In this study – based on individually titrated ultra-short laser stimulation for induction, and on bedside scoring with VAS for evaluation, of pain – we were able to evoke isolated delayed single pain responses at lower, and separately assessable immediate and delayed pain responses at higher, levels of stimulation intensity.
Influence on peripheral sensitisation and central habituation of pain perception during individual titration, was reduced by gradually increasing the duration of each laser pulse, in agreement with previous recommendations [13, 14]. To avoid epidermal overheating and tissue damage during the study intervention, and to consistently compare pain responses to the same individual level of nociceptive stimulation, we considered it important to use individually titrated, instead of predefined [3, 5, 8, 15, 16], energy levels of laser stimulation, found to induce mild pain intensity, and also to stimulate slightly different skin areas at minute-long intervals.
Thermal induction of pain mediated by Aδ- and C-fibres in humans requires skin temperature levels exceeding activation thresholds of their nociceptors, corresponding to 44.3–46.5 °C versus 41.8–42.4 °C at foot level, i.e. to a difference in heating of two to four degrees centigrade [3]. Those physiological differences at nociceptive receptor level – consistent with our main finding of single pain responses being induced by lower levels of laser energy than double pain responses – enable selective thermal activation of C-fibre nociceptors at temperature levels above their nociceptive threshold and still below that of Aδ-fibre nociceptors [3,4,5]. Moreover, they most likely explain why all single pain responses in the present study were delayed, i.e. C-fibre mediated.
Our findings of significantly higher laser energy for induction of mild pain in males studied by a female than by a male – enabled by involving more than one study investigator and interpreted to reflect corresponding differences in pain sensitivity – might indicate potential impact of experimenter gender on pain perception. However, this finding in a limited number of study participants was not a predefined aim of this study and should hence be interpreted cautiously. Follow-up studies, preferably with a paired cross-over design, are highly desirable to confirm these findings.
Our approximately one-second difference in time latency between the first and second pain responses strengthens their likelihood of reflecting Aδ- and C-fibre mediated transmission of pain [3, 7, 16], respectively, corresponding to levels of neuronal conduction velocity estimated at 11–19 [3, 15,16,17] versus 0.7–1.5 [3, 6, 13, 15, 16] m/s. Hence, the short-lasting pulses of laser stimulation used in the present and previous [3, 5,6,7,8,9] studies allow both pain components to be independently evaluated, provided that the temperature thresholds of Aδ- and C-fibre nociceptors are both exceeded. Higher separation in time between the first and second pain responses is achieved by inducing pain in the foot, as done in the present and previous [3, 7, 16] studies, instead of in the hand [1, 4,5,6, 8,9,10, 14, 15, 18, 19], considering the up to 35% more delayed transmission of second (C-fibre mediated) pain along the estimated 80–100 cm axonal distance between the plantar arc and the lumbosacral spinal cord [3, 7, 16].
Despite being widely used in research and clinical practice, established bedside tools for pain scoring have been used in few previous studies on nociceptive pain induced by laser stimulation [4, 8, 10, 14, 19], and mainly with assessment of pain intensity on eleven-level visual analogue [10] or numeric rating [4, 14, 19] scales, i.e. with lower resolution than in the present study.
We did not use neuronal or cerebrocortical electrophysiological measurements – in addition to individual pain scoring – to distinguish between first (Aδ-fibre mediated) and second (C-fibre mediated) pain responses, based on differences in their nociceptive response latency and neuronal transmission properties. Nevertheless, the approximately one-second delay of each reported single pain, and second double pain, sensation – interpreted to reflect C-fibre mediated transmission – is consistent with recent neurophysiological findings [10]. Moreover, the induction of two pain responses, consistently well separated in time at bedside, by a single nociceptive stimulus, conforms to simultaneous activation of Aδ- and C-fibre fibres.
Although we did not measure plantar skin temperature – neither before nor after pain stimulation – we do not believe potential individual differences in baseline skin temperature to have considerably influenced our results, since CO2 laser stimuli have been shown to induce similar local heating patterns at skin temperatures between 27 °C and 32 °C [3, 14].
Results reported here were obtained in more study participants compared with most previous studies based on laser-induced pain [1, 3,4,5,6,7,8,9,10, 14], except for two previous [16, 18] and one recent [19] studies in similar numbers of subjects. The risk for carry-over effects was reduced by allowing minute-long time intervals between stimulations to avoid overlapping of subsequent pain responses. Furthermore, order effects were diminished and data accuracy improved by using median values of individually calculated average data obtained under identical conditions from repeated series of pain induction. In contrast to earlier similar studies with standardized energy levels (2–4), we used individually titrated levels of laser energy to induce similar and more predictable pain responses and avoid epidermal damage. We consider this readily usable technique of selective C-fibre stimulation and evaluation to be clinically, but not necessarily scientifically, more applicable than spatial filter [20] or dissociating Aδ-fibre nerve block [21] techniques.
