Abstract
Background
Chronic itch is notoriously difficult to treat. Counter‐stimuli are able to inhibit itch, but this principle is difficult to apply in clinical practice, and the mechanisms behind counter‐stimulation‐induced itch suppression in humans are unclear.
Objectives
1) To analyse the stimulus‐response effects of transient heat stimuli on histaminergic and non‐histaminergic itch; 2) to investigate whether the antipruritic effect depends on homotopic (peripheral mediation) versus heterotopic (central mediation) counter‐stimulation relative to the itch provocation site.
Methods
18 healthy subjects (8 females, 25.7±0.8 y.o.) participated. Itch was evoked on pre‐marked areas of the volar forearms, by either histamine (1% solution), or cowhage (35‐40 spicules). In addition to the itch provocations (Experiment 1), 5‐seconds homotopic heat stimuli of 32, 40, 45 or 50°C were applied. In Experiment 2, heat stimuli were applied either homotopically, intra‐segmentally (next to the provocation site), or extra‐segmentally (dorsal forearm). Itch intensity was evaluated throughout the procedures using a digital Visual Analog Scale.
Results
Homotopic counter‐stimuli inhibited histaminergic itch by 41.27% at 45°C (p<0.01), and by 76.66% at 50°C (p<0.0001). Cowhage‐induced itch was less prone to counter‐stimulation and was only significantly diminished at 50°C by 43.60% (p=0.009). Counter‐stimulations applied heterotopically were not able to significantly inhibit itch.
Conclusions
Itch pathway‐specific effects of counter‐stimuli were observed between homo‐ and heterotopic stimulation. Histaminergic itch was robustly inhibited by short‐term homotopic noxious heat stimuli for up to 10 minutes. Non‐histaminergic itch was only weakly inhibited. The inhibitory effects exerted by the short‐term heat stimuli only occurred following homotopical counter‐stimulation.
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