Diffuse noxious inhibitory controls (DNIC). II. Lack of effect on non-convergent neurones, supraspinal involvement and theoretical implications

Effectiveness of myofascial release in the management of plantar heel pain: a randomized controlled trial

BACKGROUND

Previous studies have reported that stretching of the calf musculature and the plantar fascia are effective management strategies for plantar heel pain (PHP). However, it is unclear whether myofascial release (MFR) can improve the outcomes in this population.

OBJECTIVE

To investigate whether myofascial release (MFR) reduces the pain and functional disability associated with plantar heel pain (PHP) in comparison with a control group receiving sham ultrasound therapy (SUST).

DESIGN

Randomized, controlled, double blinded trial.

SETTING

Nonprofit research foundation clinic in India.

METHOD

Sixty-six patients, 17 men and 49 women with a clinical diagnosis of PHP were randomly assigned into MFR or a control group and given 12 sessions of treatment per client over 4 weeks. The Foot Function Index (FFI) scale was used to assess pain severity and functional disability. The primary outcome measure was the difference in FFI scale scores between week 1 (pretest score), week 4 (posttest score), and follow-up at week 12 after randomization. Additionally, pressure pain thresholds (PPT) were assessed over the affected gastrocnemii and soleus muscles, and over the calcaneus, by an assessor blinded to the treatment allocation.

RESULTS

The simple main effects analysis showed that the MFR group performed better than the control group in weeks 4 and 12 (P<0.001). Patients in the MFR and control groups reported a 72.4% and 7.4% reduction, respectively, in their pain and functional disability in week 4 compared with that in week 1, which persisted as 60.6% in the follow-up at week 12 in the MFR group compared to the baseline. The mixed ANOVA also revealed significant group-by-time interactions for changes in PPT over the gastrocnemii and soleus muscles, and the calcaneus (P<0.05).

CONCLUSIONS

This study provides evidence that MFR is more effective than a control intervention for PHP.

Acupuncture and moxibustion for lateral elbow pain: a systematic review of randomized controlled trials

Background

Acupuncture and moxibustion have widely been used to treat lateral elbow pain (LEP). A comprehensive systematic review of randomized controlled trials (RCTs) including both English and Chinese databases was conducted to assess the efficacy of acupuncture and moxibustion in the treatment of LEP.

Methods

Revised STRICTA (2010) criteria were used to appraise the acupuncture procedures, the Cochrane risk of bias tool was used to assess the methodological quality of the studies. A total of 19 RCTs that compared acupuncture and/or moxibustion with sham acupuncture, another form of acupuncture, or conventional treatment were included.

Results

All studies had at least one domain rated as high risk or uncertain risk of bias in the Cochrane risk of bias tool. Results from three RCTs of moderate quality showed that acupuncture was more effective than sham acupuncture. Results from 10 RCTs of mostly low quality showed that acupuncture or moxibustion was superior or equal to conventional treatment, such as local anesthetic injection, local steroid injection, non-steroidal anti- inflammatory drugs, or ultrasound. There were six low quality RCTs that compared acupuncture and moxibustion combined with manual acupuncture alone, and all showed that acupuncture and moxibustion combined was superior to manual acupuncture alone.

Conclusion

Moderate quality studies suggest that acupuncture is more effective than sham acupuncture. Interpretations of findings regarding acupuncture vs. conventional treatment, and acupuncture and moxibustion combined vs. manual acupuncture alone are limited by the methodological qualities of these studies. Future studies with improved methodological design are warranted to confirm the efficacy of acupuncture and moxibustion for LEP.

The visual size of one׳s own hand modulates pain anticipation and perception

How to reduce pain is a fundamental clinical and experimental question. Acute pain is a complex experience which seems to emerge from the co-activation of two main processes, namely the nociceptive/discriminative analysis and the affective/cognitive evaluation of the painful stimulus. Recently it has been found that pain threshold increases following the visual magnification of the body part targeted by the painful stimulation. This finding is compatible with the well-known notion that body representation and perceptual experience relay on complex, multisensory factors. However, the level of cognitive processing and the physiological mechanisms underlying this analgesic effect are still to be investigated. In the present work we found that following the visual magnification of a body part, the Skin Conductance Responses (SCR), to an approaching painful stimulus increases before contact and decreases following the real stimulation, compared to the non-distorted view of the hand. By contrast, an unspecific SCR increase is found when the hand is visually shrunk. Moreover a reduction of subjective pain experience was found specifically for the magnified hand in explicit pain ratings. These findings suggest that the visual increase of body size enhances the cognitive, anticipatory component of pain processing; such an anticipatory reaction reduces the response to the following contact with the noxious stimulus. The present results support the idea that cognitive aspects of pain experience relay on the multisensory representation of the body, and that could be usefully exploited for inducing a significant reduction of subjective pain experience.

Conditioned pain modulation: a predictor for development and treatment of neuropathic pain

Psychophysical evaluation of endogenous pain inhibition via conditioned pain modulation (CPM) represents a new generation of laboratory tests for pain assessment. In this review we discuss recent findings on CPM in neuropathic pain and refer to psychophysical, neurophysiological, and methodological aspects of its clinical implications. Typically, chronic neuropathic pain patients express less efficient CPM, to the extent that incidence of acquiring neuropathic pain (e.g. post-surgery) and its intensity can be predicted by a pre-surgery CPM assessment. Moreover, pre-treatment CPM evaluation may assist in the correct choice of serotonin-noradrenalin reuptake inhibitor analgesic agents for individual patients. Evaluation of pain modulation capabilities can serve as a step forward in individualizing pain medicine.

