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Near Work, Outdoor Activity, and their Association with Refractive Error

Lin, Zhong*; Vasudevan, Balamurali; Jhanji, Vishal; Mao, Guang Yun*; Gao, Tie Ying; Wang, Feng Hua; Rong, Shi Song; Ciuffreda, Kenneth J.§; Liang, Yuan Bo*

doi: 10.1097/OPX.0000000000000219
Original Articles

Purpose To assess the relationship between near work, outdoor activity, and refractive error in schoolchildren in Beijing.

Methods The Beijing Myopia Progression Study is a hospital-based myopia study, in which 386 students from primary (aged 6 to 12 years) and secondary (aged 13 to 17 years) schools in the inner city of Beijing were enrolled. Cycloplegic refraction and a detailed questionnaire probing near, intermediate, and distance visual activities were completed.

Results Three hundred seventy (95.9%) of 386 students with complete cycloplegic autorefraction and myopia questionnaire data were enrolled in this study. Children with more near work time did not exhibit a significantly more myopic refraction in both the primary and secondary school levels after adjusting for the children’s gender, outdoor activity time, and average parental refractive error. A significant association between outdoor activity time (in hours per day) and the children’s spherical equivalent (in diopters) was found in the primary school students (β = 0.27, p = 0.03) but not in the secondary school students (β = 0.04, p = 0.70) after adjusting for similar confounders. The time spent on outdoor sports and outdoor leisure in the primary school students was also significantly associated with the children’s spherical equivalent (β = 0.46, p = 0.04 and β = 0.31, p = 0.02, respectively). Primary school students with more time outdoors exhibited relatively less myopic refraction than their peers (ptrend = 0.0003), but this relation was not demonstrated in the secondary school children (ptrend = 0.53) after adjusting for similar confounders.

Conclusions Higher levels of outdoor activity were associated with less myopic refraction in primary school students in the inner city of Beijing. Near work activity was not found to be associated with refraction at either school level.

*MD, PhD

BSOptom, PhD

MD

§OD, PhD

Clinical & Epidemiological Eye Research Center, The Affiliated Eye Hospital of Wenzhou Medical University, Wenzhou, Zhejiang, China (ZL, GYM, YBL); College of Optometry, Midwestern University, Glendale, Arizona (BV); Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, China (VJ, SSR); School of Environmental Science and Public Health, Wenzhou Medical University, Wenzhou, Zhejiang, China (GYM); Handan Eye Hospital, Handan, Hebei, China (TYG); Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Science Key Lab, Beijing, China (ZL, FHW, YBL); and Department of Biological and Vision Sciences, SUNY College of Optometry, New York, New York (KJC).

Yuan Bo Liang Clinical & Epidemiological Eye Research CenterThe Affiliated Eye Hospital of Wenzhou Medical UniversityNo. 270 West College Rd Wenzhou Zhejiang 325027 China e-mail: yuanboliang@gmail.com

Myopia in schoolchildren is a critical public health problem in Asian countries.1–5 The prevalence of myopia in the Asian adult (40 years or older) population is approximately 20 to 30%.6–9 In contrast, recent studies reported that the prevalence of myopia was 45.7% in high school students in Korea,10 70.3% in 12- to 19-year-old children in Taiwan,11 and 78.4% in 15-year-old children in Guangzhou, China.12 The prevalence of myopia in Asian children is clearly much higher as compared with other parts of the world, such as Europe (20 to 30%)13,14 and Australia (11.9%).15 This steep rise in the prevalence of myopia in Asian countries suggests that rapidly changing environmental factors are predominant in determining the patterns of myopia in the younger generation.16–20

Numerous cross-sectional studies have reported that children and young adults engaged in near work were more myopic than those who spend less time on near work–related activities in kindergarten children,21 schoolchildren,22–25 and military conscripts.26 Also, children with a closer near working distance25 or a crowded living environment27 were reported to have more myopia than their counterparts. This association was further supported by longitudinal studies in whites.28–30 Another important environmental factor, outdoor activity or time spent outdoors, was reported to be less frequent among myopic children.31–34 The effect of outdoor activity was further supported in longitudinal studies that reported a reduced risk for the onset of myopia with more time spent outdoors.35–37 However, there were also cross-sectional studies that have reported lack of association between near work and myopia38–40 or outdoor activity and myopia.39,40 Furthermore, some longitudinal studies also found lack of association between near work and myopia35,36,41,42 or outdoor activity and myopia.41 Thus, the findings are equivocal.

