title: Sex ratio of multiple sclerosis in Canada: a longitudinal study.
abstract: BACKGROUND
  Incidence of multiple sclerosis is thought to be increasing, but this notion has been difficult to substantiate. In a longitudinal population-based dataset of patients with multiple sclerosis obtained over more than three decades, we did not show a difference in time to diagnosis by sex. We reasoned that if a sex-specific change in incidence was occurring, the female to male sex ratio would serve as a surrogate of incidence change.
  
  
  METHODS
  Since environmental risk factors seem to act early in life, we calculated sex ratios by birth year in 27 074 Canadian patients with multiple sclerosis identified as part of a longitudinal population-based dataset.
  
  
  RESULTS
  The female to male sex ratio by year of birth has been increasing for at least 50 years and now exceeds 3.2:1 in Canada. Year of birth was a significant predictor for sex ratio (p<0.0001, chi(2)=124.4; rank correlation r=0.84).
  
  
  CONCLUSIONS
  The substantial increase in the female to male sex ratio in Canada seems to result from a disproportional increase in incidence of multiple sclerosis in women. This rapid change must have environmental origins even if it is associated with a gene-environment interaction, and implies that a large proportion of multiple sclerosis cases may be preventable in situ. Although the reasons why incidence of the disease is increasing are unknown, there are major implications for health-care provision because lifetime costs of multiple sclerosis exceed pound1 million per case in the UK.
authors.affiliation: Wellcome Trust Centre for Human Genetics and Department of Clinical Neurology, University of Oxford, Oxford, UK.
authors.forename: Sarah-Michelle
authors.initials: SM
authors.lastname: Orton
authors.forename: Blanca M
authors.initials: BM
authors.lastname: Herrera
authors.forename: Irene M
authors.initials: IM
authors.lastname: Yee
authors.forename: William
authors.initials: W
authors.lastname: Valdar
authors.forename: Sreeram V
authors.initials: SV
authors.lastname: Ramagopalan
authors.forename: A Dessa
authors.initials: AD
authors.lastname: Sadovnick
authors.forename: George C
authors.initials: GC
authors.lastname: Ebers
mesh_terms: D000328:Adult; D017677:Age Distribution; D000368:Aged; D002170:Canada / Q000453:epidemiology; D005260:Female; D006801:Humans; D015994:Incidence; D016015:Logistic Models; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000453:epidemiology*; D012307:Risk Factors; D017678:Sex Distribution; D012744:Sex Ratio