Prevalence and determinants of visual impairment amongst school-aged children in Southern Nigeria

Vision problems, especially refractive error, amongst school-aged children are increasing globally.1,2 Low vision and blindness can impede growth in a child’s education, particularly literacy and motor development impairment.3 The Global Action Plan 2010–2019 initiative has the target to reduce vision impairment (VI) by 25%; therefore, there is a need to generate evidence to evaluate its success and other initiatives.4


Introduction
Vision problems, especially refractive error, amongst school-aged children are increasing globally. 1,2 Low vision and blindness can impede growth in a child's education, particularly literacy and motor development impairment. 3 The Global Action Plan 2010-2019 initiative has the target to reduce vision impairment (VI) by 25%; therefore, there is a need to generate evidence to evaluate its success and other initiatives. 4 refractive error. Most of the studies in Nigerian schools reported only the overall prevalence of visual impairment in children and their definition of visual impairment varied widely. Studies on the prevalence of uncorrected, presenting and best corrected visual acuity of 20/40 or worse in the better eye' for school children in Nigeria are unavailable to the best knowledge of the researcher, but are necessary information for effective school eye health programmes.
The main aim of this study was to determine the prevalence and determinants of visual impairment in school-aged children in Southern Nigeria. The results may be useful for planners of school health programmes in formulation of evidence-based policies and their implementation.

Methods
This cross-sectional analytic study design used a multistage random sampling technique in selecting 2418 children aged 6-17 years from 10 schools (both public and private) in Cross River State located in Southern Nigeria.
The eye examinations conducted on school children who participated in the study after informed consent from parents or caregivers included external and internal eye structure examinations, non-cycloplegic objective and subjective refraction. Visual acuity measurements at distance and near were done using a Logarithm of the minimum angle of resolution (LogMAR) chart. The sample size of children used for this study was computed using the formula 11 and considering an estimated proportion of persons in the population (school-age children) with eye problem as 15% 12 and 10% relative precision at a 95% confidence interval (CI). Allowance of 10% was made for non-responses and that gave a total sample size of 2418. Detailed data collection procedures as well as the sampling procedure used for this study can also be found in earlier publications. 5,13 Sampling procedure and methods of data collection The study was carried out in 10 selected schools from three local government areas (LGAs) in Cross River State. Seven public schools (4 primary, 3 secondary) and three private schools (2 primary, 1 secondary).
The first stage was the selection of one LGA from a list of LGAs in each of the three health zones in Cross River State using a simple random sampling technique by balloting, which yielded a total of three LGAs. The three LGAs selected were Calabar South in the south, Etung in the central and Ogoja in the north. The idea was to have a good spread that should be a true representation of the region concerned. In each of the selected LGAs, schools were stratified into public and private, primary and secondary schools. From the list of schools as already stratified, seven public schools (four primary and three secondary) and three private schools (two primary and one secondary) from the three selected LGAs were randomly selected for the study by balloting.
The next stage was the enrolment of children aged between 6 and 17 years from the participating schools. The population of each selected school was determined from the school register through the school head. Selection was done using simple random sampling based on a sampling probability proportionate to size. The total population of all the selected schools comprised 7922, which was used to determine the number of children enumerated for the study per school using simple proportions (school population / total population × sample size). Each school was made up of six class levels or grades with different arms per class level. One arm of each class level was randomly selected by balloting. A total of six class levels were selected per school and the total number to be enumerated per school as calculated was divided equally by six to represent each class level. The sampling frame used for the selection of children was a listing of class-based clusters within each school with the number of children in each. Only children who met the inclusion criteria (primary school children 6-11 years and secondary school children 12-17 years) were selected until the total required number was achieved per class level using the systematic sampling procedure. The next arm of the class level was generally selected if the first selected arm was not adequate.
Guidelines and recommendations from the Refractive Error Study in Children (RESC) were used in defining visual impairment 14 as presenting VA of ≤LogMAR 0.3 (20/40) on the better eye and blindness as VA of ≤LogMAR 1.0 (20/200). However, the study modified some other recommendations of the RESC, like the use of noncycloplegic refraction procedure, which also informed the definition of hyperopia used for this study. The fogging technique was used in place of cycloplegic refraction to achieve a similar relaxation of accommodation in the school children. 15 Significant refractive error was defined as myopia of ≥|−0.50D|, astigmatism of ≥|−0.75D| and hyperopia of ≥1.50D.
Assent was obtained verbally from the participants after a detailed briefing of the procedure before the procedures started, in addition to a written consent from the parents or guardians. Data were collected using intervieweradministered semi-structured questionnaires, after which participants' eyes were examined to determine the eye health status using a battery of procedures, which included VA assessment using LogMAR VA charts for distance and near vision; external eye examination with the pentorch; internal eye examination with the direct ophthalmoscope; noncycloplegic objective refraction using an autorefractor and retinoscope; subjective refraction with 2 D fogging lens; cover test via the cover-uncover technique, at distance and at near, respectively; and colour vision test using an Ishihara test chart to determine the presence or absence of colour abnormality. Diagnosis was determined based on the anatomical and aetiological considerations. Cup to disc ratio of 0.5 and above was used to diagnose glaucoma suspects. Refractive error was given as the cause of reduced vision for all eyes correcting to LogMAR 0.2 (20/32) or better with subjective refraction, even when other contributing diseases were present. Independent replicate examinations for quality assurance were carried out in four schools using 5% of the sample size to validate the data collected. Details of quality assurance measures are presented in an earlier publication. 5

