Special Issue
IMI Risk Factors for Myopia
Ian G. Morgan,1,2 Pei-Chang Wu,3,4 Lisa A. Ostrin,5 J. Willem L. Tideman,6–8 Jason C. Yam,9–11
Weizhong Lan,12–15 Rigmor C. Baraas,16 Xiangui He,17–19 Padmaja Sankaridurg,20,21
Seang-Mei Saw,22–24 Amanda N. French,25 Kathryn A. Rose,25 and Jeremy A. Guggenheim26
1
Research School of Biology, Australian National University, Canberra, ACT, Australia
State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-Sen University, Guangzhou, China
3
Department of Ophthalmology, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung, Taiwan
4
Chang Gung University College of Medicine, Kaohsiung, Taiwan
5
College of Optometry, University of Houston, Houston, Texas, United States
6
Department of Ophthalmology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands
7
Department of Epidemiology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands
8
The Generation R Study Group, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands
9
Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, China
10
Hong Kong Eye Hospital, Hong Kong, China
11
Department of Ophthalmology and Visual Sciences, Prince of Wales Hospital, Hong Kong, China
12
Aier School of Ophthalmology, Central South University, Changsha, China
13
Aier School of Optometry, Hubei University of Science and Technology, Xianning, China
14
Aier Institute of Optometry and Vision Science, Aier Eye Hospital Group, Changsha, China
15
Guangzhou Aier Eye Hospital, Jinan University, Guangzhou, China
16
National Centre for Optics, Vision and Eye Care, Faculty of Health and Social Sciences, University of South-Eastern
Norway, Kongsberg, Norway
17
Department of Preventative Ophthalmology, Shanghai Eye Disease Prevention and Treatment Center, Shanghai Eye
Hospital, Shanghai, China
18
Department of Ophthalmology, Shanghai General Hospital, Shanghai Jiao Tong University, Shanghai, China
19
Shanghai Key Laboratory of Ocular Fundus Diseases, National Clinical Research Center for Eye Diseases, Shanghai, China
20
Brien Holden Vision Institute Limited, Sydney, Australia
21
School of Optometry and Vision Science, University of New South Wales, Sydney, Australia
22
Saw Swee Hock School of Public Health, National University of Singapore (NUS), Singapore
23
Singapore Eye Research Institute, Singapore
24
Duke-NUS Medical School, Singapore
25
Discipline of Orthoptics, Graduate School of Health, University of Technology Sydney, Sydney, Australia
26
School of Optometry & Vision Sciences, Cardiff University, Cardiff, United Kingdom
2
Correspondence: Ian G. Morgan,
Research School of Biology,
Australian National University,
Canberra, ACT 2601, Australia;
ian.morgan@anu.edu.au.
Received: December 21, 2020
Accepted: December 24, 2020
Published: April 28, 2021
Citation: Morgan IG, Wu P-C, Ostrin
LA, et al. IMI Risk factors for
myopia. Invest Ophthalmol Vis
Sci. 2021;62(5):3.
https://doi.org/10.1167/iovs.62.5.3
Risk factor analysis provides an important basis for developing interventions for any
condition. In the case of myopia, evidence for a large number of risk factors has been
presented, but they have not been systematically tested for confounding. To be useful
for designing preventive interventions, risk factor analysis ideally needs to be carried
through to demonstration of a causal connection, with a defined mechanism. Statistical
analysis is often complicated by covariation of variables, and demonstration of a causal
relationship between a factor and myopia using Mendelian randomization or in a randomized clinical trial should be aimed for. When strict analysis of this kind is applied, associations between various measures of educational pressure and myopia are consistently
observed. However, associations between more nearwork and more myopia are generally
weak and inconsistent, but have been supported by meta-analysis. Associations between
time outdoors and less myopia are stronger and more consistently observed, including
by meta-analysis. Measurement of nearwork and time outdoors has traditionally been
performed with questionnaires, but is increasingly being pursued with wearable objective devices. A causal link between increased years of education and more myopia has
been confirmed by Mendelian randomization, whereas the protective effect of increased
time outdoors from the development of myopia has been confirmed in randomized clinical trials. Other proposed risk factors need to be tested to see if they modulate these
variables. The evidence linking increased screen time to myopia is weak and inconsistent,
although limitations on screen time are increasingly under consideration as interventions
to control the epidemic of myopia.
Copyright 2021 The Authors
iovs.arvojournals.org | ISSN: 1552-5783
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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1
IMI Risk Factors for Myopia
IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 2
Keywords: myopia, prevention, circadian rhythms, prevalence, diet, risk factors, education, nearwork, time outdoors, screen time, Mendelian randomization, randomized clinical trials
T
here is now an epidemic of myopia in several countries in East and Southeast Asia,1–6 In this part of the
world, the prevalence of myopia in young adults who have
completed 12 to 13 years of schooling is now 70 to 90%, up
from 20 to 30% two or three generations ago. In addition,
the prevalence of high and potentially pathological myopia
in excess of -6D of myopia7 is of the order of 10 to 20%.8–11
Some projections suggest that by the year 2050, nearly 50%
of the world’s population could be myopic, with around 10%
highly myopic.12
Epidemiologists and geneticists2,3,13,14 agree that the
speed with which the prevalence of myopia has increased
in these locations is not compatible with myopia developing
purely or predominantly due to genetic determination. But
this does not mean that genetic factors play no role, and it
has been demonstrated that genetic variation accounts for at
least 12% of the variance in mean spherical equivalent refraction (SER) in populations of European ancestry today,13 and
probably 30% or more.15 The evidence on genetic factors and
myopia has been summarized in another paper in the IMI
series.16 Although gene pools change little between generations, changes in both the natural and the social environment can take place much more rapidly. This emphasizes
the need to define the environmental exposures responsible
for the rapid increases in prevalence of both mild to moderate and high myopia, because modifiable environmental risk
factors provide an important basis for the design of preventive interventions.
Risk factors are most commonly identified by associations with the condition or disease in cross-sectional or
preferably longitudinal cohort studies on defined populations. In cross-sectional designs, the association is with
prevalence of myopia, whereas in longitudinal designs that
define the temporal sequence, the association is with incident myopia. Alternatively, associations with axial length
or changes in axial length can be studied. However, with
“observational” studies of this kind, there is inevitably risk
of confounding due to correlations between a measured
factor, and other, sometimes unmeasured factors that may
mediate the effects. Associations also raise the problem of
reverse causation. For example, in relation to myopia, the
association between less myopia and more time outdoors
could be explained by a protective effect of time outdoors,
or by myopic children having a tendency to spend less time
outdoors. Even with very careful and thoughtful statistical
analysis, it remains impossible to distinguish definitively
between simple correlation and causality, although evidence
for a plausible causal pathway increases the likelihood that
an association is causal, and help to define the direction of
causation.
More rarely, the search for risk factors makes use of
ecological comparisons, where prevalence and risk factor
exposures are compared between different populations. This
sort of design is less commonly used because of what is
known as the ecological fallacy – the false conclusions that
can be drawn from simplistic comparisons between two
locations without knowledge about other risk factors oper-
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ating within the populations. Nevertheless, in combination
with other information on risk factors, ecological comparisons can provide powerful insights.
Some fundamental aspects of study design are important for critical assessment of the literature.
• How myopia is measured and defined is clearly very
important. The gold standard is cycloplegic refraction,7,17 but many studies measure noncycloplegic
refraction, resulting in overestimation of myopia
and misclassification of other refractive categories.
When combined with imprecise estimates of risk
factors, such as nearwork and time outdoors, these
errors can contribute to failure to detect risk factors,
although they are less likely to lead to false positive
identifications. We have tried to cite data based on
cycloplegic refraction, except where no data meets
this standard.
• Reduced visual acuity has also been used as a proxy
measure of myopia, sometimes in combination with
viewing through concave and convex lenses. This
approach is particularly problematic for children of
preschool and early primary age, because of cognitive limits on performance with eye charts. These
less accurate approaches to determination of myopia
tend to be used more commonly in large surveys,
raising the question of whether smaller but methodologically better surveys would be more useful.
• Some of the limitations of these non-gold standard
approaches can be overcome if axial length (AL), or
the corneal radius of curvature (CR) are measured.
These measures are not affected by lack of cycloplegia, and AL and the AL/CR ratio correlate highly with
SER.
• Because age and years of schooling correlate highly
with refraction in most studies of refractive development in children, a study design that uses a large
homogenous sample of children of a given age or
grade, rather than one that uses a similarly large but
more heterogenous sample of children, will generally have greater power in detecting other associations.
