Prevalence and influencing factors of myopia in children: a school-based cross-sectional study in Sanya, China

doi:10.21203/rs.3.rs-4094170/v1

Background: Sanya is the southernmost city in China and has long sunshine duration and strong ultraviolet radiation intensity. The prevalence of myopia among schoolchildren in Sanya has not yet been reported.

Methods: This was a cross-sectional study in Sanya conducted by random cluster sampling from November to December 2022. This school-based study included 8 preschools, 8 primary schools and 8 secondary schools.

Students in the senior class of kindergarten (preschool), grade 3 (primary school) or grade 7 (secondary school), with or without myopia were enrolled. All the schools in each of the districts in Sanya were randomly chosen.

Results: A total of 4074 students were enrolled, and the prevalence of myopia was 28.0%. The prevalence of myopia significantly increased with educational stage: 2.8% in preschools, 21.4% in primary schools and 50.1% in secondary schools. Logistic regression analysis revealed that higher education level (odds ratio (OR)=1.513, 95% CI=1.460-1.567), female sex (OR=1.426, 95% CI=1.220-1.666), one-parent myopia (OR=2.085, 95% CI=1.733-2.509), both parents’ myopia (OR=2.800, 95% CI=2.100-3.734) and outdoor time >2 h (OR=0.720, 95% CI=0.623-0.851) were significantly related to myopia.

Conclusions: Due to its unique environmental conditions, the incidence of myopia among school-age children in Sanya was lower than that in most cities in China. Education and the environment were the two main factors affecting school myopia, and education seemed to play a more important role.

myopia incidence

spherical equivalent error

students

environment

In the past few decades, with the continuous increase in myopia incidence, myopia has become a global public health issue [1, 2]. The consequences of myopia extend beyond inconvenience and financial burden. Severe myopia can lead to serious vision problems, such as retinal detachment, neovascularization, macular degeneration, and even blindness [3, 4]. It is estimated that by 2050, more than 50% of the population in the world will suffer from myopia, with 10% having high myopia[5]. Particularly in Southeast Asia, the overall incidence of myopia is much greater, at approximately 50–80% [6, 7].

Environmental and genetic factors are considered the two major factors associated with myopia [8, 9]. Regarding environmental factors, it is widely recognized that increasing outdoor time can effectively prevent myopia [10–14]. Research has demonstrated a significant dose‒response relationship between increased outdoor light exposure and myopia control[15]. Additionally, higher ultraviolet (UV) radiation intensity may play a role in myopia control by increasing vitamin D levels [16–19].

Sanya is the southernmost city in China and is characterized by its unique geographical location, long sunshine duration and high ultraviolet intensity. There was no previous research on myopia in Sanya; therefore, the purpose of this study was to investigate the prevalence of myopia and the associated factors among school-age children in Sanya.

2.1 Population and sampling

From November 2022 to December 2022, we randomly recruited students from different schools via stratified random cluster sampling. Random numbers were generated via SPSS 25.0; these included 8 preschools (senior class of kindergarten), 8 primary schools (grade 3) and 8 secondary schools (grade 7) from four administrative districts in Sanya. A total of 4,074 students completed the examinations and questionnaire surveys. This study adhered to the principles of the Helsinki Declaration, and all participants provided written consent before the examinations.

2.2 Physical measurements

A standard tumbling E logMAR chart with a distance of 5 m was used to measure the uncorrected visual acuity of all subjects. For those who wear glasses, visual acuity will be tested. The logMAR scores were recorded.

All subjects were measured in the noncycloplegia state with an automatic computer refractometer (KR-1, Topcon, Tokyo, Japan) three times, and the average value was calculated. The spherical refractive error plus 1/2 cylindrical values were recorded as the spherical equivalent power (SER) for analysis.

For myopes, we further used an LENSTAR-900 (Haag Streit AG, Switzerland) to measure ocular biometric parameters such as axial length (AL) and the corneal radius (CR). The average value of three measurements was taken for analysis. As in previous studies, data from the right eye were used to represent individuals for analysis[20].

