Dermatologic therapy
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Itch is one of the alarm sensations that human beings have phylogenetically evolved for a defense purpose. Many theories and evidences regarding the biological, pathophysiological, and clinical aspects have been given, but an update on the neuroanatomy paths and consequent treatments is required. Some chemicals that are released after skin injury and facilitate the inflammatory process can induce itch or pain or basically lead to a sensitization of the nociceptor response. ⋯ The ambiguity of "itch unit" discharge to pruritics and algogens may be solved by the central inhibition of itch by pain: it is common knowledge that scratching relieves itching. Conversely, centrally acting pain-inhibiting opioids enhance itch by disinhibition. The relation between itch and pain is interesting in its clinical and physiopathological aspects in order to select appropriate treatment.
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The discovery of a specialized neuronal pathway for itch has markedly improved our understanding of itch processing under physiological conditions. However, the complex interactions of pain and itch are only partly understood. ⋯ There is emerging evidence that similar patterns of peripheral and central sensitization occur in chronic pain and chronic itch conditions. It will be of major interest to reveal whether the underlying mechanism for sensitization in the itch and pain pathways are also similar, as this might have major implications for therapy.
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A new pathophysiologically based classification of itch is proposed, which should help the clinician adopt a rational approach to diagnosis and management of generalized itch. Focusing on neurogenic itch (itch without visible rash), common causes are reviewed and guidelines for laboratory and radiologic investigation are proposed. ⋯ Specifically, the relative merits of broad versus narrowband ultraviolet B (UVB) are discussed and the pros and cons of doxepin, opioid antagonists, and selective serotonin reuptake inhibitors (SSRIs) such as paroxetine are considered. Attention is drawn to some novel approaches, including bright-light phototherapy and molecular adsorbent recirculating system (MARS) for selected patients with intractable itch caused by hepatic failure, and mirtazapine for nocturnal itch.