Abnormal endogenous pain modulation is a shared characteristic of many chronic pain conditions

Sedentary behaviour facilitates conditioned pain modulation in middle-aged and older adults with persistent musculoskeletal pain: a cross-sectional investigation

INTRODUCTION

Higher physical activity (PA) and lower sedentary behaviour (SB) levels have demonstrated beneficial effects on temporal summation (TS) and conditioned pain modulation (CPM) in healthy adults. This cross-sectional study investigated the relationships between PA and SB and TS/CPM responses in individuals with chronic musculoskeletal pain.

METHODS

Sixty-seven middle-aged and older adults with chronic musculoskeletal pain were recruited from the community. Questionnaires measuring demographics, pain, and psychological measures were completed. Physical activity/SB levels were measured using the International Physical Activity Questionnaire-short form and Sedentary Behaviour Questionnaire, respectively. Semmes monofilament was used to assess mechanical TS (MTS) at the most symptomatic (MTS-S) and a reference region (MTS-R); change in the pain scores (baseline-10th application) was used for analysis. Conditioned pain modulation procedure involved suprathreshold pressure pain threshold (PPT-pain4) administered before and after (CPM30sec, CPM60sec, and CPM90sec) conditioning stimulus (2 minutes; ∼12°C cold bath immersion). For analysis, PPT-pain4 (%) change scores were used.

RESULTS

PPT-pain4 (%) change scores at CPM30sec and CPM60sec demonstrated significant weak positive correlations with SB levels and weak negative correlations with PA measures. After adjusting for confounding variables, a significant positive association was found between SB (h/d) and PPT-pain4 (%) change scores at CPM30sec and CPM60sec. No significant associations between MTS and PA/SB measures.

CONCLUSION

Sedentariness is associated with higher pain inhibitory capacity in people with chronic musculoskeletal pain. The observed relationship may be characteristic of a protective (sedentary) behaviour to enhance pain modulatory mechanism. Prospective longitudinal studies using objective PA/SB measures are required to validate the observed relationship in a larger sample size.

Sensitivity to Physical Activity Predicts Daily Activity Among Pain-Free Older Adults

Objective

Prior research indicates that older adults with knee osteoarthritis have increased sensitivity to physical activity (SPA) and respond to physical activities of stable intensity with increases in pain. Whether SPA is present in healthy older adults without chronic pain and predicts functional outcomes remains relatively unexplored. The purpose of this study was to determine the degree of SPA in healthy older adults in response to a standardized walking task, and whether SPA was associated with temporal summation of pain, pain-related fear of movement, and functional outcomes.

Methods

Fifty-two older adults without chronic pain completed self-reported measures of activity-related pain and physical function, completed the Six-Minute Walk Test (6MWT), underwent quantitative sensory testing to measure temporal summation of heat pain, and wore an accelerometer for one week to measure physical activity behavior. Subjects rated overall bodily discomfort (0-100 scale) prior to and during each minute of the 6MWT. An SPA index was created by subtracting the initial bodily discomfort ratings from the peak ratings.

Results

Repeated-measures analysis of variance indicated that bodily discomfort significantly increased across the walking task, with approximately 60% of the sample experiencing SPA. Hierarchical regressions indicated that greater SPA was associated with fewer average steps per day and greater activity-related pain. Additionally, analyses revealed that temporal summation of pain and pain-related fear of movement significantly predicted the degree of SPA on the walking task.

Conclusions

These findings shed light on potential mechanisms underlying SPA in older adults and suggest that SPA might be a risk factor for reduced physical activity.

Racial differences in experimental pain sensitivity and conditioned pain modulation: a study of Chinese and Indians

Background: Substantial literature has demonstrated racial differences in pain perception and endogenous pain modulation is proposed to be a mechanism for the racial differences. Although Indians in Singapore reported higher pain severity than Chinese, the only study on racial difference in experimental pain response in Singapore did not find any difference between the two racial groups. Purpose: The aim of this study was to investigate pain sensitivity and conditioned pain modulation in Chinese and Indians in Singapore. Patients and methods: Sixty age-and sex-matched (30 Chinese 50% female, 30 Indian, 50% female) healthy adults participated in this study. Pressure pain threshold, thermal pain threshold and cold pain tolerance were measured. Conditioned pain modulation, general self-efficacy and depression were also tested, in an attempt to assess endogenous pain inhibition and psychological presentation between the two groups. Results: No difference in pain thresholds was found between the two groups. Indians demonstrated less cold pain tolerance and less efficacious conditioned pain modulation than Chinese. Conditioned pain modulation was a mediator between race and cold pain tolerance. Conclusion: These findings of racial disparities in pain tolerance and endogenous pain inhibition could possibly contribute to the higher pain severity in Indians.

The antinociceptive effect of artemisinin on the inflammatory pain and role of GABAergic and opioidergic systems

BACKGROUND

Pain is a complex mechanism which involves different systems, including the opioidergic and GABAergic systems. Due to the side effects of chemical analgesic agents, attention toward natural agents have been increased. Artemisinin is an herbal compound with widespread modern and traditional therapeutic indications, which its interaction with the GABAergic system and antinoniceptive effects on neuropathic pain have shown. Therefore, this study was designed to evaluate the antinociceptive effects of artemisinin during inflammatory pain and interaction with the GABAergic and opioidergic systems by using a writhing response test.

METHODS

On the whole, 198 adult male albino mice were used in 4 experiments, including 9 groups (n = 6) each with three replicates, by intraperitoneal (i.p.) administration of artemisinin (2.5, 5, and 10 mg/kg), naloxone (2 mg/kg), bicuculline (2 mg/kg), saclofen (2 mg/kg), indomethacin (5 mg/kg), and ethanol (10 mL/kg). Writhing test responses were induced by i.p. injection of 10 mL/kg of 0.6% acetic acid, and the percentage of writhing inhibition was recorded.

RESULTS

Results showed significant dose dependent anti-nociceptive effects from artemisinin which, at a 10 mg/kg dose, was statistically similar to indomethacin. Neither saclofen nor naloxone had antinociceptive effects and did not antagonize antinociceptive effects of artemisinin, whereas bicuculline significantly inhibited the antinocicptive effect of artemisinin.

