Role of endogenous pain modulation in chronic pain mechanisms and treatment

Pain-related anxiety promotes pronociceptive processes in Native Americans: bootstrapped mediation analyses from the Oklahoma Study of Native American Pain Risk

INTRODUCTION

Evidence suggests Native Americans (NAs) experience higher rates of chronic pain than the general US population, but the mechanisms contributing to this disparity are poorly understood. Recently, we conducted a study of healthy, pain-free NAs (n = 155), and non-Hispanic whites (NHWs, n = 150) to address this issue and found little evidence that NAs and NHWs differ in pain processing (assessed from multiple quantitative sensory tests). However, NAs reported higher levels of pain-related anxiety during many of the tasks.

OBJECTIVE

The current study is a secondary analysis of those data to examine whether pain-related anxiety could promote pronociceptive processes in NAs to put them at chronic pain risk.

METHODS

Bootstrapped indirect effect tests were conducted to examine whether pain-related anxiety mediated the relationships between race (NHW vs NA) and measures of pain tolerance (electric, heat, ischemia, and cold pressor), temporal summation of pain and the nociceptive flexion reflex (NFR), and conditioned pain modulation of pain/NFR.

RESULTS

Pain-related anxiety mediated the relationships between NA race and pain tolerance and conditioned pain modulation of NFR. Exploratory analyses failed to show that race moderated relationships between pain-related anxiety and pain outcomes.

CONCLUSION

These findings imply that pain-related anxiety is not a unique mechanism of pain risk for NAs, but that the greater tendency to experience pain-related anxiety by NAs impairs their ability to engage descending inhibition of spinal nociception and decreases their pain tolerance (more so than NHWs). Thus, pain-related anxiety may promote pronociceptive processes in NAs to place them at risk for future chronic pain.

Total sleep deprivation increases pain sensitivity, impairs conditioned pain modulation and facilitates temporal summation of pain in healthy participants

Chronic pain patients often suffer from insomnia or impaired sleep which has been associated with increased pain sensitivity, but a limited amount of studies have investigated the effects of total sleep deprivation on central pain mechanisms. Therefore, the aim of this study was to determine the effects of total sleep deprivation on temporal summation, conditioned pain modulation, thermal and pressure pain sensitivity in healthy participants. Twenty-four healthy participants took part in this two-session trial. The measurements were conducted after a night of habitual sleep (baseline) and following 24 hours of total sleep deprivation. Detection thresholds for cold and warmth and pain thresholds for cold and heat were assessed. Cuff induced pressure pain detection and tolerance thresholds, temporal summation and conditioned pain modulation were assessed with user-independent, computer-controlled cuff algometry. Conditioned pain modulation was significantly impaired, temporal summation was significantly facilitated and pain sensitivity to pressure and cold pain were significantly increased at follow-up compared with baseline. In conclusion, this study found that one night of total sleep deprivation impaired descending pain pathways, facilitated spinal excitability and sensitized peripheral pathways to cold and pressure pain. Future studies are encouraged to investigate if sleep therapy might normalize pain sensitivity in sleep-deprived chronic pain patients.

Peripheral nerve injuries, pain, and neuroplasticity

INTRODUCTION

Peripheral nerve injuries (PNIs) cause both structural and functional brain changes that may be associated with significant sensorimotor abnormalities and pain.

PURPOSE OF THE STUDY

The aim of this narrative review is to provide hand therapists an overview of PNI-induced neuroplasticity and to explain how the brain changes following PNI, repair, and during rehabilitation.

METHODS

Toward this goal, we review key aspects of neuroplasticity and neuroimaging and discuss sensory testing techniques used to study neuroplasticity in PNI patients.

RESULTS

We describe the specific brain changes that occur during the repair and recovery process of both traumatic (eg, transection) and nontraumatic (eg, compression) nerve injuries. We also explain how these changes contribute to common symptoms including hypoesthesia, hyperalgesia, cold sensitivity, and chronic neurogenic pain. In addition, we describe how maladaptive neuroplasticity as well as psychological and personality characteristics impacts treatment outcome.

DISCUSSION AND CONCLUSION

Greater understanding of the brain's contribution to symptoms in recovering PNI patients could help guide rehabilitation strategies and inform the development of novel techniques to counteract these maladaptive brain changes and ultimately improve outcomes.

Acute and chronic pain in orofacial trauma patients

Trauma or injury to the dentition and supporting tissues is associated with pain and discomfort, as expected, that may present immediately, shortly afterwards, or within a few days. Pain is an essential response to injury because it allows the organism to develop avoidance behavior to potential threats and helps the organism to avoid usage of the injured organ during the healing process. Not only does external trauma induce pain, but also essential invasive dental procedures such as extractions, dental implant insertions, root canal treatments, and oral surgeries are accompanied by similar post-surgical (post-traumatic) pain. The pain intensity after trauma varies and does not always correlate with the extent of injury. Trauma to the orofacial region or the teeth may also indirectly affect and induce pain in other orofacial structures such as the masticatory muscles, the temporomandibular joint, and even the cervical spine. In most cases, the pain will resolve as soon as healing of the affected tissue occurs or after dental and routine palliative treatment. In a limited number of cases, the pain persists beyond healing and evolves into a chronic pain state. Chronic pain in the orofacial region presents diagnostic and management challenges. Misdiagnosis or delayed diagnosis of the oral chronic pain condition may lead to unnecessary dental treatment. This article will discuss diagnosis and treatment for acute and chronic pain as well as potential mechanisms involved in the undesirable transition from acute to chronic pain.

Measuring sensory and pain thresholds by Semmes-Weinstein monofilaments in patients with leg ulcers: a pilot study

OBJECTIVE

Pain is a common and disabling symptom in patients with leg ulcers. Clinical quantification of pain mostly depends on subjective pain reports, which do not reveal underlying mechanisms. The aim of this pilot study is to identify mechanisms underlying the pain in patients with leg ulcers by documenting alterations in pain processing using quantitative sensory testing.

