Astigmatism is a common refractive error, accounting for as much as 13% of all refractive errors. The prevalence of astigmatism varies with age, with a high prevalence (approximately 20%) in the first months of life when the curvature of the cornea is very steep. Some studies have found a predominance of with-the-rule (WTR) astigmatism in infants, while others have reported a predominance of against-the-rule (ATR) astigmatism in that age group. As infants grow older, the cornea flattens and the prevalence of high degrees of astigmatism (>1 dioptre) decreases, reaching a level of 4.8% of preschool children, with most instances being WTR astigmatism. Around 63% of young adults aged 20 to 30 years exhibit 0.25 dioptre or more of astigmatism, although only a few have an astigmatism >1 dioptre. In another study, 46% of the total population had corneal astigmatism >0.5 dioptre, but only 4.7% exhibited astigmatism >1.5 dioptre, predominantly WTR. After 40 years of age, the astigmatism axis shifts, from a predominance of WTR to a predominance of ATR, probably due to changes in the corneal curvature.
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