Globally, myopia is the leading cause of distance refractive error, affecting 1.45
billion or 27% of the world's population in 2010 (myopia being defined as more than
or equal to 0.50 D of myopia).1 The number of people with myopia is expected to continue
to rise both in absolute numbers and as a percentage of the population.2 In certain
age groups in several Asian countries, the prevalence of myopia is over 80%. Among
late teenagers and young adults in Korea, Taiwan and China the prevalence is now between
84% and 97%.3–5
In addition to the increase in prevalence, there is evidence of a rise in the severity
of myopia.3 A study by Vitale et al.
6 in the United States found that the prevalence of moderate myopia, defined as between
−2.00 D and −7.9 D, nearly doubled (from 11.4% in 1971–1972 to 22.4% in 1999–2004),
and that the prevalence of high myopia, which was defined as more than 8.00 D of myopia
for this study, had increased eightfold during the same period (from 0.2% to 1.6%).
The global prevalence of high myopia (commonly defined as greater than or equal to
5.00 D of myopia) was 2.9% (224 million people) in 2010.7
Risks
High myopia is associated with an increased risk of developing sight-threatening conditions
such as myopic macular degeneration (defined as atrophic changes or choroidal neovscularisation
in the macular region in high myopia), retinoschisis, posterior staphyloma, glaucoma
retinal detachment, and cataract.8,9 A literature review found that the prevalence
of vision impairment due to pathologic myopia (high myopia with one or more typical
fundus lesions) is between 0.1% and 0.5% in European studies and between 0.2% and
1.4% in Asian studies.9 In a Japanese study, 12.2% of vision impairment was caused
by pathologic myopia.10 Myopic macular degeneration has been reported to be the major
cause of monocular blindness in Tajimi, Japan,11 and the leading cause of new cases
of blindness in Shanghai, China.12 Without interventions to slow the progress of myopia,
the prevalence of pathologic myopia can be expected to increase.
A child with myopia. PAKISTAN
Causes and cures
Myopia development and progression is considered to be multifactorial, with genetic
and environmental factors contributing, although the contribution of genetics is considered
small. Genes have been identified for myopia13,14 but genes are thought to determine
one's susceptibility to environmental factors.15 Several environmental factors have
been identified,15–18 including too much time spent on near work;18 insufficient time
spent outdoors;19 low levels of vitamin D;20 inadequate light exposure;21 and poor
diet.22 There is evidence emerging that increased time spent outdoors can reduce the
risk of developing myopia and – in those with myopia – it can reduce the rate of progression.23,24
Reduced rates of progression in summer compared to winter25 also supports the hypothesis
of the light theory. It has also been shown in experimental animal models that a defocused
retinal image can lead to axial elongation of the eye, and hence myopia. While it
seems unlikely that there will be a treatment that can completely prevent myopia development
and progression, there are some promising treatments on the horizon. These include
providing a focussed image at all retinal locations to remove the stimulus to axial
eye elongation. Executive bifocal spectacles,26 peripheral plus contact lenses,27
extended depth of focus contact lenses28 and orthokeratology29 are some of the optical
intervention methods that show some efficacy in reducing the rate of progression of
myopia (some by up to 51%). Pharmacological agents such as 0.01% atropine are being
prescribed in Asia, and show a reduction of up to 50% in the rate of progression of
myopia, although there is no reduction in the rate of axial elongation.30 Oral tablets
containing 7-methylxanthine (7-MX) have been approved for use in children in Denmark
for myopia control and shows some efficacy, but long-term studies are needed.31 While
these approaches have been tried in isolation, they may have a greater impact if different
approaches are combined.
Discussion
Uncorrected distance refractive error is already a major global health problem. It
is the main cause of vision impairment and the second highest cause of blindness.32
It has also been estimated to cost about US $202 billion33 in global lost productivity.
However, whereas uncorrected refractive error is the second major cause of blindness,
this does not include blindness from pathologic myopia; nor does the estimate of the
financial burden of uncorrected refractive error take into account blindness due to
pathologic myopia.
Myopia can no longer be considered a benign refractive error, easily corrected with
a pair of spectacles or contact lenses. It has an insidious side which, even if adequately
corrected, can sometimes progress and may lead to sight-threatening complications.
Current forms of correction for myopia (spectacles, contact lenses and refractive
surgery), do not cure the underlying myopia but provide an optical solution for clear
vision and thus do not offer any protection from the consequences of high myopia.
Given that myopia is already one of the major causes of vision impairment and blindness,
and is projected to affect almost half of the world's population within 40 years,2
urgent action is demanded from governments, non-government organisations and researchers.
Policy makers must recognise the risk of increasing myopia and ensure that appropriate
detection and treatment is available. Myopia control is possible but clinicians must
adopt myopia control strategies as soon as a child becomes myopic. Parents should
also be encouraged to monitor the time their children spend on near devices and encourage
time spent outdoors.
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Further reading:
www.mivision.com.au/high-myopia-prevalence/