global productivity lost because of uncorrected visual impairment is estimated to be 121.4 billion international dollars,12 and the global costs of facilities and personnel for establishing refractive care services are USD$20 billion.13 These figures demonstrate the clinical, public health, and economic implications of myopia as a global health problem.

Asia is the world’s largest and most populous continent, covering 8.7% of the earth’s total surface area and comprises 30% of its land area. With about 4.3 billion people, it hosts 60% of the world’s current human population.14 The prevalence of myopia in some urbanized areas of Asia has reached epidemic proportions,15 with the prevalence being as high as 40% in Japan,16 50% in Taiwan,17 and 70% in Singapore.18 However, the prevalence of myopia varies with age, and the age-specific prevalence of myopia in Asia is not well evaluated and summarized. A comprehensive overview of the age-specific prevalence of myopia in Asia is essential to guide future eye health care, intervention, and clinical management in this area and provide a perspective for other continents of the current and future challenges of myopia.

In this study, we performed a systematic review and meta-analysis aiming to estimate the age-specific prevalence of myopia in Asia.

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METHODS

Systematic Review Process

This review followed the Meta-analysis of Observational Studies in Epidemiology guidelines for the conduct of systematic reviews and meta-analyses of observational studies.19 We conducted searches in PubMed (1950 onward), Embase (1960 onward), and Web of Science (1970 onward) electronic databases. The following text words (formatted for PubMed search) were used to search for relevant references: ((“myopia”[MeSH Terms] OR “myopia”[All Fields]) OR (“refractive errors”[MeSH Terms] OR (“refractive”[All Fields] AND “errors”[All Fields]) OR “refractive errors”[All Fields] OR (“refractive”[All Fields] AND “error”[All Fields]) OR “refractive error”[All Fields])) AND ((“epidemiology”[MeSH Terms] OR “epidemiology”[All Fields] OR “prevalence”[All Fields] OR “prevalence”[MeSH Terms]) OR (“cross-sectional studies”[MeSH Terms]) OR “cohort studies”[MeSH Terms] OR “survey”[MeSH Terms]). Two authors performed the literature search independently. We compared the extracted studies and resolved inconsistencies by consensus. The electronic search was completed in September 2013. Duplicate reports were removed. Reference lists of identified reports were scanned to identify other relevant studies. This study was approved by the Singapore Eye Research Institute Institutional Review Board.

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Study Selection

Population-based surveys in Asia on the populations of any age range with response rates of at least 60% were included in the current review. “Population-based” pertains to a general population defined by geopolitical boundaries; this population is the sampling frame.20 The most commonly accepted boundaries place Asia to the east of the Suez Canal, the Ural River, and the Ural Mountains, and south of the Caucasus Mountains and the Caspian and Black seas.21 In addition, we also included the studies on military conscripts in some Asian countries where military service is mandatory. In these countries, only military conscripts with severe disabilities or serious chronic medical conditions are exempted from military service. Conscripts with visual defects or refractive errors are not exempted. Clinical- or hospital-based surveys or audits were excluded. Studies that used visual acuity as a surrogate measure for refraction or performed noncycloplegic refraction on children or teenagers younger than 20 years were excluded. Studies without a defined sampling strategy or with a response rate of less than 60% or published in languages other than English were also excluded.

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Data Extraction and Quality Assessment

A comprehensive list of variables was extracted from each eligible study: the surname of the first author, publication year; study country; study name or study population; the year the study was conducted; race/ethnicity; age range; response rate; sample size; and number (or prevalence) of myopia overall and, if available, according to age and sex.

The quality of the studies included in this review was evaluated using the methodological criteria for prevalence studies developed by Leboeuf-Yde and Lauritsen22 and Walker.23 Three aspects of each study were assessed, including (1) whether the final sample was representative of the target population; (2) quality of data (Were the data primarily from a prevalence study or were they taken from a survey not specifically designed for that purpose? Was the mode of data collection same for all subjects? Have the data been collected directly from the patient by means of a validated method?); and (3) description of the target population and the source of participant recruitment.

