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INTRODUCTION
Pruritus can be defined as an unpleasant sensation that elicits a desire to scratch. The presumed biologic purpose of pruritus is to provoke scratching to remove a parasite or other harmful pruritogen. Pruritus is the most common skin-related symptom. It often arises from a primary cutaneous disorder but can be a manifestation of an under-lying systemic disease in 15%โ50% of affected individuals, especially when generalized. Non-dermatologic conditions that can lead to generalized pruritus include hepatic, renal, or thyroid dysfunction; lymphoma, myeloproliferative neoplasms (e.g. polycythemia vera), and chronic lymphocytic leukemia; HIV or parasitic infections; and neuropsychiatric disorders (see Ch. 7).
A clinical classification scheme for chronic pruritus was proposed in 2007 by the International Forum for the Study of Itch. In this system, there are three major groups of pruritus: (1) chronic pruritus of lesional (inflamed) skin; (2) chronic pruritus of non-lesional (non-inflamed) skin; and (3) chronic pruritus presenting with secondary scratchinduced lesions (e.g. prurigo nodularis, lichen simplex chronicus). Following initial evaluation, the etiology of the pruritus is categorized as dermatologic, systemic, neurologic, psychogenic, mixed, or other/ unknown. Such classification assists in planning further evaluation and treatment of the underlying disease (if possible) as well as the pruritus itself. Recent advances in our understanding of the mechanisms underlying chronic pruritus have led to the development of targeted therapies, and additional promising drugs are expected in the near future.