Diurnal Changes of the Cornea - a Pilot Study

Higher patient expectations concerning unaided visual acuity after cataract surgery has led to an increase in use of toric intraocular lenses (toric IOLs) for the correction of corneal astigmatism. Although it was shown that toric IOLs significantly reduce astigmatism, there is still some difficulty in predicting the remaining astigmatism correctly. The main source of error appears to be the pre-operative measurement of the cornea. However, it is questionable, if this error derives from a measurement error due to the imprecise measurement technique, or natural diurnal changes of the cornea. Additionally, measurements of the cornea pose several challenges: different corneal biometry devices cannot be used interchangeably and it is difficult to decide, which device is measuring the cornea correctly as there is no gold standard.

Especially, in eyes with small corneal astigmatism the error increases. Recently it was shown that the astigmatism meridian was on median 9° off for corneas with 1.0D of astigmatism and only 4° for corneas with 2.0D of astigmatism. Similar findings were observed by Shammas et al. Norrby furthermore showed that 5% of all corneas show more than 0.5D of fluctuations between measurements at different (post-operative) time-points. The reasons are not explored, but diurnal changes, temperature and humidity potentially influencing the tear film, as well as pupil size and asphericity of the cornea could play a role. Read and Collins observed diurnal changes of corneal radii within 24 hours, but in their study only young healthy volunteers were included and only one measurement device (Scheimpflug imaging, Pentacam HR) was used. Lau and Pye and Shen et al. measured the diurnal changes of the cornea, but no data concerning corneal radii were reported. Harper et al. observed diurnal changes of the central corneal thickness. However, none of these studies investigated on diurnal changes of the corneal radii in the elderly population. Aim of this study is to gain a better understanding of the diurnal changes of the cornea to improve toric IOL power calculations.