In the emerging era of precision medicine, it is important to focus on school myopia, because different etiologies will often mean different approaches to prevention
and clinical control of progression; the risk factors, preventive approaches, and treatment will differ between the axial
myopia that develops in school-age children, and the nonaxial myopia that develops in children with keratoconus, or in
association with cataract in the elderly. Even axial myopia
is etiologically heterogeneous, consisting of a large number
(at least 200–300) of individually rare forms of myopia that
are genetically determined by specific mutations, and much
less affected by environmental factors. These are estimated
to account for myopia in fewer than 1% of any population.3
In some societies, where the total prevalence of myopia is
IMI Risk Factors for Myopia
low, school myopia affects only a small part of the population, but in East Asia and parts of Southeast Asia at the
end of senior high school, around 80% of children may be
affected by myopia.8,10,11,18
Establishing causality is crucial to translating information
about associations into preventive interventions. Identifying
a plausible mechanism linking the risk factor to the control
of eye growth is important. Whether the risk factors identified are likely to be proximal (i.e. close to the relevant
biological pathways that control eye growth, such as exposures to bright light or retinal defocus), or more likely to be
distal factors that influence exposure patterns, such as attitudes to children spending time outdoors, after school, on
the weekends, or during holidays, is an important consideration. Individual and social beliefs about the importance of
education are also important, as are legislative policies that
promote early onset of a highly competitive education.
The ultimate “gold standard” test of causality is a randomized clinical trial, but these are often not possible ethically.
For example, it would not be ethically acceptable to allow
education for some children and not for others on a randomized basis. Fortunately, there are other approaches. Where
there is sufficient information on genetic contributions to
identified risk factors, the technique of Mendelian randomization can be applied.19 There is also a range of social
“experiments” that provide information about causality. For
example, although it would be unethical to give children
different levels of education on a random basis, in most societies, variations in exposures of this kind occur “naturally”
but without randomization, and can provide insight into
causal relationships. Where policy changes that influence
access to education are involved, the technique of regression discontinuity analysis20 can be applied, both qualitatively and quantitatively, to myopia. This approach is particularly powerful when policy changes impose new patterns
of behavior on all children.
In this paper, we review the scientific evidence on risk
factors, taking account the issues discussed above. We have
not considered refraction and biometric parameters as risk
factors for myopia, because myopic shifts in refraction,
increases in axial length, and decreases in lens power occur
as part of the process of the development of an elongated
myopic eye. While this makes them potentially very useful
as predictors of subsequent myopia,21 they are unlikely to
be independent modifiable risk factors. Instead, we focus
on the strength of the evidence for potentially modifiable
risk factors, whether the associations are likely to be directly
causal, or mediated by other risk factors, and whether
the mechanism underlying any causal link is understood.
Where knowledge about risk factors has been translated into
preventive interventions, this will be noted, but the topic
will not be reviewed in detail because it has been covered
in another paper in the IMI series.22
EDUCATION AND TIME OUTDOORS: THE TWO
MAJOR RISK FACTORS FOR SCHOOL MYOPIA
In modern societies, most human myopia appears over the
time during which children attend school, whereas children
who do not go to school rarely become myopic.3 This indicates that it is the experience of the lifestyle of a school-aged
child that leads to myopia. Abolishing school or education is
certainly not an option for preventing myopia, so the problem is to determine which of the many things that change
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IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 3
in a child’s life when they start going to school actually lead
to myopia.
Education
There has been speculation about the role of education in
relation to myopia for at least several hundred years. The
association between education and myopia can be seen at a
number of levels, and has been extensively reviewed.3,23
There are three main lines of evidence:
• Although good historical data are sparse, there
appears to be very little myopia in societies in which
children do not go to school,24,25 and the prevalence of myopia increases in societies as national
education systems develop and more children attend
school and complete more years of schooling.3
• Within a given location or school system, the prevalence of myopia increases as children get older and
complete more schooling. At a given age, children
who are enrolled in more academically oriented
classes or schools, or who achieve higher grades,
tend to be more myopic.26–29 Superior academic
performance appears in children before the onset of
myopia.30 Adults who have completed more years of
schooling or have higher educational qualifications
also tend to be more myopic.31,32
• Ecological studies show that the countries that
currently have an epidemic of myopia stand out in
international comparisons of educational outcomes.
They tend to have a pattern of early onset of educational pressures, with homework starting in the
preschool years and extensive use of tutorial classes
outside of school hours.33
Despite the comprehensive nature of this evidence, the
issue of causality has constantly been debated. Although
some have argued that educational pressures cause myopia,
others have argued that those who are predisposed genetically to myopia might selectively take up educational opportunities. Just as the rapid emergence of an epidemic of
myopia in East and Southeast Asia is difficult to explain in
genetic terms, so the historical pattern of increasing myopia
over the last couple of hundred years as societies have developed school systems is similarly hard to explain in genetic
terms. It is, of course, possible to argue that high selective pressure favoring myopia-predisposing gene variants
has occurred in recent decades, but genetic analysis does
not support this hypothesis.13
The very high prevalence of myopia seen in Israeli
Jewish boys attending Orthodox or Ultra-Orthodox schools,
compared with that in their sisters, or other children receiving more secular education, is also difficult to explain in
genetic terms.34,35 Again, it is possible to postulate that there
is a sex-linked gene variant that predisposes to myopia
segregating at a high frequency in Orthodox or UltraOrthodox Jewish communities in Israel, and indeed there
are examples of rare, sex-linked forms of high myopia.36
With modern molecular genetic techniques it would be relatively straightforward to identify such a gene variant should
it exist; but to date there is no evidence that this is the case.
Further evidence on causality comes from the impact of
policy interventions on the development of myopia. In qualitative terms, these can be seen in the historical patterns of
the development of myopia in parallel with the development
IMI Risk Factors for Myopia
of school systems, and the explosion of myopia in young
adults (from 20–30% prevalence up to 70–80%) that occurred
in mainland China over 20 years after the end of the Cultural
Revolution in 1978, when there was a change in policy that
made academic performance the main criterion for access
to higher education, accompanied by a massive expansion
of enrollments in higher educational institutions.3 Similar
impacts of educational policy changes on myopia have been
documented in Singapore and Taiwan.37,38 The quantitative
technique of regression discontinuity analysis can be applied
to data of this kind, and a recent study has examined the
impact of increasing the mandatory length of schooling on
development of myopia in the United Kingdom.39 This policy
resulted in a marked decrease in mean SER. Overall, policy
innovations that have led to more children experiencing
more intense educational pressures have led to an increase
in the prevalence and severity of myopia, providing strong
evidence of causality.
The classical epidemiological evidence strongly suggests
that education has a causal role in relation to myopia. When
this information is combined with Mendelian randomization
analysis that supports a causal role,32 then the associations
are clearly causal. It should be noted that the Mendelian
randomization study does not mean that years of schooling
or myopia are strongly genetically determined, because the
known genetic variation accounts for only a low percentage of the variance in each trait. The logic is that when a
child’s genetic profile “predisposes” them to undertake more
schooling, then they are more likely to be myopic, whereas
a genetic profile that “predisposes” them to be more myopic
does not lead to them undertaking more schooling.
The mechanism by which this causal link is established
is not clear. It has generally been assumed that reading
and writing (nearwork) that are an integral part of education, provide the link. Many but not all studies have found
associations between nearwork and myopia, and, in general,
the associations have been weak and inconsistent, although
meta-analysis suggests that the effects, while small, are
real.40 In contrast, others have concluded that nearwork
plays little if any role.41 Some studies have suggested that
continuous nearwork or working distance may be more
important than total duration,42 but no randomized trials
have been conducted to evaluate if limiting the amount
of nearwork, limiting continuous periods of nearwork, or
controlling working distance reduces the development of
myopia. Nevertheless, interventions of this kind are often
considered as potential strategies for myopia control. One
possibility that could explain the weak association of nearwork with myopia is that when using imprecise questionnaires, it may be difficult to achieve statistical significance
because the data are too noisy. More quantitative measures
are now becoming available, and this may help to clarify
these issues (see below).
