Delusional disorder is a psychiatric diagnosis denoting a psychotic mental illness that involves holding one or more non-bizarre delusions in the absence of any other significant psychopathology (signs or symptoms of mental illness). In particular, a person with delusional disorder has never met any other criteria for schizophrenia and does not have any marked hallucinations, although tactile (touch) or olfactory (smell) hallucinations may be present if they are related to the theme of the delusion.

People with delusional disorder often can continue to socialize and function normally, apart from the subject of their delusion, and generally do not behave in an obviously odd or bizarre manner. This is unlike people with other psychotic disorders, who also might have delusions as a symptom of their disorder. In some cases, however, people with delusional disorder might become so preoccupied with their delusions that their lives are disrupted.

In the erotomanic subtype, the central theme of the delusion is that another person is in love with the individual. Efforts to contact the object of the delusion through telephone calls, letters, or even surveillance and stalking may be common. Behavior related to the delusion may come in conflict with the law. In the grandiose subtype, the person is convinced that he has some great talent or has made some important discovery.

Development of delusions Recently, Morimoto et al (2002) reported that 13 patients with delusional disorder were reported to have increased levels of plasma homovanillic acid (HVA) (a dopamine metabolite) compared with control subjects, and that the level of HVA is correlated with severity of psychotic symptoms. His patients responded well to treatment with low-dose haloperidol (average 2.7 mg/d) and showed decreased posttreatment plasma level of HVA, which correlated with the improvement of their symptoms.

Delusions also occur in the dementias, which are syndromes wherein psychiatric symptoms and memory loss result from deterioration of brain tissue. Because delusions can be shown as part of many illnesses, the diagnosis of delusional disorder is partially conducted by process of elimination. If the delusions are not accompanied by persistent, recurring hallucinations, then schizophrenia and schizoaffective disorder are not appropriate diagnoses. If the delusions are not accompanied by memory loss, then dementia is ruled out. If there is no physical illness or injury or other active biological cause (such as drug ingestion or drug withdrawal), then the delusions cannot be attributed to a general medical problem or drug-related causes.

The cause of delusional disorder is not known. Some studies suggest a biological component due to increased prevalence in first degree relatives of individuals with the disorder. There is a tendency for their family relationships to be characterized by turbulence, callousness, and coldness yet the significance of the patter is unclear typical defense mechanisms seen in these patients include denial, projection, and regression.

Treatment approaches may be found similar to those used to treat symptoms of schizophrenia. Antipsychotic drugs are often very effective in treating delusions. A number of new antipsychotic drugs (the so-called "atypical antipsychotics") have been introduced since 1990. The first of these, clozapine (Clozaril), has been shown to be more effective than other antipsychotics, although the possibility of severe side effects'in particular, a condition called agranulocytosis (loss of the white blood cells that fight infection)?requires that patients be monitored with blood tests every one or two weeks. Even newer antipsychotic drugs, such as risperidone (Risperdal) and olanzapine (Zyprexa), are safer than the older drugs or clozapine, and they also may be better tolerated. They may or may not treat the illness as well as clozapine, however.