Conditioned pain modulation in populations with chronic pain: a systematic review and meta-analysis.

A systematic literature review and meta-analysis were undertaken to determine if conditioned pain modulation is dysfunctional in populations with chronic pain. Studies that used a standardized protocol to evaluate conditioned pain modulation in a chronic pain population and in a healthy control population were selected and reviewed. Thirty studies were included in the final review, encompassing data from 778 patients and 664 control participants. Across all studies there was a large effect size of .78, reflecting reduced conditioned pain modulation in the patient group. Analysis of moderator variables indicated a significant influence of participant gender and age on the effect size. Methodological moderator variables of type of outcome measure, type of test stimulus, type of conditioning stimulus, and the level of conditioning stimulus pain were not significant. A risk of bias assessment indicated that poor blinding of assessors and a lack of control of confounding variables were common. It is concluded that conditioned pain modulation is impaired in populations with chronic pain. Future studies should ensure adequate matching of participant age and gender between patient and control groups, blinding of the assessors obtaining the outcome measures, and more rigorous control for variables known to influence the level of modulation.