A systematic review of the prevalence of schizophrenia

John McGrathSukanta SahaJoy WelhamDavid Chant

A systematic review of the prevalence 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. [old]

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.[old]

Understanding the prevalence of schizophrenia has important implications for both health service planning and risk factor epidemiology.[new]

Aims

To systematically identify and collate studies of the prevalence of schizophrenia

To summarize variation in time, place and person by examining the distribution of these estimates of prevalence

To explore factors which may influence prevalence estimates

Outline

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

The prevalence of schizophrenia:Hypotheses

• Sex difference Males > females

• Migrant status Migrants > native born

• Urbanicity Urban born > rural born

Types of prevalence 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• 1965-2001 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

• OverlappingAge eg all ages or age 15-54Diagnosis eg Catego S+ or Catego SPO + clinicalDiagnostic categories

eg DSMIV Schizophrenia or Schizophrenia + Delusional disorder

Site overlap eg Denmark or Copenhagen Epoch overlap eg 1990-92 or 1989-91

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 Prevalence 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% 7.7- 43.0) 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’ prevalence

ConclusionsThere is a wealth of data available on the prevalance of schizophrenia. Studies come from many countries, with many different methodological features, and conducted over several decades. The width and skew of the distributions, and the significant impact of sex, urbanicity and migrant status on these distributions, indicate substantial variations in the prevalence of schizophrenia.

Thus these data may provide leads for further research into risk factors