title: Epidemiology and heritability of astigmatism in Norwegian twins: an analysis of self-reported data.
abstract: OBJECTIVE
  To describe the occurrence and heritability of astigmatism in a population-based sample of Norwegian twins.
  
  
  METHODS
  Self-reported history of astigmatism based on questionnaire responses was used to estimate the prevalence and incidence rates of astigmatism from birth through 31 years in 8,045 twins. Kaplan-Meier analysis and log-binomial regression were used to study the conditional and relative risk of astigmatism in twin pairs by sex and zygosity. Tetrachoric correlations and structural equation models were applied to estimate the genetic and environmental sources of variations in liability for astigmatism.
  
  
  RESULTS
  Altogether 21.1% of males and 29.3% of females (p < 0.001) reported a positive history of astigmatism. Estimated incidence rates peaked in both sexes in the group aged 16-19 years and then decreased. Both conditional and relative risks of developing astigmatism were considerably higher in monozygotic than in dizygotic twins if a cotwin reported a positive history of astigmatism. The best-fitting biometrical model suggested that additive genetic, dominant genetic, and individual environmental effects explained 9% (95% CI: 0-40), 54% (95% CI: 20-69), and 38% (95% CI: 31-45) of the variation in the liability to astigmatism, respectively. No differences in heritability of astigmatism between the genders were found.
  
  
  CONCLUSIONS
  The prevalence of self-reported astigmatism in twins is comparable with previous findings from Norway. Our results suggest considerable genetic contribution to the development of astigmatism in young adult Norwegian twins mainly due to dominant genetic effects, which are similar in both males and females.
authors.affiliation: Division of Epidemiology, Norwegian Institute of Public Health, Nydalen, Oslo, Norway. andrei.grjibovski@fhi.no
authors.forename: Andrej M
authors.initials: AM
authors.lastname: Grjibovski
authors.forename: Per
authors.initials: P
authors.lastname: Magnus
authors.forename: Anna
authors.initials: A
authors.lastname: Midelfart
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Harris
mesh_terms: D000293:Adolescent; D000328:Adult; D001251:Astigmatism / Q000453:epidemiology* / Q000235:genetics*; D002648:Child; D002675:Child, Preschool; D004200:Diseases in Twins; D005260:Female; D005500:Follow-Up Studies; D020022:Genetic Predisposition to Disease; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D009664:Norway / Q000453:epidemiology; D015995:Prevalence; D012189:Retrospective Studies; D012307:Risk Factors; D012737:Sex Factors; D011795:Surveys and Questionnaires; D014427:Twins