title: Definitions of medication-overuse headache in population-based studies and their implications on prevalence estimates: a systematic review.
abstract: BACKGROUND
  Case definitions of medication-overuse headache (MOH) in population-based research have changed over time. This study aims to review MOH prevalence reports with respect to these changes, and to propose a practical case definition for future studies based on the ICHD-3 beta.
  
  
  METHODS
  A systematic literature search was conducted to identify MOH prevalence studies. Findings were summarized according to diagnostic criteria.
  
  
  RESULTS
  Twenty-seven studies were included. The commonly used case definition for MOH was headache ≥15 days/month with concurrent medication overuse ≥3 months. There were varying definitions for what was considered as overuse. Studies that all used ICHD-2 criteria showed a wide range of prevalence among adults: 0.5%-7.2%.
  
  
  CONCLUSIONS
  There are limits to comparing prevalence of MOH across studies and over time. The wide range of reported prevalence might not only be due to changing criteria, but also the diversity of countries now publishing data. The criterion "headache occurring on ≥15 days per month" with concurrent medication overuse can be applied in population-based studies. However, the new requirement that a respondent must have "a preexisting headache disorder" has not been previously validated. Exclusion of other headache diagnoses by expert evaluation and ancillary examinations is not feasible in large population-based studies.
authors.affiliation: Danish Headache Center, Department of Neurology, Glostrup Hospital, University of Copenhagen, Denmark.
authors.forename: Maria L
authors.initials: ML
authors.lastname: Westergaard
authors.forename: Ebba Holme
authors.initials: EH
authors.lastname: Hansen
authors.forename: Charlotte
authors.initials: C
authors.lastname: Glümer
authors.forename: Jes
authors.initials: J
authors.lastname: Olesen
authors.forename: Rigmor H
authors.initials: RH
authors.lastname: Jensen
mesh_terms: D003430:Cross-Sectional Studies / Q000592:standards; D015340:Epidemiologic Research Design; D051271:Headache Disorders, Secondary / Q000139:chemically induced* / Q000145:classification / Q000453:epidemiology*; D006801:Humans; D015995:Prevalence
keywords: Chronic headache; International Classification of Headache Disorders; classification; epidemiology; medication-overuse headache; prevalence