The first specific hypothesis about a more proximal mechanism was that nearwork required more accommodation
that would stimulate eye growth. This hypothesis appeared
to have gained strong support when it was shown that
atropine, a muscarinic antagonist that blocks accommodation, also blocked the development of myopia.43 This line
of research has developed into the effective control of
myopia progression with atropine,44,45 although the current
evidence suggests that the drug may block eye growth by
acting on nonmuscarinic receptors.46,47 A range of other
evidence suggests that accommodation is not involved in
the effects of atropine48–52 This triggered a search for alter-
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IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 4
native mechanisms, although it should be noted that there
is some evidence that accommodation may play some role
in early refractive development.53
Given that animal experiments have shown that imposed
hyperopic defocus stimulated eye growth,50 attention then
shifted to the lag of accommodation and resulting hyperopic defocus that occurs during nearwork. Results of a critical test of this hypothesis, namely whether lag of accommodation develops before or after the onset of myopia, have
been conflicting.54–57 In addition, reports on an association
between accommodative lag and progression of myopia are
also conflicting.58–60
A particular variant of this hypothesis is that the development of peripheral hyperopic defocus, prior to the onset
of myopia, leads to the development of myopia.61 Animal
experiments have shown that destruction of the central
retina does not prevent normal regulation of eye growth,62
demonstrating a role for the peripheral retina, although it
is less certain that the central retina has no role at all.62
The early evidence for the peripheral hyperopic defocus
hypothesis has been contested,63,64 and more recent work
suggests that peripheral hyperopic defocus does not predict
the development of myopia,63,64 but develops after the onset
of myopia in humans.65,66 This would not exclude a role for
peripheral hyperopic defocus in stimulating progression of
myopia.
More recently, Schaeffel and colleagues have suggested
that the use of black print on white paper may have a role.67
This hypothesis was based on evidence that activity in the
retinal OFF-pathway is stimulated by the use of black on
white stimuli. Because activity in the parallel ON-pathway
stimulates dopamine release,68 an increase in relative activity
in the OFF-pathway could lead to increased axial elongation,
given the evidence from animal studies that dopamine acts
as an inhibitor of eye growth.69 However, this interesting
hypothesis has not yet been tested on humans.
In summary, there is a large body of consistent evidence
suggesting that there is a causal association between more
education and more myopia. However, the mechanism
involved is not clear, although the visual tasks of reading
and writing may be contributors. Whereas this association
suggests a wide variety of potential interventions, ranging
from very distal societal interventions to regulate the amount
of homework or to reduce the competitive nature of education pathways, through to interventions to prevent continuous nearwork or increase viewing distance, none has been
validated in controlled trials.
Protection by Time Outdoors
Solid evidence that time outdoors was an important factor
in the development of refractive error only became available over the last 20 years. Before that, there was often
very weak evidence that time outdoors or physical activity was in some way protective from myopia, based generally on the lower prevalence of myopia in rural areas
and in outdoor workers.70,71 Related hypotheses were that
people would have long viewing distances outdoors and
hence use less accommodations, but there was no serious
experimentation in this area. An emphasis on lighting also
developed through the work of Cohn72 who advocated for
improved lighting in schools. This work was very influential in stimulating the development of lighting standards for
schools, but the evidence base for much of this advocacy was
weak, because methods for measuring light intensity and
IMI Risk Factors for Myopia
performing epidemiological surveys were poorly developed
at the time.
A stronger evidence base has been developed more
recently, starting with two seminal papers,26,73 and followed
by evidence from cross-sectional,74 ecological,75 and longitudinal76 studies. Since then, a large body of epidemiological evidence on the protective effects of time outdoors
has been accumulated77 and a recent systematic review and
meta-analysis has confirmed the association.78 Importantly,
increased time outdoors can reduce the impact of parental
myopia76 and higher levels of nearwork.74 The evidence
for causality now includes school-based intervention trials
that have shown that increases in time outdoors of 40 to
80 minutes per day produced significant reductions in incident myopia,79–81 consistent with the expectations from the
epidemiological data.
Rose et al.74 postulated that brighter light outdoors during
daylight hours led to more dopamine release in the retina,
which in turn inhibited axial elongation. This hypothesis has
been supported by animal experiments demonstrating that
bright light inhibits the development of form-deprivation
myopia under laboratory conditions, and that the protective effect involves D2-dopamine receptors in chickens,
monkeys, and tree-shrews.82–84 The effects of bright light
on lens-induced myopia were more limited and inconsistent; in both chickens and monkeys, the final compensation
point was not affected, but in chickens it was approached
at a slower rate, whereas no change in rate was observed in
monkeys.83,85 In contrast, bright light reduced the level of
lens-induced myopia achieved after 28 days of exposure in
tree-shrews.86
One plausible alternative hypothesis was that lower vitamin D levels, naturally observed in children who spend less
time outdoors, play a causal role in relation to myopia. It has
been shown that children or adolescents with myopia often
have lower vitamin D levels.87,88 Myopic subjects also have
less conjunctival ultraviolet autofluorescence (CUVAF)89,90
and a lower prevalence of pterygia,91 both of which are associated with UV exposures. Because of these associations, it
has been suggested that the development of CUVAF might
provide a method for quantifying time outdoors. However,
although this may provide a semiquantitative approach,
CUVAF is not observed before the age of 8 years, it depends
to some extent on skin color, and the kinetics of its development over time are not known.92
Despite these associations, a causal role for vitamin D
has not been supported by more detailed analysis, including Mendelian randomization93 and detailed longitudinal
survival analysis.94 Other hypotheses are that the protective
effects of bright outdoor light on myopia might be due to a
different balance of hyperopic and myopia defocus outdoors
as compared with indoors, or that the greater uniformity of
dioptric power outdoors may be an important factor.95 The
former is plausible in terms of the results of animal experimentation, but there is little evidence for uniformity detection of this kind. More recently, it has been suggested that
the different spatial frequency compositions of indoor and
outdoor scenes may play a role.96 These hypotheses now
need to be assessed more systematically.
The question of causality has been settled with the
randomized intervention trials in children. However, some
issues are still unclear. Initial epidemiological studies were
based on distinctions between time spent outdoors and
indoors, using an operational definition of being outdoors
(during the day) as defined by light intensities over
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IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 5
1000 Lux, based on validation studies. Animal studies
suggest that light intensities considerably higher, at least
10 to 20,000 Lux, might be required to produce significant inhibitory effects on eye growth, but there is suggestive evidence that lower light intensities (2–5,000 Lux)
may be effective in humans.97,98 One intervention trial has
even suggested that modest increases in classroom lighting
strongly inhibit the development of myopia.99 This study
has significant limitations, but requires replication because
of its significant implications for interventions. It would not
be surprising if animal experiments overestimated the light
exposures required for protection in school-aged children,
given that the stimulus for eye growth in the experiments
is strong and constant, whereas signals in humans may be
more intermittent.
It has also been suggested that the timing of the exposures,100 or their frequency,101 may also be important. There
is only limited experimental support for these ideas, and
they have not yet been tested in humans. The type of
lighting102 and parameters, such as spectral composition,103
may also be important. Studies in rhesus monkeys have
shown that rearing in narrowband long wavelength light
promotes hyperopic shifts in refraction and protects from
myopia.104,105 If more subtle spectral variations to lighting
are shown to be effective in preventing myopia, they might
provide the basis for school-based preventive strategies. It
has also been suggested that exposures to violet light may
be important for the prevention of myopia,106,107 but more
follow-up work is required. Interventions of this kind may
be particularly important if myopia prevention needs to rely
on artificial light sources.
There is also controversy over whether increased time
outdoors reduces progression as well as the onset of myopia.
The initial epidemiology did not support this possibility108 and a recent meta-analysis reached the same conclusion.78 However, there is strong evidence that the rate
of progression can be regulated, because seasonal differences in progression have been documented, with progression slower in summer than in winter. This suggests that
progression may be regulated by environmental factors, and
in a way that is generally consistent with the effects of
nearwork and time outdoors.109–112 Some epidemiological
reports have suggested that more time outdoors does slow
progression,73,98,113 and more definitive work in this area is
required.
Hagen et al.114 have raised the question of whether
controls over the development of myopia are compromised
at extreme latitudes, where hours of light are limited during
the winter months. In their study, the prevalence of myopia,
measured with cycloplegia, in Norwegian 17 to 19 year
old subjects was 16%. This is not significantly different to
the prevalence of myopia measured under cycloplegia in
samples of similar age of European ancestry in Northern
Ireland115 (18.6%) and Australia116 (17.7%), but lower than
the prevalence of myopia reported in Poland117 (34.1%), and,
as expected, somewhat lower than meta-analysis estimates
based on non-cycloplegic refractions118 (27.4%). Unfortunately, cycloplegic data on the prevalence of myopia in children of this age in Europe is very limited.
