Stress can both blunt (stress-induced analgesia, SIA) and exacerbate (stress-induced hyperalgesia, SIH) pain perception. The magnitude of effects depends on the type of stressor, the type of pain assessment, and interindividual variability. This study investigated the influence of a purely cognitive stressor on pain perception in patients with chronic musculoskeletal pain (CMP). They assessed electric pain threshold, pain tolerance, and suprathreshold ratings.
The study included 22 patients (mean 55.90 years old) with chronic (> 3 months) musculoskeletal pain and 18 pain-free controls (mean 52.94 years old). The current episode of pain must have been present for >4 weeks. The participants completed tests in the following order: 1) threshold determination, 2) test trials to calibrate stimulus intensity for suprathreshold pain testing, 3) suprathreshold pain trials 4) baseline physiological recordings, 6) Stress induction 7) suprathreshold pain trials, 8) threshold determination.
During the threshold testing, patients received electric stimuli every 2 seconds. The participants pressed a button when the stimulus had become just noticeably painful (pain threshold) and when they could no longer tolerate a higher stimulus intensity (pain tolerance). The measures were assessed three times before and after the stressor. An increase in pain threshold or tolerance indicated SIA, while a decrease indicated SIH. The stressor was completing mental arithmetic (30 tasks over 15 minutes) with progressively increasing background white noise (65 to 80 dB). Previous research has shown this stressor can induce SIA in healthy participants. Blood pressure and heart rate were measured as well.
Patients with CMP displayed significantly higher levels of pain catastrophizing and lifetime stress exposure and lower scores on the Pain-Related Self Statements Scale (active coping scale). These likely predisposed the participants to greater stress responses. There was no difference in task difficulty or the number of correct answers between groups. All three groups' perceived stress, heart rate, and blood pressure increased. Blood pressure increases were greater for the patients with CMP than the control group but there was no difference in the other measures.
Post-hoc comparisons of pain thresholds before and after the stressor showed a significant SIA effect for the control, but not for the patients with CMP. Conversely, post-hoc comparisons of pain intensity ratings before and after the stressor revealed a significant SIH effect for patients with CMP, but not for healthy participants. These findings suggest cognitive stressors may enhance pain perception for people with CMP.