Refractive Errors in University Students in Central China: The Anyang University Students Eye Study

Abstract

Purpose: To assess the prevalence of refractive errors and associated factors in university students in urban areas of Anyang, Central China.

Methods: This is a cross-sectional university-based study of 16- to 26-year-old students in China. Subjects from two universities were invited to undergo a comprehensive eye examination. Cycloplegic refraction was acquired by autorefractor with two drops of 1% cyclopentolate. The prevalence of myopia, high myopia, hyperopia, astigmatism, and anisometropia was calculated. Only data from right eyes were included in analysis.

Results: A total of 7732 eligible subjects were included, with an average age of 20.2 ± 1.4 years. Overall, the mean spherical equivalent (SE) was −2.92 ± 2.48 diopters (D). The prevalence of myopia (SE ≤ −0.50 D), emmetropia, and hyperopia (SE ≥ +0.50 D) was 83.2%, 9.5%, and 7.3%, respectively. Female sex (OR = 1.542; P < 0.001) and science and engineering students (OR = 1.219; P = 0.004) were more likely to be myopic. The prevalence of high myopia, defined using SE ≤ −5.0 D, ≤ −6.0 D, and ≤ −10.0 D, respectively, occurred in 20.2%, 11.1 %, and 0.5%. High myopia (SE ≤ −6.0 D) was statistically associated with female sex (OR = 1.202; P = 0.029) and younger age (OR = 0.896; P = 0.001). The prevalence of astigmatism (cylinder of ≤ −0.75 D) was 28.8%. Astigmatism was associated with male sex (OR = 0.824; P = 0.001) and younger age (OR = 0.925; P = 0.001).

Conclusions: A prevalence of 83.2% for myopia and 11.1% for high myopia (SE ≤ −6.0 D) was found in central Chinese university students. In the future, this generation of university students may encounter long-term, vision-threatening effects, especially pathologic myopia.

Myopia has emerged as a major worldwide public health concern because of its increasing prevalence,¹ role as a major cause of correctable visual impairment,^2,3 and its association with an increased risk of potentially blinding myopic pathologies, especially in patients with high or pathologic myopia.^1,4,5 As epidemiologic data is necessary for reasonable planning of screening and treatment services, a number of population- and school-based studies with comprehensive ocular examinations have been performed.^2,3,6–12 However, most studies have primarily included school-aged children or adults older than 40 years,^2,3,6–12 with few large-scale epidemiologic studies focused on refractive errors prevalent in university students who have higher levels of educational attainment and greater years in schooling. Although there are many studies of military conscripts that provide population prevalence of refractive errors,^13–15 only males were recruited. In addition, there are other population-based and university samples of this age,^16–24 but these studies did not perform cycloplegic refraction or only recruited a small sample size to assess the prevalence of refractive errors as well as spherical equivalent. 

Furthermore, it has been reported that the overall prevalence of myopia in East Asia, including South Korea, Japan, and China, is much higher than that in Western countries.¹ Environmental factors seem to contribute to the pathogenesis of myopia, such as more time spent in near work and less time in outdoor activities.^25–27 Due to early intense educational pressures in the school years in East Asia,²⁸ students may spend more time in sustained near work and less time in outdoor activities compared to students form Western nations, leading to the development of high prevalence of myopia. Many studies have also shown that increased education contributes to the rising prevalence of myopia.^29,30 As university students being highly educated are at particular risk of myopia and studies are needed to define the extent of the problem for this population. Furthermore, future leaders in all sorts of fields will tend to be drawn from this highly myopic group, and thus the problem of later visual impairment and blindness is particularly important. Therefore, it is highly important to perform a well-designed epidemiology study of refractive errors in Chinese university students. 

