title: Myasthenia gravis epidemiology in a national cohort; combining multiple disease registries.
abstract: OBJECTIVE
  There is a wide variation in reported prevalence and incidence of myasthenia gravis (MG). In this study, we aimed to evaluate the validity of two nationwide databases by comparing prevalence and incidence rates reported from three recent studies using the two databases as case-finding method.
  
  
  METHODS
  Two different Norwegian nationwide databases were used: the acetylcholine receptor antibody database (reference cohort) and the Norwegian Prescription Database (NorPD) (study cohort). Presence of acetylcholine receptor antibodies (AChR) is specific for MG. Up to 85% of MG patients are AChR antibody-positive. All samples from the whole country were tested at one laboratory. NorPD contains patient information on all prescriptions of pyridostigmine.
  
  
  RESULTS
  Prevalence was 131 per million in the study cohort and 145 per million estimated from the reference cohort (Jan 1, 2008). No significant difference in prevalence between the study cohort and the reference cohort was found (SIR 1.1, 95% CI 1.0-1.2). The annual incidence rate was 16.0 per million in the study cohort and 8.8 per million estimated from the reference cohort, twofold more new MG patients were found in the study cohort compared to estimated figures from the reference cohort (SIR 1.8; 1.4-2.3).
  
  
  CONCLUSIONS
  This study confirms an optimal and unbiased case finding in both databases. Our calculated prevalence and incidence rates are in line with previous population-based studies. There was good agreement in prevalence reported from the two databases. The discrepancy in incidence is expected to diminish as years of study are increasing in NorPD.
authors.affiliation: Department of Clinical Medicine, University of Bergen, Bergen, Norway.
authors.forename: J B
authors.initials: JB
authors.lastname: Andersen
authors.forename: A T
authors.initials: AT
authors.lastname: Heldal
authors.forename: A
authors.initials: A
authors.lastname: Engeland
authors.forename: N E
authors.initials: NE
authors.lastname: Gilhus
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001323:Autoantibodies / Q000097:blood; D015331:Cohort Studies; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D009157:Myasthenia Gravis / Q000150:complications / Q000453:epidemiology* / Q000276:immunology*; D009664:Norway / Q000453:epidemiology; D015995:Prevalence; D012042:Registries; D055815:Young Adult
chemical_list: D001323:Autoantibodies
keywords: acetylcholine receptor antibody test; incidence; myasthenia gravis; prevalence; pyridostigmine