The role of aberrations in refractive development

165 leaves : ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577P SO 2009 KwanBackground Axial length elongation induced by poor retinal image quality is thought to be a leading cause of myopia development. The eye's natural optical imperfections induce errors of higher order in the wavefront that cannot be easily corrected with spectacle lenses. This may constitute a form of deprivation myopia. However, previous studies reported conflicting results about the relationship between higher order monochromatic aberrations and refractive error development. The role of monochromatic aberration on refractive error development is still unclear. Purpose The objectives of this study are to investigate (1) whether different levels of refractive errors are associated with different levels of monochromatic aberrations; (2) the effect of monochromatic aberrations on the refractive error development; (3) the change of monochromatic aberrations during the refractive error development. Methods Study 1: Monochromatic aberrations of 116 subjects (19 to 29 years old) with different levels of refractive error were compared to determine whether monochromatic aberrations in the more myopic eyes were different from those in the non-myopic eyes. Study 2: In order to factor out individual variations, we investigated the relationship of monochromatic aberrations and refractive errors in a group of 26 anisometropes. In this study, we wanted to find out if monochromatic aberrations in the more myopic eye were different from those in the less myopic eye of these anisometropes. Study 3: A one year longitudinal study was carried out to determine the effect of monochromatic aberrations on the development of refractive errors and to investigate the change of monochromatic aberrations during refractive error development. Monochromatic aberrations, refractive errors and axial lengths of 964 children (7 to 9 years old) were measured, under natural accommodation at the beginning and at the end of the one year period. Study 4: A two year longitudinal study was carried out on a group of 162 children (6 to 12 years old) with closer monitoring of the monochromatic aberrations and refractive development. Monochromatic aberrations, refractive errors, corneal curvatures and axial lengths were measured under cycloplegia in each assessment, at approximately 6-monthly intervals, during the two year period.Results Study 1: Our results show that more myopic eyes had significantly smaller root mean square (RMS) value of fourth order aberrations and spherical aberration than non-myopic eyes. Fourth order aberrations and spherical aberration were significantly correlated with myopia and decreased with higher degrees of myopia. Study 2: Less myopic eyes of the anisometropes showed significantly larger total higher order aberrations, third order aberrations and spherical aberration than more myopic eyes. Study 3: Eyes with low astigmatism showed significantly smaller second order aberrations, third order aberrations, coma and total higher order aberrations than those with high astigmatism. Spherical aberration of the emmetropic eyes significantly increased after the one year period. Study 4: Spherical aberration was significantly different among myopic, emmetropic and hyperopic eyes; myopic eyes had the smallest RMS value while hyperopic eyes had the largest RMS value. Myopic eyes with more increase in myopia showed significantly smaller spherical aberration at the beginning of the study than myopic eyes with less increase in myopia. Change of spherical aberration at the end of the study was significantly larger for the hyperopic eyes than the emmetropic and myopic eyes. Both hyperopic eyes with larger spherical equivalent change and with smaller spherical equivalent change showed significant increase in spherical aberration at the end of the study. Conclusion Our results suggest that monochromatic aberrations of the eye may be associated with refractive development. There was no evidence to support the notion that high amounts of higher order aberrations drive myopia development. On the contrary, we found that small amounts of spherical aberration may be one of the risk factors for myopia development and higher amounts of spherical aberration in myopic eyes may reduce the risk of myopia. For myopic and emmetropic eyes, spherical aberration was mostly unchanged over the two year study period even though there was significant change in refractive error. For hyperopic eyes, spherical aberration was significantly increased over the two year study period irrespective of refractive error change. More investigations are required to clarify further the role of monochromatic aberrations in myopia development.School of OptometryPh.D., School of Optometry, The Hong Kong Polytechnic University, 200