Prevalence and incidence of schizophrenia spectrum disorders: implications for prevention.

OBJECTIVE: This paper reviews the historical and conceptual background to proposals about prevention of schizophrenia through intervention targeting asymptomatic, high-risk individuals. It also examines the outcomes of a hypothetical model of prevention based on a two-stage risk segmenting approach.
METHOD: The assumptions and parameters used in the model are derived from actual epidemiological and clinical research. The two risk criteria selected are: (i) genetic risk (having a parent with schizophrenia); and (ii) neurocognitive deficit (abnormal performance on the Continuous Performance Task, CPT). The parameters and risk factors are applied to a hypothetical screening program covering a population of 100000.
RESULTS: At the end of the second stage of screening the program using the risk criteria to search for preventable cases will have correctly identified only three out of 20 'true' cases and will have incorrectly assigned to treatment two non-cases. The great majority of people at risk who will eventually develop schizophrenia are likely to remain undetected by current screening or preclinical diagnostic programs, while a certain number of people actually not at risk would be falsely identified as high-risk and offered treatment.
CONCLUSIONS: Reliably identifying, with intention to treat, asymptomatic people in the community who are presumed to be at high risk of developing schizophrenia is at present epidemiologically non-viable. This caveat should not apply to strategies for early diagnosis and treatment of incipient episodes of schizophrenia where strategies to reduce the duration of untreated psychosis are likely to be both feasible and cost-effective.