Myopia affects approximately 20% to 25% of individuals in most Western populations, with a much higher prevalence (>50%) in certain Asian populations.
1–3 Both genetic and environmental factors are implicated in the etiology of myopia. The higher within-pair correlations for refractive error found in monozygotic twins compared with dizygotic twins are consistent with an important genetic component of myopia,
4–7 and parental history of myopia has been shown in some,
8,9 but not all,
10–12 studies to exert an influence on children's eye size before the onset of myopia. However, as reviewed by Morgan and Rose,
13 some researchers have argued that the interpretation of studies of twins with myopia is not straightforward, because exposure to environmental risk factors can be different in monozygotic and dizygotic twin pairs, thus violating the so-called common-environment assumption. Thus, a less controversial body of evidence for a genetic contribution to refractive error comes from the identification of several genes and genetic loci for refractive error.
14,15 As well as including loci from studies of familial high myopia, several genome-wide significant loci have been found by examining ocular refraction as a quantitative trait, covering the full spectrum from myopia to hyperopia
16–18 In those cases in which the gene concerned has been identified and replicated (e.g., the
HGF and
COL2A1 genes),
19–22 it seems unlikely that the genetic risk is manifested by a change in behavior that increases exposure to an environmental risk factor, thereby ruling out an indirect environmental effect (but this situation could hold for some of the myopia susceptibility genes that have yet to be characterized). Recent epidemiologic studies of environmental risk factors such as time spent engaged in outdoor activity and urban versus rural dwelling, have shown these variables to be strongly associated with myopia development.
23–26 However, as with the genetic studies, such epidemiology findings have not been universal: For example, a study of school children from a rural part of China found that time spent in outdoor activities was not associated with myopia.
27 In the same study of Chinese school children,
27 the intensively studied risk factor of time spent doing near work also was not associated with myopia, highlighting the difficulty researchers have faced in trying to understand the relationship between these variables.
28 Thus, although the precise mechanisms of myopia's development are still unclear, a range of genetic and environmental influences seems likely.
29