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1From the London School of Hygiene and Tropical Medicine, London, United Kingdom; and the 2National Eye Institute, National Institutes of Health, Bethesda, Maryland.
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Abstract |
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METHODS. Using the same protocol, 4082 to 6527 children aged 5 (or 7) to 15 years were examined at each site. Uncorrected and presenting visual acuities were successfully measured with retroilluminated logMAR tumbling-E charts in 3997 to 5949 children; cycloplegic autorefraction was performed and best corrected acuities assessed. All children were examined by an ophthalmologist and a cause of visual loss assigned to eyes with uncorrected acuity 
6/12. The prevalence of FLV was determined overall and by site; associations with gender, age, parental education and urban/rural location were assessed with logistic regression.
RESULTS. The prevalence of FLV ranged from 0.65 to 2.75 in 1000 children, with wide confidence intervals. The overall prevalence was 1.52 in 1000 children (95% CI 1.16–1.95). FLV was significantly associated with age (odds ratio [OR] 1.13 for each year, P = 0.01), and parental education was protective (OR 0.75 for each of five levels of education, P = 0.017). Retinal lesions and amblyopia were the commonest causes.
CONCLUSIONS. More studies are needed to determine the prevalence and causes of FLV in children so that services can be planned that promote independence, improve quality of life, and increase access to education.
The World Health Organization’s (WHO’s) International Classification of Diseases (ICD)-10 categories of visual loss define low vision as "a corrected visual acuity in the better eye of <6/18 (<20/63) down to and including 3/60 (20/400)." This definition includes all individuals, regardless of the cause of visual loss. A major limitation of the ICD-10 categories of visual loss is that they do not allow refractive errors to be assessed as a cause of visual impairment, and so the WHO recently suggested that "presenting visual acuity" (i.e., visual acuity tested with distance spectacles, if usually worn), as well as uncorrected visual acuity, be used in all population-based surveys. Most individuals who have a presenting visual acuity in the better eye of <6/18 down to and including 3/60 require spectacles, surgery (e.g., cataract surgery), or other treatment to restore sight and thus do not require assessment for low-vision interventions (e.g., optical devices).
At a meeting of low-vision specialists in 1993, it was realized that neither the original ICD-10 definition of low vision nor the revision using presenting acuity adequately identifies individuals who might benefit from low-vision services after assessment. The following definition of low vision was therefore derived for use in population-based prevalence surveys: "a person with low vision is an individual, who after refraction and medical or surgical treatment, has a best corrected visual acuity of <6/18 to light perception in the better eye, but who uses, or has the potential to use vision for the planning and/or execution of a task." However, it should be acknowledged that others may also have the potential to benefit from low-vision services (e.g., those with better visual acuity but loss of contrast sensitivity).1 This definition differs in four ways from the revised ICD-10 definition: (1) best corrected visual acuity rather than presenting vision is used; (2) a broader range of visual acuities is included; (3) individuals whose visual acuity could be improved by surgical and/or medical treatment are excluded; and (4) there is a functional component (e.g., the ability to navigate independently should also be assessed). In this article we use the term "functional" low vision (FLV) to represent the 1993 definition. Data from only two population-based surveys have been analyzed using the FLV definition to date, one in India2 and another in Pakistan3 : the former included all age groups and the latter included adults 
30 years of age.
The purpose of this study was to determine the prevalence and causes of FLV in school-age children who were examined in eight sites in Asia, Africa, and Latin America using the standard methodology developed by the Refractive Error Study in Children (RESC) Group.4 The sites were in urban and rural areas: Jhapa District in Eastern Nepal (rural)5 ; Mahabubnagar district near Hyderabad in Southern India (rural)6 ; the Liwan area of Guangzhou, China (urban)7 ; the Shunyi District near Beijing, China (semiurban)8 ; the La Florida area of Santiago, Chile (urban)9 ; the Trilokpuri segment of New Delhi, India (urban)10 ; a contiguous area within the South and West Regions of Durban, South Africa (semiurban/urban)11 ; and the Gombak District in Kuala Lumpur, Malaysia (urban).12 The surveys were conducted between 1998 and 2003.
