Myopia increases steadily with increasing age. In the United States, the prevalence of myopia has been estimated at 3% among children aged 5–7 years, 8% among those aged 8–10 years, 14% among those aged 11–12 years, and 25% among adolescents aged 12–17 years. In particular ethnic groups, a similar trend has been demonstrated, although the percentages in each age group may differ. Ethnic Chinese children have much higher rates of myopia at all ages. A national study in Taiwan found the prevalence was 12% among 6-year-olds and 84% among adolescents aged 16–18 years. Similar rates have been found in Singapore and Japan.
Different subsets of myopia have been characterized. Juvenile-onset myopia, defined as myopia with an onset between 7 years and 16 years of age, is due primarily to growth in axial length. Risk factors include esophoria, against-the-rule astigmatism, premature birth, family history, and intensive near work. In general, the earlier the onset of myopia is, the greater is the degree of progression. In the United States, the mean rate of childhood myopia progression is reported at about 0.50 D per year. In approximately 75% of teenagers, refractive errors stabilize at about age 15 or 16. In those whose errors do not stabilize, progression often continues into the 20s or 30s.
Adult-onset myopia begins at about 20 years of age, and extensive near work is a risk factor. A study of cadets at the United States Military Academy found myopia requiring corrective lenses in 46% at entrance, 54% after 1 year, and 65% after 2 years. The probability of myopic progression was related to the degree of initial refractive error. It is estimated that as many as 20%–40% of patients with low hyperopia or emmetropia who have extensive near-work requirements become myopic before age 25, compared with less than 10% of persons without such demands. Older Naval Academy recruits have a lower rate of myopia development than younger recruits over a 4-year curriculum (15% for 21-year-olds versus 77% for 18-year-olds). Some young adults are at risk for myopic progression even after a period of refractive stability. It has been theorized that persons who regularly perform considerable near work undergo a process similar to emmetropization for the customary close working distance, resulting in a myopic shift.
While there is no strict cut-off, myopic eyes with a spherical equivalent refractive error of −6.00 D or greater, or an axial length of 26.5 mm or more are said to have “high myopia,” and represent about 2% of the adult population. These eyes are at increased risk of retinal detachment, glaucoma, and choroidal neovascularization.
The etiologic factors concerning myopia are complex, involving both genetic and environmental factors. Regarding a genetic role, identical twins are more likely to have a similar degree of myopia than are fraternal twins, siblings, or parent and child. Identical twins separated at birth and having different work habits do not show significant differences in refractive error. Some forms of severe myopia suggest dominant, recessive, and even sex-linked inheritance patterns. However, studies of ethnic Chinese in Taiwan show an increase in the prevalence and severity of myopia over the span of 2 generations, a finding that implies that genetics alone are not entirely responsible for myopia. Some studies have reported that near work is not associated with a higher prevalence and progression of myopia, especially with respect to middle-distance activities such as tasks involving video displays. Higher educational achievement has been strongly associated with a higher prevalence of myopia. Poor nutrition has been implicated in the development of some refractive errors as well. Studies from Africa, for example, have found that children with malnutrition have an increased prevalence of high ametropia, astigmatism, and anisometropia. Participation in sports and time spent outdoors both appear to protect against juvenile myopia.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.