This study revealed the global patterns in health burden of URE by year, age and sex, region, and socioeconomic status. From 1990 to 2013, global DALY numbers rose, crude rates stayed stable, and age-standardized rates declined. Global DALY rates increased with age, and older females had higher DALY numbers and rates than males of the same age. Among six WHO regions, age-standardized DALY rates in Eastern Mediterranean, South-East Asia, and Africa were higher than that at a global level. Higher age-standardized DALY rates were found in countries with lower levels of socioeconomic development.
Ono et al.
18 using data from the GBD 2004 study, reported world bank regional difference, with higher DALYs caused by refractive errors in East Asia and the Pacific (including China), and South Asia (including India), which agrees with our findings. However, we had gone further and found higher URE burden in Eastern Mediterranean, South-East Asia, and Africa, after excluding the impact of population size and composition. Ono et al.
18 also reported that refractive errors burden in terms of crude DALY rates were higher in high-income countries than in middle- and low-income countries, which seems inconsistent with our analysis.
18 The difference between the terms of crude rates and age-standardized rates should be noted. Besides, the global burden of refractive errors in the GBD 2004 study was probably underestimated, based only on presenting distance visual acuity.
19 It has been estimated that 94% (386 million) of people with near-vision impairment due to uncorrected presbyopia lived in developing countries in 2005.
20 The World Health Survey conducted in 70 countries worldwide in 2003 reported the percentage of adults suffering from any distant visual difficulty being 13%, 23%, and 24% in high-, middle-, and low- income countries, respectively.
21 Like the global patterns in URE burden demonstrated in our study, older age, female sex, as well as lower socioeconomic status, were identified as risk factors for visual difficulty in the World Health Survey.
21
According to the findings of the GBD 2010 study, global number of cases of MSVI and blindness due to URE increased by 15% and 7.9%, respectively, from 1990 to 2010, less than the population growth of 30% during the same period.
2 Meanwhile, global age-standardized prevalence of MSVI and blindness due to URE declined by 25% and 33%, respectively.
2 The findings might partially explain the trends in global URE burden in terms of DALY numbers, crude rates, and age-standardized rates in the GBD 2013 study. Global health of URE is improving, which could be a consequence of eye-care development by the VISION 2020 program, the International Agency for the Prevention of Blindness, national programs, and nongovernment organizations. However, an increasing and aging population, as well as the transition to a younger onset of URE,
22 have raised DALY numbers and kept crude rates stable, implying that health improvement does not mean fewer demands of refractive services.
Age-specific analysis of the GBD 2010 study revealed that, global age-standardized prevalence of MSVI due to URE among people aged 50 years and older versus in all age was 5.3% vs. 1.5%, and blindness 0.4% vs. 0.1%.
2 With an assumed onset age of 40 to 45 years, presbyopia is nearly universal in individuals over 65-years old and is becoming a major contributor to the global burden of VI in old people.
20 Recent projections indicate that, the distribution of people with myopia will spread from 2000 with a peak in the age range of 10 to 39 years, by 2050 with a peak throughout the age range of 10 to 79 years.
23 The number of people with high myopia globally was 163 million in 2000 and is projected to increase to 938 million by 2050.
23 The pathologic changes in high myopia may increase drastically over the next few decades, largely due to the increasing number of high myopia cases and the aging population.
24 In 2010, age-standardized prevalence of VI-URE was higher in females than in males in all regions worldwide.
2 Sex inequality existed in accessibility to eye care services, with evidence that females do not access eye care as often as males especially in developing countries.
25 Greater longevity of females in many countries also contributes to a higher global burden of URE in older females.
26
Among 21 GBD regions, the highest number of people with VI-URE was found in South Asia, followed by East Asia.
2 India and China account for approximately 50% of the global VI-URE.
2,27 In our analysis, countries with lower HDI have higher age-standardized DALY rates caused by URE. Availability of eye care is a major barrier to correcting refractive errors.
28 The average number of eye doctors per million population varied with economic development, from 9 per million in low-income countries to 79 per million in high-income countries, with the lowest average number (2.7 per million) in Sub-Saharan Africa.
29 Another notable barrier is the quality of care available. Even in developed countries, refractive errors could be undetected or undercorrected in children.
30,31 This could be more challenging for less developed regions, for example, South Asia that has is a relatively younger population
2. Moreover, the cost of eye exam and spectacles impedes refractive correction to a great extent in poor regions.
32–34 For instance, 99.5% of survey participants from Timor-Leste were willing to wear spectacles if needed, whereas the proportion willing to pay at least US$1 was only 56.9%, despite the National Spectacle Program.
34
This study was subject to the limitations of the GBD 2013 study, such as data sources and statistical assumptions, which were detailed in the GBD 2013 reports.
12,13 The key limitation is the use of aggregate data for each country rather than district data, as a source of bias because of geographic variations in DALYs of URE. As annual updates of GBD data are available, analysis of global URE burden by age and sex, region, and socioeconomic status over time could be further explored.
In summary, this study suggests that the global health of URE is improving but an increasing and aging population keeps crude DALY rates stable. Health progress in URE does not mean fewer demands of refractive services. Older people and females worldwide bear higher burden of URE. The association between URE burden with socioeconomic status highlights the need to provide cost-effective refractive services for less developed countries. The findings of this study may raise public awareness of the global URE burden and are important for health policy making.