Itch

Itch is a common sensation that drives an intense urge to scratch. Itch and scratching have persisted in humans and many other species, suggesting that they play an important role in survival. This commentary discusses the function of itch as a danger signal, why itch feels unpleasant, and why we feel relief and pleasure when we scratch an itch. In addition, we explore the ways that this system becomes dysfunctional in conditions of chronic itch and important implications for treatment. This article is protected by copyright. All rights reserved.

As well-established for patients with chronic pain, patients suffering from chronic itch also exhibit signs of peripheral and central sensitization. This has been linked to parallel neuroplastic sensitization processes. However, for chronic itch, sensitization has not yet been systematically assessed, studied, and hence validated. This review (Prospero CRD42016043002) summarizes and meta-analytically evaluates whether sensory aberrations including sensitization for itch occur in chronic itch.Databases PubMed, Embase, and Cochrane Library were searched for studies investigating somatosensory sensitivity assessment by quantitative sensory testing stimuli, including experimental cutaneous chemical pruritic provocations, in patients with chronic itch from skin-/neurological conditions and compared with healthy controls. Outcomes were extracted for lesional and non-lesional skin and risk of biases were assessed. Meta-analyses were performed when sufficient quantitative data were available.Of 4,667 identified papers, 46 were included and 25 were eligible for meta-analyses. Patients (66% atopic dermatitis) were found more sensitive than the controls to histamine-evoked itch in lesional skin (SMD: 0.66 [CI: 0.16,1.15]), but not non-lesionally (SMD: -0.26 [CI: -0.58;0.06]). Cowhage did not evoke more itch in non-lesional skin of patients as compared to the controls (SMD: 0.38 [CI: -0.04,0.81]). For numerous other chemical provocations as well as for mechanical, thermal, and electrical stimulation paradigms, results were ambiguous or based on few studies.Patients with chronic itch are only robustly sensitized to various chemical pruritic stimuli when applied lesionally. More studies on somatosensory aberrations in chronic itch conditions other than atopic dermatitis are needed to establish whether sensitization is robustly present across chronic itch conditions.

Itch is often regarded as unpleasant or bothersome and is accompanied by symptoms of distress and impairments in daily life. The biopsychosocial model of chronic itch describes how psychological factors can contribute to the improvement or exacerbation of chronic itch and related scratching behavior. Recent research underlines the important role of cognitive-affective information processing, such as attention, affect, and expectancies. This may not only play a role for acute itch states, but may particularly apply to the process of itch chronification, e.g., due to the vicious cycle in which these factors shape the experience of itch. The present paper focuses on new insights into the relation between itch and the cognitive-affective factors of attention, affect, and expectancies. These factors are thought to play a possible aggravating role in itch in the long term and have received increasing attention in the recent empirical literature on maintaining and exacerbating factors for chronic physical symptoms. Possible psychophysiological and neurobiological pathways regarding these factors are discussed, as well as possible intervention methods. This article is protected by copyright. All rights reserved.

Sensitive skin is defined by the occurrence of unpleasant sensations such as tingling, burning, tautness, itching or pain. Mechanisms explaining sensitive skin are controversial, and many hypotheses have been proposed. Because sensitive skin is primarily characterized by a wide variety of neuropathic-like symptoms, it is highly likely that neurosensory dysfunction in the skin represents one of the pathological mechanisms of sensitive skin. This hypothesis does not exclude other explanations like role of keratinocyte, transient receptor potential channels, vasculature or environmental factors. Nevertheless, the role of the nervous system in the development of sensitive skin is crucial, and growing evidence supports this hypothesis. Pain and pruritus described by patients with sensitive skin corresponds to neuropathic component, and its assessment shows an increase of neuropathic measures (DN-4, Douleur Neuropathique-4) compared to control. These sensations are similar to the sensations observed in small-fiber neuropathy (SFN), which is a group of disorders that affect thin nerve fibers. One study on the pathophysiology of sensitive skin demonstrated that intraepidermal nerve fiber density, especially of peptidergic C-fibers, was lower in the sensitive skin group. A recent study showed a modification in heat-pain detection threshold in patients with sensitive skin. All these results indicate that C-fiber damage can help explain sensitive skin. Consequently, the role of the nervous system is increasingly obvious. Nevertheless, keratinocytes and other epidermal cells closely participate in sensory transduction. Therefore, the results of neurophysiological studies should be interpreted in the light of this information that the whole epidermis represents a huge polymodal nociceptor. This article is protected by copyright. All rights reserved.