title: Astigmatism and its components in 6-year-old children.
abstract: OBJECTIVE
  The purpose of the present study was to report the prevalence of refractive (RA), corneal (CA), and internal astigmatism (IA) in a population of 6-year-old children; examine their variation with gender, ethnicity, and refraction; and examine the effects of gender, ethnicity, and spherical equivalent refraction on the relationship between CA and RA in this population.
  
  
  METHODS
  The Sydney Myopia Study is a population-based survey of refraction and eye health in 6-year-old children. A random cluster design was used to recruit children from schools across Sydney, Australia, during 2003 to 2004. Data collection used a detailed questionnaire and comprehensive eye examination. Keratometric and cycloplegic autorefraction data from right eyes were analyzed.
  
  
  RESULTS
  Of 2238 eligible children, 1765 (78.9%; 50.7% boys) had parental consent to participate. Overall prevalence of RA (> or =1.0 diopter [D]) was 4.8% (95% confidence interval [CI] 3.8%-6.1%), CA (> or =1.0 D) 27.7% (CI 23.8%-32.3%), and IA (> or =1.0 D) 21.1% (CI 19.0%-23.5%). The RA axis was fairly evenly distributed, with predominance of oblique axis (39.1%; CI 35.9%-42.6%). CA axis was mainly with the rule (75.1%; CI 72.6%-77.8%), while IA axis was mainly against the rule (76.7%; CI 74.2%-79.3%). After adjustment for multiple variables, girls had significant, marginally greater mean CA and IA than boys. East Asian and South Asian children had significantly greater prevalence and mean RA and CA than European Caucasian children. There were no significant ethnic differences of mean IA. Compared to reference (spherical equivalent [SEq] 1.01-1.50 D), mean RA and CA increased significantly with more hyperopic and more myopic refractions. Mean IA was significantly greater only for hyperopic refractions (SEq > 2.00 D).
  
  
  CONCLUSIONS
  The prevalence of astigmatism found in this population of 6-year-old children was relatively low, and showed significant variation with ethnicity. The data suggest that emmetropization for RA occurs by a compensatory process between CA and IA.
authors.affiliation: Centre for Vision Research, Department of Ophthalmology, and the Westmead Millennium Institute, University of Sydney, Sydney, Australia.
authors.forename: Son C
authors.initials: SC
authors.lastname: Huynh
authors.forename: Annette
authors.initials: A
authors.lastname: Kifley
authors.forename: Kathryn A
authors.initials: KA
authors.lastname: Rose
authors.forename: Ian
authors.initials: I
authors.lastname: Morgan
authors.forename: Gillian Z
authors.initials: GZ
authors.lastname: Heller
authors.forename: Paul
authors.initials: P
authors.lastname: Mitchell
mesh_terms: D017677:Age Distribution; D001251:Astigmatism / Q000453:epidemiology*; D002648:Child; D003430:Cross-Sectional Studies; D005006:Ethnicity; D005260:Female; D006801:Humans; D008297:Male; D009517:New South Wales / Q000453:epidemiology; D015995:Prevalence; D012029:Refraction, Ocular; D017678:Sex Distribution