Modality-specific facilitation and adaptation to painful tonic stimulation in humans

To Calibrate or not to Calibrate? A Methodological Dilemma in Experimental Pain Research

To calibrate or not to calibrate? This question is raised by almost everyone designing an experimental pain study with supra-threshold stimulation. The dilemma is whether to individualize stimulus intensity to the pain threshold / supra-threshold pain level of each participant or whether to provide the noxious stimulus at a fixed intensity so that everyone receives the identical input. Each approach has unique pros and cons which need to be considered to i) accurately design an experiment, ii) enhance statistical inference in the given data and, iii) reduce bias and the influence of confounding factors in the individual study e.g., body composition, differences in energy absorption and previous experience. Individualization requires calibration, a procedure already irritating the nociceptive system but allowing to match the pain level across individuals. It leads to a higher variability of the stimulus intensity, thereby influencing the encoding of "noxiousness" by the central nervous system. Results might be less influenced by statistical phenomena such as ceiling/floor effects and the approach does not seem to rise ethical concerns. On the other hand, applying a fixed (standardized) intensity reduces the problem of intensity encoding leading to a large between-subjects variability in pain responses. Fixed stimulation intensities do not require pre-exposure. It can be proposed that one method is not preferable over another, however the choice depends on the study aim and the desired level of external validity. This paper discusses considerations for choosing the optimal approach for experimental pain studies and provides recommendations for different study designs. PERSPECTIVE: To calibrate pain or not? This dilemma is related to almost every experimental pain research. The decision is a trade-off between statistical power and greater control of stimulus encoding. The article decomposes both approaches and presents the pros and cons of either approach supported by data and simulation experiment.

Increased functional connectivity between limbic brain areas in healthy individuals with high versus low sensitivity to cold pain: A resting state fMRI study

Background

The representation of variability in sensitivity to pain by differences in neural connectivity patterns and its association with psychological factors needs further investigation. This study assessed differences in resting-state functional connectivity (rsFC) and its association to cognitive-affective aspects of pain in two groups of healthy subjects with low versus high sensitivity to pain (LSP vs. HSP). We hypothesized that HSP will show stronger connectivity in brain regions involved in the affective-motivational processing of pain and that this higher connectivity would be related to negative affective and cognitive evaluations of pain.

Methods

Forty-eight healthy subjects were allocated to two groups according to their tolerability to cold stimulation (cold pressor test, CPT, 1°C). Group LSP (N = 24) reached the cut-off time of 180±0 sec and group HSP tolerated the CPT for an average of 13±4.8 sec. Heat, cold and mechanical evoked pain were measured, as well as pain-catastrophizing (PCS), depression, anxiety and stress (DASS-21). All subjects underwent resting state fMRI. ROI-to-ROI analysis was performed.

Results

In comparison to the LSP, the HSP had stronger interhemispheric connectivity of the amygdala (p = 0.01) and between the amygdala and nucleus accumbens (NAc) (p = 0.01). Amygdala connectivity was associated with higher pain catastrophizing in the HSP only (p<0.01).

Conclusions

These findings suggest that high sensitivity to pain may be reflected by neural circuits involved in affective and motivational aspects of pain. To what extent this connectivity within limbic brain structures relates to higher alertness and more profound withdrawal behavior to aversive events needs to be further investigated.

More Insight on the Role of Personality Traits and Sensitivity to Experimental Pain

Purpose

The present study aimed to assess the influence of personality traits on the variability of sensitivity to pain in two distinct groups of healthy subjects with low versus high sensitivity to pain (LSP vs HSP, respectively).

Methods

Healthy subjects (n=156) were allocated to two groups according to their tolerability to cold stimulation (cold pressor test, CPT, 1°C). Group LSP (n=76) reached the cut-off time of 180±0 sec, and a size matched group of HSP (n=80) tolerated the CPT for an average of 10.5±3.4 sec only. Subjects from both groups completed the self-reported pain sensitivity questionnaire (PSQ), the Pain Catastrophizing Scale (PCS), and the Neuroticism Extraversion Openness - Five Factor Inventory (NEO-FFI).

