瘙痒定义为一种令人不快的、可导致强烈搔抓欲望的感觉。英文中pruritus和itching为同义词。瘙痒为皮肤科最常见症状，可伴或不伴可见皮疹。可为局限性或全身性。
阅读更多鉴别急性瘙痒和慢性瘙痒非常重要。瘙痒持续时间超过6周即为慢性瘙痒。根据病因，慢性瘙痒可分为皮肤病性的、全身性的、神经原性的、心因性/身心性、混合原因导致的及病因不明的。慢性瘙痒可令人非常苦恼，并对治疗抵抗。慢性瘙痒程度常与生活质量受损程度、耻辱感、抑郁严重程度及情绪压力相关。[1]Weisshaar E, Szepietowski JC, Darsow U, et al. European guideline on chronic pruritus. Acta Derm Venereol. 2012;92:563-581.http://www.medicaljournals.se/acta/content/?doi=10.2340/00015555-1400&html=1http://www.ncbi.nlm.nih.gov/pubmed/22790094?tool=bestpractice.com
依据目前普遍接受的临床分类，[2]Ständer S, Weisshaar E, Mettang T, et al. Clinical classification of itch: a position paper of the International Forum for the Study of Itch. Acta Derm Venereol. 2007;87:291-294.http://www.ncbi.nlm.nih.gov/pubmed/17598029?tool=bestpractice.com瘙痒患者可分为：原发疾病的炎症性皮肤瘙痒，原本正常的非炎症性皮肤瘙痒，及慢性继发性搔抓性病变。
流行病学
瘙痒很常见。德国一项涉及11,730人的大规模流行病学研究显示，慢性瘙痒（资料收集前至少6周）的时点患病率为16.8%，且患病率随年龄增长而增高，16-30岁为12.3%，61-70岁为20.3%。[3]Ständer S, Schäfer I, Phan NQ, et al. Prevalence of chronic pruritus in Germany: results of a cross-sectional study in a sample working population of 11,730. Dermatology. 2010;221:229-235.http://www.ncbi.nlm.nih.gov/pubmed/20924157?tool=bestpractice.com在一项基于自我报告的发病率研究中，瘙痒为皮肤最常见症状(7%)，而东亚男性、中东/北非男性的瘙痒报告率更高，前者为18%，后者为13%。[4]Dalgard F, Holm JO, Svensson A, et al. Self reported skin morbidity and ethnicity: a population-based study in a Western community. BMC Dermatol. 2007;7:4.http://bmcdermatol.biomedcentral.com/articles/10.1186/1471-5945-7-4http://www.ncbi.nlm.nih.gov/pubmed/17603893?tool=bestpractice.com另一项研究中，报告瘙痒的患者较为年轻，主要为女性，更为苦恼；这些患者收入较低、更缺乏社会支持、负面生活事件体验较多。[5]Dalgard F, Lien L, Dalen I. Itch in the community: associations with psychosocial factors among adults. J Eur Acad Dermatol Venereol. 2007;21:1215-1219.http://www.ncbi.nlm.nih.gov/pubmed/17894708?tool=bestpractice.com
瘙痒为许多皮肤疾病的常见症状。比如，瘙痒是特应性湿疹的主要症状，所有患者在其疾病过程中都曾有不同程度瘙痒。[6]Ständer S, Streit M, Darsow U, et al. Diagnostic and therapeutic procedures in chronic pruritus [in German]. J Dtsch Dermatol Ges. 2006;4:350-370.http://www.ncbi.nlm.nih.gov/pubmed/16638066?tool=bestpractice.com与此类似，大约70%到90%银屑病患者有瘙痒症状。[7]Gupta MA, Gupta AK, Kirkby S, et al. Pruritus in psoriasis. A prospective study of some psychiatric and dermatologic correlates. Arch Dermatol. 1988;124:1052-1057.http://www.ncbi.nlm.nih.gov/pubmed/3389849?tool=bestpractice.com[8]Yosipovitch G, Goon A, Wee J, et al. The prevalence and clinical characteristics of pruritus among patients with extensive psoriasis. Br J Dermatol. 2000;143:969-973.http://www.ncbi.nlm.nih.gov/pubmed/11069504?tool=bestpractice.com[9]Szepietowski JC, Reich A, Wisnicka B. Itching in patients suffering from psoriasis. Acta Dermatovenerol Croat. 2002;10:221-226.http://www.ncbi.nlm.nih.gov/pubmed/12588672?tool=bestpractice.com[10]Szepietowski JC, Reich A. Pruritus in psoriasis: an update. Eur J Pain. 2016;20:41-46.http://www.ncbi.nlm.nih.gov/pubmed/26415584?tool=bestpractice.com瘙痒也可见于其它系统性疾病，如慢性肾功能不全、[11]Manenti L, Tansinda P, Vaglio A. Uraemic pruritus: clinical characteristics, pathophysiology and treatment. Drugs. 