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Data Analysis

We performed the meta-analysis using Stata version 12.0 (StataCorp, College Station, TX). Heterogeneity was found to be high among the identified studies, and therefore, random-effects models were used. Age-specific pooled prevalence estimates of myopia defined by a spherical equivalent of less than −0.5 diopters (D) in the right eye were classified into seven groups: younger than 20 years, 20 to 29 years, 30 to 39 years, 40 to 49 years, 50 to 59 years, 60 to 69 years, and 70 years or older. Study year was defined as the midpoint of the study period (e.g., if the reported study period was 2009 to 2011, the study year was defined as 2010). Year of birth was calculated based on study year and age of the study subjects. We allowed for a 5-year range for each age/year of birth category (e.g., the reported prevalence for ages 43 to 54 years was pooled for the “40 to 49 years” age category).24 Statistical heterogeneity among studies was evaluated using I 2 statistic. Values of 0 to 24%, 25 to 49%, 50 to 74%, and greater than 75% denote no, low, moderate, and high heterogeneity, respectively. Sensitivity analysis was performed to investigate the contribution of each study to the heterogeneity by sequentially removing one study and reanalyzing the pooled estimate for the remaining studies. A meta-regression model was developed with age group and year of birth as the dependent variables to determine the linear age group effects on the prevalence of myopia. Regression models were also set up with age2 or year of birth2 (age or year of birth “squared”) as an independent variable to test the significance of the possible U-shaped distribution of myopia in different age groups. Publication bias was assessed using the Egger test.25

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RESULTS

Characteristics of Studies Included in the Meta-analysis

A total of 9613 unduplicated titles were identified through our literature search. After screening these titles, 512 articles were retrieved and their abstracts were reviewed. From these, 135 were selected for whole manuscript review, of which 50 unduplicated studies were identified to be included in the meta-analysis (Fig. 1). Among the 50 studies, 12 were in China (including Hong Kong),26–37 8 were in Iran,38–45 7 were in Singapore,18,46–51 5 were in India,52–56 3 were in South Korea,57–59 2 were in Taiwan,60,61 2 were in Japan,16,62 2 were in Nepal,63,64 2 were in Malaysia,65,66 and 1 each in Bangladesh,67 Pakistan,68 Indonesia,69 Myanmar,70 Laos,71 Cambodia,72 and Mongolia.73 Characteristics of the studies are presented in Table 1. The 50 studies comprised a total of 215,672 subjects aged 0 to 96 years. The response rate ranged from 66.6 to 97.6%. Common to all included studies, participants who had previous cataract surgery were excluded from the analyses. In total, 79,850 myopia cases of less than −0.5 D were included in the meta-analysis.

TABLE 1

TABLE 1

FIGURE 1

FIGURE 1

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Quality Assessment of the Included Studies

Although only population-based studies with reasonable response rates were included, we did not think that the final sample in some individual studies could be representative of the target population. The response rates were a bit low and there were significant differences in age distribution between responders and nonresponders in some individual studies. For this meta-analysis, the response rate bias is small as we are estimating the pooled age-specific prevalence rather than the overall prevalence. Therefore, we included these studies in the analysis. In addition, the data were not all primarily from a prevalence study designed for that purpose in all studies. Especially in adult studies, the study was usually designed to estimate the prevalence of other age-related eye diseases such as glaucoma, cataract, or age-related macular degeneration. The mode of data collection (e.g., refraction method, machine used for refraction) was the same for all subjects in all studies. The refraction data in each individual study have been collected directly from the patient by standardized approaches such as objective or subjective refraction. All studies described the target population and the source of participant recruitment, albeit in various extents.

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Pooled Age-Specific Prevalence of Myopia

The pooled prevalence of myopia was similar between men (28.5%; 95% confidence interval [CI], 24.2 to 32.7) and women (27.3%; 95% CI, 23.4 to 31.2) (p = 0.23).