METHODS

In nine ulcer patients the mechanical sensory thresholds and the mechanical pain thresholds were determined by Semmes-Weinstein monofilaments (SWM) at three different sites: on the contralateral (unaffected) leg, on the skin of the affected leg 10cm from the ulcer margin, and on the affected leg, close (1-2cm) to the ulcer margin. Besides the mechanical sensory thresholds and mechanical pain thresholds, pain at the site of the ulcer, using an 11-point numeric rating scale (NRS), was documented.

RESULTS

Mechanical sensory thresholds were increased in all subjects. Almost half (44%) of patients consistently showed allodynia at the unaffected site. The lowering of mechanical pain thresholds correlated with higher scores on the NRS.

CONCLUSION

All patients showed diminished touch and/or protective sensation, which might have contributed to ulcer development via (partial) loss of protective function. The allodynia at the unaffected site suggests the presence of central sensitisation of pain processing.

Conditioned pain modulation in drug-naïve patients with de novo Parkinson's disease

Background

Pain is highly prevalent in patients with Parkinson’s disease (PD), but underlying pathophysiological mechanisms are largely unclear. In many chronic pain syndromes deficits in endogenous pain inhibition have been detected that can be assessed using conditioned pain modulation paradigms. Previous studies employing this approach in medicated PD patients did not find abnormal pain inhibition. However, these results might have been confounded by residual dopaminergic medication.

Methods

An established conditioned pain modulation paradigm was used in 17 drug-naïve de novo PD patients and 17 healthy age and gender-matched controls. We tested i) whether conditioned pain modulation responses differed between the patient and control group and ii) whether pain inhibition differed between PD subtypes.

Results

PD patients and healthy controls did not differ in their conditioned pain modulation responses. Furthermore, there were no significant differences in CPM responses depending on the PD subtype. However, at a descriptive level, tremor-dominant patients showed a tendency for better descending pain inhibition compared to akinetic-rigid and mixed type patients.

Conclusions

In this first study investigating conditioned pain modulation in de novo PD patients, we found no additional impairment in descending pain modulation besides the known age-related decline. Our findings indicate that mechanisms other than an impaired descending inhibition contribute to high pain prevalence rates in PD and suggest that mechanisms underlying pain may differ between PD subtypes.

Electronic supplementary material

The online version of this article (10.1186/s42466-019-0029-x) contains supplementary material, which is available to authorized users.

From Mechanism to Cure: Renewing the Goal to Eliminate the Disease of Pain

Objective

Persistent pain causes untold misery worldwide and is a leading cause of disability. Despite its astonishing prevalence, pain is undertreated, at least in part because existing therapeutics are ineffective or cause intolerable side effects. In this review, we cover new findings about the neurobiology of pain and argue that all but the most transient forms of pain needed to avoid tissue damage should be approached as a disease where a cure can be the goal of all treatment plans, even if attaining this goal is not yet always possible.

Design

We reviewed the literature to highlight recent advances in the area of the neurobiology of pain.

Results

We discuss barriers that are currently hindering the achievement of this goal, as well as the development of new therapeutic strategies. We also discuss innovations in the field that are creating new opportunities to treat and even reverse persistent pain, some of which are in late-phase clinical trials.

Conclusion

We conclude that the confluence of new basic science discoveries and development of new technologies are creating a path toward pain therapeutics that should offer significant hope of a cure for patients and practitioners alike. Classification of Evidence. Our review points to new areas of inquiry for the pain field to advance the goal of developing new therapeutics to treat chronic pain.

Long-term effects of neonatal pain

Pain experienced during neonatal intensive care management can influence neurodevelopmental outcome and the somatosensory and/or emotional components of pain response in later life. Alterations in biological factors (e.g. peripheral and central somatosensory function and modulation, brain structure and connectivity) and psychosocial factors (e.g. gender, coping style, mood, parental response) that influence pain have been identified in children and young adults born very preterm or extremely preterm. Earlier gestational age at birth and cumulative pain exposure from tissue-breaking procedures and/or neonatal surgery influence the degree of change. In neonatal rodents, repeated needle insertion or hindpaw incision identify developmentally-regulated and activity-dependent long term alterations in nociceptive processing, and the efficacy of novel or current analgesic interventions can be compared. As prior neonatal experience and sex may influence current pain experience or the risk of persistent pain, these factors should be considered within the biopsychosocial assessment and formulation of pain in later life.

Sensory Modulation Disorder (SMD) and Pain: A New Perspective

Sensory modulation disorder (SMD) affects sensory processing across single or multiple sensory systems. The sensory over-responsivity (SOR) subtype of SMD is manifested clinically as a condition in which non-painful stimuli are perceived as abnormally irritating, unpleasant, or even painful. Moreover, SOR interferes with participation in daily routines and activities (Dunn, 2007; Bar-Shalita et al., 2008; Chien et al., 2016), co-occurs with daily pain hyper-sensitivity, and reduces quality of life due to bodily pain. Laboratory behavioral studies have confirmed abnormal pain perception, as demonstrated by hyperalgesia and an enhanced lingering painful sensation, in children and adults with SMD. Advanced quantitative sensory testing (QST) has revealed the mechanisms of altered pain processing in SOR whereby despite the existence of normal peripheral sensory processing, there is enhanced facilitation of pain-transmitting pathways along with preserved but delayed inhibitory pain modulation. These findings point to central nervous system (CNS) involvement as the underlying mechanism of pain hypersensitivity in SOR. Based on the mutual central processing of both non-painful and painful sensory stimuli, we suggest shared mechanisms such as cortical hyper-excitation, an excitatory-inhibitory neuronal imbalance, and sensory modulation alterations. This is supported by novel findings indicating that SOR is a risk factor and comorbidity of chronic non-neuropathic pain disorders. This is the first review to summarize current empirical knowledge investigating SMD and pain, a sensory modality not yet part of the official SMD realm. We propose a neurophysiological mechanism-based model for the interrelation between pain and SMD. Embracing the pain domain could significantly contribute to the understanding of this condition’s pathogenesis and how it manifests in daily life, as well as suggesting the basis for future potential mechanism-based therapies.

