Quantification of local and referred pain in humans induced by intramuscular electrical stimulation

Effects of conditioned pain modulation on Capsaicin-induced spreading muscle hyperalgesia in humans

OBJECTIVES

Muscle pain can be associated with hyperalgesia that may spread outside the area of primary injury due to both peripheral and central sensitization. However, the influence of endogenous pain inhibition is yet unknown. This study investigated how endogenous pain inhibition might influence spreading hyperalgesia in experimental muscle pain.

METHODS

Conditioned pain modulation (CPM) was assessed in 30 male volunteers by cold pressor test at the non-dominant hand as conditioning and pressure pain thresholds (PPT) at the dominant 2nd toe as test stimuli. Subjects were classified as having inhibitory or facilitating CPM based on published reference values. Subsequently, muscle pain and hyperalgesia were induced by capsaicin injection into the non-dominant supraspinatus muscle. Before and 5, 10, 15, 20, 30, 40, 50 and 60 min later, PPTs were recorded at the supraspinatus, infraspinatus and deltoid muscle, ring finger and toe.

RESULTS

Compared to baseline, PPTs decreased at the supraspinatus, infraspinatus and deltoid muscle (p≤0.03), and increased at the finger and toe (p<0.001). In facilitating CPM (n=10), hyperalgesia occurred at 5, 10, 15, 20 and 40 min (p≤0.026). In inhibitory CPM (n=20), hyperalgesia only occurred after 10 and 15 min (p≤0.03). At the infraspinatus muscle, groups differed after 5 and 40 min (p≤0.008).

CONCLUSIONS

The results suggest that facilitating CPM is associated with more spreading hyperalgesia than inhibitory CPM. This implies that poor endogenous pain modulation may predispose to muscle pain and spreading hyperalgesia after injury, and suggest that strategies to enhance endogenous pain modulation may provide clinical benefits.

Dose-Dependent Pain and Pain Radiation after Chemical Stimulation of the Thoracolumbar Fascia and Multifidus Muscle: A Single-Blinded, Cross-Over Study Revealing a Higher Impact of Fascia Stimulation

Acute low back pain can be experimentally induced by injections of hypertonic saline into deep tissues of the back, such as fascia and muscle. The current study investigated the dose-dependency of peak-pain and spatial extent of concomitant radiating pain following 50, 200 and 800 μL bolus injections of hypertonic saline (5.8%) into the thoracolumbar fascia and multifidus muscle, since data on dose-dependency is lacking in humans. Sixteen healthy subjects rated (11 female, 5 male; 23.3 ± 3.1 years, mean ± SD) intensity and spatial extent of pain. Injections into the fascia resulted in significantly higher peak-pain (+86%, p < 0.001), longer pain durations (p < 0.05), and larger pain areas (+65%, p < 0.02) and were less variable than intramuscular injections. Peak-pain ratings and pain areas were 2−3-fold higher/larger for 200 μL vs. 50 μL. In contrast, peak pain increased only marginally at 800 μL by additional 20%, while pain areas did not increase further at all in both, fascia and muscle. Thus, higher injection volumes did also not compensate the lower sensitivity of muscle. Peak-pain ratings and pain areas correlated between fascia and muscle (r = 0.530, p < 0.001 and r = 0.337, p < 0.02, respectively). Peak-pain ratings and pain areas correlated overall (r = 0.490, p < 0.0001), but a weak correlation remained when the impact of between-tissue differences and different injection volumes were singled out (partial r = 0.261, p < 0.01). This study shows dose-dependent pain responses of deep tissues where an injection volume of 200 μL of hypertonic saline is deemed an adequate stimulus for tissue differentiation. We suggest that pain radiation is not simply an effect of increased peripheral input but may afford an individual disposition for the pain radiation response. Substantially higher pain-sensitivity and wider pain areas support fascia as an important contributor to non-specific low back pain.

The Area of Pressure-Induced Referred Pain Is Dependent on the Intensity of the Suprathreshold Stimulus: An Explorative Study

OBJECTIVE

To investigate the pain referral area (number of pixels) and extent (vector length) as elicited from increasing intensities of pressure-induced pain at the shoulder.

