To the best of our knowledge, this is the first study to describe the quantitatively refractive error shift from parents to children, over a relatively wide age range of children. Wu et al20
reported that the prevalence of myopia increased from 5.8% in the grandparents’ generation to 20.8% in the parents’ generation, and then further to 26.2% in the children's generation in China, suggesting a considerable environmental effect on myopia development over this relatively short time span. Although the data spanned through three generations with a very large sample size (n>3000) and wide age range (7–17 years), only the children's non-cycloplegic refractive error, and the parents and grandparents binary refractive error state (myopia or not), were obtained. Hence, the important quantitative changes over the generations remain unknown. Three epidemiological studies reported the prevalence of myopia in urban children in China.1
The population-based prevalence of myopia increased from 5.9% at the age of 6 years, to 78.4% at the age of 15 years, in urban children of South China.1
The school-based prevalence of myopia increased from 28.9% at the age of 7 years, to 48.2% at the age of 10 years, and further to 53.1% at the age of 11 years or more in Hong Kong students.21
A recent survey on self-reported prevalence of myopia in children aged 12–19 years in Taiwan was 70.3%.4
The prevalence of myopia in the current study also increased from 39.3% at the age of 7 years, to 68.8% at the age of 10 years, and further to 89.4% at the age of 11–17 years. However, our study provides the crucial quantitative shift of refractive error in children when compared with their parents. Thus, due to the increased environmental demands (such as more near-work or less time outdoors) at age 11 years, the children would have already attained the level of myopia of their parents and furthermore, have gone even beyond that as discussed below as per the estimated model findings. We believe that it is very significant to predict the development of myopia in children at different ages, as well as the myopic shift between generations, for example, for public health management and for development of preventive measures.
There were several important findings in our study. First, with regard to the overall age range (6–17 years), the RED and the proportion of children who had a higher myopic refractive error compared with their parents, increased with the age of children. Second, by using the estimated model at the age of 11 years, the children's SE would be close to the average SE of their parents; and furthermore, half of the children would have a higher myopic refractive error than their parents. Third, by using the estimated model, the children's myopic shift and their chances of having higher myopia compared with their parents at the age of 18 years would be nearly 2.0 D and 80%, respectively. To some extent, this noticeable myopic shift found in only a span of two generations reflects an additional effect of an environmental change (such as exposure to more intensive near-work, educational attainment and less time outdoors).22–24
The generational myopic shift described and quantified in our study supports a predominant, although not necessarily exclusive, environmental effect on the increasing prevalence of myopia over the two generations.14
Indeed, previous longitudinal studies showed a predominant hyperopic shift,25
that is, a physiological decrease in the prevalence of myopia with age, among adults aged 40–60 years.27
This was principally explained by a change in the crystalline lens with age.28
However, a longitudinal clinical observation of simple myopia (SE ranged from −1.0 to −6.0 D) for more than 20 years showed an average myopic
shift of −0.60 D, −0.39 D and −0.29 D during the third (20–29 years, and so on), fourth and fifth decades, respectively; and a hyperopic
shift of +0.28 D and +0.41 D during the sixth and seventh decades, respectively.29
The myopic shift among adults younger than 45 years suggested that a decrease in the prevalence of myopia is unlikely due to physiological decrease in the ocular biometric parameters; although further direct evidence of the ocular parameters is warranted. Furthermore, the final stable myopic level was supposed to be consistent over two generations if and only if a physiological effect exists. As the time spent on reading and time outdoors were associated with longer ocular length and refractive change towards myopia,22–24
we conclude that environmental factors, such as near-work and time outdoors, play an important role in this generational myopic progression.
Although our study provides interesting findings, the representativeness of RED in this study may be limited because the participants in the BMPS were not selected randomly. The median (mean) refractive error was 0.00 (−1.01) D in an urban Beijing population aged 40–44 years,11
which was clearly more hyperopic than that of the parents (median −1.69 D; mean −2.03 D) in a similar age group in the current study. We enrolled children through flyers provided to the schools and our hospital. As most of the children were driven to participate in this study by their parents, it was possible that parents with higher myopia, higher socioeconomic status and/or higher educational attainment were more likely to join the study, and thus the generational myopic shift may have been underestimated. Furthermore, the mean non-cycloplegic SE would be approximately 0.4 D more myopic than the cycloplegic SE in the 16–45-year-old participants.30
In addition, the myopia would continue to progress after the age of 18 years in some subjects.29
As a result, the non-cycloplegic parental SE in the present study may also underestimate the generational myopic shift. Hence, these various methodological biases may contribute to the underestimation
of the generational myopic shift in the present study and thus may represent a conservative estimate of the generational myopic shift.
For the current study, the students mainly in the low grades in the elite primary or secondary schools were arbitrarily selected for participation in the BMPS. This may have led to undersampling in some age groups, as well as a bias towards higher socioeconomic status and/or educational attainment. Hence, further investigations with larger sample sizes on RED are warranted. Parallel investigations in rural children and different ethnic groups would be equally interesting. It is also noteworthy that our study had some information bias, because 23.5% of the parental refractive error was obtained through self-reporting. However, analysis for RED with both parents’ refractive error tested in the clinical centre (n=224) demonstrated a good consistency. The RED was −2.53 D, −1.97 D, 0.42 D, 0.38 D, 1.84 D and 1.50 D compared with −1.88 D, −1.63 D, 0.42 D, 0.44 D, 1.84 D and 1.53 D for all the 395 families of children in each combined age group. It will be optimal to include those aged 18–25 years as well, which would help to predict the generational myopic shift in this part of China.
In summary, this report provides information on the RED between parents and their children in urban China. At the age of 11 years, the SE of children would reach the average SE of their parents, and the estimated myopic shift at 18 years of age would be nearly 2.0 D. This generational myopic shift provides an evidence of a cohort effect on the increasing prevalence of myopia over generations of young adults, presumably due to environmental factors such as near-work.