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Educational level of subjects was classified into four categories: elementary school graduate, middle school graduate, high school graduate, and college graduate and above.

Economic status was grouped into quartiles according to annual individual earnings. Since KNHANES used a stratified, multistage sampling method, we incorporated sampling weights as well as strata for the sampled units in the statistical analysis. Continuous variables were presented as mean and standard error (SE), and categorical variables were presented as percentage and SE. To evaluate the association of myopia and AMD, multiple logistic regression analyses were performed. After calculation of the crude odds ratio, these values were adjusted for confounders that were established as risk factors in previous studies, including age, sex, smoking, hypertension, and sunlight exposure time.

The demographic characteristics of the 17,676 enrolled subjects by AMD status are summarized in Table 1. The adjusted ORs for any and early AMD were 0. In females, the adjusted ORs for any and early AMD were 0.

There was no significant association between myopia and any or early AMD in male participants. Among participants younger than 50 years, the adjusted ORs for any and early AMD were 0. In contrast, there was no significant correlation between myopia and any or early AMD in participants older than 50 years. There was no significant association between late AMD and myopia. The association between refractive error and any AMD showed positive correlation even after adjusting for potential confounders.

The odds ratio decreased by only 0. The adjusted ORs significantly showed negative correlation even after adjusting for potential confounders. In addition, these associations were revealed to be independent factors that did not change even after adjusting for various confounding factors. AMD is a disease well-known to be the leading cause of irreversible blindness and severe visual loss in older populations in both European and Asian countries.

Previously, several studies have suggested that refractive error may be associated with AMD, also among both European and Asian populations. A population-based survey of Australia showed a statistically significant increased risk of early AMD for each diopter of increase in refractive error (OR: 1.

Although the reasons for the association between refractive error and AMD remain unclear, several possible explanations for them have been proposed over the past few decades. One hypothesis is based on the difference in scleral rigidity between myopic and hyperopic eyes. Another hypothesis is the decreased intraocular concentration of vascular endothelial growth factor (VEGF) in myopic eyes.

The use of spectacles by myopic patients is also a possible reason for this association because of the attendant reduction in exposure to ultraviolet rays in sunlight, a known risk factor for AMD. According to this law, vascular tension increases as intravascular pressure increases or vascular radius increases. It is known that there is reduced blood flow in high myopic eyes, due to narrowing of the retinal vessel diameter.

Recent published study which performed Medelian randomization analysis also provided genetic evidence that there is a causal relationship between refractive error and AMD, although the causal effect size was modest (OR, 1. First, the analysis of our results according to sex showed that there was a statistically significant negative correlation of myopia with any AMD (OR, 0.

The exact explanation for these differences according to sex has not been clarified yet. We considered that this could be due to the hormonal differences between males and females. Based on a few studies, it is known that the sex and hormonal status could influence choroidal circulation.

Thus, it can be assumed that unlike in males, changes in choroidal blood flow due to hormonal changes may have affected AMD development in females; this was manifested in our results as a sex-related difference in AMD prevalence. Further detailed investigations are needed to support this explanation.

According to the analysis based on age group, our results showed that negative correlation between myopia and AMD was more significant in the younger age group ( 50 years) age group. This association between myopia and AMD was only significant in the participants in their 40s, but not in those in their 50s, 60s, or 70s. The reason for the significant result in the total analysis is that the strength of the association between myopia and AMD in the 40s age group is quite large.

Thus, it is suggested that myopia at young age has an inverse association with the occurrence of AMD. These age-related differences could be described from the aspect of posterior vitreous detachment (PVD). The prevalence of PVD significantly increases with aging. The prevalence of PVD may be more common in myopic eyes than in non-myopic eyes among younger participants. Thus, it can be suggested that young myopic subjects with PVD may be less likely to have AMD.

Another strength of our study is that we investigated the differences in sex- and age-specific associations between myopia and the prevalence of AMD, which has not been reported in previous studies.

However, our study had some limitations. Firstly, refractive errors were evaluated without cycloplegia, which may lead to overestimation of myopia. Secondly, several factors associated with refractive status, such as axial length and corneal curvature, were not available in the population-based data used in the present study.

However, to overcome these limitations, our researchers are in the process of analyzing axial length data in a population of more than 10,000 subjects collated from the KNHANES database. Thirdly, there was no statistically significant association between refractive error and late AMD, probably due to the small number of subjects with late AMD in this population.

Fourthly, this study has the risk of clumping AMD and myopic degeneration together. Because the fundus finding of myopic degeneration is similar to that of AMD, some of myopic degeneration might be interpreted as AMD. However, the prevalence of AMD was lower in myopia than in non-myopia in KNHANES database. Although the results of our study may have overestimated the prevalence of AMD in myopia, the errors inherently are most likely to reduce the chance of finding associations, so the association found in the present study may even be stronger than detected.

The confounding that myopic degeneration may be interpreted as AMD would be negative confounding which may cause underestimate of true association. Thus, this bias could support our conclusion.