Results

In comparison to the LSP group, HSP individuals had higher scores of PSQ (p<0.001), catastrophizing (p=0.001), and extraversion (p=0.01). By adjusting for age and gender, mediation analyses revealed that catastrophizing mediated the relationship between neuroticism and pain sensitivity, both in the allocation of subjects to a certain group of sensitivity to pain (LSP or HSP, B=0.02 95% CI: 0.006–0.040) and in the PSQ score (B=0.01 95% CI: 0.001–0.023).

Conclusion

These results, which were demonstrated by two different prisms (CPT and PSQ), point to the potential of the five-factor inventory and pain catastrophizing scale as tools for identifying specific personality traits associated with a high sensitivity to pain.

Design and Analysis of a Clinical Pressure Cuff Algometer, Compatible with an fMRI Machine

Chronic pain affects both adolescent and adult Canadians. To study the effect of social rejection on pain management in adolescents with chronic pain, an algometer can be used in conjunction with functional Magnetic Resonance Imaging (fMRI) to measure brain activity in real time. The algometer uses an automated pneumatic control system that follows a customizable pain schedule, controlling the amount of airflow in and out of a pressure cuff wrapped around a human participant's thigh. Plastic components allow compatibility with an fMRI environment. Measurable pain stimuli allow repeatable pressure schedules to be administered with a standard deviation between trials of 300 Pa (2.25 mmHg). A Failure Mode Effects Analysis was used to reduce participant, researcher and facility harm, with multiple safety features incorporated into the design. Through the analysis of medical standards and studies, the algometer is shown to be biologically safe to use on research subjects within the suggested usage parameters of a maximum pressure of 42.6 kPa (320 mmHg) and a pressure application period of up to one hour. This makes it feasible for research studies using fMRI machines.

Dynamic brain-to-brain concordance and behavioral mirroring as a mechanism of the patient-clinician interaction

The patient-clinician interaction can powerfully shape treatment outcomes such as pain but is often considered an intangible "art of medicine" and has largely eluded scientific inquiry. Although brain correlates of social processes such as empathy and theory of mind have been studied using single-subject designs, specific behavioral and neural mechanisms underpinning the patient-clinician interaction are unknown. Using a two-person interactive design, we simultaneously recorded functional magnetic resonance imaging (hyperscanning) in patient-clinician dyads, who interacted via live video, while clinicians treated evoked pain in patients with chronic pain. Our results show that patient analgesia is mediated by patient-clinician nonverbal behavioral mirroring and brain-to-brain concordance in circuitry implicated in theory of mind and social mirroring. Dyad-based analyses showed extensive dynamic coupling of these brain nodes with the partners' brain activity, yet only in dyads with pre-established clinical rapport. These findings introduce a putatively key brain-behavioral mechanism for therapeutic alliance and psychosocial analgesia.

Implications of rectal preconditioning for interpretation of sensory-motor data

Testing of biomechanical properties of intestine requires the tissue to be preconditioned by applying cyclic loading to obtain repeatable mechanical data. However, little is known about the mechanosensory properties during intestinal preconditioning. We aimed to study the relationship between mechanical preconditioning of the human rectum and sensory response. Three fast rectal bag distensions to the pain threshold were done in seven healthy females. A visual analog scale (VAS) was used for sensory assessment. At each distension, we determined (1) time, bag cross-sectional area (CSA), radius (r), r/r₀, pressure and tension to reach VAS = 1, 3 and 5 (pain threshold); (2) the same parameters at induced contraction start; (3) CSA where the pressure started to increase (CSA_P>baseline) and (4) the number of contractions. The time, CSA, r/r₀ and tension to reach VAS = 1 and VAS = 3 increased from distension 1 to 3 (4.9 < F < 11.5, 0.05 > P > 0.007), primarily due to difference between the first and second distension. For VAS = 5, r/r₀ was smaller in distension 3 than distension 1 (P < 0.05), whereas time, CSA and tension did not differ between distensions (P > 0.5). Compared with distension 1, CSA, r/r₀ and tension at contraction start, and CSA_P>baseline were bigger in distensions 2 and 3 (5.5 < F < 10.9, 0.05 > P > 0.009). The pressure to reach the VAS levels, the contraction numbers and pressure at contraction start did not differ among distensions (P > 0.6). During mechanical preconditioning, CSA, tension and deformation increased at sub-pain levels, reflecting sensory adaptation. The data point to acute remodeling of a strain-dependent mechanism in the rectal wall.

