Comparative psychophysical characteristics of cutaneous CO2 laser and contact heat stimulation

Abstract

Psychophysical visual analog scaling can be used to reveal critical determinants of the neural processing underlying non-painful and painful heat sensations produced by radiant and contact heat stimulation. This study determined the stimulus-response (S-R) functions of cutaneous non-painful and painful heat stimuli delivered by an infra-red CO2 laser or by a contact thermode in a series of experiments in healthy volunteers. In experiments 1 (n = 12), with the rating scale anchored at pain threshold, the S-R curve for brief (60 ms) laser pulse stimulation with a beam diameter of 10 mm was a negatively accelerating function. Transformation of laser stimulus intensity (W) into temperatures (degree C) did not change the form of the S-R curve. In experiment 2 (n = 9), using the same laser stimulus parameters as in experiment 1, but without an anchored rating scale, the form of the S-R relationship did not change. In experiment 3 (n = 9), increases of the laser pulse duration up to 5 s and the beam diameter up to 18 mm produced linear S-R curves. In contrast, in experiment 4 (n = 21), the S-R curve for cutaneous contact heat stimuli applied for 5 s with an 18 mm diameter probe was best described by a positively accelerating power function with an exponent greater than 2.0. These experiments have (1) characterized the S-R functions for different parameters of infra-red laser stimulation of the skin, and (2) have shown that the form of the S-R function for innocuous and noxious heat sensation is influenced strongly by the physical conditions of heat stimulus application, including mechanical contact with the skin.

A reliable method for the preferential activation of C- or A-fibre heat nociceptors

“…However, a potential limitation of the method employed is the necessity for contact between the heating apparatus and the paw, which will result in the activation of low-threshold mechanoceptors (LTMs). Input from LTMs may alter spinal nociceptive processing and, as a consequence, modify the withdrawal responses evoked by the noxious stimulus itself (Svensson et al, 1997). However, because the heating apparatus was fixed in place and was not moved between cycles of heating, it is reasonable to assume that the firing rates of most LTMs would have adapted in the time between initial contact with the copper disk and the start of heating.…”

The Fine Tuning of Pain Thresholds: A Sophisticated Double Alarm System

“…However the main drawback is the quasi-exclusive limitation to the C-fiber functions because of the mildness of the heating process (1–5°C/s; often ≤2°C). In addition, the method necessitates the contact of the thermode with the skin (heat transfer by conduction), eliciting two problems: (1) the concomitant activation of low threshold non-nociceptive afferents which exert an inhibitory influence on pain mechanisms [67]–[68]; (2) the subordination of the rate of thermal transfer to the quality of the thermode-skin contact given by the pressure of application, a parameter which is not easy to control [18]. Possibly as a result of these inconveniences, a low level of reproducibility of these tests is often reported [64], [66], [69]–[73].…”

Guidelines and recommendations for assessment of somatosensory function in oro-facial pain conditions - a taskforce report

“…This is in agreement with several other observations on thermal sensations in the oro‐facial region (54, 55, 59). However, radiant and contact heat stimulation yields power exponents (slope of curve in a log–log plot) close to 1 and 2, respectively, suggesting that the conduction of heat and activation of thermoreceptors differ with the two types of heat stimulation (59, 60). Interestingly, S‐R curves may also be influenced by the curve‐fitting procedures.…”