The cause per se is unknown. Delusional disorder theoretically represents a heterogeneous group of conditions that seems distinct from mood disorders and schizophrenia. Family studies show increased prevalence of delusional disorder and paranoid personality traits in relatives of delusional disorder probands but no association with mood disorders or schizophrenia. Longitudinal studies suggest that the disorder is stable and reclassified as a mood disorder or other psychotic disorder only 10-25% of the time.
Biological
Delusions can be a feature of a number of biological conditions, suggesting possibly biologic underpinnings for the disorder. Most commonly, neurological lesions associated with the temporal lobe, limbic system, and basal ganglia are implicated in delusional syndromes. Neurological observations indicate that delusional content is influenced by the extent and location of brain injury. Prominent cortical damage often leads to simple, poorly formed, persecutory delusions. Lesions of the basal ganglia elicit less cognitive disturbance and more complex delusional content. Right posterior cerebellar lesions are associated with misidentification syndromes. Excessive dopaminergic and reduced acetyl cholinergic activity have been linked to the formation of delusional symptoms.
Psychological
Psychological explanations of delusions present 3 ways of viewing the phenomena of delusions.
* Delusions as defensive: Sigmund Freud proposed that delusions serve a defensive function and protect the patient from intrapsychically unacceptable homosexual impulse through the use of reaction formation, projection, and denial.
* Delusions as the result of cognitive defects: In the 1950s, Eilhard von Domarus emphasized the use of flawed logic in the manufacturing of delusions. Patients are observed to accept ideas with too little evidence for their conclusions.
* Delusions as the result of abnormal perceptual experience: In the 1970s, Brendan Maher described delusions stemming from abnormal perceptual experience due to central nervous system (CNS), peripheral nervous system, or environmental cues. Delusions were described as created to deal with these abnormal phenomena.
Social/other
Norman Cameron defined social situations with the following characteristics as contributing to the development of delusional disorder: expectations of receiving sadistic treatment, distrust and suspicion, social isolation, jealousy, lowered self-esteem, people seeing their own defects in others, and rumination over meaning and motivation.
Associated risk factors that suggest other avenues in the pathogenesis of delusions include advanced age, social isolation, group delusions (eg, in the McCarthy era), low socioeconomic status, premorbid personality disorder, sensory isolation (particularly deafness), recent immigration, family transmission, head injury, and substance abuse disorders.