2009;69:251-263.http://www.ncbi.nlm.nih.gov/pubmed/19275270?tool=bestpractice.com血液系统恶性肿瘤或肝脏疾病。例如，血液透析患者慢性瘙痒的发生率据估计介于 25% 和 35%。[12]Weiss M, Mettang T, Tschulena U, et al. Prevalence of chronic itch and associated factors in haemodialysis patients: a representative cross-sectional study. Acta Derm Venereol. 2015;95:816-821.http://www.medicaljournals.se/acta/content/?doi=10.2340/00015555-2087&html=1http://www.ncbi.nlm.nih.gov/pubmed/25740325?tool=bestpractice.com[13]Szepietowski JC, Sikora M, Kusztal M, et al. Uremic pruritus: a clinical study of maintenance hemodialysis patients. J Dermatol. 2002;29:621-627.http://www.ncbi.nlm.nih.gov/pubmed/12432992?tool=bestpractice.com
病理生理学
这取决于其基础疾病。许多不同的介质可诱发和调节瘙痒，包括组胺、乙酰胆碱、儿茶酚胺、血液激肽、趋化因子、细胞因子（白介素2、白介素31）、神经肽、内皮素、内辣椒素、内源性大麻素、下丘脑-垂体轴激素、激肽释放酶、蛋白酶、前列腺素、白三烯B4、神经营养肽及阿片类物质。[14]Paus R, Schmelz M, Biro T, et al. Frontiers in pruritus research: scratching the brain for more effective itch therapy. J Clin Invest. 2006;116:1174-1186.http://www.jci.org/articles/view/28553http://www.ncbi.nlm.nih.gov/pubmed/16670758?tool=bestpractice.com
瘙痒的特定神经元通路已明确。瘙痒刺激主要由机械不敏感的无髓鞘C传入神经纤维传导，该纤维传导速度特别慢、神经支配面积大、经皮电阈值高。
在脊髓，瘙痒刺激由脊髓背角的特定瘙痒接受神经元传导至丘脑腹内侧核后部，后者投射至背侧岛叶皮质。脊髓内特异的瘙痒通路神经元已经明确，该神经元表达胃泌素释放肽受体。[15]Sun YG, Zhao ZQ, Meng XL, et al. Cellular basis of itch sensation. Science. 2009;325:1531-1534.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2786498/http://www.ncbi.nlm.nih.gov/pubmed/19661382?tool=bestpractice.com[16]Akiyama T, Tominaga M, Takamori K, et al. Roles of glutamate, substance P, and gastrin-releasing peptide as spinal neurotransmitters of histaminergic and nonhistaminergic itch. Pain. 2014;155:80-92.http://www.ncbi.nlm.nih.gov/pubmed/24041961?tool=bestpractice.com[17]Papoiu AD, Coghill RC, Kraft RA, et al. A tale of two itches. Common features and notable differences in brain activation evoked by cowhage and histamine induced itch. Neuroimage. 2012;59:3611-3623.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3288667/http://www.ncbi.nlm.nih.gov/pubmed/22100770?tool=bestpractice.com诱发的瘙痒刺激同时激活前扣带皮层、辅助运动区及主要位于左半球的下顶叶。
随着瘙痒感的产生，大脑内多个活化的区域对单一瘙痒中心作出应对，反应出瘙痒的多维性。[18]Steinhoff M, Bienenstock J, Schmelz M, et al. Neurophysiological, neuroimmunological, and neuroendocrine basis of pruritus. J Invest Dermatol. 2006;126:1705-1718.http://www.jidonline.org/article/S0022-202X(15)33013-X/fulltexthttp://www.ncbi.nlm.nih.gov/pubmed/16845410?tool=bestpractice.com重要的是，已证实在慢性瘙痒患者中瘙痒刺激引起的脑活动与健康对照受试者所观察到的显著不同。[19]Ishiuji Y, Coghill RC, Patel TS, et al. Distinct patterns of brain activity evoked by histamine-induced itch reveal an association with itch intensity and disease severity in atopic dermatitis. Br J Dermatol. 2009;161:1072-1080.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2784001/http://www.ncbi.nlm.nih.gov/pubmed/19663870?tool=bestpractice.com