The silent epidemic of chronic pain in older adults

Chronic pain is highly prevalent among older adults where it is associated with significant suffering, disability, social isolation, and greater costs and burden to health care systems. Pharmaceutical treatment of chronic pain in older adults is usually only partially effective and is often limited by side effects including urinary retention, constipation, sedation, cognitive impairment, and increased risk of falls. Since older adults are underrepresented in clinical trials testing treatments for chronic pain, the potential impacts of polypharmacy and frailty on reported outcomes and side effect profiles are largely unknown. Thus, for current treatments, providers and patients must balance anticipated benefits of pain reduction with the known and unknown risks of treatment. Chronic pain is also a risk factor for premature death as well as accelerated cognitive decline, suggesting potential shared mechanisms between persistent pain (or its treatment) and dementia. Cognitive decline and dementia may also impact pain perception and the ability to report pain, complicating treatment decisions. Associations between persistent pain and the risks of premature death and accelerated cognitive decline make estimates for chronic pain in these populations particularly challenging. Future research is needed to improve estimates for chronic pain in older adults, to elucidate underlying mechanisms of pain with aging, and to develop and advance safer, more effective treatment options for chronic pain in older adults.

High Temporal Summation of Pain Predicts Immediate Analgesic Effect of Acupuncture in Chronic Pain Patients-A Prospective Cohort Study

Objectives: This prospective cohort study explored whether two distinguished sensory parameters predicted acupuncture effects in chronic pain patients; namely high temporal summation of pain (TS) indicating spinal synaptic facilitation as well as a low vibration detection threshold (VDT) indicating a loss of Aβ-fiber function. Methods: Pinprick induced TS and VDT were assessed by standardized, validated methods at the most painful body site and a pain free control site in 100 chronic pain patients receiving six acupuncture sessions as part of an interdisciplinary multimodal pain treatment (IMPT). Immediate change in pain intensity after the first acupuncture session (first treatment on the first day of IMPT) was assessed by the verbal rating scale (VRS, 0-100). After 4 weeks of treatment, patients indicated in a questionnaire whether acupuncture had relieved pain immediately and whether it had contributed to overall pain reduction and well-being after IMPT. Results: Logistic regression analysis revealed an association between high TS at the control site and a reduction in pain intensity of at least 30% (VRS) after the first acupuncture (OR [95%-CI] 4.3 [1.6-11.8]). Questionnaire ratings of immediate pain relief after acupuncture were associated with high TS at the control site (OR [95%-CI] 3.8 [1.4-10.2] any pain relief, OR [95%-CI] 5.5 [1.7-17.1] over 50% pain reduction) and at the pain site (OR [95%-CI] 3.2 [1.2-8.9] any pain relief). Appraisals of the contribution of acupuncture to overall pain reduction and well-being after IMPT were not associated with TS. The VDT was not associated with any outcome. Conclusion: This explorative study provides first-time evidence that high TS, especially at a pain free control site, but not VDT, might predict immediate analgesic response to acupuncture in chronic pain patients. Thus, highly centrally sensitized chronic pain patients might respond particularly well to acupuncture.

Naloxone prolongs abdominal constriction writhing-like behavior in a zebrafish-based pain model

The ability to detect noxious stimuli is essential to survival. However, pathological pain is maladaptive and severely debilitating. Endogenous and exogenous opioids modulate pain responses via opioid receptors, reducing pain sensibility. Due to the high genetic and physiological similarities to rodents and humans, the zebrafish is a valuable tool to assess pain responses and the underlying mechanisms involved in nociception. Although morphine attenuates pain-like responses of zebrafish, there are no data showing if the antagonism of opioid receptors prolongs pain duration in the absence of an exogenous opioid. Here, we investigated whether a common opioid antagonist naloxone affects the abdominal constriction writhing-like response, recently characterized as a zebrafish-based pain behavior. Animals were injected intraperitoneally with acetic acid (5.0%), naloxone (1.25 mg/kg; 2.5 mg/kg; 5.0 mg/kg) or acetic acid with naloxone to investigate the changes in their body curvature for 1 h. Acetic acid elicited a robust pain-like response in zebrafish, as assessed by aberrant abdominal body curvature, while no effects were observed following PBS injection. Although naloxone alone did not alter the frequency and duration of this behavior, it dose-dependently prolonged acetic acid-induced abdominal curvature response. Besides reinforcing the use of the abdominal writhing-like phenotype as a behavioral endpoint to measure acute pain responses in zebrafish models, our novel data suggest a putative role of endogenous opioids in modulating the recovery from pain stimulation in zebrafish.

Evidence of impaired pain modulation in adolescents with idiopathic scoliosis and chronic back pain

BACKGROUND CONTEXT

Although 40% of adolescent idiopathic scoliosis (AIS) patients present with chronic back pain, the pathophysiology and underlying pain mechanisms remain poorly understood. We hypothesized that development of chronic pain syndrome in AIS is associated with alterations in pain modulatory mechanisms.

PURPOSE

To identify the presence of sensitization in nociceptive pathways and to assess the efficacy of the diffuse noxious inhibitory control in patients with AIS presenting with chronic back pain.

STUDY DESIGN

Cross-sectional study.

PATIENT SAMPLE

Ninety-four patients diagnosed with AIS and chronic back pain.

OUTCOME MEASURES

Quantitative sensory testing (QST) assessed pain modulation and self-reported questionnaires were used to assess pain burden and health-related quality of life.