Hagen et al.114 suggested that it might be necessary to
invoke factors other than daylight exposures to explain the
relatively low prevalence of myopia they reported, because
of the limited daylight hours available in Norway in midwinter. However, it is not clear that this is the case, because
at 60°N, where their study was performed, there are still
IMI Risk Factors for Myopia
6 hours of daylight, even in mid-winter. It is important to
note that the amount of daylight available is not necessarily made use of, either because of cultural preferences or
because of conflicts with time devoted to education, and
objective measures of light exposures may be required to
resolve this issue. Hagen et al.114 reported that Norwegian
children spent 2–4 hours/days outdoors in preschool and
throughout their school years, and that Norwegian childrearing practices place emphasis on getting even very young
infants outdoors. The study sample itself reported spending
nearly 4 hours/day outdoors. In the context of the evidence
that 2 hours outdoors per day can provide significant protection from myopia,74,76,79,81,98 this amount of time may be
sufficient to provide a large degree of control over the development of myopia, particularly since for most of the year,
there seems to be ample daylight available.
Among the other factors, Hagen et al.114 proposed
that being adapted to extreme circannual variations might
provide some protection, although there is little experimental evidence to support this idea. They also suggested that
the specific L:M cone ratios and opsin characteristics of the
population might render them less susceptible to developing
myopia. This hypothesis was based on evidence that these
characteristics have been associated with some syndromic
forms of myopia,119–121 and it has been proposed (by Neitz
and Neitz [2015] “Methods for diagnosing and treating eyelength disorders,” United States Patent US895172982) that
variations in these characteristics might play a wider role in
the etiology of myopia. In support of this idea, Hagen et al.122
presented metadata showing differences in L:M cone ratios
and opsin characteristics between Northern Europeans and
East Asians. However, a more extensive study of correlations between these characteristics and the prevalence of
myopia, taking into account other myopiagenic factors, will
be required to establish such a link. The only experimental
test of this hypothesis obtained largely negative results and
concluded that a large longitudinal study would be required
to test it more fully.123
The situation at 60°N can be contrasted with the situation at even more extreme latitudes. Early studies on Eskimo
and Inuit populations living further north at around 70°N,
where around 1 hour or less of daylight is available in midwinter, showed that the prevalence of myopia was very
low (1–2%), before the local populations had been moved
into settlements and formal education introduced.124–129 This
observation is not surprising, because if there is little pressure to become myopic, exposure to protective factors may
not be required. However, after these changes, the prevalence of myopia rapidly increased within one generation in
younger people to over 50%, suggesting that once environmental pressure to develop myopia had been introduced,
the low level of access to daylight at 70°N was insufficient
to prevent the development of myopia. It is important to note
that changes taking place were likely to place further restrictions on time outdoors, as well as introducing educational
pressures, and, indeed, some of the authors noted anecdotally that myopia still seemed to be prevented in boys who
attended school less regularly.125 These observations suggest
that further exploration at extreme latitudes of the balance
between myopiagenic factors, such as education and environmental factors such as time outdoors, would be useful.
In summary, there is considerable evidence to support
the idea that increased time outdoors delays the onset,
and perhaps slows the progression of myopia, and that
the association is causal. There is considerable evidence
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that the mechanism may involve stimulation of retinal
dopamine release by brighter light outdoors, although other
postulated mechanisms require further testing. School-based
interventions to increase time outdoors have been implemented across the school system in Taiwan, with evidence
of initial reductions in levels of reduced visual acuity, a
proxy in school-aged children for myopia.130 Promotion of
increased time outdoors is also a central part of Singapore’s
myopia prevention strategy,131 and initiatives to promote
time outdoors form part of mainland China’s myopia prevention plan.132,133
Use of Computers and Smart Phones
In the last 2 decades, use of computers and smart phones
has become a routine part of daily life, with digital devices
integrated into schooling in many countries. Dirani et al.134
have recently proposed that increased digital screen time
might now be “the single modifiable risk factor for myopia,”
accounting for “increased near-work activity and decreased
outdoor activity.” Taiwan has introduced laws controlling
the amount of digital screen time that younger children are
allowed (https://www.theatlantic.com/education/archive/
2015/01/how-taiwan-is-curbing-childrens-daily-technologyexposure/384830/, accessed October 10, 2020). How regulations of this kind could be enforced is not clear. Similarly,
in mainland China, limiting screen time in schools is being
implemented to control myopia.132,133
The current evidence implicating digital devices is sparse
and far from consistent. The epidemic of myopia appeared
well before the common use of electronic devices, because
the prevalence of myopia was already high in Taiwan and
Singapore for children born in the early 1960s,10,37 whereas
the internet did not become available to the general public
until 1993. It is certainly possible that digital devices have
now come to constitute a significant form of nearwork,
and their use may correlate closely with education and
myopia.135–144 This topic has been recently reviewed.145
However, the historical perspective is important in
considering preventive interventions. Given that the first
epidemics of myopia predated the widespread use of digital
devices, if limits are now placed on their use, children may
simply revert to traditional forms of nearwork, such as reading printed material. In addition, if digital devices encourage
even more time indoors, active steps may need to be taken
to get children to break with recently established behavior
patterns, and spend more time outdoors. Over emphasis on
digital screen time may in fact have negative consequences
if it leads to neglect of other important factors. There is
currently no evidence that time using digital devices is more
dangerous than a similar amount of time reading, but more
work in this area is clearly required.
The evidence is equivocal as to whether recent increases
in the use of digital devices are associated with increases
in the prevalence of myopia. Data from Taiwan suggest
that there has been a steady increase in the prevalence of
myopia in very recent years,38 particularly in younger children, which could be attributed to increasing screen time.
This is not inconsistent with the more recent decreases
reported after the introduction of increased time outdoors
in schools.130 In contrast, data from Hong Kong suggest that
the prevalence of myopia in 6 to 8 year old children has,
if anything, slightly decreased in the last 20 years, despite
an undoubted increase in the use of digital devices.146 It
may be that in places, such as Hong Kong, the capacity
IMI Risk Factors for Myopia
to produce more myopia has reached its limits, and more
definitive evidence may be obtained from locations where
the prevalence of myopia is much lower. Recently, the World
Health Organization (WHO) has recognized gaming disorders as a disease in the 11th revision of the International
Classification of Diseases-11, and the impacts on the development of myopia of extreme screen time on those of school
age, possibly combined with marked deprivation of time
outdoors, have the potential to be severe. Given the interest in this topic among the public, as well as public health
and education authorities, this is an area that requires more
attention.
Measurement of Nearwork and Time Outdoors
One of the problems with work in this area is that nearwork
and time outdoors have primarily been estimated with questionnaires. These are inevitably subject to problems of recall
and secondary reporting by parents or teachers. In addition,
the amount of detail that can be asked is limited; for example, it is unlikely that respondents would be able to give an
accurate picture of changes in light intensity and duration
of specific exposures.
Questionnaires started out short, with only a few questions on nearwork, and even less on time outdoors.26,73,147
The questionnaire used in the Sydney Myopia Study had a
much larger set of questions, but identified that the important factor was total time outdoors, and that indoor sport
was not protective.74 The WHO then sponsored the development of a simpler questionnaire to be used in subsequent
studies, and this has been further developed by adopting a
more diary-like format to apply time constraints to answers.
The questionnaire used in the GOALS study79 is an example
that is available online.
None of the questionnaires has been validated against
objective measures. Several attempts have been made to
assess how accurate questionnaire answers are by comparing the results to objective measurements. Limited use has
been made of objective light sensors, such as the HOBO data
logger148 and the Actiwatch,149 and the agreement between
questionnaire estimates and the more objective measurements is only limited. One of the important differences
may be that the questionnaires ask for estimates of average activity patterns, generally discriminating among weekdays, weekends, and school holidays. In contrast, objective devices collect data on specific days. Because behavior
almost certainly varies by season, in relation to weather and
in school holidays, estimates of averages are bound to differ
from specific measures. In the SCORM study,147 the questionnaires were supplemented with activity diaries, and there is
some evidence that diaries and questionnaires asking about
a specific period show somewhat better agreement.
Objective measures obtained with wearable devices are
likely to provide more reliable data. Other devices are
now available to quantify light exposures, such as the
FitSight Fitness Tracker150 and the Clouclip device.151 One
of the features of the data collected with these devices is
that the light exposures are generally significantly lower
than measures of ambient light intensities. This is probably because ambient light intensities vary depending on the
direction of collection. For example, when looking at the sky
versus toward the ground, intensities may vary by at least an
order of magnitude. Outdoors, people rarely gaze for long
periods at the horizon or the sky, but spend much more
time interacting with their peers, often with a slightly down-
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ward gaze. In this respect, devices mounted on the arms
of spectacles may have an advantage over other devices,
because they measure light intensity along the line of sight.
