Scratching and Noxious Heat Stimuli Inhibit Itch in Humans
Background: 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.
Objectives: 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.
Patients and Methods: 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.
Results: 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.
Conclusions: 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.
Itch is a complex unpleasant sensory experience with many similarities to pain. Both experiences are multidimensional with sensory discriminative, cognitive, evaluative and motivational components. Much like pain, multiple lines of evidence indicate that itch can be modulated by thermal stimuli as well as mechanical stimuli. Firstly, patients suffering from itch alleviate their itch by cooling the skin or by applying noxious heat. Secondly, Bickford observed more than 60 years ago that noxious heat as well as painful mechanical and electrical stimuli inhibit experimental histamine-induced itch. Bickford demonstrated that histamine did not cause itching or alloknesis when delivered within 10 cm of a noxious thermal stimulus and induced diminished itch when the counterstimulus was beyond 10 cm. Graham et al. demonstrated that remote noxious mechanical counterstimuli (pinprick) reduced itch when confined to the same dermatome as the pruritic stimulus. Murray and Weaver showed that a noxious cold stimulus (2 °C) reduced itch, even when delivered to the fingertips contralateral to the pruritic stimulus. More recently, several studies have shown that local noxious thermal stimuli significantly reduced experimental itch. It has been suggested that these thermal stimuli may exert a central inhibitory effect.
If thermal modulation of itch is, in part, centrally mediated, then thermal stimuli applied outside of an itchy skin region would be predicted to reduce itch. However, few quantitative psychophysical data are currently available. No previous studies have continuously measured subjects' itch perception using quantitative psychophysical techniques, such as a computerized visual analogue scale (VAS). Moreover, minimal information exists about the neural mechanisms by which scratching inhibits itch. As in the case of thermal stimuli, if scratching-induced modulation of itch relies, in part, on a central mechanism then scratching outside of the itchy area would have a similar effect. Therefore, the purpose of the current study was to use quantitative psychophysical techniques to address whether the sensory perception of itch is attenuated by interactions between both thermal and mechanical stimuli and afferent information related to itch. As pain and itch involve similar negative affective and attentional components that may influence the perception of these modalities, a second objective was to assess whether inter-individual differences in itch perception were related to inter-individual differences in pain sensitivity.
Summary and Introduction
Summary
Background: 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.
Objectives: 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.
Patients and Methods: 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.
Results: 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.
Conclusions: 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.
Introduction
Itch is a complex unpleasant sensory experience with many similarities to pain. Both experiences are multidimensional with sensory discriminative, cognitive, evaluative and motivational components. Much like pain, multiple lines of evidence indicate that itch can be modulated by thermal stimuli as well as mechanical stimuli. Firstly, patients suffering from itch alleviate their itch by cooling the skin or by applying noxious heat. Secondly, Bickford observed more than 60 years ago that noxious heat as well as painful mechanical and electrical stimuli inhibit experimental histamine-induced itch. Bickford demonstrated that histamine did not cause itching or alloknesis when delivered within 10 cm of a noxious thermal stimulus and induced diminished itch when the counterstimulus was beyond 10 cm. Graham et al. demonstrated that remote noxious mechanical counterstimuli (pinprick) reduced itch when confined to the same dermatome as the pruritic stimulus. Murray and Weaver showed that a noxious cold stimulus (2 °C) reduced itch, even when delivered to the fingertips contralateral to the pruritic stimulus. More recently, several studies have shown that local noxious thermal stimuli significantly reduced experimental itch. It has been suggested that these thermal stimuli may exert a central inhibitory effect.
If thermal modulation of itch is, in part, centrally mediated, then thermal stimuli applied outside of an itchy skin region would be predicted to reduce itch. However, few quantitative psychophysical data are currently available. No previous studies have continuously measured subjects' itch perception using quantitative psychophysical techniques, such as a computerized visual analogue scale (VAS). Moreover, minimal information exists about the neural mechanisms by which scratching inhibits itch. As in the case of thermal stimuli, if scratching-induced modulation of itch relies, in part, on a central mechanism then scratching outside of the itchy area would have a similar effect. Therefore, the purpose of the current study was to use quantitative psychophysical techniques to address whether the sensory perception of itch is attenuated by interactions between both thermal and mechanical stimuli and afferent information related to itch. As pain and itch involve similar negative affective and attentional components that may influence the perception of these modalities, a second objective was to assess whether inter-individual differences in itch perception were related to inter-individual differences in pain sensitivity.
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