Electrically evoked itch in humans

Abstract

We compared itch sensations and axon reflex flare induced by transcutaneous electrical (0.08–8 ms, 2–200 Hz) and chemical (histamine iontophoresis; 100 μC) stimulation. Stimuli were applied to non-lesional volar wrist skin in 20 healthy human subjects and 10 patients with atopic dermatitis. Intensity of evoked itch and pain sensations were rated on a numerical rating scale (NRS) of 0 (no sensation) to 10 (the maximum sensation imaginable). The axon reflex erythema was measured by laser Doppler imager and areas of alloknesis (itch evoked by light brushing) and hyperknesis (itch evoked by pricking) were assessed psychophysically. Electrical stimulation was most effective for stimulus durations ≥2 ms and frequencies ≥50 Hz. It evoked pure itch as threshold sensation in 80% of the subjects that was perceived with a delay of approximately 1 s. Itch intensities of up to 7/10 were not accompanied by an axon reflex flare. In contrast, histamine provoked a massive increase of axon reflex erythema and maximum itch ratings of 3.1±0.2. The extention of alloknesis areas (2.3±0.5 cm) evoked by electrical stimulation clearly exceeded those induced by histamine (0.7±0.3 cm). Healthy subjects and patients with atopic dermatitis did not differ significantly in their response to either stimulation. We conclude that C-fiber activation underlies the electrically evoked itch sensation. The low electrical thresholds and the absence of an axon reflex flare suggest that these fibers are not identical with the previously described mechano-insensitive histamine responsive C fibers, but represent a separate peripheral neuronal system for the induction of itch.