In adult studies, the pooled prevalence estimates of myopia were 47.3% (95% CI, 19.3 to 75.2) in those aged 20 to 29 years, 26.4% (95% CI, 17.5 to 35.4) in those aged 30 to 39 years, 28.0% (95% CI, 22.4 to 33.6) in those aged 40 to 49 years, 25.8% (95% CI, 20.4 to 31.2) in those aged 50 to 59 years, 28.2% (95% CI, 21.4 to 34.9) in those aged 60 to 69 years, and 36.3% (95% CI, 27.6 to 45.0) in those older than 70 years (Fig. 2).

FIGURE 2

FIGURE 2

When stratified by year of birth, the pooled prevalence estimates of myopia were 39.9% (95% CI, 26.3 to 53.4) in those born in 1920 to 1929, 32.2% (95% CI, 24.1 to 40.4) in those born in 1930 to 1939, 25.1% (95% CI, 19.8 to 30.3) in those born in 1940 to 1949, 23.0% (95% CI, 18.4 to 27.7) in those born in 1950 to 1959, 28.7% (95% CI, 21.9 to 35.5) in those born in 1960 to 1969, and 38.8% (95% CI, 7.2 to 70.3) in those born in 1970 to 1979 (Fig. 3).

FIGURE 3

FIGURE 3

The pooled prevalence estimates of myopia were 24.2% (95% CI, 3.3 to 44.8) in those younger than 20 years. The pooled prevalence estimates of myopia were 14.1% (95% CI, 9.0 to 19.2) in RESC (Refractive Error Study in Children).

A meta-regression model was constructed on study subjects younger than 70 years with refraction methods (subjective vs. objective), year of birth, and age group as independent variables. In this model, neither per decade increase in year of birth nor age group (per decade increase) showed a significant linear effect (both p > 0.05) on the prevalence of myopia. There was a significant U-shaped relationship between both age and year of birth and the prevalence of myopia in a regression model with age2 or year of birth2 as an independent variable (both p < 0.05). We found no significant difference in myopia prevalence measured by subjective and objective refraction (p = 0.32).

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Sensitivity Analysis and Publication Bias

Omission of individual studies revealed that no single study had a particular influence on the pooled estimate. No publication bias was detected by the Egger test (p > 0.05).

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DISCUSSION

Our meta-analysis shows that there is a large variation in age-specific prevalence of myopia in Asia. There is a U-shaped relationship between both age and year of birth and the prevalence of myopia.

Traditionally, myopia has been viewed as an Asian health problem, with the prevalence being significantly higher in Asia than in western communities.74–76 A previous meta-analysis by Kempen et al.77 on white adults aged 40 years or older estimated that the prevalence of myopia (<−1.0 D) was 25.4, 26.6, and 16.4% in the United States, Western Europe, and Australia, respectively. In our meta-analysis, the pooled prevalence of myopia (<−0.5 D) in Asian adults aged 40 years or older is about 30% (data not shown). Considering that Kempen et al.77 used a more conservative definition of myopia (spherical equivalent < −1.0 D), there is no evidence showing that myopia is more prevalent in Asia compared with the whites in United States and Western Europe in adults older than 40 years. However, in specific ethnic groups (e.g., Japanese) or regions (e.g., Chinese in Singapore) in Asia, the prevalence of myopia is still higher compared with the whites living in western countries. The magnitude of difference is even larger in the younger generation of an urban Asian population compared with whites of similar age. The differences in the younger generation may be more marked because of the recent cohort effect in urban Asian cities such as Singapore whereby the education system has been more rigorous in the past few decades.78–80 Middle-aged to elderly adults in urban Asian cities may not have experienced a rigorous schooling system during their childhood. For example, the pooled analysis of RESC studies on schoolchildren aged 5 to 15 years was 14% whereas the prevalence was reported to be 1.5% in Sydney schoolchildren,81 5% in schoolchildren living in urban cities in the United States,82 and 2.8% in Northern Ireland.83 However, reports from other Asian countries have not always revealed a high prevalence of myopia. In Cambodia, the prevalence (≤−0.5 D) was only 5.5 to 6.0% in children aged 12 to 14 years.72 In Laos, the prevalence of myopia (≤−0.5 D) was only 0.8% in children aged 6 to 11 years.71 In rural Nepal, only less than 3% of Nepalese children aged 5 to 15 years were affected by myopia (≤−0.5 D).63 These low figures contrasted with the higher prevalence of myopia in children with similar age observed in other urban Asian communities such as Singapore. Thus, myopia is perceived to be a recent public health problem in some urbanized areas in Asia.84 A systematic review and meta-analysis of the age-specific prevalence in western populations may be useful to elucidate the differences in myopia prevalence between Asians and western populations.

