A person with this illness holds a false belief firmly, despite clear evidence or proof to the contrary. In schizophrenia, delusions are described as "bizarre" (examples: feeling controlled by an outside force, or having thoughts inserted into your head). In delusional disorder, the delusions involve circumstances that could occur in reality even though they are unlikely (for example, the family next door plotting to kill you). A religious or cultural belief that is accepted by other members of the person's community is not a delusion.

There are several types of delusions. Themes of persecution are common. Other types are erotic, grandiose, jealous, or somatic (that is, delusions about the body). People with delusional disorder usually do not have hallucinations or a major problem with mood. Unlike people with schizophrenia, they tend not to appear odd or exhibit odd emotions.

When hallucinations do occur, they are part of the delusional belief. For example, someone who has the delusion that internal organs are rotting may hallucinate smells or sensations related to that delusion.

Since people with delusional disorder are aware that their beliefs are unique, they generally do not talk about them. If their functioning is impaired, it is usually a direct result of the delusion. Therefore, the disorder may be detected only by observing behavior that is a consequence of the belief. For example, a person who fears being murdered may quit a job or stay home with all the shades drawn, never venturing out.

The lifetime risk of delusional disorder is less than one in 1,000. Its cause is not known.

Treatment for this disorder is challenging, especially if the delusion is long lasting. Antipsychotic medications can be helpful. Since the patient may not believe he or she has a mental disorder, he or she may even refuse psychotherapy. However, support, reassurance, and pointing out the difference between the symptoms and reality can all be helpful if the person is willing to meet with a therapist. Educating the family about how to respond to the person's needs can be useful.

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