A general problem with all devices is that wearing them may
affect behavior, and in the case of the Clouclip device, children without glasses are required to wear frames. Protection
from the damaging effects of UV exposures is often required
outdoors, and it has been shown that the use of sunglasses
and hats results in only slight reductions in exposure.
Attempts to quantify nearwork through measurement of
viewing distance have been less common. An early instrument does not appear to have been used for research
purposes.152 However, the Clouclip device has this capacity, as does the RangeLife.153 The Clouclip device has been
independently validated for distance measurements.154
Wearable sensors are likely to be used more systematically in the future, but the logistics of their use on large
samples is likely to be very challenging. Because they give
a discrete sample in time, some sort of experience sampling
regime may need to be applied to estimate longer-term
patterns of use. With measurement along the line of sight,
interpretation of the results in terms of viewing distance may
be relatively straightforward, but the interpretation of this as
nearwork may be more complicated.
One of the most fundamental problems with objective
measures of activities may be that changes in well-measured
parameters still need to be translated into changes in refraction and axial length. Although quality data are currently in
short supply, initial data suggests that as children progress
through schooling, the amount of nearwork they perform
increases, whereas the time spend outdoors decreases. From
this pattern, it would simplistically be expected that myopic
refractive shifts and perhaps progression of myopia would
increase as children enter higher year grades of schooling,
but in fact these changes generally decrease after the early
primary years. It seems likely that age limits the plasticity of axial growth rates, complicating the interpretation of
the results by requiring age-specific translation of exposures
into refractive and biometric changes. These are challenges
that still need to be addressed, but appropriately used, objective devices have the potential to make a significant contribution.
OTHER RISK FACTORS
FOR
MYOPIA
A range of other risk factors reported to be associated
with myopia have been documented, but whether they are
independently associated with myopia, mediated by other
factors, or are surrogates of other factors is generally not
clear. Given the strength and consistency of the evidence for
education and time outdoors as risk factors, it is particularly
important to consider whether any of the other associations
with myopia are mediated by these two exposures.
Perhaps the most common approach is to put all the
risk factors significantly associated with myopia on univariable analysis into a multivariable regression, and label
all those that remain significant as independent. However,
this approach has significant limitations, related to variable
collinearity, the need to include all relevant variables, and
inaccurate measurement of variables.155 In practice, statistical adjustment tends to perform poorly because exposures are difficult to measure and because models typically assume simple linear relationships between variables.
Patterns of confounding can be complex, and it is unlikely
that all relevant confounders are known, let alone measured.
IMI Risk Factors for Myopia
Approaches based on “mediation analysis” (inclusion and
removal of variables to look for changes in the associations
between the dependent variable and independent variables)
can suffer from similar problems.156 The issues surrounding analysis of interactions between variables are similarly
complex, and there is considerable debate about when
additive and multiplicative models should be considered.157,158 A crucial part of any analysis requires careful
thought about plausible causal mechanisms, and careful
statistical testing of specific hypotheses.
Basic Birth Parameters
Sex. Many studies have compared the prevalence of
myopia in male and female subjects. In older studies, the
prevalence in male subjects tends to be higher, whereas
more recent studies more commonly report higher prevalences in female subjects. For example, the Blue Mountains
Eye Study reported that the prevalence of myopia was higher
in older male adults than in female adults,159 but the situation was reversed in the Sydney Myopia Study on children.160
Similarly, the Liwan Eye Study reported that sex differences
in older adults were marginal,161 but in more recent cohorts
in China, girls are more likely to be myopic than boys.11,162
The extremely large difference in the prevalence of myopia
in girls and boys in Orthodox Jewish communities in Israel,
where the boys undergo very intensive education from an
early age, shows this trend in reverse,34,35 and contrasts
with the similarity of boys and girls receiving more secular
education. This variability does not suggest a direct biological link between sex and myopia, but rather suggests that
the associations may be mediated by social factors, such as
access to education for girls, which varies markedly between
locations and has improved considerably in many places
in recent decades. The relationship is highly confounded,
and may be influenced by differential engagement of the
sexes in outdoor and nearwork activities, irrespective of
whether they are biologically or socially determined. Some
links to growth spurts or puberty163,164 have been reported,
and these may explain some of the differences in prevalence of myopia between girls and boys, who will be at
different stages of puberty and growth spurts at the same
age.
Ethnicity. Ethnicity or race has often been proposed
as a risk factor for myopia, and indeed as evidence for
genetic determination of myopia. It is important to note that
the terms race and particularly ethnicity cover both genetic
differences, which are small in magnitude compared with
the genetic commonalities across all human populations, but
can be measured very precisely, and cultural differences, that
can be large, but are harder to quantify.
Epidemiological evidence shows major differences
between ethnic groups in the prevalence of myopia, but
more detailed analysis shows that these differences may
be mediated by environmental exposures. For example,
the prevalence of myopia is high in the three major
ethnic groups resident in Singapore, Chinese, Indian, and
Malay,9,165 but in India and Malaysia, the population prevalence is much lower.166–169 This suggests that it is the environment of Singapore, probably the education system and
the limited time spent outdoors, that is responsible for
the higher prevalences.2,3,5 The prevalence of myopia is
higher in children of Chinese ethnicity in Singapore, but
the gap has narrowed over recent years. In addition, it is
known that Chinese children currently have higher engage-
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ment in education, and currently achieve higher outcomes,
whereas children of Malay ethnicity report spending more
time outdoors. Adjustment for the differences in educational achievements narrows the gap between the ethnic
groups, but adjustment for time outdoors has not yet been
performed.
Consistent with the epidemiological analysis, genetic
studies have not found major differences between East
Asian and European ethnic groups in the levels of myopiaassociated single-nucleotide polymorphisms (SNPs).170 It
should be noted, however, that the East Asian sample was
relatively small and the study did not include analysis of the
sex chromosomes. Nevertheless, genetic factors accounted
for a lower percentage of variance in the East Asian sample,
as would be expected if environmental factors played a
larger part in East Asia. Although genetic aspects of race
and ethnicity are not modifiable, cultural aspects of ethnicity
are potentially more modifiable, although the difficulties of
changing cultural patterns of behavior should not be underestimated.
Parental Myopia. One of the best documented risk
factors for myopia is having parents with myopia. Although
the consistent impact of parental myopia can be explained
by parents with myopia passing on genetic variants that
predispose their children to myopia, it is also likely that
parents with myopia will be more well educated on average. Hence, parents with myopia may also pass on a myopiagenic lifestyle, in addition to shared genes. The conclusion
that myopia must be a genetic phenomenon alone, because
it runs in families, is simplistic, but this idea still persists.171
A purely genetic explanation for rare, monogenic
(syndromic) forms of myopia is clear, but the impact of
parental myopia is also seen for school myopia. Studies
covering a range of different ethnic groups have shown that
having one or two parents with myopia increases the risk
of myopia in children,26,172–178 although the relative risk is
naturally lower in populations with a high baseline prevalence of myopia.
So far, using data from risk factor questionnaires, there
is no evidence that children with parents with myopia are
more exposed to risk factors, such as nearwork and limited
time outdoors. However, a recent study found that children
with parents with myopia had a greater risk of myopia even
after accounting for the increased risk conferred by the SNPs
they inherited (having parents with myopia and inheriting
myopia-predisposing SNPs were independently associated
with myopia).179 This implies that environmental risk factors
may also be involved. Similar conclusions were reached by
Enthoven et al.180 More accurate objective measures of nearwork and time outdoors may be required to measure differences in environmental exposures between children with
and without parents with myopia.
Birth Order. Associations between myopia and birth
order have been reported in several cohort studies, with firstborn children tending to be more myopic.181 In educational
studies, it is well-documented that first-born children generally get more education,182 which would tend to produce
more myopia. A subsequent study on the UK Biobank
dataset showed that the association between myopia and
birth order was reduced but not eliminated after adjusting
for years of education.183 In addition, in China, children from
one child families were more myopic than children with
siblings, which the authors attributed to greater parental
support for their child’s education.184 However, the sociology of these differences is very complex, and more work
IMI Risk Factors for Myopia
needs to be done to establish whether birth order is an independent risk factor.
Date or Season of Birth. Season of birth has also
been associated with myopia in several studies. There is
a higher prevalence of high myopia in children born in
Israel185 and the United Kingdom186 in the summer months,
but differences in the prevalence of mild myopia were slight
and inconsistent, as were correlations with photoperiod. In
the Israeli study, the sample consisted of young male adults
(military conscripts), whereas the UK sample covered the
age 18 to 100 years. A more recent paper from the UK TEDS
study reported that children born in the summer months
were more myopic, but again perinatal photoperiod effects
were not significant.144 The authors proposed a link to the
age of starting school, with children born in the summer
months tending to start school younger by up to 1 year
because of age cutoffs for school enrollment, and progression of myopia tending to be more rapid at younger ages.
