The visual experience of a child plays a significant role in his/her mental, social, emotional, physical and intellectual development. Refractive error has been one of the leading causes of visual impairment among school children. This has made the prevalence of refractive error amongst school children a topic of public health interest in research. As such, many studies, including the current research has been conducted to determine the prevalence of refractive error among this population in different geographical areas.
In this study, the prevalence of refractive error was evaluated amongst primary school children between the ages of 5 and 9years, attending primary schools in Ado-Odo / Ota LGA of Ogun state, Nigeria. Out of the 217 pupils recruited, 205 pupils were screened, using pin hole, auto-refractor and subjective refraction method to determine the presence of refractive error. About 63 (30.74%) of the study population (205 children screened) were diagnosed of refractive error of (≥ ± 0.50D SE), but none of them had corrective glasses on and none had record of any previous eye screening. The prevalence of 30.74% refractive error seen in this study is similar to the result of a study conducted in Western Saudi Arabia among 3-10 years old children which reported a prevalence of 34.9% of uncorrected refractive error among this population (2). Also, this prevalence is comparable to what was reported in Imphal, Manipur India, where the prevalence of 29.14% refractive error among school children was also reported (8). These similarities may be explained by the fact that both studies adopted a similar working definition for refractive error, that is: Myopia (visual acuity ≤6/9 in any eye for far vision that improves after pinhole testing), hyperopia (visual acuity of < N5 at near that improves after pinhole testing) (2)(8). However, the prevalence of 30.7% as reported in this study is higher compared to the prevalence of 13.5%, 24.6.9% and 22.5% reported in Al Hassa, Saudi Arabia, North-West Rajasthan and Abia State, Nigeria respectively(9) (10)(11). The reason for these variations may be explained by differences in the methodologies used, definition of working terms (such as hyperopia, myopia and astigmatism), age range, sample size, geographical areas, environmental factors, genetics, diet and ethnicity/tribe, of the studied population in these studies.
From the study, the prevalence of refractive error varied according to type. For instance, astigmatism (≥-0.75DC) was found present among 3.4% of the entire population of pupils studied. This was less than what was gotten in Abia State, Nigeria where astigmatism was the least prevalent (7.8%) refractive error present in school children between 5-15 years of age (12). Myopia (≥ 0.50Ds) was the most common type of refractive error encountered, constituting 24.89% in both eyes. This reflects in its totality about 80.96% of the entire refractive error encountered in the study. This is higher than the prevalence of myopia (65.7%) reported in a study conducted in Saudi Arabia among 6-14 year olds school children living in Saudi Arabia (9). It is also higher than the prevalence reported in the studies conducted in Malaysia, Nepal and Jordan which found that myopia represented 77.5%, 59.8%, and 63.5% of screened errors respectively among 6-14 year olds school children, making it also the most prevalent refractive error among these populations (9). Hyperopia on the other hand, was the least refractive error encountered with a prevalence rate of 2.44%. This is lower in comparison to the prevalence of hyperopia (58.1%) reported in Zahedan District, Southeastern Iran, among 5-to-15 year -old children (13). However, the findings of this study is higher when compared to the prevalence of hyperopia reported in Northwest Ethiopia wherein hyperopia of 1.47% was found (14). Similarly, the prevalence of hyperopia in this study is also higher than the prevalence reported in a study conducted in Qassim, Saudi Arabia, in which a prevalence rate of 0.7% was obtained (15). Variation in the prevalence rate of this studies maybe attributable to the difference in sample size, population studied and the operational definition of hyperopia as employed by the studies.
This study also found an age variation in the distribution of refractive error across the studied population. Among the affected population, refractive error was most prevalent among 9 year olds (49.01%) compared to the rest of the population. Myopia, as a refractive error was also found to vary with age. It increased from 0% in the 5 year olds to 49.01% in the 9 year olds. This observation of variation in myopia with age was not unexpected, because many studies in the past demonstrated a correlation between age and myopia (16). The most notable factor in this correlation between age and myopia is the growth-related increase in axial length. Increased axial length during the first and second decades of life has been documented to result in increase in myopia as found in cross-sectional and prospective studies. When compared to other studies there seem to be an agreement that myopia varies with age, however there were significant variations in its distribution across specific ages and/or age groups. For instance, the current study recorded a 0%, 11.76% and 49.01% prevalence of myopia (≥ 0.50) in 5, 7 and 9 year-old children respectively. Variations in distribution of myopia was also noted in the study by Jokbe et al., conducted on German children and adolescents, which recorded prevalence of myopia as 0% in children aged 2–6 years, 5.5% in children aged 7–11 years, 21.0% in adolescents (aged 12–17 years) and 41.3% in adults aged 18–35 years (17). Reasons for these variations in the distribution of myopia across the age groups may be explained by the growing demand for more near work as children grow in age and in their academic demand. Also the high increase in the usage of electronic gadgets to play games by children especially the older age group has also increased the rate of near work among this population.
Finally, the difference in the distribution of refractive error among the different sexes was in drastic contrast to other studies. Whereas, the current study indicates a higher prevalence of refractive error amongst the male pupils (19.51%) than female pupils (11.22%), studies by other researchers had indicated a higher prevalence of refractive error among females than males (13) (2) (3). However, this is similar to the work of Shukla et al who reported a higher prevalence of refractive error among male than female pupils (18). The findings here are in line with previous studies; while few studies have shown this relation in school-age children, most studies in older age groups agree that the prevalence of myopia is higher in males (19). This may be due to the higher frequency of male to female ratio of the studied population. However, factors like gestational period weight, family history of refractive error and duration of near work among the children were not studied. These factors may have been contributory to the variations in prevalence of refractive error amongst the male and female children.