Background: There is an compensatory effect between corneal astigmatism (CA) and internal astigmatism (IA). And the refractive astigmatism (RA) decreased fast in the early 3 years. Nevertheless, little attention was paid to the change in astigmatism in preschool children. Thus, we aimed to investigate the age-dependent distribution of astigmatism and its component in Chinese preschool children from year 3 to year 6 and also, to study the relationship between ocular biometry and IA.
Methods: Automated refraction, keratometry and axial length (AL) were performed in this cross-sectional study. The IA was determined as the vector difference between RA and CA. The magnitude and type (with-the-rule WTR, against-the-rule ATR, oblique) of RA, CA and IA were investigated according to the age. Spearman correlation was adopted to detect the factors that may influence IA.
Results: A total of 2315 subjects (2315 right eyes) (47.38% female) with a mean age of 4.52±0.88 years (range 3-6 years) were included. The magnitude and the ratio of three kinds of CA components did not change with aging (p=0.1233 and p>0.1, respectively). Though the axial distribution of RA remained unchanged with aging, a significant reduction in RA was seen in year 6 compared to that of year 3 and year 4 (p=0.036 and 0.041, respectively). Conversely，the ATR component in IA was lower in year 6 (68.02%) than it in year 3 (81.92%) and year 4 (77.12%) (both p<0.001) accompanied by increased frequency of oblique astigmatism in year 6 than it in year 3 and year 4 (p<0.001 and p=0.012,respectively). Negative relationship was found between IA and AL.
Conclusion: RA decreased from year 3 to year 6. The shift of the axis in IA from ATR to Oblique and WTR may account for the reduction in RA. Eyes with shorter AL had larger IA.
Keywords: Internal astigmatism; corneal astigmatism; axial length; with-the-rule.