This subtype of depression is listed as severe episode with psychotic symptoms in ICD-10 and major depression with psychotic features
(mood-congruent and mood-incongruent) in DSM-IV. It is also commonly cited in the literature as psychotic or delusional depression. On the basis of its presenting symptoms, it was found in the ECA study to cover 14% of all major depressions, representing their most severe form .
Clinically this subtype is identified by the presence of delusions in conjunction with psychomotor disturbances, vegetative symptoms and occasional hallucinations. Depending on the delusional content, distinction is made between mood-congruent and mood-incongruent forms.
It has long been a controversial issue and is still debated whether delusions and other psychotic features in depression denote a qualitatively distinct psychopathological entity or merely manifest a greater severity of the depressive disorder continuum .
Demographic and clinical characteristics (phenomenology, course and prognosis), family history, treatment response and neurobiological markers have been used as variables to validate the diagnostic autonomy of delusional from non-delusional depression [3,10,49-51]. Findings derived from the community survey of the ECA study indicated that delusional depression is different on a number of variables from other subtypes of MD, and the differences are not accounted for by demographic factors, symptom profile and severity . The majority of studies have failed to substantiate a clear and significant difference in many other variables [51,52]. Some authors reported higher association of delusional depression with a number of neuro-biological markers (lower levels of serum dopamines-hydroxylase (DBH) and cerebrospinal fluid (CSF) 3-methoxy-4-hydroxyphenylglycol (MHPG); higher cortisol non-suppression to DST and higher hospitalization rates) . Whereas it may still be argued that the reported differences are at best state indicators of greater severity rather than distinguishing nosological markers , it should be noted that premorbid vulnerability factors for delusional-psychotic depression have been identified  and that the homogeneity of type and content of delusions from episode to episode has been reported by independent authors. These finding may to a certain degree validate the categorization of delusional depression as a distinct subtype of major depression and warrant further investigations [5, 7,10,55,56].
The mood-incongruent psychotic major depression is still debated with regard to its proper nosological placement. It may be close to schizoaffective disorder as defined by Research Diagnostic Criteria (RDC), but it differs from it in many other respects and particularly in premorbid social adjustment variables. It seems that mood incongruent major depression has a boundary problem and represents a largely heterogeneous clinical condition consisting of different cases which resemble cases of other categories (major depression, schizoaffective bipolar delusional states and even paranoid schizophrenia with affective component) qualifying for placement in either or in both the mood and delusional disorders .
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