Table 1.

Types of itch

SystemicItch from noncutaneous organ systems (eg, cholestasis, kidney disease, myeloproliferative disorders, hyperthyroidism)
  • Central nervous system transmission

  • No peripheral nerve input

  • Causes include hematologic, renal, hepatic, and drug-induced

PsychogenicItch from a disorder of the mind (eg, delusions of parasitosis, formication)
  • Causes include obsessive-compulsive disorder, depression, anxiety, somatic symptom disorders, psychosis, substance use

NeuropathicItch from central or peripheral nerve damage (eg, postherpetic neuralgia, brachioradial pruritus, notalgia paresthetica)
  • Similar causes to neuropathic pain

PruritoceptiveDermatologic itch (eg, xerosis, scabies, urticaria, reactions to insect bite)
  • Transmitted by slow, unmyelinated group C nerve fibres (nerve roots in the epidermis, dedicated to itch and separate from pain-conducting group C nerve fibres)

  • Poorly understood

  • Keratinocytes interact with pruritogens such as histamine (and many others)

  • Data from Twycross et al,3 Nowak and Wong,4 and Yosipovitch et al.5