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 Table of Contents  
Year : 2022  |  Volume : 34  |  Issue : 1  |  Page : 1-15

Global prevalence and causes of visual impairment and blindness in children: A systematic review and meta-analysis

1 Department of Optometry, School of Paramedical Sciences, Mashhad University of Medical Sciences, Mashhad, Iran
2 Iranian Research Center on Aging, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
3 Health in Emergency and Disaster Research Center, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
4 Refractive Errors Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
5 Rehabilitation Research Center, Department of Optometry, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran
6 Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran
7 Department of Basic Sciences, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran, Iran

Date of Submission25-Apr-2021
Date of Decision06-Dec-2021
Date of Acceptance09-Dec-2021
Date of Web Publication16-Apr-2022

Correspondence Address:
Mehdi Khabazkhoob
Department of Basic Sciences, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/joco.joco_135_21

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Purpose: To determine the global prevalence and common causes of visual impairment (VI) and blindness in children.
Methods: In this meta-analysis, a structured search strategy was applied to search electronic databases including PubMed, Scopus, and Web of Science, as well as the list of references in the selected articles to identify all population-based cross-sectional studies that concerned the prevalence of VI and blindness in populations under 20 years of age up to January 2018, regardless of the publication date and language, gender, region of residence, or race. VI was reported based on presenting visual acuity (PVA), uncorrected visual acuity (UCVA), and best corrected visual acuity (BCVA) of equal to 20/60 or worse in the better eye. Blindness was reported as visual acuity worse than 20/400 in the better eye.
Results: In the present study, 5711 articles were identified, and the final analyses were done on 80 articles including 769,720 people from twenty-eight different countries. The prevalence of VI based on UCVA was 7.26% (95% confidence interval [CI]: 4.34%–10.19%), PVA was 3.82% (95% CI: 2.06%–5.57%), BCVA was 1.67% (95% CI 0.97%–2.37%), and blindness was 0.17% (95% CI: 0.13%–0.21%). Refractive errors were the most common cause of VI in the subjects of selected articles (77.20% [95% CI: 73.40%–81.00%]). The prevalence of amblyopia was 7.60% (95% CI: 05.60%–09.10%) and congenital cataract was 0.60% (95% CI: 0.3%–0.9%).
Conclusion: Despite differences in the definition of VI and blindness, based on PVA, 3.82%, and based on BCVA, 1.67% of the examined samples suffer from VI.

Keywords: Blindness, Children, Low vision, Visual impairment

How to cite this article:
Yekta A, Hooshmand E, Saatchi M, Ostadimoghaddam H, Asharlous A, Taheri A, Khabazkhoob M. Global prevalence and causes of visual impairment and blindness in children: A systematic review and meta-analysis. J Curr Ophthalmol 2022;34:1-15

How to cite this URL:
Yekta A, Hooshmand E, Saatchi M, Ostadimoghaddam H, Asharlous A, Taheri A, Khabazkhoob M. Global prevalence and causes of visual impairment and blindness in children: A systematic review and meta-analysis. J Curr Ophthalmol [serial online] 2022 [cited 2022 May 23];34:1-15. Available from: http://www.jcurrophthalmol.org/text.asp?2022/34/1/1/343341

  Introduction Top

Visual impairment (VI) in childhood has a negative and sometimes irreversible impact on children's psychological, educational, and social performance, which can persist into adulthood and affect individuals' quality of life.[1] Given the significant burden of VI, its causes, and visual complications, the VISION 2020 Initiative was implemented by the World Health Organization (WHO) to eliminate preventable blindness on a global level.[2],[3] According to WHO estimates at the beginning of the VISION 2020 program, about 19 million children under the age of 15 years were visually impaired and 1.4 million children had irreversible blindness, and it was predicted that half of the blindness cases were preventable.[4] The reported prevalence of blindness in low and middle-income countries ranges from 0.2 to 7.8/10,000 people, and in developed and industrialized countries, the annual incidence is 6/10,000 in the under-15 age group.[5],[6] According to available information, the causes of VI differ by the residence location of the studied population (urban versus rural) or in different countries (developed, under developed, or developing) as well as the prevention strategies within each health system. Nevertheless, Courtright et al. suggest that retinal disorders, glaucoma, corneal ulcers due to vitamin A deficiency, cataract, and neural causes are the most common causes of VI in low and middle-income countries.[5] This is while neurological disorders are one of the major causes of VI in industrialized countries, and in countries such as England, 75% of blindness cases are due to unpreventable causes.[7],[8] A large amount of information on VI in children has been generated from population-based and clinic-based studies, studies in schools for blind children, different age groups (3–5 years, 7-years, 3–10 years, under-15-years, 5–15 years, etc.) as well as different settings such as high-income and low-income countries, but due to the mentioned differences, it is not possible to make global policies or evaluate measures that have been taken in this regard. Given the lack of cohesive results on the prevalence of VI as well as the differences in the causes of VI in different parts of the world, it seems necessary to have an estimate of the global prevalence and causes of VI in children to inform policies, especially the Vision 2020 Initiative. Therefore, the present study aims to determine the overall prevalence and causes of VI in children in the world.

