Itch Sensitization Research Articles

Generalized and symptom‐specific sensitization of chronic itch and pain

Physicians are frequently confronted with patients reporting severe itch and pain. Particularly in patients suffering from persistent itch and pain, central and peripheral sensitization processes are assumed to be involved in the long-term maintenance and aggravation of the symptoms. The present study explores generalized and symptom-specific sensitization processes in patients suffering from persistent itch and pain. Specifically, it examines whether patients with chronic itch and pain are more sensitive to somatosensory stimuli (generalized sensitization) and simultaneously perceive somatosensory stimuli as a symptom of their main physical complaint, e.g. pain in chronic pain patients (symptom-specific sensitization). Thresholds for different mechanical and electrical sensory stimuli of Quantitative Sensory Testing were determined in 15 female patients suffering from chronic itch associated with atopic dermatitis, 15 female chronic pain patients diagnosed with fibromyalgia, and 19 female healthy controls. Intensities of itch and pain sensations were rated on a visual analogue scale. As expected, the patient groups had significantly lower tolerance thresholds for the somatosensory stimuli applied than the healthy controls, supporting generalized sensitization. Moreover, patients with chronic itch consistently reported more itch, while patients with chronic pain partly reported more pain in response to analogous somatosensory stimuli than the healthy controls and the other patient group, indicating symptom-specific sensitization. The present study provides preliminary support that both generalized and symptom-specific sensitization processes play a role in the regulation and processing of somatosensory stimulation of patients with chronic itch and pain.

Scratching and noxious heat stimuli inhibit itch in humans: a psychophysical study

Patients who suffer from chronic itch employ creative techniques to alleviate their itch, often using painful thermal stimuli, such as hot and very cold showers, as well as mechanical stimuli, such as scratching. The present study examined whether the sensory perception of itch is attenuated by remote interactions between both thermal and mechanical stimuli and afferent information related to itch. Itch was induced with histamine iontophoresis in 21 healthy young subjects. Repetitive thermal stimuli including innocuous warmth, innocuous cool, noxious cold and noxious heat as well as scratching were applied 3-cm distal to the area of histamine iontophoresis. Subjects rated their perceived intensity of histamine-induced itch with a computerized visual analogue scale. Itch intensity ratings were significantly reduced during each period of scratching and repeated noxious heat and cold. Innocuous cooling and warming did not significantly alter itch intensity ratings. Inter-individual differences in histamine-induced itch sensitivity were unrelated to inter-individual differences in pain sensitivity. The present psychophysical study demonstrates that repetitive noxious thermal and scratching stimuli inhibit itch and do not require direct physical interaction with the area of the skin from which itch originates.

Frontiers in pruritus research: scratching the brain for more effective itch therapy

This Review highlights selected frontiers in pruritus research and focuses on recently attained insights into the neurophysiological, neuroimmunological, and neuroendocrine mechanisms underlying skin-derived itch (pruritogenic pruritus), which may affect future antipruritic strategies. Special attention is paid to newly identified itch-specific neuronal pathways in the spinothalamic tract that are distinct from pain pathways and to CNS regions that process peripheral pruritogenic stimuli. In addition, the relation between itch and pain is discussed, with emphasis on how the intimate contacts between these closely related yet distinct sensory phenomena may be exploited therapeutically. Furthermore, newly identified or unduly neglected intracutaneous itch mediators (e.g., endovanilloids, proteases, cannabinoids, opioids, neurotrophins, and cytokines) and relevant receptors (e.g., vanilloid receptor channels and proteinase-activated, cannabinoid, opioid, cytokine, and new histamine receptors) are discussed. In summarizing promising new avenues for managing itch more effectively, we advocate therapeutic approaches that strive for the combination of peripherally active antiinflammatory agents with drugs that counteract chronic central itch sensitization.

Itch and Atopic Dermatitis : The Role of Sensitization for Itch

Itch and Atopic Dermatitis : The Role of Sensitization for Itch

Neuronal sensitization for itch in patients with chronic pruritus

Itch is one of the major symptoms of various skin diseases. Although specific neuronal pathways for itch were identified both peripherally and centrally, they still fail to explain itchy skin observed in patients with chronic pruritus. In this study, sensitivity to itchy and painful stimuli in patients with atopic dermatitis was investigated. Histamine‐prick evoked enormous itch in their lesional skin, while less itch in their non‐lesional skin than healthy subjects. Flare reaction was not significantly different between their non‐lesional and lesional skin, rather smaller than healthy subjects. Mechanical (pin‐pricks), electrical, heat and chemical (injection of pH3 solution) stimuli evoked intense itch in their lesional skin and partly also in their non‐lesional skin, while only pain in healthy subjects. Itch was also, but not intensely, evoked in healthy subjects by injection of pH3 solution after sufficient histamine stimuli. These results confirm the presence of itchy skin with hyperkinesis (excessive itch by itchy stimuli) and allokinesis (itch by non‐itchy stimuli) in patients with atopic dermatitis, which is so intense that painful stimuli cannot suppress but evoke itch, and suggest that neuronal sensitization is involved in their itch not only peripherally but also centrally.