CONCLUSIONS

It seems that antinocicptive effects of artemisinin are mediated by GABAA receptors.

Acquisition of analgesic properties by the cholecystokinin (CCK)/CCK2 receptor system within the amygdala in a persistent inflammatory pain condition

Pain is associated with negative emotions such as anxiety, but the underlying neurocircuitry and modulators of the association of pain and anxiety remain unclear. The neuropeptide cholecystokinin (CCK) has both pronociceptive and anxiogenic properties, so we explored the role of CCK in anxiety and nociception in the central amygdala (CeA), a key area in control of emotions and descending pain pathways. Local infusion of CCK into the CeA of control rats increased anxiety, as measured in the light-dark box test, but had no effect on mechanical sensitivity. By contrast, intra-CeA CCK infusion 4 days after Complete Freund's Adjuvant (CFA) injection into the hindpaw resulted in analgesia, but also in loss of its anxiogenic capacity. Inflammatory conditions induced changes in the CeA CCK signaling system with an increase of CCK immunoreactivity and a decrease in CCK1, but not CCK2, receptor mRNA. In CFA rats, patch-clamp experiments revealed that CCK infusion increased CeA neuron excitability. It also partially blocked the discharge of wide dynamic range neurons in the dorsal spinal cord. These effects of CCK on CeA and spinal neurons in CFA rats were mimicked by the specific CCK2 receptor agonist, gastrin. This analgesic effect was likely mediated by identified CeA neurons projecting to the periaqueductal gray matter that express CCK receptors. Together, our data demonstrate that intra-CeA CCK infusion activated a descending CCK2 receptor-dependent pathway that inhibited spinal neuron discharge. Thus, persistent pain induces a functional switch to a newly identified analgesic capacity of CCK in the amygdala, indicating central emotion-related circuit controls pain transmission in spinal cord.

Development and Characterization of An Injury-free Model of Functional Pain in Rats by Exposure to Red Light

We report the development and characterization of a novel, injury-free rat model in which nociceptive sensitization after red light is observed in multiple body areas reminiscent of widespread pain in functional pain syndromes. Rats were exposed to red light-emitting diodes (RLED) (LEDs, 660 nm) at an intensity of 50 Lux for 8 hours daily for 5 days resulting in time- and dose-dependent thermal hyperalgesia and mechanical allodynia in both male and female rats. Females showed an earlier onset of mechanical allodynia than males. The pronociceptive effects of RLED were mediated through the visual system. RLED-induced thermal hyperalgesia and mechanical allodynia were reversed with medications commonly used for widespread pain, including gabapentin, tricyclic antidepressants, serotonin/norepinephrine reuptake inhibitors, and nonsteroidal anti-inflammatory drugs. Acetaminophen failed to reverse the RLED induced hypersensitivity. The hyperalgesic effects of RLED were blocked when bicuculline, a gamma-aminobutyric acid-A receptor antagonist, was administered into the rostral ventromedial medulla, suggesting a role for increased descending facilitation in the pain pathway. Key experiments were subjected to a replication study with randomization, investigator blinding, inclusion of all data, and high levels of statistical rigor. RLED-induced thermal hyperalgesia and mechanical allodynia without injury offers a novel injury-free rodent model useful for the study of functional pain syndromes with widespread pain. RLED exposure also emphasizes the different biological effects of different colors of light exposure. PERSPECTIVE: This study demonstrates the effect of light exposure on nociceptive thresholds. These biological effects of red LED add evidence to the emerging understanding of the biological effects of light of different colors in animals and humans. Understanding the underlying biology of red light-induced widespread pain may offer insights into functional pain states.

Fatiguing Trunk Flexor Exercise Decreases Pain Sensitivity in Postpartum Women

Background

Low back pain (LBP) is common in the general population and among postpartum women. Abdominal muscle exercise is often used to treat LBP, but it is unknown if fatiguing abdominal muscle exercise can produce exercise-induced hypoalgesia (EIH).

Objectives

To assess pressure pain thresholds (PPTs) at rest and following fatiguing trunk flexor exercise (EIH) in (1) nulligravid and postpartum women to evaluate the impact of pregnancy and childbirth and (2) nulligravid women and men to examine sex differences.

Methods

Seventy healthy adults (31 postpartum women, 23 nulligravid women, 16 men) participated. Postpartum and nulligravid women were tested twice (16-18 weeks apart) to identify changes in EIH with postpartum recovery. PPTs were measured at the nailbed and superior rectus abdominis before and after exercise to investigate systemic and local EIH, respectively. Rectus abdominis muscle thickness was assessed with ultrasound.

Results

Postpartum women reported lower PPTs than nulligravid women at the abdomen (p < 0.05) whereas postpartum women had lower PPTs at the nailbed during the first session only. Men reported higher nailbed PPTs (p = 0.047) and similar PPTs at the abdomen than women (p = 0.294). All groups demonstrated EIH at the abdomen (p < 0.05). Systemic EIH was absent in postpartum and nulligravid women (p > 0.05), while men demonstrated hyperalgesia. Local EIH was positively associated with muscle thickness for men and women, which was not significant at the second timepoint.

Limitations

Acute exercise response may not reflect changes that occur with exercise training.

Conclusion

Fatiguing trunk flexor exercise produced local EIH for all groups including postpartum and nulligravid women. Clinically, trunk exercises may be useful for acute pain relief for clinical populations that are characterized by pain and/or weakness in the abdominal region muscles in populations with abdominal pain syndromes.

