Definitions - Pruritus is defined as an unpleasant sensation within the skin that produces the desire to scratch. "Itch" is a primary sensation resulting from stimulation of free, unmyelinated penicillate nerve endings associated with the epidermis. The pruritic threshold is the level of stimulation required to elicit pruritus, this varies from patient to patient. Pruritic stimuli have a summation effect with more stimuli enhancing the sensation of pruritus. Epicritic itch is a spontaneous, sharp, well-demarcated, "pricking" pruritus which is transmitted over A delta fibers (myelinated, conduct at 15 m/sec). Protopathic itch is a poorly localized, "burning" pruritus that is fransmitted over C fibers (nonmyelinated, conduct at 2 m/sec). Referred itchis a sensation of pruritus that occurs somewhere else on the body when a primary itch is scratched; in humans, it is usually ipsilateral and occurs at a higher segmental level. Paroxysmal pruritus is bursts of severe itching with an increased pain threshold at the site so pleasure rather than pain is elicited.
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