Global disease burden of uncorrected refractive error among adolescents from 1990 to 2019

Uncorrected refractive error (URE) is an underappreciated but widespread public health problem that significantly diminishes both quality of life and productivity. Among the global population with vision impairment and blindness in 2015, URE resulted in moderate to severe visual impairment in 116.3 million people and blindness in 7.4 million people. With an extremely high prevalence, URE ranks first among visual impairment causes, and the affected population is expected to continually grow [1]. Being at the critical period of eye development,children and adolescents could be sensitive to environmental factors and at high risk of suffering from refractive disorders. Especially during the coronavirus disease 2019 (COVID-19) pandemic period, these school-aged children could be vulnerable to undiagnosed myopia owing to the home confinement policy and not attending class where myopic individuals can be highlighted through routine lessons (e.g. copying from the board) [2]. Access to ophthalmology outpatient services including optometry were also limited during the peaks of the pandemic, and the need for these services is now backlogged.

Given the difficulties that people with URE face in activities of daily living and social functioning, it is not surprising that URE can be associated with lower quality of life [3] and productivity loss [4]. The adverse effects of URE can be more serious in children, whose poor-quality visual experience due to URE can lead to unreversible amblyopia [5] and inadequate knowledge acquisition from the outer world during development and maturation [6]. There is no doubt that good visual performance has a profound effect on children’s cognitive and psychosocial development [7, 8].

Although there have been many studies on the epidemiology and etiology of refractive error, there are few evaluations quantifying its health burden. The Global Burden of Diseases Study (GBD study) [9] developed disability-adjusted life years (DALYs) to estimate the disease burden of many conditions, including visual impairment due to URE. DALYs are determined by the number of disabled years weighted by the level of disability caused by a disability or disease. Previous research has examined the DALYs of URE among people of all ages [10]. However, an aged population was generally affected by presbyopia and the previous study failed to fully describe the URE burden among adolescents. As most URE in adolescents is preventable and controllable, in this study, we focused on the disease burden of URE in adolescents [11‐13]. The aim of this study was to describe the temporal trends of the global disease burden of URE among adolescents and its distribution across different age brackets, sexes, and regions and to explore the risk factors for DALYs due to URE in adolescents, including national-level data on demographic, socioeconomic and educational factors.

This research has been conducted as part of the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD), coordinated by the Institute for Health Metrics and Evaluation. The GBD was partially funded by the Bill & Melinda Gates Foundation; the funders had no role in the study design, data analysis, data interpretation, or writing of the report.

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.

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, levels of income and SDI. 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. Stepwise linear multiple regression revealed that after eliminating collinearity, only primary school dropout rates and urbanization rates were significantly correlated with the disease burden of URE.

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 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 visual impairment 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, 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. The loss of accommodative ability and lens opacity with aging can lead to presbyopia and crystalline source refractive error among older people, which constitutes a large part of visual impairment due to 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]. As estimated by one previous study, the median annual cost of refractive correction was 226.48 dollars (including eye exams and eyeglasses) in the United States, which could be a considerable sum for low-income adults [24]. However, the sex discrepancy of spectacle coverage has been reported as less extensive than expected [25, 26], 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, 27]. The increasing study burden among these school-age children may explain the growing prevalence rate of visual impairment associated with URE [21, 28].

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 visual impairment due to URE in univariate linear regression. The association of disease burden of URE with HDI and SDI might change with economic status. 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 visual impairment due to URE. By multiplying prevalence, the labor force participation rate, the employment rate, a disability weight and the GDP per capita, the potential productivity loss caused by 244 million uncorrected presbyopia cases among people aged < 50 years were estimated to reach US $11.023 billion (0.016% of global GDP) [29]. The prevalence of presbyopia is estimated to be higher in regions with longer life expectancies, whereas a greater burden of visual impairment 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 [30]. 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 [31]. For adolescents 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 [32]. This process can be greatly influenced by environmental risk factors. Low outdoor time, dim light exposure, close use of electronic screen (phones / laptops), education and living in an urban environment has been successively suggested as possible risk factors for myopia in adolescents in recent studies [13, 33]. 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 visual impairment has been documented in almost all major population groups [21, 28, 34]. 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 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 [35, 36] and in urban areas of developing countries [37]. 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 [38, 39].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 visual impairment among adolescents [13, 33]. 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 [40‐42]. 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) [41] and heavy academic pressure, all of which may affect refractive errors. Recent interventional prospective studies have shown that encouraging outdoor activity and exposure.

This study has several limitations. First, the data of URE burden in this study included myopia, hypermetropia, astigmatism and etc., thus, URE subtypes were not investigated. Second, HDI and SDI were used as indicators of socioeconomic condition, but these indices could not fully describe social resources. Third, the measurement of HDI and SDI are very similar and they may show strong correlation. Therefore, we applied stepwise linear multiple regression to eliminate collinearity. Fourth, besides HDI, SDI, education and urbanization, there are still numbers of risk factors which may relate to DALYs not included in this study due to lack of available data.

Despite the limitations mentioned above, the study findings can be useful for developing targeted strategies to address the severe visual impairment 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.

In conclusion, the global visual impairment burden of URE among adolescents remained stable without significant alleviation from 1990 to 2019. Older age and female sex were associated with a higher burden of URE. The DALY rates rapidly increased in the 5–9 age group and reached a plateau at 15–19 years old adolescents. High socioeconomic development status (indicated by HDI and SDI), low primary school dropout rates and high urbanization rates were associated with increasing DALY rates. Based on the results, we recommend a universal screening program for 15–19 years old adolescents and an early warning system (EWS) for the 5–9 age group is also necessary for timely detection and control of refractive errors. Moreover, we should be aware of the growing disease burden of URE among adolescents along with the process of urbanization and universal education. These findings can provide information for policymakers to craft more targeted strategies to prevent, screen for and control adolescents’ visual impairment due to URE.