Altered central pain processing after pancreatic surgery for chronic pancreatitis

BACKGROUND

Chronic abdominal pain is common in chronic pancreatitis (CP) and may involve altered central pain processing. This study evaluated the relationship between pain processing and pain outcome after pancreatic duct decompression and/or pancreatic resection in patients with CP.

METHODS

Patients with CP underwent quantitative sensory testing. Pain processing was measured via electrical pain detection (ePDT) and electrical pain tolerance (ePTT) thresholds in dermatomes C5 and L4. Inhibitory descending pain control mechanisms were assessed using the conditioned pain modulation (CPM) paradigm. Healthy controls and patients with CP were compared, and patients with CP and a poor pain outcome (visual analogue scale (VAS) score greater than 30) were compared with those with a good pain outcome (VAS score 30 or less).

RESULTS

Forty-eight patients with CP had lower ePDT, ePTT and CPM responses compared with values in 15 healthy controls (P < 0·030). The sum of ePDT values was lower in patients with a poor pain outcome than in those with a good outcome (median 7·1 versus 11·2 mA; P = 0·008). There was a correlation with the VAS score and the sum of ePDT values (rs  = -0·45, P = 0·016) and ePTT values (rs  = -0·46, P = 0·011), and CPM response (rs  = -0·43, P = 0·006) in patients with CP.

CONCLUSION

After pain-relieving pancreatic surgery, patients with CP exhibit altered central pain processing compared with that in healthy controls. Poor pain outcomes are associated with more central sensitization and more pronociceptive descending pain modulation, and this should be considered when managing persistent pain after pain-relieving surgery for CP.

The influence of intensity and duration of a painful conditioning stimulation on conditioned pain modulation in volunteers

BACKGROUND AND METHOD

The aim was to investigate influence from variations in intensity of a painful conditioning stimulation (CS) on early (0-6 min) and prolonged (6-12 min) conditioned pain modulation (CPM) in volunteers during concurrent exposure to test stimuli (TS). CS was applied to either forearm using painful heat with an intensity of 2/10 and 5/10, respectively, rated on a 0-10 numerical pain rating scale. At a second session, CS with an intensity of 7/10 was applied to the arm using a tourniquet. Threshold and suprathreshold painful heat and pressure as well as painful repeated monofilament pricking (RMP) were assessed as TS.

RESULTS

Regardless of TS, there was no significant difference in the magnitude of CPM within the same stimulus modality during the various intensities and phases of the CS. Significant modulation of heat pain thresholds (HPTs) was found during the early phase at 5/10 and 7/10, but not at 2/10. Only at 5/10 the prolonged CS resulted in a significant additional increase in HPT. During the early CS phase, CPM of suprathreshold heat pain was found at 2/10 and 5/10. The prolonged CS resulted in a significant additional temperature increase at 5/10. Only during the early phase significant CPM of pressure pain thresholds were found for all three pain intensities in conjunction with a significant CPM of suprathreshold pressure pain at 5/10. There was no CPM of RMP.

CONCLUSION

The CS intensity and the duration of CPM modulated pain sensitivity differentially across TS modalities.

The important role of CNS facilitation and inhibition for chronic pain

Multiple studies have demonstrated that the pain experience among individuals is highly variable. Even under circumstances where the tissue injuries are similar, individual pain experiences may vary drastically. However, this individual difference in pain sensitivity is not only related to sensitivity of peripheral pain receptors, but also to variability in CNS pain processing. Peripheral impulses derived from tissue receptors undergo modification in dorsal horn neurons that can either result in inhibition or facilitation of pain. Such influences are particularly apparent in inflammation where not only peripheral, but also central, pain modulatory mechanisms can significantly increase nociceptive pain. Emotional state, level of anxiety, attention and distraction, memories, stress, fatigue and many other factors can either increase or reduce the pain experience. Increasing evidence suggests that 'bottom-up' and 'top-down' modulatory circuits within the spinal cord and brain play an important role in pain processing, which can profoundly affect the experience of pain.

The role of descending inhibitory pathways on chronic pain modulation and clinical implications

The treatment and management of chronic pain is a major challenge for clinicians. Chronic pain is often underdiagnosed and undertreated, and there is a lack of awareness of the pathophysiologic mechanisms that contribute to chronic pain. Chronic pain involves peripheral and central sensitization, as well as the alteration of the pain modulatory pathways. Imbalance between the descending facilitatory systems and the descending inhibitory systems is believed to be involved in chronic pain in pathological conditions. A pharmacological treatment that could restore the balance between these 2 pathways by diminishing the descending facilitatory pain pathways and enhancing the descending inhibitory pain pathways would be a valuable therapeutic option for patients with chronic pain. Due to the lack of evidence for pharmacological options that act on descending facilitation pathways, in this review we summarize the role of the descending inhibitory pain pathways in pain perception. This review will focus primarily on monoaminergic descending inhibitory pain pathways and their contribution to the mechanism of chronic pain and several pharmacological treatment options that enhance these pathways to reduce chronic pain. We describe anatomical structures and neurotransmitters of the descending inhibitory pain pathways that are activated in response to nociceptive pain and altered in response to sustained and persistent pain which leads to chronic pain in various pathological conditions.