METHODS

Patients underwent a detailed pain assessment using a standard and validated quantitative sensory testing (QST) protocol. The measurements included mechanical detection thresholds (MDT), pain pressure threshold (PPT), heat pain threshold (HPT), heat tolerance threshold (HTT), and a conditioned pain modulation (CPM) paradigm. Altogether, these tests measured changes in regulation of the neurophysiology underlying the nociceptive processes based on the patient's pain perception. Funding was provided by The Louise and Alan Edwards Foundation and The Shriners Hospitals for Children.

RESULTS

Efficient pain inhibitory response was observed in 51.1% of patients, while 21.3% and 27.7% had sub-optimal and inefficient CPM, respectively. Temporal summation of pain was observed in 11.7% of patients. Significant correlations were observed between deformity severity and pain pressure thresholds (p=.023) and CPM (p=.017), neuropathic pain scores and pain pressure thresholds (p=.015) and temporal summation of pain (p=.047), and heat temperature threshold and pain intensity (p=.048).

CONCLUSIONS

Chronic back pain has an impact in the quality of life of adolescents with idiopathic scoliosis. We demonstrated a high prevalence of impaired pain modulation in this group. The association between deformity severity and somatosensory dysfunction may suggest that spinal deformity can be a trigger for abnormal neuroplastic changes in this population contributing to chronic pain syndrome.

Descending Control Mechanisms and Chronic Pain

PURPOSE OF REVIEW

The goal of the review was to highlight recent advances in our understanding of descending pain-modulating systems and how these contribute to persistent pain states, with an emphasis on the current state of knowledge around "bottom-up" (sensory) and "top-down" (higher structures mediating cognitive and emotional processing) influences on pain-modulating circuits.

RECENT FINDINGS

The connectivity, physiology, and function of these systems have been characterized extensively over the last 30 years. The field is now beginning to ask how and when these systems are engaged to modulate pain. A recent focus is on the parabrachial complex, now recognized as the major relay of nociceptive information to pain-modulating circuits, and plasticity in this circuit and its connections to the RVM is marked in persistent inflammatory pain. Top-down influences from higher structures, including hypothalamus, amygdala, and medial prefrontal areas, are also considered. The challenge will be to tease out mechanisms through which a particular behavioral context engages distinct circuits to enhance or suppress pain, and to understand how these mechanisms contribute to chronic pain.

Conditioned Pain Modulation in Sexual Assault Survivors

Sexual assault (SA) is associated with an increased risk of chronic pain, but the mechanisms for this relationship are poorly understood. To explore whether disrupted descending inhibition is involved, this study used a conditioned pain modulation task to study the inhibition of pain and the nociceptive flexion reflex (NFR; a correlate of spinal nociception) in 32 pain-free SA survivors. This group was compared with 32 pain-free, trauma-exposed persons without SA and a group of 40 pain-free persons who reported no trauma exposure. Conditioned pain modulation was assessed from painful electric stimulations (test stimulus) delivered to the ankle before, during, and after participants submerged their hand in painful 10°C water (conditioning stimulus). Pain ratings and NFR were assessed in response to test stimuli. All groups demonstrated significant inhibition of pain during conditioned pain modulation. However, only the no trauma exposure group demonstrated significant inhibition of NFR. The persons without SA group showed no inhibition of NFR, whereas the SA group showed significant facilitation of the NFR. These findings suggest that trauma exposure may impair inhibitory cerebrospinal circuits, but that SA may specifically promote facilitation of spinal nociception. Perspective: This study suggests that trauma exposure disrupts the cerebrospinal inhibition of spinal nociception, but that exposure to SA further promotes chronic pain risk by facilitating spinal nociception. This finding help may help to elucidate the pain risk mechanisms in trauma survivors.

Mechanism-based pain management in chronic pancreatitis - is it time for a paradigm shift?

INTRODUCTION

Pain is the most common symptom in chronic pancreatitis and treatment remains a challenge. Management of visceral pain, in general, is only sparsely documented, and treatment in the clinic is typically based on empirical knowledge from somatic pain conditions. This may be problematic, as many aspects of the neurobiology differ significantly from somatic pain, and organs such as the gut and liver play a major role in tolerability to analgesics. On the other hand, clinical awareness and new methods for quantitative assessment of pain mechanisms, will likely increase our understanding of the visceral pain system and guide more individualized pain management. Areas covered: This review includes an overview of known pain mechanisms in chronic pancreatitis and how to characterize them using quantitative sensory testing. The aim is to provide a mechanism-oriented approach to analgesic treatment, including treatment of psychological factors affecting pain perception and consideration of side effects in the management plan. Expert opinion: A mechanism-based examination and profiling of pain in chronic pancreatitis will enable investigators to provide a well-substantiated approach to effective management. This mechanism-based, individualized regime will pave the road to better pain relief and spare the patient from unnecessary trial-and-error approaches and unwanted side effects.

Association Between Pain Sensitization and Disease Activity in Patients With Rheumatoid Arthritis: A Cross-Sectional Study

OBJECTIVE

Pain sensitization may contribute to pain severity in rheumatoid arthritis (RA), impacting disease activity assessment. We examined whether pain processing mechanisms were associated with disease activity among RA patients with active disease.

METHODS

The study included 139 subjects enrolled in the Central Pain in Rheumatoid Arthritis cohort. Subjects underwent quantitative sensory testing (QST), including assessment of pressure pain thresholds (PPTs) at multiple sites, conditioned pain modulation, and temporal summation. RA disease activity was assessed using the Clinical Disease Activity Index (CDAI) and its components. We examined cross-sectional associations between QST measures and disease activity using linear regression.

RESULTS

Low PPTs (high pain sensitization) at all sites were associated with high CDAI scores (P ≤ 0.03) and tender joint counts (P ≤ 0.002). Associations between PPTs and patient global assessments were also seen at most sites. High temporal summation at the forearm (also reflecting high pain sensitization) was significantly associated with high CDAI scores (P = 0.02), patient global assessment scores (P = 0.0006), evaluator global assessment scores (P = 0.01), and tender joint counts (P = 0.02). Conversely, conditioned pain modulation (a measure of descending inhibitory pain pathways) was associated only with tender joint count (P = 0.03).