As far as we know, few large-scale studies on prevalence of refractive errors in mainland China have involved university students, with the exception of one in Shanghai.³¹ Although the sample size was large, the noncycloplegic autorefraction measured in their study may have overestimated the prevalence of myopia, since cycloplegic refraction is suggested as the gold standard for epidemiologic studies of both children and young adults in other studies.^32–36 Although Sanfilippo et al.³⁷ and Krantz et al.³⁸ considered cycloplegia not necessary in epidemiologic studies of refraction in adults, the poor cycloplegic regime of 1% tropicamide used in their studies instead of the rigorous regime of 1% cyclopentolate might explain this erroneous conclusion.³² Besides, cycloplegia by 1% cyclopentolate for those university students was also proved to be necessary in our previous paper, which reported a similar prevalence of myopia based on noncycloplegic refraction with theirs (95.3% vs. 95.5%).³⁹ 

Therefore, this is the first well-designed and large-scale study with comprehensive ophthalmologic examinations and cycloplegic data among university students in mainland China. In this paper, we report the prevalence of refractive errors, as well as the potential impact of demographic factors, with data on other ophthalmic diseases to be presented elsewhere. 

Methods

Study Population

The Anyang University Students Eye Study (AUSES) is a university-based, cross-sectional study of eye diseases among university students in Anyang, Henan Province, located in central China. Students were recruited from the only two public universities in Anyang: Anyang Normal University and Anyang Institute of Technology, which are respectively ranked approximately 460th and 600th out of the 1243 full-time universities in China, according to the Ministry of Education of the People's Republic of China. Both are multidisciplinary universities with undergraduate and graduate degree programs across various disciplines including economics, computing, engineering, foreign languages, chemistry, and art. Anyang Normal University houses 21 colleges with approximately 21,000 undergraduates, while Anyang Institute of Technology houses 13 colleges with over 19,000 undergraduates. Excluding those students who were in their final-year internships, there were approximately 30,000 undergraduates studying on-campus. Enrollment at both the two universities is accessible to a broad range of students throughout mainland China via the national unified college entrance examination. 

Ethics committee approval was obtained from the Institutional Review Board of Beijing Tongren Hospital, Capital Medical University. All participating students gave their informed consent before participating in the study, according to the tenets of the Declaration of Helsinki. 

Sample Size

According to the results of a school-based study that was recently conducted in Donghua University in Shanghai, the prevalence of myopia and high myopia in young Chinese adults were 95.5% and 19.5%, respectively. Thus, assuming a design effect of 2.0, a tolerated error of (0.1 × [incidence of myopia or high myopia]), and a response rate of 80%, we calculated that at least 3965 students should be included in our study (utilizing the equation below). However, since this previous study used noncycloplegic refraction, it might have overestimated the prevalence of myopia and high myopia in young adults, which could have led to an underestimation of the necessary sample size. Thus, we aimed to recruit one-third of all on-campus students at both universities. Using cluster sampling, we included all first-year, second-year, and third-year undergraduates from an array of randomized, selected colleges within both universities in Anyang. Ultimately, we invited 9710 undergraduates to participate in our study; a total of 7971 students underwent our examinations (response rate = 82.1%) from September 2016 to June 2017.  

