Segmental Chiropractic Spinal Manipulation Does not Reduce Pain Amplification and the Associated Pain-Related Brain Activity in a Capsaicin-Heat Pain Model

Abstract

Musculoskeletal injuries lead to sensitization of nociceptors and primary hyperalgesia (hypersensitivity to painful stimuli). This occurs with back injuries, which are associated with acute pain and increased pain sensitivity at the site of injury. In some cases, back pain persists and leads to central sensitization and chronic pain. Thus, reducing primary hyperalgesia to prevent central sensitization may limit the transition from acute to chronic back pain. It has been shown that spinal manipulation (SM) reduces experimental and clinical pain, but the effect of SM on primary hyperalgesia and hypersensitivity to painful stimuli remains unclear. The goal of the present study was to investigate the effect of SM on pain hypersensitivity using a capsaicin-heat pain model. Laser stimulation was used to evoke heat pain and the associated brain activity, which were measured to assess their modulation by SM. Eighty healthy participants were recruited and randomly assigned to one of the four experimental groups: inert cream and no intervention; capsaicin cream and no intervention; capsaicin cream and SM at T7; capsaicin cream and placebo. Inert or capsaicin cream (1%) was applied to the T9 area. SM or placebo were performed 25 min after cream application. A series of laser stimuli were delivered on the area of cream application (1) before cream application, (2) after cream application but before SM or placebo, and (3) after SM or placebo. Capsaicin cream induced a significant increase in laser pain (p < 0.001) and laser-evoked potential amplitude (p < 0.001). However, SM did not decrease the amplification of laser pain or laser-evoked potentials by capsaicin. These results indicate that segmental SM does not reduce pain hypersensitivity and the associated pain-related brain activity in a capsaicin-heat pain model.

Keywords: SMT; manual therapy; pain; pain modulation; spine.

Figure 1

Flow diagram of enrollment, allocation, and analyses. SM, spinal manipulation; LEP, laser-evoked potential.

Figure 2

Experimental protocol. Three blocks of 30 laser stimuli were applied to the back, for a total of 90 stimuli. After each block, participants were instructed to rate pain verbally. Inert or capsaicin cream was applied to the stimulation area after block 1 and removed at the end of the experiment. Between blocks 2 and 3, participants received either no intervention, a placebo intervention or a spinal manipulation at T7.

Figure 3

Capsaicin pain ratings. Mean pain ratings for capsaicin pain for the three groups with capsaicin cream. Data from each participant are represented by linked colored points and the mean of these data points for each block is represented by gray bars.

Figure 4

Heat pain amplification induced by capsaicin. Mean laser pain ratings during block 1 and block 2 (before and after cream application, but before the intervention) for the four experimental groups. Data from each participant are represented by colored points and the mean of these data points for each block is represented by gray bars. ***P < 0.001.

Figure 5

Effect of SM on heat pain amplification. Mean laser pain ratings for the three groups with capsaicin cream during block 2 and block 3 (before and after the intervention). Data from each participant are represented by linked colored points and the mean of these data points for each block is represented by gray bars. Note that the inert cream group did not report heat pain amplification and received no intervention, so it is not included in this analysis.

Figure 6

Aδ fiber laser-evoked potentials. (A) Average waveforms for the N2 and P2 at Cz with a nose reference of the 77 participants included in the Aδ fiber LEP analysis, time-locked to the onset of laser stimulation. (B) Average topographic maps for the Aδ-N2 and Aδ-P2 in the four groups.

Figure 7

Amplification of Aδ fiber LEP by capsaicin. Average Aδ fiber N2-P2 peak to peak amplitude values during block 2 for the four experimental groups. These values are relative to baseline (block 2 minus block 1). Data from each participant are represented by colored points and the mean of these data points for each block is represented by gray bars. ***P < 0.001.

Figure 8

Effect of SM on Aδ fiber LEP. Average Aδ fiber N2-P2 peak to peak amplitude values for the three groups with capsaicin cream before (block 2) and after (block 3) the intervention. Data from each participant are represented by linked colored points and the mean of these data points for each block is represented by gray bars. *P < 0.05.