Refractive error and visual impairment in primary s

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Introduction
Refractive error (RE) is an optical defect of the eye that prevents parallel light rays from being brought to a clear focus on the retina and is a common cause of visual impairment (VI) and blindness worldwide. 1It is estimated that 2.3 billion people live with RE globally. 2In 2006, 153 million people had uncorrected refractive error (URE) of whom 13 million were children and 45 million were visually impaired. 3,4The resulting VI can lead to health, socio-economic and quality-of-life implications. 1In addition, 90% of people with URE live in developing countries such as Nigeria. 5efractive errors can be easily and effectively corrected with a pair of spectacles, contact lenses or refractive surgery, with spectacles being the most common and cost-effective form of correction.
Studies on the prevalence of RE and VI have been conducted in various provinces of Nigeria, such as Western, 6 Northern, 7 Southeastern 8 and South-South, 9 and have yielded different results.Many of these studies have documented region-specific prevalence and causes of VI, with most being conducted on older children and adults.There is a need for local studies to ascertain the prevalence of RE and VI peculiar to children in each community, as this varies from one locality to another for a variety of reasons.These factors include tribal, ethnic, geographical and socio-economical differences, which may have an impact on RE and VI.The aim of this study was to determine the prevalence of RE and VI in Onitsha, Anambra State, Nigeria.Data on RE and VI will be useful for planning, implementing and monitoring refraction and other eye care services in this group.In addition, the information can be used as baseline data for evaluating existing RE and VI programmes in this region.public schools in Onitsha North and South, the estimated pupil school population being 13 296 in 2009 which was projected to be 15 324 by the year 2017. 10A multistage sampling method was used to select school children in the study population and the baseline sample size was determined using the equation 11 : 2 (1.0 -P) (P) / ([B][P]) 2 [ Eqn 1]   where P is the anticipated prevalence of RE, B is the desired error bound (10%) and Z = 1.96% confidence interval.The prevalence estimates from previous studies conducted in Nigeria ranged between 7.3% and 22.5%. 12A 15% prevalence rate, which is the average of the prevalence estimates from previous studies, was used.The minimum sample size required was, therefore, calculated to be 968, which was increased to 1000 to adjust for anticipated absenteeism and non-participation rate, which should not exceed 10%, to ensure minimal bias in the study results.
Children aged between 5 and 15 years whose parents or legal guardians provided informed consent were included in the study.The other inclusion criteria were children who gave verbal assent and/or signed assent, children who were Nigerian citizens, as indicated by the class register and the class teacher.Children younger than 5 years and older than 15 years, those who were not Nigerian citizens and those whose parents or legal guardians could not provide informed consent were excluded from the study.Ethical approval to conduct the study was obtained from the University of KwaZulu-Natal's Biomedical Research and Ethics Committee (BE620/16) and Onitsha Zone Education boards.The research protocol adhered to the provisions of the Declaration of Helsinki for research involving human subjects.The heads of the identified schools were contacted to request their permission to engage with the children in their schools.Each school provided a room in which the equipment could be set up for testing, and the teachers were informed that the children would be called out of class to have their eyes tested in the assigned rooms.

Pilot study
Prior to the main study, a pilot study was conducted among 50 primary school children outside the study area to check the appropriateness of the study procedures, methods and logistics.All queries that arose from the pilot study were addressed, and the procedures were modified accordingly before the main study was performed.

Data management and analysis
Class enumeration and clinical examination data forms were reviewed for completeness in the field before they were captured into the computer.Assistance of a statistician was sought for the data analysis, which was analysed using descriptive and inferential statistics in the Statistical Package for Social Sciences (SPSS) version 24.Ranges, means, standard deviations, frequencies and correlations were determined.Chisquare and Pearson's correlation tests were used to investigate relationships between age and gender with VI and RE.

Ethical consideration
The study was approved by the Biomedical Research and Ethics Committee, University of KwaZulu-Natal, the Department of Education and Training, and the heads of the identified schools, Onitsha, Nigeria, and the study adhered to the tenets of the Declaration of Helsinki.