Reappraising neuropathic pain in humans--how symptoms help disclose mechanisms

Neuropathic pain--that is, pain arising directly from a lesion or disease that affects the somatosensory system--is a common clinical problem, and typically causes patients intense distress. Patients with neuropathic pain have sensory abnormalities on clinical examination and experience pain of diverse types, some spontaneous and others provoked. Spontaneous pain typically manifests as ongoing burning pain or paroxysmal electric shock-like sensations. Provoked pain includes pain induced by various stimuli or even gentle brushing (dynamic mechanical allodynia). Recent clinical and neurophysiological studies suggest that the various pain types arise through distinct pathophysiological mechanisms. Ongoing burning pain primarily reflects spontaneous hyperactivity in nociceptive-fibre pathways, originating from 'irritable' nociceptors, regenerating nerve sprouts or denervated central neurons. Paroxysmal sensations can be caused by several mechanisms; for example, electric shock-like sensations probably arise from high-frequency bursts generated in demyelinated non-nociceptive Aβ fibres. Most human and animal findings suggest that brush-evoked allodynia originates from Aβ fibres projecting onto previously sensitized nociceptive neurons in the dorsal horn, with additional contributions from plastic changes in the brainstem and thalamus. Here, we propose that the emerging mechanism-based approach to the study of neuropathic pain might aid the tailoring of therapy to the individual patient, and could be useful for drug development.

Evaluation of the Bonapace Method: a specific educational intervention to reduce pain during childbirth

OBJECTIVE

As pain during childbirth is very intense, several educational programs exist to help women prepare for the event. This study evaluates the efficacy of a specific pain management program, the Bonapace Method (BM), to reduce the perception of pain during childbirth. The BM involves the father, or a significant partner, in the use of several pain control techniques based on three neurophysiological pain modulation models: (1) controlling the central nervous system through breathing, relaxation, and cognitive structuring; (2) using non-painful stimuli as described in the Gate Control Theory; and (3) recruiting descending inhibition by hyperstimulation of acupressure trigger points.

METHODS

A multicenter case control study in Quebec on pain perception during labor and delivery compared traditional childbirth training programs (TCTPs) and the BM. Visual analog scales were used to measure pain perception during labor. In all, 25 women (TCTP: n = 12; BM: n = 13) successfully reported their perceptions of pain intensity and unpleasantness every 15 minutes.

RESULTS

A POSITIVE CORRELATION BETWEEN THE PROGRESSION OF LABOR AND PAIN WAS FOUND (PAIN INTENSITY: P < 0.01; pain unpleasantness: P < 0.01). When compared to TCTP, the BM showed an overall significant lower pain perception for both intensity (45%; P < 0.01) and unpleasantness (46%; P < 0.01).

CONCLUSION

These significant differences in pain perception between TCTP and the BM suggest that the emphasis on pain modulation models and techniques during labor combined with the active participation of a partner in BM are important variables to be added to the traditional childbirth training programs for childbirth pain management.

Mechanisms of topical analgesics in relieving pain in an animal model of muscular inflammation

OBJECTIVE

To investigate the possible mechanisms of topical analgesics in relieving pain in an animal model of muscular inflammation.

METHODS

Adult Sprague-Dawley rats of both sexes were injected with complete Freund's adjuvant to induce inflammation in the anterior tibialis muscle of left hindlimb. One of two types of topical analgesics: Xiaotong Tiegao (XTT), a Tibetan herb compound, or Capzasin (CAP), a cream containing 0.1% capsaicin, was applied to the skin over the inflamed anterior tibialis muscle. The following experiments were performed: pain behavioral tests, evaluation of plasma extravasation in the affected limb, and electrophysiological recordings of afferent nerve fibers.

RESULTS

The behavioral experiments demonstrated that applications of either type of topical analgesic to the skin over the inflamed muscle significantly reduced muscular inflammatory pain, as indicated by the increased weight bearing capacity on the affected hindlimb (with latencies of 10 minutes for XTT and 1-2 hours for CAP). Meanwhile, both analgesics caused plasma extravasation in the affected skin. Electrophysiological recordings from the afferent fibers in the related cutaneous nerve indicated that topical analgesics selectively activated C-fibers, but not A-fibers innervating the same region of receptive field. The latency and duration of C-fiber activation was similar to those of the reduction of muscular inflammatory pain. On the contrary, topical analgesics substantially decreased C-fiber afferent spontaneous firing in the nerve innervating the inflamed muscle. Moreover, denervation of the affected skin blocked the analgesic effects of both topical analgesics in muscular inflammatory pain.

CONCLUSION

This study suggests that topical analgesics may reduce the nociceptive input from inflamed muscles via a reflex mechanism by activating the cutaneous nociceptive afferents.

Changes in the modulation of spinal pain processing are related to severity in irritable bowel syndrome

BACKGROUND

In irritable bowel syndrome (IBS) patients can be divided in two groups according to inhibition or facilitation of the RIII nociceptive spinal reflex induced by rectal distension. We further investigated the differences in pain processes in these two groups and their relationship to clinical symptoms.

METHODS

This study included 10 female IBS-C patients with facilitation (Group F) and 10 patients with inhibition (Group I) of the RIII reflex recorded on the left lower limb during slow-ramp rectal distension, and 11 healthy female volunteers. Diffuse noxious inhibitory control (DNIC)-induced inhibition was assessed by measuring the effects of noxious cold stimulation of the right hand on the RIII reflex and the concomitant sensation of pain. Functional magnetic resonance imaging (fMRI) was performed to compare the changes in brain activity induced by painful and non painful rectal distension. Irritable bowel syndrome symptom severity, mood, anxiety, and catastrophizing were also systematically assessed.