DESIGN

Cross-sectional design.

SETTING

Clinical laboratory setting.

PARTICIPANTS

Twenty-two healthy men and women participated in two experimental sessions.

METHODS

Delayed onset of muscle soreness (DOMS) was induced in the dominant shoulder and assessed 24 hours later. Participants rated the level of DOMS on a 6-point Likert scale. Four different intensities (pressure pain threshold [PPT]+20%, PPT+30%, PPT+40%, and PPT+50%) were applied to the infraspinatus in a randomized, balanced fashion for 60 seconds from low to high intensity or vice versa. The resulting location, area, and extent of referred pain as drawn by the participants on a digital body chart were extracted and expressed in pixels. The extent of pain was defined as the vector length extending from the ipsilateral earlobe to the most distal location of the pain.

RESULTS

The referred pain area from PPT+20% was smaller than PPT+30%, PPT+40%, and PPT+50%. The extent of referred pain did not differ between the pressure pain intensities.

CONCLUSIONS

Pressure intensity at PPT+30%, but no more, produces the greatest referred pain area as compared with the traditional pressure intensity of PPT+20%. Thus, the intensity of PPT+30% may be ideal for exploring the mechanisms of referred pain. The extent of the pain represents an independent expression of the intensity of the provoking stimulus and may be more closely related to the location of the stimulus.

A task-relevant experimental pain model to target motor adaptation

KEY POINTS

Motor adaptation is thought to be a strategy to avoid pain. Current experimental pain models do not allow for consistent modulation of pain perception depending on movement. We showed that low-frequency sinusoidal stimuli delivered at painful intensity result in minimal habituation of pain perception (over 60 s) and minimal stimulation artefacts on electromyographic signals. When the amplitude of the low-frequency sinusoidal stimuli was modulated based on the vertical force participants applied to the ground with their right leg while standing upright, we demonstrated a strong association between perceived pain and motor adaptation. By enabling task-relevant modulation of perceived pain intensity and the recording electromyographic signals during electrical painful stimulation, our novel pain model will permit direct experimental testing of the relationship between pain and motor adaptation.

ABSTRACT

Contemporary pain adaptation theories predict that motor adaptation occurs to limit pain. Current experimental pain models, however, do not allow for pain intensity modulation according to one's posture or movements. We developed a task-relevant experimental pain model using low-frequency sinusoidal electrical stimuli applied over the infrapatellar fat pad. In fourteen participants, we compared perceived pain habituation and stimulation-induced artefacts in vastus medialis electromyographic recordings elicited by sinusoidal (4, 10, 20 and 50 Hz) and square electrical waveforms delivered at constant peak stimulation amplitude. Next, we simulated a clinical condition where perceived knee pain intensity is proportional to the load applied on the leg by controlling sinusoidal current amplitude (4 Hz) according to the vertical force the participants applied with their right leg to the ground while standing upright. Pain ratings habituated over a 60 s period for 50 Hz sinusoidal and square waveforms but not for low-frequency sinusoidal stimuli (P < 0.001). EMG filters removed most stimulation artefacts for low-frequency sinusoidal stimuli (4 Hz). While balancing upright, participants' pain ratings were correlated with the force applied by the right leg (R²  = 0.65), demonstrating task-relevant changes in perceived pain intensity. Low-frequency sinusoidal stimuli can induce knee pain of constant intensity for 60 s with minimal EMG artefacts while enabling task-relevant pain modulation when controlling current amplitude. By enabling task-dependent modulation of perceived pain intensity, our novel experimental model replicates key temporal aspects of clinical musculoskeletal pain while allowing quantification of neuromuscular activation during painful electrical stimulation. This approach will enable researchers to test the predicted relationship between movement strategies and pain.

