Myopia is one of the most common visual disorders affecting both children and adults.1 An uncorrected refractive error may impose significant burden on children in terms of social capability and education achievement. A high degree of myopia further increases the chance of developing irreversible visual impairment due to pathologic changes in the retina and increased risk of glaucoma and cataracts.2–4 Nowadays, uncorrected refractive error is increasingly recognized as a significant cause of avoidable visual impairment worldwide, as suggested by the inclusion of uncorrected refractive error as one of the prioritized eye diseases of Vision 2020: The Right to Sight—a global initiative launched by a coalition of non-government organizations and the World Health Organization.5 It is imperative to plan corresponding public health strategies based on the knowledge of the prevalence of refractive error worldwide.
It has been well recognized that the myopia rates in East Asians, particularly the Japanese and Chinese populations, are much higher than in the European-derived population.1 However, although numerous studies claim to report the prevalence of refractive error in children living in mainland China, most are based on subjects with unknown representation. Problems in non-uniform definition and measurement methods further hinder a valid comparison across studies.
Environmental exposure is recognized as a major risk factor for myopia. A higher prevalence of myopia in urban settings, compared with rural settings, has been consistently reported in several studies.6 To plan and provide better eye health services for the world’s largest population, this article intends to review the available data on the prevalence of myopia in Chinese children living in mainland China; particular consideration is given to the evidence of urban-rural differences and the implications.
History of Studies on Prevalence of Myopia in China
Data detailing the rates of myopia in children were limited and of variable quality before the 1990s (Table 1). Most of the previous reports were difficult to interpret in part due to their equivocal descriptions of study design. Furthermore, much of these data were available only in Chinese biomedical journals and therefore difficult for international readers to access.
In the early twentieth century, it was believed that the Chinese population had a high prevalence of myopia. This view stemmed from two 1920s findings in which the prevalence of myopia was 53% in a study by Li7 and 58.4% in another by Rush.8 Due to the obscure sampling characteristics, the representativeness of these findings is unclear. In another investigation conducted in 1936, Rasmussen showed that a 65% prevalence of myopia in the Chinese population.9 However, this study was limited by selective sampling and subjective estimation of myopia based on the records of people who required prescription glasses. Therefore, the magnitude of myopia before the establishment of the People’s Republic of China (1949) remains unclear.
Surveys on myopia in school-age children took the epidemiologic spotlight in the 1950s and early 1960s when researchers found visual acuity a quick and convenient approach for identifying children with refractive error, although this method may be insufficient for a precise classification of myopia in children. Xu found a prevalence rate of 5.3% among children in primary schools (equivalent to age 7 to 12 years) and 25.5% in college students in their survey conducted in 1957. In the same manuscript, they further reported that the prevalence of myopia increased to 7.8% in primary schools and 46.4% in college students in 1964 survey.10
During the Cultural Revolution between 1966 and 1976, the education system in China suffered. School curriculums were frequently nullified, and the plan to eradicate illiteracy fell behind schedule. We did not find relevant literature from this particular period of time. However, it is believed that the generation that did not complete formal education during the Cultural Revolution might have a lower risk of myopia.11 Morgan and Rose suggested that this decline in the rate of myopia is an indication of the sensitivity of myopia to environmental change.6
After the repudiation of the Cultural Revolution, uniform national university entry examinations were reintroduced in December 1977. Interest in receiving higher education has been reignited among the Chinese population. In July 1986, a 9-year compulsory education program was implemented, symbolizing the resumption of intensive reading demands imposed upon school-age children thereafter.
