title: Obsessive-compulsive disorder as a risk factor for schizophrenia: a nationwide study.
abstract: OBJECTIVE
  Despite a remarkable co-occurrence of obsessive-compulsive disorder (OCD) and schizophrenia, little is known about the clinical and etiological relationship of these 2 disorders. Exploring the degree to which these disorders share etiological factors might provide useful implications for clinicians, researchers, and those with the disorders.
  
  
  OBJECTIVE
  To assess whether patients with OCD experience an enhanced risk of developing schizophrenia and schizophrenia spectrum disorders and to determine whether a family history of OCD constitutes a risk factor for schizophrenia and schizophrenia spectrum disorders.
  
  
  METHODS
  Using individual data from longitudinal nationwide Danish registers, we conducted a prospective cohort study with 45 million person-years of follow-up. All survival analyses were adjusted for sex, age, calendar year, parental age, and place of residence at the time of birth. A total of 3 million people born between January 1, 1955, and November 30, 2006, were followed up from January 1, 1995, through December 31, 2012. During this period, 30 556 people developed schizophrenia or schizophrenia spectrum disorders.
  
  
  METHODS
  The presence of a prior diagnosis of OCD and the risk of a first lifetime diagnosis of schizophrenia and a schizophrenia spectrum disorder assigned by a psychiatrist in a hospital, outpatient clinic, or emergency department setting. Incidence rate ratios (IRRs) and accompanying 95% confidence intervals are used as measures of relative risk.
  
  
  RESULTS
  The presence of prior diagnosis of OCD was associated with an increased risk of developing schizophrenia (IRR = 6.90; 95% CI, 6.25-7.60) and schizophrenia spectrum disorders (IRR = 5.77; 95% CI, 5.33-6.22) later in life. Similarly, offspring of parents diagnosed as having OCD had an increased risk of schizophrenia (IRR = 4.31; 95% CI, 2.72-6.43) and schizophrenia spectrum disorders (IRR = 3.10; 95% CI, 2.17-4.27). The results remained significant after adjusting for family history of psychiatric disorders and the patient's psychiatric history.
  
  
  CONCLUSIONS
  A diagnosis of OCD was associated with higher rates of schizophrenia and schizophrenia spectrum disorders. The observed increase in risk suggests that OCD, schizophrenia, and schizophrenia spectrum disorders probably lay on a common etiological pathway.
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark2National Centre for Register-Based Research, Aarhus University, Aarhus, Denmark.
authors.forename: Sandra M
authors.initials: SM
authors.lastname: Meier
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark2National Centre for Register-Based Research, Aarhus University, Aarhus, Denmark.
authors.forename: Liselotte
authors.initials: L
authors.lastname: Petersen
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark2National Centre for Register-Based Research, Aarhus University, Aarhus, Denmark.
authors.forename: Marianne G
authors.initials: MG
authors.lastname: Pedersen
authors.affiliation: Clinic for Anxiety Disorders, Aarhus University Hospital, Risskov, Denmark4Department of Psychology and Behavioral Sciences, Aarhus University, Aarhus, Denmark.
authors.forename: Mikkel C B
authors.initials: MC
authors.lastname: Arendt
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark2National Centre for Register-Based Research, Aarhus University, Aarhus, Denmark.
authors.forename: Philip R
authors.initials: PR
authors.lastname: Nielsen
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark5Department of Biomedicine, Aarhus University, Aarhus, Denmark.
authors.forename: Manuel
authors.initials: M
authors.lastname: Mattheisen
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark6Research Department P, Aarhus University Hospital, Risskov, Denmark.
authors.forename: Ole
authors.initials: O
authors.lastname: Mors
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark2National Centre for Register-Based Research, Aarhus University, Aarhus, Denmark.
authors.forename: Preben B
authors.initials: PB
authors.lastname: Mortensen
mesh_terms: D003718:Denmark / Q000453:epidemiology; D005192:Family Health / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D009771:Obsessive-Compulsive Disorder / Q000150:complications* / Q000453:epidemiology* / Q000523:psychology; D011446:Prospective Studies; D012042:Registries; D012307:Risk Factors; D012559:Schizophrenia / Q000150:complications / Q000453:epidemiology* / Q000209:etiology*