December 15, 2021
By Noel A. Brennan, MScOptom, PhD, and Mark A. Bullimore, MCOptom, PhD
Outdoors, sunlight, near work, education, screen time, COVID-19… What does science tell us about environmental influences on myopia development?
Stakeholders in the myopia field are eager to identify risk factors for myopia development to aid in reducing the individual and societal burden of myopia. Numerous risk factors, including those mentioned above, have been identified. However, the statistical significance of such associations does not necessarily confer a causal nature. Typically, causality is demonstrated through intervention studies, with controlled, randomized, masked trials being the gold standard. Attributing cause and effect to factors that have not been subjected to this scientific rigor may be attempted through other statistical and logical arguments but should be done with considerable caution.
Genetics Play A Limited Role
A number of studies have linked myopia in children with myopia in their parents.1-4 However, many overestimate the hereditary influence since it is difficult to separate genetic predisposition from family behavioral habits when one or both parents are myopic. Myopic parents may simply be more inclined to indulge in activities that create an environment more conducive to myopia development. Genome-wide association studies show an association with refractive development, but only a modest impact has been demonstrated to date.5-7 Further, it is evident that genetics alone cannot explain the boom of myopia incidence seen around the world in the last 50 years.8
Understanding the Protective Benefits of Outdoor Time
This leads us to the question of environmental influence. Time spent outdoors has been convincingly shown to be a protective factor in the onset of myopia in intervention studies.9-11 There is also an evidence base, albeit weaker, to show that it slows the progression of myopia10 — although not all experts are aligned on this point. Of all options for restricting the extent of myopia development, increasing time spent outdoors prior to myopia onset may be the most effective. Delaying onset by a year in an 8-year-old might remove up to one diopter of final myopia (assuming no later acceleration effect or longer progression when onset is delayed). It may take more than three years of treatment with an optical modality to achieve this degree of myopia control.
Many attribute the influence of time spent outdoors to the exposure to sunlight; however, this has not been directly substantiated. Ngo et al. point to animal studies, arguing that they demonstrate that light level accounts for the protective effects of outdoor activity.12 Flitcroft makes the more nuanced argument that light is an open loop system and retinal light levels are independent of focus. The major difference between indoors and outdoors is the structure of the visual environment. The outdoor visual environment is optically uniform, while the structure of the indoor environment presents a wide range of peripheral defocus — both myopic and hyperopic errors depending on the task being performed.12 Both longer viewing distances outdoors and sunlight are likely to play a role in delaying myopia onset, but the relative contributions are yet to be clearly established. The role of different wavelengths on ocular growth is also an area of interest.13,14 Further research is needed before fully understanding the critical ingredient in outdoor activity.
More Education Predicts Higher Myopia
In many instances, it is virtually impossible or hugely unethical to conduct intervention studies. Mountjoy and coworkers subverted the need for an intervention study in examining the cause and effect between myopia and the genetic tendency toward education by using reverse Mendelian randomization.15 Without going into the details of this analysis, they were able to demonstrate statistically that the number of years of education is predictive of myopic refraction. It is an interesting claim since, assuming relative consistency in the starting age of education across populations, differences in years of education between people must occur at the end of the education period, that is, during high school or post-secondary education years. Yet, differences in the ultimate degree of myopia and population prevalence will be primarily impacted by the age of onset of myopia, with the most affected having onset before the teenage years. The missing link here is that the number of years of education an individual undertakes is likely to be highly correlated with the intensity of educational activity during those early influential years and inversely related to the amount of time spent outdoors.
The Impact of Near Work and Digital Device Use is Less Clear
The evidence that excessive near work leads to myopia onset or progression is less conclusive.16 Certainly, myopes have been found to spend more time at near work and tend to have shorter working distances.17 But, in the absence of intervention studies, cause and effect have not been demonstrated.
Similarly, despite the considerable press being given to screen time and digital devices, a causal role for excessive use of digital devices and screen time on myopia onset and progression is yet to be established.18,19 Indeed, there is reason to argue against such an impact. The timelines of the myopia epidemic and the introduction of handheld digital devices do not match.22 Further, the optical distance across the visual field is inconsistent with the peripheral defocus theory of myopia development.20,21
The COVID-19 Pandemic Has Affected Myopic Progression
Several recent studies have also considered the effect of the COVID-19 pandemic on myopia incidence and progression.22-25 It seems reasonable to deduce from these reports that changes in behaviors associated with COVID-19 lockdowns have had an impact, with higher myopia rates evident in Chinese populations.
So, what is the clinical relevance of the issues discussed here?
There is sufficient evidence to support the following positions:
- Genetics play a limited role in myopia development.
- Spending more time outdoors is protective against myopia onset.
- More education is associated with more myopia.
- The COVID-19 pandemic has most probably increased the incidence of myopia through lifestyle changes.
While associations have been established, there is insufficient evidence to say that:
- Sunlight is responsible for the protective effect of outdoor time.
- Excessive use of digital devices influences myopia development.
- Excessive near work leads to myopia onset or progression.
Note: providing educational needs are met, there is very little downside to sending kids outside more and getting them off their screens.
