Refractive error (RE) is one of the most common ocular conditions, and uncorrected refractive error (URE) is a major public health challenge. Worldwide, URE is the leading cause of vision impairment (VI) and the second leading cause of blindness.1,2 The impact of URE is profound, as not only do strong socioeconomic factors such as poverty and the inability to access treatment influence the correction of RE but URE can also contribute to the individuals’ and their families’ socioeconomic status.3 Vision impairment due to URE have been observed to have extensive social and economic impact, for example, limiting educational and employment opportunities of economically active persons, healthy individuals, and communities. Smith et al. indicated that the global economy loses $269 billion annually as a result of lost productivity due to URE.4
Traditionally, the World Health Organization (WHO) used categories of VI that referred to best-corrected visual acuity in the better eye rather than “real world” or presenting visual acuity.5 It was subsequently recognized that unless URE were included among the causes, VI at a global level was significantly underestimated,6 thus limiting the relative importance of RE in causing blindness and VI. From the year 2000, a series of studies using a survey methodology, referred to as Refractive Error Study in Children (RESC), were performed in populations with different ethnic origins and cultural settings: a rural district in eastern Nepal7; a semi-rural county outside of Beijing, China8; an urban area of Santiago, Chile9; an urban and a semi-rural area of KwaZulu-Natal, South Africa10; a rural district near Hyderabad, India11; and an urban area of New Delhi, India.12 These studies utilized data on presenting vision and confirmed the need for RE correction for children. In addition, Resnikoff et al.1 in 2008 released data on the global prevalence of URE in 2004 providing VI estimates of 153 million people and blindness estimates of 8 million. The methods involved in this paper prevented a meaningful analysis of temporal change in the cause-specific contribution to blindness and visual impairment prevalence. However, it focused increased attention on URE as part of the eye health agenda and elevated the need for data for URE based on presenting vision.
We conducted, as part of the Global Burden of Disease, Risk Factors and Injuries Study 2010 (GBD), a systematic review of all available population-based prevalence studies performed worldwide between 1990 and 2010. The results informed a meta-analysis to estimate the number of people affected by blindness and VI globally, regionally and by cause, and yielded estimated temporal trends in prevalence from 1990 to 2010, and investigated regional differences worldwide. The present paper specifically details the contribution of URE to blindness and VI when compared to other major eye diseases as part of this meta-analysis.2,13
The methodology utilized in this study has been described extensively in other Global Burden of Disease 2010 (GBD) papers.2,13,14 A summary of this is presented below: We estimated 1990 to 2010 trends in VI causes and their uncertainties, by sex and severity of VI, for 21 GBD subregions.14 A systematic review of medical literature from 1 January 1990 to 31 January 2012 identified indexed articles containing data on incidence, prevalence, and causes of blindness and VI. Only high-quality cross-sectional population-based representative studies were selected from which a database of age- and sex-specific data of prevalence of four distance and one near vision loss categories (presenting and best-corrected acuity) could be extracted.14
The studies that were included in the GBD Vision Loss database met the following requirement criteria14:
- The reported prevalence of blindness and/or VI must be measured from random sample cross-sectional surveys of representative populations of any age of a country or area of a country. Studies using hospital/clinic case series, blindness registries, and interview studies self-reported vision status were not included.
- The definitions of VI or blindness must be clearly stated, using thresholds of visual acuity, in the better eye
- Best-corrected and/or presenting visual acuity must be stated.
- The procedures used for measurement of visual acuity must be clearly stated.
Additional data sources were identified through personal communications with researchers, including enquiries about additional data from authors of published studies. These data were used only if information about the study population and measurement methods were available.
We identified 14,908 relevant manuscripts using Medline, Embase, and the WHO library information system. Most (13,574) articles were rejected during the abstract and title review process by two independent reviewers. A further 1130 articles were rejected by the consensus panel and finally 252 articles were used for our analysis (references can be found in the Web Appendix at www.anglia.ac.uk/verugbd). Search terms included concepts to describe “blindness,” “VI,” “population,” “eye,” “survey,” and a list of conditions affecting the eye including URE. We supplemented the published study data with unpublished microdata sourced through personal communication with the principal investigators identified in the literature search. We estimated the contribution of six causes of VI: cataract, glaucoma, macular degeneration, diabetic retinopathy, trachoma, and URE. We also estimated the fraction of VI that had other causes. We made estimates for moderate and severe VI (MSVI defined as presenting visual acuity <6/18 but ≥3/6013) and blindness (blindness defined as presenting visual acuity <3/60). Our analysis was carried out in three steps: (1) data identification and access, described previously13,14; (2) estimation of cause fractions for each cause, by severity of VI, sex, age, and world region (for each cause, we used the subset of studies for which causal data were available); and (3) application of cause fractions to the prevalence of all-cause presenting VI, which were estimated as described previously.2 For the statistical analysis, the DisMod-MR model from the GBD study was used to calculate the fraction of VI due to URE and the other causes. It has been described in detail previously.2,13,15 DisMod-MR is a negative binomial regression model including the following elements: covariates that predict variation in the true proportion of VI from each disease (e.g., year); fixed effects that adjust for definitional differences (e.g., whether the causes of presenting vs. best-corrected VI were reported); a hierarchical model structure which fits random intercepts in individual countries derived from the data observed in the country, in its region, and in other regions based on the availability and consistency of country- and region-specific data; age-specific fixed effects allowing for a nonlinear age pattern; and a fixed effect for data on males. We used a specific set of parameters for each cause of VI.
