The diurnal variation of corneal topography and aberrations
Purpose: To investigate if diurnal variation occurs in corneal topography. Methods: Seventeen young subjects had corneal topography measured 3 times a day (around 9 AM, 1 PM, and 5 PM) on 3 days of the week (Monday, Tuesday, and Friday). Corneal tangential and refractive power were analyzed. To investigate diurnal variation in corneal aberrations, the corneal height data were converted into corneal wavefront error and expressed as Zernike wavefront coefficients. Results: A significant diurnal change in corneal tangential power maps was found in 15 of the 17 subjects. This change typically consisted of a horizontal band of distortion in the superior and, to a lesser extent, inferior cornea, increasing throughout the day. Corneal refractive power showed small but significant diurnal changes occurring in the best fit sphere. Corneal wavefront error analysis revealed significant diurnal changes occurring in astigmatism 90/180 degrees and astigmatism 45/135 degrees. Two higher-order aberrations, primary vertical coma and trefoil along 30 degrees, also showed significant diurnal change. The combination of these 2 aberrations represents a "wavelike" distortion increasing throughout the day and returning to baseline the next morning. Conclusion: We have shown that significant diurnal change occurs in certain corneal aberrations. The changes appear to be related to forces from the eyelids on the anterior cornea. These findings may have important implications for customized refractive corrections and refractive error development.
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