Delusions of parasitosis. A dermatologist's guide to diagnosis and treatment.

Delusions of parasitosis is a rare psychiatric disorder in which the patient has a fixed, false belief that he or she is infested by parasites. Even though it is a psychiatric disorder, these patients usually present to a dermatologist because they are convinced that they have a dermatological problem. Patients with delusions of parasitosis generally reject psychiatric referral. The diagnosis of delusions of parasitosis can often be made on the basis of the history alone, but it is important to make sure that the patient does not have a organic skin disorder, and the delusion is not secondary to another mental or physical illness. The current treatment of choice is the antipsychotic medication pimozide. The principal difficulty in management is convincing patients to take the drug. This results from the obvious discrepancy between the patients' belief system and the clinician's understanding of the situation. The most common adverse effects of pimozide are extrapyramidal symptoms such as stiffness and, less frequently, a special inner sense of restlessness called akathisia. Effective treatment of such extrapyramidal reactions includes benztropine 1 to 2 mg up to 4 times daily as needed, or diphenhydramine 25 mg 3 times daily as needed. Pimozide can have cardiotoxic effects at high dosages. Traditionally, it has been recommended to check pretreatment and post-treatment electrocardiograms even for patients who have no history or cardiac conduction abnormalities or arrhythmia. However, a more recent publication questions the need for this if the patient takes < 10 mg/day, is not elderly, and has no history of cardiac arrhythmia. Effective dosages of pimozide for delusions of parasitosis have ranged from 1 to 10 mg/day. The lowest effective dosage of pimozide should be used for the shortest possible duration to minimize the risk of tardive dyskinesia developing in these patients. Significant improvements in quality of life have been achieved with successful treatment with pimozide. Atypical antipsychotics such as risperidone with a much safer adverse effect profile may prove to be effective for the treatment of delusions of parasitosis in the future.