The Measurement of Itch

Abstract

Itch is a uniquely cutaneous sensation related to the superficial organisation of pain sensory nerves in the skin (see Chap. 6; see also Chap. 10 in Part 1 of this volume). It has been suggested that its anatomical localisation and relationship to pain and scratch are likely to have evolved as an integrated response to irritants such as parasites, successful removal of which by scratching away the superficial layers of the skin is signalled when itch is replaced by pain (Shuster 1981). Itch and skin appearance are the main symptoms of cutaneous disease and the treatment of itch is therefore a major concern. Although treatment of itch is primarily that of the causal disease, e.g. the itch of scabies or the rash of dermatitis herpetiformis, anti-pruritics are used during the early stages of response of several diseases and in those diseases where control is incomplete (e.g. see Chap. 32). Thus, the development of more potent and specific anti-pruritic drugs and the selection of the most satisfactory drugs from amongst those already available is desirable and to this end methods for the measurement of itch are required. Since by its very nature sensation cannot be measured directly, either the sensation is recorded subjectively, but directly by the individual experiencing it or else its consequences are measured objectively, but indirectly. The former integrates quality and magnitude of stimulus with its interpretation which includes cortical modification; the latter simply measures the consequence of a response which, depending on the type of measurement, may be subject to cortical modulation.