The Myopia Prevalence and Association With Physical Activity Among Primary School Students Aged 6–12 Years: A Cross-Sectional Study in Tianjin, China

Abstract

Purpose: This study aimed to investigate the prevalence of myopia and determine the association between physical activity and risk of myopia among primary school students in Tianjin, China.

Methods: A cross-sectional study was conducted among subjects from nine primary schools. All of the subjects underwent visual acuity and spherical equivalent (SE) with noncycloplegic autorefraction measurement. Myopia was defined as an SE refraction ≤−0.50D and an uncorrected visual acuity <5.0 in either eye. Physical activity was measured via the Physical Activity Questionnaire for Children. Data were analyzed using the Pearson χ² test and binary logistic regression. Stratification analysis by sex was also performed.

Results: A total of 2976 participants (1408 boys and 1568 girls) aged six to 12 years (mean age 8.82 years) were included in this study. The overall prevalence of myopia was 52.92%. When stratified according to physical activity, myopia prevalence significantly decreased with increasing physical activity levels (χ² trend test = 127.63, P < 0.001). In the binary logistic regression analysis, after adjusting for age, sex, and school region, the odds ratio for the association between physical activity and myopia was 0.762 (95% confidence interval, 0.675–0.862, P < 0.001). When stratified by sex, the significant statistical association between physical activity and myopia both can be found in two groups (P < 0.05).

Conclusions: Higher levels of physical activity were independently associated with decreased risk of myopia. The significant reverse statistical association between physical activity and myopia can be found in male or female groups.

Translational Relevance: Taking part in physical activities may be an effective way to reduce the prevalence of myopia.

Introduction

Myopia is one of the most commonly diagnosed refractive disorders. It has been estimated that myopia will affect nearly five billion people by the year 2050 and become a major public health challenge.¹ Myopia has a negative impact on the health of children worldwide.² Studies show myopia progresses at the fastest rate at six to seven years of age and tends to slow down after 11 to 12 years of age.^3,4 As the most common cause of visual disorder in children, correctable visual impairment, mainly myopia, has become a significant public health issue worldwide in China. The prevalence in children five to 14 years of age is 36.7% in Beijing⁵ and 53.9% in children six to 12 years of age in Tianjin.⁶ In central China, the Anyang Childhood Eye Study showed that the prevalence rates of myopia were 3.9% and 67.3% at mean ages of 7.1 years and 12.7 years, respectively.⁷ Prevention of myopia has become an important public health priority in China. It is urgent and critical to identify potential risks and preventive factors to develop more effective prevention strategies for the development of myopia in primary school students. 

The etiology of myopia still remains unclear. It has been speculated that education; lifestyle changes such as reduced physical activity, reduced outdoor time, and more close-up work; and genetics might be the driving force behind the rapid increase in myopia.^8–10 Physical activity is an essential part of childhood physical and mental development. Some studies have described physical activity as a potential contributor to development and progression of myopia.^11,12 Physical activity exerts myriad positive effects on child health and may protect against eye diseases.¹³ Some studies demonstrated a reverse association between physical activity and myopia but also attributed the results to time spent outdoors and not physical activity per se.^14–20 One cross-sectional study found no relation.²¹ 

Physical activity influences the body and brain via several different mechanisms, including effects on body fatness, blood pressure, lipid and lipoprotein metabolism, vascular function, central and peripheral growth factors, inflammation, oxidative stress, and insulin resistance.^22,23 Thus growth hormones or other systemic mechanisms induced as a result of physical activity may be involved in the regulation of the growth of the eyes. Although the influence of physical activity on the development and progression of myopia in children has been studied to some extent, the published data focused on this issue in primary school students are sparse. This cross-sectional study was conducted to examine the prevalence of myopia among elementary school students in Tianjin, China, and to explore the association between physical activity and risk of myopia. 

