Association of Myopia in Elementary School Students in Jiaojiang District, Taizhou City, China

Background The aim of our study was to evaluate the prevalence of myopia in elementary school students and to assess the risk factors for myopia. Methods This school-based cross-sectional study was performed on students from two elementary schools in Jiaojiang, Taizhou City, China. A total of 556 students, whose age ranged from 9 to 12 years, were included. The uncorrected visual acuity and noncycloplegic refractive error tests were performed to determine the myopia. Each student was asked to fulfill the questionnaire about the possible factors associated with myopia. Multivariate logistic analyses of risk factors were conducted. Results The overall prevalence of myopia among those students was 63.7%, ranged from 53.4% in grade 4 to 72.5% in grade 6. Multivariate logistic analysis showed that adjusting the height of desks and chairs according to the changing height and the presence of myopia in parents were significantly associated with myopia in these students, respectively. Conclusions Our results showed that myopia among elementary school students was associated with environmental and hereditary factors.


Introduction
Myopia, also known as short-sightedness, is one of the leading causes of visual disability that develops primarily during childhood when excessive elongation of the eyes results in blurry distance vision and clear close vision [1]. e increasing prevalence of myopia is a global health and social problem [2]. Researchers have estimated that about 50% of the world's population will be myopic and about 10% will be high myopic by 2050 [3]. e "myopia boom" is particularly prominent in urban areas of East and Southeast Asia, where 80% to 90% of high school graduates have myopia and about 20% have high myopia [4,5]. As the most common visual impairment in children, myopia poses an enormous personal and social burden [6]. Additionally, children with high myopia are at high risk of developing irreversible visual impairment or blindness mostly due to retinal detachment, glaucoma, and myopic macular degeneration [7,8].
Most myopic individuals are associated with excessive axial elongation, and very few occur as a result of disproportionately high corneal power [9]. For adults aged 50 or older, myopia can also be rarely caused by nuclear cataracts [9]. Both environmental and genetic factors impose a significant risk of myopia [10]. e identified genetic variants could explain about 12% of the variance of the refractive error trait [11,12]. Tideman et al. found that different genetic loci were associated with different ages of axial length (AL) and corneal radius (CR) ratio [13]. Among those younger than 10 years, three loci (GJD2, CHRNG, and ZIC2) were associated with AL/CR. In people aged 10 to 25 years, four loci (BMP2, KCNQ5, A2BP1, and CACNA1D) were associated; and in adults (>25 years of age), 20 loci were associated. Environmental factors such as high levels of education, lack of outdoor exposure, and excessive near-work activities are the most established risk factors for myopia [1,5]. A Mendelian randomization study by Mountjoy et al. also showed that more time in education may be a causal risk factor for myopia [14]. Since refractive error correction could not prevent the myopic pathologies, preventing the myopia and particularly high myopia at the early age is of great significance [1,4]. Each year of delay in the age at onset could substantially reduce the chance of developing high myopia in adulthood [15]. With the aim of discovering potentially effective prevention methods during childhood, in this cross-sectional study, we collected children in elementary schools to evaluate the prevalence of myopia in these young populations and assess the protective and risk factors for myopia.

Materials and Methods
Two elementary schools (school A and school B) in Jiaojiang District, Taizhou City, Zhejiang Province, China, were included. Students from grades 4 to 6 were enrolled from September to October 2019. Two or three classes were randomly selected in each grade, and all students in selected classes were enrolled.
Each participant was asked to fulfill the customized questionnaire, including the characteristics of students and possible factors associated with myopia. e uncorrected visual acuity (UCVA) and noncycloplegic refractive error tests were performed by pediatric ophthalmologists from Taizhou Municipal Hospital. e UCVA was tested using the Standard Logarithmic Visual Acuity E Chart, and noncycloplegic refractive error was tested using the RM-800 Auto Refractometer (Topcon Medical Systems, Inc). e UCVA less than 5.0 and spherical equivalent refraction less than −0.50 diopter in at least one eye were used to define the myopia.
Statistical analyses were conducted using the Statistical Package for the Social Sciences (SPSS, version 21.0; IBM, Chicago). e chi-squared tests were used to evaluate the associations between factors and myopia. e parameters with a univariate association were selected as candidate variates for multivariate logistic analysis. e odds ratio (OR) and 95% confidence intervals (CIs) were calculated. A P value of less than 0.05 was considered statistically significant.

