This community-based study showed the prevalence of high myopia and associated factors including physical, ocular and demographic factors among adult Japanese for the first time. The factors associated with myopia were widely investigated as we analyzed biochemistry tests, blood pressure, height, body weight, habit of smoking, alcohol intake, past medical history, and present diseases. In this current study, we found that high myopia is more prevalent in women, younger age, and it has higher IOP.
High myopia affects approximately 1-4% of adults aged ≥40 years, and its prevalence was higher in some studies of East Asian adults and adolescents.[19-26] Our findings showed that the prevalence of high myopia was 5.0%, which was no less than the generally affected rates, although Chikusei-city is in a rural area where its prevalence has usually been lower compared to urban areas.[23] In our study, the high myopia rate in the older population was relatively low and the younger generation had a higher prevalence of high myopia, which may be reflect cohort effects. Although the reason for this cohort effects is unknown, the prevalence of high myopia could be expected to increase in future.
Some previous studies reported the relationship of height[27-29] and BMI[15, 30-32] with myopia; however, we did not find such association. A possible reason for this discrepancy is confounding; the previous reports did not adjust for age and any other confounding factors. In fact, BMI and height were shown to be associated with high myopia in unadjusted models, and adjustment for age resulted in the elimination of this significance in our study, presumably because age is a strong predisposing factor for BMI and height reflecting cohort effects.
As for the other laboratory factors, there was no significant difference. A few studies suggested that hyperglycemia and hyperlipidemia led to myopic shift, whereas other studies revealed that the refractive shift was more likely hyperopic with hypoglycemia.[33-36] Further analysis is needed to elucidate the influence of metabolic shift.
In terms of alcohol intake and smoking history, both factors did not show any associations with high myopia according to our results. Also, liver functions, represented by GOT, GPT, and GGTP, did not show any relationship to high myopia. Previous reports also found that there were no significant trends observed between smoking and refractive errors.[37]
The percentage of high myopia in men and women was 3.8% and 5.9%, respectively, indicating significant gender difference. It has been reported that female sex had a predisposition of high myopia.[38] Likewise, female sex was proven to be high risk for myopic complications usually caused by high myopia.[39] Hyman L et al. reported that female sex is independently associated with faster myopic progression,[40] Although there have been no reports describing causal relationship between gender and high myopia.
The mean IOP linearly increased parallel to the myopic progression. It has been reported that IOP was associated with central corneal thickness, age, and blood pressure.[41, 42] Even after adjustment for these factors, we still found that high IOP was significantly associated with high myopia,while there are conflicting evidences regarding relationship of high myopia and IOP.[43-45]
This study has several limitations. First, since this was a cross-sectional study, the causal relationships cannot be determined. Second, some known risk factors of myopia such as natural guardians, near work, outdoor activities, and academic backgrounds were unavailable in this study. Moreover, neither measuring of the axial length nor lens examinations has been performed. With respect to the lenses, factors of cataracts remain to be considered. We also should consider environmental differences across generations which might have affected the result of associations of age with high myopia.
As discussed, the factors associated with high myopia were undetermined. This study did not suggest that height, BMI, blood glucose, hypocholesteremia, liver dysfunction, kidney dysfunction, smoking, and alcohol intake were associated with high myopia, whereas women, young age and high IOP were found to be related to high myopia. Meanwhile, high IOP and young age were found to be risk factors for high myopia, which may indicate the path for future studies concerning myopia control.