Data analysis
The analysis of data was performed using Statistical Package for Social Sciences (SPSS) version 20. Socioeconomic status (SES) of each child's family was constructed from a combination of parents' occupation and education. Parents with no formal education were scored as 0, and those with primary, secondary and tertiary education were scored as 1, 2 and 3, respectively. Occupational categories were also scored accordingly.
From the composite scores, parents were classified into low, medium and high SES. For the purpose of logistic regression analysis, dichotomous categorical variables were recoded to 0 and 1, where 0 referred to the reference category and 1 referred to the other category. Socioeconomic status was treated as an ordinal variable. The hypothesis on the relationship between independent variables (age, sex, school type and SES) and the occurrence of visual impairment in school-aged children while controlling for potential confounders was examined using unconditional logistic regression analysis.
The parents or caregivers of the study participants provided consent through a consent form sent home with their children or wards. Assent was also obtained from the school children before participation in the study.

Ethical considerations
Ethical clearance to conduct the study was obtained from the Cross River State Health Research Ethics Committee (CRS-HREC) (clearance number CRS/MH/CGS/E-H/018/ VOLII/081).

Results
A sample size of 2110 (87.3% response rate) children aged 6-17 years, from both public (government-owned) and private schools was evaluated. More than half of the children (52.9%) were females, 59.2% were in primary schools (6-11-year-olds) and 76.4% were in public schools (Table 1) Table 4).
Parents' or guardians' highest level of education and occupation were used as a proxy to measure their SES into high, medium and low SES. From the logistics regression analysis (Table 5), age group, sex, SES and school type were found to be significantly associated with visual impairment. Children aged 12-17 years were two times more likely to have presenting VA of LogMAR 0.3 or worse than those aged 6-11 years. Females were more likely than males to have a visual impairment. Children whose parents are within the medium and high SES were about two times more likely to have impaired vision than those whose parents were in the low socio-economic class. This means that the risk of visual impairment was significantly associated with SES of parents. LGA, local government area.
http://www.avehjournal.org Open Access Children who attended private schools were more likely to have visual impairment than those in the public schools (p < 0.001, Table 5). In this study, all the children enrolled were refracted irrespective of VA, which could account for the higher prevalence of visual impairment reported, when compared with other studies in Nigeria, Ghana and South Africa. 6,7,18,19 However, higher percentages for countries such as China (27%, 16.6% and 0.46%) 9 and Malaysia (17.1%, 10.1% and 1.4%) 10 were reported in the literature. The difference in results is not unconnected with the variation in study areas, definitions of visual impairment and research protocols. What is rather consistent in all the studies is that about two-thirds of the visual impairment in school-aged children were because of refractive error, followed by amblyopia. 6,7,18,19,20       The main causes of VI found in this study were refractive error (70.7%), which is a treatable cause of VI; glaucoma suspects (38%) and amblyopia (3.0%), which can usually be prevented through early diagnosis and treatment; corneal opacity (1.5%), which is also a preventable and treatable cause of VI; cataract (0.5%), which is treatable through surgery; and retinal anomaly (0.5%) that may be preventable via early diagnosis. Similar studies in Nigeria also found errors of refraction to be the main cause of impaired vision in school-aged children. 