Delusional disorder is a mental illness in which the patient presents with delusions, but with no accompanying prominent hallucinations, thought disorder, mood disorder, or significant flattening of affect. Delusions are a specific symptom of psychosis. Delusions can be bizarre or non-bizarre in content. Non-bizarre delusions are fixed false beliefs that involve situations that could potentially occur in real life; examples include being followed or poisoned. Apart from their delusions, people with delusional disorder may continue to socialize and function in a normal manner and their behavior does not generally seem odd or bizarre. However, the preoccupation with delusional ideas can be disruptive to their overall lives. For the diagnosis to be made, auditory and visual hallucinations cannot be prominent, though olfactory or tactile hallucinations related to the content of the delusion may be present.
To be diagnosed with a delusional disorder, the delusion(s) cannot be due to the effects of a drug, medication, or general medical condition, and delusional disorder cannot be diagnosed in an individual previously properly diagnosed with schizophrenia. A person with delusional disorder may be high functioning in daily life. Recent and comprehensive meta analyses of scientific studies point to an association between a deterioration in aspects of IQ in psychotic patients, in particular perceptual reasoning.
According to German psychiatrist Emil Kraepelin, patients with delusional disorder remain coherent, sensible and reasonable. The Diagnostic and Statistical Manual of Mental Disorders (DSM) defines six subtypes of the disorder characterized as erotomanic (believes that someone is in love with them), grandiose (believes that they are the greatest, strongest, fastest, richest, and/or most intelligent person ever), jealous (believes that the love partner is cheating on them), persecutory (delusions that the person or someone to whom the person is close is being malevolently treated in some way), somatic (believes that they have a disease or medical condition), and mixed, i.e., having features of more than one subtype. Delusions also occur as symptoms of many other mental disorders, especially the other psychotic disorders.
The DSM-IV, and psychologists, generally agree that personal beliefs should be evaluated with great respect to cultural and religious differences, since some cultures have widely accepted beliefs that may be considered delusional in other cultures.
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http://apt.rcpsych.org/content/19/3/212 recent developments in the management of delusional disorders by christopher f. Fear advances in psychiatric treatment may 2013, 19 (3) 212-220; doi: 10.1192/apt.bp.111.010082