The total prevalence of VI and its uncertainty were estimated using prevalence data of blindness and moderate and severe VI (MSVI) based on presenting visual acuity and best-corrected visual acuity.13 This model implicitly estimated the difference between the prevalence of blindness (and of MSVI) based on presenting visual acuity and on best-corrected visual acuity prevalence, respectively. We interpreted this difference as the fraction of VI caused by URE.
For the presentation of the data, we age-standardized prevalence using the WHO reference population.16 We also calculated the estimated numbers of people with VI and blindness due to refractive error, which reflected each region’s population size and age structure.
A total of 243 high-quality, population-based studies remained after application of the above rigorous selection criteria and review by an expert panel.2,13,14 URE was the second leading worldwide cause of blindness (after cataract) contributing in 1990 to 19.9% (95% confidence interval [CI]:14.9–24.9%) of all blindness and in 2010 to 20.9% (95% CI: 15.2–25.9%). In 1990 and 2010, the proportion of MSVI due to URE was 51.1% (95% CI: 45.6–56.0%) and 52.9% (95% CI: 47.2–57.3%), respectively, and as such remains the leading cause of all MSVI worldwide. In 2010, URE was the leading cause of MSVI in all regions with the proportion ranging between 43.2 and 48.1%, except in South Asia (Table 1). The proportion in South Asia where there is a relatively younger population was 65.4% (95% CI: 62.0–72.0%). South Asia also had a proportion of 36.0% of blindness due to URE compared to a low of 13.1% in North Africa/Middle East and Eastern Sub-Saharan Africa (Table 2).
The prevalence of blindness in all ages due to uncorrected refractive error decreased from 0.2% (0.2%; 95% CI: 0.1–0.2%) in 1990 to 0.1% (0.1%; 95% CI: 0.1–0.1%) in 2010, a 33% reduction. The prevalence of MSVI due to uncorrected refractive error decreased from 2.1% (2.1%; 95% CI: 1.6–2.4%) in 1990 to 1.5% (1.5%; 95% CI: 1.3–1.9%) in 2010, a 25% reduction.
In 2010, out of 32.4 million blind and 191 million moderate and severely vision impaired worldwide,13 an estimated 6.8 million (95% CI: 4.7–8.8 million) people were blind (Table 2) and 101.2 million (95% CI:87.8–125.5 million) moderate and severely vision impaired (Table 1) due to URE, whereas in 1990, 6.3 million (95% CI: 4.4–8.1 million) were blind and 87.9 million (95% CI: 69.9–103.3 million) were moderate and severely vision impaired out of a total of 31.8 million blind and 172 million with moderate and severe VI. This represents an increase in the estimated number of people blind (7.9%) and visually impaired (15.1%), whereas the global population increased by 30% from 1990 to 2010 (Figs. 1 and 2).
Globally, the age-standardized prevalence of blindness and MSVI combined among people aged 50 years and older declined substantially from 1990 to 2010 from 7.5% (95% CI: 6.1–8.5%) to 5.7% (95% CI: 5.0–6.9%), respectively. URE contributed to the largest decline in this prevalence (20% for blindness and 45% for MSVI). Regionally, the percentage reduction in age-standardized prevalence of URE as a cause for adult blindness and MSVI combined was most marked in Tropical Latin America (35.1%), Central Asia (35.8%), and high-income Asia Pacific (33.3%), and least marked in eastern Sub-Saharan Africa (16.8%), Oceania (19.8%), and western Sub-Saharan Africa (19.8%) (Figs. 1 and 2).
Age-standardized prevalence of refractive error–related blindness worldwide was 0.4% (95% CI: 0.3–0.5%) in adults aged 50+ years in 2010 and a reduction of 33% to 0.6% (95% CI: 0.4–0.7%) for 1990 (Table 2). The age-standardized prevalence of refractive error–related MSVI worldwide decreased to 5.3% (95% CI: 4.6–6.5%) in 2010 from 6.9% (95% CI: 5.6–8.0%) in 1990 (Table 1).
In 2010, the global age-standardized prevalence of refractive error blindness was the same in men and women (0.1% [95% CI: 0.1–0.2%]), whereas the age-standardized prevalence of VI was greater in women (1.6% [95% CI: 1.4–2.0%]), than in men (1.4% [95% CI: 1.2–1.8%]). This disparity in VI due to refractive error between men and women existed in all regions of the world.