Summary. The factors discussed in the section are set
at birth, and are not modifiable per se. However, if the differences in the prevalence of myopia that emerge during childhood associated with these factors are mediated by cultural
or social attitudes or rules that lead to differential exposures,
it may be possible to devise interventions to limit the development of myopia.
Other Personal Factors
Height. Height is similar to myopia in that it has quite
a high heritability, although not as high as that of myopia.187
Like myopia, it is also subject to environmental influences,
with significant increases in height seen in many populations over the past century.188 These have been generally
attributed to more adequate nutrition. Rare and often deleterious mutations can also cause extreme variation in height.
It has been argued that associations between height and
myopia might be expected, given that taller people have
longer axial lengths (see for example ref. 189), but this
argument does not take into account that “emmetropization” mechanisms190 should produce substantial convergence of refractive status, despite differences in body stature.
Although it has been reported that that height is a risk
factor for myopia in children,191 the evidence on this is
inconsistent.192 In fact Rosner et al.193 reported that Israeli
male military conscripts who were not myopic, were taller
and weighed more than those who were myopic – the
reverse of some expectations. Another inconsistency lies in
the difference in prevalence of myopia between male and
female subjects, with a higher prevalence of myopia being
commonly reported in girls in recent studies (see above),
despite their smaller stature and shorter axial lengths.194
In general, there appears to be a tight biological link
between height and axial length, but not with refraction.
Social factors affecting nutrition and education may be
significant confounders. Mean height varies considerably
between populations, (https://worldpopulationreview.com/
country-rankings/average-height-by-country), but the countries known to have a high prevalence of myopia do not
stand out through differences in height in the way that they
do in relation to educational achievement.33
Intelligence. Higher intelligence or IQ, and some
other cognitive measures, are generally associated with
myopia.195–197 Initially, this link was conceptualized in terms
of dominant genetic effects within a rather simplistic big
brain-big eye hypothesis,198 although it is not clear that
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bigger brains are associated with higher intelligence, or that
bigger, rather than relatively elongated, eyes are associated
with myopia.
Intelligence or general cognitive function show high heritability in twin studies, although not as high as the heritability of myopia.199 Genetic variants with large effects
on intelligence or cognitive capacity are rare and deleterious, providing an interesting analogy between rare mutations that cause intellectual disability and rare mutations
that cause early onset high myopia. However, whether intelligence or cognitive capacity exert effects independent of
education and perhaps time outdoors is not clear. As is the
case with myopia, there is considerable evidence that these
traits can be modified environmentally199–202 and a longterm trend toward increasing population IQ levels has been
reported,201,202 although it is much less dramatic than the
changes in myopia in East and Southeast Asia.
In the SCORM study, both academic grades and IQ
scores were reported to be independently associated with
myopia,29,196 and the same result has been obtained in
a very large study of Israeli conscripts.28 Both cognitive
performance and years of education were associated with
myopia in the Gutenberg Health Survey, but the association with years of education was stronger.31,195 Williams
et al.203 reported that the phenotypic correlation between
myopia and IQ was low but significant, and that most of
it could be explained by genetic differences, although the
proportion of variance explained by genetic factors was
small for both phenotypes. This is an area in which thoughtful mediation analysis or a Mendelian randomization analysis would be particularly useful. The potentially bidirectional links among intelligence, cognition, education, and
academic performance are not well understood. In addition,
whatever subsequent research reveals about these links, it
does not seem likely that this research will lead to interventions to prevent myopia.
Physical Activity. A number of papers have reported
associations between increased physical activity and less
myopia, but this association is confounded, given that
increased physical activity is often performed outdoors. A
systematic review has concluded that although most studies
reported a negative association between increased physical
activity and myopia, most did not rule out mediation by time
outdoors, and several concluded that the important factor
was time outdoors.74,204,205 A recent detailed investigation
concluded that there was no significant protective association of increased physical activity with myopia,206 whereas
a more recent paper has reported more robust associations
but without ruling out time outdoors.207 Further studies with
more objective measures of activity and time outdoors are
important because interventions aimed at promoting indoor
physical activity rather than time outdoors may have little
effect in preventing myopia, although they may be easier to
implement.
Sleep. Associations between sleep and myopia have also
been reported, but the evidence is quite inconsistent.208–213 A
large longitudinal study from Shanghai reported consistent
significant associations of going to sleep late with greater
myopia prevalence at baseline, incident myopia, and myopic
shift in refraction, after adjustment for several variables
including age, but did not find that sleep duration was an
important factor.214 The authors noted that going to sleep
late was more prevalent in children who lived in urban
areas, were older, had more parents with myopia, had better
educated parents, tended to wake up late, spent more time
IMI Risk Factors for Myopia
reading and on screens, and spent less time outdoors – all
characteristics that were also identified as risk factors for
myopia. The analysis is thus highly confounded, and the
evidence on causality is not strong. The authors suggested
that their results might also implicate circadian rhythms.
Children who have heavy study loads after school are probably likely to get less sleep, both because there is less time
available, and also because mental activity close to bed
time can disrupt sleep. This suggests that lack of sleep is
more likely to be a problem in the senior years of school,
when homework loads in many parts of East and Southeast Asia are very high. However, sleep deprivation may be
less common in the early primary years, when myopia first
appears.
Summary. Many of the associations reported in this
section are not consistent across studies, suggesting that
direct biological links may not be involved. In most cases,
causality has not been demonstrated. The inconsistent findings suggest that many of the associations are affected by
social factors and could have arisen due to confounding.
There is too little data related to the role of circadian rhythms
to make any firm conclusions, however, one of the benefits of natural daytime light is to maintain healthy diurnal
rhythms. Thus, the effects of outdoor time on myopia may
be related to whether diurnal rhythms of ocular growth
are disrupted or not, and this may again be related to
seasonal behavioral changes. Physical activity would seem
to be a readily modifiable factor, but the available evidence
currently does not suggest that interventions based on
increasing physical activity, without increasing outdoor time,
are likely to be effective.
Family Characteristics and Environment
Socio-economic Status. Since James Ware reported to
the Royal Society in 1813 on the greater need for and use
of corrections for near-sightedness in “persons of the higher
ranks in life” as compared to “persons in the inferior stations
of life,”215 a large body of evidence has been accumulated
showing that family income, as well as parental education
and parental myopia, are associated with an increased prevalence of myopia in children. Other research has consistently shown that young adults engaged in continuing study
or in occupations that involve nearwork indoors have a
higher prevalence of myopia.70 These associations have been
observed in a wide range of populations.216–219 Exceptions
to this observation are rare,220,221 and may possibly be associated with recent groups of migrants on low incomes pursuing intensive education for their children.
The possibility of a link between income and myopia has
also been suggested by the recent epidemic of myopia in
parts of East and Southeast Asia that have seen marked
increases in per capita income, producing some of the
wealthiest countries in the world. Jan et al.222 have shown
that, in mainland China, increases in the prevalence of
visual impairment (a proxy measure for myopia) between
provinces correlate with increases in gross domestic product (GDP) per capita at the province level. The potential for confounding in these analyses is obvious, and it
is hard to understand how rising income could translate
directly into biological changes in eye growth. Income is,
however, a possible covariate of both education and nearwork. Although the association between socio-economic
status (SES) and myopia is generally strong within a society at a given time, high per capita incomes were achieved
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in many Western societies with only modest prevalences of
myopia, well before East Asian societies achieved similar
income levels, but with much higher prevalences of myopia.3
Within East and Southeast Asia, the prevalence of myopia is
now similar in China, Japan, South Korea, and Singapore,3
but per capita income and GDP are still much lower in
China (https://en.wikipedia.org/wiki/List_of_countries_by_
GDP_(nominal)_per_capita, accessed May 12, 2020). A more
consistent association is with the intensity of the education system as shown in the PISA studies of educational
outcomes.33 This is an area in which more quantitative analysis would be very useful.
As another example of the potential for confounding, Rahi and colleagues reported that maternal height
and age were associated with more myopia.223 In the
United Kingdom, height differs by SES, with mean heights
greater in higher SES groups.195,224 The same is true for
maternal age, with women in higher SES groups tending to have children later in life (https://www.ons.gov.uk/
peoplepopulationandcommunity/birthsdeathsandmarriages/
livebirths/articles/anoteonchildbearingbysocioeconomicstatusandcountryofbirthofmother/2016#socio-economic-statusand-average-age-of-mother-for-uk-and-non-uk-bornwomen, accessed May 12, 2020). Given that children
from higher SES groups are generally more myopic, these
associations could have arisen due to confounding.
