East Asia’s high levels of childhood myopia, also known as nearsightedness, has long been a mystery: It affects as many as 90% of urban 18-year-olds. Researchers have identified possible causes including intensive studying, less time spent outside—even the Chinese language itself. And now a new study has made the mystery even stranger.
Researchers compared 9,400 students in a middle-income area of Shaanxi province with about 10,100 in neighboring Gansu province, China’s second poorest province, and found that students in the wealthier area were twice as likely to be nearsighted. Even after adjusting for factors like the amount of time the students spent reading, playing outside, or whether their parents wore glasses, “living in the middle class was associated with a 69% increased risk for nearsightedness,” the researchers said.
“We could not explain much of the large variation in prevalence of clinically significant myopia between middle income Shaanxi and low-income Gansu,” the researchers concluded in the study, this week in the Journal of the American Academy of Ophthalmology.
What exactly is it about life in more rural areas that protects children against myopia? The use of blackboards is one possibility, according to the study. In lower-income schools, students own fewer books and are more likely to use blackboards in the classroom, which appeared to have a “protective effect” against myopia, possibly because they require more focusing at a distance.
“We’re still on the hunt for a plausible explanation and think the results merit more study into whether using blackboards versus books may be partially responsible for protecting eyes against nearsightedness, and what other factors may play a role,” said Nathan Congdon, the project’s lead investigator, a professor at the Zhongshan Ophthalmic Center at Sun Yat-sen University in Guangzhou, China.