CONCLUSION

High pain sensitization is associated with elevations in disease activity measures. Longitudinal studies are underway to elucidate the cause-effect relationships between pain sensitization and inflammatory disease activity in RA.

Duration of symptoms is associated with conditioned pain modulation and somatosensory measures in knee osteoarthritis

Impaired descending pain inhibition has been linked to chronic pain and poorer patient outcomes. Vibration perception threshold (VPT) has also been observed and linked to radiographic stage of OA. However, it is not known how duration of symptoms may influence these neurophysiological measures. Our purpose was to evaluate the relationship between duration of symptoms and conditioned pain modulation (CPM), a measure of descending pain inhibition, and VPT, and to determine whether these relationships differed in men and women seeking orthopedic care. We evaluated 18 men and 27 women with moderate to severe knee OA. We assessed CPM using a submaximal-effort tourniquet test: Pressure pain threshold (PPT) at the symptomatic knee was evaluated before and after a noxious stimulus. CPM impairment was indicated by a ratio of pre-to-post stimulus PPT ≥1. VPT was assessed using a biothesiometer at the medial femoral condyle. We used chi-square, t-tests and Pearson correlations to address study questions. 72% of men, but only 44% of women had CPM impairment. Duration of symptoms was associated with CPM impairment in women (R = 0.566, p = 0.003) but not men (R = 0.366, p = 0.135). Duration of symptoms was also associated with VPT in both men (R = 0.580, p = 0.012) and women (R = 0.406, p = 0.039). These results suggest that longer duration of knee OA may predict more severe pain sensitization and that important sex differences exist in descending pain inhibition in people with chronic knee OA that may affect disease and course of treatment in male and female patients. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Comparisons of Conditioned Pain Modulation and Physical Activity Between Hispanic and Non-Hispanic White Adults

It is well-documented that adults of racial/ethnic minorities experience pain more frequently and suffer from more severe pain compared to non-Hispanic White (NHW) adults. These observations are consistent with laboratory findings that adults of racial/ethnic minorities show increased sensitivity to laboratory pain stimuli compared to NHW adults. Research generally shows that central pain inhibitory processing, as quantified using conditioned pain modulation (CPM), serves as a risk factor of clinical pain. Currently, racial/ethnic differences in CPM are poorly understood, and research suggests that physical activity (PA) may help understand potential racial/ethnic differences in CPM. However, very little is known regarding CPM in Hispanic compared to NHW adults. Therefore, the present study compared CPM and PA between Hispanic and NHW adults. Twenty-one young, healthy Hispanic and 21 NHW adults completed validated questionnaires to assess PA, pain catastrophizing, and dispositional optimism. The participants then completed the CPM test to quantify changes in pain ratings to electrical stimuli delivered to the ankle during concurrent application of pressure pain applied to the finger compared to baseline. Results indicated that Hispanic and NHW adults exhibited comparable CPM responses and PA levels, along with similar levels of pain catastrophizing and dispositional optimism (p > 0.05). These results suggest that young, healthy Hispanic and NHW adults may possess a similar risk of clinical pain when they are comparable in PA, pain catastrophizing, and dispositional optimism. More research is needed to explore the role of PA in racial/ethnic disparities in clinical pain and central pain inhibitory processing.

An investigation into the noradrenergic and serotonergic contributions of diffuse noxious inhibitory controls in a monoiodoacetate model of osteoarthritis

Osteoarthritis (OA) is a debilitating conditioning with pain as the major clinical symptom. Understanding the mechanisms that drive OA-associated chronic pain is crucial for developing the most effective analgesics. Although the degradation of the joint is the initial trigger for the development of chronic pain, the discordance between radiographic joint damage and the reported pain experience in patients, coupled with clinical features that cannot be explained by purely peripheral mechanisms, suggest there are often other factors at play. Therefore, this study considers the central contributions of chronic pain, using a monoiodoacetate (MIA) model of OA. Particularly, this study explores the functionality of descending controls over the course of the model by assessing diffuse noxious inhibitory controls (DNIC). Early-phase MIA animals have a functional DNIC system, whereas DNIC are abolished in late-phase MIA animals, indicating a dysregulation in descending modulation over the course of the model. In early-phase animals, blocking the actions of spinal α₂-adrenergic receptors completely abolishes DNIC, whereas blocking the actions of spinal 5-HT₇ receptors only partially decreases the magnitude of DNIC. However, activating the spinal α₂-adrenergic or 5-HT₇ receptors in late-phase MIA animals restored DNIC-induced neuronal inhibition. This study confirms that descending noradrenergic signaling is crucial for DNIC expression. Furthermore, we suggest a compensatory increase in descending serotonergic inhibition acting at 5-HT₇ receptors as the model progresses such that receptor activation is sufficient to override the imbalance in descending controls and mediate neuronal inhibition. NEW & NOTEWORTHY This study showed that there are both noradrenergic and serotonergic components contributing to the expression of diffuse noxious inhibitory controls (DNIC). Furthermore, although a tonic descending noradrenergic tone is always crucial for the expression of DNIC, variations in descending serotonergic signaling over the course of the model mean this component plays a more vital role in states of sensitization.

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.

Can disbudding of calves (one versus four weeks of age) induce chronic pain?

BACKGROUND

Calf disbudding is a welfare-relevant invasive husbandry procedure. While it is widely recognized that disbudding elicits acute pain, its chronic consequences are currently unknown.

OBJECTIVES

Primary aim of this study was to investigate whether disbudding leads to chronic pain in calves; secondary aim was to evaluate the influence of age at disbudding (1 week versus 4 weeks).

