This study used DALYs for the first time to elucidate the disease burden associated with URE among adolescents, exploring temporal trends from 1990 to 2019 and the global distribution by age, sex, WHO regions, level of healthcare and socioeconomic factors. From 1990 to 2019, the world DALY numbers due to URE increased by 8%, while the world DALY rates slightly decreased over time after adjusting for increases in population. The global DALY rates of URE increased by age. Females had higher DALY rates than males of the same age. Regarding regional distribution, the disease burden in the Eastern Mediterranean, high-income and middle SDI regions was highest. Further research revealed that the disease burden of URE was associated with HDI, SDI, primary school dropout rates and urbanization rate.
Refractive errors are the result of a mismatch between the axial length of the eye and its optical power, creating blurred vision. Eye growth, namely, emmetropization, is believed to be completed at the age of 13 years, while axial length growth in myopic eyes may continue[14]. Myopes typically exhibit early progression quickly, and the progression then slows down; in general, their refractive error eventually stabilizes during late adolescence, e.g., 15–21 years of age[15]. High incidence and high degrees of astigmatism were demonstrated to exist among children, especially newborns. As children grow older, the cornea flattens with significantly reduced astigmatism and stabilizes before adulthood[16]. Therefore, in this study, adolescence was defined as ages below 20 years.
The results showed fluctuation in the prevalence and disease burden of URE among adolescents from 1990 to 2019. The temporal variation in the disease burden of URE was largely in accordance with the variations observed in prevalence. However, compared to prevalence, DALYs are more suitable indicators to measure the influence of a disease on quality of life. The total DALY numbers due to URE increased by 8% from 1990 to 2019, while the DALY rates remained stable after adjusting for global population growth. The high disease burden of URE among adolescents persists, suggesting that current health policies to control URE have failed to alleviate the additional disease burden caused by increasing prevalence. According to Holden et al[17], the global myopic population will continue to expand and reach 4.758 billion (49.8% of the world population) by 2050, which means healthcare planners will face an even heavier burden of VI related to URE.
Sex inequality in the global burden of URE exists even among adolescents and has persisted since 1990, with females bearing a significantly higher burden of URE than males. Females had a higher burden of URE than males for all age groups in 2019, as has been reported previously[18]. Many studies have revealed a higher prevalence of refractive disorders among young females than in males. This may reflect different environmental risk factors, such as the tendency of girls to spend a greater number of hours engaged in near vision activities and significantly fewer hours outdoors than boys[19–22]. The longer life expectancy of females in most cultures could also contribute to a higher global burden of URE. Moreover, the inequality of social, cultural, and economic status between men and women is believed to reduce access to eye care services, including refractive correction for women[23]. However, the sex discrepancy of spectacle coverage has been reported as less extensive than expected[24, 25], which implies that the gender inequality of resource distribution may not be the primary cause.
The age-specific variation curve of DALY rates due to URE showed that the steepest increase in disease burden occurred in the 5–9 age group, followed by relatively smooth growth in the 10–14 age group; DALY rates then seemingly reached a plateau at 15–19 years of age. This trend was nicely consistent with the growth law of the ocular axis and myopia, which typically exhibits fast progression early at the age of 6–8 years, followed by slowing and eventual stabilization of refractive error in late adolescence[14, 15, 26]. The increasing study burden among these school-age children may explain the growing prevalence rate of VI associated with URE[21, 27].
Regional inequality was also obvious, with a higher disease burden among adolescents in the WHO Eastern Mediterranean Region, Region of Americas and South-East Asia Region. Our results also revealed that regions with middle to high income and SDI have higher DALY rates. The HDI and SDI, two widely used indexes of socioeconomic development, were significantly positively correlated with VI due to URE in univariate linear regression. We compared the results of our disease burden analysis focused on adolescents to similar studies among individuals of all ages. Our results turned out to contrast with the findings of Lou et al., in which age-standardized DALY rates were inversely associated with HDI[10]. These differences could be attributed to the diverse spectrum of diseases at different ages. The aging process of ocular refractive structures, such as loss of lens power and lens opacity, can lead to presbyopia and crystalline source refractive error among older people, which constitutes a large part of VI due to URE. A total of 244 million uncorrected presbyopia cases among people aged < 50 years were associated with a potential productivity loss of US $11.023 billion (0.016% of global GDP)[28]. The prevalence of presbyopia is estimated to be higher in regions with longer life expectancies, whereas a greater burden of VI resulting from uncorrected presbyopia occurs in less developed countries. The low amounts of available eye care resources and poor optical correction rates in these areas were proven to be the underlying causes[29]. Moreover, the quantity and quality of cataract surgery that could correct refractive errors to some extent were also positively associated with GDP per capita and HDI[30]. For children under 20 years of age, a recent meta-analysis by Hashemi et al. estimated global and regional prevalence figures of refractive errors: the estimated pooled prevalence of astigmatism (> 0.50 D, 14.9%) was higher than that of myopia ( ≤ − 0.50 D, 11.7%) and hyperopia ( ≥ + 2.0 D, 4.6%). The prevalence and severity of astigmatism and hyperopia often decrease during emmetropization, while myopia tends to exhibit fast progression in school-age children[31]. This process can be greatly influenced by environmental factors[13, 32]. Among these factors, education as an element of SDI and HDI plays an important role. The tendency for schooling to lead to an increased prevalence of myopia and VI has been documented in almost all major population groups[21, 27, 33]. This result agrees with our finding that primary school dropout rates were inversely correlated with the DALY rates due to URE in both Pearson correlation and univariate linear regression analyses.
Additionally, variations in other parameters, such as living environment, diet and lifestyle accompanying societal development may also contribute to the high prevalence of URE and VI among adolescents[13, 32]. Therefore, we explored the effects of primary school dropout rates and urbanization rates on URE burden with multiple linear regression analysis. A heavier disease burden due to URE in young people was suggested to occur among higher urbanized countries with lower primary school dropout rates. Potential mechanisms were enumerated as follows: first, primary and middle students could be more susceptible to myopia due to environmental factors such as heavy educational stress and fewer outdoor activities[21]. Second, findings from many studies indicated that the burden of myopia may be heavier among urban residents, which is consistent with our results[34–36]. Urban areas are usually characterized by more severe environmental pollution (less green space, ambient light exposure), different lifestyles (lower levels of time outdoors and higher levels of indoor activities)[35] and heavy academic pressure, all of which may affect refractive errors. Recent interventional prospective studies have shown that encouraging outdoor activity and exposure to sunlight effectively prevents the development of myopia[11, 12]. Third, significant inequities in resources for refractive error correction still exist in developed countries with high urbanization rates[37, 38] and in urban areas of developing countries[39]. As a result, the need for refractive error correction in many people, especially migrants, remains unmet, although resources are adequate in these regions. This is partially attributable to socioeconomic barriers, such as low income, lower rates of health care coverage, fewer visits to health services, and language barriers[40, 41].
This study has several limitations. First, the accuracy of the disease burden information was affected the limitations of the data source; the data were not collected first-hand. Second, data from 204 countries and territories were obtained in the GBD database, and the completeness of the study was limited by the unavailability of data from the remaining countries and territories.
Despite the limitations mentioned above, the study findings can be useful for developing targeted strategies to address the severe VI resulting from URE. The estimated large disease burden reveals a challenging task for healthcare policymakers, and considerable efforts will be required to scale up refractive error services for adolescents.