The prevalence of myopia is increasing worldwide and is particularly high in East Asian countries. It can lead to axial elongation and induce various vision-threatening complications, as mentioned in the introduction, even in individuals of working age. Therefore, myopia should not be ignored, and prevention is better than treatment. Both hereditary and environmental factors contribute to myopia progression; these were analyzed in the present study and are interpreted in the following paragraphs.
With regard to parents’ myopia status, the rate of high myopia in the participants (27.5%) was much higher compared with fathers with high myopia (3.3%) and mothers with high myopia (6.0%), which is compatible with previous reports that the prevalence of both myopia and high myopia is increasing rapidly. Family history of myopia is a risk factor for adult-onset myopia[21–23], and Parssinen et al indicated that women with myopic parents showed higher refractive power and faster/earlier myopia progression than did those with non-myopic parents . Our results indicated that mothers may contribute more on hereditary myopia compared with fathers. After subgroup analysis based on sex, high myopia in mothers was only a risk factor for high myopia in women but not in men. Thus, further studies are necessary to identify the roles played by sex in hereditary myopia. Conversely, the great difference in myopia prevalence between the two generations suggests that genetics does not have a major effect on myopia prevalence; environmental factors may be the major cause.
One recent meta-analysis showed that more time spent on near work was associated with higher odds of myopia. Higher educational level is associated with more myopic refraction[14, 24], which may be linked to longer time spent on near work. This may partially explain why there is high myopia prevalence in East Asia, where students spend much more time on studying and after-school classes because there is fierce competition for higher education. In the present study, univariate analyses showed that the high myopia group spent more time on near work at all four educational stages, and multivariate analysis indicated that high myopia group undertook more after-school class in senior high school.
In contrast, longer time spent outdoors is a protective factor for myopia[10, 25, 26]. This is an important factor in public health efforts. Further studies have found that the bright light outdoors increase dopamine receptor activity, with resultant decreases in myopic shift and ocular elongation[27–29]. In Taiwan, after outdoor activities were implemented in the Taiwan Student Vision Care Program, the rate of vision impairment decreased. Recently, Wu et al found that activities undertaken in lower outdoor light intensity, such as in hallways or under trees, also provide some protective effects on myopia development. In the present study, although this did not reach significance in multivariate analyses, both outdoor and indoor sports time were significantly less in primary school in high myopia group (Table 3).
The age at first spectacle use has been discussed in previous studies. In a UK-based study, almost half of the individuals with myopia wore glasses only after the age of 17 years . Rafael et al reported that the subjects who were older at their first spectacle use tended to develop lower refractive error, but this relationship was not significant. Another study indicated that subjects who developed myopia after the age of 20 years had low myopia. Wearing spectacles to correct myopic refractive error is more common in East Asia than in Western countries. The present study revealed that the mean refractive error of the first prescription in Taiwan was − 1.62 diopters, at a mean age of 13 years. This is compatible with the clinical guidelines for childhood vision care in Taiwan, which suggests that the first spectacles should be prescribed once children having myopia of around − 1.50 diopter who need refractive assistance in life. However, one study showed that refractive cutoffs ≤ -0.75 diopters of myopia showed significantly greater improvement in vision than children failing to meet the criteria. Table 4 shows that younger age at first spectacle use was a predictor of more severe myopia progression in the future, with a high OR. Based on the results, another interesting issue is that should we postpone glasses use or under-correction for myopic children who need refractive assistance. First, it’s an important fact that early age at first spectacle use is the result of early myopia onset. In this study, we use the age of wearing first spectacle to estimate the age of myopia onset. Early age at first spectacle use indicated the early age of myopia onset. Secondly, as for the consideration of glasses use to control myopia progression, this question is still controversial and under investigation. The manipulations of optic correction in spectacles, including undercorrection, full correction, multifocal or bifocal, are investigated to control myopia progression, but several studies show a variety of results, ranging from decreasing to worsening myopia progression [36–40].
Based on the present results, high myopia is much more prevalent now than in the previous generation and mother may play a more important role, which has not been fully described in available literature. Compared with non-high myopia, the high myopia group had early age and higher refractive power of the first spectacle used as predisposing factors, less outdoor activity in primary school and longer near-work time in all educational stages, primary through tertiary. After multivariate analyses, early age and higher refractive power of the first spectacle, mother with high myopia, and after-school classes in senior high school were significant risk factors for high myopia.
At this point, we would like to emphasize the feasibility of questionnaire use in epidemiological studies and the importance of early myopia control. The present study was the first report to analyze the risk associated with first spectacle use at different ages regarding final myopic refractive error. Early age at first spectacle use is linked to earlier myopia onset. These results demonstrate that earlier age at myopia onset increases the risk of developing high myopia in adulthood. That is, for myopia onset, latter would be better.
There are some limitations in the current study. First, the questionnaire collected self-reported data, so the memory recall of participants was a major limitation. Second, female participants (71.3%) were more than male participants (28.7%). In this regard, the participants were selected from among those who came for a general ocular health check-up or accompanied patients. There are 3 reasons to explain sex disparity. One is that females are more careful about their health; another is that females are more willing to accompany their family or friends to the hospital; the third reason is that female were more willing to receive our invitation and complete the questionnaire. Thirdly, the participants’ purposes of spectacle use may be not all for myopia, perhaps for astigmatism. Lastly, the time of after-school classes and sport team attendance did not be quantified. The result indicated that after school class attendance only in senior high school was associated with high myopia but not other periods. Perhaps the time and strength of after-school classes in senior high school were much more and higher than other periods, because the stress of university entry system was upmost in senior high school. Further prospective longitudinal studies with larger samples and objective refraction examination are warranted to precisely identify the risk factors for myopia onset and progression.