KEY RESULTS

Unlike the patients of Group I and healthy volunteers, Group F patients displayed no inhibition of the RIII reflex or of concomitant pain sensation during immersion of the hand in ice-cold water. The reduction of the inhibition induced by heterotopic noxious stimuli was directly correlated with the severity of IBS symptoms, but not with psychological symptoms. The fMRI study showed that non-painful and painful rectal distension induced similar changes in brain activity in the two groups of patients.

CONCLUSION & INFERENCES

Alterations of the modulation of spinal pain processing in IBS correlates with symptom severity but not with psychological factors or brain activity.

Placebo and nocebo: how to enhance therapies and avoid unintended sabotage to pain treatment

SUMMARY There is good evidence showing that placebo and nocebo responses do not only reflect a psychological reappraisal of an unchanged nociceptive activity. There are several scientific evidences indicating that placebo or nocebo responses trigger changes in the brain that activate descending modulatory mechanisms, affecting the nociceptive signal early in the CNS. Among the psychological factors that trigger a placebo or nocebo response, conditioning and expectation have been demonstrated to greatly affect the outcomes of pain perception, but also the response to treatment. Placebo or nocebo responses can be triggered without the administration of an inert substance in several therapeutic contexts and will affect the treatment outcome. In this article, we will describe different experimental situations where psychological factors produce physiological changes of the nociceptive signal in the brain, and how these changes are reflected in the spinal cord. Finally, we will discuss the importance of better understanding placebo and nocebo mechanisms in clinical contexts for pain treatment.

The prevalence of widespread central hypersensitivity in chronic pain patients

BACKGROUND

Chronic pain is associated with generalized hypersensitivity and impaired endogenous pain modulation (conditioned pain modulation; CPM). Despite extensive research, their prevalence in chronic pain patients is unknown. This study investigated the prevalence and potential determinants of widespread central hypersensitivity and described the distribution of CPM in chronic pain patients.

METHODS

We examined 464 consecutive chronic pain patients for generalized hypersensitivity and CPM using pressure algometry at the second toe and cold pressor test. Potential determinants of generalized central hypersensitivity were studied using uni- and multivariate regression analyses. Prevalence of generalized central hypersensitivity was calculated for the 5th, 10th and 25th percentile of normative values for pressure algometry obtained by a previous large study on healthy volunteers. CPM was addressed on a descriptive basis, since normative values are not available.

RESULTS

Depending on the percentile of normative values considered, generalized central hypersensitivity affected 17.5-35.3% of patients. 23.7% of patients showed no increase in pressure pain threshold after cold pressor test. Generalized central hypersensitivity was more frequent and CPM less effective in women than in men. Unclearly classifiable pain syndromes showed higher frequencies of generalized central hypersensitivity than other pain syndromes.

CONCLUSIONS

Although prevalent in chronic pain, generalized central hypersensitivity is not present in every patient. An individual assessment is therefore required in order to detect altered pain processing. The broad basic knowledge about central hypersensitivity now needs to be translated into concrete clinical consequences, so that patients can be offered an individually tailored mechanism-based treatment.

Understanding central mechanisms of acupuncture analgesia using dynamic quantitative sensory testing: a review

We discuss the emerging translational tools for the study of acupuncture analgesia with a focus on psychophysical methods. The gap between animal mechanistic studies and human clinical trials of acupuncture analgesia calls for effective translational tools that bridge neurophysiological data with meaningful clinical outcomes. Temporal summation (TS) and conditioned pain modulation (CPM) are two promising tools yet to be widely utilized. These psychophysical measures capture the state of the ascending facilitation and the descending inhibition of nociceptive transmission, respectively. We review the basic concepts and current methodologies underlying these measures in clinical pain research, and illustrate their application to research on acupuncture analgesia. Finally, we highlight the strengths and limitations of these research methods and make recommendations on future directions. The appropriate addition of TS and CPM to our current research armamentarium will facilitate our efforts to elucidate the central analgesic mechanisms of acupuncture in clinical populations.

Personalized pain medicine: the clinical value of psychophysical assessment of pain modulation profile

Experimental pain stimuli can be used to simulate patients' pain experience. We review recent developments in psychophysical pain testing, focusing on the application of the dynamic tests-conditioned pain modulation (CPM) and temporal summation (TS). Typically, patients with clinical pain of various types express either less efficient CPM or enhanced TS, or both. These tests can be used in prediction of incidence of acquiring pain and of its intensity, as well as in assisting the correct choice of analgesic agents for individual patients. This can help to shorten the commonly occurring long and frustrating process of adjusting analgesic agents to the individual patients. We propose that evaluating pain modulation can serve as a step forward in individualizing pain medicine.