  Methods Top

The entire process of this study was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.[9] All population-based cross-sectional studies concerning the prevalence of VI and blindness in individuals under 20 years of age were reviewed regardless of publication and language, gender, region of residence, and race. The search strategy and entry terms showed in Appendix 1. Of studies conducted on the same population, the one with a higher quality was included in this review. Also, we included studies that were performed in all age groups and used the prevalence rates reported for the under-20-year age groups. We excluded articles that did not have one or more of the inclusion criteria. The outcome of interest was the prevalence of VI and blindness and the causes of VI in the population. In the selected papers, cases of VI were identified using measurements based on different units including feet, logMAR, and meters. For this reason, and to facilitate the presentation of the results, all measurements were converted to feet.

The prevalence of VI in this study was calculated based on uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), and presenting visual acuity (PVA) as reported in previous studies.[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40] The participant's PVA was considered UCVA in participants without glasses and visual acuity with present glasses in individuals with glasses. According to previous studies, the prevalence of VI was reported based on visual acuity cut-point of 20/40 or worse and 20/60 or worse in the better eye (according to the WHO guidelines, VI based on PVA, UCVA, and BCVA was considered as visual acuity in the better eye of equal to 20/60 or worse). The prevalence of blindness was determined based on: (1) BCVA of 20/200 or worse in the better-seeing eye, and (2) BCVA of 20/400 or worse in the better-seeing eye (according to the WHO guidelines, blindness was defined as visual acuity worse than 20/400 in the better eye). We excluded the studies that specifically investigated the VI and blindness in the schools for the blind.

To ensure the correct selection of articles related to the topic of the research and in accordance with the inclusion criteria, two researchers (E.H. and M.S.) independently selected the articles; they were not blinded to the names of the authors, the journal titles, or study results. The kappa agreement index between researchers was 80.2%. Cases of controversy between the researchers were decided through discussion or by consulting a third person. The two researchers independently extracted the required data based on predefined variables. We used the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist[41] to perform a qualitative assessment of the selected articles in terms of methodology and report. Present key elements of study design, describe the setting, locations, and relevant dates, including periods of recruitment, give the eligibility criteria, and the sources and methods of selection of participants, clearly define all outcomes, and report numbers of outcome events or summary measures were assessed. The studies were categorized as low risk of bias if they reported all items, as moderate risk of bias if they reported all items but one, and as high risk of bias otherwise. To examine the inconsistency of the articles, the k-square test was used at a 5% confidence interval (CI). In order to quantitatively analyze the heterogeneity of the results, we used the I-square test based on the Higgins classification. According to which, an I-square more than 75% was considered as heterogeneity. The variables investigated in this study included the name of the first author, the year of publication, the country of the study, the mean age and gender distribution of study subjects, sample size, the prevalence VI (based on UCVA, PVA, and BCVA) and blindness with their 95% CI, and the prevalence of the most important causes of VI and blindness. One of the PRISMA checklist items is calculating publication bias. In our study, publication bias was not assessed because the prevalence is always a positive number between zero and one, and cannot be negative; therefore, all studies were distributed on the right side of the vertical line, and this leads to asymmetry in the funnel plot which is not related to publication bias. Data analysis was performed using Stata Software version 11 (StataCorp, College Station, TX, USA). The data was analyzed using the random-effects model at a 95% confidence level.