\(\def\upalpha{\unicode[Times]{x3B1}}\)\(\def\upbeta{\unicode[Times]{x3B2}}\)\(\def\upgamma{\unicode[Times]{x3B3}}\)\(\def\updelta{\unicode[Times]{x3B4}}\)\(\def\upvarepsilon{\unicode[Times]{x3B5}}\)\(\def\upzeta{\unicode[Times]{x3B6}}\)\(\def\upeta{\unicode[Times]{x3B7}}\)\(\def\uptheta{\unicode[Times]{x3B8}}\)\(\def\upiota{\unicode[Times]{x3B9}}\)\(\def\upkappa{\unicode[Times]{x3BA}}\)\(\def\uplambda{\unicode[Times]{x3BB}}\)\(\def\upmu{\unicode[Times]{x3BC}}\)\(\def\upnu{\unicode[Times]{x3BD}}\)\(\def\upxi{\unicode[Times]{x3BE}}\)\(\def\upomicron{\unicode[Times]{x3BF}}\)\(\def\uppi{\unicode[Times]{x3C0}}\)\(\def\uprho{\unicode[Times]{x3C1}}\)\(\def\upsigma{\unicode[Times]{x3C3}}\)\(\def\uptau{\unicode[Times]{x3C4}}\)\(\def\upupsilon{\unicode[Times]{x3C5}}\)\(\def\upphi{\unicode[Times]{x3C6}}\)\(\def\upchi{\unicode[Times]{x3C7}}\)\(\def\uppsy{\unicode[Times]{x3C8}}\)\(\def\upomega{\unicode[Times]{x3C9}}\)\(\def\bialpha{\boldsymbol{\alpha}}\)\(\def\bibeta{\boldsymbol{\beta}}\)\(\def\bigamma{\boldsymbol{\gamma}}\)\(\def\bidelta{\boldsymbol{\delta}}\)\(\def\bivarepsilon{\boldsymbol{\varepsilon}}\)\(\def\bizeta{\boldsymbol{\zeta}}\)\(\def\bieta{\boldsymbol{\eta}}\)\(\def\bitheta{\boldsymbol{\theta}}\)\(\def\biiota{\boldsymbol{\iota}}\)\(\def\bikappa{\boldsymbol{\kappa}}\)\(\def\bilambda{\boldsymbol{\lambda}}\)\(\def\bimu{\boldsymbol{\mu}}\)\(\def\binu{\boldsymbol{\nu}}\)\(\def\bixi{\boldsymbol{\xi}}\)\(\def\biomicron{\boldsymbol{\micron}}\)\(\def\bipi{\boldsymbol{\pi}}\)\(\def\birho{\boldsymbol{\rho}}\)\(\def\bisigma{\boldsymbol{\sigma}}\)\(\def\bitau{\boldsymbol{\tau}}\)\(\def\biupsilon{\boldsymbol{\upsilon}}\)\(\def\biphi{\boldsymbol{\phi}}\)\(\def\bichi{\boldsymbol{\chi}}\)\(\def\bipsy{\boldsymbol{\psy}}\)\(\def\biomega{\boldsymbol{\omega}}\)\(\def\bupalpha{\unicode[Times]{x1D6C2}}\)\(\def\bupbeta{\unicode[Times]{x1D6C3}}\)\(\def\bupgamma{\unicode[Times]{x1D6C4}}\)\(\def\bupdelta{\unicode[Times]{x1D6C5}}\)\(\def\bupepsilon{\unicode[Times]{x1D6C6}}\)\(\def\bupvarepsilon{\unicode[Times]{x1D6DC}}\)\(\def\bupzeta{\unicode[Times]{x1D6C7}}\)\(\def\bupeta{\unicode[Times]{x1D6C8}}\)\(\def\buptheta{\unicode[Times]{x1D6C9}}\)\(\def\bupiota{\unicode[Times]{x1D6CA}}\)\(\def\bupkappa{\unicode[Times]{x1D6CB}}\)\(\def\buplambda{\unicode[Times]{x1D6CC}}\)\(\def\bupmu{\unicode[Times]{x1D6CD}}\)\(\def\bupnu{\unicode[Times]{x1D6CE}}\)\(\def\bupxi{\unicode[Times]{x1D6CF}}\)\(\def\bupomicron{\unicode[Times]{x1D6D0}}\)\(\def\buppi{\unicode[Times]{x1D6D1}}\)\(\def\buprho{\unicode[Times]{x1D6D2}}\)\(\def\bupsigma{\unicode[Times]{x1D6D4}}\)\(\def\buptau{\unicode[Times]{x1D6D5}}\)\(\def\bupupsilon{\unicode[Times]{x1D6D6}}\)\(\def\bupphi{\unicode[Times]{x1D6D7}}\)\(\def\bupchi{\unicode[Times]{x1D6D8}}\)\(\def\buppsy{\unicode[Times]{x1D6D9}}\)\(\def\bupomega{\unicode[Times]{x1D6DA}}\)\(\def\bupvartheta{\unicode[Times]{x1D6DD}}\)\(\def\bGamma{\bf{\Gamma}}\)\(\def\bDelta{\bf{\Delta}}\)\(\def\bTheta{\bf{\Theta}}\)\(\def\bLambda{\bf{\Lambda}}\)\(\def\bXi{\bf{\Xi}}\)\(\def\bPi{\bf{\Pi}}\)\(\def\bSigma{\bf{\Sigma}}\)\(\def\bUpsilon{\bf{\Upsilon}}\)\(\def\bPhi{\bf{\Phi}}\)\(\def\bPsi{\bf{\Psi}}\)\(\def\bOmega{\bf{\Omega}}\)\(\def\iGamma{\unicode[Times]{x1D6E4}}\)\(\def\iDelta{\unicode[Times]{x1D6E5}}\)\(\def\iTheta{\unicode[Times]{x1D6E9}}\)\(\def\iLambda{\unicode[Times]{x1D6EC}}\)\(\def\iXi{\unicode[Times]{x1D6EF}}\)\(\def\iPi{\unicode[Times]{x1D6F1}}\)\(\def\iSigma{\unicode[Times]{x1D6F4}}\)\(\def\iUpsilon{\unicode[Times]{x1D6F6}}\)\(\def\iPhi{\unicode[Times]{x1D6F7}}\)\(\def\iPsi{\unicode[Times]{x1D6F9}}\)\(\def\iOmega{\unicode[Times]{x1D6FA}}\)\(\def\biGamma{\unicode[Times]{x1D71E}}\)\(\def\biDelta{\unicode[Times]{x1D71F}}\)\(\def\biTheta{\unicode[Times]{x1D723}}\)\(\def\biLambda{\unicode[Times]{x1D726}}\)\(\def\biXi{\unicode[Times]{x1D729}}\)\(\def\biPi{\unicode[Times]{x1D72B}}\)\(\def\biSigma{\unicode[Times]{x1D72E}}\)\(\def\biUpsilon{\unicode[Times]{x1D730}}\)\(\def\biPhi{\unicode[Times]{x1D731}}\)\(\def\biPsi{\unicode[Times]{x1D733}}\)\(\def\biOmega{\unicode[Times]{x1D734}}\)\begin{equation}n = {{{{\left( {{Z_{1 - {\rm{\alpha }}/2}}} \right)}^2}P\left( {1 - P} \right)} \over {{d^2}}}*{\rm{deff}}\end{equation}