The perception and endogenous modulation of pain

Pain is often perceived an unpleasant experience that includes sensory and emotional/motivational responses. Accordingly, pain serves as a powerful teaching signal enabling an organism to avoid injury, and is critical to survival. However, maladaptive pain, such as neuropathic or idiopathic pain, serves no survival function. Genomic studies of individuals with congenital insensitivity to pain or paroxysmal pain syndromes considerable increased our understanding of the function of peripheral nociceptors, and especially of the roles of voltage-gated sodium channels and of nerve growth factor (NGF)/TrkA receptors in nociceptive transduction and transmission. Brain imaging studies revealed a "pain matrix," consisting of cortical and subcortical regions that respond to noxious inputs and can positively or negatively modulate pain through activation of descending pain modulatory systems. Projections from the periaqueductal grey (PAG) and the rostroventromedial medulla (RVM) to the trigeminal and spinal dorsal horns can inhibit or promote further nociceptive inputs. The "pain matrix" can explain such varied phenomena as stress-induced analgesia, placebo effect and the role of expectation on pain perception. Disruptions in these systems may account for the existence idiopathic pan states such as fibromyalgia. Increased understanding of pain modulatory systems will lead to development of more effective therapeutics for chronic pain.

Neural and psychosocial contributions to sex differences in knee osteoarthritic pain

People with osteoarthritis (OA) can have significant pain that interferes with function and quality of life. Women with knee OA have greater pain and greater reductions in function and quality of life than men. In many cases, OA pain is directly related to sensitization and activation of nociceptors in the injured joint and correlates with the degree of joint effusion and synovial thickening. In some patients, however, the pain does not match the degree of injury and continues after removal of the nociceptors with a total joint replacement. Growth of new nociceptors, activation of nociceptors in the subchondral bone exposed after cartilage degradation, and nociceptors innervating synovium sensitized by inflammatory mediators could all augment the peripheral input to the central nervous system and result in pain. Enhanced central excitability and reduced central inhibition could lead to prolonged and enhanced pain that does not directly match the degree of injury. Psychosocial variables can influence pain and contribute to pain variability. This review explores the neural and psychosocial factors that contribute to knee OA pain with an emphasis on differences between the sexes and gaps in knowledge.

Acute pressure on the sciatic nerve results in rapid inhibition of the wide dynamic range neuronal response

Background

Acute pressure on the sciatic nerve has recently been reported to provide rapid short-term relief of pain in patients with various pathologies. Wide dynamic range (WDR) neurons transmit nociceptive information from the dorsal horn to higher brain centers. In the present study, we examined the effect of a 2-min application of sciatic nerve pressure on WDR neuronal activity in anesthetized male Sprague–Dawley rats.

Results

Experiments were carried out on 41 male Sprague–Dawley albino rats weighing 160–280 grams. Dorsal horn WDR neurons were identified on the basis of characteristic responses to mechanical stimuli applied to the cutaneous receptive field. Acute pressure was applied for 2 min to the sciatic nerve using a small vascular clip. The responses of WDR neurons to three mechanical stimuli applied to the cutaneous receptive field were recorded before, and 2, 5 and 20 min after cessation of the 2-min pressure application on the sciatic nerve. Two-min pressure applied to the sciatic nerve caused rapid attenuation of the WDR response to pinching, pressure and brushing stimuli applied to the cutaneous receptive field. Maximal attenuation of the WDR response to pinching and pressure was noted 5 min after release of the 2-min pressure on the sciatic nerve. The mean firing rate decreased from 31.7±1.7 Hz to 13±1.4 Hz upon pinching (p < 0.001), from 31.2±2.3 Hz to 10.9±1.4 Hz (p < 0.001) when pressure was applied, and from 18.9±1.2 Hz to 7.6±1.1 Hz (p < 0.001) upon brushing. Thereafter, the mean firing rates gradually recovered.

Conclusions

Our results indicate that acute pressure applied to the sciatic nerve exerts a rapid inhibitory effect on the WDR response to both noxious and innocuous stimuli. Our results may partially explain the rapid analgesic effect of acute sciatic nerve pressure noted in clinical studies, and also suggest a new model for the study of pain.

Is the deficit in pain inhibition in fibromyalgia influenced by sleep impairments?

It has been proposed that a deficit in inhibitory conditioned pain modulation (ICPM) underlies the pathophysiology of fibromyalgia (FM), but there is high variability in ICPM efficacy in this syndrome that remains poorly understood. Based on emerging data showing that age, anxiety, depression and sleep can modulate ICPM efficacy, the main objective of this study was to determine the clinical correlates of experimentally-induced pain perception in FM. Fifty FM patients and 39 healthy controls (HC) were tested. Anxiety, depression, sleep and FM symptoms were measured with questionnaires or interview-type scales. Experimental pain testing consisted of two tonic heat pain stimulations separated by a 2-minute cold pressor test (CPT). Thermal pain thresholds and tolerance were higher in HC compared to FM patients. Pain ratings during the CPT were lower in HC relative to FM patients. ICPM efficacy was stronger in HC compared to FM patients. Finally, sleep quality was the only factor significantly related to ICPM efficacy. To our knowledge, this is the first study to report this association in FM. Future studies will need to replicate this finding, to determine whether impaired sleep is primary or secondary to deficient pain inhibition, and to characterize the neurobiological mechanisms underlying this association.

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

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.

Has central sensitization become independent of nociceptive input in chronic pancreatitis patients who fail thoracoscopic splanchnicectomy?

BACKGROUND AND OBJECTIVES

Central sensitization due to visceral pancreatic nociceptive input may be important in chronic pancreatitis pain. We investigated whether bilateral thoracoscopic splanchnicectomy (BTS) to reduce nociceptive input in chronic pancreatitis patients (CPP) with poor pain control affects supraspinal and spinal sensitization.