  Results Top

In the present study, 5711 studies were identified; 5211 articles by searching electronic databases and 500 articles through the lists of references of selected articles and other sources. After removing redundant articles, the title and abstract of 4381 articles were reviewed, and 4231 articles were excluded after applying the exclusion criteria, and thus, 150 papers were eligible for full-text review. After reviewing the full text of the articles, 70 articles were excluded from the study for not meeting the inclusion criteria, lack of access to the full text of the article, nonoriginal paper (letter, commentary, review), and finally, data for this study were extracted from 80 articles [Figure 1].
Figure 1: Flow of information through the different phases of the systematic review

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As shown in [Table 1], the final 80 papers comprised 769,720 people from a total of 28 different countries.[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46],[47],[48],[49],[50],[51],[52],[53],[54],[55],[56],[57],[58],[59],[60],[61],[62],[63],[64],[65],[66],[67],[68],[69],[70],[71],[72],[73],[74],[75],[76],[77],[78],[79],[80],[81],[82],[83]
Table 1: Summary of studies results

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Among the selected articles, the studies by Razavi et al.[75] in Iran with 123 people and Beiram[84] with 127,426 people in Sudan had the smallest and the largest sample sizes, respectively.

The overall prevalence of VI was 12.72% (95% CI: 9.26%–16.19%) based on a UCVA of 20/40 or worse in the better eye, and 7.26% (95% CI: 4.34%–10.19%) based on a UCVA of 20/60 or worse in the better eye [Figure 2]. The prevalence was 7.34% (95% CI: 5.53%–9.15%) based on a PVA of 20/40 or worse in the better eye and 3.82% (95% CI: 2.06%–5.57%) with a PVA of 20/60 or worse in the better eye, and 2.91% (95% CI: 2.31%–3.51%) based on a PVA worse than 20/60 in the better eye [Figure 3]. The prevalence of VI based on a BCVA of 20/40 or worse in the better eye was 0.77% (95% CI: 0.56%–0.97%), 1.67% (95% CI 0.97%–2.37%) based on a BCVA of 20/60 or worse in the better eye, and 0.88% (95% CI: 0.63%–1.12%) based on a BCVA worse than 20/60 in the better eye [Figure 4].
Figure 2: Overall prevalence and subgroups of uncorrected visual acuity based on uncorrected visual acuity

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Figure 3: Overall prevalence and subgroups of presenting visual acuity (PVA) based on PVA

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Figure 4: Overall prevalence and subgroups of best corrected visual acuity (BCVA) based on BCVA

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Based on criteria worse than 20/200 in better eye and worse than 20/400 in the better eye, the blindness prevalence was 0.15% (95% CI: 0.06%-0.25%) and 0.17% (95% CI: 0.13%-0.21%), respectively [Figure 5]. [Table 2] summarizes the prevalence of UCVA, BCVA, PVA VI, and blindness in the six regions of the WHO. The highest rate of VI based on UCVA of 20/40 or worse in the better eye was 20.10% (95% CI: 13.75%–26.45%) in the Pacific Region, and based on UCVA of <20/60 in the better eye was 15.72% (95% CI: 14.74%–16.70%) in the Americas. The highest prevalence of VI based on PVA of 20/40 or worse in the better eye, 20/60 or worse in the better eye, and worse than 20/60 in the better eye in the Pacific Region was 10.87% (95% CI: 7.26%–14.48%), 8.03% (95% CI 1.00% -20.84%) in the Americas, and 11.59 (95% CI: 10.65–12.53) in the Eastern Mediterranean Region, respectively. The highest prevalence of VI based on a BCVA of 20/40 was 0.91 (95% CI: 0.54–1.27) in the Pacific Region. The highest rates of blindness were 1.91 (95.1% CI: 1.78–5.58) in the African Region based on worse than 20/200 and 1.94 (95% CI: 0.27%–3.61%) in the Eastern Mediterranean Region with criteria worse than 20/400.
Figure 5: Overall prevalence and subgroups of blindness

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Table 2: Prevalence of visual impairment and blindness in the six regions of the World Health Organization

