Based on the data from large population-based multi-age group studies, we presented
prevalence estimations for myopia, hyperopia and astigmatism in Hainan, the tropical island of China with children aged 6 to 15 years old. This is the first report about refractive errors in this emblematic population of children and adolescents in farthest south China. It has revealed myopia to be the most common type of refractive error.
In our study, we found that prevalence of myopia, high myopia and uncorrected visual acuity impairment was lower in the first 3 years of primary school, but increased with age and grade as intensive continuing education increased. However, hyperopia prevalence declined with age. The average SE results were -0.98 D (±1.37), -1.16 D (±1.42), and -1.39D (±1.50) in the above three age groups respectively. In addition, low myopia is the most common form of myopia, but the prevalence of high myopia increased with advancing age.
Therefore, compared with recently published 5 year child and adolescent myopia data from other countries, the prevalence of myopia in our sample was significantly higher than Netherlands (2.4%) (7), Saudi Arabia (2.7%) (12), Norway(13.0%) (10), Colombia (14.9%) (13), North India (21.1%) (14), Denmark (17.9%), Spain (20.0%) (9) and Poland (16.33%) (15). However, it is similar to France (42.7%) (16), but lower than in Korea (51.9%) (3) (see Table 3). Compared with different regions across China, the prevalence of myopia in our study within the same age range of children was lower than that of Feng Hua (87.65%, Eastern China) (17)，Guangzhou (69.9%, Southern China) (17)，Beijing (70.9%, Northern China) (18)，Qingdao (52.02%, Eastern China）(18), Chongqing (54.9%, Western China), and Tianjin (53.9%, Northern China) (19), but higher than Mangshi (35.9%,Western Rural China) (20) and Tibet（28.51%, Plateau of China）(20). (Table 3)(Figure. 5).
The overall prevalence of high myopia was very low (0.96%), lower than many studies with a similar age group，such as studies from Korea (5%) (3), Spain (3.6%)(9)，Indonesia (8.54%) (9, 21), and the aforementioned Chinese cities. The overall prevalence of hyperopia (SE≥+0.50 D) (13.80%) is significantly higher than Indonesia (0.73%) (9), America(5%)(22) and Chongqing (3%, Western China) (23); Significantly lower than Norway (57.0%) (10), Brazil (59.8%) (24), and Colombia (32.3%) (13). However, it is similar to Korea (13.4%) (3). A thorough comparison of refractive error based on studies published during the last 5 years is summarized in Table3.
Mean SE in our study was better than Guangzhou (-1.7± 1.9)(17) ,Tianjin (-0.99 ± 1.69 D) (19) in 6-12 year-olds, and Yiwu (-2.61 ±2.01)(25) in 12 year-olds. It was however worse than Poland (+0.55±1.23) (15) and Norway (+0.51 ±1.29) (10). In this study, the trends of different SE were also calculated. The trends showed a close ratio between SE≤-0.5 and SE≤-1.00, a low ratio of SE≤-5.00 and SE≤-6.00. This indicated another feature that although myopia was common, it could develop into high myopia slowly. (Figure.5).
The exact mechanism associated with myopia was undetermined, but could be explained by many related factors, such as genetic factors, environment, lower refractive status at baseline, shorter reading distance, outdoor exposure and so on. Overall, increased incidence of myopia remains a global public health challenge, which necessitates novel therapeutic methods to curb its progression.
Even though genetic factors are considered to be important in myopia development, especially high myopia (7, 26), a multitude of studies on the large increase in incidence propose a much stronger effect of environmental factors in younger students. Some researchers hold the view that intensive continuing education and limited time outdoors are major risk factors (18). However, little evidence of the relationship between time spent at work and myopia was reported in Norway (10). This is important as Norway has a low myopia prevalence and being outdoors is a part of growing up. Many of the studies on myopia suggest that longer outdoor light exposure time correlated with a significant reduction in myopia prevalence and incidence among school students (9). High levels of daylight exposure were considered to be the environmental factor of greatest importance in preventing myopia (9). Haikou City, as the capital of Hainan Province, located in low-latitude tropical regions in China, experiences China’s longest hours of sunshine and great radiant energy. The average sunshine hours are more than 2000 hours per year. The hypothesis that daylight exposure explains why prevalence of myopia in our study was lower than other regions of China cannot explain why it was higher than other countries at the same latitude, such as North India (14). In addition, at the same latitude, the incidence rate of myopia was higher than in other Asian children of the same age, but with lower high myopia prevalence (3)Therefore, daylight exposure may be an influential factor, but it does not fully explain myopia development.
Results of another study showed that increased computer use is related to myopia development before children reach 10 years of age. outdoor exposure may be important for intervention against myopia because it could mitigate near work activities, including computer use, reading time and distance (27).A systematic review of several studies has indicated an association between screen time and myopia, but Meta-analysis suggested that screen time was not related with prevalent and incident myopia. So, there is still a debate whether digital screen time would induce the higher risk of myopia (28). However, there is no question that more near work and less time spent outdoors would be affected by the increased use of digital devices.
As mentioned in the literature review, previous studies have noted the importance of UVB exposure in myopia. Increased UVB exposure reduces myopia, especially in adolescence (29). Violet light suppressed the axial length elongation in the chick myopia model, and myopia suppressive gene EGR1 was upregulated as revealed by expression microarray analyses (30). Several reports have shown that the increasing quartile of total UVB can contribute to decreased prevalence of myopia (31).
Hainan, with the longer time of UV exposure, is the smallest and southernmost province of China. As compared to many studies conducted in other provincial capital cities of China, low overall myopia prevalence (45.97%) and low overall high myopia prevalence (0.96%) were found in our screening population. In addition, numbers of nonmyopic students newly developing myopia annually are lower. The possible reasons for this difference may be associated with environmental influences. It is likely that the UVB exposure is one of the important factors in myopia development.
Our study contributes new knowledge to the field attributed to the latest and large population-based screening methods. When we defined refractive errors, monocular myopia, hyperopia and astigmatism were taken into consideration rather than performing statistical analyses on one of two eyes. Our approach thus represents the overall data unlike reports in other published papers. Our calculated myopia prevalence would therefore be artificially lower than now reported if we simulated the alternative approach from such studies.
Although myopia was defined as combined spherical equivalent with normal UCVA to reduce over-measuring of myopic magnitude, there is still a notable limitation of the present study. There is lack of cycloplegic refraction that might lead to over-estimation of myopia and under-estimation of hyperopia due to accommodation. The difference in refractive error prior to, and after cycloplegia was about 0.37 diopters in a small sample; consistent with other research findings (32). Regardless, the difference between pre- and post-cycloplegia was small and may not impose a significant clinical influence under real-world conditions in a large sample population. We also need further investigations to acquire data on ocular biometry and more detailed information about students. We certainly aim for this to be our next research focus.
Genetic and environmental risk factors may be taken into consideration to explain how refractive errors develop differently. Our results presented a higher myopia prevalence than European countries, which indicated the potential presence of a genetic predisposition to myopia in Asian populations. All in all, myopia in Asians is a serious health problem. Genetic heterogeneity, variation in circannual adaptation and environmental factors including timing and behavior patterns of exposure to myopia-generation are related to greater shift towards myopia. Therefore, it is important to dedicate effective control methods to slow myopia progression.