METHODS

Seventeen CPP were studied preoperatively and 6 weeks after BTS. Pressure pain thresholds (PPT) were measured in clavicle and pancreatic dermatomes reflecting supraspinal and spinal central sensitization, respectively. Patients with increased PPT after BTS (hypoalgesic) were compared to those without (hyperalgesic) and PPT vs. pain numeric rating scale (NRS) changes compared.

RESULTS

After BTS, ten patients showed C5 PPT increases (hypoalgesic; median change 87 kPa), 7 patients had unaltered/lower PPT (hyperalgesic; -135 kPa). Preoperative pain NRS was similar between groups (4 vs. 5, P = 0.2). After BTS hypoalgesic group NRS was lower (1 vs. 6; P = 0.008) and NRS change greater (-2 vs. 0; P = 0.005). Whole group NRS and C5 PPT change correlated significantly and negatively (r = 0.53; P < 0.05), but not for pancreatic PPT.

CONCLUSIONS

Reduced supraspinal-but not spinal-central sensitization after BTS was associated with significantly reduced pain scores in a majority of CPP. A subgroup showed no reductions in supraspinal central sensitization after BTS, coupled to no significant pain NRS reduction. Our results suggest that a subgroup of CPP has altered pain processing that may be independent of ongoing peripheral nociceptive input, resulting in persisting pain despite BTS. If confirmed, these results indicate the importance of sensory testing for indications and management of pain treatments.

Subgroups of musculoskeletal pain patients and their psychobiological patterns - the LOGIN study protocol

BACKGROUND

Pain conditions of the musculoskeletal system are very common and have tremendous socioeconomic impact. Despite its high prevalence, musculoskeletal pain remains poorly understood and predominantly non-specifically and insufficiently treated.The group of chronic musculoskeletal pain patients is supposed to be heterogeneous, due to a multitude of mechanisms involved in chronic pain. Psychological variables, psychophysiological processes, and neuroendocrine alterations are expected to be involved. Thus far, studies on musculoskeletal pain have predominantly focused on the general aspects of pain processing, thus neglecting the heterogeneity of patients with musculoskeletal pain. Consequently, there is a need for studies that comprise a multitude of mechanisms that are potentially involved in the chronicity and spread of pain. This need might foster research and facilitate a better pathophysiological understanding of the condition, thereby promoting the development of specific mechanism-based treatments for chronic pain. Therefore, the objectives of this study are as follows: 1) identify and describe subgroups of patients with musculoskeletal pain with regard to clinical manifestations (including mental co-morbidity) and 2) investigate whether distinct sensory profiles or 3) distinct plasma levels of pain-related parameters due to different underlying mechanisms can be distinguished in various subgroups of pain patients.

METHODS/DESIGN

We will examine a population-based chronic pain sample (n = 100), a clinical tertiary care sample (n = 100) and pain-free patients with depression or post-traumatic stress disorder and pain-free healthy controls (each n = 30, respectively). The samples will be pain localisation matched by sex and age to the population-based sample. Patients will undergo physical examination and thorough assessments of mental co-morbidity (including psychological trauma), perceptual and central sensitisation (quantitative sensory testing), descending inhibition (conditioned pain modulation, the diffuse noxious inhibitory control-like effect), as well as measurement of the plasma levels of nerve growth factor and endocannabinoids.

DISCUSSION

The identification of the underlying pathophysiologic mechanisms in different subgroups of chronic musculoskeletal pain patients will contribute to a mechanism-based subgroup classification. This will foster the development of mechanism-based treatments and holds promise to treat patients more sufficient.

Comparison of a single vs. a four intradermal sterile water injection for relief of lower back pain for women in labour: a randomised controlled trial

OBJECTIVE

sterile water injections are a simple, safe, effective, non-pharmacological technique for relieving back pain in labour, however the number of injections required to achieve optimal analgesia is unknown. The objective of this trial was to evaluate the degree and duration of analgesia provided by a single injection of sterile water, compared to four injections.

DESIGN

randomised controlled non-inferiority trial.

PARTICIPANTS AND SETTING

three hundred and five women in labour at term, requesting analgesia for back pain were recruited from two metropolitan hospitals in Brisbane, Australia.

INTERVENTION

participants were randomly assigned to receive either one (n=147) or four (n=158) sterile water injections.

OUTCOME MEASURES

difference in self-reported pain measured using a visual analogue scale (VAS) between baseline and 30 mins post-intervention. The clinically acceptable margin of difference was defined as ≤ 1 cm on the VAS between the single injection compared to four injection technique. Secondary outcomes include VAS score on injection and 10, 60, 90 and 120 mins post-intervention, analgesia use, mode of birth and maternal satisfaction.

FINDINGS

the mean difference in the pre and post (30 mins) injection scores between two groups was -1.48 cm (95% CI -2.10, -0.86) in favour of the FI technique, however the injection pain associated with the FI was significantly greater than that of the SI technique (p<0.001). There were no significant differences between the two groups in terms of other analgesic use, mode of birth or maternal satisfaction.