Taken together, the rates of myopia in Chinese children before the 1990s are similar to what have been reported in white populations. Given the Chinese have much lower schooling intensive as well as lower rate of myopia before 1990s, these findings support the association between myopia and schooling.12,13–15
Studies on Prevalence of Myopia after the 1990s
In part supported by a contract between the National Eye Institute in the U.S. and the World Health Organization, the Refractive Error Study in Children (RESC) studies have provided unprecedented comparative data on the prevalence of refractive error in school-age children across different geographic locations and countries.16 Surveys have been conducted in three areas of China: the Shunyi District,17 a semi-rural area outside Beijing in northern China; Guangzhou,18 a large metropolitan city in southern China; and Yangxi,19 a rural county located in the western part of Guangdong province. These three studies allow a valid comparison across rural and urban settings in northern and southern China where the same definition and protocol were adopted. The subjects were identified using random clustering sampling in geographically defined areas in both Shunyi and Guangzhou. The resident registry was used to identify and enumerate the eligible subjects. Therefore, in order to examine eligible children, our study team visited more than 71 schools and set up 19 temporary clinics in the community in the Guangzhou study. Due to the high rates of school attendance and pragmatic reasons, in the study in Yangxi, we identified subjects from schools-randomly selected classes from all the classes in the junior high schools in the entire county (class as cluster unit) and then examined all students in the selected classes.19 The subjects were aged 5 to 15 years old in Shunyi and Guangzhou, and aged 13 to 17 years old in Yangxi county. These three studies all achieved reasonably good response rates (95.9% in Shunyi, 86.4% in Guangzhou, and 97.6% in Yangxi). The refraction status was assessed by autorefractor and retinoscopy using 1% cyclopentolate as the cycloplegic agent and light reflex as the indicator for successful cycloplegia. Myopia was defined based on the spherical equivalent. Biometric data were not reported in these studies.
Another prevalence survey on myopia was conducted in Hong Kong using a school-based study design.20 One school was randomly selected from each of the 19 school districts; two to three classes were randomly selected from each school. The subjects were aged 5 to 16 years old although the age-specific rates were reported as aged ≤5, 7, 8, 9, and 10 years and ≥11 years. Drops of 0.5% tropicamide and 0.5% phenylephrine were used as cycloplegic agents; thus it is possible that this did not attain satisfactory cycloplegia, particularly in younger children. Refraction was determined by an autorefractor. Biometric data are available for the study cohort. The cohort was re-examined after 12 month and thus provides longitudinal data as well.
A school-based study comparing the prevalence rate of myopia in schoolchildren in urban and rural Xiamen in the Fujian province of southeast China was also conducted.21 Children aged 8 to 9 years attending the 2nd grade of primary school were examined although the sample size was small (n = 132 in city, and n = 104 in countryside). One percent cyclopentolate was used as the cycloplegic agent, and an autorefractor was used for refraction measurement. Biometric data as well as interview-based questionnaire data are available from this cohort.
Urban and Rural Differences
The age-specific prevalence data of myopia in these studies are summarized in Fig. 1. In general, myopia is essentially absent (<5%) in those aged 5 to 6 years in both urban and rural settings of all three sites of the RESC studies. This has been consistently found in RESC studies in India,22 Nepal,23 Chile,24 South Africa,25 and Malaysia26 when the same protocol (1% cyclopentolate as the cycloplegic agent and refraction determined by retinoscopy and autorefractor) was used; however, the rates of myopia was as high as 17.0% in Hong Kong20 (being similar to Singaporean children).27 The earlier start of formal education in Hong Kong and Singapore could probably explain the high rate in preschool-aged children; however, the fact that 0.5% tropicamide was used as a cycloplegic agent may inflate this rate when substantial residual accommodation is present.20 In fact, the same group of researchers found <5% suffered from myopia when they specifically looked into the preschool group and used 1% cyclopentolate as the cycloplegic agent.28 In China, children normally start formal primary school at the age of 6 to 7 years; before that time, kindergarten or preschool attendance is voluntary, where only a limited amount of near-work may be required. The relatively low prevalence in both rural and urban settings in China, similar to what has been found in other countries, suggests the children may start from a very similar refraction status before the onset of myopia and formal schooling.
The prevalence of myopia starts to rise and is consistently higher in urban settings and at the age of 7 years for Chinese children (Fig. 1). The study in rural southern China (Yangxi county) also demonstrates that the prevalence was lower than what was found in semi-rural China (Shunyi) and substantially below that in urban Guangzhou. This study in Yangxi county further demonstrates that the rate of myopia in schools located in an urban center nearly doubles that of those in rural countryside schools. The urban–rural difference was not attributable to gender, grade level (age), or parental education.19 A similar pattern of urban-rural differences was also observed in 2nd-grade Chinese children in urban city private schools vs. those attending government schools in the countryside of Xiamen City,21,27 as well as in Indian children in RESC studies22,29 and urban private schools vs. rural schools in Nepal.30 Other studies further demonstrate in Indians, Malays, and Chinese, that people of the same ethnicity may have different rates of myopia if the people live in different countries.31,32
Prevalence of myopia was observed to increase substantially in Eskimos when formal education became available.33,34 This in part suggests the role of education and urbanization on myopia although this increase may also be attributable to nutrition, other lifestyle changes or even cohort differences. An increase in the myopia rate linked with environmental changes has also been reported in a Taiwanese population.35
The urban–rural difference in myopia is suggested by longitudinal data as well. The Shunyi cohort (those aged 5 to 13 years) was re-examined after approximately 2 years and thus provides useful population-based longitudinal data for children living in semi-rural northern China.36 A rate of −0.17 D per year was observed in this semi-rural children cohort. This is considerably lower than in other studies: −0.40 D per year found in a 1-year study of 4973 children in Hong Kong initialized at ages 5 to 16 years20; and −0.56 to −0.65 D per year found in 153 myopic Singaporean children aged 6 to 12 years in a contact lens trial.37 The considerable differences in myopic progression in Shunyi and Hong Kong suggest an urban-rural difference and support the difference identified in cross-sectional studies.
