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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

Following nociceptive heat or laser stimulation, an early contralateral and later vertex potential can be recorded. Although more indicative of the nociceptive input, the acquisition of the contralateral N1 after contact heat stimulation (contact heat-evoked potentials [CHEPs]) remains difficult. An advantage of contact heat is that the baseline skin temperature preceding peak stimulation can be controlled. Increasing the baseline temperature may represent a novel strategy to improve the acquisition of CHEPs without resulting in more subjective pain to stimulation. A study was undertaken in 23 healthy subjects to examine the effects of increasing the baseline temperature but not the perceived intensity of contact heat stimulation. A combined standard averaging and single-trial analysis was performed to disclose how changes in averaged waveforms related to latency jitter and individual trial amplitudes. By increasing the baseline temperature, the acquisition of N1 was improved among subjects with a low-amplitude response (greater than -4μV) following 35°C baseline temperature stimulation (P<.05). Based on standard averaging, N2/P2 amplitudes were also significantly increased with and without an accompanying change in the rating of perceived pain when the baseline temperature was increased (P<.05). In contrast, automated single-trial averaging revealed no significant difference in N2 amplitude when the baseline temperature was increased to 42°C and the peak temperature reduced. These findings suggest that 2 mechanisms underlie the improved acquisition of CHEPs: increased synchronization of afferent volley, yielding larger-amplitude evoked potentials in response to the same rating of intensity; and reduced inter-trial variability. [ABSTRACT FROM AUTHOR]