Pruritus

Basics

Description

  • Unpleasant yet common sensation that provokes a desire to scratch
  • Chronic if persisting for 6 wk or longer:
    • Incidence increases with age
    • Affects ∼22% of people during their lifetime
  • Generally accepted to be mediated by unmyelinated C-fibers in upper portion of dermis (distinct from those for pain):
    • Transmitted to dorsal horn of spinal cord
    • Via spinothalamic tract to cerebral cortex
  • Peripheral mediators (eg, histamine and peptides such as substance P that release histamine) stimulate C-fibers and induce itching
  • Prostaglandins (PGE2,PGH2) lower threshold to pruritus
  • No single pharmacologic agent effectively treats all causes of pruritus “Itch–scratch–itch” cycle:
    • Itching triggers scratching
    • Scratching damages skin and stimulates nerve endings, thereby producing even greater itching
  • Pruritus accounts for about 1% of all physician visits

Etiology

  • Dermatologic mechanisms:
    • Histamine-mediated pruritus (mast cell activation)
    • Cytokine-mediated pruritus (IL-2, IL-4, IL-13, IL-31, TSLP)
    • Eczematous, psoriatic, or urticarial inflammation causing neural sensitization
  • Neurolopathic mechanisms:
    • Central nervous system dysregulation affects pruritic pathways
  • Systemic mechanisms:
    • Uremic pruritus:
      • Retention of pruritogenic substances such as urea, creatinine, and parathyroid hormone (PTH) contributes to nerve and skin irritation
    • Cholestatic pruritus:
      • Accumulation of bile acids in the skin stimulates sensory nerve endings
    • Hematologic disorders:
      • Excess mast cell and basophil activation, histamine release, and microvascular dysfunction
    • Paraneoplastic syndromes:
      • Tumor-driven cytokine imbalance, neuroinflammation, and pruritogenic factors enhance systemic inflammation and nerve hyperstimulation
  • Infectious causes:
    • Cytokine and immune dysregulation: Systemic infections (HIV, COVID-19, hepatitis B/C, tuberculosis)
    • Microbial toxins, metabolic byproducts, or direct nerve invasion (eg, varicella-zoster, syphilis, leprosy)
    • Helminth infections (schistosomiasis, strongyloidiasis) drive eosinophil degranulation, histamine release, and immune complex deposition
  • Psychogenic causes:
    • Somatoform disorders
    • Stress-related neuroinflammation and pruritus amplification
  • Medication-induced pruritus:
    • Direct histamine release (opiates, vancomycin, IV contrast)
    • Opioid receptor dysregulation
    • Immune-related hypersensitivity (β-lactam antibiotics, sulfonamides, NSAIDs)
    • Chemotherapeutic agents induce nerve dysfunction leading to a chronic itch

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