Speaker
Description
Introduction: Chronic itch is a clinically significant condition for which effective treatments remain limited. Experimentally induced itch using histamine and cowhage provide a controlled and reproducible way to study the mechanisms of itch, helping identify and test new therapeutic targets. However, histamine and cowhage differ in several characteristics, including their ability to elicit secondary neurogenic flares (vascular changes of the skin). This study aimed to compare three different methodologies used to assess neurogenic flare: Laser speckle contrast imaging (FLPI-2), optical coherence tomography angiography (OCTA), and infrared-thermography (IR), and to study if they could differentiate between histamine and cowhage induced neurogenic flare.
Methods: Four 4x4cm areas on the forearms of 9 participants were selected. Two areas were treated with histamine or cowhage application for 10 and 5 minutes respectively, while two other areas were used as controls. The blood perfusion and temperature were assessed using FLPI-2, OCTA, and IR-thermography before and after histamine or cowhage application. The mean superficial blood perfusion (SBP) was extracted from the FLPI data, the mean temperature was extracted from the IR data, while the Vessel Area Density (VAD) was extracted from the OCTA data.
Results: All three methodologies showed a significant difference in flare from baseline (BL) to post-treatment (PT) measurements for histamine (OCTA, Thermography, FLPI-2: p<0.0001). Meanwhile no significant change from BL to PT was seen for cowhage (OCTA: p=0.26; Thermography: p=0.88; FLPI-2: p=0.54). One-way ANOVA indicated a significant difference between cowhage and histamine at PT (OCTA: p=0.0024; IR: p<0.0001; FLPI-2: p<0.001) and a paired t-test showed that histamine caused a larger flare than cowhage (OCTA: 95% CI [8.13-21.07%], p=0.0008; IR: 95% CI [1.49-2.60 °C], p<0.0001; FLPI-2: 95% CI [91.20-180.73 A.U.] p<0.001). No baseline differences were observed between cowhage and histamine (OCTA: 95% CI [-7.43-2.78%], p=0.3242; IR: 95% CI [-0.12-0.44 °C], p=0.2292; FLPI-2: 95% CI [-15.20-15.20 A.U.], p=1.0).
Conclusions: This pilot study demonstrates that all three methodologies were able to detect histamine-induced neurogenic flare, showing a significant increase from baseline to post-treatment measurements. None of the methods detected a flare response to cowhage. These findings support the potential clinical use of OCTA, IR thermography and FLPI-2 for assessing vascular changes associated with itch-related conditions. Each technique assesses distinct aspects of neurogenic flare, which can provide insight into underlying differences of peripheral pathological mechanisms and how pharmaceutical treatment act in different layers of the skin. Hence, the presentation will explore how the techniques can differentiate neurogenic flares.