Smoking. Maternal smoking was associated with a
lower risk of myopia in the SCORM study from Singapore,
but there was no association with paternal smoking, and
the number of mothers who smoked was small.225 In the
subsequent STARS study, a stronger negative association
with maternal and paternal smoking was reported.226 A similar protective relationship was reported in a sample from a
pediatric ophthalmology clinic, which largely persisted after
adjustment for a range of factors, including child’s nearwork
activity and parental myopia and education.227 A detailed
study from South Korea reported consistent results for
exposure to passive smoke estimated from urinary cotinine
level,228 supporting the suggestion that nicotinic pathways
are involved in the regulation of eye growth. In contrast, Rahi
et al. reported an association between maternal smoking in
early pregnancy and more myopia.223 Although some of the
associations reported are substantial, given the associations
of smoking with SES and education, and lower gestational
weight, these studies are at high risk of confounding.
Diet. Over the ages during which myopia develops
in children, diets are largely set by family characteristics,
including family wealth and cultures. Changes in diet have
often accompanied economic development, as reflected in
the secular increases in height that have been reported in
many parts of the world. It should be noted that there
is a need to carefully distinguish between dietary change
associated with increased height as compared to that associated with an increase in obesity. Nevertheless, Cordain
et al., taking a broad anthropological perspective, argued
that dietary change could have contributed to the increased
prevalence of myopia, and supported this argument with
a plausible hypothesis linking insulin resistance, chronic
hyperinsulinemia, increased circulating IGF-1, decreased
circulating growth hormone, and decreased retinoid receptor signaling to increases in scleral growth.229 However,
expected associations of height, weight, body mass index
(BMI), and obesity with myopia have not been consistently
observed. Improved diet has been associated with greater
height and axial length, but, as noted above, this does not
IMI Risk Factors for Myopia
appear to have produced increased myopia because of the
powerful eye growth control mechanisms that exist.
International variations in mean height do not parallel
variations in the prevalence of myopia. Similarly, international variations in the prevalence of people in the
overweight and obesity categories do not parallel the international distribution of myopia, with none of the countries
with a high prevalence of myopia making the list of the
top 20 countries ranked by percentage of obesity (https:
//www.who.int/gho/ncd/risk_factors/overweight/en/,
accessed January 30, 2019). Thus, there is little support for
a tight biological link between diet and myopia.
Another problem in this area is the sheer diversity of
the components of diet and the difficulty of measuring
lifetime exposures. Few dietary nutrients and micronutrients have been examined in detail. However, over 50 years
ago, Gardiner explored the relationship between diet and
myopia, particularly protein, with suggestive results, but
this work does not appear to have been followed up.230–233
More recently, studies examining dietary zinc and myopia
suggested no association.234,235 At present, there is no strong
evidence implicating dietary change in the epidemic of
myopia.
Summary. The association of family income with
myopia in children is largely consistent. Although it is difficult to test formally, it seems likely that most of the data can
be explained by associations between family income and
education of the children, rather than a direct link between
income and education. However, further work is needed
for a more comprehensive understanding of the causal and
noncausal pathways linking family income to myopia.
Aspects of the Lived Environment
Urban/Rural Differences. Urban-rural differences in
the prevalence of myopia have been frequently reported,
with large differences appearing when the level of economic
development is markedly different in the different locations. Studies from mainland China,11,162,236–238 Taiwan,239
and India166,168,240 have shown marked differences in the
prevalence of myopia, with the prevalence higher in urban
than rural areas. It has generally been assumed that these
differences can be explained by differences in educational
outcomes and time spent outdoors, but this assumption has
never been systematically tested. However, a detailed analysis of data from the ALSPAC study has suggested other
factors, such as population density, might be more important, at least in the prosperous Avon Valley region.241 Population density has also been invoked as a factor in an
Australian study242 and in China.243 In the latter study, the
prevalence of myopia was high across a wide range of population densities, suggesting that other factors were more
important.
Even within cities, regional differences in prevalence
of myopia have been reported. The Sydney Myopia Study
reported that the prevalence of myopia was highest in inner
city areas.242 Access to green space has also been linked to
lower use of spectacles, as a proxy for myopia,244 but there
are many confounding effects in studies of this kind, such as
where do higher SES families live, and where do the families of children achieving higher educational outcomes live.
It does seem plausible that greater access to green space for
play might provide an opportunity for more time outdoors
and the prevention of myopia, but other factors, including
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IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 11
safety, weather, pollution, and cultural attitudes, may determine whether it is used effectively.
Pollution. Pollution is one of the factors that has
increased markedly since the Second World War in parts of
East and Southeast Asia. One of the problems in this area is
that there are many forms of pollution, but most attention
has been devoted to air pollution. In international terms,
air pollution is more extreme in many cities in South Asia
and the Middle East than in Chinese cities, although their
prevalence of myopia is much lower than in Chinese cities
(https://www.who.int/airpollution/data/cities/en/, accessed
May 12, 2020). Increased use of spectacles, presumably for
myopia, has also been associated with traffic-related pollution,245 but the effect is weak and may be related to the
association between urban residence and more myopia, as
well as links to SES, area of residence, and education, rather
than to a direct effect of pollution. An association between
myopia and traffic pollution was also reported from Taiwan.
These studies are also highly confounded. The Taiwanese
group has reported that concentrated atmospheric pollution
applied to the eyes in animal experiments promotes the
development of myopia,246 but whether this simply represents a form of form-deprivation myopia is not clear.
Housing. Type of housing, particularly its size, has also
been suggested as a factor, particularly the idea that living
in small apartments might promote myopia. However, the
results in this area are currently inconsistent. In Singapore,
more spacious housing was associated with more myopia,27
possibly because of a causal chain involving SES, housing, and its associations with education. In contrast, in both
Sydney242 and Hong Kong,247 small apartment dwelling has
been associated with more myopia. A detailed study in Hong
Kong has suggested that home size and aspects of the home
defocus environment may be associated with myopia.248
Circadian Rhythms. A large body of evidence from
animal experiments supports the idea that there are circadian or diurnal rhythms in parameters, such as axial length
and choroidal thickness, and that abnormal light exposures, such as constant light and dark, lead to changes
in eye growth in animals.249,250 In addition, studies examining gene expression in animal models of myopia have
reported changes in expression of mRNAs associated with
circadian clock genes,251,252 and genomewide association
studies (GWAS) have reported SNPs in similar genes associated with myopia.13
A fundamental problem in interpreting these observations is that dopaminergic function, through its interaction
with melatonin, is an integral part of circadian and diurnal
pathways. Given the evidence for a major role of dopamine
release in the control of eye growth,253 it is difficult to
determine whether changes in light-regulated dopamine
release or perturbations of broader circadian pathways have
a primary role in leading to excessive axial elongation. In the
animal experiments, it is possible that changes in dopamine
release led to changes in the expression of clock genes, and
it is equally possible that mutations in clock genes may lead
to perturbed dopamine synthesis and release.
An environmental exposure that disrupts circadian
rhythms in humans, leading to the development of myopia,
has not been identified. An early report that children
who slept with night lights became very myopic generated considerable interest.254 However, attempts to replicate this finding in a range of populations found little or
no effect.255–261 One epidemiological phenomenon that may
give some support to this hypothesis is the emergence of an
IMI Risk Factors for Myopia
IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 12
epidemic of myopia in Inuit and Eskimo populations when
they were moved into settlements and received somewhat
rudimentary education, far less intensive than that required
to produce an epidemic of myopia in East and Southeast
Asia.124,125,128
Based on evidence that brief exposure of chickens to light
at night disrupted growth rhythms,262 it has been suggested
that increased use of artificial lighting and the consequent
rise of light pollution might be a cause of myopia, although
rhythms in humans seem to be more robust.263 However,
light pollution maps show similar levels of light pollution
in East Asia, Europe, and North America (https://www.
lightpollutionmap.info, accessed October 12, 2020) although
the prevalences of myopia in these regions are quite different. It is therefore difficult to attribute the increased prevalence of myopia in East Asia to increased light pollution, and
other factors seem likely to play a major role.
Kearney et al.264 have recently reported that myopes in
the NICER study have higher morning levels of serum melatonin, although this finding was not replicated in a US
study.263 More recently, this group reported that circadian
rhythms in melatonin levels were not altered in myopes as
compared with emmetropes.265 In contrast, a more recent
paper reported that melatonin levels were lower, and that
there were phase shifts in rhythms.266 At this stage, it is not
clear whether these observations suggest a role for circadian rhythms, or whether the changes in melatonin levels
are secondary to changes in dopamine metabolism.
