What causes formication?

Causes of Formication

Formication is most commonly caused by substance use (particularly stimulants), medication side effects, neurological disorders, psychiatric conditions, or dermatological issues. The sensation of insects crawling on or under the skin without actual infestation has several distinct etiologies that can be identified through careful evaluation.

Substance-Related Causes

• Recreational drugs: Cocaine, amphetamines, and other stimulants are well-documented causes of formication 1
• Gasoline inhalation: Can lead to organic lead encephalopathy with formication as a symptom 2
• Substance withdrawal: Particularly from alcohol or stimulants

Medication-Induced Formication

Medications that alter neurotransmitters can trigger tactile hallucinations, with the following being most commonly implicated 1:

1. Anti-Parkinsonian agents: Most frequently reported class
2. Antidepressants: Particularly those affecting serotonin and norepinephrine
3. Prescription stimulants: Similar mechanism to recreational stimulants
4. Antihypertensives: Particularly propranolol
5. Antiepileptics: Various mechanisms

Neurological Disorders

• Multiple sclerosis: Formication is reported as one of the neuropathic sensory symptoms in MS patients, described as "formicatory pain" 3
• Peripheral neuropathy: Particularly diabetic neuropathy
• Perineural tumor involvement: Squamous cell carcinoma with perineural spread can present with formication 4

Psychiatric Conditions

• Delusional infestation (DI): A monosymptomatic hypochondriacal psychosis characterized by fixed, false belief of being infested, accompanied by formication 1, 5
• Psychosis: Can occur in various psychiatric disorders
• Anxiety disorders: Can manifest as somatic symptoms including skin sensations

Dermatological Causes

• Skin infections: Bacterial infections can cause localized formication 6
• Lichen sclerosus: Can cause dysesthesia including formication-like symptoms 6
• Cutaneous ulceration: Associated with altered sensation in affected areas 6

Diagnostic Approach

When evaluating a patient with formication:

1. Medication review: Identify any medications known to cause tactile hallucinations
2. Substance use history: Specifically inquire about stimulants, alcohol, and inhalants
3. Neurological examination: Look for signs of peripheral neuropathy or central nervous system disorders
4. Skin examination: Evaluate for dermatological conditions that might cause abnormal sensations
5. Psychiatric assessment: Screen for delusional disorders, particularly if the patient has a fixed belief about infestation

Management Strategy

Treatment should target the underlying cause:

1. For substance-induced formication:

   • Discontinuation of the offending substance
   • For organic lead encephalopathy, chelation therapy can reverse symptoms 2

2. For medication-induced formication:

   • Dose adjustment or medication change when possible
   • Consider alternative treatments for the underlying condition

3. For neurological causes:

   • Disease-specific treatment (e.g., MS therapy)
   • Neuropathic pain medications may help (gabapentin, pregabalin)

4. For psychiatric causes:

   • Antipsychotics for delusional infestation (olanzapine has shown success) 5
   • Cognitive behavioral therapy for anxiety-related formication

5. For dermatological causes:

   • Treat underlying skin condition
   • Topical treatments for symptom relief

Special Considerations

• Formication without other psychotic features may indicate a non-psychiatric cause
• Patients with multiple types of hallucinations (visual, auditory) alongside formication are more likely to have a psychiatric or substance-related cause 1
• Persistent formication despite treatment of apparent causes warrants further investigation for undiagnosed conditions

By systematically evaluating these potential causes, the underlying etiology of formication can be identified and appropriately treated in most cases.