The relationship between sensitivity to pain and conditioned pain modulation in healthy people

BACKGROUND

The relationship between sensitivity to pain and conditioned pain modulation (CPM) - a paradigm reflecting the activity of the endogenous descending analgesic system - is still unclear. This study aimed at investigating CPM magnitude in two distinct subgroups of healthy subjects, presenting low vs. high sensitivity to pain (LSP vs. HSP, respectively), by employing two different thermal paradigms of CPM.

METHOD

Ninety-five healthy subjects (out of 293 tested) were identified as LSP (n = 48) or HSP (n = 47) according to their tolerance time to noxious cold stimulation (Cold Pressor Test, 1 °C). All subjects were exposed to two different paradigms of CPM: 1) Fixed temperature 'test-pain' (TP) where phasic, fixed painful heat stimuli of 47 °C were administered before and during a prolonged 'conditioning stimulus' (cold water at 12 °C for 30 s); and 2) Individually based 'pain-60' where TP was determined as the temperature that induced pain at a magnitude of 60 on a 0-100 rating scale (with the same conditioning stimulus).

RESULT

Using both thermal paradigms, LSP subjects showed decreased CPM magnitudes in comparison to HSP (p < 0.0001 in both paradigms). Within each group, no differences in the magnitudes of CPM were found between the two paradigms.

CONCLUSION

These findings show that regardless of the thermal CPM paradigm employed, healthy individuals exhibiting low sensitivity to pain have a low pain inhibition profile and vice-versa. It is suggested that in healthy subjects, pain sensitivity predisposes the magnitude of CPM and not the other way around.

Effect and neurophysiological mechanism of acupuncture in patients with chronic sciatica: protocol for a randomized, patient-assessor blind, sham-controlled clinical trial

Background

Sciatica is a relatively frequent illness that easily becomes a chronic and relapsing condition. Although numerous systematic reviews have analyzed various therapies for sciatica, the validity of their included studies is limited. Considering the limitations of conventional treatment options for sciatica, acupuncture is a possible option; however, evidence supporting its efficacy and mechanism in patients with sciatica is lacking. The aim of this proposed protocol is to investigate the effect and neurophysiological mechanism of acupuncture in patients with chronic sciatica.

Methods/design

This study is a randomized, patient-assessor blind, two-arm, parallel, non-penetrating, sham-controlled clinical trial. Eligible participants will include adults (aged 19–70 years old) with a clinical diagnosis of chronic sciatica (40 mm or more of a 100-mm visual analog scale (VAS) for bothersomeness) blinded to the treatment received. Patients will be randomly allocated into the acupuncture treatment group (manual acupuncture plus electroacupuncture (EA), n = 34) or the sham acupuncture control group (sham acupuncture plus placebo EA without electrical stimulation, n = 34). Groups will receive treatment twice a week for a total of eight sessions over 4 weeks. Functional magnetic resonance imaging will be implemented at baseline and endpoint to investigate the mechanism of acupuncture. The primary outcome measure is the VAS for bothersomeness and secondary outcomes include the VAS for pain intensity, Oswestry Disability Index, EuroQol 5-Dimension, Coping Strategy Questionnaire, Beck’s Depression Inventory, and State-Trait Anxiety Inventory. Adverse events will be assessed at every visit.

Discussion

The results of this trial (which will be available in 2020) should provide important clinical evidence for the effect of acupuncture and demonstrate how acupuncture can be helpful for the treatment of chronic sciatica.

