Characteristics of astigmatism in a Hong Kong Chinese population

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Characteristics of astigmatism in a Hong Kong Chinese population


Author: Leung, Tsz Wing
Title: Characteristics of astigmatism in a Hong Kong Chinese population
Degree: Ph.D.
Year: 2014
Subject: Astigmatism -- China -- Hong Kong
Hong Kong Polytechnic University -- Dissertations
Department: School of Optometry
Pages: xiv, 140 leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: Purpose: Astigmatism is a common refractive error in the Asian populations; however, its biometric characteristics in this ethnicity receive little attention. Furthermore, although astigmatism is primarily corneal in nature, little is known about the corneal shape and the posterior ocular structures in astigmatic eyes. Consequently, the main purposes of this thesis were to determine: 1) the characteristics of astigmatism as a function of age in a Hong Kong Chinese clinical population; 2) the effects of age and myopic astigmatism on the corneal shape factors; and 3) the effect of myopic astigmatism on the retinal thickness and visual resolution. Methods: Experiment 1: To determine the prevalence of astigmatism, data were retrieved from all new clinical records during 2007 at the Optometry Clinic of The Hong Kong Polytechnic University. Only subjects who had visual acuity better than or equals to 6/9 in both eyes and completed subjective refraction were analyzed (age range = 3~84yrs; n = 2759). Experiments 2 & 3: The corneal shape factors as a whole (mean-P) and at semi-meridian region (semi-P) were measured by the Pentacam’s Scheimpflug-based tomography. Exp. 2 focused on younger age groups which were known to show high prevalences of myopia and astigmatism in Exp.1: 10-15yrs (n=32), 20-25yrs (n=37) and 40-45yrs (n=31), in each age group the subjects were subdivided into myopic astigmatic (MA) and emmetropic (EM) groups; Exp. 3 aimed to characterize the corneal shape and ocular aberrations in myopes with (MA) and without astigmatism (simple myope, SM): 50-70yrs (n=29). Experiment 4: This experiment aimed to determine the relationship between ocular structure and visual resolution in myopic astigmats. The retinal thicknesses of 20 young adults (MA, n=10; EM, n=10), aged 18 to 35 years, were measured by the Spectralis HRA+OCT spectral-domain optical coherence tomography. The resolution acuities at the fovea and 15° eccentricity of the temporal and superior retina of these subjects were determined by a psychophysical experiment.
Results: Experiment 1: The prevalence of astigmatism (Cyl≥1.00D) was high in this Hong Kong Chinese clinical population. The manifest astigmatism was mainly corneal in nature, associated with myopia and hyperopia, bilaterally mirror symmetric in axis, and shifted from predominantly WTR to ATR with age. Experiment 2: Age and myopic astigmatism had significant effects on the corneal shape. Mean-P and semi-Ps were less prolate in the older age groups. MA had more prolate temporal semi-Ps than EM in all the three age groups. In addition, the symmetry of the corneal shape along the horizontal meridian was significantly affected by age and refractive errors. Experiment 3: Compared to simple myopes, subjects with against-the-rule myopic astigmatism had less prolate nasal semi-Ps. Astigmatism was associated with the variation of several Zernike coefficient indexes. Using data from all subjects, temporal and inferior semi-Ps were associated with the spherical and total higher order aberrations, respectively. Experiment 4: When compared to emmetropes, subjects with myopic astigmatism had significantly thinner parafoveal thickness at the inner temporal, outer temporal, outer nasal and outer inferior retinal quadrants, and lower resolution acuity at the temporal retina. Using data from all subjects, the horizontal resolution acuity was significantly correlated with the local retinal thickness at 15° eccentricity of the temporal retina. Conclusion: These results provide important characteristics of astigmatism in the Hong Kong Chinese population and suggest that factors including age, myopia, corneal shape and retinal structures need to be taken into account when studying the mechanism underlying astigmatism.

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