John McGrath Sukanta Saha Joy Welham David Chant A systematic review of the incidence of schizophrenia

Schizophrenia - the most common form of psychosis – is characterised by disorders of cognition (eg paranoia), affect (mood), communication (thought disorder) and perception (eg hallucinations) – leading to a loss of contact with reality along with various forms of impaired behavior. Because schizophrenia (a) has an early onset and (b) has a relapsing or chronic course, it is a significant public health problem. It ranks in the top 10 leading causes of disability, with a burden of disease comparable to cancer and greater than heart disease. Previously the incidence of schizophrenia has been thought to be relatively uniform across time and place. The identification of variations in the incidence of schizophrenia is important in unravelling the causes of the disorder

Aims To systematically review studies of the incidence of schizophrenia To examine variation in time, place and person by examining the distribution of these rates To explore a limited number of a priori hypotheses

Outline Research questions Methods & Systematic reviews Key results Caveats and Conclusions

The incidence of schizophrenia: Hypotheses Sex difference Males > females Migrant status Migrants > native born Urbanicity Urban born > rural born

Types of incidence studies ‘Core studies’ Sentinel surveys Register based studies Migrant studies Cohort studies Other special groups

Methods: systematic review Electronic data search –Medline, PsychoInfo, Embase, LILAC inclusive (schizo* OR psycho*) AND incidence OR prevalence) also Review article bibliography Wrote to authors Screen abstract and reviewed papers to cull irrelevant citations

Rate items and discrete data Non-overlapping Sex Male, Female Overlapping Age eg all ages or age Diagnosis eg Catego S+ or Catego SPO + clinical Diagnostic categories eg DSMIV Schizophrenia or Schizophrenia + Delusional disorder Site overlap eg Denmark or Copenhagen Epoch overlap eg or

Data analysis: example cumulative distribution Rate per 100,000

Results Electronic search = 834 potential papers Manual checking = 249 potential papers Letters from 52 authors, who provided an additional 41 references Of potentially relevant papers, 74% were identified from electronic sources

Results (2) Rates based on 176,056 potentially overlapping incident cases After review 158 were included from 32 countries Types of studies –Core studies = 100 –Migrant studies = 24 –Cohort studies = 23 –Other special groups = 14

Core Incidence Studies

Sex differences

Male : female rate ratio

Migrant status

Urban-rural differences

Cohort studies –Australia (n=2) –Denmark (n=3) –Finland (n=5) –Israel (n=2) –Italy (n=1) –Sweden (n=2) –The Netherlands (n=2) –USA (n=3) –United Kingdom (n=3)

Other special groups –over age 65 –twins –various ethnic and/or religious subgroups –students –deaf individuals –workers in a radiation contamination zone

Key findings Most of the distributions are ‘data rich’ Variation –Asymmetrical –Long upper tail (>25% rates) –Median 15.2 (10-90% ) per 100,000 –Five-fold range within the 10-90% quantiles

Key findings (2) Males > females Migrants > native born Urban > mixed urban/rural Schizophrenia has a varied and detailed epidemiological landscape

Caveats Comparisons in systematic reviews should be planned, based on directional hypotheses & limited to a reasonable number Systematic reviews are best suited to hypothesis-generation Geographical boundaries are administrative Mostly ‘treated’ incidence

Conclusions There is a wealth of data available on the incidence of schizophrenia. Studies come from many countries, with many different methodological features, and conducted over several decades. The width and skew of the rate distribution, and the significant impact of sex, urbanicity and migrant status on these distributions, indicate substantial variations in the incidence of schizophrenia. Thus these data may provide leads for further research into risk factors