Results
Of the 1020 primary school children aged between 5 and 15 years from 17 primary schools in Onitsha North and South who were enumerated, 998 (97.8%) participated in the study.

Causes of visual impairment
Of the 97 children with uncorrected VA of 20/40 or worse in the better eye, 84 (86.6%) were because of URE and thus improved to ≥ 20/32 after refraction, while 13 (13.4%)could not be improved to ≥ 20/32 in the better eye.The prevalence of VI (best corrected VA of 20/40 or worse in the better eye) was therefore 13 (1.3%)(Table 4).

Discussion
Except for a relatively large number of 8-to 10-year-olds and a small number of 15-year-olds, the age distribution of the enumerated population was reasonably uniform.The high response rate in this study could be in part, owing to the fact that there are no school eye screening programmes in Onitsha, so the children felt it was an opportunity to get their eyes tested.The endorsements by the education board secretaries, co-operation of the class teachers and Parent-Teacher Association (PTA) also contributed to the high response rate.
The prevalence of RE among primary school children aged between 5 and 15 years tested in Onitsha, Anambra state was 9.7%.This falls within the World Health Organization (WHO) prevalence range of 2% -10% reported among children in this age group worldwide and warrants regular vision screening.Table 5 6,7,8,9,13,14, 15,16,17,18,19,20,21,22,23,24,25,26,27 provides an overview of   selected studies on the prevalence of REs among children of various ages in a range of African countries.
The prevalence of 9.7% for URE is higher than the 2.2% reported in Bayelsa, Nigeria, 9 but is comparable to that reported elsewhere in Nigeria, specifically the 7.3% and 8.7% in Lagos, 28 as well as the 7.4% in Enugu 29 and 8.9% 30 in Ibadan.However, the prevalence is lower than 22.5% and 58.0% reported in South-South Nigeria and Abia State, 8 Nigeria, respectively.The low prevalence recorded in Bayelsa study could be because of the fact that it was a vision screening programme, and some children with low magnitude of RE and normal VA could have been missed.In addition, the current study was school-based while that of Opubiri et al. 9 was hospital based among 4-to 15-year-olds, which could have accounted for this difference.It is well known that most hospital-based studies would generally record higher rates of conditions than in the general population. 9Furthermore, the sample size and different definitions used for RE could have accounted for the differences between them.For example, the current study used a sample size of 998 and VA of 20/40 or worse in the better eye for the definition of RE, while a sample size of 4225 and VA of 20/32 or worse were used in the study in Abia State. 8mparison of the current study with studies in Africa shows that the prevalence of 9.7% for URE is close to 7.6% and 9.4% reported in Ethiopia (Table 5).The differences observed in the prevalence could be because of different operational definitions and methodologies considered by authors and differences in demographic variables.Moreover, lifestyle and environmental as well as genetic factors have been reported to contribute to variations in the prevalence of RE. 31 Recently, increased outdoor activities and exposure to high light intensity, common among rural dwellers, have been suggested to be factors influencing the prevalence of RE in rural areas. 32On the contrary, lifestyle factors such as increased near work and indoor activities common among urban dwellers have also been known to increase the prevalence of RE in urban areas. 32However, other studies have shown the prevalence of RE to be high in urban and highly developed areas compared to rural and less developed areas. 33Although racial and ethnic differences are the most important reasons for differences in the prevalence of RE, studies have also shown that genetically determined factors (such as eye pigmentation) may theoretically interact with environmental influences (such as outdoor light exposure) to impact RE development in black people. 2 Various studies have shown that gender differences at the age of onset of puberty affect development of RE and with differences between biometric ocular parameters of males and females being reported, which suggests a possible correlation between gender and RE. 34Refractive error was found to be significantly higher in females (56.7%) than males (43.3%) (p = 0.04) in the present study, a result similar to those observed in Lagos, 6 South-South Nigeria, 9 Kebbi State Nigeria 7 and Ethiopia. 13udies have shown that the human eye grows by 5 mm from birth to 6 years, and by an additional 1 mm after 6 years until the age of 12 years. 6The prevalence of RE has been reported to increase to 2% by the age of 6 years and to 15% by the age of 15 years in the general population, 7 indicating the possibility of a relationship between increase in age and RE.This study found RE to increase with age and was highest (48.9%) among children 11 to 13 years old.Similar findings were reported by Assefa et al. 13 in Ethiopia and by Opubiri et al. 9 in South-South Nigeria.However, a study in Enugu State, Nigeria, by Chuka-Okosa 13 showed no significant association between RE and age.The large age range of 12-21 years used in the Enugu study could be responsible for the observed differences.
