Environmental and familial risk factors for psychotic and non-psychotic severe depression

Severe unipolar depression can be classified as either psychotic depression (PD) or non-psychotic depression (non-PD). A number of biological and clinical differences have been detected between PD and non-PD, but it remains unknown whether risk factors for the two subtypes also differ. The aim of the present study was therefore to investigate whether a number of potential risk factors influenced the risk of developing PD and non-PD to different extents.

This is a register-based historical prospective cohort study following all 2.4 million individuals born in Denmark between 1955 and 1990. During follow-up 2183 and 9101 individuals were registered in the Danish Psychiatric Central Research Register with PD and non-PD respectively. The association between risk factors and the development of PD and non-PD was estimated by survival analysis (Poisson regression) and expressed as incidence rate ratios (IRR).

The most consistent finding of the study was that of a general overlap in familial and environmental risk factors for PD and non-PD. However, a parental history of bipolar disorder was a risk factor for PD (mother, IRR=1.66, p=0.003. Father, IRR=1.56, p=0.040) and not for non-PD (mother, IRR=0.92, p=0.430. Father, IRR=1.08, p=0.552). Conversely, a positive family history of schizophrenia was associated with neither PD nor non-PD LIMITATIONS: Diagnoses were assigned as part of routine clinical practice.

Our findings justify the distinction between PD and non-PD in the current diagnostic manuals. Furthermore, the fact that parental bipolar disorder and not schizophrenia was a risk factor for PD supports the Kraepelinian dichotomy.