Schizophrenia and depression: challenging the paradigm of two separate diseases--a controlled study of schizophrenia, depression and healthy controls.

BACKGROUND: We studied descriptive and causal associations between schizophrenia, depressive symptoms and episodes of depression.
METHODS: Untreated psychotic, depressive and negative symptoms were assessed retrospectively from onset until first admission using the IRAOS in a population-based sample of 232 first episodes of schizophrenia. A representative subsample of 130 patients, studied retrospectively until onset and followed up prospectively over 6 months after first admission, were compared with 130 age- and sex-matched healthy population controls and with 130 equally matched first admissions for unipolar depressive episodes.
RESULTS: The lifetime prevalence of depressive mood (>or=2 weeks) at first admission for schizophrenia was 83%. The most frequent initial symptom of schizophrenia was depressive mood, appearing more than 4 years before first admission and followed by negative symptoms and functional impairment. Showing considerable overlap in symptoms and functional impairment at their initial stages, schizophrenia and unipolar depression became clearly distinguishable with the emergence of psychotic symptoms. In the first psychotic episode 71% presented clinically relevant depressive symptoms, 23% fulfilled the ICD-10 criteria for a depressive episode. With remitting psychosis the prevalence of depression, too, decreased. The high frequency of depressive symptoms at the prepsychotic prodromal stage and their increase and decrease with the psychotic episode suggests that depression in schizophrenia might be expression of an early, mild stage of the same neurobiological process that causes psychosis.
CONCLUSIONS: The high prevalence of depression in the population and the diversity of its causes prompted us to speculate about a hierarchical model of preformed dimensional patterns of psychopathology.