This article is sponsored by Johnson & Johnson Vision.
- Jones LA, Sinnott LT, Mutti DO, et al. Parental History of Myopia, Sports and Outdoor Activities, and Future Myopia. Invest Ophthalmol Vis Sci 2007;48:3524-32.
- Jones-Jordan LA, Sinnott LT, Manny RE, et al. Early Childhood Refractive Error and Parental History of Myopia as Predictors of Myopia. Invest Ophthalmol Vis Sci 2010;51:115-21.
- O’Donoghue L, Kapetanankis VV, McClelland JF, et al. Risk Factors for Childhood Myopia: Findings from the Nicer Study. Invest Ophthalmol Vis Sci 2015;56:1524-30.
- Wu LJ, You QS, Duan JL, et al. Prevalence and Associated Factors of Myopia in High-School Students in Beijing. PLoS One 2015;10:e0120764.
- Verhoeven VJ, Hysi PG, Wojciechowski R, et al. Genome-Wide Meta-Analyses of Multiancestry Cohorts Identify Multiple New Susceptibility Loci for Refractive Error and Myopia. Nat Genet 2013;45:314-8.
- Huang Y, Kee CS, Hocking PM, et al. A Genome-Wide Association Study for Susceptibility to Visual Experience-Induced Myopia. Invest Ophthalmol Vis Sci 2019;60:559-69.
- Tedja MS, Haarman AEG, Meester-Smoor MA, et al. IMI – Myopia Genetics Report. Invest Ophthalmol Vis Sci 2019;60:M89-M105.
- Morgan IG, French AN, Ashby RS, et al. The Epidemics of Myopia: Aetiology and Prevention. Prog Retin Eye Res 2018;62:134-49.
- Wu PC, Tsai CL, Wu HL, et al. Outdoor Activity During Class Recess Reduces Myopia Onset and Progression in School Children. Ophthalmology 2013;120:1080-5.
- Deng L, Pang Y. Effect of Outdoor Activities in Myopia Control: Meta-Analysis of Clinical Studies. Optom Vis Sci 2019;96:276-82.
- Xiong S, Sankaridurg P, Naduvilath T, et al. Time Spent in Outdoor Activities in Relation to Myopia Prevention and Control: A Meta-Analysis and Systematic Review. Acta Ophthalmol 2017;95:551-66.
- Ngo C, Saw SM, Dharani R, Flitcroft I. Does Sunlight (Bright Lights) Explain the Protective Effects of Outdoor Activity against Myopia? Ophthal Physiol Opt 2013;33:368-72.
- Hung LF, Arumugam B, She Z, et al. Narrow-Band, Long-Wavelength Lighting Promotes Hyperopia and Retards Vision-Induced Myopia in Infant Rhesus Monkeys. Exp Eye Res 2018;176:147-60.
- Gawne TJ, Siegwart JT, Jr., Ward AH, Norton TT. The Wavelength Composition and Temporal Modulation of Ambient Lighting Strongly Affect Refractive Development in Young Tree Shrews. Exp Eye Res 2017;155:75-84.
- Mountjoy E, Davies NM, Plotnikov D, et al. Education and Myopia: Assessing the Direction of Causality by Mendelian Randomisation. BMJ 2018;361:k2022.
- Gajjar S, Ostrin LA. A Systematic Review of near Work and Myopia: Measurement, Relationships, Mechanisms and Clinical Corollaries. Acta Ophthalmol 2021; PAP.
- Wen L, Cao Y, Cheng Q, et al. Objectively Measured near Work, Outdoor Exposure and Myopia in Children. Br J Ophthalmol 2020;104:1542-7.
- Lanca C, Saw SM. The Association between Digital Screen Time and Myopia: A Systematic Review. Ophthal Physiol Opt 2020;40:216-29.
- Foreman J, Salim AT, Praveen A, et al. Association between Digital Smart Device Use and Myopia: A Systematic Review and Meta-Analysis. Lancet Digit Health 2021; S2589-7500(21)00135-7.
- Brennan NA, Cheng X. Commonly Held Beliefs About Myopia That Lack a Robust Evidence Base. Eye Contact Lens. 2019;45:215-225.
- Flitcroft DI. The Complex Interactions of Retinal, Optical and Environmental Factors in Myopia Aetiology. Prog Retin Eye Res 2012;31:622-60.
- Hu Y, Zhao F, Ding X, et al. Rates of Myopia Development in Young Chinese Schoolchildren During the Outbreak of Covid-19. JAMA Ophthalmol 2021;139:1115-1121.
- Ma M, Xiong S, Zhao S, et al. Covid-19 Home Quarantine Accelerated the Progression of Myopia in Children Aged 7 to 12 Years in China. Invest Ophthalmol Vis Sci 2021;62:37.
- Xu L, Ma Y, Yuan J, et al. Covid-19 Quarantine Reveals That Behavioral Changes Have an Effect on Myopia Progression. Ophthalmology 2021;128:1652-4.
- Wang J, Li Y, Musch DC, et al. Progression of Myopia in School-Aged Children after Covid-19 Home Confinement. JAMA Ophthalmol 2021;139:293-300.