The interaction of near work and outdoor activity in the development and/or progression of myopia in Chinese children is inconclusive. Several studies, including the Strabismus, Amblyopia and Refractive Error in Singaporean children (STARS) and the Xichang Pediatric Refractive Error Study (X-PRES) in a rural area of China, have reported that neither near work nor outdoor activity was associated with myopia and/or myopic progression in either middle school Chinese children or preschool Singaporean Chinese children.39,40 In Taiwan, Wu et al.34 reported that more frequent outdoor activity was associated with a lower prevalence of myopia in rural Chinese primary school children. Whereas, recently, Guo et al.24 reported that less outdoor activity and more indoor studying were associated with myopia and longer ocular axial length in grade 1 primary school students in both urban and rural Beijing. However, only noncycloplegic autorefraction was performed in that study. Thus, in light of the available evidence, it is possible that outdoor activity may play a role for a certain age range of schoolchildren with different near work demands.

The Beijing Myopia Progression Study (BMPS), a 3-year cohort study, has primarily aimed to investigate the possible relation between near work–induced transient myopia (NITM) and permanent myopia in school-aged children in the inner city of Beijing, China.43 The school-aged children of BMPS were recruited from the inner city of Beijing, where children experience tremendous near work loads. In addition, besides the student’s refractive error, the parental refractive error, a significant confounder for the children’s myopia, was assessed.15,40,44,45 A standardized myopia questionnaire, which was developed by the Sydney Myopia Study (SMS) group, was used to acquire the information on near work/outdoor activity, number of books read per week, habitual reading distance, and so on.31 In the current study, the aim was to examine the possible association between near work, outdoor activity, and their interaction, with respect to myopia in schoolchildren in the inner city of Beijing, China.

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METHODS

Subjects

The study design, procedures, and baseline characteristics of BMPS have been reported elsewhere.43 In brief, students from elite primary and secondary schools in Beijing examined in the ophthalmology and optometry clinics were considered for enrollment. The detailed inclusion criteria were (1) children from elite primary (aged 6 to 12 years) and secondary (aged 13 to 17 years) schools in the inner city of Beijing; (2) best-corrected visual acuity of 0.1 LogMAR (log minimum angle of resolution) or better; and (3) willingness to cooperate and return for several scheduled visits. The exclusion criteria were (1) presence of amblyopia and/or strabismus; (2) history of intraocular surgery or penetrating ocular trauma; and (3) severe medical/ocular health problems, for example, severe cardiovascular disease, mental disease, iridocyclitis, and neovascular glaucoma. The parents of these students were also invited to join the study.

The study followed the tenets of the Declaration of Helsinki and was approved by the Beijing Tongren Hospital Ethics Committee. All participants (children and their parents) signed written informed consent.

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Myopia Questionnaire

The myopia questionnaire used in the SMS46 (available at http://www.cvr.org.au/sms.htm) was translated by professionals into Chinese with minor modifications. For very young students who could not read or understand the questionnaire very well (e.g., primary school students in grade 1), help was obtained from their respective parent(s) to complete the questionnaire.

The activities were grouped into near work, intermediate distance, and outdoor activities.31 Average hours spent on near work activity (<50 cm working distance) were summed from questions regarding drawing, homework, reading, and handheld computer use. Intermediate distance activity included television watching, videogame playing, and desktop computer use. Time spent on outdoor activities was based on questions about playing outdoors, family picnics and barbeques, bicycle riding, hiking, and outdoor sports. Time engaged in indoor sport activities was also estimated. Activity levels were graded as low, moderate, and high using population tertiles of the average daily hours spent on these different activities.

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Refractive Error

All students received a cycloplegic autorefraction (Accuref-K9001, Shin Nippon, Japan), whereas the parents received a noncycloplegic autorefraction. Cycloplegic autorefraction was performed 20 minutes after instilling three drops of cyclopentolate 1% (Cyclogyl, Alcon). Three readings were obtained in each eye and averaged. This information was used to determine the distance refractive error.