METHODS

Thirty-four Holstein bull calves, enrolled in a prospective, controlled, randomized trial, underwent either early disbudding (ED) or late disbudding (LD) or sham treatment (control) at 1 and 4 weeks of age. Pain scoring and quantitative sensory tests (von Frey filaments and algometry) were performed at regular intervals up to 105 days of life; neurophysiological determination of nociceptive reflex thresholds and conditioned pain modulation (CPM) up to 90 days. Data was analyzed with the Brunner and Langer model followed by post-hoc test, with level of significance set at 0.05.

RESULTS

Thirty-one calves completed the data collection. Signs of chronic sensitization were observed in 38% of disbudded calves (4 in ED and 4 in LD). Pain scores (until 105 days) and allodynia scores (until 60 days) were significantly higher while pressure thresholds (until 105 days) were significantly lower in both groups ED and LD compared to control. Late disbudded calves showed impairment of trigeminal CPM. Lower laser-evoked trigeminal reflex thresholds were found in clinically painful calves compared to control.

CONCLUSIONS

The sequelae of disbudding can extend beyond the acute post-procedural phase. Chronic trigeminal sensitization, independently from the age at disbudding can affect individual calves.

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.

Dynamic of the somatosensory system in postherpetic neuralgia

Introduction:

In postherpetic neuralgia (PHN) different types of patients can be distinguished regarding their predominant peripheral nociceptor function.

Objective:

The aim was to examine somatosensory profiles in the course of disease with special regard to the different subtypes existing in PHN.

Methods:

Twenty patients with PHN (7 men and 13 women, age 67 ± 9.6 years) were examined at baseline (disease duration 18.1 ± 26 months) and follow-up (31.6 ± 23.8 months later) with quantitative sensory testing (protocol of the German Research Network on Neuropathic Pain).

Results:

Fourteen (70%) PHN patients presented with impaired (iPHN) and 6 (30%) with preserved (pPHN) C-fiber function. Groups did not differ regarding age, disease duration, or pain intensity at baseline. Both groups did not differ regarding change in pain intensity (−0.5 ± 2.3 vs −1.7 ± 2.6 numerical rating scale, P = n.s.) at follow-up. Impaired PHN improved in thermal and mechanical detection thresholds as well as allodynia independent from change in pain intensity. By contrast, pPHN showed an increase in mechanical pain sensitivity (1.4 ± 2.5 vs −0.4 ± 2.2, P < 0.05) and a trend towards a stronger loss of detection (66% vs 33%, P = n.s.) on follow-up.

Conclusion:

Results demonstrate that patients with preserved C-fiber function are more predisposed to develop signs of central sensitization as demonstrated by an increased mechanical pain sensitivity. Impaired C-fiber function is able to improve even in chronic cases, but a functional loss is unlikely to play a role here. The knowledge of development of somatosensory profiles in the course of the disease offers possibilities to optimize a mechanism-based treatment.

Reduced endogenous pain inhibition in adolescent girls with chronic pain

Background and aims Chronic pain is affecting a growing number of individuals including adolescents. Different endogenous pain inhibitory systems could confer protection against development of chronic pain. Decreased pain perception can be observed following intense pain (i.e. conditioned pain modulation - CPM) or after physical exercise (i.e. exercise-induced analgesia - EIA). Reduced effectiveness of pain inhibitory mechanisms have been reported in several chronic pain conditions. However, the extent of these dysfunctions has not been thoroughly investigated in adolescents suffering from chronic pain. Our hypothesis was that adolescents suffering from chronic pain have less effective CPM and EIA than pain-free teenagers. Methods Twenty-five healthy adolescent girls and 16 teenage girls with chronic pain participated in this study. Only girls were included in this investigation, since chronic pain is more prevalent in females. The effectiveness of CPM was assessed by comparing heat pain stimulations (individually adapted to induce mild pain intensity) performed with a thermode before and after a cold pressor test (CPT; 2 min, 10 °C). EIA was evaluated by comparing pain intensity produced by an ice cube placed on the forearm before and after a graded exercise test on a cycle ergometer. Results Pain intensity produced by heat pain stimulations decreased following CPT in healthy (p<0.05), but not in chronic pain adolescent girls (p=0.4). Pain intensity induced by the ice cube was reduced after exercise in healthy (p<0.05), but not in chronic pain adolescents (p=0.9). The effectiveness of CPM and EIA was inferior in teenage girls suffering from chronic pain compared to healthy participants (p<0.05). Conclusions Endogenous pain inhibitory mechanisms triggered by intense pain or by physical exercise are effective in healthy adolescent girls. Teenage girls living with chronic pain do not show diminished pain perception after a CPT or a graded exercise test. These results suggest that pain inhibitory mechanisms such as CPM and EIA are ineffective in adolescent girls suffering from chronic pain. Implications In a wider context, the findings of the present research could help understand better the mechanisms involved in the development of chronic pain. Improved comprehension of this subject might help prevent chronic pain conditions and thus, reduce the negative impacts of this burden.

Association of Serum Serotonin and Pain in Patients with Chronic Low Back Pain before and after Spinal Surgery

Introduction

In this study we are aiming to evaluate the changes of serum serotonin and its association with pain in patients suffering from chronic low back pain before and after lumbar discectomy surgery.

Patients and Methods

A prospective study was performed on the patients referring to the outpatient clinic in Besat hospital, Hamadan University of Medical Sciences, Hamadan, Iran, during 2016. A 2 mL fasting blood sample was collected from each patient at preoperative day 1 and postoperative day 14 and they were measured for level of serum serotonin. Besides, all patients were asked for severity of their low back pain in preoperative day 1 and postoperative day 14 and scored their pain from zero to ten using a Numerical Rating Scale.

