Drug-Induced Formication

The false sensation of bugs crawling on or within the skin is a sensory hallucination commonly associated with psychostimulant drugs. It was first reported in chronic cocaine users in 1889.¹ Drug-induced formication has been referred to as "coke bugs," "meth mites," and "amphetamites," depending on which drug caused the hallucination. Patients with this disorder often have self-induced dermatosis caused by intense picking and scratching of the skin. The lesions may appear as multiple well-circumscribed, erythematous papules and partially healed scabs in easy-to-reach areas, such as the face, scalp, neck, anterior thighs, and arms (lesions on the dorsal forearms are often worse on the side opposite the patient's dominant hand).

Patients with drug-induced formication often have an associated fixed delusion of parasitosis and claim to have seen "bugs" or "worms" crawling under or out of their skin. The woman in Photo A complained of bugs in her skin. In addition to her face, she had lesions on her arms, shoulders, and neck; no other areas were affected. She reported a history of methamphetamine abuse concurrent with her alleged infestation. The man in Photo B thought he had a "worm infestation" in his neck and used a pair of household scissors to cut out the imagined worms; his toxicology screening was positive for cocaine, opiates, and benzodiazepines.

Delusions of parasitosis that are not related to drug abuse are more common among middle-aged and elderly women.^1,2 Patients with this disorder may be fully functional, but have a fixed false belief that they are infested with parasites. They may claim to have the parasites in a collection of material (typically pieces of skin, scabs, hair, lint, and other debris) and demand that it be observed under a microscope or sent to the laboratory for identification. This clinical presentation is called the "matchbox sign" because, historically, patients used a matchbox to collect the material, although now small plastic bags are most frequently used for this purpose.³ The matchbox sign suggests that the patient has seen other clinicians who have discounted a parasitic infection as a cause of their lesions.

Delusional parasitosis may be a shared illness, a folie deux, in which the patient's accompanying close contact also believes in the infestation. Family members and friends who do not share the delusion can be of help in the patient's evaluation and treatment. Reassuring patients that no parasitosis exists can be quite difficult because they typically refuse to believe that their disease is functional rather than organic. Although results of objective tests (such as complete blood cell count, skin biopsy, and stool evaluation for ova and parasites) may satisfy the clinician, patients frequently deny negative test results and may even have their delusion reinforced by the clinician's willingness to order such tests.

Referral for psychiatric therapy must be approached with tact in patients with delusional parasitosis. Management of drug-induced formication obviously involves discontinuation of the offending drug. Pimozide is recommended for non-drug-induced delusions of parasitosis.^1-3

References:

REFERENCES:1. de Leon J, Antelo RE, Simpson G. Delusion of parasitosis or chronic tactile hallucinosis: hypothesis abouttheir brain physiopathology. Compr Psychiatry. 1992;33:25-33.
2. Wilson FC, Uslan DZ. Delusional parasitosis. May Clin Proc. 2004;79:1470.
3. Anonymous. The matchbox sign. Lancet. 1983;2:261.