The eye’s shape depends on growth that occurs primarily during infancy, and to a lesser extent through adolescence. We think that growth is ruled in part by genetic instructions that humans have evolved over many millennia; if the genetic blueprint is defective, eyesight can certainly suffer. But growth of the eye also depends heavily on external cues — what scientists call visual feedback. The bombardment of light, with its colors and contrasts, and use of the eyes (reading, computer work, etc.) help guide proper or improper eye growth.
Scientists are now convinced that something about the visual environment and the use of the eys in this environment has changed drastically in recent decades, and those changes are driving the onslaught of nearsightedness seen in teens and young adults. From the early 1970s to the turn of the century, myopia prevalence in the United States rose from 25 percent to nearly 42 percent among people ages 12 to 54, a substantial shift in just one generation. The rate among U.S. young adults is 38 percent, up from 28 percent in the 1970s. On the other side of the globe, myopia rates in Singapore, which has gone from a sleepy port city to a center for international commerce, have risen from 43 percent among military conscripts (all young men) in the late 1980s to more than 80 percent today.
Meanwhile, older generations haven’t experienced a sharp rise in the disorder. The rate in people over age 40 inChina and the United States is at about one-fourth.
Rural vs urban life
Studies suggest that rates of nearsightedness differ in ethnically related populations living in rural versus urban areas (data from country to country may not be comparable). City living appears to have a detrimental effect on visual problems (I. Morgan and K. Rose/Progress in Retinal and Eye Research 2005).
Because such increases also have not shown up in rural areas, scientists think the trend reflects new behaviors among young urbanites. With more people moving to cities, the trend is likely to worsen. For some, nearsightedness will be a mere inconvenience. But others, who develop high-degree myopia, will have worsening vision over time and a greater risk of cataracts, glaucoma or a detached retina later in life. Of those young men in Seoul and students in Shanghai who are nearsighted, roughly one in five already has high-degree myopia.
This graph shows the prevalence of myopia in China, Vietnam, India and Nepal whether people live in a rural or urban area. We see that the people who live in rural areas (with a school system probably more demanding) have a higher prevalence.
“There will be an epidemic of pathological myopia and associated blindness in the next few decades in Asia,” says Seang-Mei Saw, a physician and epidemiologist at the National University of Singapore.
The new wave isn’t genetic, Morgan says. “The gene pool can’t change that much in a generation, not even in several,” he says.
The other behavioral change that may not mesh well is near work. Human forebears didn’t read, and even those who chipped arrow points or did other fine work probably didn’t do it all day, every day. Frequent near work arrived with civilization; in many societies, it came about in the last century or two. A lot of myopia develops during childhood, and there may be some science behind the stereotypical bookworm with thick glasses. Myopia can also show up in adulthood, depending on the quantity of near work done. This is called occupational myopia.
Recent work by several researchers argues that “reading, writing and computer work will contribute to myopia, and that children who regularly spend much time on computers have a higher risk of myopia.”
A world tour…
Source: Epidemiology (http://en.wikipedia.org/wiki/Myopia)
The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion. The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors. Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.
The prevalence of myopia has been reported as high as 70–90% in some Asian countries, 30–40% in Europe and the United States, and 10–20% in Africa. Myopia is less common in African people. In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.
In some parts of Asia, myopia is very common. Singapore is believed to have the highest prevalence of myopia in the world; up to 80% of people there have myopia, but the accurate figure is unknown. China’s myopia rate is 31%: 400 million of its 1.3 billion people are myopic. The prevalence of myopia in high school in China is 77.3%, and in college is more than 80%. However, some research suggests the prevalence of myopia in India in the general population is only 6.9%.
A recent study involving first-year undergraduate students in the United Kingdom found 50% of British whites and 53.4% of British Asians were myopic.
Myopia is common in the United States, with research suggesting this condition has increased dramatically in recent decades. In 1971-1972, the National Health and Nutrition Examination Survey provided the earliest nationally representative estimates for myopia prevalence in the U.S., and found the prevalence in persons aged 12–54 was 25.0%. Using the same method, in 1999-2004, myopia prevalence was estimated to have climbed to 41.6%.
In Australia, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%. In one recent study, less than one in 10 (8.4%) Australian children between the ages of four and 12 were found to have myopia greater than −0.50 diopters. A recent review found 16.4% of Australians aged 40 or over have at least −1.00 diopters of myopia and 2.5% have at least −5.00 diopters.
Epidemic myopia in Asia
Source: http://blogs.discovermagazine.com/80beats/2012/05/12/why-are-90-of-asian-schoolchildren-nearsighted-from-doing-what-youre-doing-now/#.UTNmJ5aEXjY (Why Are 90% of Asian Schoolchildren Nearsighted? From Doing What You’re Doing Now – By Sarah Zhang)
The sheer prevalence of nearsightedness, or myopia, among Asian schoolchildren (in Singapore, China, Taiwan, Hong Kong, Japan, and Korea) is stunning: 80 to 90% according to a recent review in the journal Lancet. In comparison, that number is just 20 to 30% in the UK. Myopia has also been on the rise in both Asia and Europe over the past few years.
In Singapore, myopia has shot up in the last 30 years among all three major ethnic groups—Chinese, Indian, and Malay—which highly suggests a environmental cause. Singaporean schoolchildren who read more than two books per week were also more likely to have myopia. How one reads physically, may have an impact too: ultra-orthodox Jewish boys, who study the Torah intensely and at a close distance while swaying, have higher myopia numbers than the girls, who don’t. Together, these observational studies suggest that high myopia rates in Asian schoolchildren are likely related to their intense educational systems.
Change in prevalence of myopia among three ethnic groups in Singapore. The following numbers Figures are approximate and are taken from the illustration above.
China India Malaysia
1987-1992 48% 29% 25%
1996-1997 80 % 70% 65%
2009-2010 85% 75% 70%
Also adapted from : http://www.sciencenews.org/view/feature/id/347738/description/Urban_Eyes – By Nathan Seppa