Trial registration

ClinicalTrials.gov, ID: NCT03350789. Registered on 15 November 2017.

Electronic supplementary material

The online version of this article (10.1186/s13063-018-3164-8) contains supplementary material, which is available to authorized users.

Experimental pain phenotyping in community-dwelling individuals with knee osteoarthritis

Pain among individuals with knee osteoarthritis (OA) is associated with significant disability in older adults, and recent evidence demonstrates enhanced experimental pain sensitivity. Although previous research showed considerable heterogeneity in the OA clinical pain presentation, less is known regarding the variability in responses to experimental pain. The present study included individuals with knee OA (n = 292) who participated in the Understanding Pain and Limitations in Osteoarthritic Disease study and completed demographic and psychological questionnaires followed by a multimodal quantitative sensory testing (QST) session. Quantitative sensory testing measures were subjected to variable reduction procedures to derive pain sensitivity index scores, which in turn were entered into a cluster analysis. Five clusters were significantly different across all pain sensitivity index variables (P < 0.001) and were characterized by: (1) low pain sensitivity to pressure pain (N = 39); (2) average pain sensitivity across most modalities (N = 88); (3) high temporal summation of punctate pain (N = 38); (4) high cold pain sensitivity (N = 80); and (5) high sensitivity to heat pain and temporal summation of heat pain (N = 41). Clusters differed significantly by race, gender, somatic reactivity, and catastrophizing (P < 0.05). Our findings support the notion that there are distinct subgroups or phenotypes based on experimental pain sensitivity in community-dwelling older adults with knee OA, expanding previous findings of similar cluster characterizations in healthy adults. Future research is needed to further understand the pathophysiological mechanisms underlying pain within these subgroups, which may be of added value in tailoring effective treatments for people with OA.

Alterations in pain responsiveness and serum biomarkers in juvenile myoclonic epilepsy: an age- and gender-matched controlled pilot study

Introduction: Serum levels of several biomarkers along with sensory responsiveness were investigated in juvenile myoclonic epilepsy patients in comparison with healthy controls. Methods: Ten epileptic patients (36.1 ± 3.4 years) and ten gender- and age-matched healthy controls were recruited. Mechanical sensitivity, cold pressor tolerance and serum levels of BDNF, CGRP, PGE2, S100B and TNF-α were investigated. Results: Mechanical sensitivity to pinprick was lower in patients (p < 0.05) while cold pain tolerance threshold was higher. Serum level of BDNF was higher in patients compared with controls (p < 0.01). The same pattern was evident for CGRP (p < 0.05). Serum level of PGE2 was lower in patients (p < 0.01). Conclusion: Juvenile myoclonic epilepsy patients had an altered serum biomarker pattern and sensory perception in comparison with controls.

The lateral prefrontal cortex mediates the hyperalgesic effects of negative cognitions in chronic pain patients

Although high levels of negative affect and cognitions have been associated with greater pain sensitivity in chronic pain conditions, the neural mechanisms mediating the hyperalgesic effect of psychological factors in patients with pain disorders are largely unknown. In this cross-sectional study, we hypothesized that 1) catastrophizing modulates brain responses to pain anticipation and 2) anticipatory brain activity mediates the hyperalgesic effect of different levels of catastrophizing in fibromyalgia (FM) patients. Using functional magnetic resonance imaging, we scanned the brains of 31 FM patients exposed to visual cues anticipating the onset of moderately intense deep-tissue pain stimuli. Our results indicated the existence of a negative association between catastrophizing and pain-anticipatory brain activity, including in the right lateral prefrontal cortex. A bootstrapped mediation analysis revealed that pain-anticipatory activity in the lateral prefrontal cortex mediates the association between catastrophizing and pain sensitivity. These findings highlight the role of the lateral prefrontal cortex in the pathophysiology of FM-related hyperalgesia and suggest that deficits in the recruitment of pain-inhibitory brain circuitry during pain-anticipatory periods may play an important contributory role in the association between various degrees of widespread hyperalgesia in FM and levels of catastrophizing, a well-validated measure of negative cognitions and psychological distress.

