The current study compared the pain responses of children with chronic pain and matched controls from the community to standardized pain tasks involving thermal heat, pressure and cold pain stimuli. Our results indicated that children with chronic pain did not differ from controls in pain intensity or pain tolerance across all three pain modalities when exposed to open-ceiling trials. However, surprisingly, children with chronic pain were significantly more likely to tolerate a fixed 1 min cold pressor trial compared with controls (). All but two children with chronic pain (92%) were able to complete the trial compared with only 61.5% (n=16) of control children. The findings for the fixed trial are even more surprising given that for the open cold pressor trial, children with chronic pain and control children were equally likely to belong to the low tolerance group (ie, tolerance times of 49 s or less). Despite differences in pain tolerance for the fixed trial, pain intensity ratings did not differ between children with chronic pain and controls, and mean tolerance times for noncompleters did not differ. Additional findings indicated that children with chronic pain exhibited a trend toward higher heart rate at baseline, before the start of the laboratory pain tasks, and after the pain tasks were completed, relative to controls (). Neither blood pressure nor ratings of anxiety differed between groups at baseline. However, children with chronic pain reported more anxiety and depressive symptoms than controls.
Overall, our findings suggest that children with chronic pain do not exhibit heightened pain responsivity compared with their healthy counterparts when exposed to a variety of pain modalities during ‘pure’ assessments of pain tolerance (ie, open ceiling trials). These results are consistent with other investigations reporting no differences in pain threshold (10
), pain tolerance (16
) or pain intensity ratings (14
) between these groups of children. Our results partly agree with those of Zohsel et al (12
), who found no differences in heat pain threshold between children with chronic pain and controls, but did find lower mechanical pain thresholds among children with chronic pain. However, our findings conflict with studies reporting lower pressure pain thresholds (1
) and lower cold pain tolerance (14
) for children with chronic pain compared with controls. Differences in assessment protocol (eg, stimulus intensity) or sample characteristics (eg, age range) may account for the divergent findings. For example, previous studies have found lower cold pain tolerance for children with JIA relative to controls (14
), but our sample of children with chronic pain did not include any participants with JIA. We did not assess pain threshold in the current study; previous work in our laboratory has indicated that some children naturally use distraction to cope with pain and that instructions to attend to pain sensation necessitated by the assessment of pain threshold may interfere with the use of distraction, thereby altering the ability to tolerate pain (30
). Thus, studies on pain threshold in children that do not control for natural coping style during pain exposure may introduce confounds that complicate the interpretation of results. For these reasons, we chose to assess pain intensity and pain tolerance times, rather than pain threshold.
Our results for the fixed trial are consistent with those of Zohsel et al (11
), who reported that children with RAP evidenced less perceptual sensitization than controls in response to repetitive mechanical and tonic heat stimulation. However, even though children with chronic pain exhibited greater tolerance than controls for the fixed trial in the present study, pain ratings for the fixed trial did not differ between groups. These findings suggest that factors other than sensory aspects of pain may have a greater impact on pain tolerance among children with chronic pain, relative to controls. Although we did not explicitly test which of these other contextual factors were at play, one possibility is that children with chronic pain have a greater need to act in ways that would be viewed favourably by others (ie, high social desirability). Thus, children with chronic pain may have wanted to please the experimenter by completing the entire trial. In one of the few studies examining social desirability among children with chronic pain, children with high social desirability scores reported fewer anxiety/depression symptoms compared with lower scorers, but no differences were reported for somatic symptoms, pain duration or pain-related disability (31
). Nevertheless, a control group was not included in this work and, therefore, it is unknown whether children with chronic pain differ from healthy children in social desirability. A related explanation is that children with chronic pain may be more compliant with laboratory procedures given their history of undergoing frequent medical procedures for their pain. Earlier, we found that among such patients, the average number of medical visits for pain in the previous year was 6.26 (32
Another possibility is that pain tolerance among children with chronic pain may be especially influenced by the presence and/or behaviour of the experimenters during pain stimulation. Experimenters (who were aware of to which group each child belonged) may have sent subtle cues that differed between groups even though the protocol was standardized and experimenters were highly trained. Previous work has demonstrated the effect of parent presence/behaviours on children’s experimental pain responsivity. Zohsel et al (12
) found that children with migraines had similar heat pain thresholds to controls when tested alone, but migraineurs showed more perceptual sensitization to tonic heat stimulation when their mother was present. Walker et al (33
) reported that compared with a no instruction control condition, symptom complaints by children with RAP and healthy children following induction of visceral discomfort increased when mothers devoted attention to their symptoms and were reduced when mothers distracted the children; the effect of mother attention on symptom complaints was greater for girls with RAP than for boys with RAP or healthy children (33
). We were not able to test for sex differences in pain responsivity due to the small number of boys in the current study. However, the proportion of girls and boys among fixed trial noncompleters was similar across groups. For the chronic pain group, one boy (50%) and one girl were noncompleters, and for the control group four boys (40%) and six girls (60%) were noncompleters.
