Some aspects of the experimental induction and measurement of itch

Two different rating scales--a visual analogue scale (VAS) connected to a chart-recorder, and Pain-Track, a micro-computerized system with a 7-step-graded, fixed-point, non-verbal scale (FPNVS)--were evaluated for their capacity to assess experimental, histamine-induced itch continuously in 38 healthy subjects. The consequences for itch perception of using different injection sequences of various histamine concentrations were also investigated. A linear dose-response relationship was shown with random injection order for all subjective variables studied (itch latency and duration, maximal itch intensity, 'total itch index') with the VAS, but only for itch duration and 'total itch index' with the FPNVS. Using the VAS and injecting histamine solutions with increasing concentration, a significant dose-response curve was obtained for maximal itch intensity and 'total itch index', but when the same histamine stimuli were presented in the reverse (i.e. decreasing) order, there was no dose-response relationship. This indicates that central nervous system interaction may be unequally activated, depending on the order of different injected histamine stimuli. The objective variable flare was unaffected by the injection sequence. It is concluded that random injection order should be used in the assessment of itch sensation, in order to avoid systematic errors. The fact that the FPNVS did not discriminate as well as the VAS could indicate that our experimental stimuli were too weak to be properly discriminated with a 7-step-graded scale.