The essential feature of delusional disorder (American Psychiatric Association, 1994) is the presence of one or more non-bizarre delusions that persist for at least 1 month. A delusion is defined as a false belief, based on faulty or incorrect interpretation of external reality that is not consistent with the patients' cultural background or intelligence and cannot be corrected by reasoning. A delusion that is frequently referenced in the dermatological literature is delusion of parasitosis (Driscoll et al., 1993). The delusion may be associated with tactile or olfactory hallucinations that are related to the theme of the delusion; for instance, a crawling sensation under the skin in association with delusions of parasitosis, or a delusion that one is emitting a foul odour from an orifice of the body. This can occur in conjunction with delusions of reference where the patient believes that everyone around him is talking about the odour that he is emitting. Certain hallucinations related to touch may in part be related to an organic brain syndrome or peripheral degenerative changes that may cause some cutaneous dysaesthesias. A delusional disorder may coexist with a major depressive episode where the patient experiences delusions that are congruent with his or her depressed mood, and typically represent a more severe form of depressive disease. BDD is often associated with delusions of disfigurement. If the delusion or hallucination becomes more bizarre and if they are clearly implausible and not derived from normal life experience (e.g. a complaint that aliens are putting electricity through the body and causing the patient to feel a stinging sensation in the skin) the diagnosis of schizophrenia should be considered.
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