Factors Influencing the Visual Acuity of Primary School Pupils

To investigate the factors influencing the visual acuity of primary school pupils, an epidemiological study of 480 pupils in the 6th grade (11-12 years of age) was conducted in 8 primary schools in Sapporo City, Japan. Questionnaires were used to inquire into their current and past visual acuity and related factors. Dividing the subjects into those whose visual acuity of both eyes was 0.7 or more and those whose visual acuity of at least one eye was less than 0.7, odds ratios of various factors were calculated. Lifestyle and dietary factors showed no significant odds ratios. Parental myopia and age of parents at the birth of subjects showed significant odds ratios. Visual acuity of the pupils whose parents were myopic or older than 30 years when they were born tended to have worse visual acuity as they got older. Till the ages 11 or 12, hereditary factors seem much more contributory to visual acuity than environmental ones. J Epidemiol, 2000 ; 10 : 179-182


INTRODUCTION
Recently in Japan the lifestyle of children has changed.Video games have become popular; children have to study hard in order to enter first-rank schools and they often stay up till late at night.Such circumstances may be bad for their health, and might have some influence on their visual acuity.It is said that visual acuity of children is influenced by hereditary and environmental factors.We conducted an epidemiological study concerning visual acuity of primary school pupils and various factors.

SUBJECTS AND METHODS
Of the 192 primary schools in Sapporo City, 8 were randomly selected.A questionnaire was sent to 752 pupils in the 6th grade (11-12 years old) in these primary schools in April 1996.Using the questionnaire their visual acuity of both eyes at each of the grades at primary school, factors of lifestyle, dietary factors, family history and so on were investigated.(We asked the subjects to respond according to their records of the annual health check-up for visual acuity.)The visual condition of their parents were also asked about; i.e., whether they had myopia, hyperopia, presbyopia, or normal vision.The questionnaires were filled out by the pupils and their parents, and returned to us through their teachers.
Odds ratios were calculated for each factor with the method of Mantel and Haenzsel u, after stratifying the subjects by gender.The chi-square test was employed for testing statistical significance.Logistic regression analysis was employed as a multivariate analysis.

RESULTS
A total of 480 pupils gave eligible answers (64 .0%);they had complete data of visual acuity from the 1st grade to the 6th grade.In 345 of them (71.9%), the visual acuity of both eyes with neither glasses nor contact lenses was 0.7 or more.The visual acuity of at least one eye of the remaining subjects was less than 0 .7.In this report the subjects were divided according to this criterion .Table I shows the percentages of those whose visual acu-Table 1.The percentages of those whose visual acuity of both eyes was better than 0.7 (Sapporo City, 1992).
Table 2. Odds ratios of the risk factors for the visual acuity of at least one eye in the 6th grade being less than 0.7.
ity was better than 0.7 in Sapporo City 2).The percentage at the 6th grade is the same as the present figure.
Table 2 shows the odds ratios of various factors.Parents' myopia and parents' being 30 years of age or older at birth of the subjects were significant.The factors relating to lifestyle and foods showed no significance, though the factor "distance from TV" showed borderline significance.
Table 3 shows the changes in visual acuity of the subjects by the visual condition of their parents.Those whose parents were myopic tended to have worse visual acuity than those who had at least one parent whose vision was normal.
There was a tendency for those whose parents were 30 years of age or older at their birth to have worse visual acuity (Table 4).The correlation coefficient between the age of the father and that of the mother was 0.62.But the correlation coefficient between age (year) and the visual acuity [myopia (1) or not (0)] in fathers was 0.18, and that in mothers was 0.25.
In the logistic regression analysis (the outcome variable, visual acuity of both eyes was more than 0.7 or not), myopia of the father was significant in the 3rd and 6th grades (Table 5).

DISCUSSION
Visual acuity over 0.7 is necessary for pupils to see the letters written on the blackboard ".Therefore, in the present report we employed the visual acuity 0.7 as the cut-off value.
It is reported that bad visual acuity of Japanese pupils in early grades is partly due to hyperopia, but the percentage of myopia increases to be the main cause as they grow up ").Though the exact causes for the bad visual acuity of the present subjects were not ascertained, many of them seem to have suffered from myopia.
In the present study hereditary factors seem much more contributory to visual acuity than environmental ones.It is reported that in adults there is no compelling evidence that suggests there is a significant increase in the risk of the onset of myopia or its progression from the use of VDTs ~.In the present study, "the distance from the TV" showed borderline significance.However, it is hard to tell whether the distance from the TV is a cause or a result of bad visual acuity, since *P<0 .05#0.05<P<0.10Foods; very frequent + frequent vs intermediate + occasional + rare.
Visual Acuity of Pupils Table 3.The percentages of those whose visual acuity of both eyes was better than 0.7 (n, in 6th grade).2).Similarly, this might be a result of instruction by parents, teachers or doctors.Several studies employing monozygotic and/or dizygotic twin pairs 8-10) suggested the significant hereditary role in determining the ocular refraction; according to the study concerning a set of triplets (two of whom were monozygotic) 9), there were discordances between the monozygotic sisters and the dizygotic sister, though all of the three lived in the same environment.However, the difference in age of subjects might account for the difference in results; several studies whose subjects were students suggested the significance of the role of environment.Marumoto 11) reported that the failing eyesight of young students results from postural problems such as shortness of viewing distance and increased neck flexion.In high school and university students there was a positive correlation between studying time and myopia 12, 13).To the contrary, there seems to be a tendency that a study whose subjects are pupils, like the present one, concludes that hereditary factors are more contributory than environmental ones.Zadnik 14) reported that in children 6-14 years of age a parental history of myopia was more contributory than near work, but near work was a significant factor as well.Though a study of Hong Kong Chinese 15) showed that there was no significant relation between parental myopia and 12-yearold children's myopia, the number of the subjects in this study was only 83 and actually the observed number of myopic children whose parents were myopic was higher than the expected number.The use of more subjects might provide significant results.
Thus, the contributions of hereditary factors and environmental ones may be different by age; in primary school pupils the former plays a more important role in being myopic.
The fact that only the father's myopia remained significant in the logistic regression analysis can not be explained, though it is possible that men who graduated from university many late in life, and father's school career might have something to do with the result.There does not seem to be large difference between the paternal contribution to children's visual acuity and the maternal one.The high correlation between fathers' and mothers' ages may have influenced the results of the present logistic regression analysis.
The influence of parental age when subjects were born seems to be weaker than that of parental myopia, judging from the present results.But since the coefficients between age and visual acuity were low both in fathers and in mothers, the two factors may be independent of each other.Several disorders, e.g., Down's syndrome, are related to parental age at birth.Disadvantage due to the oldness of parents might lead to the promotion of cell aging or vulnerability.No reports have referred to parental age as a risk factor.It will be interesting to further investigate the relation between parental age and the visual condition of children.

Table 4 .
The percentages of those whose visual acuity of both eyes was better than 0.7 (n, in 6th grade).
*P<0 .05myopic children are usually told not to watch TV within a short distance.Short time lengths for playing video games and watching TV had rather high odds ratios (1.51 and 1.38, respectively; Table