Pruritus

Basics

Unpleasant yet common sensation that provokes a desire to scratch
Chronic if persisting for more than 6 wk (incidence increases with age)
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 (e.g., histamine and peptides such as substance P that release histamine) stimulate C-fibers and induce itching
Prostaglandins (PGE₂, PGH₂) lower threshold to pruritus
Opiates cause pruritus by acting on central receptors
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

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