Schizophrenia: who is at risk? Who is a case?

It is now almost unanimously acknowledged that schizophrenia is of multifactorial origin (i.e. that the clinical expression of the vulnerability induced by genetic factors depends on the interactions between this genetic liability and environmental risk factors). The list of putative environmental risk factors keeps on lengthening, including perinatal events with potential harmful neurodevelopmental impact, or later risk factors such as substance use or social factors. Although research on genetic and environmental risk factors for schizophrenia is of major importance to elucidate the etiology of the disorder, the usefulness of this body of knowledge with respect to prevention should be considered cautiously. Indeed, there is a huge gap between determining who is at increased risk for the disease and predicting who will actually develop schizophrenia amongst all subjects at increased risk. How a case of schizophrenia is defined is another major issue for etiological research and also for prevention. A growing body of evidence suggests the existence of a symptomatic continuum between subjects from the general population and subjects with clinical psychosis with respect to the occurrence of psychotic experiences. Thus, establishing a boundary between psychosis and normality is, by its very essence, an arbitrary process. Creating a new diagnostic category of "prodromal psychosis" is moving the boundary to include subjects with less severe symptoms. This strategy has major clinical implications because the question "who is a case?" is often equivalent in clinical practice to the question "who needs treatment?".