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      Expectations of increased and decreased pain explain the effect of conditioned pain modulation in females

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          Abstract

          Objective

          Chronic pain is believed to be related to a dysfunction of descending pain modulatory mechanisms. Functioning of descending pain modulation can be assessed by various methods, including conditioned pain modulation (CPM). CPM refers to the inhibition of one source of pain by a second noxious stimulus, termed the conditioning stimulus. This procedure can activate an endogenous pain inhibitory mechanism that inhibits early nociceptive processing. Chronic pain and anxiety disorders are more prevalent among females and it has been hypothesized that females react with more negative emotions towards unpleasant stimuli and this might be part of the explanation of greater pain sensitivity in females. The present study investigated whether expectations modulate the effect of conditioning stimulation on pain, subjective stress, and heart rate. In addition, we investigated whether the modulation of CPM by expectations differed between males and females.

          Methods

          Seventy-two subjects (including 36 women) received six noxious heat stimuli to the forearm. During three of these stimuli, a conditioning stimulus (cold-water bath) was applied to the contralateral arm in order to activate CPM. One third of the subjects were told that this would reduce pain (analgesia group), one-third that it would increase pain (hyperalgesia group), and one third received no information about its effect (no info group).

          Results

          Information that conditioning stimulation decreased or enhanced pain had the corresponding effect in females, but not in males. Conditioning stimulation increased stress, but not heart rate in females in the hyperalgesia group. A higher expectation of analgesia and lower stress during conditioning stimulation was associated with larger inhibitory CPM.

          Conclusion

          These results suggest that reduced inhibitory CPM can be due to contextually induced cognitive and emotional factors and not necessarily a dysfunction of descending inhibitory pathways.

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          Most cited references 63

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          The validation of visual analogue scales as ratio scale measures for chronic and experimental pain.

          Visual analogue scales (VAS) of sensory intensity and affective magnitude were validated as ratio scale measures for both chronic and experimental pain. Chronic pain patients and healthy volunteers made VAS sensory and affective responses to 6 noxious thermal stimuli (43, 45, 47, 48, 49 and 51 degrees C) applied for 5 sec to the forearm by a contact thermode. Sensory VAS and affective VAS responses to these temperatures yielded power functions with exponents 2.1 and 3.8, respectively; these functions were similar for pain patients and for volunteers. The power functions were predictive of estimated ratios of sensation or affect produced by pairs of standard temperatures (e.g. 47 and 49 degrees C), thereby providing direct evidence for ratio scaling properties of VAS. Vas sensory intensity responses to experimental pain, VAS sensory intensity responses to different levels of chronic pain, and direct temperature (experimental pain) matches to 3 levels of chronic pain were all internally consistent, thereby demonstrating the valid use of VAS for the measurement of and comparison between chronic pain and experimental heat pain.
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            Diffuse noxious inhibitory controls (DNIC). I. Effects on dorsal horn convergent neurones in the rat.

            (1) Sixty-eight convergent dorsal horn neurones have been recorded at the lumbar level in anaesthetized intact rats. All cells received prominent A alpha and C fibre afferents and correspondingly could be activated by high and low threshold stimuli applied to the peripheral excitatory receptive field. (2) The activity of 67/68 of these neurones was powerfully inhibited by noxious stimuli applied to various parts of the body. Since non-noxious stimuli were ineffective in this respect, the term "diffuse noxious inhibitory controls" (DNIC) is proposed. (3) DNIC could be evoked by noxious pinch applied to the tail, the contralateral hind paw, the forepaws, the ears and the muzzle; the most effective areas were the tail and muzzle. Noxious heat applied to and transcutaneous electrical stimulation of the tail were extemely effective in eliciting DNIC as was the intraperitoneal injection of bradykinin. (4) DNIC strongly depressed by 60-100% both the C fibre response following suprathreshold transcutaneous electrical stimulation and the responses to noxious radiant heat. (5) The spontaneous activity and the responses to low threshold afferents induced either by A alpha threshold electrical or natural stimulation were also powerfully inhibited. (6) In the majority of cases, long lasting post-effects directly related to the duration of conditioning painful stimulus were observed.
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              Prediction of chronic post-operative pain: pre-operative DNIC testing identifies patients at risk.

              Surgical and medical procedures, mainly those associated with nerve injuries, may lead to chronic persistent pain. Currently, one cannot predict which patients undergoing such procedures are 'at risk' to develop chronic pain. We hypothesized that the endogenous analgesia system is key to determining the pattern of handling noxious events, and therefore testing diffuse noxious inhibitory control (DNIC) will predict susceptibility to develop chronic post-thoracotomy pain (CPTP). Pre-operative psychophysical tests, including DNIC assessment (pain reduction during exposure to another noxious stimulus at remote body area), were conducted in 62 patients, who were followed 29.0+/-16.9 weeks after thoracotomy. Logistic regression revealed that pre-operatively assessed DNIC efficiency and acute post-operative pain intensity were two independent predictors for CPTP. Efficient DNIC predicted lower risk of CPTP, with OR 0.52 (0.33-0.77 95% CI, p=0.0024), i.e., a 10-point numerical pain scale (NPS) reduction halves the chance to develop chronic pain. Higher acute pain intensity indicated OR of 1.80 (1.28-2.77, p=0.0024) predicting nearly a double chance to develop chronic pain for each 10-point increase. The other psychophysical measures, pain thresholds and supra-threshold pain magnitudes, did not predict CPTP. For prediction of acute post-operative pain intensity, DNIC efficiency was not found significant. Effectiveness of the endogenous analgesia system obtained at a pain-free state, therefore, seems to reflect the individual's ability to tackle noxious events, identifying patients 'at risk' to develop post-intervention chronic pain. Applying this diagnostic approach before procedures that might generate pain may allow individually tailored pain prevention and management, which may substantially reduce suffering.
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                Author and article information

                Journal
                J Pain Res
                J Pain Res
                Journal of Pain Research
                Dove Medical Press
                1178-7090
                2012
                17 August 2012
                : 5
                : 289-300
                Affiliations
                Department of Psychology, Faculty of Health Sciences, University of Tromsø, Tromsø, Norway
                Author notes
                Correspondence: Espen Bjørkedal, Department of Psychology, Faculty of Health Sciences, University of Tromsø, Tromsø N-9037, Norway, Tel +47 7764 6779, Fax +47 7764 5291, Email espen.bjorkedal@ 123456uit.no
                Article
                jpr-5-289
                10.2147/JPR.S33559
                3442740
                23049277
                © 2012 Bjørkedal and Flaten, publisher and licensee Dove Medical Press Ltd.

                This is an Open Access article which permits unrestricted noncommercial use, provided the original work is properly cited.

                Categories
                Original Research

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