A randomized clinical trial on the effect of a lidocaine patch on shoulder pain relief in laparoscopic cholecystectomy

The incidence of laparoscopy-related shoulder pain reaches 90% in women. We evaluated the effect of lidocaine patch 5% on the shoulder pain after laparoscopic cholecystectomy (LC) in female patients. Total 63 female patients were randomly allocated to patch group (n = 31) and control group (n = 32). Patch group received lidocaine patch 5% and dressing retention tape on both shoulder, and control group received only dressing retention tape. Abdominal and shoulder pains were evaluated with rating on numeric rating scale (0 = no pain and 10 = the worst pain) at baseline and at 30 min, 6 h, 24 h, and 48 h after surgery. There were no significant differences in patient characteristics and operation details. The overall incidence of shoulder pain was significantly lower in patch group than in control group (42% vs. 78%, P = 0.005). The severity of shoulder pain also was significantly reduced in patch group compared to control group at 24 h and 48 h after surgery (P = 0.01 and P = 0.015, respectively). Complications related to lidocaine patch were not found except nausea. Lidocaine patch 5% reduced the incidence and severity of postoperative shoulder pain in female patients undergoing LC without complications.

Designing and conducting proof-of-concept chronic pain analgesic clinical trials

Introduction:

The evolution of pain treatment is dependent on successful development and testing of interventions. Proof-of-concept (POC) studies bridge the gap between identification of a novel target and evaluation of the candidate intervention's efficacy within a pain model or the intended clinical pain population.

Methods:

This narrative review describes and evaluates clinical trial phases, specific POC pain trials, and approaches to patient profiling.

Results:

We describe common POC trial designs and their value and challenges, a mechanism-based approach, and statistical issues for consideration.

Conclusion:

Proof-of-concept trials provide initial evidence for target use in a specific population, the most appropriate dosing strategy, and duration of treatment. A significant goal in designing an informative and efficient POC study is to ensure that the study is safe and sufficiently sensitive to detect a preliminary efficacy signal (ie, a potentially valuable therapy). Proof-of-concept studies help avoid resources wasted on targets/molecules that are not likely to succeed. As such, the design of a successful POC trial requires careful consideration of the research objective, patient population, the particular intervention, and outcome(s) of interest. These trials provide the basis for future, larger-scale studies confirming efficacy, tolerability, side effects, and other associated risks.

Analgesic effect of trigger point injection and EMLA for shoulder pain in patients undergoing total laparoscopic hysterectomy: A randomized controlled study

BACKGROUD

The purpose of this study was to evaluate the effects of trigger point injection (TPI) and eutectic mixture local anesthetics (EMLA) cream on the postoperative shoulder pain in patients undergoing total laparoscopic hysterectomy.

METHODS

In this randomized, single-blinded, and controlled study, total 75 patients were randomly allocated to TPI group (n = 25), EMLA group (n = 25), and control group (n = 25). TPI group received TPIs with 2 mL of 0.2% ropivacaine, and EMLA group received an occlusive dressing with EMLA cream 2 g on both shoulders. Overall, abdominal, and shoulder pains were evaluated at rest and in motion on postoperative day 3.

RESULTS

The incidence of shoulder pain was significantly reduced in EMLA group (56%) compared to control (88%) or TPI (88%) groups (P = .025 in both); the severity of shoulder pain was mitigated in EMLA and TPI groups compared to control group (P < .001, each). Consequently, the overall pain decreased in EMLA group and TPI group (P = .023). The patients with exercise habit (n = 31) showed lower incidence of pain than patients without exercise habit (n = 26) (P = .002, P = .005, and P = .037 in overall, abdominal, and shoulder pain, respectively). TPI or EMLA treatments decreased shoulder pain irrespective of exercise habit (P = .001 and P < .001, respectively), but decreased overall pain only in patients without exercise habit (P = .019). Lastly, EMLA lowered overall pain score at the time of first analgesic request in ward compared to control group (P = .02).

CONCLUSIONS

TPI and EMLA with occlusive dressing effectively reduced the shoulder pain after total laparoscopic hysterectomy.