Myopia was the most prevalent (46.4%)URE found in this study.This could be because of the fact that Onitsha is mainly an urban area and children in urban environment engage more in indoor and near work activities, such as higher usage of computers, smartphones and video games. 31,32Studies in Abia State 8 and Bayelsa State, Nigeria, 9 also found myopia to be the most prevalent refractive condition among primary school children aged between 7 and 17 years, and between 5 and 21 years.Although both studies were conducted in urban environments, variations in the prevalence rates could be attributed to the differences in age ranges, sample sizes and methodologies used.For instance, the current study used a sample size of 998 and an age range of 5-15 years, while the study by Ahuama and Atowa 8 used a sample size of 4225 and an age range of 7-17 years.In addition, the present study was school based while Opubiri et al. 9 was hospital based.
Studies in Tanzania and South Africa by Wedner et al. 14 and Naidoo et al., 15 respectively also showed myopia to be the most prevalent refractive condition among children aged between 5 and 15 years.Higher prevalence findings were reported in Singapore, 35 China, 36 Vietnam, 37 Egypt 38 and Sudan. 16The higher prevalence of myopia recorded in those studies was attributed to the high prevalence of myopia in Asians and Arab children.Myopia is a common visual problem in schoolchildren globally, with early detection and management being highly indicated for educational, behavioural and quality-of-life benefits.
In this study, myopia was found to increase with age, starting from 8 to 10 years, which could possibly be because this age group is mostly in grade 5 and 6 which is the grade for preparing and writing entrance examination to high school in Nigeria leading to increased academic demand; thus, increasing the onset of myopia.A possible reason could be the onset of juvenile myopia, defined as myopia, with an onset between 7 and 16 years of age, primarily because of axial elongation that is usually caused by intensive near work. 6Ahuama and Atowa 8 also reported myopic progression starting from 12 to 17 years in Abia State, Nigeria, with similar findings being recorded in the RESC survey in China, 26 Chile 39 and South Africa, 15 with the upward trend in myopia started from 13 to 14 years in the later.
Myopia was found to be significantly associated with males (p = 0.03), which could be because of the fact that the male children engage more in indoor activities, such as computer video games, chatting on phones, reading and writing, unlike the females who do more outdoor activities, such as domestic chores.Msiska et al. 17 also found myopia to be significantly associated with males, while Wedner et al. 14 and Kawuma and Mayeku 18 reported contrary results in Tanzanian and Ugandan children, respectively.Muma et al. 19 and Kumah et al. 20 did not find any significant association between gender and myopia in Kenya and Ghanaian children, respectively, which could be because of the diverse age groups in the study samples.The prevalence of astigmatism varies in studies across populations, with racial factors being reported to be among the reasons for the differences observed in the prevalence of astigmatism worldwide. 40Early detection and correction of astigmatism are important, as most asthenopic symptoms that could lead to poor school performance are usually because of astigmatism.The prevalence of astigmatism of −0.50 D or worse found in this study was high.School children in urban areas engage more in near work and astigmatism is associated with increased near work.The prevalence is lower than 38.8% reported in Lagos, Nigeria by Faderin and Ajaiyeoba. 6However, it is more comparable to 6.1% and 7.8% recorded in South-South Nigeria and Abia State, Nigeria, respectively (Table 5).The reasons for the discrepancies could be the inclusion of diverse age and ethnic groups in their study samples.
With-the-rule astigmatism was the most common type found in this study and this could be because it is commonly found in children 16 Similar findings were reported by Atif et al. 16 among Sudanese children aged between 5 and 15 years.Opubiri et al. 9 and Naidoo et al. 15 found astigmatism to increase with an increase in age in South-South Nigeria and South Africa, respectively.The present study did not find astigmatism to increase with age, the variability in findings possibly being attributed to ethical and racial differences.
There was no gender difference in the prevalence of astigmatism in this study, this being similar to a report by Muma et al. 19 in Kenya.
The prevalence of hyperopia was low (Table 5), which could be because of the study setting.Children in urban areas are more actively involved in near work and less outdoor activities, thus reducing their prevalence of hyperopia. 19The prevalence of hyperopia found in this study is higher than those reported in other Nigerian studies but lower than and comparable to those conducted in other African countries (Table 5).This wide variation could in part be because of the variations in the definitions of hyperopia, and/or age groups and ethnicities used in the various studies.For example, Ahuama and Atowa 8 defined hyperopia as +0.50 D and higher, while the current study defined it as 2 D and higher (Table 5).The study by Ahuama and Atowa 8 also involved rural and older school children (7-17 years), which could have increased the prevalence of hyperopia, as it is well reported that hyperopia is more prevalent among children in the rural areas. 17,19Hyperopia was found to decrease with increasing age and was highest among children 5-7 years old.This is possibly because of the fact that this younger age group is prone to be more hyperopic, as their crystalline lens