Human subject research approval for the RESC study protocol was obtained from the WHO Secretariat Committee on Research Involving Human Subjects. Implementation of the study at each location was approved by the appropriate human subject review committee. The research protocol adhered to the provisions of the Declaration of Helsinki for research involving human subjects.
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Methods |
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Sample Selection
Each of the RESC study populations was obtained by random sampling of geographically defined clusters. The originally calculated sample size of 5194 children aged 5 to 15 years (per study site) was based on estimating a prevalence of refractive errors of 22% within a 20% error bound with 95% confidence with upward adjustment to accommodate nonparticipation (10%) and cluster sampling (25%).4 As there were no reliable population-based data from the study areas that could be used to modify the calculation, this sample size, or larger, was used in planning each survey.
Enumeration
Using house-to-house visits within the randomly selected clusters, information on the name, age, gender, and schooling of each eligible child, along with the educational level of the parents, was obtained from an interview with an adult family member. Children temporarily absent from the community were included in the enumeration of eligible children, but those away from home for 6 months or more were not. Nonresident visitors were also excluded. In the Mahabubnagar district, Southern India, and the Gombak district, Kuala Lumpur, 5- and 6-year-old children were excluded after pilot exercises demonstrated particular difficulty in obtaining reliable visual acuity measurements among these young children.
Clinical Examination
Within study sites, clinical examinations were generally performed at one or more locations (e.g., health posts, schools). Written consent for the examination was obtained from a parent or guardian. Presenting and uncorrected monocular visual acuities were measured at 4 m with a retroilluminated logMAR chart with five tumbling-E optotypes on each line (Precision Vision, La Salle, IL) and recorded as the smallest line read with one or no errors. After distance visual acuity measurement and evaluation of ocular motility, cycloplegia was induced with 2 drops of 1% cyclopentolate administered 5 minutes apart, with a third drop after 20 minutes if necessary. Cycloplegia and pupil dilation were evaluated after an additional 15 minutes. Pupillary dilation of 6 mm or more with an absent light reflex was considered complete cycloplegia. Refractive error was determined by retinoscopy and a handheld autorefractor (Retinomax K-Plus; Nikon, Tokyo, Japan). Best corrected visual acuity was measured after cycloplegic refraction in all children with uncorrected visual acuity of 6/12 (20/40) in either eye.
The external eye and anterior segment (eyelid, conjunctiva, cornea, iris, and pupil) were examined with a magnifying loupe followed by handheld slit lamp and indirect ophthalmoscopic examination of the media and fundus. The study protocol stipulated that a principal cause of visual impairment be assigned for all eyes with uncorrected visual acuity 6/12 (from an eight-item checklist), which means that all children defined as having FLV (i.e., best corrected visual acuity of <6/18 to light perception in the better eye, from untreatable causes) had a cause assigned for each eye. Amblyopia was assigned as the cause of impairment in eyes with no apparent organic lesion and best corrected visual acuity 6/12 if one or more of the following criteria were met: (1) esotropia, exotropia, or vertical tropia at 4 m fixation, or exotropia or vertical tropia at 0.5 m; (2) anisometropia of 2.00 spherical equivalent diopters or more; or (3) bilateral ametropia of at least +6.00 spherical equivalent diopters. At the discretion of the examiner, eyes that did not meet these explicit criteria were also deemed to have amblyopia as the principal cause of low vision.
Quality Assurance
Interobserver reproducibility was monitored throughout the course of each study. For visual acuity in right eyes, unweighted 
statistics ranged from 0.53 to 0.83, with 0.0% to 4.1% of repeat measurements differing by two or more lines. In left eyes, statistics ranged from 0.53 to 0.81 with 0.0% to 3.3% of measurements differing by two or more lines. Children with uncorrected visual acuity of 6/12 (in either eye) and approximately 5% to 10% of other children had test-retest evaluations of uncorrected visual acuity. The repeat testing was conducted independently by a second examiner who was masked to the findings from the initial testing.
Data Management, Definitions, and Analysis
Enumeration and examination data forms were reviewed in the field for accuracy and completeness before computer data entry at a central study headquarters at each site. Measurement data ranges, frequency distributions, and consistency among related measurements were checked with data-cleaning programs.