PERSPECTIVE

This article highlights the presence of alterations in pain-anticipatory brain activity in FM. These findings provide the rationale for the development of psychological or neurofeedback-based techniques aimed at modifying patients' negative affect and cognitions toward pain.

Adaptability to pain is associated with potency of local pain inhibition, but not conditioned pain modulation: a healthy human study

This study investigated the relationship between pain sensitivity, adaptability, and potency of endogenous pain inhibition, including conditioned pain modulation (CPM) and local pain inhibition. Forty-one healthy volunteers (20 male, 21 female) received conditioning stimulation (CS) over 2 sessions in a random order: tonic heat pain (46 °C) on the right leg for 7 minutes and cold pressor pain (1 °C to 4 °C) on the left hand for 5 minutes. Participants rated the intensity of pain continuously using a 0 to 10 electronic visual analogue scale. The primary outcome measures were pressure pain thresholds (PPT) measured at the heterotopic and homotopic location to the CS sites before, during, and 20 minutes after CS. Two groups of participants, pain adaptive and pain nonadaptive, were identified based on their response to pain in the cold pressor test. Pain-adaptive participants showed a pain reduction between peak pain and pain at end of the test by at least 2 of 10 (n=16); whereas the pain-nonadaptive participants reported unchanged peak pain during 5-minute CS (n=25). Heterotopic PPTs during the CS did not differ between the 2 groups. However, increased homotopic PPTs measured 20 minutes after CS correlated with the amount of pain reduction during CS. These results suggest that individual sensitivity and adaptability to pain does not correlate with the potency of CPM. Adaptability to pain is associated with longer-lasting local pain inhibition.

Experimental pain phenotype profiles in a racially and ethnically diverse sample of healthy adults

OBJECTIVE

To examine patterns of interindividual variability in experimental pain responses emerging from multiple experimental pain measures in a racially/ethnically diverse sample of healthy adults and to examine the association between the derived phenotype profiles with demographic, psychological, and health-related measures.

METHODS

Two hundred and ninety-one participants underwent heat, cold, pressure, and ischemic pain assessments, and completed several psychological and health-related assessments. The experimental pain measures were subjected to a principal component analysis and factor scores were used to compute Pain Sensitivity Index scores. The scores were subsequently submitted to a cluster analysis to identify patterns of pain sensitivity across experimental pain modalities.

RESULTS

The sample was equally composed of non-Hispanic whites, African Americans, and Hispanic whites. Sensitivity scores were computed for heat pain, pressure pain, cold pain, ischemic pain, and temporal summation of heat pain. Five distinct clusters were characterized by high heat pain sensitivity, low ischemic pain sensitivity, low cold pain sensitivity, low pressure pain sensitivity, and high temporal summation. Cluster membership was significantly different by sex as well as somatic reactivity and catastrophizing, although cluster differences were most pronounced between the heat pain-sensitive individuals vs the cold pain-insensitive individuals.

CONCLUSIONS

Our findings highlight the importance of phenotyping individuals to account for interindividual differences in pain responses. Our findings also replicate previously reported pain phenotypes, which are not solely related to demographic, psychosocial, or health-related factors in our healthy participants. Future studies designed to elucidate the biological underpinnings of pain sensitivity profiles would be of substantial value.