Results

Forty patients with the mean age of 47 ± 13 yrs/old (range 25–77) including 15 (37.5%) males were enrolled into the study. The overall mean score of preoperative pain was significantly decreased from 7.4 ± 2.18 (range 4–10) to the postoperative pain score 3.87 ± 2.92 (range 0–10) (P < .001). The overall levels of pre- and postoperative serum serotonin were 3.37 ± 1.27 (range 1.1–6.4) and 3.58 ± 1.32 (range .94–7.1) ng/mL, respectively, with no significant difference (P = .09). The levels of pre- and postoperative serum serotonin were significantly higher in males and patients older than 50 yrs/old compared to the females and patients younger than 50 yrs/old, respectively (P = .03 and .005, respectively). A significant inverse correlation between the postoperative levels of pain and serum serotonin was observed (r = -.36 and P = .02).

Conclusion

A negative medium strength linear relationship may exist between the postoperative serum serotonin and low back pain.

Sustained exposure to acute migraine medications combined with repeated noxious stimulation dysregulates descending pain modulatory circuits: Relevance to medication overuse headache

Background

Loss of conditioned pain modulation/diffuse noxious inhibitory controls has been demonstrated in patients with migraine and medication overuse headache. We hypothesized that exposure to acute migraine medications may lead to dysregulation of central pain modulatory circuits that could be revealed by evaluating diffuse noxious inhibitory controls and that prior noxious stimulus is required for a loss of the diffuse noxious inhibitory control response in rats exposed to these medications.

Methods

Rats were “primed” by continuous infusion of morphine or one of two doses of sumatriptan. Diffuse noxious inhibitory control was evaluated at the end of drug-priming (day 7) and again after sensory thresholds returned to baseline (day 21). The Randall-Selitto hindpaw pressure test was used as the test stimulus and forepaw capsaicin injection served as the conditioning stimulus.

Results

Morphine-primed rats showed opioid-induced hyperalgesia accompanied by a loss of diffuse noxious inhibitory controls on day 7. Sumatriptan-primed rats did not develop hyperalgesia or loss of diffuse noxious inhibitory controls on day 7. Morphine-primed and high-dose sumatriptan-primed rats only had a loss of diffuse noxious inhibitory control on day 21 if they received a capsaicin injection on day 7.

Conclusions

Prolonged exposure to migraine treatments followed by an acute nociceptive stimulation caused long-lasting alterations in descending pain modulation, shown by a loss of diffuse noxious inhibitory controls. Morphine was more detrimental than sumatriptan, consistent with clinical observations of higher medication overuse headache risk with opioids. These data suggest a mechanism of medication overuse headache by which migraine medications combined with repeated episodes of pain may amplify the consequences of nociceptor activation and increase the probability of future migraine attacks as well as risk of medication overuse headache.

Pathophysiology of chronic pain in cerebral palsy: implications for pharmacological treatment and research

The high prevalence of chronic pain in individuals with cerebral palsy (CP) across the lifespan has been well documented, as has its negative impact on quality of life. However, without an understanding of the underlying (possibly unique) pathophysiology of pain in CP, identification of more effective management options, such as innovative and individualized pharmacological approaches to non-opioid pain treatment, will be significantly hindered. We review, briefly, what is known about chronic pain in CP and present what we need to know with respect to the neurobiology of pain and new developments in pain treatment research that might be applied to CP.

WHAT THIS PAPER ADDS

Pain conditions in cerebral palsy have differing mechanisms and will not respond to the same treatments. Novel analgesics under development include inhibitors of ion channels, nerve growth factor, and calcitonin gene-related peptide.

General versus pain-specific cognitions: Pain catastrophizing but not optimism influences conditioned pain modulation

Background

Previous studies found evidence that dispositional optimism is related to lower pain sensitivity. Recent findings suggest that temporarily increasing optimism by means of imagining a positive future may also have pain‐alleviating effects.

Objectives

The present experiment was designed to investigate conditioned pain modulation (CPM) as a potential underlying mechanism of this pain‐alleviating effect of induced optimism.

Methods

For this purpose, 45 healthy participants were randomized into an optimistic or neutral imagery condition. Additionally, participants completed questionnaires on dispositional optimism, pain catastrophizing and pain expectations. CPM was assessed by delivering a series of five heat pain stimuli on the nondominant hand before and during immersion of the dominant hand in water of 5°C for 70 s.

Results

A clear CPM effect was found, that is heat pain reports were lower during simultaneous cold water stimulation. Although the optimism manipulation successfully increased optimism, it did not affect pain ratings or CPM. Post hoc analyses indicated that dispositional optimism was not associated with the magnitude of CPM, but pain catastrophizing and pain expectations did significantly correlate with the CPM effect.

Conclusion

Pain‐specific but not general cognitions appear to influence endogenous pain modulation.

Significance

Conditioned pain modulation is not the underlying mechanism of the pain‐alleviating effects of induced optimism. However, pain‐specific cognitions including pain catastrophizing and pain expectations affect endogenous pain modulation which should be taken into account in treatment and CPM research.

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.

Salivary beta-endorphin and substance P are not biomarkers of neuropathic chronic pain propensity

Objective

The pathophysiology of chronic pain is complex, with most of our knowledge being derived from preclinical studies. The search for biomarkers mirroring the pathophysiology of chronic pain is ongoing, and there is an increasing interest in saliva as a diagnostic tool. Given what is known about salivary substance P and salivary gland innervation, we hypothesized that salivary substance P and/or beta-endorphin might reflect the basal activity of these neuropeptides in the central nervous system, thereby perhaps mirroring a general propensity to chronic pain. Based on this overall hypothesis, our aim was to compare salivary levels of these neuropeptides in chronic neuropathic pain patients with healthy controls. An additional aim was to relate salivary levels to plasma levels.

Materials and methods

We compared salivary concentrations of beta-endorphin and substance P in 14 chronic neuropathic pain patients with concentrations in 18 healthy controls using a Luminex technology kit. Salivary-to-plasma quotients were also calculated.