2.3 Definition of myopia

As suggested by previous studies, noncycloplegia combined with visual acuity can more accurately indicate myopia [24]. Therefore, myopia was defined in this study as a mean spherical equivalent power (SER) ≤ -0.5 D and VA (logMAR) <5.0. Considering that the age of preschool students was 3 to 5 years, the definition of myopia in preschool was a mean spherical equivalent (SER) ≤ -0.5 D and a logMAR < 4.8. According to the severity, myopia was divided into three grades: low myopia (-3.00 D ≤ SER ≤ -0.50 D), moderate myopia (-6.00 D < SER < -3.00 D), and high myopia (≤ -6.00 D).

2.4 Questionnaire

The questionnaire included information on the myopia status of the parents, the duration of electronic work, the duration of sleep, and the duration of outdoor activities. The duration of electronics is divided into two levels,<1 h, and ≥1 h [21]; the duration of sleep is divided into three levels,<9 h,9~10 h, and >10 h [22],and the duration of outdoor activities is divided into two levels,<2 h, and ≥2 h [23]. Parents completed the questionnaire with the assistance of their children.

2.5 Statistical analysis

The statistical analysis was performed using SPSS 25.0 (Chicago, IL, USA). The data are presented as the mean and standard deviation (SD) and as the frequency and percentage, appropriately. We subsequently used the chi-square test to analyze the differences in the prevalence of diabetes among different education levels between boys and girls. We used nonparametric tests (Kruskal‒Wallis tests) to analyze the differences in ocular biological parameters such as AL and CR among patients at different educational stages. In addition, we used binary logistic regression to identify potential risk factors associated with myopia. All P values were calculated two-sided and were considered to be significant when the P value was less than 0.05.

3.1 Subject characteristics

As Table 1 showed a total of 4074 students (aged 4~15 years) were enrolled; these included 1074 preschool, 1359 primary schools, and 1641 secondary schools, with 2249 boys and 1825 girls. The average ages were 5.47±0.51 years in preschool, 8.56±0.52 years in primary school and 12.54±0.53 years in secondary school.

Table 1 The characteristics of the subjects

Characteristic	Preschool	Primary school	Secondary school	P value
Sex.	1074	1359	1641
Boys	605	760	884
Girls	469	599	757
Age (year)	5.47 ± 0.51	8.56 ± 0.52	12.54 ± 0.53	<0.001

3.2 Prevalence of myopia

Table 2 shows that the prevalence of myopia among children was 28.0% in total, 2.8% in preschool, 21.4% in primary school, and 50.1% in secondary school. The difference between the three groups was statistically significant (p<0.001). The prevalence of myopia in preschool girls was lower than that in boys (2.4% vs 3.1%, P=0.433), while that in girls was greater than that in boys in primary school (23.7% vs 19.6%, P=0.067) and was significantly greater than that in boys in secondary school (56.4% vs 44.7%, P<0.001).

Table 2 The prevalence of myopia among children in different grades and by sex

Grades	Prevalence % (n)			P value
Grades	All	Boys	Girls	P value
Preschool	2.8 (30)	3.1 (19)	2.4 (11)	0.433
Primary school	21.4 (291)	19.6 (149)	23.7 (142)	0.067
Secondary school	50.1 (822)	44.7 (395)	56.4 (427)	<0.001

The severity categories of myopia are shown in Table 3. The prevalence of low myopia (18.3%) was relatively greater than that of moderate myopia (8.5%), followed by high myopia (1.2%).

Table 3 The prevalence of myopia in patients in different categories

Grades	Prevalence % (n)				P value
	Non myopia	Low- myopia	Moderate- myopia	High- myopia
Preschool	97.2 (1044)	2.6 (28)	0.2 (2)	0.0 (0)	<0.001
Primary school	78.6 (1068)	17.6 (239)	3.67 (50)	0.14 (2)	<0.001
Secondary school	49.9 (819)	29.2 (479)	18.0 (295)	2.92 (48)	<0.001

3.3 Comparison of ocular biological parameters among myopic patients: SER, AL, and CR

The average SERs were -1.77±0.90 D in preschool, -1.93±1.16 D in primary school and -2.99±1.73 D in secondary school (p<0.001). The average CRs were 43.05±1.44 D in preschool, 43.2±1.55 D in primary school and 42.8±1.56 D in secondary school (p=0.19). The average AL was 24.07±0.74 mm in preschool, 24.31±0.97 mm in primary school and 24.82±1.05 mm in secondary school (p<0.001). More details are shown in Figures 1~3.

