It is now widely accepted that environmental factors play an important role in the development of myopia, and that the myopia epidemic can be attributed to rapid environmental changes over the last century. However more research is needed to fill the gaps in our understanding on the link between environmental factors and myopia so that we can introduce strategies to reduce the risk of myopia development and progression.
Major environmental factors
that have been consistently associated with increased myopia risk include decreased time spent outdoors, higher education, and near work. These environmental factors for myopia are often not independent of one another. For example, children who spend more time studying indoors might tend to do so at the expense of outdoor activities.
every additional hour of time spent outdoors per week results in a 2 per cent decrease in the odds of developing myopia
TIME SPENT OUTDOORS
The evidence for the inverse relationship between time spent outdoors and myopia, though emerged relatively recently, is extensive and undeniable. In a metaanalysis of seven cross-sectional studies, Sherwin et al.1 estimated that every additional hour of time spent outdoors per week results in a 2 per cent decrease in the odds of developing myopia. Studies from Asia-Pacific2-4 have generally reported that non-myopic children tend to spend approximately two to three more hours per week outdoors than myopes. Randomised controlled trials5,6 in East Asia found that intervention programs encouraging children to spend more time outdoors significantly reduced myopia incidence, progression, and axial growth. A metaanalysis7 estimated that this interventional increase in time spent outdoors would reduce myopic progression by -0.3D per year over the course of three years, and the odds of rapid myopia progression (defined as ≥0.5D per year) would be reduced by 54 per cent, compared to having no intervention. These studies demonstrated that increased outdoor time is protective against myopia, rather than myopes having an aversion to being outdoors.
The inverse association between myopia and time spent outdoors begs the question of whether it is the engagement in physical activity, increase in vitamin D, or exposure to high lighting levels that is protective against myopia. In the Singapore cohort study of risk factors for myopia,2 participation in indoor sports activities was inversely not found to be associated with myopia, whereas participation in outdoor activities was. This strongly suggests that physical activity itself is not protective against myopia. Findings from a prospective study4 at the Queensland University of Technology, Brisbane, confirmed that myopes tend to spend less time outdoors than emmetropes, as measured with actigraphy watches, but levels of physical activity were similar between the two groups of individuals. Vitamin D has been independently associated with myopia in several studies; however, a longitudinal study8 found no effect of vitamin D on myopia after adjustment for sun exposure, suggesting that vitamin D may just be a proxy of time spent outdoors. A recent Mendelian randomisation study9 found that having genetic variants that increased natural levels of vitamin D did not alter the risk of myopia. It is therefore unlikely that vitamin D is directly inversely associated with myopia.
Myopia resulting from less time spent outdoors is most likely attributed to reduced lighting levels indoors. This has been demonstrated in animal experiments, where exposing chicks to low lighting levels (500 lux or less) results in longer axial lengths and more myopic refraction compared to those exposed to high lighting levels equivalent to that of outdoor daytime (30,000 lux or more). This relationship appears to be regulated by the neurotransmitter dopamine, which is released in the retina in response to bright light exposure and is known to regulate eye growth. The light/dopamine hypothesis has been confirmed in animal studies in which subconjunctival injections of dopamine agonists inhibited axial elongation induced by form deprivation, while in chicks, the administration of intravitreal dopamine antagonists negated the effects of bright lights, leading to accelerated axial growth.
The light/dopamine hypothesis could partly explain variations in myopia prevalence between urban and rural regions. Children in urban areas may be more likely to spend more time indoors due to the differences in indoor and outdoor facilities available in urban and rural areas.
Cultural difference is also likely to be a factor in the amount of time spent outdoors. For example, children of Malay ethnicity in Singapore spend significantly more time outdoors than those of Chinese descent.2 The Sydney Myopia Study10 also reported that children of European Caucasian descent spend significantly more time outdoors than those of East Asian descent. As we might guess from this information, East Asians, including Chinese people, indeed have the highest myopia prevalence of all the major ethnic groups in Singapore11 and Sydney,10 respectively.
Education, whether measured in terms of duration (years of formal education),12 academic scores,13 or highest level achieved,12 has consistently been found to have a positive association with myopia. However, the direction of causality in the relationship between education and myopia has remained uncertain for a long time. Is myopia a result of spending more time engaging in academic activities, or do myopes just have a tendency to spend more time on near work and studying because of their reduced distance vision? Recently, using Mendelian randomisation, researchers14 in the United Kingdom were able to show that having a genetic predisposition for higher educational attainment increased the risk of myopia, with every additional year spent in formal education resulting in a -0.27D change in refractive state. On the other hand, having a genetic predisposition for myopia was not associated with the level of educational attainment. This was the first study to provide concrete evidence that increased education is a causal factor for myopia.
NEAR WORK AND HYPEROPIC DEFOCUS
While near work has long been thought of as a risk factor for myopia, the epidemiological evidence behind this has been inconsistent. The Sydney Adolescent Vascular and Eye Study3 reported that increased near work was an independent risk factor for myopia in 12-year-old children. However, the link between near work and myopia was not significant in children of 17 years of age.
The underlying mechanism for the association between near work and myopia is also not well understood. The demand for long durations of near focusing is thought to result in hyperopic defocus at the central15 and/or peripheral retina,16 stimulating axial growth. Findings from animal experiments support this hyperopic defocus theory. Based on this concept, in theory, myopic defocus should have the opposite effect, or at least slow down axial growth and minimise myopia progression. Indeed, animal studies have shown that myopic defocus retards axial elongation. Conversely in humans, producing a myopic defocus in children’s eyes by under-correcting their myopia has proven to be counter-productive, speeding up myopia progression instead.17 Yet, if the hyperopic defocus theory is erroneous, why do optical methods of myopia control that are based on this theory, such as multifocal and peripheral defocus modifying lenses, have some evidence of efficacy (albeit small) in slowing myopia progression?