12,21,22,23,24 Two cases of monocular phthisis bulbi (also preventable) were seen in a particular LGA located in the central senatorial district of Cross River State, which requires further investigation to establish its relationship with the environmental condition of the area. The number of children who fell into the vision criteria of 1.0 LogMAR or worse was 4 (0.2%), and three them were girls. No child met the category of 'presenting visual acuity of less than 1.3 LogMAR in the better eye'. Monocular blindness which is defined as vision criteria of 1.0 LogMAR or worse in one eye was 20 (0.95%). In this study, the prevalence of blindness is similar to results from other studies, 6,12,18 although the definition of blindness used in these studies varied considerably.

Discussion
The low prevalence of blindness found in this study and others could be attributed to the fact that most school children with blindness may likely stay at home to avoid stigmatisation. 6 This therefore calls for a population-based visual impairment study in children. After best correction, the overall prevalence of presenting VA of 0.3 LogMAR (20/40) or worse decreased from 153 (7.2%) to 4 (0.19%), which may imply that the delivery of accessible refractive error services with provision of affordable spectacles would significantly reduce the magnitude of VI in schoolaged children.
Children of school age whose parents were classified as high SES were more likely than those with low SES to have VI. This is closely linked with the association between school type (private vs. public) and prevalence of refractive error; those in private schools were two times more likely to have VI (p < 0.001) than those in the public schools. Private schools almost always have higher school or tuition fees when compared to public schools; therefore, parents in higher SES tend to be more likely to afford to send their children to private schools. A similar study in Calabar Municipality comparing students from private and public secondary schools also found that refractive errors were significantly more amongst children whose parents are in the high socio-economic class. 25 Studies carried out elsewhere also found that children of parents of upper to middle SES, as well as children in private and urban schools, have higher refractive error. 26,27 However, other studies found no significant relationship between these variables. 28,29 Significantly, the prevalence of VI was more amongst the girls than the boys (p < 0.01). This could be because of the fact that about 70% of those with VI also had refractive error. Girls have been known to be more involved in activities that require near work than boys, like reading and doing house chores, while boys are more likely to engage in outdoor activities. 5 Older children (12-17 years old) were more likely to have vision of 0.3 LogMAR or worse than children aged 6-11 years. Similar studies found the same result. 26,30,31 The reason may be that older children engage in more near work, and also refractive error (myopia) increases with age.
In the absence of adequate resources, eye care programme targeted at school children could focus on older children 12 years and above.

Conclusion
The major causes of visual impairment amongst schoolaged children were refractive error, glaucoma suspects, amblyopia and corneal opacity. Most of these causes are preventable and about 99.8% of them are correctible with spectacles, and only three out of those who need spectacles wore them; this implies that the uptake of refractive services amongst school-aged children is low. Without screening programmes at schools, children with visual problems may go undetected. Provision of spectacles will greatly reduce the prevalence of impaired vision amongst school-aged children. Age, SES of parents, sex and school type were the independent predictors of VI in school children. Government policy on eye and physical health examination and treatment before admittance to school and periodically during school years is recommended and school eye health should be fully integrated into school health programmes.