An alternative explanation is that pain tolerance for the fixed trial was more heavily influenced by expectations engendered by the task instructions in children with chronic pain than controls. For instance, children with chronic pain may have interpreted the instructions as indicating that they should be able to tolerate the full trial. This desire to maintain stoicism in the face of pain is somewhat distinct from a more general need to appear ‘good’ in front of others. Previously, we found that attempts to hide pain from others was the most common theme from our narrative analysis of children’s experience with chronic pain, reported by 62% of children (32
). Another possibility is that informing children of the amount of time they would be exposed to pain may have differentially influenced responses. An earlier study involving adults found that time estimates for a cold pressor trial were more accurate among those given a time-specific goal for tolerance compared with those not given a time-specific goal (34
). A specific time goal may have distracted subjects from their pain by focusing their attention instead on the passage of time. Given their experience with frequent pain episodes, it may be that children with chronic pain are more adept at using this type of distraction compared with healthy children. Relatedly, the fixed ceiling may have increased the perception of controllability and/or predictability of pain exposure, thereby decreasing its noxiousness among children with chronic pain. In adults, the desire for control and prediction of having control was associated with level of pain during dental treatment (35
). Given that clinical pain episodes are often difficult to control and/or predict, the fixed trial in particular may have been comparatively ‘easy’ for children with chronic pain to endure.
Although we did not assess which, if any, of these cognitive factors may have influenced children in the current study, one clinical implication of our findings is that if such factors are confirmed to influence pain responses among children with chronic pain, it follows that efforts to actively change such cognitions may lead to changes in pain-related behaviour. This notion is a fundamental principle underlying cognitive-behavioural treatment approaches to chronic pain that seek to alter patients’ cognitions about pain to affect behaviour change. An additional clinical implication of our results is that behavioural interventions may incorporate laboratory-based pain exposures to teach and reinforce changes in pain tolerance over time. Newly developed behavioural approaches to pediatric chronic pain have begun to include similar, task-based pain exposures (36
). Giving children with chronic pain concrete evidence of increased pain tolerance during controlled pain exposures may lead to improvements in pain self-efficacy and a greater sense of mastery over pain.
Our finding that relative to controls, children with chronic pain exhibited a trend toward increased physiological arousal as indexed by higher heart rate in anticipation of and following pain induction is in line with previous investigations. One study found that a clinical group of children with RAP and anxiety disorders evidenced higher heart rate at baseline compared with controls; elevated heart rate was also reported in the clinical group during the CPT (16
). Others have similarly found increased physiological responses following pain induction among children with RAP relative to control children (17
). Our findings contradict one investigation in which children with RAP did not differ from controls in blood pressure or heart rate responses following the CPT (20
). Despite exhibiting heightened physiological arousal before pain stimulation, children with chronic pain in the present study did not report increased anticipatory anxiety relative to control children. This result is somewhat surprising given that children with chronic pain reported more anxiety symptoms (as well as depressive symptoms) compared with controls – findings that have been commonly reported in the literature (37
). On the other hand, we have previously found in a sample of healthy children that anxiety symptoms are not directly related to pain-related anticipatory anxiety (38
Limitations to the present study should be noted. Children with chronic pain who agreed to undergo pain induction may differ from those who did not participate. It is possible that the current sample of children with chronic pain may have been less anxious or more compliant than those who refused participation. An important factor differentiating the chronic pain and control samples is that the former was recruited among patients from a tertiary chronic pain clinic whereas the latter was recruited in a different way from the wider community. Thus, group differences may have as much to do with selection factors such as parental concern, solicitude, fear of medical problems, or family resources and access to medical care, as with children’s pain symptoms. As noted above, we did not include a measure of social desirability to assess the possible effects of this construct on pain responses. We also did not ask about children’s thoughts and expectations regarding the pain tasks, and such qualitative data would have provided information on why children with chronic pain behaved differently from controls in relation to the fixed trial. As mentioned above, we were not able to test for sex differences due to the small number of boys. However, the sample composition was dictated by the higher proportion of girls in the chronic pain group – such sex differentials in a pediatric chronic pain sample have been previously reported (32
). The present study tested the effects of acute pain induction and the results may not hold for tasks designed to induce sensations that more closely reflect the experience of chronic pain. Finally, our assessment of physiological responses was limited to a single measure of heart rate and blood pressure at baseline and postsession; future studies may employ more sophisticated systems to continuously assess multiple indices of physiological response during pain induction.
Overall, our findings indicate no differences between healthy children and children with chronic pain when exposed to laboratory pain tasks with open ceilings. However, when exposed to a fixed trial of cold pain, children with chronic pain were more likely to complete the entire trial than were healthy children, despite reporting similar pain intensity ratings as the healthy control group. The chronic pain group also evidenced a trend toward higher baseline and postsession heart rates and endorsed more anxious and depressive symptoms than the control group, suggesting higher levels of general arousal and mood disturbances. Nevertheless, the lack of group differences in anticipatory anxiety suggest that the laboratory pain procedures in themselves were not additionally stressful for the children with chronic pain. Our findings indicate that children with chronic pain may be particularly sensitive to contextual factors such as social desirability, pain-related social expectations and perceptions of controllability/predictability engendered by specific types of pain induction. Future work may use qualitative data to explore relevant themes in relation to pain coping, perceived control and other salient factors. Such information, together with quantitative data on acute pain responses, may paint a more comprehensive picture of the unique and specific characteristics that influence the pain responses of children with chronic pain, and may inform the development of interventions that seek to alleviate pain in this population.