 

Ocular Examination

Students who participated in AUSES underwent a comprehensive, standardized examination procedure at their respective school clinics, including distant visual acuity, dominant eye, noncycloplegic and cycloplegic refraction, vertometer, intraocular pressure, accommodative response, ocular biometry, digital fundus photography, and optical coherence tomography. Additionally, a detailed questionnaire was required for each participating student. In general, the questionnaires were designed to collect students' information including personal information, age of myopia onset, age of wearing first spectacles, expenditure on myopia prevention or treatment, hours per week spent in near work and outdoor activity, habits of reading and wearing spectacles, visual quality of life, and parents' information such as refractive and socioeconomic status, education and so on. 

During the examination of each subject, one drop of 1% cyclopentolate (Alcon) was administered first, followed by one drop of Mydrin P (Santen Pharmaceutical Co., Ltd., Shiga, Japan) and a second drop of 1% cyclopentolate at 5-minute interval.⁷ Thirty minutes after the last drop, if pupillary light reflex was still present or the pupil size was less than 6.0 mm, a third drop of 1% cyclopentolate was administered and the examination was repeated 15 minutes later. If complete cycloplegia was not achieved after the third drop, subjects did not undergo cycloplegic refraction. Cycloplegic refraction was measured by an autorefractor (HRK7000 A; Huvitz, Gunpo, South Korea) three times consecutively with average data used for analysis. All 3 readings should be at most 0.50 D apart in both the spherical and cylinder components, otherwise the measurements had to be repeated. Data and details of other AUSES examinations are not formally reported in this article. 