We found that in 2010, 6.8 million people were blind and 101.2 million people were vision impaired due to URE worldwide, a total of 108 million blind or MSVI. Further, URE is the leading cause of moderate and severe vision impairment.
Our results vary from some of the previous estimates by Resnikoff et al.1 and Dandona and Dandona.17 In 2008, Resnikoff et al.1 reported that 153 (123–184) million had VI (blindness and MSVI) of which 8 million were blind, whereas Dandona and Dandona17 using population estimates for 2004 and 2002 stated that 98 (82–117) million had VI (blind and MSVI). Dandona and Dandona17 only considered nine studies for their analysis and utilized only published data and excluded data from studies on children or those older than 60 years. Our estimates fall within the range suggested by Dandona and Dandona17 except their estimates are for 2002. We found a 7.9% increase in the number of individuals blind due to URE and a 15% increase for those with MSVI from 1990 to 2010 compared to the Resnikoff et al. study. The inconsistency in comparing these results may be explained by the variation in methodology and the greater degree of granularity in our analysis by presenting data in 5-year age groups and by sex, by calculating time-series estimates for the period 1990 to 2010, and by disaggregating the estimates for 190 countries in 21 regions. Thus, we believe our estimates of prevalence of VI have increased detail and show temporal changes with more accuracy (Fig. 3).
It is encouraging to note that the prevalence of blindness and MSVI due to uncorrected refractive error decreased by 33 and 25%, respectively, from 1990 to 2010. The total number of persons blind or vision impaired due to URE grew relatively less (7.9 and 15%, respectively) compared to the population growth of 30%. This could be a consequence of increased focus on URE service delivery programs and human resource development by VISION 2020, national programs, non-government and development organizations, and professional associations. The change from best-corrected to presenting visual acuity in determining the magnitude of URE possibly contributed to this by elevating its relative importance to blindness and VI and thus motivating increased investment in refractive error programs.
Unlike the previous reviews of URE by Dandona and Dandona17 and Resnikoff et al.,1 our analysis provided data on sex variations with age-standardized prevalence of VI greater in women than in men in all regions of the world. A meta-analysis of population-based surveys on blindness prevalence in Asia, Africa, and the developed countries in 2000 indicated that women bear approximately two-thirds of the burden of blindness in the world.18 The excess blindness in women occurred in all the regions studied, but the factors behind the disparity vary by geopolitical regions.18 Mganga et al.19 postulated that in developed countries, the overall excess blindness in women was due to the fact that there are more numbers of elderly women than elderly men. In less-developed countries, the greater longevity of women contributes, but access to services is also a major factor, highlighting the need to address specific strategies to reach women, particularly in societies where barriers to women accessing eye care exist.19
South Asia had the highest prevalence of MSVI of all regions. However, it should be considered that the basic studies were conducted in different time periods and at different locations, and there could have been a regional disparity in the estimates. The large countries such as India and others could present large regional differences in prevalence and causes of VI even at a given point in time.
The design of our study had potential limitations. As also pointed out in our previous reports on the global prevalence of vision loss,3,14 a major limitation was that many country-years remained without data or only had sub-national data. Only a few national studies reporting VI for all ages and all causes were available. The increased number and broader distribution of recent data sources underscores an increase in population-based studies conducted in the 2000s compared to the previous decades; however, there remains a dearth of such information from certain world regions such as Central Africa and Central and Eastern Europe, the Caribbean, and Latin America.14 Some data sources did not report prevalence by age. To use these data, we imputed age-specific cause fractions, assuming that the age pattern of vision impaired in the study matched the modeled age pattern of vision impaired in the country where the study was conducted.3 Finally, some studies had a relatively small sample size; therefore, the confidence intervals of the cause-specific prevalence estimate were relatively large. Our methods, however, took into account sample size, so that studies with small sample sizes influenced the estimates less than studies with large sample sizes. The lack of data for near VI due to presbyopia during the period of our study remains a major limitation of our study.
In 2010, 6.8 million people were blind and 101.2 million vision impaired due to URE with increasing numbers from 1990 (6.3 million were blind and 88.0 million were vision impaired). In 2010, uncorrected refractive error continues as the leading cause of vision impairment and the second leading cause of blindness worldwide, affecting a total of 108 million people or 1 in 90 persons. The most frequent cause for MSVI and the second most common cause for blindness was URE. Our data again emphasizes that globally one of the most simple, effective, and cost effective ways to improve the burden of vision loss would be to provide access to affordable adequate spectacles to correct refractive errors with the appropriate human resources.
Funding for the study was received from Bill & Melinda Gates Foundation, Brien Holden Vision Institute, Fight for Sight, and Fred Hollows Foundation.
The authors have no proprietary or commercial interest in any materials discussed in this article.
Received: April 10, 2015; accepted October 20, 2015.
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