Effects of nerve growth factor experimentally-induced craniofacial muscle sensitization on referred pain frequency and number of headache days: A double-blind, randomized placebo-controlled study

OBJECTIVE

To assess if repeated intramuscular injections of nerve growth factor into the temporalis and masseter muscles increase mechanical sensitivity and entropy scores. Furthermore, to investigate if increased mechanical sensitivity would lead to increased prevalence of referred pain in the studied individuals. Finally, if increased muscle sensitization would lead to an increase in number of headache days during the experimental period.

METHODS

The present double-blind, randomized placebo-controlled study recruited 16 healthy participants who were injected with nerve growth-factor, on 2 days, into the masseter and temporalis muscles and isotonic saline on the contralateral side. Mechanical sensitivity was assessed at seven different time-points (total of 21 days) by application of three different forces to 15 different sites of both muscles. Participants were asked after each force application if they experienced referred pain and were asked to keep a headache diary during the experimental period.

RESULTS

In summary, a) repeated intramuscular injections of nerve-growth-factor caused an increase in mechanical sensitivity for the masseter but not the temporalis muscle, and an increase in entropy scores when compared to the isotonic saline side. b) Both referred pain frequency and number of headache days were not increased following nerve-growth-factor injections.

CONCLUSIONS

These findings support the idea that mechanical sensitization in the masseter and temporalis muscles differs following injections of nerve growth factor. Furthermore, referred pain and headache frequency do not seem to be related to nerve growth factor sensitization in this model. These findings support the idea that in healthy individuals referred pain may be an epiphenomenon of the muscle in response to noxious input.

Possible Mechanisms of Pain Perception in Patients with Episodic Tension-Type Headache. A New Experimental Model of Myofascial Pain

A new experimental human model of myofascial pain using intramuscular infusion of a combination of bradykinin, serotonin (5-hydroxytryptamine), histamine, and prostaglandin E2 was applied to patients with episodic tension-type headache (ETTH) in order to examine pain perception. Fifteen patients with ETTH and 15 healthy controls completed the randomized, balanced, double-blinded, placebo-controlled study. Pain intensity, punctate hyperalgesia and allodynia, and pain quality were recorded. The combination induced a moderate and prolonged pain in both patients (median 51 min) ( P = 0.001) and controls (median 22 min) ( P = 0.001). Patients reported more pain than controls both after the combination ( P = 0.045) and after placebo ( P < 0.001). The McGill pain score [PRI(R)] was significantly higher in patients ( P = 0.002) and in controls ( P = 0.001), whereas pain quality and hyperalgesia were similar after the combination compared with placebo in the two groups. Due to side-effects nine subjects did not complete the study. The increased pain response, but similar qualitative pain perception, in ETTH patients may be explained by sensitization of peripheral nociceptors even though central mechanisms may also be involved.

Assessment of deep tissue hyperalgesia in the groin - a method comparison of electrical vs. pressure stimulation

BACKGROUND

Deep pain complaints are more frequent than cutaneous in post-surgical patients, and a prevalent finding in quantitative sensory testing studies. However, the preferred assessment method - pressure algometry - is indirect and tissue unspecific, hindering advances in treatment and preventive strategies. Thus, there is a need for development of methods with direct stimulation of suspected hyperalgesic tissues to identify the peripheral origin of nociceptive input.

METHODS

We compared the reliability of an ultrasound-guided needle stimulation protocol of electrical detection and pain thresholds to pressure algometry, by performing identical test-retest sequences 10 days apart, in deep tissues in the groin region. Electrical stimulation was performed by five up-and-down staircase series of single impulses of 0.04 ms duration, starting from 0 mA in increments of 0.2 mA until a threshold was reached and descending until sensation was lost. Method reliability was assessed by Bland-Altman plots, descriptive statistics, coefficients of variance and intraclass correlation coefficients.

RESULTS

The electrical stimulation method was comparable to pressure algometry regarding 10 days test-retest repeatability, but with superior same-day reliability for electrical stimulation (P < 0.05). Between-subject variance rather than within-subject variance was the main source for test variation. There were no systematic differences in electrical thresholds across tissues and locations (P > 0.05).

