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The inhibition of
pain, as well as itch, was effectively induced by conditioning pain stimulation
while conditioning itch stimulation was unable to cause a reduction in pain or
itch.
There is little information
regarding endogenous descending control of itch. In chronic pain, the
modulation of lowered conditioned pain reveals a reduction in descending pain
inhibition. It has also been indicated that patients with constant irritation
also possess reduced endogenous descending inhibition of itch and pain. In this
study, Hjalte H. Andersen et al. investigated the level of itch modulation by
itch conditioning and pain stimuli.
A total of 26 healthy volunteers were enrolled. The criteria for evaluation of
endogenous modulation of itch or pain in the study include 1) modulation of
contralateral irritation induced by itch and pain 2) modulation of ipsilateral
itch produced by pain 3) modulation of contralateral pain caused by pain and
itch. The cold pressor-induced pain and histamine-evoked itch were used as
conditioning stimuli, whereas electrical stimulation paradigms designed to
evoke itch or pain were used as test stimuli.
The results of study specify that there was a significant reduction in pain
(conditioned pain modulation-effect) caused by the conditioning pain stimulus
(P < .001) but not by the conditioning itch stimulus (negative control
condition). Reduction in itch (conditioned itch modulation-effect) was induced
by contra- as well as ipsilateral applied conditioning pain (both P< .001),
whereas conditioning itch stimulation was induced by only marginally reduced
itch. Heterotopic conditioning pain stimulation evoked endogenous descending
itch inhibition through mechanisms that are independent of segmental gating.
It was concluded that there occurs a hierarchical prioritisation favouring
pain-induced central descending modulation of itch as well as pain in humans,
but there is a need of future studies that address potential aberrations in the
pain-evoked descending modulation of itch in chronic itch patients.
The Journal of Pain
Modulation of itch by conditioning itch and pain stimulation in healthy humans
Hjalte H.Andersen et al.
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