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Data Analysis

All analyses were conducted using only complete data sets from the children. Spherical equivalent (SE) refractive error was calculated as a sphere + 1/2 cylinder. The SEs of the right and left eyes were highly correlated; the Pearson correlation coefficients of the SE were 0.95, 0.96, and 0.93 for the children, fathers, and mothers, respectively. Therefore, for simplicity, only data for the right eyes were reported. Average parental refractive error was defined as the mean of the noncycloplegic SE of the father and mother combined (right eyes) of each. Myopes were classified as having an SE less than or equal to −0.5 diopters (D); emmetropes were classified as −0.50 < SE ≤ +0.50 D; and hyperopes were classified as SE > +0.50 D.43 Activity hours were first analyzed as the average daily hours of activity reported, and then stratified by school level and placed into population tertiles. The association between SE and near work activity levels was assessed after adjusting for the children’s age, gender, average parental refractive error, and outdoor activity time. The association between SE and outdoor activity levels was also assessed after adjusting for the children’s age, gender, average parental refractive error, and near work time. The joint effect of near work and outdoor activity level for the SE was assessed using general linear models after adjusting for the children’s age, gender, and average parental refractive error. Statistical analysis was performed using the Statistical Analysis System for Windows version 9.1.3 (SAS Inc., Cary, NC).

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RESULTS

The 370 (95.9%) of 386 students with complete cycloplegic autorefraction and myopia questionnaire data were analyzed (95.2% [200/210] and 96.6% [170/176]) in the primary and secondary schools, respectively. There were 200 (54.1%) primary school students and 170 (45.9%) secondary school students, aged 8.4 ± 1.2 and 14.2 ± 1.6 years, respectively. The median (low quartile, upper quartile) refractive error was −0.50 (−1.94, 0.88) and −2.75 (−4.25, −1.50) D, respectively. Overall, 172 (46.5%) boys and 198 (53.5%) girls were included, of which 252 (68.1%) were myopes, 35 (9.5%) were emmetropes, and 83 (22.4%) were hyperopes. There were 59 (16.0%), 144 (38.9%), and 167 (45.1%) students with none, one, or both myopic parents, respectively (Table 1).

TABLE 1

TABLE 1

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Near Work Activity

As shown in Table 1, the time spent on near work activities in the overall combined students was 4.04 ± 1.71 (range, 0.36 to 10.29) h/d. Secondary school students spent significantly longer time on near work activities per day than primary school students (∼68 minutes more, p < 0.001). There was no significant difference in the amount of time spent on near work between boys and girls (3.88 vs. 4.18 h/d, p = 0.09). Similarly, there was no significant difference in the amount of time spent on near work among students with both, one, or no myopic parents (4.15, 3.98, and 3.87 h/d, p = 0.49).

When stratifying by school level (primary and secondary), no significant associations between the time spent on near work (in hours per day) and the children’s SE (in diopters) were found in either school level after adjusting for the children’s gender, average parental refractive error, and time spent on outdoor activity (β = −0.03, p = 0.77 and β = −0.01, p = 0.93, respectively). When the total near work time was separated into the time spent on handheld computer use, study and homework, and reading for fun, no significant associations were found in the primary (β = 0.16, p = 0.60; β = 0.05, p = 0.77; and β = −0.16, p = 0.55, respectively) and secondary (β = 0.33, p = 0.18; β = −0.19, p = 0.11; and β = 0.16, p = 0.43, respectively) school children.

Children with a high level of near work time did not exhibit significantly more myopic refraction than children with moderate and low levels of near work time in both the primary and secondary levels after adjusting for the children’s age, gender, average parental refractive error, and time spent on outdoor activity (ptrend = 0.94 and ptrend = 0.63, respectively), as shown in Table 2.

TABLE 2

TABLE 2

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Outdoor Activity

Table 1 presents the time spent on outdoor activities in the overall combined students, which was 1.87 ± 1.25 (range, 0.05 to 6.29) h/d. There was no significant difference in the amount of time spent on outdoor activity between boys and girls (1.94 vs. 1.82 h/d, p = 0.35). Similarly, no significant difference was observed in the amount of time spent on outdoor activity between primary and secondary school students (1.97 vs. 1.76 h/d, p = 0.12).

When stratifying by school level (primary and secondary), a significant association between the time spent on outdoor activity (in hours per day) and the children’s SE (in diopters) was found in the primary school students (β = 0.27, p = 0.03), but not in the secondary school students (β = 0.04, p = 0.70), after adjusting for the children’s gender, average parental refractive error, and time spent on near work. When the total outdoor activity time was divided into outdoor sports and outdoor leisure, the time spent on outdoor sports and outdoor leisure in the primary school students was significantly associated with the children’s SE (β = 0.46, p = 0.04 and β = 0.31, p = 0.02, respectively).

