Placebo response in neuropathic pain after spinal cord injury: a meta-analysis of individual participant data

Abstract

Catherine R Jutzeler,1–3 Freda M Warner,1,2 Jacquelyn J Cragg,1,3 Jenny Haefeli,4 J Scott Richards,5 Sven R Andresen,6 Nanna B Finnerup,7,8 Catherine Mercier,9 John LK Kramer1,2 1Faculty of Medicine, ICORD, University of British Columbia, Vancouver, BC, Canada; 2Faculty of Education, School of Kinesiology, University of BC, Vancouver, BC, Canada; 3Faculty of Medicine, Spinal Cord Injury Center, University Hospital Balgrist, University of Zurich, Zurich, Switzerland; 4Weill Institute for Neurosciences, Department of Neurological Surgery, Brain and Spinal Injury Center, University of California, San Francisco, CA, USA; 5Department of Physical Medicine and Rehabilitation, University of Alabama at Birmingham, Birmingham, AL, USA; 6Spinal Cord Injury Centre of Western Denmark, Department of Neurology, Regional Hospital of Viborg, Viborg, Denmark; 7Danish Pain Research Centre, Department of Clinical Medicine, Aarhus University, Aarhus, Denmark; 8Department of Neurology, Aarhus University Hospital, Aarhus, Denmark; 9Center for Interdisciplinary Research in Rehabilitation and Social Integration, Québec, QC, Canada Background: Understanding factors associated with high placebo responses in clinical trials increases the likelihood of detecting a meaningful treatment effect. The aim of the present study was to identify subject-level factors that contribute to placebo variability in patients with neuropathic pain due to spinal cord injury (SCI).Methods: Multiple regression analysis of patient data from randomized, double-blind, placebo-controlled trials (duration >4 weeks) involving individuals with SCI was performed. Patient demographics, as well as injury and pain characteristics were examined for their association with changes in pain rating from baseline to the end of the trial (i.e., placebo response). The overall effect of individual predictors was quantified with meta-analysis statistics.Results: A total of 276 patients with SCI from six studies were included in the analysis. Based on the meta-analysis of subject-level predictors, larger placebo responses were associated with male subjects (β=0.635; standard error [SE]=0.262; p=0.016) and higher baseline pain (β=−0.146; SE=0.073; p=0.044). There were no significant effects for injury characteristics (i.e., severity, level, and time since injury) or pain characteristics (i.e., location and evoked). No significant publication bias was detected.Conclusion: The current meta-analysis of individual patient data demonstrated the importance of sex and baseline pain intensity on changes in pain ratings in the placebo arm of SCI central neuropathic pain randomized controlled clinical trials. Overall, our findings indicate that placebo responses occur independent of injury characteristics. Keywords: placebo response, clinical trial, spinal cord injury, neuropathic pain

Keywords

• Placebo response
• clinical trial
• spinal cord injury
• neuropathic pain