Effects of mood on pain responses and pain tolerance: an experimental study in chronic back pain patients.

Although chronic pain and depression commonly co-occur, causal relationships have yet to be established. A reciprocal relationship, with depression increasing pain and vice versa, is most frequently suggested, but experimental evidence is needed to validate such a view. The most straightforward approach would be a demonstration that increasing or decreasing depressed mood predictably modifies pain responses. The current experiment tested whether experimentally induced depressed and happy mood have differential effects on pain ratings and tolerance in 55 patients suffering from chronic back pain. Participants were randomly assigned to depressed, neutral (control) or elated mood induction conditions. They completed a physically passive baseline task prior to receiving mood induction, then a clinically relevant physically active task (holding a heavy bag) to elicit pain responses and tolerance. Measures were taken immediately after the baseline task and immediately after the mood induction to assess the changes in mood, pain ratings and tolerance before and after the experimental manipulation. Results indicate that the induction of depressed mood resulted in significantly higher pain ratings at rest and lower pain tolerance, whilst induced happy mood resulted in significantly lower pain ratings at rest and greater pain tolerance. Correlations between changes in mood on the one hand and changes in pain response and pain tolerance on the other hand were consistent with these findings. It is concluded that, in chronic back pain patients, experimentally induced negative mood increases self-reported pain and decreases tolerance for a pain-relevant task, with positive mood having the opposite effect.