Children with a high level of outdoor time exhibited significantly less myopic refraction than children with moderate and lower levels of outdoor time in the primary school level (ptrend = 0.005) but not in the secondary school level (ptrend = 0.16). This trend in the primary school level was still significant after adjusting for the children’s age, gender, average parental refractive error, and time spent on near work (ptrend = 0.0003), as shown in Table 3.

TABLE 3

TABLE 3

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Intermediate Distance Activity and Indoor Sports

The intermediate distance activities conducted in the overall combined students was 1.68 ± 1.39 (range, 0.14 to 7.93) h/d; it was 1.43 and 1.97 h/d in the primary and secondary school students, respectively. However, the mean SE (in diopters) was not associated with either the intermediate distance activity time (in hours per day) (β = 0.07, p = 0.41) or levels of these activities (β = −0.08, p = 0.44).

The time students spent on indoor sports was 0.19 ± 0.32 (range, 0 to 2.0) h/d; it was 0.20 and 0.17 h/d in the primary and secondary school students, respectively. However, the mean SE (in diopters) was not associated with either indoor sports time (in hours per day) (β = −0.15, p = 0.71) or level of these activities (β = 0.08, p = 0.61).

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Interaction between Near Work and Outdoor Activity

After adjusting for the children’s age, gender, and average parental refractive error, as shown in Table 4, no significant trend toward a more myopic SE was found with increasing levels of near work activity in the low (<1.18 h/d), moderate (1.18 to 2.01 h/d), or high (2.01 to 6.29 h/d) levels of outdoor activity in the combined students. A significant trend toward more hyperopic SE with increasing levels of outdoor activity was found only in the low level of near work activity subgroup (<3.08 h/d) (mean SE, −1.96, −0.37, and −1.10 D; trend p = 0.043). No interaction between near work and outdoor activity was observed (p = 0.21).

TABLE 4

TABLE 4

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DISCUSSION

Although several studies have reported the association and interaction of near work and outdoor activity and permanent myopia among Chinese children, the results have been equivocal. For example, Dirani et al.33 found a positive relation between outdoor activity and myopia in children from the Singapore Cohort Study of the Risk Factors for Myopia (SCORM), with Chinese teenagers having the highest representation. In Taiwan, Wu et al.34 reported that more frequent outdoor activity was associated with a lower prevalence of myopia in rural Chinese primary school children. In a recent Beijing study, Guo et al.24 reported that less outdoor activity and more indoor studying were associated with myopia and a longer ocular axial length in primary level students. In contrast, in X-PRES and STARS, negative results were reported related to both near work and outdoor activity for Chinese (or Singapore Chinese) children.39,40

The current study performed in Beijing is the only investigation that reports on the effect and possible interaction of outdoor activity with near work and myopia in both primary and secondary school students among the Chinese population. Furthermore, in the present study, the actual parental refractive error, one of the important risk factors for the children’s myopia, was obtained directly. It was found that children who spent more time outdoors tended to have a less myopic refraction in the low near work activity subgroup. However, these results were still equivocal because the effects do not follow a clear dose-response; and furthermore, the interaction between near work and time outdoors was not significant. Moreover, from current cross-sectional data, the cause or effect of any particular activity for myopia development was not clear. In the present study, it was also found that the children’s SE was significantly associated with the time spent on outdoor activities (including outdoor sports and outdoor leisure) in the primary school students. Furthermore, children who spent more time on outdoor activities had less myopic refraction in the primary student level after adjusting for the children’s gender, near work time, and average parental refraction. However, this was not observed in the secondary school students. This finding was consistent with three previous Chinese studies. In Taiwan, Wu et al.34 reported that more frequent outdoor activity was associated with a lower prevalence of myopia in rural Chinese schoolchildren aged 7 to 12 years (age range of primary school students in the present study was 6 to 13 years). Very recently, Guo et al.24 also reported that less outdoor activity was associated with myopia in primary school students in grades 1 to 4 (aged 5 to 13 years) in Beijing. In the X-PRES study, Lu et al.39 reported that the time spent on outdoor activities was not associated with myopia in junior middle school children (mean age, 14.6 years; mean age of secondary school students in the present study was 14.2 years). This suggests that outdoor activity was mainly effective in children younger than 13 years.