As defined herein, children were considered to have FLV if they had a best corrected visual acuity of <6/18 down to and including light perception in the better eye and visual loss was not due to a treatable cause in one or both eyes (e.g., cataract). Prevalence rates of FLV were calculated, and associations between FLV and the child’s gender, age, parental education (based on the parent with the highest level of schooling and categorized as no education or 1 to 6, 7 to 9, 10 to 12, or >12 years of schooling), and location (rural, semiurban, or urban) were explored by using logistic regression. Statistical analyses were performed with commercial software (Stata Statistical Software, ver. 8.013 ).
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Results |
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The causes of low vision are shown in Table 4 . Retinal lesions, mainly retinal dystrophies, were the commonest cause, followed by amblyopia. Two children with nonsurgically treated cataract have been included, as both had other ocular conditions likely to result in a poor visual outcome should surgery be performed. There were 11 children with "other" causes: 5 had nystagmus, 1 had coloboma, 2 had unspecified developmental abnormalities, 2 had lesions of the central nervous system, and 1 had an anterior staphyloma (one eye only, with a corneal opacity in the fellow eye). In 11 eyes (6 children) the examining ophthalmologist could not determine the cause of visual loss.
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Discussion |
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Reliable population-based data on the prevalence of blindness in children are limited, but the evidence suggests a close association with under-5 mortality rates (U5MRs).14 In countries with very low U5MRs (i.e., Western Europe, North America, and the industrialized countries of Southeast Asia), the prevalence of blindness is approximately 0.3 in 1000 children, but can be as high as 1.5 in 1000 children in the poorest countries in Africa and Asia, which have high U5MRs. Indeed, the International Vitamin A Consultancy Group has recently suggested that U5MRs be used as a proxy to indicate whether vitamin A deficiency (a major cause of blindness and mortality in children) is likely to be a public health problem.15 In this study the prevalence of FLV also seems to reflect levels of development, as the prevalence was higher in rural areas than in most of the urban areas and lower in countries with higher development indices (e.g., urban China, Malaysia), but differences were not statistically significant. In the multivariate analyses, which pooled data across survey sites, children were significantly less likely to have FLV if their parents were educated, reflecting the findings of many studies of morbidity and mortality in children.16
As the number of children identified with FLV in this study is very small, the data on causes should be interpreted with caution, particularly as a cause was not documented in six children. However, children from rural sites were less likely to be amblyopic, possibly reflecting the lower prevalence of refractive errors in children from rural populations compared with urban areas.6 7 11 Children in rural locations were more likely to have visual loss from "other" causes, the majority of which were not preventable or treatable.
The pooled data from these surveys seem to suggest that the prevalence of FLV in children is approximately twice the prevalence of severe visual impairment and blindness (using presenting visual acuity in the better eye). This finding is not consistent with the WHO’s rule of thumb, which says that 95% of the number of those who are blind can be used to estimate the number of people with FLV.17 However, the confidence intervals around the pooled estimates of FLV as well as severe visual impairment and blindness in our study are wide, which makes these comparisons unreliable.
As provision of low-vision services is a priority of WHO’s global initiative, VISION2020—the Right to Sight, more data would be useful for planners, particularly as children with low vision require frequent and long-term follow-up. Data on the prevalence and magnitude of FLV would also be useful for Ministries of Education and other organizations supporting inclusive education, particularly as the Millennium Development Goal of universal primary education provides an impetus and opportunity for improving access to education for visually impaired children in resource-poor countries.
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Appendix 1 |
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Footnotes |
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Supported by the World Health Organization (WHO) under Contract N01-EY2103 from the National Eye Institute; WHO New Delhi, India; and other nongovernment organizations.
Submitted for publication July 30, 2007; revised September 26, October 19, and November 1, 2007; accepted January 11, 2008.
Disclosure: C.E. Gilbert, None; L.B. Ellwein, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 
1734 solely to indicate this fact.
Corresponding author: Clare E. Gilbert, International Centre for Eye Health, Clinical Research Unit, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK; clare.gilbert{at}lshtm.ac.uk.
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