As the global myopia crisis continues to grow, researchers are working to identify and quantify the influences of various environmental factors on the disease. Previous epidemiological studies have pointed to higher occupational status, education, near work and school attendance as major associations, while other studies demonstrated that less time spent outdoors seems to be a contributing factor. There’s still no consensus, however, on which factors are key and how they influence myopia.

A recent study attempted to answer these questions, looking specifically at associations of near work, outdoor time and parental myopia with myopia prevalence in school-aged children. The team reanalyzed questionnaire data from a 1986 study conducted previously in Finland by one of the same authors. They found positive associations with the investigational factors in addition to a higher prevalence among girls.

The 1986 study included 4,961 school-aged children in first, fifth and eighth grade (aged seven, 11 and 15, respectively). Each child underwent vision screening before answering a questionnaire. A total of 4,352 questionnaires (87.7%) were returned. The questionnaire included items such as daily time spent on near work and outdoor activities (excluding time spent at school), TV usage and parental myopia.

Upon reanalysis, the researchers reported the prevalence of myopia as 3%, 15% and 27% among each age group, respectively. If the amount of daily near work at home was an hour or less, myopia prevalence decreased to 0.5%, 3.3% and 17.6%, respectively.

Adjusted myopia risk scores for each daily near-work hour were 1.476, 1.346 and 1.206, respectively, for the three age groups. Adjusted myopia risk scores for each daily hour of outdoor time were 0.764 in 11-year-olds and 0.840 in 15-year-olds.

The researchers reported that if the ratio of near work and outdoor time was ≤0.5 or >1.5, myopia prevalence was 1.4% vs. 5.6%, 6.3% vs. 24.7% and 15.9% vs. 36.9%, respectively. “Outdoor time prevented myopia at different levels of near work, although less at the highest levels and near work increased risk of myopia with the level of outdoor time.”

Homework time may also play a role. “If the amount of daily near work was low (≤1 hour, excluding time at school), the prevalence of myopia was uncommon in the seven-year-olds (0.5%) and 11-year-olds (3.3%),” the researchers wrote. “In Finland, students in the early school grades are not usually given much homework, and there is little educational competition. In many East and South Asian countries, where the prevalence of myopia is high, young children do more homework. In Singapore, schoolchildren did approximately twice as much near work as same-age Finnish school children.”

Additionally, they found that girls experienced a higher rate of myopia than boys. “This is explained by more near work and less outdoor time among the girls,” the researchers wrote.  

Parental myopia status also affected childhood myopia prevalence, but this finding was not necessarily genetic. “Having two myopic parents roughly doubled the risk of myopia compared with one myopic parent in the 11- and 15-year-olds,” the researchers noted. “However, while parental myopia in this study was also strongly associated with myopia in their children, the link is obviously not solely genetic but also environmental, including factors such as parental education and socioeconomic status.”

Whatever the reason, younger age of myopia onset was the most significant factor contributing to a higher rate of myopic progression and increased prevalence of adulthood myopia, hence increasing the risk of myopia-related ocular complications. “Thus, if the onset of myopia could somehow be delayed, complications associated with high myopia could be significantly reduced,” the study authors concluded.

Pärssinen O, Kauppinen M. Associations of near work time, watching TV, outdoors time, and parents’ myopia with myopia among school children based on 38-year-old historical data. Acta Ophthalmologica. July 21, 2021. [Epub ahead of print].