Chronic exposure to insufficient sleep alters processes of pain habituation and sensitization

Chronic pain conditions are highly comorbid with insufficient sleep. While the mechanistic relationships between the 2 are not understood, chronic insufficient sleep may be 1 pathway through which central pain-modulatory circuits deteriorate, thereby contributing to chronic pain vulnerability over time. To test this hypothesis, an in-laboratory model of 3 weeks of restricted sleep with limited recovery (5 nights of 4-hour sleep per night followed by 2 nights of 8-hour sleep per night) was compared with 3 weeks of 8-hour sleep per night (control protocol). Seventeen healthy adults participated, with 14 completing both 3-week protocols. Measures of spontaneous pain, heat-pain thresholds, cold-pain tolerance (measuring habituation to cold over several weeks), and temporal summation of pain (examining the slope of pain ratings during cold water immersion) were assessed at multiple points during each protocol. Compared with the control protocol, participants in the sleep-restriction protocol experienced mild increases in spontaneous pain (P < 0.05). Heat-pain thresholds decreased after the first week of sleep restriction (P < 0.05) but normalized with longer exposure to sleep restriction. By contrast, chronic exposure to restricted sleep was associated with decreased habituation to, and increased temporal summation in response to cold pain (both P < 0.05), although only in the past 2 weeks of the sleep-restriction protocol. These changes may reflect abnormalities in central pain-modulatory processes. Limited recovery sleep did not completely resolve these alterations in pain-modulatory processes, indicating that more extensive recovery sleep is required. Results suggest that exposure to chronic insufficient sleep may increase vulnerability to chronic pain by altering processes of pain habituation and sensitization.

Motor corticospinal excitability: a novel facet of pain modulation?

Introduction

Increase in excitability of the primary motor cortex (M1) is associated with pain inhibition by analgesics, which is, in turn, associated with the psychophysical antinociceptive pain modulation profile. However, the relationship between neurophysiological M1 excitability and psychophysical pain modulation has not yet been explored.

Objectives

We aim to study these relationships in healthy subjects.

Methods

Forty-one young healthy subjects (22 women) underwent a wide battery of psychophysical testing that included conditioned pain modulation (CPM) and pain temporal summation, and a transcranial magnetic stimulation neurophysiological assessment of the motor corticospinal excitability, including resting motor threshold, motor-evoked potentials (MEPs), and cortical silent period.

Results

Increased motor corticospinal excitability in 2 parameters was associated with more efficient CPM: (1) higher MEP amplitude (r = -0.574; P _{_Bonferroni} = 0.02) and (2) longer MEP duration (r = -0.543; P _{_Bonferroni} = 0.02). The latter also correlated with the lower temporal summation magnitude (r = -0.421; P = 0.007); however, on multiplicity adjustment, significance was lost.

Conclusions

Increased corticospinal excitability of the primary motor cortex is associated with more efficient inhibitory pain modulation as assessed by CPM, in healthy subjects. Motor-evoked potential amplitude and duration may be considered as an additional, objective and easy to measure parameter to allow for better individual assessment of pain modulation profile.

Substituting Sedentary Time With Physical Activity in Fibromyalgia and the Association With Quality of Life and Impact of the Disease: The al-Ándalus Project

OBJECTIVE

There is an overall awareness of the detrimental health effects of sedentary time (ST) in fibromyalgia; however, data are limited on how replacement of ST with physical activity (PA) of different intensity may be related to health in this condition. The aim of this study was to examine how a substitution of ST with light PA (LPA) or moderate-to-vigorous PA (MVPA) is associated with quality of life and disease impact.

METHODS

This study comprised 407 women with fibromyalgia, mean ± SD age 51.4 ± 7.6 years. The time spent in ST and PA was measured with triaxial accelerometry. Quality of life and disease impact were assessed using the Short Form 36 (SF-36) health survey and the Revised Fibromyalgia Impact Questionnaire (FIQR), respectively. The substitution of ST with an equivalent time of LPA or MVPA and the associated outcomes were examined using isotemporal substitution analyses.

RESULTS

Substituting 30 minutes of ST with LPA in the isotemporal model was associated with better scores in bodily pain (B = 0.55), vitality (B = 0.74), and social functioning (B = 1.45) according to the SF-36, and better scores at all of the domains (function, overall impact, symptoms, and total impact) of the FIQR (B ranging from -0.95 to -0.27; all P < 0.05). When ST was replaced with MVPA, better physical role (B = 2.30) and social functioning (B = 4.11) of the SF-36 and function of the FIQR (B = -0.73) were observed (all P < 0.05).

CONCLUSION

In regression models, allocation of time of sedentary behavior to either LPA or MVPA was associated with better quality of life and lower disease impact in women with fibromyalgia.

Evaluating psychosocial contributions to chronic pain outcomes

The biopsychosocial model of pain dominates the scientific community's understanding of chronic pain. Indeed, the biopsychosocial approach describes pain and disability as a multidimensional, dynamic integration among physiological, psychological, and social factors that reciprocally influence one another. In this article, we review two categories of studies that evaluate the contributions of psychosocial factors to the experience of chronic pain. First, we consider general psychosocial variables including distress, trauma, and interpersonal factors. Additionally, we discuss pain-specific psychosocial variables including catastrophizing, expectations, and pain-related coping. Together, we present a diverse array of psychological, social, and contextual factors and highlight the need to consider their roles in the development, maintenance, and treatment of chronic pain conditions.

Developing an optimized strategy with transcranial direct current stimulation to enhance the endogenous pain control system in fibromyalgia

INTRODUCTION

Fibromyalgia affects more than 5 million people in the United States and has a detrimental impact on individuals' quality of life. Current pharmacological treatments provide limited benefits to relieve the pain of fibromyalgia, along with a risk of adverse effects; a scenario that explains the increasing interest for multimodal approaches. A tailored strategy to focus on this dysfunctional endogenous pain inhibitory system is transcranial direct current stimulation (tDCS) of the primary motor cortex. By combining tDCS with aerobic exercise, the effects can be optimized. Areas covered: The relevant literature was reviewed and discussed the methodological issues for designing a mechanistic clinical trial to test this combined intervention. Also, we reviewed the neural control of different pathways that integrate the endogenous pain inhibitory system, as well as the effects of tDCS and aerobic exercise both alone and combined. In addition, potential neurophysiological assessments are addressed: conditioned pain modulation, temporal slow pain summation, transcranial magnetic stimulation, and electroencephalography in the context of fibromyalgia. Expert commentary: By understanding the neural mechanisms underlying pain processing and potential optimized interventions in fibromyalgia with higher accuracy, the field has an evident potential of advancement in the direction of new neuromarkers and tailored therapies.