For now, hyperopic defocus during near work remains our best working theory for the link between near work and myopia. Nonetheless, more research is needed to fill our gaps in our understanding of this link, or there might be other previously unknown mechanisms awaiting discovery.
OTHER ENVIRONMENTAL FACTORS
Many other environmental risk factors of myopia have been suggested by researchers, including increased screen time (e.g. television and computers or other personal electronic devices) and living in cities with high population densities. However, the evidence supporting these as risk factors for myopia is weak, with most studies reporting a null relationship.13,18 Furthermore, these postulated risk factors are highly associated with the two established major risk factors: increased education and less time spent outdoors.
Increasingly educated and urbanised populations worldwide are not only inevitable, but may also be regarded as societal progress. However, unintended side effects of this advancement, such as myopia, should be addressed concurrently before the problem escalates beyond control. Since reducing educational attainment is unlikely to be a socially acceptable public health intervention for lowering myopia risk, increasing time spent outdoors may be a useful intervention to tackle the problem of myopia. Such intervention studies are underway in Singapore and China. However, the high incidence of skin cancer in Australia means that any such intervention in Australia would need to be carefully targeted to ensure it does not significantly increase the risk of sunexposure- related diseases.
Dr. Samantha Lee is a Postdoctoral Research Fellow at the Lions Eye Institute in Perth, Western Australia. She completed her PhD in early 2017 at the School of Optometry and Visual Science, Queensland University of Technology in Brisbane under the supervision of Professor Joanne Wood and Dr. Alex Black. Her main research interests include visual impairment, and epidemiology of glaucoma and myopia Mr. Gareth Lingham is an orthoptist at the Lions Eye Institute in Perth, Western Australia. He is currently completing his PhD through the University of Western Australia under the supervision of Professor David Mackey, Dr. Seyhan Yazar, and Professor Robyn Lucas. His current research focusses on environmental determinants of myopia risk in epidemiological studies, particularly time spent outdoors and sun exposure.
- Sherwin JC, Reacher MH, Keogh RH, et al. The Association between Time Spent Outdoors and Myopia in Children and Adolescents: A Systematic Review and Meta- Analysis. Ophthalmology 2012;119:2141-51.
- Dirani M, Tong L, Gazzard G, et al. Outdoor Activity and Myopia in Singapore Teenage Children. Br J Ophthalmol 2009;93:997-1000.
- French AN, Morgan IG, Burlutsky G, et al. Prevalence and 5- to 6-Year Incidence and Progression of Myopia and Hyperopia in Australian Schoolchildren. Ophthalmology 2013;120:1482-91.
- Read SA, Collins MJ, Vincent SJ. Light Exposure and Physical Activity in Myopic and Emmetropic Children. Optom Vis Sci 2014;91:330-41.
- He M, Xiang F, Zeng Y, et al. Effect of Time Spent Outdoors at School on the Development of Myopia among Children in China: A Randomized Clinical Trial. JAMA 2015;314:1142-8.
- Wu PC, Chen CT, Lin KK, et al. Myopia Prevention and Outdoor Light Intensity in a School-Based Cluster Randomized Trial. Ophthalmology 2018;125:1239-50.
- 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.
- Guggenheim JA, Williams C, Northstone K, et al. Does Vitamin D Mediate the Protective Effects of Time Outdoors on Myopia? Findings from a Prospective Birth Cohort. Invest Ophthalmol Vis Sci 2014;55:8550-8.
- Cuellar-Partida G, Lu Y, Kho PF, et al. Assessing the Genetic Predisposition of Education on Myopia: A Mendelian Randomization Study. Genet Epidemiol 2016;40:66-72.
- Rose KA, Morgan IG, Ip J, et al. Outdoor Activity Reduces the Prevalence of Myopia in Children. Ophthalmology 2008;115:1279-85.
- Koh V, Yang A, Saw SM, et al. Differences in Prevalence of Refractive Errors in Young Asian Males in Singapore between 1996-1997 and 2009-2010. Ophthalmic Epidemiol 2014;21:247-55.
- Mirshahi A, Ponto KA, Hoehn R, et al. Myopia and Level of Education: Results from the Gutenberg Health Study. Ophthalmology 2014;121:2047-52.
- Mutti DO, Mitchell GL, Moeschberger ML, et al. Parental Myopia, near Work, School Achievement, and Children’s Refractive Error. Invest Ophthalmol Vis Sci 2002;43:3633-40.
- Mountjoy E, Davies NM, Plotnikov D, et al. Education and Myopia: Assessing the Direction of Causality by Mendelian Randomisation. BMJ 2018;361:k2022.
- Charman WN. Near Vision, Lags of Accommodation and Myopia. Ophthalmic Physiol Opt 1999;19:126-33.
- Charman WN, Radhakrishnan H. Peripheral Refraction and the Development of Refractive Error: A Review. Ophthalmic Physiol Opt 2010;30:321-38.
- Adler D, Millodot M. The Possible Effect of Undercorrection on Myopic Progression in Children. Clin Exp Optom 2006;89:315-21.
- Guo Y, Liu LJ, Xu L, et al. Outdoor Activity and Myopia among Primary Students in Rural and Urban Regions of Beijing. Ophthalmology 2013;120:277-83.