The results of binary logistic regression analysis of factors associated with myopia are shown in Table 4. Educational stage (OR=1.513, P<0.001), sex (OR=1.426, P<0.001), one-parent myopia (OR=2.085, P<0.001), both parental myopia (OR=2.800, P<0.001), and a duration of outdoor education ≥2 hours (OR =0.728, P<0.001) were associated with myopia.

Table 4 Potential factors associated with myopia

Variables	OR	95%CI	P value
Educational stage	1.513	1.460, 1.567	＜0.001
Gender
Boys	1
Girls	1.426	1.220, 1.666	＜0.001
Parents myopia
None	1
One	2.085	1.733, 2.509	＜0.001
Both	2.800	2.100, 3.734	＜0.001
Electronics time
<1 h	1
≥1 h	1.118	0.927, 1.347	0.243
Sleep time
＜9 h	1
9~10 h	0.864	0.732, 1.021	0.086
>10 h	0.990	0.612, 1.600	0.966
Outdoor time
<2 h	1
≥2 h	0.728	0.623, 0.851	＜0.001

4.1 Low prevalence of myopia in Sanya

To the best of our knowledge, this is the first study on the prevalence of myopia among school-age children in Sanya. Our findings revealed an overall myopia incidence of 28.0% in this population, with rates of 2.79% in preschool (age: 5.47±0.51 years), 21.4% in primary schools (age: 8.56±0.52 years), and 50.1% in secondary schools (age: 12.54±0.53 years). Notably, the overall myopia incidence in Sanya is significantly lower than that reported in most cities in China. Shi X et al. conducted a comprehensive analysis of the spatiotemporal incidence of myopia in China and revealed an incidence ranging from 23.8% to 52.5% in 1995, which subsequently increased to 39.8% to 66.4% in 2014[25].

The incidence of myopia among preschool, primary school, and secondary school students in Sanya is also lower than that reported in other regions [26-30]. For instance, a study conducted in Yiwu, Zhejiang, in 2020 by Wang et al. revealed a myopia prevalence of 12% in preschool, 36.9% in third-grade primary school, and approximately 63.6% in secondary school. Similarly, a study conducted by Liu et al. in Chengdu in 2019 reported an overall myopia rate of 38.1% among 3- to 14-year-old school-age children, with rates of 8.6% in preschool, 37.2% in third-grade primary school, and 66.2% in first-grade junior high school. In Chongqing, the overall myopia rate was 33.9%, with 24.8% in the third grade and 63.6% in the sixth grade. Moreover, 37.7% of primary school students in Hong Kong exhibited myopia, while in Taiwan, 42.0% of primary school students had myopia.

In addition, our analysis of myopia distribution revealed similar findings to those of other studies, where the majority of students exhibited low to moderate myopia, while high myopia was less common [31]. Although the incidence of high myopia increases with age, it remains lower than that in other regions [32]. To summarize, the overall incidence of myopia and the prevalence of high myopia among school-age children in Sanya are lower than those observed in other regions.

As widely acknowledged, light exposure has been identified as an independent factor in the prevention and control of myopia progression [10-15]. The underlying mechanism for this protective effect is thought to involve the stimulation of dopamine release in the retina, which in turn inhibits the elongation of the ocular axis length, a structural characteristic associated with myopia. This mechanism helps prevent and control the onset and progression of myopia[32-35].

Sanya, the southernmost city in China, experiences an average annual sunshine duration of 2534 hours, which surpasses that of many other Chinese cities, such as Beijing (2414 hours), Shanghai (1629 hours), Guangzhou (1748 hours), Chengdu (1061 hours), and Nanjing (1800 hours) (data sourced from the China Meteorological Administration)[36]. This prolonged sunshine duration is considered one of the potential contributing factors to the decreased incidence of myopia in Sanya. Moreover, previous research has demonstrated that the chromaticity and spectral components of light can impact myopia. The spectral composition of light exerts a significant influence on ocular axis length. For instance, exposure to ultraviolet B (UVB) radiation has been suggested to potentially inhibit myopia development through enhanced vitamin D synthesis. Increased levels of vitamin D resulting from daylight exposure may regulate scleral growth by exerting an antiproliferative effect[16-19]. Sanya exhibits an average UV intensity of 1680-1826 kwh/m2 (Level 4), which surpasses that of many Chinese cities, such as Chengdu (<1095 kwh/m2, Level 1), Shanghai (1095-1387 kwh/m2, Level 2), and Beijing (1387-1680 kwh/m2, Level 3) (data sourced from the China Meteorological Administration)[37]. We posit that prolonged sunshine duration, high light intensity, and strong ultraviolet light intensity in Sanya play a role in the low incidence of myopia.