When pain gets stuck: the evolution of pain chronification and treatment resistance

It is well-recognized that, despite similar pain characteristics, some people with chronic pain recover, whereas others do not. In this review, we discuss possible contributions and interactions of biological, social, and psychological perturbations that underlie the evolution of treatment-resistant chronic pain. Behavior and brain are intimately implicated in the production and maintenance of perception. Our understandings of potential mechanisms that produce or exacerbate persistent pain remain relatively unclear. We provide an overview of these interactions and how differences in relative contribution of dimensions such as stress, age, genetics, environment, and immune responsivity may produce different risk profiles for disease development, pain severity, and chronicity. We propose the concept of "stickiness" as a soubriquet for capturing the multiple influences on the persistence of pain and pain behavior, and their stubborn resistance to therapeutic intervention. We then focus on the neurobiology of reward and aversion to address how alterations in synaptic complexity, neural networks, and systems (eg, opioidergic and dopaminergic) may contribute to pain stickiness. Finally, we propose an integration of the neurobiological with what is known about environmental and social demands on pain behavior and explore treatment approaches based on the nature of the individual's vulnerability to or protection from allostatic load.

The Contribution of Endogenous Modulatory Systems to TMS- and tDCS-Induced Analgesia: Evidence from PET Studies

Chronic pain is an important public health issue. Moreover, its adequate management is still considered a major clinical problem, mainly due to its incredible complexity and still poorly understood pathophysiology. Recent scientific evidence coming from neuroimaging research, particularly functional magnetic resonance (fMRI) and positron emission tomography (PET) studies, indicates that chronic pain is associated with structural and functional changes in several brain structures that integrate antinociceptive pathways and endogenous modulatory systems. Furthermore, the last two decades have witnessed a huge increase in the number of studies evaluating the clinical effects of noninvasive neuromodulatory methods, especially transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), which have been proved to effectively modulate the cortical excitability, resulting in satisfactory analgesic effects with minimal adverse events. Nevertheless, the precise neuromechanisms whereby such methods provide pain control are still largely unexplored. Recent studies have brought valuable information regarding the recruitment of different modulatory systems and related neurotransmitters, including glutamate, dopamine, and endogenous opioids. However, the specific neurocircuits involved in the analgesia produced by those therapies have not been fully elucidated. This review focuses on the current literature correlating the clinical effects of noninvasive methods of brain stimulation to the changes in the activity of endogenous modulatory systems.

Enhanced facilitation and diminished inhibition characterizes the pronociceptive endogenous pain modulatory balance of persons living with HIV and chronic pain

Chronic pain in persons living with HIV (PLWH) may be related to alterations in endogenous pain modulatory processes (e.g., high facilitation and low inhibition of nociception) that promote exaggerated pain responses, known as hyperalgesia, and central nervous system (CNS) sensitization. This observational study examined differences in endogenous pain modulatory processes between 59 PLWH with chronic pain, 51 PLWH without chronic pain, and 50 controls without HIV or chronic pain. Quantitative sensory testing for temporal summation (TS) of mechanical and heat pain as well as conditioned pain modulation (CPM) were used to assess endogenous pain facilitatory and inhibitory processes, respectively. Associations among TS, CPM, and self-reported clinical pain severity were also examined in PLWH with chronic pain. Findings demonstrated significantly greater TS of mechanical and heat pain for PLWH with chronic pain compared to PLWH without chronic pain and controls. CPM effects were present in controls, but not in either PLWH with or without chronic pain. Among PLWH with chronic pain, greater TS of mechanical pain was significantly associated with greater average clinical pain severity. Results of this study suggest that enhanced facilitation and diminished inhibition characterizes the pronociceptive endogenous pain modulatory balance of persons living with HIV and chronic pain.

Endogenous Pain Modulation Profiles Among Individuals With Chronic Pain: Relation to Opioid Use

It is generally assumed that individuals exhibiting high pain inhibition also tend to exhibit low pain facilitation, but little research has examined this association in individuals with pain. The aims of this cross-sectional study were 1) to examine the association between measures of conditioned pain modulation (CPM) and temporal summation (TS) in individuals with chronic pain, and 2) to examine whether this association was moderated by demographic (age, sex), psychological (depression, catastrophizing), or medication-related (opioid use) variables. Individuals (N= 190) with back or neck pain completed questionnaires and underwent a series of quantitative sensory testing procedures assessing CPM and TS. Results indicated that individuals with higher levels of CPM showed lower levels of TS, r = -.20, P < .01. Analyses, however, revealed that the magnitude of this association was substantially weaker among opioid users (r= -.08, NS) than nonusers (r= -.34, P < .01). None of the demographic or psychological variables included in our study influenced the association between CPM and TS. The magnitude of CPM was lower for opioid users than nonusers, suggesting that opioid use might dampen the functioning of endogenous pain-inhibitory systems and possibly contribute to a discordance between measures of pain inhibition and pain facilitation. PERSPECTIVE: Results of the present study indicated that greater endogenous pain-inhibitory capacity is associated with lower levels of pain facilitation. This association, however, was not significant among opioid users, suggesting that opioids might compromise the functioning and interrelationship between endogenous pain modulatory systems.

Oculomotor Disturbances in Patients with Chronic Nonspecific Spinal Pain

Objective

There is increasing evidence that the cerebellum has a role in pain processing. The present study investigates whether chronic pain patients, who are likely to have altered pain processing, exhibit signs of subtle cerebellar dysfunction. We used oculomotor tasks to assess dysfunction of the associated neuronal networks, including the cerebellum.

Methods

Thirty patients with chronic nonspecific spinal pain and 30 age- and sex-matched controls were enrolled. Using a head-mounted eye tracker (EyeSeeCam), eye movements were quantified during predictable and unpredictable saccade and smooth pursuit tasks in the horizontal plane.

Results

The initial latency and the velocity variability of smooth pursuit were significantly increased in the chronic spinal pain patients compared with controls (initial latency: 198 ± 20 vs 185 ± 11 ms, P < 0.01; slow phase velocity standard deviation: 3.31 ± 1.02 vs 2.70 ± 0.83°/s, P < 0.05). Moreover, the latency of predictable saccades was prolonged in patients (rightward: 161 ± 20 vs 152 ± 12 ms, P < 0.05; leftward: 164 ± 22 vs 153 ± 18 ms, P = 0.05).