CONCLUSION

the four injection technique was associated with increased level of analgesia at 30 mins post-intervention compared to the single injection, but also a greater degree of injection pain.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Self-injurious behaviour in intellectual disability syndromes: evidence for aberrant pain signalling as a contributing factor

BACKGROUND

In most individuals, injury results in activation of peripheral nociceptors (pain-sensing neurons of the peripheral nervous system) and amplification of central nervous system (CNS) pain pathways that serve as a disincentive to continue harmful behaviour; however, this may not be the case in some developmental disorders that cause intellectual disability (ID). Moreover, individuals affected by ID disorders may initiate self-injurious behaviour to address irritating or painful sensations. In normal individuals, a negative feedback loop decreases sensation of pain, which involves descending inhibitory neurons in the CNS that attenuate spinal nociceptive processing. If spinal nociceptive signalling is impaired in these developmental disorders, an exaggerated painful stimulus may be required in order to engage descending anti-nociceptive signals.

METHODS

Using electronic databases, we conducted a review of publications regarding the incidence of chronic pain or altered pain sensation in ID patients or corresponding preclinical models.

RESULTS

There is a body of evidence indicating that individuals with fragile X mental retardation and/or Rett syndrome have altered pain sensation. These findings in humans are supported by mechanistic studies using genetically modified mice harbouring mutations consistent with the human disease. Thus, once self-injurious behaviour is initiated, the signal to stop may be missing. Several developmental disorders that cause ID are associated with increased incidence of gastroesophageal reflux disease (GERD), which can cause severe visceral pain. Individuals affected by these disorders who also have GERD may self-injure as a mechanism to engage descending inhibitory circuits to quell visceral pain. In keeping with this hypothesis, pharmacological treatment of GERD has been shown to be effective for reducing self-injurious behaviour in some patients. Hence, multiple lines of evidence suggest aberrant nociceptive processing in developmental disorders that cause ID.

CONCLUSIONS

There is evidence that pain pathways and pain amplification mechanisms are altered in several preclinical models of developmental disorders that cause ID. We present hypotheses regarding how impaired pain pathways or chronic pain might contribute to self-injurious behaviour. Studies evaluating the relationship between pain and self-injurious behaviour will provide better understanding of the mechanisms underlying self-injurious behaviour in the ID population and may lead to more effective treatments.

Manual and electroacupuncture for labour pain: study design of a longitudinal randomized controlled trial

Introduction. Results from previous studies on acupuncture for labour pain are contradictory and lack important information on methodology. However, studies indicate that acupuncture has a positive effect on women's experiences of labour pain. The aim of the present study was to evaluate the efficacy of two different acupuncture stimulations, manual or electrical stimulation, compared with standard care in the relief of labour pain as the primary outcome. This paper will present in-depth information on the design of the study, following the CONSORT and STRICTA recommendations. Methods. The study was designed as a randomized controlled trial based on western medical theories. Nulliparous women with normal pregnancies admitted to the delivery ward after a spontaneous onset of labour were randomly allocated into one of three groups: manual acupuncture, electroacupuncture, or standard care. Sample size calculation gave 101 women in each group, including a total of 303 women. A Visual Analogue Scale was used for assessing pain every 30 minutes for five hours and thereafter every hour until birth. Questionnaires were distributed before treatment, directly after the birth, and at one day and two months postpartum. Blood samples were collected before and after the first treatment. This trial is registered at ClinicalTrials.gov: NCT01197950.

The Pain-Relieving Effects Induced by Electroacupuncture with Different Intensities at Homotopic and Heterotopic Acupoints in Humans

The purpose of the present study was to investigate the relationship between the distribution of the analgesic area when different intensities of stimulation were applied to homotopic and heterotopic acupoints. The experiments were performed on volunteers; the left sural nerve was stimulated by a volley of eight rectangular pulses delivered at a frequency of 400 Hz. Electromyographic reflex responses (EMGs) were recorded from the ipsi-lateral biceps femoris muscle. Electroacupuncture stimuli were given on the skin overlying the Zusanli point (St 36), and the strengths chosen were times of the threshold eliciting a nociceptive reflex (TRIII) response (0.6, 0.8, 1.0, TRIII and Tsupra-RIII , respectively). The effects of homotopic and heterotopic stimuli applied to St 36 on the pain sensation and the RIII reflex elicited by stimulation of the left sural nerve were observed to explore the pain-relief and RIII reflex-inhibition produced by stimulation of the St 36 point with different intensities. Both the nociceptive reflex and painful sensation evoked by stimulating the sural nerve were similarly inhibited by electroacupuncture at less than TRIII applied to the ipsi-lateral acupoint. In other words, acupuncture with an innocuous intensity can produce homotopic pain-alleviating effects and reflex suppression. With contra-lateral electroacupuncture at the St 36 acupoint, innocuous intensities cannot produce heterotopic pain-relieving effects; these inhibitions were only observed at electroacupuncture intensities similar to the TRIII threshold. These results suggest that local acupuncture-induced analgesia is effective with activation of large afferent fibers, whereas heterotopic acupuncture-induced analgesia is only effective with intensities strong enough to excite small afferent fibers. Local analgesic effects of acupoint stimulation involve segmental inhibition, whereas systemic analgesic effects of acupoint stimulation are involved in contra-lateral effects. The latter may recruit the diffuse noxious inhibitory controls (DNIC) system. The specific function of an acupoint is determined by the anatomical relationship between the disease focus and the segmental location of the acupoint.

Effects of dexmedetomidine on conditioned pain modulation in humans

BACKGROUND

Systemic administration of dexmedetomidine (DEX; selective α(2) -adrenoceptor agonist) is found to inhibit diffuse noxious inhibitory control in rats, now referred to as conditioned pain modulation (CPM) in humans. The present study was designed to investigate the effect of intravenous administration of DEX on CPM in humans.

