Delusions of Parasitosis

 

Salient features

 

·       Fixed, false belief that the skin is infested with parasites

 

·       May experience sensations of biting, crawling, or stinging

 

·       Need to distinguish from substance-induced formication

 

·       Requires treatment with an antipsychotic medication

 

Introduction

 

Delusions of parasitosis is classified as a somatic type of delusional disorder (previously referred to as monosymptomatic hypochondriacal psychosis) characterized by a false, fixed belief that their skin is infested by parasites, in the absence of any objective evidence of infestation.

 

Epidemiology

 

The average age of onset is 55–60 years. Among individuals over 50 years of age, women experience the disorder twice as often as men; however, prior to age 50, men and women are equally affected.  Younger patients with this disorder are usually of a lower socio-economic status and may have a history of substance abuse, while older patients are frequently of a higher socio-economic status.

Clinical features

 

Patients with delusions of parasitosis do not meet criteria for a diagnosis of schizophrenia. However, they should fulfill the DSM-5™ diagnostic criteria for a delusional disorder which are: (1) the delusion must be present for ≥1 month; (2) the patient does not exhibit impaired functioning or bizarre behavior apart from the impact of the delusion; and (3) the delusion cannot be attributable to the effects of a substance, medication, medical condition, or other psychiatric disorder. The delusional belief is “encapsulated”, i.e. there is a narrow and specific focus on skin infestation. Patients may be able to state that others view their beliefs as irrational (“factual insight”), but they lack the true insight necessary to personally accept the valid explanation.

Individuals with delusions of parasitosis typically present with a history of symptoms for months or even years. They have often already been evaluated by many physicians and have tried to eradicate their alleged “parasites” by methods such as using pesticides, hiring exterminators, or changing their residence. Patients frequently bring in bits of skin, lint, and other samples that they believe represent “parasites”, which is referred to as the “matchbox sign”. They may report cutaneous sensations of crawling, biting, or stinging.

 

Skin findings in delusions of parasitosis range from none to excoriations, lichenification, prurigo nodularis, and deep, gouged-out, irregular appearing ulcers. All of these are self-induced, usually resulting from the patient’s efforts to dig out “parasites”. The onset of the initial pruritus or paresthesia may be related to xerosis or, in fact, to a previously treated infestation. The skin lesions typically spare areas out of reach for the patients such as the interscapular area.

 

One intriguing aspect of this disorder is the potential for a shared delusional system whereby the patient’s close contacts come to believe in the delusion as well. Folie à deux (“craziness for two”) is the term used to describe two people who share the same delusion. Occasionally, larger numbers of people harbor the same delusion, which may include the patient’s parent(s) and children.

 

 

Samples of alleged “parasites” brought in by a patient (“matchbox sign” or “specimen sign”).

 

Differential diagnosis

 

Delusions of parasitosis are distinct from formication, which represents a tactile hallucination involving the sensation of “bugs” crawling within or biting the skin. Formication does not involve a fixed, false belief that the sensations are caused by a skin infestation. Formication and/or delusions of skin infestation can result from use of drugs, especially amphetamines and cocaine. As noted above, patients with delusions due to the effects of a substance, medication, medical condition (e.g. true skin infestation, neurologic disorder), or other psychiatric disorder (e.g. schizophrenia, depression with psychotic features) are excluded from diagnosis of delusions of parasitosis.

 

Management

 

One of the most challenging aspects of management is getting patients with delusions of parasitosis to agree to take an antipsychotic medication. The first step is to establish rapport and address their concerns seriously without challenging their beliefs, making sure to do a thorough dermatologic examination. When discussing the diagnosis with the patient, communicate it in a matter-of-fact manner and refrain from making any statements that may be misinterpreted by the patient as supporting his/her delusional ideation. Often, it is more feasible to present the antipsychotic medication as one that may work empirically for symptoms of formication and agitation rather than to confront the individual about psychiatric issues. If the medication is presented in an objective and pragmatic manner, the patient may have less difficulty accepting it as a therapy.

 

The treatment of choice for delusions of parasitosis has traditionally been pimozide. As part of the discussion on therapy, the dermatologist should explain to the patient that he or she does not have Tourette syndrome (the FDA-approved indication for pimozide) or schizophrenia. There have been multiple reports of successful treatment of delusions of parasitosis with atypical antipsychotic medications (e.g. risperidone, olanzapine, aripiprazole), which have more favorable side-effect profiles. Patients with delusions of parasitosis can often be successfully tapered off medication after 2–6 months of therapy.