Sustained deep-tissue pain alters functional brain connectivity

Recent functional brain connectivity studies have contributed to our understanding of the neurocircuitry supporting pain perception. However, evoked-pain connectivity studies have employed cutaneous and/or brief stimuli, which induce sensations that differ appreciably from the clinical pain experience. Sustained myofascial pain evoked by pressure cuff affords an excellent opportunity to evaluate functional connectivity change to more clinically relevant sustained deep-tissue pain. Connectivity in specific networks known to be modulated by evoked pain (sensorimotor, salience, dorsal attention, frontoparietal control, and default mode networks: SMN, SLN, DAN, FCN, and DMN) was evaluated with functional-connectivity magnetic resonance imaging, both at rest and during a sustained (6-minute) pain state in healthy adults. We found that pain was stable, with no significant changes of subjects' pain ratings over the stimulation period. Sustained pain reduced connectivity between the SMN and the contralateral leg primary sensorimotor (S1/M1) representation. Such SMN-S1/M1 connectivity decreases were also accompanied by and correlated with increased SLN-S1/M1 connectivity, suggesting recruitment of activated S1/M1 from SMN to SLN. Sustained pain also increased DAN connectivity to pain processing regions such as mid-cingulate cortex, posterior insula, and putamen. Moreover, greater connectivity during pain between contralateral S1/M1 and posterior insula, thalamus, putamen, and amygdala was associated with lower cuff pressures needed to reach the targeted pain sensation. These results demonstrate that sustained pain disrupts resting S1/M1 connectivity by shifting it to a network known to process stimulus salience. Furthermore, increased connectivity between S1/M1 and both sensory and affective processing areas may be an important contribution to interindividual differences in pain sensitivity.

Disentangling linear and nonlinear brain responses to evoked deep tissue pain

Pain stimuli evoke widespread responses in the brain. However, our understanding of the physiological significance underlying heterogeneous response within different pain-activated and -deactivated regions is still limited. Using functional magnetic resonance imaging, we evaluated brain responses to a wide range of stimulus intensity levels (1 innocuous, 7 painful) in order to estimate region-specific stimulus-response functions, which we hypothesized could illuminate that region's functional relationship to pain. Linear and nonlinear brain responses to pain were estimated through independent Legendre polynomial transformations of pain ratings within a general linear model. This approach identified at least 5 different, regionally specific activity profiles in the brain. Linearly increasing (eg, primary somatosensory/motor cortex, insulae) and intensity-independent (eg, secondary somatosensory cortex) activation was noted in traditional pain-processing areas, potentially reflecting sensory encoding and all-or-none salience responses, respectively. Multiple activity profiles were seen in areas of the default mode network (DMN): intensity-independent deactivation (eg, posterior cingulate cortex), linearly decreasing (eg, contralateral inferior parietal lobule), and quadratic (U-shaped; eg, medial prefrontal cortex). The latter observation suggests that: (1) different DMN subregions exhibit functional heterogeneity and (2) some DMN subregions respond in a percept-related manner to pain, suggesting closer linkage between the DMN and pain processing than previously thought. Future studies should apply a similar approach using innocuous stimuli of multiple intensities to evaluate whether the response profiles reported here can also be generalized to nonpainful somatosensory processing.

Evoked pain analgesia in chronic pelvic pain patients using respiratory-gated auricular vagal afferent nerve stimulation

OBJECTIVE

Previous vagus nerve stimulation (VNS) studies have demonstrated antinociceptive effects, and recent noninvasive approaches, termed transcutaneous-vagus nerve stimulation (t-VNS), have utilized stimulation of the auricular branch of the vagus nerve in the ear. The dorsal medullary vagal system operates in tune with respiration, and we propose that supplying vagal afferent stimulation gated to the exhalation phase of respiration can optimize t-VNS.

DESIGN

Counterbalanced, crossover study.

PATIENTS

Patients with chronic pelvic pain (CPP) due to endometriosis in a specialty pain clinic. INTERVENTIONS/OUTCOMES: We evaluated evoked pain analgesia for respiratory-gated auricular vagal afferent nerve stimulation (RAVANS) compared with nonvagal auricular stimulation (NVAS). RAVANS and NVAS were evaluated in separate sessions spaced at least 1 week apart. Outcome measures included deep-tissue pain intensity, temporal summation of pain, and anxiety ratings, which were assessed at baseline, during active stimulation, immediately following stimulation, and 15 minutes after stimulus cessation.

RESULTS

RAVANS demonstrated a trend for reduced evoked pain intensity and temporal summation of mechanical pain, and significantly reduced anxiety in N = 15 CPP patients, compared with NVAS, with moderate to large effect sizes (η(2) > 0.2).