Febrile Diseases. Using data from the UK Biobank,
Guggenheim et al. reported associations between several
childhood diseases and myopia. From a list including pneumonia, encephalitis, meningitis, rheumatic fever, measles,
rubella, mumps, diphtheria, and pertussis, myopia was associated with rubella, and mumps and pertussis were associated with any myopia, whereas measles, rubella, and pertussis were associated with high myopia.273 The authors argued
against a link to educational disruption or limited time
outdoors, because not all serious childhood diseases were
linked to myopia. This link, whatever its causes, is unlikely to
explain the emergence of the epidemic of myopia, because,
in general, childhood vaccination has increased over time in
many countries, including in East and Southeast Asia since
the Second World War, yet the prevalence of myopia has
increased. However, these findings may have clinical implications that need to be explored.
Fertility Treatment. The British TEDS study has documented a standard range of social variables, with level of
maternal education, summer birth, and hours spent playing
computer games surviving full multivariate regression analysis, with associations with SES, educational attainment, reading enjoyment, and cognitive variables showing associations
at multiple stages in the life-course analysis. A unique feature
of the analysis was the protective associations of fertility
treatment detected in the final analysis.197 The authors ruled
out associations with parental education, and the explanation for this finding remains obscure.
Miscellaneous Risk Factors
POPULAR BELIEFS ABOUT
Allergic Conjunctivitis, Hay Fever, and Kawasaki
Disease. In 2011, Herbort et al. proposed an association
There are many popular beliefs about the causes of myopia
around the world, which have presumably arisen because
the development of myopia and its progression is often
observed by parents, who naturally seek explanations. In the
Western world, a common belief is that reading in dim light,
or under the bed-clothes causes vision to deteriorate, but this
outcome, and these behaviors might indeed be common in
those who like reading books, and read a lot, without indicating a causal connection. Scientific evidence in this area is
very limited, and although animal experiments suggest that
chickens exposed to constant dim light may slowly develop
myopia, objective measurements on children suggests that
children with myopia are less exposed to dim lights as well
as brighter lights than nonmyopic children.274 We have not
attempted a systematic survey in this area, but in China, there
seems to be many beliefs of this kind, perhaps because the
prevalence of myopia has increased so conspicuously. One
commonly encountered belief is that myopia is associated
with reading and writing postures that violate the “foot, fist,
inch” rule, that is the eyes should be one foot from the book,
the chest should be one fist from the desk, and the fingers
should be one inch from the nib of the pen. This is a variant on the idea that bad posture while reading leads to the
development of myopia, which has widespread currency,
but has never been rigorously tested. A similar common
belief is that reading while riding on public transport is
dangerous, but again this has never been tested. Other ideas
include the development of myopia in children who read
on their back, or their front, or who read extracurricular books with font sizes greater than standard text-books.
These proposed factors need to be subjected to thorough
epidemiological investigation. If they stand up to scrutiny,
they need to be evaluated in carefully designed randomized
clinical trials. Unfortunately, several such recommendations
of myopia with inflammatory conditions affecting the choriocapillaris.267 An association between myopia and ocular
inflammatory conditions, such as uveitis, was subsequently
demonstrated,268 and a higher risk of myopia was associated with allergic conjunctivitis, and less so allergic rhinitis,
atopic dermatitis, and asthma.246 A large population-based
study using the US National Health and Nutrition Examination Survey (NHANES) dataset showed that hay fever was
also associated with a higher prevalence of high myopia.269
A recent report has also associated increased myopia with
Kawasaki disease,270 which has conjunctivitis as one of its
core diagnostic criteria.
These associations raise the intriguing possibility of a link
between ocular allergic responses and the development of
myopia. Using an animal model, Wei et al. have proposed
a potential molecular mechanism involving increased tumor
necrosis factor (TNF)-alpha and interleukins.246 It does not
seem likely that a link between ocular inflammation and
myopia can explain the epidemic of myopia in East and
Southeast Asia, because there is no parallel between the
international distribution of myopia and that of allergic
rhinoconjunctivitis in children.271 One possibility is that eye
rubbing may lead to myopic refractions through corneal
changes, as may be the case with keratoconus,272 but a US
study on hay fever did not support this hypothesis.269 The
possibility that children with these conditions tend to spend
less time outside should be examined. It is also plausible that
allergic conditions might add to the incidence and progression of myopia, without being the primary determinant of
myopia onset. Another possible factor may be the drugs used
to control allergies, although there is currently no evidence
for this.
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THE
CAUSES
OF
MYOPIA
IMI Risk Factors for Myopia
IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 13
TABLE. Summary of Factors Associated With Myopia
Factor
Major factors
Education
Time outdoors
Screen time
Basic birth factors
Sex
Ethnicity
Parental myopia
Birth order
Birth season
Other personal factors
Height
Intelligence
Physical activity
Sleep
Family characteristics
Socio-economic status
Smoking
Diet
Environment
Urban/rural
Pollution
Housing
Circadian rhythms
Night light
Light spectrum
Miscellaneous factors
Allergic conjunctivitis, hay fever, Kawasaki
disease, febrile diseases
Fertility treatment
Common beliefs
Reading in dim light, under bed-clothes or
in transport
Posture in reading/writing and holding pen,
font size in book
Evidence/Causal Relationship
Confounding Issues
Strong and causal
Strong and causal
Equivocal
Time outdoors
Role of light (intensity, duration, spectrum)
Nearwork
Weak
Inconsistent
Strong
Weak
Weak
Weak
Moderate
Moderate
Weak
Social factors
Education, time outdoors
Time outdoors
Educational pressures
Moderate
Weak
Weak
Education
Education, SES
Education, SES
Moderate
Weak
Weak
Weak
Negative
Weak
Education, SES, time outdoors
SES
Education, SES
Dopamine
have been written into China’s National Myopia Prevention
Plan as advice to parents, without a solid scientific basis.
CONCLUSIONS
This overview of risk factors for myopia has identified education and limited time outdoors as major risk factors for
myopia. These two factors offer the prospect of identifying evidence-based approaches to the control of myopia,
such as increased time outdoors and, possibly, decreased
nearwork time. How these two factors act to regulate eye
growth is largely unknown, but in the case of time outdoors
it appears to involve regulation of the rate of dopamine
release, and possibly other factors. Animal studies relevant to
these pathways have been reviewed in another article in this
series.16 To date, only the negative (protective) association
of increased time outdoors with myopia has been translated
into a proven preventive intervention.
Myopia is often described as a complex multifactorial
condition, and many other risk factors for myopia have been
proposed. The Table lists these factors, and the quality of
the evidence that currently documents them. The majority of
them may involve more distal social factors, such as parental
and social attitudes to education, provision of educational
opportunities, and organization of school systems, and may
be mediated by the exposures to educational pressures and
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Social factors
Cultural attitudes or genetics
Genetics or myopiagenic environments
Years of education
Years of education
Limited data
Weak
Limited data, time outdoors
Weak
Limited data
Weak
Limited data
Weak
Limited data
time outdoors that children receive. So far, few have been
translated into a preventive intervention that has been validated in a controlled trial, although several have obvious
potential.
Future studies in this area need to become more rigorous. Cycloplegia needs to follow the required standard.
Statistical adjustment for potential confounders, and mediation analysis, need to become more systematic, and to
be conducted with greater thought about potential causal
pathways. Measurement of the major identified risk factors,
education or nearwork, and time outdoors, needs to become
more accurate. New studies should therefore collect data on
education, nearwork exposures and time outdoors, ideally
using the objective sensors that are becoming available.
Where possible, the powerful techniques of Mendelian
randomization and regression discontinuity analysis should
be applied. These improvements are required if studies on
risk factors are going to provide a reliable basis for the development of future preventive interventions.
Acknowledgments
The authors thank Monica Jong for facilitation of the process.
Supported by the International Myopia Institute. The publication costs of the International Myopia Institute reports were
IMI Risk Factors for Myopia
IOVS | Special Issue | Vol. 62 | No. 5 | Article 3 | 14
supported by donations from the Brien Holden Vision Institute,
Carl Zeiss Vision, CooperVision, Essilor, and Alcon.
Disclosure: I.G. Morgan, None; P.-C. Wu, None; L.A. Ostrin,
None; J.W.L. Tideman, None; J.C. Yam, None; W. Lan, None;
R.C. Baraas, None; X. He, None; P. Sankaridurg, BHVI (E), coinventor on multiple patents related to myopia (P); S.M. Saw,
None; A.N. French, None; K.A. Rose, None; J.A. Guggenheim,
None
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