Date Published: April 26, 2019
Publisher: Public Library of Science
Author(s): Hongyu Guan, Ning Neil Yu, Huan Wang, Matthew Boswell, Yaojiang Shi, Scott Rozelle, Nathan Congdon, Yingfeng Zheng.
Various types of near work have been suggested to promote the incidence and progression of myopia, while outdoor activity appears to prevent or retard myopia. However, there is a lack of consensus on how to interpret these results and translate them into effective intervention strategies. This study examined the association between visual acuity and time allocated to various activities among school-going children.
Population-based survey of 19,934 students in grade 4 and 5 from 252 randomly selected rural primary schools in Northwest China in September 2012. This survey measured visual acuity and collected self-reported data on time spent outdoors and time spent doing various types of near activities.
Prolonged (>60 minutes/day) computer usage (-0.025 LogMAR units, P = .011) and smartphone usage (-0.041 LogMAR units, P = .001) were significantly associated with greater refractive error, while television viewing and after-school study were not. For time spent outdoors, only time around midday was significantly associated with better uncorrected visual acuity. Compared to children who reported no midday time outdoors, those who spent time outdoors at midday for 31–60 minutes or more than 60 minutes had better uncorrected visual acuity by 0.016 LogMAR units (P = .014) and 0.016 units (P = .042), respectively.
Use of smart phones and computers were associated with declines in children’s vision, while television viewing was not. Statistically significant associations between outdoor time at midday and reduced myopia may support the hypothesis that light intensity plays a role in the protective effects of outdoor time.
Various types of near work[1–4] have been suggested to promote the incidence and progression of myopia[2,5–8], while outdoor activity appears to prevent or retard myopia. However, there is a lack of consensus on how to interpret these results and to translate them into effective intervention strategies[10–14].
Among the 19,934 children in our study (mean age 10.6 +/-1.15 years, 48% girls), the mean LogMAR UCVA was -0.172, equivalent to a Snellen fraction of approximately 6/9. A total of 4939 children (24.8%) failed vision testing, and among them, 4659 (94.3%) underwent refraction. The prevalence of myopia was 18.1% (3607 of 19,934 students).
Our models of UCVA and myopia both point to consistent conclusions that time using computers and smartphones is associated with more myopic refractive error, while television viewing and after-school study are not. Evidence about the impact of smart phone and computer use on myopia has been inconsistent[13,14,23,24]. It may be that, as prevalence of use of these devices among school-age children continues to rise in China and elsewhere, the association is becoming clearer. It should be noted, though, that in the present cohort, daily use of both smartphones and computers was less common than the frequency of television watching and after-school study. Given the greater expected viewing distance for television, our failure to find an association with visual acuity or myopia, as opposed to computers and smartphones (generally used at closer distances), is consistent with the prevailing hypothesis that viewing distance plays a role in the influence of near work on myopia, due perhaps to greater peripheral defocus at closer working distance. We speculate that the lack of an observed association between after-school study time and myopia or decline in vision might be due to afternoon study and tutorial classes, which some children may not have considered as “reading after school”. Once again, evidence on the impact of reading and studying on myopia incidence and progression has been somewhat inconsistent[13,14].