Definitions

The spherical equivalent (SE) was calculated by the standard formula of the algebraic sum of the dioptric powers of the sphere and half of the cylinder (sphere + 0.5 × cylinder). The prevalence of refractive errors, utilizing different definitions, was calculated based on cycloplegic refraction data. Myopia was defined as SE ≤ −0.5 D, −0.75 D, and −1.0 D, and mild hyperopia and hyperopia as SE ≥ + 0.5 D and ≥ + 2.0 D, respectively. High myopia was defined as SE ≤ −5.0 D, < −6.0 D or < −10.0 D. Astigmatism was defined as ≤ −0.75 D and ≤ −1.0 D of cylinder, while anisometropia was defined as ≥1.0 D of the SE difference between the two eyes. 

Data Management and Statistical Analysis

All examination data were independently entered twice into a database using commercial software (Epidata software 3.1; The Epidata Association, Odense, Denmark) by two independent individuals. When there were discrepancies between the database entries, the raw data were checked. Statistical software (SAS version 9.4; SAS, Inc., Cary, NC, USA) was used for cleaning, logically checking, merging data, and analysis. Continuous variables were expressed as mean ± SD. We used χ² tests to compare categorical variables between groups. An independent t-test and ANOVA were utilized to compare the mean SE between male and female subjects. A multivariate logistic regression analysis using a stepwise backward method was employed to explore the factors associated with myopia, hyperopia, high myopia, astigmatism, and anisometropia. Age, sex, grade, and discipline prevalence estimates of refractive errors were calculated and analyzed. Unadjusted prevalence ratio was used for analyzing factors associated with refractive errors if its prevalence was high, otherwise, odds ratio was applied. The prevalence ratio of myopia was also calculated as a subsequent step in the analysis. Since large correlation coefficients for cycloplegic SE were observed between the two eyes (r = 0.90), only data from right eyes were included in the analyses. A two-sided P value < 0.05 was considered statistically significant. 

Results

A total of 7971 university students were recruited (response rate 82.1%), while 178 (2.2%) of them did not complete all the examinations because of refusal of cycloplegia. As students aged 16, 25, and 26 years only totaled 61 (0.8%), they were excluded from the study population, leaving 7732 (97.0%) subjects included for this analysis. Table 1 shows a comparison of age, sex, and year level (year at university) between included and excluded subjects, with no significant differences between them for all these demographic parameters (P > 0.05 for all). 

Table 1

 

Comparison of Subjects Included in and Excluded From Data Analyses by Age, Sex, and Year Level

 

The respective mean SE values for all subjects, male subjects, and female subjects were −2.92 ± 2.48 D (median, −2.88; range, −14.50 to +9.63 D), −2.71 ± 2.50 D (median, −2.63; range, −14.38 to +9.63 D), and −3.04 ± 2.45 D (median, −3.00; range, −14.50 to +9.63 D; Fig. 1); male subjects demonstrated a significantly less myopic mean SE than female subjects (P < 0.001). After division by age, female subjects were more myopic than male subjects at all the same ages (Fig. 2). 

Figure 1

 

Distribution of refractive errors, in diopters, for (A) all subjects, (B) male subjects, and (C) female subjects from the Anyang University Students Eye Study.

Figure 2

 

Distribution of mean SE refractive error and 95% confidence intervals, stratified by age and sex, of subjects in the Anyang University Students Eye Study.

