Delusions of parasitosis

Published on 19/03/2015 by admin

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Last modified 22/04/2025

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Delusions of parasitosis

Jillian W. Wong Millsop and John Y.M. Koo

Evidence Levels:  A Double-blind study  B Clinical trial ≥ 20 subjects  C Clinical trial < 20 subjects  D Series ≥ 5 subjects  E Anecdotal case reports

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(From Robles, D.T., Romm, S., Combs, H., Olson, J., Kirby, P., 2008. Delusional disorders in dermatology: a brief review. Dermatology Online Journal 14 (6), 2. ©2008.)

Delusions of parasitosis is a form of delusional disorder somatic type (known also as a monosymptomatic hypochondriacal psychosis), in which patients have a cutaneous dysesthesia that causes them to pick at their skin continuously, in order to ‘extract’ an organism or ‘foreign body’ they believe is present in their skin. The cutaneous findings that result from these attempts to dig out the suspected parasites range from normal skin to excoriations, picker’s nodules, and frank ulcerations. Patients develop elaborate and complex delusional ideations associated with their condition, and their fixed beliefs cannot be argued with reason. These patients often collect ‘samples’ in bottles, Ziploc® bags, jars, or slides of what is often lint, hair, debris, dead skin, and even common insects found in the home, using these specimens to provide ‘evidence’ to physicians of the alleged underlying cause of their condition. Many of these patients can also have tactile hallucinatory experiences that are compatible with their delusion. The most characteristic hallucinatory symptom they may experience is formication, which manifests as sensations of cutaneous crawling, biting, or stinging. Many also complain of pruritus. The condition has a bimodal age distribution, occurring in younger adults (men and women) and the elderly (mostly women). There can be secondary psychopathologies in delusions of parasitosis, such as depression and anxiety; these can be severe enough to cause the patient to commit suicide.

Management strategy

The physician should first establish therapeutic rapport with the patient. This begins with a thorough skin examination that sometimes entails deciding whether performing a biopsy may be worthwhile to establish rapport as patients frequently insist upon having a biopsy. To develop trust, it is imperative that the dermatologist examine these patients thoroughly and carry out a detailed medical history to exclude a frank skin condition (e.g., scabies incognito) or other possible organic conditions, such as substance abuse (e.g., cocaine), neurological (e.g., multiple sclerosis), endocrine (e.g., diabetes mellitus), hematologic/oncologic (e.g., lymphoma), nutritional (e.g., B12 or folate deficiency), infectious (e.g., AIDS), cardiovascular (e.g., congestive heart failure) or renal disorders. The condition can also develop after the patient, relative, or pet has had a true parasitic infection, and so ruling out the presence of a real infestation is warranted. Importantly, when patients do not know the reason why they are itching, the clinician should consider a diagnosis other than delusions of parasitosis, because patients with delusions typically ‘know’ that infestation is causing their symptoms.

Examining various ‘specimens’ that the patient may bring to the dermatologist’s office under the microscope will demonstrate to the patient that his or her concerns are being taken seriously. One should not make any comments that may reinforce their delusional ideation, such as a statement that an organism responsible for the condition was found; an agreement such as this on the part of the clinician may ultimately render the patient more difficult to deal with by making them even more firmly fixated on their erroneous belief systems. On the other hand, by definition, rational argument or trying to talk a patient out of a delusion is both not possible (if the patient has a real delusion) and counterproductive. Dermatologists can acknowledge that they know the patient’s sensations and suffering are real, and that they will do everything they can to help. It is also important for the dermatologist to treat secondary skin changes in these patients. By doing so, the dermatologist will help to forge a therapeutic alliance with the patient. Dermatologists should consider soothing baths and topical agents such as steroid–anesthetic combinations or creams with menthol.

The most effective way to reverse delusional ideation is to start the patient on an antipsychotic medication. If this is described as an antipsychotic agent, however, few patients will accept the treatment. On the other hand, if this option is offered in a neutral way, emphasizing possible symptom reduction, such as reduced crawling, biting, or stinging sensations, while avoiding arguing over pathophysiology or the mechanism of action of these medications, patients may be eager to accept.

The medication traditionally used to treat delusions of parasitosis is the antipsychotic agent pimozide, a neuroleptic. This medication generally works very well, whether patients have classic delusions of parasitosis or merely formication, but are not delusional. The starting dose of pimozide is deliberately kept low at 0.5–1 mg/day to minimize the risk of side effects. The dose is gradually increased until the optimal clinical response is attained, as evidenced by reduced mental preoccupation, formication, and agitation. The dose of pimozide can be increased by as little as 0.5–1 mg increments, and as slowly as on a biweekly to monthly basis until significant clinical response is noted, which is usually evident by the time the dose is 3–5 mg daily. It is very rare that a patient will require a dose of more than 5 mg daily, and the use of more than 10 mg daily is almost unheard of in the treatment of delusions of parasitosis. Once the patient reaches a stable, well-tolerated dose, and agitation, mental preoccupation, and symptoms of formication have subsided, this dose should be maintained for a few months. During this time, if the patient continues to experience improvement, the dosage of pimozide can then be gradually reduced by as little as 1 mg every 2 to 4 weeks until the minimum necessary dosage is determined or the patient is tapered off the pimozide altogether.

