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G. Heidary, G.–S. Ying, M.G. Maguire, T.L. Young; The impact of sphere on cylinder type and severity in a high myopia cohort . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2747.
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Purpose: Previous studies have been equivocal regarding the relationship between myopia and astigmatism. We examined patients with familial high myopia to determine whether the degree of myopia influences the presence and degree of astigmatism, and to assess risk factors of astigmatism. Methods: We performed a retrospective study on 217 individuals (range 0.5–91 yrs., mean age 37.1 yrs.) from families with two or more subjects from successive generations with myopia of at least –5 D or greater in one eye. Mean myopic sphere was –10 D and the mean age of myopia onset was 7 yrs. Refractive error measurements were obtained using cycloplegic retinoscopy in individuals < 15 yrs., and dry manifest refractions in those older. Risk factors for the presence and degree of astigmatism were assessed by generalized linear regression. The association between the degree of myopia and cylinder power was examined by correlation analysis. Data from both eyes were included, and the inter–eye correlation was adjusted by using generalized estimating equations. Results: The prevalence of astigmatism (>0.5 D of cylinder) was 61.3%. This did not differ with gender or age. With–the–rule astigmatism was most common (51.3%), and the majority of astigmats had between 0.5 and 1.5 D of cylinder (63.8%). No statistically significant association was found between the presence of astigmatism and risk factors including the degree of sphere, age of myopia onset, gender, and age. In those patients with astigmatism, however, there was a moderate correlation between the degree of sphere and cylinder power (r=–0.34, p=0.0008). The degree of sphere was also associated with the type of astigmatism (p=0.01), with a higher proportion of oblique astigmatism in patients with –15 D or greater of sphere. Male gender (p=0.01) and younger age (p=0.05) were associated with higher cylinder power. Conclusions: In severely myopic patients, there is a high prevalence of astigmatism that is predominantly with–the–rule. The degree of myopia is correlated with astigmatism severity and astigmatism type but is not a risk factor for the presence of astigmatism.
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