We searched the Medline and Online Mendelian Inheritance in Man (OMIM) databases using the search terms “myopia”, “high myopia”, and “pathological myopia”, alone or in combination with “prevalence”, “epidemiology”, “genetics”, and “prevention”. We made a separate search for “stationary night blindness”. Names of authors and reference lists from relevant article lists were used as the basis for further searches. Where possible, review articles or meta-analyses that contain comprehensive
SeriesMyopia
Introduction
Myopia (short-sightedness or near-sightedness) is often regarded as a benign disorder, because vision can be corrected with glasses, contact lenses, and refractive surgery. Nevertheless, myopia has emerged as a major public health concern for three reasons: first, in developed countries in east and southeast Asia, such as Singapore, China, Taiwan, Hong Kong, Japan, and Korea, the prevalence of myopia has rapidly increased in the past 50–60 years.1, 2 In urban areas in these countries, 80–90% of children completing high school are now myopic, whereas 10–20% can have high myopia.3 These changes are not restricted to urbanised east Asia, since the prevalence of myopia is also increasing in North America,4 albeit more slowly, and probably in Europe as well. Second, the WHO recognises that myopia, if not fully corrected (uncorrected or under-corrected refractive error) is a major cause of visual impairment.5 Finally, people with high myopia are at a substantially increased risk of potentially blinding myopic pathologies, which are not prevented by optical correction.6
These factors call for adequate diagnosis and correction of myopic refractive errors, effective treatment of myopic pathologies, and, above all, prevention of myopia. Fortunately, our understanding of the cause of myopia has substantially progressed, leading to promising approaches to prevention, and so has our understanding of pathological myopia and its treatment.
Section snippets
Biological basis and definition
Refractive status is a complex variable, determined by the balance of the optical power of the cornea and the lens, and the axial length of the eye (with its component parts anterior chamber depth, lens thickness, and vitreal chamber depth). Myopia usually results from an eye that has become too long, particularly through elongation of the vitreal chamber.
Most children are born hyperopic, with a normal distribution of refractive errors.7 During the first year or two after birth, the
Epidemiology of myopia
Striking evidence exists for rapid increases in the prevalence of myopia, which has been considerably reviewed.1, 2 Rapid change was first noted in Inuits in North America as the populations moved into settlements,13 but it has been best documented in Singapore14, 15, 16, 17 and China (Taiwan3, 18 and Guangzhou19, 20) where the prevalence of myopia in different population-based birth cohorts can be compared. The data from Taiwan18 show that the prevalence of myopia has reached a plateau at a
Causes of myopia
50 years ago, myopia was believed to be genetic, with only minor environmental influences.25 However, results from experimental studies, including in primates, support the evidence of environmental factors from human epidemiology. These studies show that changes in visual experience by fitting of diffusers or both positive and negative lenses over the eyes can generate signals that promote eye growth, leading to myopia, as well as signals that slow eye growth.26
These models are relevant to
Environmental risk factors for myopia
The importance of environmental risk factors is strongly supported by experimentation with animals, and by the rapid changes in the prevalence of myopia. Associations of myopia with years of schooling and school results have been consistently reported.1 The very high prevalence of myopia in boys attending Orthodox schools in Israel compared with that seen in girls attending Orthodox schools in Israel and in all students attending Israeli secular schools is particularly striking.30 The rise in
Genetic risk factors for myopia
One key indicator of a genetic basis is familial clustering. In the case of myopia, sibling risk ratios are generally high, and even higher for high myopia.47 However, families share environments as well as genes, and sibling similarities in postulated myopigenic environmental factors are often higher than the sibling risk for myopia itself.48
Heritability values for myopia in twin studies have generally been high.49 Although apparently less ambiguous, twin heritability analysis depends on the
Ocular morbidity of myopia
Myopia is associated with other ocular disorders such as cataract63 and glaucoma,64 whereas it is negatively associated with age-related macular degeneration,65 but the causal connections are unknown. However, the major risk associated with myopia is the association between high myopia and ocular pathologies.
Pathological myopia associated with high myopia is particularly important because, in addition to the changes in overall myopia, in the urban centres of east Asia, the prevalence of high
Pathological myopia
Pathological myopia was originally described as high myopia accompanied by characteristic degenerative changes in the sclera, choroid, and retinal pigment epithelium, with compromised visual function.66 Not all highly myopic eyes develop pathological myopia, and attempts have been made to define highly myopic eyes at high risk as those with an axial length of more than 3 (SD) from the mean for emmetropic eyes. Although issues associated with differing definitions of high myopia and signs of
Interventions to control myopia
Interventions to control myopia are of two kinds. Those aimed at prevention of myopia need to be minimally invasive, since they would be applied to children who do not require glasses. Once myopia is developed, progression can continue throughout childhood and, particularly in high myopia, throughout adult life. In this situation, more invasive interventions are possible. Myopic pathologies increase with greater myopic refractive error, and even partial prevention of progression can provide
Conclusions and future priorities
Myopia is an increasingly widespread condition around the world, but particularly in east Asia. Effective reduction of visual impairment is available with optical correction by spectacles, contact lenses, and refractive surgery.
From a clinical perspective, the major priorities for future research lie with the prevention of incident myopia and myopic progression leading to the development of high myopia, and several evidence-based approaches are currently under trial. Health behaviour programmes
Search strategy and selection criteria
References (101)
- et al.
How genetic is school myopia?
Prog Retin Eye Res
(2005) - et al.
Refractive and ocular findings in the newborn
Am J Ophthalmol
(1951) - et al.
Prevalence and risk factors for refractive errors in the Singapore Malay Eye Survey
Ophthalmology
(2008) - et al.
Homeostasis of eye growth and the question of myopia
Neuron
(2004) - et al.
Monocular axial myopia associated with neonatal eyelid closure in human infants
Am J Ophthalmol
(1981) - et al.
Outdoor activity reduces the prevalence of myopia in children
Ophthalmology
(2008) - et al.
Dopaminergic agents affect the ability of brief periods of normal vision to prevent form-deprivation myopia
Exp Eye Res
(2007) - et al.
The heritability of ocular traits
Surv Ophthalmol
(2010) - et al.
The GEnes in Myopia (GEM) study in understanding the aetiology of refractive errors
Prog Retin Eye Res
(2010) - et al.
Genome-wide association studies reveal genetic variants in CTNND2 for high myopia in Singapore Chinese
Ophthalmology
(2011)