Prevalence and diagnosis of schizophrenia based on register, case record and interview data in an isolated Finnish birth cohort born 1940-1969.

Introduction: Schizophrenia occurs worldwide but the prevalence varies markedly. In Finland, schizophrenia is most prevalent in the northeastern region. Our aims were to reassess the register-, case record- and interview-based lifetime prevalence in a genetically homogeneous birth cohort from an isolate population with earlier reported high prevalence of schizophrenia and a chromosome linkage to chromosome 1q.
Methods: We identified all patients with a diagnosis of schizophrenia [International Classification of Diseases (ICD)-8, ICD-9 or ICD-10 codes], born 1940-1969 in the isolate (n=282) and alive (n=237) in 1998 using the Hospital Discharge, Disability Pension and Free Medicine Registers. The corresponding birth cohort of 14,817 persons and 12,368 alive in 1998 was identified from the National Population Register. We validated 69% of the register diagnosis by making DSM-IV consensus diagnoses, and interviewed 131 (55%) of the 237 patients with SCID-I and SCID-II.
Results: The register-based lifetime prevalence was 1.5% for schizophrenia and 1.9% for schizophrenia spectrum psychotic disorders: in birth cohorts born 1945 to 1959, the latter prevalence was especially high (2.4%). Of those with a register diagnosis of schizophrenia spectrum psychotic disorder, 69% or 63% also received a record-based consensus diagnosis or SCID interview diagnosis of schizophrenia, and the prevalence was 0.9-1.3 and 0.7-1.2%, respectively, when we reassessed most of the register-based cases. The cumulative incidence of schizophrenia spectrum psychotic disorders in the total birth cohort was 1.9%.
Conclusion: In this isolate, the register, DSM-IV consensus and SCID interview-based lifetime prevalence of schizophrenia was internationally high. For genetic research work, the register diagnosis should be reassessed using either structured interview or the best estimate consensus diagnosis.