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August 31, 2017
Myopia is the medical term for the common vision condition known as nearsightedness, in which close vision is sharp, but objects farther away appear blurred. Nearsightedness occurs when the eyeball is too long, the cornea is steeply curved, the lens stays focused on near vision, or a combination of all these factors. This causes light rays to focus at a point near the front of the retina rather than directly on its surface, resulting in distant objects being out of focus.
Like all refractive errors, myopia is measured in units called diopters, which are measurements of optical lens power. This is determined by the distance from the lens surface at which light is brought into perfect focus. A minus sign in an eyeglass prescription indicates nearsightedness, with negative numbers escalating with myopia severity.
-0.25 to -3.00 diopters: mild nearsightedness
-3.25 to -6.00 diopters: moderate nearsightedness
-6.25 to -10.00 diopters: severe nearsightedness
-10.25 diopters or higher: extreme nearsightedness
In 2010, myopia was the leading cause of distance refractive error, affecting 1.45 billion or 27% of the world’s population. In the U.S. this figure was slightly less in 2010, with myopia impacting nearly 24% of the population or 34.1 million people. The number of people with myopia is increasing, however, the prevalence differs greatly by country. The National Eye Institute projects myopia will impact 39 million Americans by 2020 and 44.5 million by 2050. Recent studies indicate the prevalence of myopia is increasing globally in young people, but especially in East and Southeast Asian countries.
In Taiwan, the prevalence of myopia among 16- to 18-year-olds increased from 74% in 1983 to 84% in 2000
In Japan, the prevalence among 7-year-olds increased from 39% in 1984 to 59% in 1996.
Other surveys have shown an extremely high prevalence among teenagers in Korea (96.5%), Shanghai, China (81.6%), and Singapore (95.5%).
A 2017 Chinese study on 37,424 14-16 year-olds indicated a significant increase in the prevalence of myopia from 55.95% in 2006 to 65.48% in 2015. The moderate high myopia subgroup increased the most (18.34%).
Multiple studies have shown an increase in myopia among children is caused by a decrease in time spent outdoors.
While moderate myopia typically does not have serious repercussions, the degenerative changes associated with high myopia can result in varying degrees of sight loss. High myopia (also called pathological myopia) is generally defined as a refractive error exceeding -6.00 diopters and/or an axial length longer than 26.5 mm. Axial length is the measurement of the eyeball from front to back.
Pathological changes in high myopia begin in childhood and become more apparent in adulthood. About 30% of all cases of pathological myopia occur at birth, with 60% of individuals diagnosed between the ages of 6 and 13. Excessive axial elongation results in chorioretinal stretching and subsequent thinning. High myopia is associated with an increased risk of the following sight-threatening conditions.
Myopic macular degeneration: As the eyeball grows and stretches, this may cause layers under the retina to atrophy or crack. The cracks can facilitate the growth of abnormal blood vessels under the retina, which can hemorrhage and scar in a similar manner as in wet macular degeneration. Raised, pigmented, round, or elliptical lesions called Fuch’s spots can form, which cause blind spots in central vision. These spots most often impact people in their 40s to 60s, however only about 5% of those with pathological myopia are affected.
Glaucoma: Population-based studies indicate the risk of primary open angle glaucoma (the most common type) increases with a higher degree of myopia, across many geographic regions. A large study on 32,918 older adults (ages 57-79) found the prevalence of newly detected glaucoma increased with higher degrees of myopia across all age groups.
Cataract: This common clouding of the eye’s lens affects a majority of people age 80 and older, however, high myopia raises the risk, especially if it has an earlier onset. One theory is that increasing axial length deprives the posterior surface of the lens of nutrients, leading to reduced clarity. Other studies suggest axial elongation causes damage to the light-receptor cells of the retina, thereby contributing to cataract formation. Moreover, the risk of retinal detachment during cataract surgery is 5-10 times higher in people with high myopia.
Retinal detachment: Thinning layers of the eye may lead to degenerative changes in the peripheral retina. These retinal holes and lattice degeneration increase the risk of retinal detachment. The lifetime risk for retinal detachment in individuals with high myopia is about one in 20.
Posterior staphyloma: This condition can be congenital or occur as a result of pathological myopia. When the eye elongates, it stretches and thins the retina and the sclera of the eye. Staphylomatous protrusion in the back of the eye can contribute to visual distortion. By age 60, 50% of people with staphylomas in both eyes are legally blind.
Myopia is a complex trait influenced by genetic and environmental factors, as well as gene-environment interactions. The link between parental myopia and a child’s risk of developing the condition is not well understood. A 2017 study on 31,677 participants and 8,393 cases of myopia, confirmed that parental myopia has a significant association with a child’s risk of developing the condition. Moreover, if both parents are myopic, children are at even greater risk. Multiple population-based genetic studies have attempted to identify specific genes linked to myopia, with a wide array of results. Many studies on non-syndromic high myopia have focused on genes involved in the scleral extracellular matrix. A recent study on 14 Caucasian families with autosomal dominant high myopia identified 104 potential new gene variants for high myopia.
While research studies are promising, it could be years before scientists uncover conclusive evidence that might lead to more exacting risk predictors and preventive interventions. Until then, experts suggest encouraging children to spend time outside rather than playing computer games all day long. Annual dilated eye exams are recommended for all people with myopia, especially those with more severe cases.