The overall prevalence of myopia, defined as SE ≤ −0.5 D, −0.75 D, and −1.0 D, was 83.2% (95% CI, 82.4–84.0), 80.1% (95% CI, 79.2–81.0), and 77.0% (95% CI, 76.0–77.9), respectively (Table 2). As shown in Table 2, females had a significantly greater prevalence of myopia (85.1% vs. 80.0%, P < 0.001), relative to males. In our multiple logistic regression analysis, female sex (OR, 1.542; 95% CI,1.350–1.762; P < 0.001) and science and engineering students (OR, 1.219; 95% CI, 1.067–1.394) were associated with higher risk of myopia (SE ≤ −0.5 D), while age (P = 0.108) and year level (P = 0.641) were not significantly associated with risk of myopia. The prevalence ratio of myopia was also calculated as a subsequent step in the analysis. For sex, the prevalence ratio is 1.06 (85.1/80.0), indicating females are more myopic than males. For discipline, the prevalence ratio is 1.003 (83.3/83.1), indicating science and engineering students are more myopic than liberal art students. The prevalence of mild hyperopia (≥ +0.5 D and < +2.0 D) and hyperopia (≥ +2.0 D) were 6.3% (95% CI, 5.8–6.9) and 0.9% (95% CI, 0.7–1.2; Table 3). Female sex (OR, 0.675; 95% CI, 0.557–0.817; P < 0.001) and science and engineering students (OR, 0.772; 95% CI, 0.636–0.937; P = 0.009) were less likely to have mild hyperopia, while age (P = 0.371) and year level (P = 0.694) were unassociated with a change in risk of hyperopia. 

Table 2

 

Prevalence (With 95% CI) of Myopia Stratified by Age, Sex, Year Level, and Discipline in the Anyang University Students Eye Study

 

Table 3

 

Prevalence (With 95% Confidence Interval) of Hyperopia, Astigmatism, and Anisometropia Stratified by Age, Sex, Year Level, and Discipline in the Anyang University Students Eye Study

 

High myopia defined as SE ≤ −5.0 D, ≤ −6.0 D, and ≤ −10.0 D, respectively, occurred in 20.2% (95% CI, 19.3–21.1), 11.1% (95% CI, 10.4–11.8), and 0.5% (95% CI, 0.3–0.6) of the subjects (Table 2). High myopia (SE ≤ −6.0 D) was significantly associated with female sex (OR, 1.202; 95% CI: 1.019–1.419; P = 0.029) and younger age (OR, 0.896; 95% CI: 0.840–0.957; P = 0.001), reflecting a lower prevalence of high myopia with increasing age in our subjects aged 17 to 24 years. There was no association of high myopia with year level (P = 0.058) and discipline (P = 0.244). 

The mean astigmatic error of all included students was −0.53 ± 0.54 D (median, −0.50; range, −6.75 to 0; Fig. 3). The overall prevalence of astigmatism, defined as cylinder ≤ −0.75 D and ≤ −1.0 D, was 28.8% (95% CI: 27.8–29.8) and 16.5% (95% CI: 15.6–17.3; Table 3). In multiple logistic regression analysis, female sex was found to be less likely to have astigmatism (OR, 0.824; 95% CI: 0.738–0.921; P = 0.001) and those of younger age were more likely to have astigmatism (OR, 0.925; 95% CI: 0.885–0.967; P = 0.001), while year level (P = 0.129) and discipline (P = 0.437) were not associated with risk of astigmatism (cylinder ≤ −0.75 D). 

Figure 3

 

Distribution of refractive astigmatism, in diopters, of subjects in the Anyang University Students Eye Study.

The distribution of anisometropia is shown in Figure 4, with an overall prevalence of 20.9% (95% CI: 20.0–21.8; Table 3). Anisometropia was associated with female sex (OR, 1.265; 95% CI: 1.114–1.436; P < 0.001), higher-level grades (OR, 1.127; 95% CI: 1.029–1.234; P = 0.010), science and engineering students (OR, 1.141; 95% CI: 1.011–1.289; P = 0.033) and younger age (OR, 0.884; 95% CI: 0.841–0.929; P < 0.001). 

Figure 4

 

Distribution of anisometropia, in diopters, of subjects in the Anyang University Students Eye Study.

