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.