Painful stimuli evoke itch in patients with chronic pruritus - Central sensitization for itch

Background: Central sensitization for pain is important for patients with chronic pain. The authors investigated a possible role of central sensitization for itch in patients with chronic pruritus. Methods: Noxious stimuli were applied in lesional and visually nonlesional skin areas of 25 patients with atopic dermatitis, in lesional skin areas of 9 patients with psoriasis vulgaris, and in 20 healthy subjects. The stimuli included mechanical pinpricks, electrical stimuli, contact heat, and injection of low-pH solution. Intensities of itch and pain were assessed separately on a numeric rating scale. Results: All the noxious stimuli primarily evoked pain in control subjects and patients with psoriasis vulgaris. In patients with atopic dermatitis, however, itch was evoked instead of burning pain. In their lesional skin, itch was the predominant sensation. Chemical stimuli evoked intense itch in lesional and visually healthy skin areas (the area under the curve of itch rating compared with the control, mean +/-SEM, 668+/-166 and 625+/-192 vs 38+/-23; p<0.001; p<0.01). Chemically induced itch also was observed in healthy subjects after a conditioning histamine stimulus of 15 minutes, but not after a conditioning histamine stimulus of 2 minutes. Conclusion: The chronic barrage of pruriceptive input may elicit central sensitization for itch so that nociceptive input no longer inhibits itch but on the contrary is perceived as itch. In contrast to the well-known A-fiber-mediated alloknesis and hyperknesis, this type of central sensitization appears to be elicited by C-nociceptors.