Methods

Study Population

This was a school-based and cross-sectional study using data from the Tianjin Child and Adolescent Research of Eye. The current study started from August to October in 2022. A cluster sampling method was used for the selection of children. Nine elementary schools were randomly selected from elementary schools in Tianjin, China. All children aged six to 12 years in the selected schools were invited to participate in this study except for children who underwent previous laser refractive surgery or cataract surgery or used low-dose atropine or myopia control spectacles. The study protocol was approved by the Medical Ethics Committee of Tianjin Medical University Eye Hospital, China (No. 2020KY-39) and the Ethical Committee of Tianjin Medical University General Hospital (No. IR2022-YX-087-01) and conducted according to the Declaration of Helsinki. All participants were aware of the research purpose by oral instruction. Application for exemption from informed consent signature has been submitted to and approved by the Medical Ethics Committee of Tianjin Medical University Eye Hospital. 

Data Collection

Students aged 10 to 12 years or their parents/guardians (for students aged six to nine years) were asked to complete a self-administered questionnaire. The information on student name, date of birth, sex (male/female), school region (six central districts/four districts adjacent to the center/suburb), and class were obtained from the selected classes of the school. Physical activity levels were self-reported by respondents. 

Visual acuity test and refraction error measurement All the students in the selected schools were examined for vision and refraction following the standard study protocol by certified opticians who had undergone standardized training. Visual acuity testing and noncycloplegic autorefraction were carried out for both eyes of all participants. The uncorrected visual acuity (UCVA) was tested 5 m away from a standard logarithmic visual acuity E chart, and the results were obtained. UCVA was recorded as the smallest size that the subject can identify in all four directions. The test was conducted monocularly under room light during the daytime, starting with the oculus dexter (OD) while the oculus sinister (OS) was occluded with a noncontact black spoon-shaped eye occlude from a distance of 5 m. When the OD test was finished, the OS was tested after five to 10 seconds of break time. The break was given to each child for the OS to recover from the previous occlusion. Acuity was tested with and without refractive correction for those wearing spectacles. The results were recorded as corrected visual acuity for the participants who wore contact lenses or orthokeratology lenses. 

Noncycloplegic refractive error was measured using the Tianle RM-9600 noncycloplegic autorefractor (Shanghai, China). Three readings of refractive error were taken from each eye, and their average was entered for analysis. Each participant was re-examined if the differences between any two of the three results obtained greater than 0.50 diopters (D). 

Noncycloplegic autorefraction tests were carried out to record the sphere and the cylinder. The spherical equivalent (SE) was calculated according to the standard formula of the algebraic sum of the dioptric powers of sphere and half of the cylinder (sphere + 0.5 × cylinder). In our study, myopia was defined as an SE refraction ≤ −0.50D and a UCVA <5.0 in either eye.²⁴ 

Physical Activity Questionnaire

Participants completed the Physical Activity Questionnaire for Children (PAQ-C). The PAQ-C is a self-administered, seven-day recall questionnaire that assesses participation in different physical activities, as well as activity during physical education, lunch break, recess, after school, in the evenings, and at weekends.²⁵ The PAQ-C has acceptable reliability and convergent validity.^26,27 The PAQ-C is recommended for individuals who could be between six to 14 years old depending on the assessment data during the school year. Each item is scored according to a five-point scale (1–5), with “1” indicating low and “5” a high level of physical activity. Once a value from 1 to 5 for each of the nine items (items 1 to 9) used in the PA composite score is obtained, the mean of these nine items is taken, which results in the final PAQ-C activity summary score. The average score denotes the PAQ score. A high score indicates higher levels of physical activity. The following grades are classified according to the summary score; grade 1 (first quartile: 0 ≤ physical activity level < 1.25); grade 2 (second quartile: 1.25 ≤ physical activity level < 2.5); grade 3 (third quartile: 2.5 ≤ physical activity level < 3.75); grade 4 (fourth quartile: 3.75 ≤ physical activity level < 5).²⁵ 

Statistical Analysis

All statistical analyses were performed by the Statistical Package for Social Science (SPSS) version 25.0 (IBM Corp., Armonk, NY, USA) and Origin 2022 version software for the graph drawing. The continuous variables were described as the mean ± standard deviation, and the categorical variables were presented as n(%). The continuous variables were first tested for normality, and those that did not conform to the normal distribution were analyzed by nonparametric tests. One-way analysis of variance was used to compare the differences among various physical activity groups. The χ² test was applied to compare the distribution of categorical data. Binary logistic regression was used to investigate the association between physical activity and myopia. The odds ratio (OR) and 95% confidence interval (95% CI) were used to describe the strength of the association. All statistical analyses were two-sided, and P < 0.05 was considered statistically significant. 

