The validity of the severity-psychosis hypothesis in depression

Psychotic depression (PD) is classified as a subtype of severe depression in the current diagnostic manuals. Accordingly, it is a common conception among psychiatrists that psychotic features in depression arise as a consequence of depressive severity. The aim of this study was to determine whether the severity of depressive and psychotic symptoms correlate in accordance with this "severity-psychosis" hypothesis and to detect potential differences in the clinical features of PD and non-psychotic depression (non-PD).

Quantitative analysis of Health of the Nation Outcome Scales (HoNOS) scores from all patients admitted to a Danish general psychiatric hospital due to a severe depressive episode in the period between 2000 and 2010 was performed.

A total of 357 patients with severe depression, of which 125 (35%) were of the psychotic subtype, formed the study sample. Mean HoNOS scores at admission differed significantly between patients with non-PD and PD on the items hallucinations and delusions (non-PD=0.33 vs. PD=1.37, p<0.001), aggression (non-PD=0.20 vs. PD=0.36, p=0.044) and on the total score (non-PD=10.55 vs. PD=11.87, p=0.024). The HoNOS scores on the two items "depression" and "hallucinations and delusions" were very weakly correlated.

Diagnoses were based on normal clinical practice and not formalized research criteria.

The symptomatology of PD and non-PD differs beyond the mere psychosis. Furthermore, severity ratings of depressive and psychotic symptoms are very weakly correlated. These findings offer further support to the hypothesis stating that the psychotic- and non-psychotic subtypes of depression may in fact be distinct clinical syndromes.