We investigated changes in the SE refractive error and astigmatism in a pediatric referral population in Korea with longitudinal follow-up. The results of this study demonstrated an overall negative shift in SE refractive error and increasing tendency in astigmatism from age 3 to 16 years. Moreover, the initial degree of SE refractive error showed a significant association with changes in SE refractive error, and the initial degrees of SE refractive error and astigmatism showed a significant association with changes in astigmatism.
Changes in SE Refractive Error
We suggest that the negative shift may be the result of complex interactions between genetic and environmental factors. Previous studies have shown that the prevalence of myopia and the rate of myopic progression in East Asian children are higher than those in Western children.1 Intense nearsighted work,5 limited outdoor activity,7 and growing up in an urban environment7 are factors suspected to contribute to myopic shift. Unlike previous studies,8,9 an initial increase in the mean SE refractive error was not observed in our study, and there was a greater annual decrease in SE refractive error throughout the follow-up period. In the myopia group, a slightly increased SE refractive error was observed between age 3 and 4 years. After this point, the value started to decrease continuously.
In the current study, the initial degree of SE refractive error affected the negative shift in SE refractive error. Compared with the emmetropia and hyperopia groups, the myopia group was more likely to have a significant negative shift in SE refractive error, consistent with previous reports showing significantly greater myopic shift in children with myopia.1,10 These findings indicate that initially myopic eyes are more likely to have a faster rate of change toward high myopia. However, the annual rate of change was so variable that the refractive outcome could hardly be predicted for an individual child.
Previous studies have provided conflicting data regarding the influence of sex on changes in SE refractive error. From the age of 5 to 15 years, several studies found a higher prevalence of myopia among girls and a faster rate of progression among girls with myopia.11 Another study found no difference in the progression of myopia based on sex.10 In the current study, there was no difference in the mean SE refractive error or changes in SE refractive error due to sex.
There are several reports on refractive error changes in children with strabismus. Longitudinal studies of accommodative esotropia have revealed that refractive errors show a slow negative shift over time.12,13 Regarding exotropia, a significant myopic shift over time compared with similarly aged non-strabismic children was reported in a population-based study.14 In the current study, compared to the orthotropia and exotropia groups, the esotropia group exhibited slower changes in SE refractive error. Because esotropia occurs more frequently in children with hyperopia, the hyperopic refractive error in the esotropia group might have caused the negative shift to be less than that in the other groups, as discussed previously. However, in the final multivariable model adjusting for the initial extent of SE refractive error, ocular alignment was not a significant predictor of SE refractive error.
Changes in Astigmatism
Many potential risk factors for astigmatism are still not well understood. Findings in some studies have suggested that astigmatism is dominantly inherited,15 whereas others have shown low heritability.16 Saw et al.17 suggested that environmental influences have a major impact on astigmatism. Thus, the relative contribution of genetic and environmental influences to astigmatism requires further analysis. In previous studies, investigators have consistently found that there is a rapid decline in astigmatism in the first 2 years of life,18,19 followed by slower changes occurring between ages 2 and 6 years.1 However, findings in other studies lead to inconsistent conclusions concerning later developmental changes in astigmatism. Anstice20 reported a significant decrease in astigmatism up to age 14 years. In contrast, a longitudinal study of astigmatism in Tohono O'odham Native American children21 showed that highly astigmatic children aged 3 to 11 years and children older than 11 years show a small increase in astigmatism with age. In the current study, an increasing tendency in astigmatism during the follow-up period was noted.
We evaluated the effects of potential risk factors on an increasing tendency in astigmatism. Any association between astigmatism and myopia remains controversial. In the current study, the change in astigmatism in the myopia group was significantly greater than that in the hyperopia group. Gwiazda et al.22 suggested that spherically asymmetric forces operative in tense ciliary muscles or zonules could induce astigmatism associated with the development of myopia. Moreover, optical blurring caused by uncorrected astigmatism may trigger myopic development. Although the initial degree of astigmatism was not associated with changes in the SE refractive error, greater astigmatism was associated with more myopic refraction in the current study. Our results support the hypothesis that an increase in myopia in children can enhance the development and progression of astigmatism.
The initial degree of astigmatism showed a significant association with changes in astigmatism. The low astigmatism group was more likely to have a significant increase in astigmatism than were the moderate and high astigmatism groups. These findings indicate that eyes with moderate to high astigmatism are relatively stable initially and that eyes with low astigmatism are more likely to have an increasing tendency in astigmatism.
Findings in previous studies on differences in astigmatism between sexes have been inconsistent. Several large population-based studies reported slightly higher prevalence rates of refractive astigmatism in girls than in boys,23 although several other studies reported no difference based on sex.1,24 In our study, the mean astigmatism and changes in astigmatism throughout the follow-up period were not significantly different between boys and girls.
There have been few studies on the association between strabismus and astigmatism. In previous studies, astigmatism increased the risk of developing exotropia.25 Longitudinal changes in astigmatism according to ocular alignment have not been reported. In the current study, ocular alignment was significantly correlated with changes in astigmatism in a univariable linear mixed model. However, in the final multivariable model adjusting for the initial extents of SE refractive error and astigmatism, ocular alignment was not a significant predictor of astigmatism.
Our study had several limitations. First, our population was a referral population; therefore, referral bias may have been present, which limits the generalizability of the findings. Although referral bias is indeed a limitation, the study design did allow changes in refractive error to be examined in a large group of patients over an extended follow-up period. Second, this study was retrospective in design; thus, there were different follow-up intervals for different patients. Third, several risk factors known to influence changes in refractive error were not included: the parental history of refractive errors, environmental factors, age when spectacles were first prescribed, and amount of correction. Longitudinal studies of accommodative esotropia have revealed that changes in refractive errors were mostly related to the age when spectacles were first prescribed and the amount of correction.12,13 Further study of the effect of these risk factors on changes in refractive error is needed.
We found that a pediatric referral population in Korea showed a negative shift in SE refractive error and increasing tendency in astigmatism during childhood. Changes in refractive error may be influenced by the initial degree of SE refractive error and astigmatism.