Results
A total of 556 students (310 in school A and 246 in school B) were included in this study. e prevalence of myopia was 63.7%, with 64.8% in school A and 62.2% in school B. ere is no statistical difference in the prevalence of myopia between the two schools (P � 0.520). e prevalence of myopia showed statistically different among grade 4, grade 5, and grade 6 in school B (P < 0.001) and total (P � 0.001), respectively.
e average age of students with myopia was higher than those of normal students in school B and total (both P < 0.001). No statistical difference in the proportion of myopia was found between males and females ( Table 1). Table 2 shows the associations between factors studied and the prevalence of myopia in primary school students. e frequencies of changing class seats and adjusting the height of desks and chairs were statistically associated with the presence of myopia (P < 0.05). Sleeping time more than 8 h and the presence of myopia in parents were also found to be associated with the prevalence of myopia (P < 0.05). No other factor showed a univariate association.
After adjusting the age and gender, adjusting the height of desks and chairs according to the changing height and the presence of myopia in parents were still associated with the presence of myopia (all P < 0.05, Table 3). Comparing with never adjusting the height of desks and chairs, adjusting the height of desks and chairs once a year and once a semester in total (OR � 0.37, 95% CI � 0.21-0.67, P � 0.001; OR � 0.60, 95% CI � 0.35-0.97, P � 0.037) and adjusting the height of desks and chairs once a year in school B (OR � 0.26, 95% CI � 0.11-0.62, P � 0.003) were protective factors. Parents having no myopia was a protective factor for myopia in total (OR � 0.51, 95% CI � 0.35-0.74, P < 0.001), school A (OR � 0.56, 95% CI � 0.34-0.93, P � 0.026), and school B (OR � 0.45, 95% CI � 0.25-0.83, P � 0.009), respectively.

Discussion
In this study, we identified that adjusting the height of desks and chairs according to the changing height and the presence of myopia in parents were associated with myopia in elementary school students. e prevalence of myopia in our study was 63.7%, which was similar to the myopia prevalence of 66.5% among      [16]. e prevalence of myopia was found to be positively associated with grade and age. For the intervention of myopia, spectacles and contact lenses are considered as the mainstay to improve distance vision [9]. Pharmacological intervention includes nonselective muscarinic antagonist atropine and the M1 receptor-specific antagonist pirenzepine, which are also used to control myopia [17,18], whereas refractive surgeries including keratorefractive procedures and intraocular procedures are used to correct refractive error [19][20][21]. Although the symptoms of myopia can be alleviated with those management practices, the risk of complications from potentially blinding conditions such as retinal detachments increase with the longer AL associated with high myopia [7,22]. e prevention or delay of myopia by controlling environmental and genetic risk factors at the early age should be the priority for myopia control. Parents having no myopia were identified to be a protective factor for myopia, suggesting hereditary factors may play an important role in myopia. Verhoeven et al. had identified multiple susceptibility loci for refractive error and myopia [11]. Multiple studies have suggested the family history of myopia was significantly associated with myopia [23,24]. In our study, adjusting the height of desks and chairs according to the changing height was also shown to be a protective factor, possibly due to the rapid change of stature in students of this age.
e prevalence and the associations should be interpreted with caution because of the several limitations in this study. First, because of the relatively small sample size, some variates may not show a significant difference between myopic students and normal students, such as outdoor activities. Second, recall bias could exist in this cross-sectional study; hence, a longitudinal cohort trial was needed to further confirm the associations. ird, only two primary schools were included in this study, which led to a selection bias.

Conclusions
Our results showed that the prevalence of myopia among elementary school students was associated with environmental and hereditary factors.

Data Availability
e data used to support the findings of this study are presented in the tables.

Conflicts of Interest
e authors declare that they have no conflicts of interest.