Background: Despite the common clinical assumption that psychosis is an indicator of severity in depression, it is not known what determines the presence of psychotic features in major depression. Our aim was to answer the question: Is depression severity the sole cause of psychotic symptoms during an episode of unipolar major depression?
Methods: In a sample of 585 patients from the UK, meeting criteria for both DSM-IV and ICD-10 major recurrent depression, we assessed measures of severity of depression and the presence of psychotic features, both within and between subjects.
Results: Within patients, psychotic episodes tended to be more severe than non-psychotic episodes. However, between patients there was wide variation in severity in both those that did, and did not, experience psychotic episodes.
Limitations: We used retrospective interview information together with case note data. Our cases may not be typical of usual case loads because they were selected to avoid family history of bipolar illness or schizophrenia.
Conclusions: Individuals with a predisposition to psychotic features tend to display such features during more severe episodes of depression. However, patients with no history of psychosis may experience non-psychotic depressive episodes of equal or greater severity, in terms of depressive symptomatology, compared to patients with psychotic depression. Thus, there is individual variation in susceptibility to psychosis during mood episodes and severity is not the sole determinant.