FIGURE 2 :
FIGURE 2: Distribution of refractive error by gender.

TABLE 1 :
the 998 children examined, uncorrected VA of 20/32 or better in the better eye was found in 901 (90.3%) children, 97 (9.7%) had uncorrected VA of 20/40 or worse in the Complete cycloplegia was achieved in 85 (87.6%) right eyes and 79 (81.4%) left eyes.However, pupil dilation with light reflexes present was found in 11 (11.3%)right eyes and 9 (9.3%) left eyes, while the absence of light reflex without full pupil dilation was found in 12 (12.4%)right eyes and 18 (18.6%)left eyes.Therefore, 72 (74.2%) children satisfied one or both criteria for cycloplegia dilation in both eyes.Distribution of uncorrected, presenting and best corrected visual acuity.
better eye and 20 (2%) wore spectacles.Eighty-four (86.6%) children improved to ≥ 20/32 in the better eye after refraction and 13 (13.4%)hadbestcorrectedVA of 20/40 or worse in the better eye, including one child (1%) with no light perception in one eye.This results in the prevalence of uncorrected VA of 20/40 or worse in the better eye of 9.7%, of presenting VA of 7.7%, and best corrected of 1.3% (Table1).Refractive errorNinety-seven (9.7%) children who had VA of 20/40 or worse in either eye underwent cycloplegic refraction.2%)lefteyes with auto-refraction.These results indicate that significant astigmatism in either eye was present in 35 (36.1%) children.The prevalence of hyperopia ranged between 17.5% and 21.6% when measured with retinoscopy and auto-refraction, respectively.With retinoscopy, the prevalence of myopia ranged from 6.2% in 5-to 7-year-olds to 15.5% in 8-to 10-year-olds (Table2).With auto-refraction, the prevalence of myopia was 49.5%, while auto-refraction and retinoscopy results could not be obtained in 21 (21.6%)eyes because of poor co-operation, fixation and claustrophobia.Of the 97 children who had RE, 45 (46.4%) had myopia, 35 (36.1%) had astigmatism and 17 (17.5%)hadhyperopia(Figure2).Forty-two (43.3%) children who had RE were males, while RE was highest (45.4%) among the 11-to 13-yearolds and least common (4.1%) among children aged between 14 and 15 years (Figure3).Refractive error was found to be VA, visual acuity; N, number.

TABLE 4 :
Causes of visual impairment.

TABLE 3 :
Distribution of types of astigmatism.
FIGURE 3: Distribution of refractive error by age group.

TABLE 5 :
Prevalence of refractive error studies in children in Africa.