Delusions of parasitosis

Management Strategy

The management strategy for secondary DOP is a referral to psychiatry. For suspected primary DOP, it is still imperative that the dermatologist carry out a detailed medical history to exclude a bona fide skin condition (e.g., scabies incognito) or other possible organic conditions, such as substance abuse, neurologic, endocrine, hematologic/oncologic, nutritional (e.g., B ₁₂ or folate deficiency), or renal (e.g., uremic pruritus) 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.

Second, a thorough physical examination should be performed, including examining ‘specimens’ that the patient may bring. These specimens can be observed under the microscope, which help 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; this may ultimately render the patient more difficult to deal with by making him or her even more firmly fixated on the erroneous beliefs. On the other hand, by definition, rational argument or trying to talk a patient out of a delusion is both not possible and counterproductive. Dermatologists can acknowledge the patient’s sensations and suffering as real and that they will do everything they can to help.

It is also important for the dermatologist to treat secondary skin damages in these patients to create a therapeutic alliance with the patient. Dermatologists should consider soothing baths and topical agents such as creams with menthol or the use of occlusive dressin gs to enhance healing and limit access to involved parts of the skin.

The most effective way to reverse delusional ideation is to start the patient on an antipsychotic medication. If this option is offered in a neutral way, emphasizing possible symptom reduction, such as reduced crawling, biting, or stinging sensations, while avoiding discussing pathophysiology or the mechanism of action of these medications, patients may be willing to accept the medication on a ‘trial and error’ basis.

The medication historically used to treat DOP is pimozide. The fact that pimozide is US Food and Drug Administration (FDA)–indicated only for Tourette syndrome with no psychiatric indications makes pimozide uniquely acceptable to those patients who are often intensely opposed to accepting a psychiatric medication. This medication generally works very well, whether patients have classic DOP or formication without delusions. The trapezoid dosing plan is used to describe the use of pimozide with regard to its dose titrations when treating DOP. The dose of pimozide can be increased by as little as 0.5 mg increments and as slowly as on a biweekly to monthly basis until significant clinical response is noted, which is usually evident at a dose of 3 mg daily or less. It is very rare that a patient will require a dose of more than 5 mg daily. If the patient continues to experience improvement, the dosage of pimozide can eventually be gradually reduced by as little as 0.5 mg every 2–4 weeks until the minimum necessary dosage is determined, or the patient is slowly tapered off the pimozide altogether.

If the clinical state deteriorates again in the future with a new episode of a delusional belief 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.

Because pimozide blocks dopaminergic receptors, there is the possibility that extrapyramidal side effects such as stiffness or akathisia may develop. 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 up to four times a day as needed. Side effects are not a reason for discontinuing treatment provided they are well controlled. Acute dystonic reaction and tardive dyskinesia are other potential adverse consequences associated with pimozide, although with the relatively low dosages used, these are extremely rare.

The metabolism of pimozide is catalyzed mainly by the cytochrome P450 3A4 (CYP 3A4) enzymatic system and, to a lesser extent, by cytochrome P450 1A2 (CYP 1A2) and cytochrome P450 2D6 (CYP 2D6). Individuals with genetic variations resulting in poor CYP 2D6 metabolism (approximately 10% of the population) exhibit higher pimozide concentrations than extensive CYP 2D6 metabolizers. The time to achieve steady-state pimozide concentrations is expected to be longer (about 2 weeks) in poor CYP 2D6 metabolizers because of the prolonged half-life. Alternative dosing strategies are recommended in patients who are genetically poor CYP 2D6 metabolizers. At doses above 4 mg/day, CYP 2D6 genotyping should be performed. In poor CYP 2D6 metabolizers, pimozide doses should not exceed 4 mg/day, and doses should not be increased earlier than 14 days (see FDA package insert for further information).

Because pimozide can theoretically prolong the QT interval or cause arrhythmias, it is advisable to consider checking electrocardiograms (ECGs) pretreatment and, periodically, during treatment, especially in older patients or those with a history of cardiac arrhythmia. Caution must also be exercised in prescribing pimozide for those with hepatic or renal dysfunction. Hypokalemia or hypomagnesemia is a contraindication to therapy. Therefore, a basic metabolic panel and magnesium (Mg) level should be checked before starting therapy.

Do not prescribe pimozide if a patient has a history of hypersensitivity to it or any components. Further contraindications include history of central nervous system (CNS) depression, history of cardiac arrhythmias, congenital long QT syndrome, and concurrent use with QTc-prolonging agents. Drugs that are inhibitors of CYP3A4 should not be used concurrently. Do not prescribe pimozide if a patient is on a selective serotonin reuptake inhibitor (SSRI) or stimulant.

Atypical antipsychotics, such as risperidone and olanzapine , have also been used successfully to treat patients with DOP. Atypical antipsychotics block more 5-HT _-2 (serotonin) receptors than D ₂ 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 DOP. Thus, atypical antipsychotics, by blocking both serotonin and dopaminergic receptors, are thought to be an effective choice for treating this condition. Furthermore, the burden of side effects with atypical antipsychotics may be reduced compared with that of older typical antipsychotics. Having the patient agree to go on risperidone or olanzapine, however, may be more difficult than pimozide; the primary indication for risperidone and olanzapine is schizophrenia, with the latter indication likely being unacceptable to many patients with DOP.

Nevertheless, over the past several years, clinicians have reported many successful cases 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, and work up to the effective dose for that particular patient with the usual maximal dose of 5 mg daily. While not widely studied in DOP, the recommended dose for olanzapine ranges from 2.5 mg/day to 7.5 mg/day. The dose should not exceed 10 mg/day.

Even with a skillful interpersonal approach, 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 intense suffering and misery, they are encouraged to try the medication with the spirit of ‘trial and error’. Even though managing psychotic patients in dermatologic settings has many challenges, when a dermatologist succeeds in connecting with these patients and dramatically reversing their very miserable situation, these patients often turn out to be the most grateful patients a dermatologist can have in their career.