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[Table 3] presents the prevalence of the causes of VI and blindness. In the selected articles, refractive errors, with a prevalence of 77.20% (95% CI: 73.40%–81.00%), were the most common cause of VI. Amblyopia, retinal disorders, congenital cataract, and corneal opacities were other causes of visual impairment, and cataract, glaucoma, and refractive errors were the most common causes of blindness.
Table 3: The proportion (%) of causes of visual impairment and blindness in the reviewed articles

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  Discussion Top

Our study is the first to generate a more accurate estimate of the global prevalence of VI in children using credible population-based studies. We also presented the prevalence of VI and blindness based on different definitions. Studies in the under-20 year's old groups and especially studies in the under-15 year's old groups were the most important reason for choosing 20 years-old as a cut-off. Our results indicated that the lowest prevalence of BCVA VI was 0.057% in the study by Flanagan et al.[58] in Ireland and the highest prevalence was 7.29% in a study by Maul et al. in Chile.[39] The lowest and highest prevalence of VI based on PVA was, respectively, 19.29% in the study by Adhikari et al.[43] and 0.1% in the study by Congdon et al.[27] Despite the lower prevalence of VI in children compared to adults (3.82% versus 35.8%[10]), the number of years lost due to disabilities caused by vision impairment in children imposes a large burden on societies, especially in less developed countries. In a systematic review, Köberlein et al.[92] reported that the direct costs of VI included hospitalization, utilization of medical services, purchase of medical products, and the recurrence of VI. They showed that in several population-based studies using representative populations in the United States, the annual cost was 12,175-14,029 dollars for a patient with moderate VI, and 14,882–24,180 dollars for a blind person.[92] The high cost of treatment and follow-up on the one hand, and the mental burden, the educational failure, and in general, the reduced quality of life for children on the other hand justify the importance of determining estimates of the trend of the prevalence of VI and its causes in children.

In addition to imposing costs, the burden of disease is an important issue. In a retrospective study, examining data from 195 countries between 1995 and 2017, the disability-adjusted life year (DALY) number of refractive errors in school children was higher than preschool and teenagers.[93]

Determining the prevalence of VI and its most important causes are necessary to apply policies and strategies to prevent and eliminate the preventable causes of VI. Our findings showed that refractive errors were the most common cause of VI in most articles reviewed in this meta-analysis, such that 29 articles described refractive errors as the cause or one of the causes of VI with rates ranging between 48.3% in the study by Zainal et al.[90] and 96.8% in the study by He et al.[28] Failure to use the protocol recommended for Refractive Error Study in Children (the RESC Protocol which suggests the use of cycloplegic refraction) in some studies has led to different estimates of the prevalence of refractive errors. In the RESC study, the following definition is defined to determine the refractive error Cycloplegic Refraction: In eyes with successful cycloplegia, refraction is performed with either an autorefractor or retinoscope. Autorefraction is carried out according to the manufacturer instruction manual, including daily calibration. Retinoscopy is carried out using a streak retinoscope in a semi-dark room, with the examiner at a distance of 0.75 meters and a +1.50 diopter lens in the trial frame. Therefore, not using the same definition in studies has led to different estimates in the reports. In studies on similar age groups in geographic regions close to each other, different definitions of refractive errors have been used, and the prevalence of refractive errors, as a cause of VI, is significantly different.[51] Another cause of the difference in the prevalence of refractive errors can be the difference between the studied age groups in the reviewed articles. In studies conducted in age groups over 7 years, the refractive errors as a cause of VI is higher than in studies where the average age of the participants is <7 years. In studies such as those by Sapkota et al.[25] and Paudel et al.[15] where the average age is 10 years and older, over 90% of VI is due to refractive errors. The age-related increase in the prevalence of myopia is one of the major causes of the high prevalence of refractive errors in studies that sampled older age groups. The meta-analysis by Rudnicka et al.[94] in the Middle East Region suggested a significant age-related increase in the prevalence of myopia, such that rates changed from 3.5% in the 5-year age group to more than 47% in the 18-year age group. In a trend analysis from 1990 to 2017, the prevalence of children aged 1–14 years with refractive disorders was 1.8% (95% uncertainty interval [UI]: 1.5–2.1). In school children, teenagers, and preschool children, the prevalence was 2.1% (95% uncertainty interval [UI]: 1.5–2.8), 2% (95% UI: 1.4–2.7) and 1.6% (95% UI: 1.2–2), respectively.[93] Another cause of difference in the results of these studies can be race and ethnic differences, and thus, genetic and lifestyle differences. In the meta-analysis by Rudnicka et al.,[94] the prevalence of myopia in the East Asian Region was more than 80% while it was <5.5% in black African children of the same age group. This racial difference has also been observed with other causes of VI such as amblyopia.

