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Formication: Causes of the "Bugs Crawling on Skin" Sensation

Formication (from Latin formica, ant) is the tactile hallucination of insects crawling on or under the skin. It is a symptom, not a diagnosis, with diverse medical and psychiatric causes.

Medical Causes

1. Neurological

  • Peripheral neuropathy: Diabetes (most common), B12 deficiency, alcohol, chemotherapy, HIV.
  • Radiculopathy: Cervical/lumbar nerve root compression.
  • Multiple sclerosis: Demyelinating lesions in sensory pathways.
  • Stroke/TIA: Thalamic or sensory cortex involvement.
  • Chiari malformation, syringomyelia.

2. Dermatological

  • Xerosis (dry skin), eczema, psoriasis, contact dermatitis.
  • Scabies, lice, cutaneous larva migrans (true infestation → real crawling).
  • Brachioradial pruritus (cervical spine + sun exposure).

3. Systemic / Metabolic

  • Chronic kidney disease (uremic pruritus)
  • Liver disease (cholestasis → bile salt deposition)
  • Thyroid dysfunction (hyper/hypo)
  • Malignancy (paraneoplastic pruritus, lymphoma)
  • Electrolyte imbalance (Ca2+, PO4)

4. Medication/Substance-Induced

  • Stimulants: Cocaine, amphetamines, methylphenidate (dopaminergic).
  • Opioids: Withdrawal or high-dose therapy.
  • SSRIs/SNRIs: Initiation or withdrawal.
  • Antibiotics: Fluoroquinolones (rare).
  • Hormonal: Estrogen, testosterone.

Psychiatric Causes

  • Delusional parasitosis (Ekbom syndrome): Fixed false belief of infestation. Formication + conviction. Antipsychotics (pimozide, risperidone) effective.
  • Anxiety disorders: Somatic hypervigilance, panic attacks.
  • OCD: Contamination obsessions.
  • Depression with somatic features.

Diagnostic Approach

  1. History: Onset, distribution, timing, medications, substance use, psych history.
  2. Exam: Skin (excoriations, burrows), neuro (sensory, reflexes, cranial nerves).
  3. Labs: CBC, CMP, TSH, B12/folate, HbA1c, ESR/CRP, urine tox.
  4. Skin scraping (scabies), biopsy if atypical.
  5. EMG/NCS if neuropathy suspected.
  6. Brain/spine MRI if central cause considered.

Treatment

  • Treat underlying cause (glucose control, B12 replacement, medication adjustment).
  • Symptomatic: Gabapentin/pregabalin (neuropathic), antihistamines (limited), topical menthol/capsaicin, moisturizers.
  • Psychiatric: CBT, SSRIs for anxiety/OCD; low-dose antipsychotics for delusional parasitosis.

When to Seek Care

  • Persistent > 2 weeks
  • Associated weakness, numbness, vision changes
  • Skin breakdown from scratching
  • Functional impairment
  • Fixed belief in infestation despite evidence

Formication is distressing but usually treatable once the driver is identified. Don't self-diagnose—see a clinician.

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