2.2. Assessment of heat pain sensitivity

In the capsaicin experimental pain model, chemical methods of nociceptor stimulation were used to produce stable and long-lasting hyperalgesia with a low potential for skin injury, in the original publication supplemented by heat stimulation.⁵⁸ Topical application of 150 mg capsaicin cream (0.2%, manufactured by the local Hospital Pharmacy) onto a 3 × 3 cm² skin area was used. Subsequently, the area was covered with a plaster for 30 minutes.

Quantitative sensory testing (QST) was performed at baseline and after application of capsaicin. A clinically established QST test battery proposed by the German Research Network on Neuropathic Pain^63,64 was used. For the present report, pain thresholds to noxious heat were selected. They were assessed using a 3 × 3 cm thermode (TSA 2001—II; Ramat Yishai, Israel) on a 9 cm² skin area at the inside of the forearm without any superficial veins or birth marks. Heat pain thresholds (HPTs) were measured by increasing the temperature of the thermode by 1°C/s, starting at 32°C, until the subject indicated pain, which triggered the reversal of the temperature ramp back to the baseline. According to the published instructions for the QST test battery,^59,63,64 the HPT was defined as the mean of 3 measurement repetitions. During testing, the room temperature was kept at 20 to 25°C.

Data were preprocessed according to the QST test battery instructions,^59,63,64 which included uniform direction along increasing stimulus intensity as HPT_T = HPT − 32°C, where the subscript T denotes the data transformation. The values of HPT_T were mapped onto the distribution of the reference group that consists of 180 healthy subjects, in whom a data set of 1080 QST parameter values has been obtained. This serves as the reference for all QST-based diagnoses.⁵² Therefore, according to the QST standard procedure, the individual QST parameter values were z-transformed as , with QST reference values with regard to the sex, age, and tested body site of the actual subject taken from.⁵² The signs of the z-scores, zHPT_T, were adjusted to denote that a z-score >0 indicates high sensitivity and z-score <0 indicates low sensitivity, according to the standardized instructions. The effect of capsaicin was quantified as the difference between the measurement after capsaicin application and the measurement without the presence of capsaicin, ie, .