4.2 Other Factors Associated with Myopia

4.2.1 Education and Myopia

Previous studies have consistently identified education and outdoor time as the two primary factors influencing myopia in school-age children. It has been observed that the prevalence of myopia is generally greater among children attending school than among those not enrolled[38]. Furthermore, there was a clear association between duration of education and incidence of myopia. Studies have consistently shown that as the number of years of education increases, the risk of developing myopia also increases[39-42]. Sanya, similar to other cities, also follows a nine-year compulsory education system. The educational pressure experienced in primary and junior high schools in Sanya is comparable to that in other cities.

In line with the findings of previous research, our study revealed a significant correlation between the prevalence of myopia and the duration of education.

Furthermore, we conducted a comparative analysis of spherical equivalent refraction (SER), corneal radius (CR), and axial length (AL) among students from three different grade levels. Our findings revealed a consistent trend where as age and years of education increased, there was a corresponding increase in the SER and AL.

Notably, in our study, we observed a significant increase in the prevalence of myopia, from 2.8% in preschool to 21.4% in the third grade of elementary school. We believe these findings are associated with early educational pressure and the prevalence of extracurricular tutoring classes in primary school. This finding is consistent with the findings of a cohort study conducted by Li SM et al., which followed school-age children from the first grade to the sixth grade. Their study reported a gradual increase in the incidence of new myopia each year, particularly after the third grade[43].

Furthermore, Wang et al. conducted a study on the progression of myopia after COVID-19 home confinement and observed that students in grades 1 to 2 experienced a more significant myopic shift than did those in grades 3 to 6. These findings suggest that myopia development is critical in children younger than grade 3, typically aged 6 to 8 years, and may be more sensitive to environmental changes[44]. We propose implementing interventions for preventing the development of myopia throughout primary school, with a particular emphasis on the pregrade 3 period. These measures could encompass reducing educational pressure, limiting extracurricular tutoring classes, and promoting increased outdoor time.

4.2.2 Age and myopia

There are many studies on the relationship between age and myopia in school-aged children[27,30]. However, it is important to acknowledge that age always correlates with an increase in years of education. In light of this, the abovementioned study[44] provides a more comprehensive analysis of the impact of age on myopia incidence. This highlights that the critical period of 6 to 8 years is particularly significant for myopia development and is more sensitive to environmental changes.

4.2.3 Gender and Myopia

A comparative analysis was also conducted to examine the prevalence of myopia among boys and girls. The findings revealed that during preschool, the incidence of myopia was lower among girls than among boys. However, in primary and secondary schools, the incidence of myopia among girls gradually surpassed that among boys (23.7% vs 19.6% in primary school; 56.4% vs 44.7% in secondary school). The difference in myopia rates may be attributed to girls spending less time outdoors than boys after entering primary school[43].

4.2.4 Parental refraction status and myopia

Myopia is typically a result of a combination of genetic and environmental factors. Genetic influences are commonly acknowledged to contribute to the development of myopia. To date, approximately 200 genetic loci associated with myopia have been identified[45]. Our study showed that parental myopia is an independent risk factor for myopia. The odds ratio (OR) was found to be 2.08 when one parent was myopic and 2.8 when both parents were myopic. In addition, the living habits and reading habits of myopic parents may influence the eye habits of their children[46].

4.2.5 Daily habits and myopia

In conjunction with the questionnaire, we conducted an analysis of outdoor time, electronic phone time and sleep duration. Our findings align with numerous previous research findings[10-14]. We found that an APD ≥2 hours was an independent protective factor against myopia (OR=0.728, P<0.001). In addition, our study revealed no correlation between myopia and electronic time, which is inconsistent with the findings of previous studies[47,48]. In a recent Japanese study conducted in 2022 focused on myopia among preschool children aged 4 to 6 years, it was found that an average duration of follow-up of at least 1 hour per day was an independent risk factor for myopia progression. A possible explanation is that we included only the electronic time in this questionnaire rather than all nearwork time, such as the duration of homework.

