The role of itch and pain modulation in the prediction of phototherapy outcomes: a prospective cohort study.

OBJECTIVES: Ability to predict which chronic itch patients will benefit from particular treatments is a challenge. Common features in itch and pain in respect to sensory elicitation, and mechanisms of processing including sensitization and inhibition at the peripheral and central levels, may serve to understand variability in treatment outcomes. As such this study aimed to explore whether phototherapy outcomes can be predicted by psychophysical parameters of pain and itch modulatory processing. METHODS: Prospective cohort study on chronic-itch patients (n = 44) assessed before 20 treatments of NB UVB. Level of itch and pain reduction following painful stimulation (reflecting the 'pain inhibits pain' phenomenon) used to assess the top-down modulation response efficacy. Magnitude of Conditioned Pain Modulation (CPM) for itch (CPM-itch) and for pain (CPM-pain) (reflecting inhibition) and magnitude of temporal summation (TS) of pain (reflecting ascending facilitation pathways) assessed to predict treatment effect. RESULTS: Higher improvement of itch symptoms following phototherapy was correlated with more efficient CPM-itch (r = 0.62, p < .001), but not magnitude of CPM-pain or level of temporal summation. DISCUSSION: Findings emphasize the role of descending inhibition pathways in determining phototherapy efficacy in chronic itch patients. Such an evaluation-based approach may contribute to better patient selection for phototherapy improving patients' disease outcomes.

Enhanced Itch Intensity Is Associated with Less Efficient Descending Inhibition Processing for Itch But Not Pain Attenuation in Chronic Dermatology Patients.

OBJECTIVES: The study aims were 1) to investigate the direction of mutual inhibitory pathways on itch intensity by utilizing conditioned pain modulation paradigms for pain and itch attenuation and 2) to explore whether itch severity is affected by the individual pain sensitivity profile, as well as pain scores reported during the tests and the past week. DESIGN: Cross-sectional. SETTING: Testing was conducted at the Department of Dermatology, Rambam Health Care Campus. SUBJECTS: Forty patients suffering from chronic skin disorders associated with itch and treated in the Dermatology Clinic at Rambam Health Care Campus participated in the study. METHODS: Efficacy of descending inhibition was evaluated by two conditioned pain modulation (CPM) paradigms: by pruriception (CPMItch) induced by cold and heat as counterstimuli to inhibit itch intensity and by nociception (CPMPain). Severity and interference of clinical pain were assessed using the Brief Pain Inventory (BPI). RESULTS: Robust CPMItch responses were obtained following the various noxious stimulations. No associations were observed between CPMPain and CPMItch, itch severity, skin disease severity, and clinical pain symptoms. According to the linear regression model, itch severity was independently associated with less efficient CPMItch (B = -0.750, P < 0.001) and more efficient CPMPain (B = 0.031, P = 0.016), which affects itch in opposing manners. CONCLUSIONS: Findings indicate that the intrinsic capacity to inhibit pain and itch by exposure to exogenous noxious stimuli autonomously affects itch intensity in an opposing manner. These findings may shed new light on the mutual mechanistic similarity and dissimilarity between pain and itch and their hierarchy.