Results

Characteristics of Study Participants

A total of 3767 children aged six to 12 years were eligible, and 2976 (79.00% participation rate) completed all examinations and were included in this study. Among all children, 1575 had myopia; the overall prevalence of myopia was 52.92% (95% CI, 48.99%–56.20%). The students were stratified into two groups: myopia group and nonmyopia group. There were significant differences between the two groups in terms of age, sex, school region, UCVA, SE, and almost all dimensions of physical activity (all P < 0.05). Basic characteristics of the study population were summarized in the Table. 

Table.

 

Basic Characteristics of Participants With and Without Myopia Subjects (Univariate)

 

Changes of Myopia Prevalence by Physical Activity Categories

When stratified according to physical activity, myopia prevalence significantly decreased with increasing physical activity level (χ² trend test = 127.63, P < 0.001), ranging from 58.31% in the first quartile to 49.26% in the fourth quartile. The prevalence in females was significantly higher than that in males (55.62% vs. 50.11%, χ² = 9.617, P = 0.002). In the female group, myopia prevalence significantly decreased with increasing grade (χ² trend test = 1.590, P = 0.662), ranging from 60.02% in the first quartile to 53.83% in the fourth quartile. In the male group, myopia prevalence significantly decreased with increasing grade (χ² trend test = 6.388, P = 0.011), ranging from 53.62% in the first quartile to 41.52% in the fourth quartile (Fig. 1). 

The Association Between Levels of Physical Activity and Myopia

The association between levels of physical activity and myopia is shown in Figure 2. There was a significant association between higher physical activity and lower risk of having myopia (OR = 0.762; 95% CI, 0.675-0.862; P = 0.033). After adjusting for age, sex, and school region compared with physical activity in the first quartile as the reference group, there was a significant association between myopia and physical activity in the second quartile (OR = 0.876; 95% CI, 0.758-0.983; P = 0.023), third quartile (OR = 0.708; 95% CI, 0.535-0.890; P = 0.009), and fourth quartile (OR = 0.676; 95% CI, 0.495-0.849; P = 0.003). 

Figure 1.

 

Trend of myopia prevalence by physical activity categories.

Figure 2.

 

Binary logistic regression analysis of the association between levels of physical activity and myopia.

Figure 3.

 

Association between physical activity and myopia stratified by sex.

Associations Between Physical Activity and Myopia Stratified by Sex

When stratified by sex, whether in the male or female group, in binary logistic regression analysis, a significant statistical association between physical activity and myopia can be found (P < 0.05). In the male group, after adjusting for covariates, compared with physical activity in the first quartile, there were significant statistical associations between myopia and physical activity in the second, third, and fourth quartiles (P for trend < 0.001). In the female group, after adjusting for the same factors, compared with physical activity in the first quartile, there were significant statistical associations between myopia and physical activity in the second, third, and fourth quartiles (P for trend < 0.001) (Fig. 3). 

Discussion

In our study, the overall prevalence of myopia was 52.9% in primary school students aged six to 12 years in Tianjin, China. After adjusting for age, sex, and school region, higher levels of physical activity were associated with a decreased risk of myopia. When stratified by sex, a significant statistical association between physical activity and myopia can be found in both the male and female groups (P < 0.05). 

The current study revealed the prevalence of myopia was 52.92% among primary school students aged six to 12 years. By comparison, the prevalence of myopia reported in this study is higher than those found in other countries, of which myopia prevalence was 11.4% for ages zero to 17 years in Germany,²⁸ 17.5% for ages six to 13 years in Canada,²⁹ 24.8% in age 12 years old in Malaysia,³⁰ and 23.5% for ages seven to 12 years in Korean children³¹ but lower than 69.5% in children aged seven to nine years in Singapore.¹⁸ 

The discrepancies in the prevalence of myopia might be due to differences in patterns of outdoor activities, socioeconomic settings, education levels, urbanization rates, and differences in environmental and genetic factors.^32–34 The overall prevalence of myopia was relatively high in the present study, suggesting that myopia is a serious public health problem among school children in Tianjin. Taken together, these results highlight the emergent need for efforts to control myopia in school-aged children. 

