Magnitude, response, and psychological determinants of placebo effects in chronic low-back pain: a randomised, double-blinded, controlled trial

Denervation of the lumbar zygapophyseal joints by medial branch radiofrequency neurotomy has shown some benefit in treating chronic low-back pain. Before denervation, a diagnosis is made by one or more blinded injections on separate occasions to ascertain whether the relevant joints are contributing to the pain. Placebo injections have been advocated in a diagnostic regime that also includes local anaesthesia, with a decision to proceed to neurotomy based on response to local anaesthesia and not to placebo. Objectives: We investigated the magnitude of and response rate to placebo injections, and the roles of expectation, desire for pain relief, and anxiety as determinants of response to placebo. Methods: One hundred twenty patients were randomised to receive placebo and local anaesthetic injections on alternate occasions in a double-blind manner. A smaller control group with 2 local anaesthetic injections was also used. Responses to placebo were characterised, including magnitude and frequency. Results: This study demonstrated very large response to placebo injections, both response rate (78%) and magnitude (effect size d = 1.85). Expectation and anxiety were important modulators of response to placebo in this setting, with support given to expectation as a dynamic modulator of placebo responses. Large response to placebo (both in rate and magnitude) was observed when participants reported the belief that they were in the placebo arm. Conclusion: This study demonstrated large placebo responses in the context of injections for low-back pain and further characterised the importance of expectation and anxiety as important psychological mediators.