We also analyzed the relationship between sleep duration and myopia. Previous studies have shown inconsistent results [49,50]. Jee reported that sleep duration > 9 hours significantly reduces the risk of myopia (OR=0.59). However, Qu et al. found no correlation between sleep duration and myopia in a study of 1831 students aged 11-18 years[51]; similarly, a 4-year study by Wei SF et al. involving 2893 Chinese primary school students revealed that sleep duration has no relationship with myopia[52]. Our findings are consistent with these findings. Perhaps sleep duration combined with sleep quality or other indicators of sleep quality can better reflect the relationship between sleep and myopia. However, further research is needed to explore these aspects and enhance our understanding of this relationship.

Limitations

There are also several limitations in our study. First, regarding the design of the questionnaire mentioned earlier, it would have been beneficial to include the duration of near work rather than solely focusing on the time spent using electronic products. Furthermore, due to resource constraints and other factors, we were only able to measure ocular biological parameters (such as AL and CR) in students diagnosed with myopia. However, we recognize the need for improvement in our subsequent cohort studies to address these limitations and obtain a more comprehensive dataset.

Our study revealed a lower prevalence of myopia among school-age children in Sanya than in most other cities in China. This difference may be attributed to the unique environmental conditions in Sanya, which include factors such as long daylight duration and strong light intensity.

Despite the advantageous environmental factors in Sanya, as education increases, the prevalence of myopia also tends to increase rapidly. Education seems to have a greater impact on myopia than on the environment.

Efforts should be made to reduce educational pressure in primary school, particularly for children younger than grade 3 (aged 6-8 y), as this period is considered critical for the development of myopia.

Ethics approval and consent to participate

The study was conducted in accordance with the ethical standards of the Declaration of Helsinki and was approved by the Human Ethics Committee of Sanya Maternal and Child Health Hospital. Written informed consent forms were obtained from the children’s parents before the eye examinations.

Consent for publication

Not applicable.

Availability of data and materials

All data generated or analysed during this study are included in this published article.

Competing interest

The authors declare that they have no competing interests.

Funding:

This study was supported in part by Pudong New Area Municipal Health Bureau (PW2020E-3).

Authors` contributions:

Cheng Fang was a major contributor in writing the manuscript. Yao Jiao, Hui Wei and Xiaoyan Wang were responsible for data collection. Ting Qiu and Huijia Zheng analyzed the data. Shijian Liu and Hong Liu revised the manuscript. All authors read and approved the final manuscript.

Acknowledgements:

We appreciated Shijian Liu and Hong Liu for reviseing the manuscript.

Author details:

1 Department of Ophthalmology, Sanya Maternal and Children’s Hospital, managed by Shanghai Children’s Medical Center, School of Medicine, Shanghai Jiao Tong University, Sanya, China

2 Department of Ophthalmology, Fujian Children's Hospital, Fuzhou, China

3 Department of Ophthalmology, Shanghai Children's Medical Center, Shanghai, China

4 Big Data Center, Sanya Maternal and Children’s Hospital, managed by Shanghai Children’s Medical Center, School of Medicine, Shanghai Jiao Tong University, Sanya, China

5 Department of Clinical Epidemiology and Biostatistics, Children Health Advocacy Institute, Shanghai Children’s Medical Center, School of Medicine, Shanghai Jiao Tong University, Shanghai, China.

Corresponding Author:

Xiaoyan Wang, BS, Department of Ophthalmology, Sanya Maternal and Child Health Hospital, No. 339 Yingbin Road, Jiyang District, Sanya City, 572000, China ([email protected]).

Hong Liu^1,3, PhD, Department of Ophthalmology, Shanghai Children's Medical Center, No. 1678, Dongfang Road, Pudong District, Shanghai, 200120, China ([email protected]).

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No competing interests reported.

Prevalence and influencing factors of myopia in children: a school-based cross-sectional study in Sanya, China

Status:

Version 1

Abstract

Figures

1. Introduction

2. Methods

3. Results

4. Discussion

Conclusions

Declarations

References

Additional Declarations

Status:

Version 1