CONCLUSION

Chronic pain disorders such as CPP are in great need of effective, nonpharmacological options for treatment. RAVANS produced promising antinociceptive effects for quantitative sensory testing (QST) outcomes reflective of the noted hyperalgesia and central sensitization in this patient population. Future studies should evaluate longer-term application of RAVANS to examine its effects on both QST outcomes and clinical pain.

Efficacy of augmenting a subacromial continuous-infusion pump with a preoperative interscalene block in outpatient arthroscopic shoulder surgery: a prospective, randomized, blinded, and placebo-controlled study

PURPOSE

This study's purpose was to determine the effectiveness of adding a preoperative interscalene brachial plexus block to standard postoperative management, including oral narcotics and a subacromial bupivacaine infusion pump, after arthroscopic shoulder surgery.

METHODS

After performing a prospective power analysis and obtaining institutional board approval, we conducted a randomized placebo-controlled trial of 53 patients separated into a preoperative interscalene brachial plexus group and a control group. Group 1 received an interscalene block with 30 mL of 0.5% ropivacaine. Group 2 received a placebo with 10 mL of saline solution. All patients postoperatively received an arthroscopically placed subacromial infusion pump catheter for 72 hours and oral narcotics. Pain scores on a visual analog scale (VAS) and narcotic pill use were recorded at 6, 12, 20, 32, 40, 52, 60, 72, and 80 hours.

RESULTS

Preoperative pain scores between groups were not significant (P > .05). A statistically significant difference was found for decreased pain scores at 6 hours after discharge in patients receiving an interscalene block (P = .001) (VAS of 30.9 in group 1 v 61.8 in group 2). There was also a decrease in the number of narcotic pills taken at the 6-hour time interval (P = .1) (0.6 pills v 1.1 pills). Group 1 had a rebound phenomenon 20 hours after discharge. Pain scores spiked as the effects of the block wore off (P = .08) (net change in VAS score increase, 25.0 v 10.3). No other statistical or clinical differences were observed.

CONCLUSIONS

The addition of a preoperative interscalene block to a postoperative subacromial infusion pump provided significant improvement of pain control only at 6 hours after discharge. Twelve hours after discharge, this benefit had disappeared. A rebound phenomenon of increased pain at 20 hours was seen after the interscalene block had worn off. After 20 hours, no statistically significant or clinically applicable differences were found.

LEVEL OF EVIDENCE

Level I, randomized controlled trial.

Somatosensory symptoms and signs and conditioned pain modulation in chronic post-stroke shoulder pain

Persistent shoulder pain is a common complication after stroke. Its etiology and underlying mechanisms are not well understood and treatment is generally unsatisfactory. The objective of this study was to assess the role of central sensitization and disinhibition in chronic stroke patients with chronic PSSP (n = 19), pain-free stroke patients (n = 29), and healthy controls (n = 23). Positive and negative somatosensory symptoms and signs were assessed using clinical examination and electrical and mechanical quantitative sensory testing (QST). Conditioned pain modulation (CPM) was assessed by comparing QST thresholds before and after applying a cold pressor test. Sensory abnormalities were more frequently observed and more severe in patients with PSSP, including positive signs such as allodynia at the affected side and generalized hyperalgesia at the unaffected side. CPM was similar in stroke patients and healthy controls. This study showed that chronic PSSP was associated with several positive and negative somatosensory signs, implicating a role for central sensitization and possibly for disinhibition. Since the causal relationship remains unclear, and may be related to either neuroplasticity induced by ongoing nociception as well as to the neuropathic brain lesion, prospective studies are warranted.

PERSPECTIVE

The assessment of somatosensory symptoms and signs and endogenous pain modulation demonstrated a role for central sensitization and possibly for disinhibition in chronic PSSP. Prevention and treatment of PSSP could benefit from a more detailed analysis of both peripheral and central pain mechanisms.

