Corneal topography, near work and eyelid forces

The cornea is the most powerful refractive component of the eye and as such, subtle changes in corneal shape can cause substantial changes in the optical characteristics of the eye. Monocular diplopia has previously been linked to corneal distortion following near work in various studies but has not been investigated in detail. The work reported in this thesis has investigated the optical effects of corneal distortions caused by eyelid forces and demonstrated that several corneal higher and lower order Zernike wavefront aberrations can change following reading. Measuring subtle changes in corneal topography requires the highest possible instrument accuracy, while software analysis tools should be able to detect and highlight those subtle changes with high reliability. The effect of ocular microfluctuations on the qualitative and quantitative analysis of corneal topography was investigated. A technique was developed to measure tilt, displacement, and cyclotorsion in multiple videokeratographs from the same cornea. This information was used to reposition each videokeratograph according to the average position of a sample of multiple measurements. The corneal topography of ten subjects was measured 20 times each, using videokeratoscopy. The RMSE calculated from difference between single videokeratographs and the average videokeratograph decreased by an average of 24.6 % for the ten subjects' data. The method can improve the precision performance of videokeratoscopy in multiple measurements of corneal topography. A study was undertaken, to investigate whether there are significant changes in corneal topography during accommodation in normal corneas and corneas that are pathologically thinner due to keratoconus. This was done to eliminate the possibility that changes in corneal aberrations associated with near work could be at least partly due to corneal changes caused by the effects of accommodation. A videokeratoscope was modified to present an accommodation stimulus that was coaxial with the instrument's measurement axis. Six subjects with normal corneas and four subjects with keratoconus were studied. In the initial analysis it was found that a number of the subjects showed significant changes in corneal topography as accommodation changed. However further analysis showed a significant group mean excyclotorsion of the topography maps for both accommodation stimuli compared with the 0 D stimulus. When the excyclotorsion was accounted for, no clear evidence of statistically significant changes in corneal topography as a result of accommodation were found. A small ocular excyclotorsion typically accompanies accommodation and this changes the relative orientation of the topography of the cornea. To investigate the effects of eyelid pressure on corneal shape and corneal aberrations during reading, twenty young subjects with normal ocular health were recruited. Cornea1 topography of one eye was measured with a videokeratoscope prior to reading and then again after a 60 minute reading task. Twelve of the twenty corneas showed significant changes in central topography immediately following reading. The location of the changes corresponded closely to the position and angle of the subject's eyelids during reading. Within the central region of the cornea there were significant changes in corneal wavefront Zernike coefficients, the root-mean-square error, overall refractive power and astigmatism. The changes observed in corneal topography appear to be directly related to the force exerted by the eyelids during reading. These findings may have important implications for the definition of refractive status and may also aid in the understanding of the relationship between reading and the development of refractive errors. To study whether corneal distortions after reading significantly differ between refractive error groups, corneal aberrations were measured before and after a period of reading, for a group of ten young progressing myopes and a group of ten young stable emmetropes. The major difference between the two groups was the location and magnitude of the corneal distortions, which had a significantly larger effect on central corneal optics in the myopic group compared to the emmetropic group. A significantly smaller palpebral aperture for the myopic group in the reading gaze position was the cause of this difference. The experiments described in this thesis have shown that numerous corneal characteristics can change due to eyelid forces during near work. The eye was shown to undergo a small cyclotorsion during higher levels of accommodation. There was a shift in direction of against the rule astigmatism of the cornea following reading and a change was found for primary vertical coma and trefoil. The changes in corneal shape following reading appear to be different in myope versus emmetropic refractive error groups. These findings are important for our understanding of the stability of the refractive error of the eye and could have important implications for refractive error development