Internal optics failed to compensate for increased corneal astigmatism in myopic children over a 2-year period

Purpose : It has been shown that the internal optics can partially compensate for corneal aberrations to maintain the overall aberration of the entire eye at a lower level. However, optical compensation reported in previous studies was based on cross-sectional data collected from adults with fully developed eyes. Longitudinal evidence from children whose eyes are rapidly developing is still lacking. This study investigated longitudinal interactions of corneal and internal aberrations on the ocular aberrations in myopic children.

Methods : Myopic Chinese children (n = 69, age: 8-10 years) were included. Their ocular and corneal wavefront aberrations were measured using an aberrometer incorporated with a corneal topographer at baseline and after two years. Internal aberration was calculated by directly subtracting the corneal aberration from the ocular aberration. Compensation factor (CF) was calculated, using root mean square error (RMSE), to indicate the effectiveness of internal optics to counterbalance the corneal aberrations: CF = 1 – (ocular aberration RMSE / corneal aberration RMSE). Axial length was measured using a partial coherence interferometer.

Results : Ocular lower-order astigmatism RMSE was significantly increased by 0.21±0.03 μm during myopic eye growth, accompanied by 0.18±0.03 μm increase in corneal lower-order astigmatism (paired t-tests, P < 0.001). The CF was dropped from 0.51 to 0.38, suggesting that compensation from the internal optics became less effective. When investigating individual Zernike coefficients, there was a significant increase in negative corneal H/V astigmatism (paired t-test, P < 0.001) but no significant change for the compensatory positive internal H/V astigmatism (unpaired t-test, P = 0.07), resulting in a more negative ocular H/V astigmatism (paired t-test, P < 0.001). While myopia progressed over the two-year follow-up period, H/V astigmatic change was not significantly associated with axial elongation (Pearson’s correlation, P = 0.26).

Conclusions : The increased ocular lower-order astigmatism indicates that the internal optics failed to compensate for the increased corneal astigmatism in children during myopic eye growth. Further works are required to understand the associated mechanism underlying the astigmatic changes in children.