METHODS

There were two sequential sessions in this double blind, randomized study. The first session was the control with normal saline infusion (N(1st), L(1st), H(1st)). During the second session, three types of agents were infused: normal saline (N(2nd)); a low plasma concentration of DEX (0.04 ng/mL; L(2nd)); and a high plasma concentration of DEX (0.08 ng/mL; H(2nd)). The amplitude of somatosensory evoked potentials (ampSEP)s and the visual analogue scale of tooth pain (VASt) induced by electrical tooth stimulation were evaluated with and without conditioning CO(2) laser stimulation of the hand. The inhibition rate (% inhibition) was calculated [= (1-[ampSEP or VASt with conditioning stimuli]/[ampSEP or VASt without conditioning stimuli]) × 100] to compare the magnitude of the DEX effects on CPM.

RESULTS

The inhibition rates of ampSEPs and VASt in Types N, L and H varied significantly, demonstrating a dose-dependent reduction of CPM effects of ampSEP and VASt during randomized DEX administration, consistent with results from animal studies.

CONCLUSIONS

The present study shows that systemic administration of an α(2) -adrenoceptor agonist (DEX), less than the clinical dose, inhibited CPM in humans. These results may provide some mechanistic insight into why many chronic pain patients show impaired CPM.

Time dependant differences in pain sensitivity during unilateral ischemic pain provocation in healthy volunteers

Plurisegmental endogenous pain inhibitory mechanisms related to diffuse noxious inhibitory controls (DNIC) were demonstrated in animal experiments to act on multireceptive neurons of the entire cord outside the conditioned segment without any side differences. Human experiments have demonstrated altered pain sensitivity to pressure, heat and electrical stimulation during heterotopic noxious conditioning stimulation (HNCS). The purpose of the study was to examine if side and/or time differences in pain thresholds and suprathreshold pain sensitivity for pressure and heat, respectively, could be detected during HNCS. Perception thresholds to pressure and heat pain as well as the sensitivity to suprathreshold pressure (SPP) and heat pain (SHP) were assessed in 18 healthy volunteers bilaterally at the thighs before, during and following ischemia-induced pain of the left forearm (HNCS). The assessments started with either the right (10 subjects) or the left thigh (8 subjects). During HNCS the pressure pain threshold increased significantly (p<0.001) on both sides alike. No significant difference in the magnitude of the altered pressure pain threshold was seen between sides for the first or the lastly assessed side. On the lastly assessed side only SPP and SHP increased significantly on both sides alike (p<0.02 and p<0.03, respectively), without magnitude differences between sides. During unilateral HNCS of the left arm, a time factor was demonstrated only for alterations in suprathreshold pain sensitivity, without any differences in magnitude between sides. Therefore, the results have implications for future design of HNCS-related experimental and clinical studies.

Heterotopic ischemic pain attenuates somatosensory evoked potentials induced by electrical tooth stimulation: Diffuse noxious inhibitory controls in the trigeminal nerve territory

The purpose of this study was to determine whether the late component of somatosensory evoked potentials (SEP) induced by electrical tooth stimulation and pain intensity are inhibited by heterotopic ischemic stimulation. The tourniquet pressure with 50 mmHg greater than the individual's systolic pressure was applied to the left upper arm for 10 min as ischemic conditioning stimulation. The late component of SEP and visual analogue scale (VAS) were recorded at 4 times and both were significantly decreased when ischemic conditioning stimulation was applied. The maximum reductions in SEP amplitude and the VAS value were 26.1% and 21.2%, respectively, during ischemic conditioning stimulation. After-effect was observed 5 min after removal of the conditioning stimulation. The present study revealed that heterotopic ischemic stimulation attenuated the late component of SEP induced by electrical tooth stimulation, triggering diffuse noxious inhibitory controls (DNIC) and after-effects in the trigeminal nerve territory. It was also suggested that the DNIC effect differs, depending on the intensity, kind, and quality of the test and conditioning stimuli.

Hypoalgesia to pressure pain in referred pain areas triggered by spatial summation of experimental muscle pain from unilateral or bilateral trapezius muscles

Animal and human experimental studies have suggested the importance of spatial summation in the nociception processing and in the activation of descending inhibition. However, the relationship between the areas (size) of muscles stimulated and the recruitment of descending inhibition has not been addressed. Consequently, we tested whether bilateral versus unilateral injection of hypertonic saline into trapezius muscles caused hypoalgesia to pressure pain (pressure pain thresholds, PPTs) in the local pain areas (the trapezius muscles) and the referred pain areas (the posterolateral neck muscles). Two groups of volunteers participated. One group received a unilateral injection (one injection) and the other group bilateral injections (two injections). In the bilateral group, hypertonic saline was injected in one trapezius first, and 45 s later, while pain was still present from the first injection, a second injection was performed into the contralateral trapezius muscle. The saline-evoked time to maximal pain was significantly shorter after the second injection than after the first injection. More subjects developed referred pain after the bilateral compared with the unilateral injection. In the referred pain areas, the PPTs 7.5 and 15 min after the second injection were significantly increased compared with the first injection, while no changes in the PPT were observed in local and referred pain areas after unilateral injection. This suggests that the induction of descending inhibition was triggered by spatial summation during the later phase of experimentally induced muscle pain. The present experimental model might be used for further investigation of descending inhibition related to the spatial characteristics of nociceptive stimuli in humans.