Protocol

Each participant was examined during 3 successive visits separated by at least 3 days. Visits 1 and 2 were for recording the histamine flare at various locations, whereas quantitative sensitivity testing (QST) was performed on visit 3. The same investigator (the first author, I.C.-R.) performed all examinations between 8 and 12 am, in a quiet temperature-controlled room (22°C–24°C), with the participant lying comfortably on a hospital bed.

On visit 1, 4 pairs of skin zones were chosen, 1 on each forearm and 1 on each calf. Zones within each pair were at least 10 cm apart and were both allocated to receive histamine iontophoresis, with a current intensity of 20 μA on 1 zone and 100 μA on the other. On the upper and lower limbs, treated zones at both intensities were placed as symmetrically as possible and their location recorded on transparent film. A total of 8 histamine flares were thus recorded: 2 intensities on each of the 4 limbs. The order of rotation between limbs and for intensities on limbs was randomized. The whole procedure including setup, acquisition of baseline, iontophoresis, acquisition of post-iontophoresis images required approximately 30 minutes, making for a total visit duration of 4 hours.

For each participant, visit 1 protocol was repeated identically on visit 2, so as to assess the intraindividual variability. In particular, the recording zones were repositioned as exactly as possible at the same spot (made possible by the aforementioned recording on transparent film), with intensity allocation and rotation order identical to those of visit 1.