Conclusions

Our results show that chronic spinal pain patients display subtle but significant oculomotor changes as compared with healthy controls. Considering the networks involved in the generation of saccades and smooth pursuit, the results would be consistent with a dysfunction of cerebellar regions, especially parts of the cerebellar hemispheres. Alternatively, they could also point toward a dysfunction in the frontal eye field and/or pontine oculomotor nuclei.

Evidence that Dry Eye Represents a Chronic Overlapping Pain Condition

Recent data suggest that corneal somatosensory dysfunction may be the underlying cause of severe dry eye symptoms in the absence of ocular surface pathology seen in a subset of patients diagnosed with “dry eye syndrome.” This subset of patients tends to demonstrate a unique constellation of symptoms that are persistent, more severe, and generally respond poorly to current dry eye therapies targeting inadequate or dysfunctional tears. A growing body of literature suggests that symptoms in these patients may be better characterized as neuropathic ocular pain rather than dry eye. In these patients, dry eye symptoms are often associated with numerous comorbid pain conditions and evidence of central pain processing abnormalities, where eye pain is just one of multiple overlapping peripheral manifestations. In this review, we discuss the concept and potential mechanisms of chronic overlapping pain conditions as well as evidence for considering neuropathic ocular pain as one of these overlapping pain conditions.

Wireless transcutaneous electrical nerve stimulation device for chemotherapy-induced peripheral neuropathy: an open-label feasibility study

Chemotherapy-induced peripheral neuropathy (CIPN) occurs in approximately 68% of patients who receive neurotoxic chemotherapy and lasts at least 6 months post-chemotherapy in approximately 30% of individuals. CIPN is associated with decreased quality of life and functional impairments. Evidence suggests that CIPN symptoms are caused, in part, by enhanced excitability and impaired inhibition in the central nervous system. Transcutaneous electrical nerve stimulation (TENS) decreases pain by counteracting both of these mechanisms and is efficacious in other conditions associated with neuropathic pain. This single-arm study (n = 29) assessed the feasibility of investigating TENS for CIPN after chemotherapy completion using a wireless, home-based TENS device. Eighty-one percent of eligible patients who were approached enrolled, and 85% of participants who received the TENS device completed the primary (6-week) study term. Qualitative interview data suggest that use of the device on the continuous setting that automatically alternates between 1-h stimulation and rest periods for 5 h/day would be acceptable to most participants. Significant (i.e., p < 0.05) improvements were observed with the EORTC-CIPN20 (percent change from baseline: 13%), SF-MPQ-2 (52%), numeric rating scale of pain (38%), tingling (30%), numbness (20%), and cramping (53%), and UENS large fiber sensation subscore (48%). Preliminary data that support the reliability and construct validity of the UENS for CIPN in cancer survivors are also provided. Together these data suggest that it is feasible to evaluate TENS for CIPN using a wireless, home-based device and that further evaluation of TENS for CIPN in a randomized clinical trial is warranted.

Pain Modulation: From Conditioned Pain Modulation to Placebo and Nocebo Effects in Experimental and Clinical Pain

Accumulating evidence reveal important applications of endogenous pain modulation assessment in healthy controls and in patients in clinical settings, as dysregulations in the balance of pain modulatory circuits may facilitate pain and promote chronification of pain. This article reviews data on pain modulation, focusing on the mechanisms and translational aspects of pain modulation from conditioned pain modulation (CPM) to placebo and nocebo effects in experimental and clinical pain. The specific roles of expectations, learning, neural and neurophysiological mechanisms of the central nervous system are briefly reviewed herein. The interaction between CPM and placebo systems in pain inhibitory pathways is highly relevant in the clinic and in randomized controlled trials yet remains to be clarified. Examples of clinical implications of CPM and its relationship to placebo and nocebo effects are provided. A greater understanding of the role of pain modulation in various pain states can help characterize the manifestation and development of chronic pain and assist in predicting the response to pain-relieving treatments. Placebo and nocebo effects, intrinsic to every treatment, can be used to develop personalized therapeutic approaches that improve clinical outcomes while limiting unwanted effects.

Lateralized kappa opioid receptor signaling from the amygdala central nucleus promotes stress-induced functional pain

The response of diffuse noxious inhibitory controls (DNIC) is often decreased, or lost, in stress-related functional pain syndromes. Because the dynorphin/kappa opioid receptor (KOR) pathway is activated by stress, we determined its role in DNIC using a model of stress-induced functional pain. Male, Sprague-Dawley rats were primed for 7 days with systemic morphine resulting in opioid-induced hyperalgesia. Fourteen days after priming, when hyperalgesia was resolved, rats were exposed to environmental stress and DNIC was evaluated by measuring hind paw response threshold to noxious pressure (test stimulus) after capsaicin injection in the forepaw (conditioning stimulus). Morphine priming without stress did not alter DNIC. However, stress produced a loss of DNIC in morphine-primed rats in both hind paws that was abolished by systemic administration of the KOR antagonist, nor-binaltorphimine (nor-BNI). Microinjection of nor-BNI into the right, but not left, central nucleus of the amygdala (CeA) prevented the loss of DNIC in morphine-primed rats. Diffuse noxious inhibitory controls were not modulated by bilateral nor-BNI in the rostral ventromedial medulla. Stress increased dynorphin content in both the left and right CeA of primed rats, reaching significance only in the right CeA; no change was observed in the rostral ventromedial medulla or hypothalamus. Although morphine priming alone is not sufficient to influence DNIC, it establishes a state of latent sensitization that amplifies the consequences of stress. After priming, stress-induced dynorphin/KOR signaling from the right CeA inhibits DNIC in both hind paws, likely reflecting enhanced descending facilitation that masks descending inhibition. Kappa opioid receptor antagonists may provide a new therapeutic strategy for stress-related functional pain disorders.

Age and gender differences in mechanically induced intraoral temporal summation and conditioned pain modulation in healthy subjects

OBJECTIVE

The aim of this study was to investigate intraoral temporal summation (TS) and conditioned pain modulation (CPM) and compare the outcome with TS and CPM induced in the forearm. In addition, we aimed to study the effect of age and gender on intraoral and forearm TS and CPM.

