From an optical point of view, we can well explain the role played by various optical elements of eyeball, that is, eye axis growth is the main cause of myopic progression. Controlling the growth of eye axis is the most important means to control the progression of myopia[15, 16]. It is well known that high myopia can easily lead to many complications, and eventually lead to serious visual impairment[17, 18]. A large-scale cross-sectional epidemiological survey of myopia in Shanghai found that the prevalence of high myopia among college students in China was as high as 10%-20%. Previous studies have shown that the age of 9 to 16 is the fastest growing period of myopia in adolescents. Given the reliability of the longitudinal study and the growing period of myopia, it is extremely significant for us to conduct this longitudinal study on adolescents aged 10–15.
Donovan et al compared the progression of myopia among adolescents (9.3 years old) in Asian urban areas with that in Europe. They found that the progression of myopia among adolescents in Asia was faster (-0.82D vs. -0.55D), and the progression speed of myopia decreased significantly with the increase of age. Hong Kong scholars reported that the annual growth of myopic adolescents was − 0.63D, while those who were not myopic was − 0.29D. The Singapore Cohort Study of the Risk Factors for Myopia (SCORM) found that the annual growth rates of myopia in adolescents aged 7–9 years were − 0.80D, − 0.66D and − 0.57D, respectively. Thorn et al found that the evolution of myopia follows the rule that there was no myopia in the early stage, the occurrence of myopia was in the student period, and finally myopia was stable. After at least six years of follow-up, the Correction of Myopia Evaluation Trial (COMET) found that the average age at which myopia slowed down was 11.95 years old, the average age at which myopia was stable was 15.61 years old. In addition, they compared the stable time and myopia degree of five different races, and found that African-Americans had the earliest stabilization of myopia, and the myopia degree was the lowest. In this study, there was significant myopia progression in all age groups after two-year follow-up, with an average increase of − 0.56D. The myopia progression in 6th grade was the fastest (F = 8.236, P = 0.003). However, age was not a risk factor for myopia progression (OR = 0.94, 95% CI: 0.62–1.32, P = 0.424). By comparing with the domestic and foreign literatures mentioned above, we can draw the following conclusions: In recent years, the progression of myopia among Chinese adolescents has obviously accelerated; the age of 13 may be a turning point in the progression of myopia; and the stable age of myopia was over 15 years old. Although compared with other ethnic adolescents in Europe and America, Chinese Han adolescents was indeed developing faster, this rapid growth might not be explained only by genetic factors. The social, living, learning and other environmental factors might be the important factors contributing to this difference.
At present, the role of gender in the progression of myopia was still controversial. A longitudinal studies of Australian adolescents aged 12 and 17 showed that girls' myopia progressed faster. The COMET found that boys' myopic progression was about 0.16D slower than girls' each year. They suggested that if gender played a role in myopia, they would occur in the early stage of myopia, and it would not last long. Some scholars have explained that this may be related to the faster and earlier growth of girls, the relatively rapid development of eyeball and their susceptibility to environmental factors, as well as the fact that girls were quiet, eager to learn and having less outdoor sports than boys[26, 27]. In this study, although we found that both boys and girls showed significant myopia progression after 2 year follow-up. the SE of both boys and girls decreased significantly after two year of follow-up, the growth rate between them was not significant.We believe that the hypothesis of COMET is reasonable. Due to the old age of the adolescents selected in this study, the progression of myopia might be less affected by gender.
Similar to our previous cross-sectional study, the proportion of two parent myopia was still the highest, followed by one parent, and no parent myopia was the lowest (χ2 = 27.919, P༜0.001). In addition, in our previous cross-sectional studies, although we did not find a correlation between one parent myopia with the prevalence of myopia, there was a positive correlation between two parent myopia with it. However, in the longitudinal study, we found there was no significant correlation between the progression of myopia and one parent myopia (OR = 1.14, 95% CI: 0.74–1.84, P = 0.532) and two parent myopia (OR = 1.58, 95% CI: 0.76–2.16, P = 0.163). Perhaps we can hypothesize that the effect of parent myopia on children might only occur in the initial stage of myopia. With the deepening of myopia, the impact would be less obvious.
By comparison, we found that the results of this study on near work and outdoor activity time were in good agreement with those reported in ACES. A meta-analysis of seven cross-sectional studies reported a 2% reduction in the odds of myopia per additional hour of time spent outdoors per week. A three-year prospective study in Guangzhou found that a 40-minute outdoor activity class added to each school day could effectively reduce the incidence of myopia(30.4% vs 39.5%); however, they did not find that such interventions reduced the growth of myopia. Other studies have suggested that there might be a threshold of 10 to 14 hours spent outdoors per week required to prevent myopia[29, 30]. In our cross-sectional study, outdoor activity time was an independent protective factor for myopia. However, in the longitudinal study, we did not find that the progression of myopia was related with near work time and outdoor activity time. Thus, our results are basically consistent with the above results. Perhaps, outdoor activities could only slow down the onset of myopia, but it has no effect on the progression of myopia. It was also possible that the effect of outdoor activities on the progression of myopia needed to reach a certain threshold. In other words, the threshold for controlling myopia progression might be higher for children who were already myopic. However, due to the influence of learning pressure, outdoor activity time of Chinese adolescents was far from the threshold.
At present, the influence of near work on myopia progression was still controversial. In this study, we still haven't found that near work time could effect the progression of myopia. Perhaps, the hypothesis that near work related behavior played a more important role in the occurrence and progression of myopia in ACES might be reasonable. This provided a good direction for our further research. Although the design of this experiment was precise, there was still a memory bias in the information collected through the questionnaire. Recently, an intelligent, wearable, and real-time myopia monitoring device “Clouclip-(TM)” developed by Aier ophthalmology school of Central South university and China Glasson Technology Co., Ltd had aroused widespread concern. It would be of great help to myopia prevention and control.