Volunteers with high versus low alpha EEG have different pain-EEG relationship: a human experimental study

The alpha rhythm (7.5-12 Hz) is one of the fundamental features of the human EEG which usually has maximum amplitude over occipital regions. It is well recognized that individuals have highly different magnitudes of alpha EEG. This study examined occipital alpha EEG activity during different levels of experimental tonic cuff-pressure pain. The aim was to study the pain reactions and pain-EEG relationship in subjects with high alpha (H(alpha)) and low alpha (L(alpha)) EEG. Tonic experimental cuff-pressure induced pain, and high density EEG (124 channels) were used. The pain-EEG responses for the high alpha (above 600 microV(2) in total alpha power at baseline), and low alpha (below 600 microV(2) in alpha power at baseline) subjects were analyzed. Forty healthy volunteers were included and received tonic pain for 3 min at three intensities (VRS2) intense, but no pain, (VRS4) slight pain, and (VRS6) moderate pain. There were no differences in stimulus intensities to reach the three ratings between the H(alpha) and the L(alpha) groups. The H(alpha) and L(alpha) groups are highly different in alpha1(PO3), alpha1(PO4), alpha1(PO7) and alpha1(PO8) EEG powers. A positive correlation (P = 0.008) between alpha2(PO3) EEG and average subjective pain ratings was specific for the L(alpha) group. The H(alpha) group showed alpha1 desynchronization as pain increased, but no significant correlation between alpha1 EEG powers and average subjective pain ratings. The differences between the L(alpha) and the H(alpha) in alpha EEG powers and the different pain-EEG responses may be related to different degrees of attention, fear of pain and pain related coping strategies.

Testing of a new pneumatic device to cause pain in humans

BACKGROUND

Surgical pain typically combines superficial and deep pain. We wished to generate pain that resembled surgical pain, reliably and reproducibly, in volunteers.

METHODS

We constructed a computer-controlled pneumatic device to apply pressure to the anterior tibia. The reproducibility of the pain was tested by rating the pressure that caused pain rated 4-5 on a visual analogue scale (VAS) on days 0, 7, and 24 in 10 volunteers. The effect of remifentanil (0.025, 0.05, 0.075, and 0.1 microg kg-1 min-1) on pain tolerance in another set of volunteers (n=11) was used as an indirect measure of the reliability of pain production.

RESULTS

The pressure needed (0.7 (0.3) to 0.9 (0.4) atm (mean (SD)) to induce pain rated 4-5 (VAS) did not vary, showing long-term reproducibility of the method. When pressure was applied to cause increasing pain in volunteers (n=11) 0.05 microg kg-1 min-1 remifentanil increased pain tolerance by 50%. An approximate doubling of the dose (0.1 microg kg-1 min-1) increased pain tolerance significantly more. The linear logarithmic dose-effect relationship shows that the device causes pain reliably, and this can be reduced with opioid treatment.

CONCLUSION

This pneumatic device can apply pain reliably and reproducibly.

Induction and assessment of muscle pain, referred pain, and muscular hyperalgesia

Muscle pain can be induced and assessed experimentally by a variety of methods. Ischemic and exercise-induced muscle pain are typical endogenous pain models; external stimulation with mechanical, electrical, and chemical modalities constitute the exogenous models. These models are a good basis to study the muscle sensitivity, muscle pain responses under normal and pathophysiologic conditions, and drug efficacy on specific muscle pain mechanisms. When evaluating muscle pain in clinical or experimental settings, it is important to assess parameters related to the pain intensity, pain quality, referred and local distribution, and the deep tissue sensitivity in local and referred areas. The experimental test paradigm must include different stimulation modalities (multimodal) to obtain sufficiently advanced and differentiated information about the human nociceptive system under normal and pathophysiologic conditions because the different stimuli activate different receptors, pathways, and mechanisms. This may be a useful approach in future mechanism-based classification and treatment of muscle pain. Similarly, the multimodal approach is important in clinical studies to provide evidence for which specific muscle pain modalities and mechanisms are affected and how they are modulated by pharmacologic approaches.