It is debatable whether the decreasing trend in myopia prevalence with increasing age observed in cross-sectional studies is explained by an age or cohort effect. In an early study, Mutti and Zadnik85 reanalyzed the prevalence data of three population-based surveys in the United States and concluded that this trend is more likely to be explained by longitudinal age effect rather than cohort effect. Our meta-regression analysis showed that either per decade increase in year of birth or age group showed a nonsignificant effect on the prevalence of myopia (both p > 0.05). However, we tested for age group effect under strict assumptions of linearity (meta-regression). The age group effect for the whole Asian population was not linear but followed a U-shaped distribution. Based on the analysis in this study, we cannot determine whether a cohort effect exists in Asian populations.

There are conflicting results on whether there are any sex differences in the prevalence of myopia. A number of studies have shown a higher prevalence of myopia in female subjects than in male subjects.86–88 Our meta-analysis did not find any significant difference for the prevalence of myopia between men and women. Currently, there is no plausible biological argument to explain the differences in the prevalence between sexes observed in some studies.

There are several strengths of our meta-analysis, namely, the large number of subjects from all studies, the wide age range of the participants, and the strict inclusion criteria whereby only the studies of good quality were included. However, our analysis included a number of limitations that should be considered when interpreting the results. First, we assumed that the contributing studies in the meta-analysis are representative of the general populations in Asia. However, this may not be true as population-based data were not available in many Asian countries such as Thailand or countries located in the middle of Asia. The exclusion of studies published in a language other than English may also lead to bias as English is not the first language in most Asian countries. Therefore, the prevalence estimates may not represent the countries/ethnic groups, of which population-based data were not available. Second, prevalence surveys are often conducted in areas where it is thought there is a high disease burden. Therefore, population-based studies that are not nationally/regionally representative may provide biased results. Third, it is difficult to compare the prevalence of myopia between Asians and western populations precisely because of the methodological issues attributed to differences in age, definition of myopia, and sampling strategy. Fourth, as the age ranges in different ethnic groups were different, we were unable to assess the ethnic differences in Asians. Fifth, not all urban areas showed a high estimate of the prevalence of myopia, and therefore, the association between urbanization and the risk of myopia still needs to be elucidated. Finally, we cannot separate axial myopia and myopia caused by cataract in this analysis as we did not have individual data of each included study.

In summary, within Asia, there is a large variation in the age-specific prevalence of myopia. Overall, we showed that the prevalence of myopia in middle-aged to elderly Asian adults is similar to published literature in Western populations. The prevalence of myopia is high in adults older than 70 years, possibly attributed to cataract in elderly adults. More research is needed in Asia regarding the prevalence rates for myopia in countries where population-based data are unavailable to more accurately estimate the extent of the problem, which could direct epidemiological research and health policy decision making in the future.

Seang-Mei Saw

Singapore Eye Research Institute

16 Medical Drive (MD 3), 117597

Singapore

e-mail: seang_mei_saw@nuhs.edu.sg

Received May 17, 2014; accepted October 7, 2014.

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Keywords:

myopia; Asia; prevalence; meta-analysis

© 2015 American Academy of Optometry