The influence of expectation on spinal manipulation induced hypoalgesia: an experimental study in normal subjects

Abstract

Background: The mechanisms thorough which spinal manipulative therapy (SMT) exerts clinical effects are not established. A prior study has suggested a dorsal horn modulated effect; however, the role of subject expectation was not considered. The purpose of the current study was to determine the effect of subject expectation on hypoalgesia associated with SMT.

Methods: Sixty healthy subjects agreed to participate and underwent quantitative sensory testing (QST) to their leg and low back. Next, participants were randomly assigned to receive a positive, negative, or neutral expectation instructional set regarding the effects of a specific SMT technique on pain perception. Following the instructional set, all subjects received SMT and underwent repeat QST.

Results: No interaction (p = 0.38) between group assignment and pain response was present in the lower extremity following SMT; however, a main effect (p < 0.01) for hypoalgesia was present. A significant interaction was present between change in pain perception and group assignment in the low back (p = 0.01) with participants receiving a negative expectation instructional set demonstrating significant hyperalgesia (p < 0.01).

Conclusion: The current study replicates prior findings of c- fiber mediated hypoalgesia in the lower extremity following SMT and this occurred regardless of expectation. A significant increase in pain perception occurred following SMT in the low back of participants receiving negative expectation suggesting a potential influence of expectation on SMT induced hypoalgesia in the body area to which the expectation is directed.

Figure 1

Effect of Instructional Set on Expected Pain in the Low Back. Change in expected pain in the low back following instructional set. Positive values indicate expectation of less pain. A statistical interaction occurred with participants receiving a positive expectation instructional set reporting expectations for less pain with quantitative sensory testing (QST) following spinal manipulative therapy (SMT) and those receiving a negative expectation instructional set reporting expectations for greater pain. Error bars represent 1 standard error of the mean (SEM). * indicates significant change at p ≤ 0.05.

Figure 2

Change in Pain Perception in the Low Back and Lower Extremity by Expectation Instructional Set. Change in pain perception in the low back and lower extremity following spinal manipulative therapy (SMT). Positive numbers indicate hypoalgesia, while negative numbers indicate hyperalgesia. A significant interaction was present in the low back suggesting that post SMT pain perception was dependent upon the group to which the participant was randomly assigned. Follow up pairwise comparison indicated a significant increase in pain perception in subjects receiving a negative expectation instructional set. No interaction was observed in the lower extremity of participants; however, a significant main effect occurred suggesting hypoalgesia regardless of group assignment. Error bars represent 1 standard error of the mean (SEM). * indicates a statistically significant change in pain perception in the low back following SMT at p ≤ 0.05.