In this study, the overall prevalence of myopia in children aged 9 to 18 was 81.5%, which was similar to another cross-sectional survey conducted in school-based children in Tianjin.14 However the prevalence was higher than that of 63.1% in East China15, 34.3% in Northwest China16, 55.8% in South China9, and other countries17-19. These results elucidated that there is an urgent need to control the high prevalence of myopia in school-aged students in Tianjin. On the other side, prevalence of high myopia in our study was not notably higher than those reported in other studies15,20.
In our study population, myopia prevalence increased rapidly from 57.2% in grade 4 to 85.4% in grade 7, slowly increased to 93.6% in grade 10 and then remained in the high level. The prevalence of high myopia increased dramatically from 0.7% in grade 4 to 18.5% in grade 7. Longitudinal study in China showed that the incidence of myopia was approximately 20% to 30% each year, and progression of myopia was found to be as early as grade 1 to 23. The increasing prevalence with age or grade may mainly be due to risk factors change such as education onward and increased academic burden for students instead of age only. Study showed that new high myopia was strongly associated with education-related parameters21.
Besides, environmental factors were also strong influencing factors for myopia development. Increasing evidence suggests that light intensity is associated with myopia and refractive error change. Li22 recorded light exposure patterns and self-reported time outdoors and found that more time spent outdoors was associated with lower risk of myopia (OR=0.82, 95% CI=0.70, 0.95, p=0.009). A recent prospective study by Sánchez23 revealed that children with shorter daylight hours had higher increase in SER over 18 months (p<0.001). Dopamine (DA) pathway is one of the links between light and myopia. After Stone et al first described that reduced DA and its metabolite 3,4-dihydroxyphenylacetic (DOPAC) acid were detected in myopic eyes compared with control eyes24, extensive studies focused on relationship between dopamine level and eye growth or myopia. In eyes, neurotransmitter DA is released by amacrine cell and plays an important role in the signaling cascade which controls eye growth25. The synthesis and excretion of DA in retina is control by both light exposure and retinal circadian clock26. Light can rapidly activate amacrine cells and then accelerate the rate of DA formation27. Animal experiment results showed that Intravitreal and topical application of DA inhibit the progression of myopia induced by form-deprivation28,29. Meanwhile this inhibit effect was also detected by increased light intensity in mice models30. These results suggests that exposure to light by spent more time on outdoor activity could protest myopia progress by alter DA levels in retina during eye development. In our study population, students spent more than 1h/d outdoor had lower rate of myopia and high myopia and larger SE value, which was consistent with the hypothesis described above.
Near work over 2h/d was associated with higher prevalence of myopia and lower SE in our study. However, whether near work interacts with myopia and eye development remains in dispute. Results similar to ours were found in some studies4,31-33 but not in others34,35. To date, near work is not strictly defined and thus measures of near viewing behavior were not homogeneous between studies36. Near work habit such as continuous reading time, close screen distance, head tilt while writing and reading, desk lighting type, and discontinuing near work period is associated with myopia33,37. Near work related parameters act as independent factors or not should be examined.
In the past decades, electronic device is becoming increasingly common in students and the influence of screen watching on visual acuity is investigated. Liu38 revealed that under the condition of coronavirus (COVID-19) and digital devices became common learning tools, time of television, computer, and mobile phone was positively associated with myopia progression. Yang39 found same trend among preschool-aged children. Mechanisms beyond screen use and myopia or SE change may be the interaction between focusing errors and lags of accommodation40-42.
Our results showed that not enough sleep increased risk of myopia, which is consistent with previous studies43-45. Axial length (AL) and choroidal thickness had significant diurnal variation46 which was regulated by melatonin and melanopsin47. Melatonin and melanopsin play an important role in circadian rhythm regulation. Chakraborty48 evaluated sleep time and urinary melatonin level of human participants and found that myopia was associated with delayed sleep onset, shorter sleep duration, and lower urinary melatonin level, suggesting that abnormal circadian rhythms may be a risk factor of myopia.
Currently, results of influence of diet on myopia are inconsistent in database. We only detected association between myopia and sugary drink but not breakfast and milk. Berticcat7 reported increased probability of myopia for girls with elevated consumption of refined carbohydrates in France children. However, no evidence of carbohydrate intakes and SE was discovered in Singapore children by Chua49. Meanwhile the lack of associations between dietary habits and myopia also occurs in several other studies49,50. Relationship between dietary intakes and myopia development needs further discussion.
In areas with high prevalence of myopia such as Asian countries, prevention is an urgent task. A variety of studies investigated lifestyle (mainly including outdoor, near work, and screen use)51-54, but how these factors work together and lead to myopia or promote myopia progression is intricated. Though many efforts have been tried to reduce the incidence of myopia and the effect of myopia prevention was checked55, most of them implemented only a few intervention measures which may not be suitable for every single person in practice. In addition, how to identify target population with high risk of early-onset myopia is another significant important issue56 because delivery of accurate early intervention could be twice as effective. One of the aims of this study is to attempt to identify if the risk of myopia or SE value is different among children with different lifestyles. Our results revealed that children having longer near work and screen use and consumed sugary drink had higher risk for myopia and lower SE value compared with others. Thus, students exposed to these risk factors should be paid more attention for myopia prevention. Whether individual combined with universal intervention strategy is better than universal strategy needs more evidence.
The main advantage of our study is that this survey is part of 8th CNSSCH and standard protocol ensured data quality. Meanwhile we try to classify children into subgroups using LCA model according to lifestyle and found that myopia risk was different among groups. A few disadvantages existed. Non-cycloplegic SE was used to evaluate myopia status and may lead to overestimate of myopia57. However, in such a large-scale myopia epidemiological survey, non-cycloplegic SE was most frequently used58-61 concerning of the resistance to cycloplegia and time efficacy62. Our study design is cross-sectional research so that causal relationship between lifestyle and myopia cannot be testified.
In conclusion, our findings suggest that prevalence of myopia in children aged 9 to 18 in Tianjin is high. Lifestyle is associated with myopia and SE value. More efforts should be implemented precisely to students with different lifestyle.