Near work activity hours were much longer in children living in Beijing compared with any other reported city. In the present hospital-based study, it was found that primary and secondary school students in urban Beijing spent 3.51 and 4.65 h/d, respectively, on near work activities. A school-based study reported that the near work time was 5.3 h/d in grades 1 and 4 students in Beijing.24 Although the near work time here was less than that in the latter study, it is noteworthy that a different activity questionnaire and a different activity definition were used in the present study. Times spent were 2.29 and 2.74 h/d in the 6- and 12-year-old children, respectively, in the SMS,31 approximately 2.3 h/d in the 13- to 17-year-old children in X-PRES,39 less than 1.5 h/d in the 13-year-old children in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error study (CLEERE) and in grades 1 through 8 schoolchildren in the Orinda Longitudinal Study of Myopia (OLSM).36,38

Several studies have suggested that near work may increase the risk for myopia. For example, in a population study in Newfoundland, Canada, the refraction became more myopic by 0.43 and 0.30 D with each hour increase in near work after controlling for age, gender, and education for participants aged 5 to 14 years and 15 to 30 years, respectively.47 In Singapore, children who read more than two books per week had a 1.43 and 3.05 higher odds for low/moderate myopia and high myopia, respectively, compared with children who read less than two books per week22,23 or had axial lengths that were 0.2 mm longer, vitreous chambers that were 0.2 mm deeper, and refractions that were 0.3 D more negative.48 Mavracanas et al.49 reported on 1738 Greek high school students aged 15 to18 years, in which a significantly higher proportion of myopic students studied more than 5 h/d compared with that of nonmyopic ones (43.14 vs. 28.62%, p < 0.001). In the SMS, a population-based study, Ip et al.25 reported that a close reading distance (<30 cm) and continuous reading (>30 minutes) independently increased the odds of having myopia. Recently, Guo et al.24 reported that the prevalence of myopia was associated with study time in primary school students in Greater Beijing. This was further supported in three longitudinal studies in whites.28–30 However, in the present study, the near work activity was not significantly related to myopia. However, this could also represent a “saturation” effect rather than a lack of effect. For example, if one-half hour of near work represented a physiological “saturation limit,” then any value greater than one-half hour, such as 1, 2, or 3 hours, would produce the same effect. Thus, testing should be extended to include small and progressive increments that would demonstrate if such an effect may be present. Lastly, in the present study, no association between intermediate distance activity, indoor sports, and refractive error was found. This was consistent with data from SMS as reported by Rose et al.31

Although there were several important findings in our study, there were some potential limitations. First, the BMPS was hospital based, rather than population based. As a result, the study participants were more likely to have myopia or have higher myopic refraction compared with the other natural population-based studies. The general higher myopic refraction may make the possible effect of near work/outdoor activity difficult to ascertain because it may represent an upper limit refractive saturation effect. Second, the activities were self-reported by the students (or with the help of parents in very young students). Although this method was predominant in previously reported studies, the estimation of activity time could be subject to recall bias. Third, because of the relatively small sample size and considerable response variability, any effect of reducing myopic refraction of outdoor activity among secondary school children, or the effect of aggregating myopic refraction and near work, may not have been observed. Lastly, the cause or effect of a particular activity for myopia development was not clear from the current cross-sectional data.

Yuan Bo Liang

Clinical & Epidemiological Eye Research Center

The Affiliated Eye Hospital of Wenzhou Medical University

No. 270 West College Rd

Wenzhou Zhejiang 325027

China

e-mail: yuanboliang@gmail.com

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ACKNOWLEDGMENTS

The authors thank Hong Jia Zhou (research assistant of The Affiliated Eye Hospital, School of Optometry and Ophthalmology, Wenzhou Medical University), Dr. Xiao Dong Yang, and Dr. Qian Jia (Beijing Tongren Hospital, Capital Medical University) for their invaluable assistance in data collection. The authors also thank Yi Peng, research assistant at the Department of Ophthalmology and Vision Science, The Chinese University of Hong Kong, China, for her statistical help.

This study was supported by the Beijing Science and Technology Novel Star Program (2009B44).

Received October 8, 2013; accepted January 3, 2014.

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Keywords:

near work; outdoor activity; refractive error; myopia; transient myopia

© 2014 American Academy of Optometry