Discussion

To our knowledge, this study provides the first large-scale, cycloplegic refraction data on the prevalence of refractive errors in university students in mainland China. The prevalence of myopia and high myopia was high in these university students. The mean SE in these students was −2.92 D; myopia (SE ≤ −0.50 D) and high myopia (SE ≤ −6.0 D) occurred in 83.2% and 11.1% of the subjects respectively. Besides, astigmatism and anisometropia was present in 28.8% and 20.9% of subjects, respectively. 

As compared to another university-based study conducted in East China, the respective prevalence of myopia and high myopia were reported as 95.5% and 19.5%,³¹ much higher than ours. Although it is possible that the difference in school rankings may contribute to this difference, we suspect it is mostly caused by the noncycloplegic refraction used in their study. Many studies have demonstrated that noncycloplegic refraction may overestimate the prevalence of myopia.^32,35,40,41 Furthermore, in our previous study comparing the differences between cycloplegic and noncycloplegic refraction among these university students, a similar prevalence of 95.3% was found if myopia was defined by noncycloplegic refraction, while it decreased to 83.2% with myopia defined by cycloplegic refraction.³⁹ Thus, the prevalence of myopia and high myopia seems to be overestimated in their study. The prevalence of myopia in our study was higher than that reported in university students from Europe and the United States,^16–22 while consistent with prevalence reported in other studies in Asia, such as those in Taiwan,²³ and Singapore (Table 4).²⁴ We hypothesize that this may due to differences in the educational system between Eastern and Western cultures, which have been widely reported.^28,42 Eastern students are likely to spend more time on their studies, leaving little time for outdoor activities and making them more susceptible to myopia.^28,43 

Table 4

 

Prevalence of Refractive Errors in University Students Both the Anyang University Students Eye Study and Previous Reports

 

The major finding in our study was that the prevalence of myopia in our group of Chinese university students was extraordinarily high. It's comparable with previous school-based investigations in mainland China showing a notable increase in prevalent myopia in the younger generation, such as in grade 1 (3.9%) and grade 7 (67.3%) in the Anyang Childhood Eye Study,⁶ as well as in grade 9 (52.7%) in a rural area of southern China.³ It is suggested that higher educational attainment and greater years of schooling may have contributed to the higher prevalence of myopia, which has also been broadly reported.^23,44,45 Moreover, the current prevalence of myopia was also much higher than the studies of older populations, such as the Handan Eye Study (26.7% in individuals aged older than 30 years)⁹ and the Beijing Eye Study (22.9% in individuals aged older than 40 years).¹⁰ We hypothesize that this may be due to the development of particularly intensive mass education systems in the recent 4 decades in China. It has been reported that the prevalence of myopia has rapidly increased, associating with rapid changes in access to education in East Asia.²⁸ Increasing the prevalence of myopia in young adults may increase potential future visual disability, especially the sight-threatening complications associated with high myopia. 

As high myopia may be closely related to certain complications, the prevalence of high myopia is important. The prevalence of high myopia (SE ≤ −6.0 D) was 11.1% in our present study. This is much higher than the prevalence in studies of older populations in East Asia, including the Handan Eye Study (1.3% in those aged older than 30 years),⁹ the Beijing Eye Study (2.6% in those aged older than 40 years)¹⁰ and the Singapore Epidemiology of Eye Disease Study (8.4% in those aged older than 40 years).¹² High myopia was defined as SE < −5.0 D in the Singapore Epidemiology of Eye Disease Study, while it was defined as SE < −6.0 D for the other two studies. However, some studies have reported that the prevalence of high myopia is >10.0% in the younger population in Asia,^13–15,23 consistent with our data. Upon evaluating those prior studies, an elevated prevalence of high myopia has been documented in the Asian younger generation. Since high myopia can lead to vision-threatening ocular disorders, including cataract, glaucoma, retinal detachment, and myopic macular degeneration,⁵ those who are at high risk of its development should be closely followed. 