STUDY DESIGN

Mechanical stimulation was induced with # 5.46 von Frey filament applying 26 grams of force. A single stimulus, followed by a train of 30 successive stimuli, was applied intraorally and to the dominant forearm. CPM was assessed with the TS test as the painful stimulus and with immersion of the nondominant hand in a hot water bath as the conditioning stimulus.

RESULTS

Gender was significantly associated with TS but not with CPM measures. Females had significantly lower mean TS measured in the face and in the dominant forearm compared with males. Age was significantly associated with CPM, but not with TS measures. In both sites examined, older patients had significantly lower mean CPM compared with younger patients.

CONCLUSIONS

Mechanical TM elicited in the oral cavity can be used as test stimulus for CPM testing. Intraoral modulation, both TS and CPM, has an extent similar to that of the standard cutaneous extremity. TS was lower in females, and CPM was reduced with age.

Neuropathic pain-induced enhancement of spontaneous and pain-evoked neuronal activity in the periaqueductal gray that is attenuated by gabapentin

Neuropathic pain is a debilitating pathological condition that is poorly understood. Recent evidence suggests that abnormal central processing occurs during the development of neuropathic pain induced by the cancer chemotherapeutic agent, paclitaxel. Yet, it is unclear what role neurons in supraspinal pain network sites, such as the periaqueductal gray, play in altered behavioral sensitivity seen during chronic pain conditions. To elucidate these mechanisms, we studied the spontaneous and thermally evoked firing patterns of ventrolateral periaqueductal gray (vlPAG) neurons in awake-behaving rats treated with paclitaxel to induce neuropathic pain. In the present study, vlPAG neurons in naive rats exhibited either excitatory, inhibitory, or neutral responses to noxious thermal stimuli, as previously observed. However, after development of behavioral hypersensitivity induced by the chemotherapeutic agent, paclitaxel, vlPAG neurons displayed increased neuronal activity and changes in thermal pain-evoked neuronal activity. This involved elevated levels of spontaneous firing and heightened responsiveness to nonnoxious stimuli (allodynia) as well as noxious thermal stimuli (hyperalgesia) as compared with controls. Furthermore, after paclitaxel treatment, only excitatory neuronal responses were observed for both nonnoxious and noxious thermal stimuli. Systemic administration of gabapentin, a nonopioid analgesic, induced significant dose-dependent decreases in the elevated spontaneous and thermally evoked vlPAG neuronal firing to both nonnoxious and noxious thermal stimuli in rats exhibiting neuropathic pain, but not in naive rats. Thus, these results show a strong correlation between behavioral hypersensitivity to thermal stimuli and increased firing of vlPAG neurons in allodynia and hyperalgesia that occur in this neuropathic pain model.

Negative Affectivity, Depression, and Resting Heart Rate Variability (HRV) as Possible Moderators of Endogenous Pain Modulation in Functional Somatic Syndromes

Background: Several studies have shown that patients with functional somatic syndromes (FSS) have, on average, deficient endogenous pain modulation (EPM), as well as elevated levels of negative affectivity (NA) and high comorbidity with depression and reduced resting heart rate variability (HRV) compared to healthy controls (HC). The goals of this study were (1) to replicate these findings and (2) to investigate the moderating role of NA, depression, and resting HRV in EPM efficiency within a patient group with fibromyalgia and/or chronic fatigue syndrome (CFS). Resting HRV was quantified as the root mean square of successive differences between inter-beat intervals (RMSSD) in rest, a vagally mediated time domain measure of HRV.

Methods: Seventy-eight patients with fibromyalgia and/or CFS and 33 HC completed a counter-irritation paradigm as a measure of EPM efficiency. Participants rated the painfulness of electrocutaneous stimuli (of individually calibrated intensity) on the ankle before (baseline phase), during (counter-irritation phase) and after (recovery phase) the application of a cold pain stimulus on the forearm. A larger reduction in pain in the counter-irritation phase compared to the baseline phase reflects a more efficient EPM.

Results: In contrast to our expectations, there was no difference between pain ratings in the baseline compared to counter-irritation phase for both patients and HC. Therefore, reliable conclusions on the moderating effect of NA, depression, and RMSSD could not be made. Surprisingly, patients reported more pain in the recovery compared to the counter-irritation and baseline phase, while HC did not. This latter effect was more pronounced in patients with comorbid depression, patients who rated the painfulness of the counter-irritation stimulus as high and patients who rated the painfulness of the electrocutaneous stimuli as low. We did not manage to successfully replicate the counter-irritation effect in HC or FSS patients. Therefore, no valid conclusions on the association between RMSSD, depression, NA and EPM efficiency can be drawn from this study. Possible reasons for the lack of the counter-irritation effect are discussed.

Subliminal (latent) processing of pain and its evolution to conscious awareness

By unconscious or covert processing of pain we refer to nascent interactions that affect the eventual deliverance of pain awareness. Thus, internal processes (viz., repeated nociceptive events, inflammatory kindling, reorganization of brain networks, genetic) or external processes (viz., environment, socioeconomic levels, modulation of epigenetic status) contribute to enhancing or inhibiting the presentation of pain awareness. Here we put forward the notion that for many patients, ongoing sub-conscious changes in brain function are significant players in the eventual manifestation of chronic pain. In this review, we provide clinical examples of nascent or what we term pre-pain processes and the neurobiological mechanisms of how these changes may contribute to pain, but also potential opportunities to define the process for early therapeutic interventions.

Chronic pain patients can be classified into four groups: Clustering-based discriminant analysis of psychometric data from 4665 patients referred to a multidisciplinary pain centre (a SQRP study)

OBJECTIVE

To subgroup chronic pain patients using psychometric data and regress the variables most responsible for subgroup discrimination.

DESIGN

Cross-sectional, registry-based study.

SETTING AND SUBJECTS

Chronic pain patients assessed at a multidisciplinary pain centre between 2008 and 2015.

