Is there a difference between heat-capsaicin induced low back pain and placebo for neural oscillations and inflammatory blood markers? An experimental randomized crossover study.

Purpose: Low back pain is difficult to study due to its heterogeneity. Inducing back pain experimentally, with an established model such as heat-capsaicin, would beneficially control for some variability. How heat-capsaicin affects neurophysiological factors relevant to back pain is currently unknown, therefore, this study used a randomized crossover design with the aim to explore the differences between heat-capsaicin and placebo on brain activity and blood markers.

Methods: 18 healthy participants completed two sessions: heat-capsaicin (45°C heat + capsaicin) and placebo (reduced heat + placebo). Pre- and post-pain-induction/placebo, electroencephalogram and blood draws were taken, and perceived pain was rated with a 100 m visual analog scale. Band power was calculated for theta (4-8 Hz), alpha (8-13 Hz), beta (13-30 Hz), gamma1 (30-58 Hz), and gamma2 (62-100 Hz) for six brain regions. An immune assay was run on plasma in duplicate for cytokines IL-1β, IL-6, IL-10, and TNFα. A repeated measures ANCOVA was run for all variables comparing between conditions (heat-capsaicin, placebo) with baseline measures as covariates. A Pearson's correlation was used to determine the relationship between perceived pain ratings and brain wave and blood biomarkers.

Results: The heat-capsaicin model induced transient mild to moderate pain which was significantly higher than placebo (24.50 vs. 0.39; p < 0.001). Brain wave and blood biomarkers were not significantly different between heat-capsaicin and placebo (p ≥ 0.05) or correlated to perceived pain ratings (p ≥ 0.15).

Conclusion: Levels of perceived pain did not relate to neurophysiological changes that may occur immediately after heat-capsaicin exposure. Although changes have been found with other pain models and clinical low back pain, a statistically significant systematic response was not measurable using blood cytokine markers immediately after pain induction and may take longer to develop.