Results

We found no significant difference between the groups' salivary concentrations of substance P and beta-endorphin. No correlation was found between salivary and plasma concentrations of each neuropeptide, which we hypothesize might point to local production of beta-endorphin and/or substance P in the salivary glands. Given high substance P salivary-to-plasma quotients, such a local production seems more likely for substance P than for beta-endorphin.

Conclusions

Propensity to neuropathic chronic pain was not substantiated by our analysis of salivary levels of substance P and/or beta-endorphin. However, we report salivary-to-plasma quotients that give potentially important physiological insight about these neuropeptides.

The added value of bedside examination and screening QST to improve neuropathic pain identification in patients with chronic pain

Background

The assessment of a neuropathic pain component (NePC) to establish the neurological criteria required to comply with the clinical description is based on history taking, clinical examination, and quantitative sensory testing (QST) and includes bedside examination (BSE). The objective of this study was to assess the potential association between the clinically diagnosed presence or absence of an NePC, BSE, and the Nijmegen–Aalborg screening QST (NASQ) paradigm in patients with chronic (≥3 months) low back and leg pain or with neck shoulder arm pain or in patients with chronic pain due to suspected peripheral nerve damage.

Methods

A total of 291 patients participated in the study. Pain (absence or presence of neuro-pathic pain) was assessed independently by two physicians and compared with BSE (measurements of touch [finger, brush], heat, cold, pricking [safety pin, von Frey hair], and vibration). The NASQ paradigm (pressure algometry, electrical pain thresholds, and conditioned pain modulation) was assessed in 58 patients to generate new insights.

Results

BSE revealed a low association of differences between patients with either absent or present NePC: heat, cold, and pricking sensations with a von Frey hair were statistically significantly less common in patients with present NePC. NASQ did not reveal any differences between patients with and without an NePC.

Conclusion

Currently, a standardized BSE appears to be more useful than the NASQ paradigm when distinguishing between patients with and without an NePC.

Conditioned pain modulation identifies altered sensitivity in extremely preterm young adult males and females

BACKGROUND

Conditioned pain modulation is a potential biomarker for risk of persistent pain. As early-life experience can alter subsequent somatosensory processing and pain response, we evaluated conditioned pain modulation after extremely preterm birth.

METHODS

This observational study recruited extremely preterm (<26 weeks gestation; n=98) and term-born control (n=48) young adults (19-20 yr) from the longitudinal EPICure cohort. Pressure pain threshold (PPT; variable test stimulus lower leg) was measured before, during, and after a conditioning stimulus (contralateral hand immersion; 5°C water; 30 s). Questionnaires assessed current pain, medication use, anxiety, and pain catastrophising.

RESULTS

For participants tolerating conditioning, there were significant main effects of extremely preterm status, sex, and time on PPT during and after hand immersion. Inhibitory modulation was evoked in 64/98 extremely preterm (3, no change) and 38/48 term-born control (3, facilitation) subjects. The conditioned pain modulation effect (percentage change in PPT) did not differ between the extremely preterm and term-born control groups {53% [95% confidence interval (CI): 41-65] vs 57% [95% CI: 42-71]}. Reduced cold tolerance (<20 s) hampered conditioned pain modulation quantification in a higher proportion of extremely preterm participants [extremely preterm vs term-born control: 31/98 (32%) vs 7/48 (15%); P=0.03]. One-third of extremely preterm females withdrew the hand before parallel PPT (<15 s), and had lower baseline PPT than term-born control females [4.9 (95% CI: 4.8-5.1) vs 5.3 (95% CI: 5.1-5.5) ln kPa; P=0.02]. Higher anxiety, pain catastrophising, and medication use correlated with pain intensity, but not conditioned pain modulation effect.

CONCLUSIONS

Cold conditioning evoked inhibitory modulation in the majority of young adults and identified a subgroup of extremely preterm females with increased baseline sensitivity. Early-life experience and sex/gender should be considered when evaluating persistent pain risk with conditioned pain modulation.

Persistent Post-Mastectomy Pain: Risk Factors and Current Approaches to Treatment

Persistent Post-Mastectomy Pain (PPMP) is a common condition that can follow surgeries for breast cancer, the most common cancer in women. Because of the frequency of PPMP and its potential severity, it has received increasing research attention. This manuscript reviews the recent research literature, beginning with a brief history and then relevant medical, surgical, demographic, and psychosocial risk factors. Subsequently, social, psychological, and functional sequelae that have been linked to PPMPS are considered, as is research on current pharmacological, psychological, and rehabilitative approaches to treatment. The review concludes with a discussion of directions for future research and treatment that might reduce the incidence and impact of PPMP on breast cancer survivors. PERSPECTIVE: This article describes current research literature involving mechanisms, risks, and treatments related to persistent post-mastectomy pain. Implications of research findings also are discussed for pre- and post-surgical approaches to pain management, current treatments, and promising research directions.

Cuff Pressure Algometry in Patients with Chronic Pain as Guidance for Circumferential Tissue Compression for Wearable Soft Exoskeletons: A Systematic Review

In this article, we report on a systematic review of the literature on pressure-pain thresholds induced and assessed by computerized cuff pressure algometry (CPA). The motivation for this review is to provide design guidance on pressure levels for wearable soft exoskeletons and similar wearable robotics devices. In our review, we focus on CPA studies of patients who are candidates for wearable soft exoskeletons, as pain-related physiological mechanisms reportedly differ significantly between healthy subjects and patients with chronic pain. The results indicate that circumferential limb compression in patients most likely becomes painful at ∼10-18 kPa and can become unbearable even below 25 kPa. The corresponding ranges for healthy control subjects are 20-42 kPa (painful limits) and 34-84 kPa (unbearable levels). In addition, the increase of pain with time tends to be significantly higher, and the adaptation to pain significantly lower, than in healthy subjects. The results of this review provide guidance to designers of wearable robotics for populations with chronic pain regarding rates and magnitudes of tissue compression that may be unacceptable to users.