Reliability of Measuring the Area and Intensity of Secondary Hyperalgesia Induced by High Frequency Stimulation

This prospective cohort study is designed to evaluate the method of assessing the areas and intensity of secondary hyperalgesia following High Frequency Stimulation (HFS), and will consist of two experimental days separated by a minimum of two weeks. During the two each session (Day1 and Day2), subjects will go through the HFS procedure on the same forearm. Then, after a 30 minutes break, the area and intensity of secondary hyperalgesia induced by HFS will be assessed three times (Time9-14), with a two-minute break between each assessment.

Before the experiment, subjects will fill two questionnaires by email : the Spielberger Trait anxiety inventory and the Pain catastrophization scale.The first experimental day, the patients will be seated on a chair and informed consent will be gained. Subjects will provide demographic information to the researcher (age, gender, size) (Time1). Then, the forearm skin of the selected arm will be marked with a pen to improve the precision of subsequent measures. The subject will then observe a period of rest for 5 minutes, to provide a baseline measure of heart rate variability.

Next, the patients will be asked to rate the pain induced by pin-prick stimulation in the area surrounding the electrode (Time2).

Following standardized explanation of the HFS procedure, the subject will fill the Spielberger State anxiety inventory (Time5), and HFS will be applied on the volar forearm (6-10 cm distal to the cubital fossa) (Time6). To avoid any confounding effect of handedness, the arm onto which HFS will be applied (dominant vs nondominant) will be counterbalanced across participants. In summary, HFS will consist of 12 trains of 42 Hz electrical pulses (pulse width: 2 ms) lasting 1 s each. The time interval between each train will 10 s. The intensity of stimulation will be 5mA. Heart rate variability during the HFS procedure will be monitored.

After a 25 minutes break the heart rate variability will be assessed a third time during 5 minutes. During the break, subjects will be asked to stay seated, to fill the Pain Catastrophizing scale (short-version) and give a rating from 0-100 for the pain and stress experienced during the HFS procedure (Time7).

Following this break, the area of secondary hyperalgesia will be assessed with a calibrated 128 mN pinprick probe (The Pin Prick, MRC Systems GmbH, Heidelger, Germany) (Time9). Pokes of 1-second duration will be provided at a frequency of 0.5 Hz, starting well outside and approaching the stimulation site with steps of 0,5 cm from 4 different directions towards the application site. Participants will be asked to keep their eyes closed during the entire mapping procedure. Then, participants will rate the pain induced by pin-prick stimulation in the area surrounding the electrode (Time10). This procedure will be conducted three times, with a two-minute break between each assessment (Time 11-14).

The second experimental day (Day2), the procedure will be similar to the first, but informed consent and the Spielberger Trait anxiety inventory and the Pain catastrophization scale will not be filled.

Inclusion criteria

• Male or female sex
• age>18 years and <40 years
• be fluent either in English or French
• ability to provide informed consent

Exclusion criteria

• experiencing a pre-existing pain condition
• self-reported medication (except contraceptives) consumption within 48 hours before experimental day and/or self-reported recreational drug use
• presenting any medical conditions, including heart, neurological, dermatological and psychiatric diseases
• participation to more than 6 hours per week of sport
• exhibiting sign of damage at or near the ventral forearm
• tattoos on the tested forearm
• Failure to cooperate to the tests