In this study, the association between physical activity and myopia was observed, and physical activity was inversely associated with the risk of myopia, suggesting a protective effect of physical activity on the development and progression of myopia in children aged six to 12 years. The exact mechanisms for the association remain elusive. A possible biological explanation is that specially designed sports activities include the alternation of far-sightedness and near-sightedness, which can replace the conventional regulation function training on children's kinetic visual acuity, uncorrected distance visual acuity, axial length, and accommodative facility, improve the regulatory ability of ciliary muscle, and promote the benign development of dynamic vision. Open sports are usually accompanied by visual tracking of the target object, and closed sports can also integrate the use of the eyes in a reasonable way to achieve full exercise of visual function.^35,36 Physical exercise relieves eye fatigue caused by long-term attention to static objects and improves the overall health level of the body to promote the development of visual function of teenagers and prevent the decline of myopia and other visual functions.³⁷ In terms of activities and participation, regular participation in sports activities is an effective means to prevent myopia. It plays a positive role in improving the myopia of teenagers.^38,39 Physical activities may effectively reduce the probability of becoming myopic in children aged six to 12 years and promote eye health. 

In our study, the prevalence of myopia was consistently higher in females than males in different physical activity categories. The sex disparities in myopia prevalence have also been observed in previous studies.^39–41 We also investigated potential underlying mechanisms for these gender differences in the association between physical activity and myopia. Possibilities for the observed sex differences might be decided by biological factors, outdoor activity time, and a large quantity of near-vision work.^41–44 Large international studies show that girls have better reading scores and attitudes than boys around the world.⁴⁵ Boys spend more time on games and computers than girls.^46–48 The increase in working hours on near work is an important risk factor for myopia, but this effect can be compensated for by increasing outdoor time.²² 

Strengths and Limitations

The strength of the study was the relatively large sample size with a representative sample of school students of all grades and the adjustment for potential confounders allowing us to evaluate the association between physical activity and myopia. However, there are also several limitations. First, the current study is based on a cross-sectional design. No conclusion on causality can be made. Longitudinal studies are needed to clarify a true cause-effect association. Second, physical activity that was mainly based on validated questionnaires and the subjective measures may be less accurate than objective measures. Third, the study is limited in that the risk factors evaluated are minimal, as well as the absence of risk factors such as cognitive skills, amount of near work (reading, writing, television, and computer habits), reduced outdoor time, and parental refractive status in association with myopia because of insufficient information. Last, in consideration of subject compliance and longer examination time, computer-assisted optometry after non-ciliary muscle paralysis has been conducted to screen out children and adolescents who may suffer from myopia. Although our myopia definition was based on the combination of spherical equivalent and visual acuity to minimize the overestimated prevalence of myopia from using noncycloplegic refractive error for defining the myopia, our estimate of myopia can still be biased. 

Conclusions

The prevalence of myopia (52.92%) in children aged six to 12 years is high in China. Higher levels of physical activity were independently associated with decreased risk of myopia. The significant statistical association between physical activity and myopia can be found in both the male and female groups. Taking part in physical activities may be an effective way to reduce the prevalence of myopia. Future longitudinal study is needed to evaluate the progression of myopia in school-aged children and to develop intervention strategies to prevent or slow down the progression of myopia. 

Acknowledgments

The authors appreciate all of participants who enrolled in this study. 

Supported by National Key Research and Development Program of China (Grant Number 2021YFC2401404), National Natural Science Foundation of China (Grant Numbers 82020108007, 81830026). 

Disclosure: F. Ma, None; J. Yang, None; J. Yuan, None; B. Du, None; T. Li, None; Q. Wu, None; J. Yan, None; Y. Zhu, None; X. Meng, None; Y. Liu, None; R. Wei, None; G. Huang, None; H. Yan, None