If the clinical state deteriorates again in the future with a new episode of exacerbation of a delusional belief system and formication, the patient can be restarted on pimozide and again treated on a time-limited fashion to control the particular episode. Most patients can be treated on an episodic basis and can be tapered off pimozide after several months, but some require long-term low-dose maintenance treatment.

As pimozide blocks dopaminergic receptors, there is the possibility that extrapyramidal side effects such as stiffness in the muscles or akathisia, an inner sensation of restlessness, may develop. Acute dystonic reaction and tardive dyskinesia are other potential adverse consequences associated with pimozide, although with the relatively low dosages used to treat delusions of parasitosis, these side effects are rarely encountered. If they do develop, however, they can usually be controlled with anticholinergic agents such as benztropine (Cogentin) 1–2 mg up to four times a day as needed, or diphenhydramine (Benadryl) 25 mg four times a day as needed for stiffness or restlessness. Akathisia and pseudoparkinsonian side effects are not a reason for discontinuing treatment with pimozide provided they are kept under control with one of these agents.

Because pimozide can theoretically prolong the QT interval or cause ventricular arrhythmias, it is advisable to consider checking pretreatment – and, periodically, post-treatment – electrocardiograms (ECGs), especially in older patients or those with a history of cardiac arrhythmia. It should be noted that in most cases, however, the risk of ECG abnormalities, including prolongation of the QT interval, is minimal with doses at or less than 5 mg daily, provided that they are not elderly and have no history of arrhythmia. Caution must also be exercised in prescribing pimozide for those with hepatic or renal dysfunction.

More recently, atypical antipsychotics, such as risperidone, and olanzapine, have been used successfully to treat patients with delusions of parasitosis. Atypical antipsychotics block more 5-HT-2 (serotonin) receptors than D2 receptors. Serotonin has been shown to be a key player in some states of psychosis, most cases of obsessive–compulsive disorder, and self-mutilation, which can all potentially manifest in patients with delusions of parasitosis. Thus, atypical antipsychotics, by blocking both serotonin and dopaminergic receptors, are thought theoretically to be an effective choice for treating this condition. Furthermore, the burden of side effects with atypical antipsychotics may be reduced compared to that of older typical antipsychotics. Having the patient agree to go on risperidone or olanzapine, however, may be more difficult than with pimozide, because pimozide’s primary indication is Tourette syndrome, whereas the primary indication for risperidone and olanzapine is schizophrenia, the latter indication being not acceptable to most delusions of parasitosis patients. Nevertheless, over the past several years, clinicians have reported many cases with good success using atypical antipsychotics. It is usually advisable to start at low doses and titrate upward as needed. (For example, for risperidone, start from 0.5 mg once to twice daily up to the usual maximum dose of 5 mg daily.) To date, however, there have been no randomized double-blind, placebo-controlled trials comparing the efficacy of pimozide with the atypical antipsychotics, and most of the medical literature on atypical antipsychotics is limited to case reports.

At any point, if feasible, it may be beneficial to try to refer the patient to a psychiatrist. However, many of these patients cannot be managed by psychiatrists because of their refusal to believe their condition is psychiatric in nature. Therefore, for the dermatologist to be willing to use antipsychotic medications is likely to be the only way that most of these patients can receive the treatment they need. At the same time, the most difficult aspect of managing patients with delusions of parasitosis is trying to obtain their cooperation in taking the medication. This difficulty arises as a result of the difference between the patients’ belief system and the physician’s understanding of the patient’s experience. Even with all the interpersonal skillfulness as described earlier, patients may be reluctant to take a psychotropic medication. It may be helpful to emphasize to them that these medications have worked well with patients with similar symptoms and that, in light of their suffering, they have nothing to lose by trying the medication with the spirit of ‘trial and error.’ Even though managing psychotic patients in dermatological settings has many challenges, when a dermatologist manages to connect with at least some of these patients and dramatically reverse their very miserable situation, those patients often turn out to be the most grateful patients one can have in a dermatologist’s career.

Specific investigations

These additional tests may be considered, depending on the patient’s clinical presentation:

First-line therapies

image Pimozide B

Second-line therapies

image Risperidone D
image Olanzapine D
image Quetiapine E
image Aripiprazole E
image Trifluoperazine E
image Haloperidol E
image Sulpiride E
image Fluphenazine E
image Flupenthixol E
image Promazine E

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