METHODS

Data from the Swedish quality registry for pain rehabilitation (SQRP) were retrieved and analysed by principal component analysis, hierarchical clustering analysis, and partial least squares-discriminant analysis.

RESULTS

Four subgroups were identified. Group 1 was characterized by low "psychological strain", the best relative situation concerning pain characteristics (intensity and spreading), the lowest frequency of fibromyalgia, as well as by a slightly older age. Group 2 was characterized by high "psychological strain" and by the most negative situation with respect to pain characteristics (intensity and spreading). Group 3 was characterized by high "social distress", the longest pain durations, and a statistically higher frequency of females. The frequency of three neuropathic pain conditions was generally lower in this group. Group 4 was characterized by high psychological strain, low "social distress", and high pain intensity.

CONCLUSIONS

The identification of these four clusters of chronic pain patients could be useful for the development of personalized rehabilitation programs. For example, the identification of a subgroup characterized mainly by high perceived "social distress" raises the question of how to best design interventions for such patients. Differentiating between clinically important subgroups and comparing how these subgroups respond to interventions is arguably an important area for further research.

Assessment and manifestation of central sensitisation across different chronic pain conditions

Different neuroplastic processes can occur along the nociceptive pathways and may be important in the transition from acute to chronic pain and for diagnosis and development of optimal management strategies. The neuroplastic processes may result in gain (sensitisation) or loss (desensitisation) of function in relation to the incoming nociceptive signals. Such processes play important roles in chronic pain, and although the clinical manifestations differ across condition processes, they share some common mechanistic features. The fundamental understanding and quantitative assessment of particularly some of the central sensitisation mechanisms can be translated from preclinical studies into the clinic. The clinical perspectives are implementation of such novel information into diagnostics, mechanistic phenotyping, prevention, personalised treatment, and drug development. The aims of this paper are to introduce and discuss (1) some common fundamental central pain mechanisms, (2) how they may translate into the clinical signs and symptoms across different chronic pain conditions, (3) how to evaluate gain and loss of function using quantitative pain assessment tools, and (4) the implications for optimising prevention and management of pain. The chronic pain conditions selected for the paper are neuropathic pain in general, musculoskeletal pain (chronic low back pain and osteoarthritic pain in particular), and visceral pain (irritable bowel syndrome in particular). The translational mechanisms addressed are local and widespread sensitisation, central summation, and descending pain modulation.

Significance

Central sensitisation is an important manifestation involved in many different chronic pain conditions. Central sensitisation can be different to assess and evaluate as the manifestations vary from pain condition to pain condition. Understanding central sensitisation may promote better profiling and diagnosis of pain patients and development of new regimes for mechanism based therapy. Some of the mechanisms underlying central sensitisation can be translated from animals to humans providing new options in development of therapies and profiling drugs under development.

Stress-Induced Chronic Visceral Pain of Gastrointestinal Origin

Visceral pain is generally poorly localized and characterized by hypersensitivity to a stimulus such as organ distension. In concert with chronic visceral pain, there is a high comorbidity with stress-related psychiatric disorders including anxiety and depression. The mechanisms linking visceral pain with these overlapping comorbidities remain to be elucidated. Evidence suggests that long term stress facilitates pain perception and sensitizes pain pathways, leading to a feed-forward cycle promoting chronic visceral pain disorders such as irritable bowel syndrome (IBS). Early life stress (ELS) is a risk-factor for the development of IBS, however the mechanisms responsible for the persistent effects of ELS on visceral perception in adulthood remain incompletely understood. In rodent models, stress in adult animals induced by restraint and water avoidance has been employed to investigate the mechanisms of stress-induce pain. ELS models such as maternal separation, limited nesting, or odor-shock conditioning, which attempt to model early childhood experiences such as neglect, poverty, or an abusive caregiver, can produce chronic, sexually dimorphic increases in visceral sensitivity in adulthood. Chronic visceral pain is a classic example of gene × environment interaction which results from maladaptive changes in neuronal circuitry leading to neuroplasticity and aberrant neuronal activity-induced signaling. One potential mechanism underlying the persistent effects of stress on visceral sensitivity could be epigenetic modulation of gene expression. While there are relatively few studies examining epigenetically mediated mechanisms involved in visceral nociception, stress-induced visceral pain has been linked to alterations in DNA methylation and histone acetylation patterns within the brain, leading to increased expression of pro-nociceptive neurotransmitters. This review will discuss the potential neuronal pathways and mechanisms responsible for stress-induced exacerbation of chronic visceral pain. Additionally, we will review the importance of specific experimental models of adult stress and ELS in enhancing our understanding of the basic molecular mechanisms of pain processing.

Preoperative Norepinephrine Levels in Cerebrospinal Fluid and Plasma Correlate With Pain Intensity After Pediatric Spine Surgery

PURPOSE

Catecholamines were found to be involved in descending pain modulation and associated with perioperative pain. The purpose of the present study was to investigate the relationship between preoperative concentrations of catecholamines and postoperative pain intensity of pediatric patients.

METHODS

Fifty adolescents with idiopathic scoliosis scheduled for elective spinal fusion surgery were enrolled in this prospective cohort study. Preoperative plasma and cerebrospinal fluid (CSF) samples were collected and analyzed by mass spectrometry. Pain intensity was assessed during the acute postoperative period and in the intermediate period.

RESULTS

Preoperative plasma concentrations of norepinephrine (NE) and normetanephrine (NME), as well as the CSF concentration of NE, were significantly correlated with the presence of pain six weeks after surgery (r = 0.48, 0.50, and 0.50, respectively; p < .002). We also found that preoperative NE levels in CSF were significantly higher in patients reporting moderate to severe pain intensity than in patients with mild pain during the first day following surgery (0.268 ± 0.29 ng/mL vs. 0.121 ± 0.074 ng/mL, p = .01), as well as between patients reporting pain and painless patients at 6 weeks postsurgery (0.274 ± 0.282 ng/mL vs. 0.103 ± 0.046 ng/mL respectively, U = 69.5, p = .002).

CONCLUSIONS

These results support the potential role of catecholamine levels in predicting postoperative pain intensity.

