L-menthol as a Topical Counter-irritant to TRPA1-induced Neurogenic Inflammation and Pain

Rating of the overall pain intensity on a visual analog scale (VAS ranging from "No pain" = 0, to "worst imaginable pain" =10) once every minute

Speckle contrast flowmetry (MoorFLPI, Moor Instruments, Devon, UK). The assessment is conducted with a 30-cm distance between the head of the laser and the application area with an exposure time of 8.3ms. Single frame images are analyzed on appertaining software (MoorFLPI Review V 4.0, Moor Instruments), upon which the arithmetic mean flux (arbitrary units) is calculated. The longitudinal analysis of the spatial dispersion of neurogenic inflammation is performed using the line histogram tool. A 7-cm line centered in the area of application was placed longitudinally along the volar forearm and the perfusion intensity is recorded every 2.5 mm.

Heat pain threshold assessments is performed with a Medoc Pathway (Medoc Ltd, Ramat Yishay, Israel) equipped with a 3 × 3 cm advanced thermal stimulator probe where the baseline temperature was set to 32 °C. Ramp stimuli of 1 °C/s are delivered and the subjects are asked to identify the heat pain threshold upon which the temperature is returned to the baseline at a rate of 5 °C/s. The test result is calculated as the arithmetic mean of the outcome from three repeated stimuli. Heat hyperalgesia is considered to be a significant drop in heat pain threshold

To evaluate the mechanical pain threshold (MPT) an electronic von Frey transducer and an electronic coVAS connected to a SENSE-Box setup is used (both items: Somedic, Hörby, Sweden). Five ramp stimuli from 0-110 g at a rate of 5g/1 sec with the default stimulus probe is conducted at different locations within the application area. The subjects are instructed to continuously rate the perceived pain on the coVAS ranging from "No pain" (0) to "worst imaginable pain" (10). The arithmetic mean of the VAS scores yields three MPTs (fixed to VAS = 0.5, 1 and 2) and an area-under-the-curve (AUC).