In the present study, female sex was associated with greater myopia risk, agreeing with previous studies.^2,46,47 It is possible that females spent more time on near work, lectures, doing practical near work, and less time on outdoor activities, as other studies reported.^48–50 Previous studies have showed that the risk of myopia increased with age,^2,46,51 but we did not find the association. We hypothesized that this is due to the narrow age range and cohort differences are hard to pick up over short time frames. Besides, the prevalence of myopia might also be relatively stable over ages in our study. Studies have shown that once myopia occurs in school-aged children,⁵² it progresses quickly until early adulthood, when it slows down.⁵³ 

Astigmatism of at least 0.75 D in the cylinder was found in 28.8% of subjects. The prevalence of astigmatism in our study population was higher than reported in rural Southern China (25.3%)³ and the Shunyi District (9.5%),⁵⁴ but lower than observed in Guangzhou (42.7%)² and Shandong (36.3%)⁵⁵ in mainland China. Anisometropia is a significant risk factor for amblyopia, if untreated in children.⁵⁶ The prevalence of anisometropia was 20.9% in the present study. Our study showed higher prevalence of anisometropia than the Shandong Children Eye Study (7.0%; 4–18 years)⁵⁵ and Shanghai (4.9%; 3–10 years)⁵¹ in mainland China. It is likely that the prevalence of myopia was high in the present study. As reported in many previous studies that a positive association between the prevalence of anisometropia with the myopia prevalence rate.^56–58 

The strengths of our study included a large sample size, a high participation rate, and the use of a standard criterion for the measurement of refractive errors using cycloplegia, as well as the fact that few studies have investigated both refractive errors and other eye diseases in university students in mainland China. Many factors have contributed to the success of the high response rate, including the support of local government officials, the cooperation of teachers and of students' unions in the two universities, the good reputation of our eye center, a series of lectures at each selected college on the necessity of eye examinations and appropriate eye protection, accessible and convenient communication about these examinations, positive feedback among students about the examinations, and convenient examination locations and times. Besides, free postmydriatic sunglasses and reading glasses were provided to alleviate hesitancy in participation caused by concern about side effects related to pupillary dilation and cycloplegia. 

However, our study also had several limitations. First, it was conducted as a university-based investigation rather than a population-based investigation. Thus, the prevalence of myopia and high myopia may be overestimated if we apply these results to Chinese young adults in general, as university students experience higher levels of educational attainment and greater years in schooling than their nonuniversity peers. This study, however, provides the first large-scale, cycloplegic refraction data on the prevalence of refractive errors in university students in mainland China. Second, although students were enrolled from across China and their universities ranked in the middle of all full-time Chinese universities, we only performed a single-center study in central China, rather than a multicenter one. Therefore, our study sample might not be representative of all universities across the whole country. Third, as a cross-sectional study, it was unable to provide any longitudinal information. Therefore, it is uncertain whether the refractive errors change over time in university students. 

In conclusion, this university-based study discovered that myopia and high myopia affected 83.2% and 11.1% of mainland Chinese university students, respectively. Our findings may suggest that female sex was associated with myopia and high myopia. Besides, high-pressure educational systems and exposure to rapid urbanization, accompanied by concomitant changes in lifestyle, might be important risk factors for myopia in China. Since the prevalence of myopia and high myopia was high in the young adults in our study, long-term vision-threatening effects may face the future adult population. 

Acknowledgments

Supported by the Integration, Translation, and Development on Ophthalmic Technology (Jingyiyan 2016-5), the Capital Health Research and Development of Special Project (2016–4–2056), and the Major International (Regional) Joint Research Project of the National Natural Science Foundation of China (81120108007). 

Disclosure: S. Wei, None; Y. Sun, None; S. Li, None; J. Hu, None; X. Yang, None; C. Lin, None; K. Cao, None; J. Du, None; J. Guo, None; H. Li, None; N. Wang, None