Treatment response and central pain processing in Anterior Cutaneous Nerve Entrapment Syndrome: An explorative study

BACKGROUND

10-30% of chronic abdominal pain originates in the abdominal wall. A common cause for chronic abdominal wall pain is the Anterior Cutaneous Nerve Entrapment Syndrome (ACNES), in which an intercostal nerve branch is entrapped in the abdominal rectus sheath. Treatment consists of local anaesthetics and neurectomy, and is ineffective in 25% of cases for yet unknown reasons. In some conditions, chronic pain is the result of altered pain processing. This so-called sensitization can manifest as segmental or even generalized hyperalgesia, and is generally difficult to treat.

OBJECTIVE

The aim of this study was to assess pain processing in ACNES patients responsive and refractory to treatment by using Quantitative Sensory Testing, in order to explore whether signs of altered central pain processing are present in ACNES and are a possible explanation for poor treatment outcomes.

METHODS

50 patients treated for ACNES with locally orientated treatment were included. They were allocated to a responsive or refractory group based on their response to treatment. Patients showing an improvement of the Visual Analogue Scale (VAS) pain score combined with a current absolute VAS of <40mm were scored as responsive. Sensation and pain thresholds to pressure and electric skin stimulation were determined in the paravertebral bilateral ACNES dermatomes and at four control areas on the non-dominant side of the body, i.e. the musculus trapezius pars medialis, musculus rectus femoris, musculus abductor hallucis and the thenar. The ACNES dermatomes were chosen to signal segmental hyperalgesia and the sum of the control areas together as a reflection of generalized hyperalgesia. Lower thresholds were interpreted as signs of sensitized pain processing. To test for alterations in endogenous pain inhibition, a conditioned pain modulation (CPM) response to a cold pressor task was determined. Also, patients filled in three pain-related questionnaires, to evaluate possible influence of psychological characteristics on the experienced pain.

RESULTS

Patients refractory to treatment showed significantly lower pressure pain thresholds in the ACNES dermatomes and for the sum of as well as in two individual control areas. No differences were found between groups for electric thresholds or CPM response. Duration of complaints before diagnosis and treatment was significantly longer in the refractory compared to the responsive group, and refractory patients scored higher on the pain-related psychological surveys.

CONCLUSION AND IMPLICATIONS

In this hypothesis-generating exploratory study, ACNES patients refractory to treatment showed more signs of sensitized segmental and central pain processing. A longer duration of complaints before diagnosis and treatment may be related to these alterations in pain processing, and both findings could be associated with less effective locally orientated treatment. In order to validate these hypotheses further research is needed.

REGISTRATION NUMBER

NCT01920880 (Clinical Trials Register; http://www.clinicaltrials.gov).

Loss of Temporal Inhibition of Nociceptive Information Is Associated With Aging and Bodily Pain

An age-related decline in endogenous pain inhibitory processes likely places older adults at an increased risk for chronic pain. Limited research indicates that older adults may be characterized by deficient offset analgesia, an inhibitory temporal sharpening mechanism that increases the detectability of minor decreases in noxious stimulus intensity. The primary purpose of the study was to examine age differences in offset analgesia in community-dwelling younger, middle-aged, and older adults. An additional aim of the study was to determine whether the magnitude of offset analgesia predicted self-reported bodily pain. Eighty-seven younger adults, 42 middle-aged adults, and 60 older adults completed 4 offset analgesia trials and 3 constant temperature trials in which a noxious heat stimulus was applied to the volar forearm for 40 seconds. The offset trials consisted of 3 continuous phases: an initial 10-second painful stimulus, either a 1.0°C or .4°C increase in temperature from the initial 10-second painful stimulus for 10 seconds, and either a 1.0°C or .4°C decrease back to the initial testing temperature for 20 seconds. During each trial, subjects rated pain intensity continuously using an electronic visual analog scale (0-100). All subjects also completed the Short-Form Health Survey-36 including the Bodily Pain subscale. The results indicated that older and middle-aged adults showed reduced offset analgesia compared with younger adults in the 1.0°C and .4°C offset trials. Furthermore, the magnitude of offset analgesia predicted self-reported bodily pain, with those exhibiting reduced offset analgesia reporting greater bodily pain. Dysfunction of this endogenous inhibitory system could increase the risk of developing chronic pain for middle-aged and older adults.

PERSPECTIVE

Older and middle-aged adults showed reduced offset analgesia compared with younger adults. The significant association between reduced offset analgesia and pain in daily life supports the notion that pain modulatory deficits are associated with not just a chronic pain condition but with the experience of pain in general.

Evoked Pressure Pain Sensitivity Is Associated with Differential Analgesic Response to Verum and Sham Acupuncture in Fibromyalgia

OBJECTIVE

Fibromyalgia is a chronic pain condition with few effective treatments. Many fibromyalgia patients seek acupuncture for analgesia; however, its efficacy is limited and not fully understood. This may be due to heterogeneous pathologies among participants in acupuncture clinical trials. We hypothesized that pressure pain tenderness would differentially classify treatment response to verum and sham acupuncture in fibromyalgia patients.

DESIGN

Baseline pressure pain sensitivity at the thumbnail at baseline was used in linear mixed models as a modifier of differential treatment response to sham versus verum acupuncture. Similarly, needle-induced sensation was also analyzed to determine its differential effect of treatment on clinical pain.

METHODS AND PATIENTS

A cohort of 114 fibromyalgia patients received baseline pressure pain testing and were randomized to either verum (N = 59) or sham (N = 55) acupuncture. Participants received treatments from once a week to three times a week, increasing in three-week blocks for a total of 18 treatments. Clinical pain was measured on a 101-point visual analog scale, and needle sensation was measured by questionnaire throughout the trial.

RESULTS

Participants who had higher pain pressure thresholds had greater reduction in clinical pain following verum acupuncture while participants who had lower pain pressure thresholds showed better analgesic response to sham acupuncture. Moreover, patients with lower pressure pain thresholds had exacerbated clinical pain following verum acupuncture. Similar relationships were observed for sensitivity to acupuncture needling.

CONCLUSIONS

These findings suggest that acupuncture efficacy in fibromyalgia may be underestimated and a more personalized treatment for fibromyalgia may also be possible.