Scratching and noxious heat stimuli inhibit itch in humans: a psychophysical study

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Abstract
Patients who suffer from chronic itch employ creative techniques to alleviate their itch, often using painful thermal stimuli, such as hot and very cold showers, as well as mechanical stimuli, such as scratching. The present study examined whether the sensory perception of itch is attenuated by remote interactions between both thermal and mechanical stimuli and afferent information related to itch. Itch was induced with histamine iontophoresis in 21 healthy young subjects. Repetitive thermal stimuli including innocuous warmth, innocuous cool, noxious cold and noxious heat as well as scratching were applied 3-cm distal to the area of histamine iontophoresis. Subjects rated their perceived intensity of histamine-induced itch with a computerized visual analogue scale. Itch intensity ratings were significantly reduced during each period of scratching and repeated noxious heat and cold. Innocuous cooling and warming did not significantly alter itch intensity ratings. Inter-individual differences in histamine-induced itch sensitivity were unrelated to inter-individual differences in pain sensitivity. The present psychophysical study demonstrates that repetitive noxious thermal and scratching stimuli inhibit itch and do not require direct physical interaction with the area of the skin from which itch originates.