The fact that people exhibit different patterns or rates of myopia in urban and rural settings appears to suggest an impact of environmental effects, given that the genetic backgrounds are very similar among people living in the urban and rural areas of the same country. However, the urban–rural difference may be a surrogate for other “myopigenic” environmental risk factors because the education, socioeconomic, and nutrition status of the people should also tend to be different in urban and rural environments. The study in Xiamen found that children in the city spend more time on near-work activities and less time on outdoor activities outside school than children in the countryside: the average time was 2.2 h of near-work per day in the city vs. 1.6 h/d in the countryside, 5.6 h of outdoor activity per week in the city vs. 15 h/week in the countryside.21 These discrepancies were also consistent when the same research group compared Chinese children in Singapore to those in Xiamen.27 However, this association is not always consistent—based on population-based data in Sydney and Singapore, Australian Chinese students did more near-work activities than Singaporean children and spent more time outdoors (13.75 vs. 3.05/week); time spent outdoors was identified as the most significant risk factor that explains the lower rate of myopia in young Australian children.38 More interestingly, based on the same Australian children, the near-work time was found to poorly correlate with myopia, however, the distance of close-up reading and intensity (continuous reading) were important independent risk factors.39 These studies underscore the need for quantifying not only the time spent on near-work but also the intensity and distance.
Outdoor activities have been increasingly recognized as a protective factor for myopia based on cross-sectional data in children living in Australia, Singapore, the U.S., and Turkey.38,40,41 This association has been further demonstrated in longitudinal data in the Orinda longitudinal study and thus somehow suggests a causal effect of the outdoor exposure on myopia.42 The recent work based on 6- and 12-year-old Australian children further confirms the protective effect as total time spent outdoors rather than sport per se.43 Children living in rural environments may have more outdoor space and better “accessibility” to outdoor activities; this may in part explain the relatively lower rate of myopia as well. Interestingly, one recent report based on 12-year-old Australian children demonstrated the independent effects of higher population density (apartment-style housing) as a risk factor for myopia independent of near-work and outdoor activities.44 This study is one of several that document the independent effects of the urban environment on myopia.
Although there have been increasing amounts of evidence suggesting risk factors for myopic development, one has to be cautious that the majority of data are derived from cross-sectional data and the risk factors are mainly collected by questionnaire. Casual effect is difficult to prove with cross-sectional data while retrospectively collected risk factors may be subject to recall bias. Lack of a standardized protocol may further hinder a valid comparison across studies. In order to address this problem, the World Health Organization convened a group of myopia researchers in Singapore and recently proposed a questionnaire for myopia-related risk factor collection. This tool has been adopted in several ongoing WHO-sponsored longitudinal studies on myopia in children.
Finally, the unmet need for correcting refractive error in children is widespread. Only two thirds of children with visual acuity of 20/40 or worse were wearing spectacles in urban areas,18 and this proportion was only one third among children living in rural China.17,19 If higher visual acuity cutoffs were adopted or those with under-correction were included, the situation could have been even worse. Barriers to the use of spectacles include parental awareness of vision problems, attitude toward the need for spectacles, cost, and concerns that wearing glasses may cause myopic progression.45 Given that quality optometric service may be not uniformly available and accessible to children, an effective school screening program in combination with spectacle provision that involving the training of optometric practitioners as well as health education may be in serious demand in rural areas.
No conflicting relationship exists for any authors.
Department of Preventive Ophthalmology
Zhongshan Ophthalmic Center
People’s Republic of China
e-mail: [email protected]
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