According to our findings, amblyopia is the second leading cause of VI after refractive errors in the reviewed papers. In countries where such screening programs have been in effect for a longer time, the prevalence of amblyopia, as one of the most important preventable causes of VI has been reported. In the absence of apparent strabismus, amblyopia is usually not easily identifiable in children, thus, only properly designed and implemented screening programs by trained people will be effective for the timely diagnosis of amblyopia. Otherwise, childhood amblyopia will continue until they reach adulthood and will lead to a decline in the quality of life in adolescence and older age. Findings by Høeg et al.[95] show that the prevalence of amblyopia in the Danish 20 to 29-year old population, who had been screened by the national screening program for children and treated in childhood was 0%, and in cohorts over 50 years of age, the rate was more than 1.5%. This significant difference clearly shows the impact of the implementation and expansion of screening program in recent years compared to previous years.

Based on our findings, the overall global prevalence of blindness in the under 20-year population was 0.17%. The definition by the WHO is based on BCVA <0.05 (20/400). The prevalence of blindness in the studies using this criterion was estimated at 4.5%. These definitions in different countries have always led to various estimates of blindness. For example, blindness is defined as a visual acuity of ≤0.02 (20/1000) in Germany and ≤0.05 in Israel.[56],[96] Rosenberg and Klie[97] have shown that changing the definition of blindness from ≤0.1 to <0.1 can reduce the diagnosis of blindness by up 32%. Establishing national registries for the blind is very important and effective in determining the prevalence and causes of blindness. Unfortunately, few countries have established reliable registries so far, and in other countries, relevant information, such as the prevalence and causes of blindness, is generated from surveys or studies in schools for the blind, and due to methodological errors in these studies, the results are interpreted with caution. This lack of consistency in the definition and diagnosis of blindness and the lack of registries has led to overestimation or underestimation of global blindness. Despite these differences, we determined the prevalence of blindness based on different diagnostic criteria by referring to the most reliable survey articles and excluding studies performed at schools for the blind. Studies have shown that despite the reduction in age-standardized prevalence of blindness and VI over the past 20 years, based on corrected vision, cataract is still the most important cause of blindness in the world, such that in 2015, Khairallah et al.[98] reported that more than 33% of the world's blindness was due to cataract between 1990 and the end of 2010. In our study, cataract was the most common cause of blindness and the third most common cause of VI in the reviewed studies. Due to lack of information such as nonreporting standard error or CI, meta-analysis of other causes was not possible for the authors. In 2002, Zainal et al.[90] reported the highest prevalence of cataract (3.92%) in children younger than 19 years of age. In determining the cause of blindness and comparing it among different populations, the study of the economic status of the countries and the availability of public health services plays an important role. In countries where access to cataract surgery due to lack of equipment, lack of experienced specialists, and financial inability of people for access to surgery, cataract plays a major role in blindness. In light of this discussion, to reduce preventable blindness, it is necessary to conduct nationwide surveys to determine the existence and availability of surgical facilities and to give priority to raising public awareness for the utilization of healthcare services.

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Conflicts of interest

There are no conflicts of interest.

  Appendix Top

Appendix 1: Search methods

The search strategy was created using the following phrase

(Vision impairment or Low Vision or Visual Disorders or Visual Disorder or Visual Impairments or Vision Disability or Visual disability or Vision Disabilities or Day Blindness or Reduced Vision or Subnormal Vision or Diminished Vision or vision impaired or Visual defect or Visual loss or Visually impaired or Visually impaired persons or blindness or Acquired Blindness or Complete Blindness) and (prevalence or epidemiology or cross-sectional stud* or observational stud* or survey). Three international databases including Scopus, Web of Science, and PubMed were searched for publications indexed up to January 2018. To access more articles and to ensure the correctness of the search strategy in the databases, we also reviewed the reference lists of the selected articles as well as Google Scholar.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2], [Table 3]


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