Basic Mechanisms of Itch and Advances in Clinical Management.

Abstract

Chronic pruritus requires comprehensive assessment and management due to its profound impact on quality of life. The etiologies of pruritus are diverse, encompassing dermatologic, neuropathic, systemic, psychogenic, and mixed causes. Pruritogens activate C and Aδ fibers through histaminergic and non-histaminergic pathways. Increasing evidence highlights the role of Th2 cytokines, particularly interleukin (IL)-4, IL-13, and IL-31, which directly stimulate sensory neurons and perpetuate the itch-scratch cycle. Itch perception arises from dynamic interactions between the immune system and the peripheral and central nervous systems. Signals transmitted by sensory fibers are processed in the dorsal horn and relayed to thalamic and cortical centers, reinforcing chronicity. Diagnostic evaluation begins with detailed history and examination, complemented by laboratory testing such as complete blood count, kidney, liver, glucose, and thyroid function tests to uncover systemic or neurologic contributors. For dermatologic conditions, topical immunomodulators-including corticosteroids, calcineurin inhibitors, and PDE4 inhibitors- are first-line options for localized itch. Extensive itch may require systemic immunosuppressants or phototherapy. Biologic agents targeting IL-4/IL-13 and IL-31 pathways have revolutionized treatment for atopic dermatitis and prurigo nodularis, while JAK-STAT inhibitors also offer significant antipruritic effect in atopic dermatitis and beyond. Neuropathic itch may respond to topical anesthetics: menthol, pramoxine, capsaicin, or compounded ketamine-amitriptyline-lidocaine formulations, as well as systemic gabapentinoids and antidepressants. Systemic or intractable pruritus may benefit from kappa opioid receptor agonists. New drugs targeting the mast cell such as BTK inhibitors, c-KIT inhibitors, and MRGPRX2 antagonists have robust anti-pruritic effects in mast cell-mediated diseases. Supportive measures, including psychosocial interventions, remain integral to long-term management.