A New Experimental Model of Muscle Pain in Humans Based on Short-Wave Diathermy: Preliminary Results

MISTA, CHRISTIAN ARIEL; LAUGERO, SILVIO; ADUR, JAVIER; ANDERSEN, OLE KÆSELER; BIURRUN MANRESA, JOSÉ ALBERTO

Boston

Conferencia; 17th World Congress on Pain; 2018

International Association for the Study of Pain (IASP)

Aim of Investigations Experimental models of pain in humans are crucial for understanding pain mechanisms. These models enable the exploration of pain effects under controlled scenarios in healthy and pathological conditions and consequently, they are relevant from both research and clinical points of view (Klein et al., 2008). In relation to muscle pain, most of the commonly used models require invasive procedures, such as injection of algesic substances, or entail a long developing time in case of models based on routines of muscle exercise (Graven-Nielsen, 2006). In this work, a new experimental muscle pain model is presented, based on deep heat stimulation using shortwave diathermy (SWD), i.e., by means of electromagnetic energy through oscillating high-frequency non-ionizing electromagnetic fields (Goats, 1989; Shields et al., 2002). The proposed procedure is projected as a novel, simple and non-invasive alternative to existing muscle pain models. Methods SWD was induced in the wrist extensor muscle group of the dominant arm of ten healthy volunteers, using a continous wave with a frequency of 27.12 MHz. Irradiation intensity was fixed at a constant value, and SWD was maintained until tolerance threshold for heat pain was reached. A visual analog scale (VAS) was used to quantify the heat sensation profile and the time to reach tolerance threshold was recorded. A McGill questionnaire and pain drawing areas were used to describe pain at the tolerance threshold. Quantitative evaluation of the model was performed through assessment of pressure pain threshold (PPT) and pinprick stimulation (PPS, 50 g) over extensor carpi radialis brevis (ECRB) muscles in both the dominant and the non-dominant arm (as a control) at baseline, and then 0, 30 and 60 minutes after SWD. A 2-way repeated measures ANOVA (with factors time and arm) was used to evaluate differences in PPT and PPS, calculated as percentage of change from baseline. Values are reported as mean ± standard deviation and a p values smaller than 0.05 were regarded as statistically significant.ResultsTime to reach heat pain tolerance during SWD was 160.4 ± 128.8 s. Immediately after SWD, 60% of the participants described the sensation at tolerance threshold as burning, 30% as hot, and the remaining 10% described it as scalding. With regards to pain intensity at tolerance threshold, participants described it as mild (40%), distressing (40%), discomforting (10%), and 10% as no pain at all. In relation to quantitative measures, RM ANOVA revealed a significant difference in PPT between arms (p < 0.035), reflecting a decrease in PPT of 16.1 ± 6.1% in the irradiated arm vs. a decrease of 3.60 ± 5.6% in the control arm. No significant differences due to time or significant interactions were found for PPT. Additionally, no significant changes related to time or arm were found for PPS. Conclusions This new model represents a promising tool for investigating muscle pain mechanisms. The main advantages of the model are its non-invasiveness and the convenience of the time frame in which pain is developed. Further work is required in order to fully describe the effects of changes in the model parameters (e.g. SWD intensity, application time, type of applicator) and to explore the possibility of applying the model to other muscles (e.g. in the lower limb).