The role of environmental factors and hereditary predisposition in the causation of low myopia

Effect of Myopic Undercorrection on Habitual Reading Distance in Schoolchildren: The Hong Kong Children Eye Study

INTRODUCTION

This study aimed to evaluate the habitual reading distance among non-myopic children and also myopic children with undercorrection and with full correction.

METHODS

This was a population-based cross-sectional study with a total of 2363 children aged 6-8 years who were recruited from the Hong Kong Children Eye Study. Cycloplegic autorefraction, subjective refraction, habitual visual acuity, and best corrected visual acuity were measured. The entire reading process (9 min) was recorded using a hidden video camera placed 5 m away from the reading desk. Reading distances were taken at 6, 7, 8, and 9 min after the child began reading and were measured using a customized computer program developed in MATLAB. The main outcome was the association of habitual reading distances with refraction status. Habitual reading distances of children were documented via video camera footage.

RESULTS

The habitual reading distances of undercorrected myopic children (23.37 ± 4.31 cm) were the shortest when compared to non-myopic children (24.20 ± 4.73 cm, P = 0.002) and fully corrected myopic children (24.81 ± 5.21 cm, P < 0.001), while there was no significant difference between the last two children groups (P = 0.17). A shorter reading distance was associated with myopia (OR 1.67; 95% CI 1.11-2.51; P = 0.013) after adjusting for age, sex, height, near work time, outdoor time, and parental myopia. The association of reading distance with myopia did not hold after undercorrected myopic children were excluded (OR 0.97, 95% CI 0.55-1.73; P = 0.92). A shorter reading distance correlated with poorer vision under habitual correction (β = - 0.003, P < 0.001).

CONCLUSION

A shorter reading distance was present among undercorrected myopic children. Myopia undercorrection is not recommended as a strategy for slowing myopic progression.

The importance of genes and environment for ocular refraction and its determiners: a population based study among 20-45 year old twins

AIMS

To estimate the heritability for ocular refraction and its determiners in a population based cohort of 20-45 years old twins.

METHODS

114 twin pairs (53 monozygotic and 61 dizygotic) participated. Refraction was determined in cycloplegia and eye dimensions were measured with ultrasound. Educational length was assessed. The heritability was estimated employing aetiological model fitting. Evidence of gene-environment interaction was analysed. Correlations between intrapairwise differences in educational length and in refraction were evaluated.

RESULTS

The heritability was between 0.89 and 0.94 (95% CI: 0.82, 0.96) for refraction, total refraction, axial length, and radius of corneal curvature. Phenotypic variation was mostly due to additive genetic effects. Refraction revealed evidence of gene-environment interaction (r = -0.29 to -0.32; p <0.05). The heritability for anterior chamber depth and lens thickness was between 0.88 and 0.94 (95% CI: 0.81, 0.96) and dominant genetic effects were the most likely explanation. There was no correlation between age and intrapairwise differences in refraction. The dizygotic twins had significant larger intrapairwise differences in educational length (p <0.05), but the differences were not correlated with differences in refraction.

CONCLUSIONS

The results indicate a high heritability for ocular refraction and its determiners and thus suggest that environmental impact on refraction is not significant. However, the epidemiological association between educational length (near work) and myopia, the evidence of increasing myopia prevalence within a few generations, and the theory of gene-environment interaction imply that some individuals might be genetically liable to develop myopia if exposed to certain environmental factors.

The influence of study habits on myopia in Jewish teenagers

The prevalence and degree of myopia were measured in 870 teenagers, males and females. We found a statistically significant higher prevalence and degree of myopia in a group of 193 Orthodox Jewish male students who differed from the rest in their study habits. Orthodox schooling is characterized by sustained near vision and frequent changes in accommodation due to the swaying habit during study and the variety of print size. A possible myopic effect of this unique visual demand is postulated.