CONCLUSION

The presented tissue-specific direct deep tissue electrical stimulation technique has equal or superior reliability compared with the indirect tissue-unspecific stimulation by pressure algometry. This method may facilitate advances in mechanism based preventive and treatment strategies in acute and chronic post-surgical pain states.

Painful heat attenuates electrically induced muscle pain in men and women

Background and purpose

Women exhibit higher prevalence of most painful disorders. Several explanations have been proposed for this discrepancy, one being that endogenous pain modulatory pathways, which affect incoming nociceptive signals, act differently in men and women. A less efficient pain inhibitory system has been proposed as a contributing factor to explain why women exhibit higher prevalence of most painful disorders. The present study determined whether muscle pain, induced experimentally by electrical stimulation, is inhibited by a painful heat stimulus. This conditioned pain modulation (CPM) paradigm was used to determine whether women show signs of reduced inhibition compared to men.

Methods

Forty self-reported healthy individuals (20 female, 20 male) participated in a cross-over design with painful and non-painful heat as a conditioning stimulus. Test stimuli were painful intramuscular electrical stimulation of the tibialis anterior muscle at two intensities; low (1.1 × pain threshold) and high (1.6 × pain threshold). Painful conditioning was contact heat (45–49 ° C) to the contralateral forearm. Nonpainful conditioning was contact heat at 35 °C. Ten test stimuli were delivered in three blocks (before, during and after conditioning) in two sessions (painful and non-painful conditioning). The women were tested during days 12-14 of the menstrual cycle. This interval corresponds to the ovulatory phase of the menstrual cycle, the interval during which women are reported to show the largest inhibitory effects.

Results

Test stimuli were rated significantly lower during painful conditioning, compared with before conditioning. This was found for both low and high test stimulus intensities. Anonspecific attenuation was seen during non-painful conditioning for the low test stimulus intensity. Test stimuli were rated significantly lower also 3 min after conditioning, compared with before conditioning. The inhibitory effects were not different between men and women. Similar findings were obtained also if six non-CPM-responders (subjects rating test stimuli higher during conditioning than before conditioning) were excluded.

Conclusions and implications

The present findings indicate that painful contact heat inhibits electrically induced muscle pain and that inhibition was not different between men and women, when women were tested in the interval 12-14 days after their last menstruation. Some inhibition of muscle pain was seen during non-painful conditioning, indicating that nonspecific inhibitory effects were triggered. Also the nonspecific inhibitory effects were similar in men and women.

Translational musculoskeletal pain research

Diagnosis and management of musculoskeletal pain is a major clinical challenge. Fundamental knowledge of nociception from deep somatic structures and related mechanisms of sensitisation have been characterised in animals but the translation into clinical sciences is still lacking. Development and refinement of mechanism-based quantitative sensory testing in healthy volunteers and pain patients have provided new opportunities to assess pain and hyperalgesic reactions. The current technologies can provide information about, for example, peripheral and central sensitisation, descending pain control, central integration and structure specific sensitisation. Such a mechanistic approach can be used for differentiated diagnosis and for target validating new and existing analgesics. Mechanistic pain assessment of new compounds under development provides opportunities for target validation in proof-of-concept studies, which generate information to be used for selecting the most optimal patients for later clinical trials. New safe and efficient compounds are highly needed in the area of musculoskeletal pain management.

Reduction of human experimental muscle pain by alfentanil and morphine

Musculoskeletal pain is a major clinical problem. By using various experimental models in humans, the understanding of the basic mechanisms behind muscle pain can increase, thereby giving hope for new and optimized treatment. Opioids are increasingly often used to treat muscle pain. There are, however, a limited number of previous studies on opioids and muscle pain, most of them using a relative low, single dose. Therefore, we wanted to further study the effect of two rather high doses of alfentanil (25 and 75ng/ml) and morphine (0.14 and 0.28mg/kg) in human volunteers. The study consisted of two parallel studies with morphine and alfentanil, respectively, and was conducted as randomized, double-blinded, placebo-controlled, 3-way cross-over. We used intramuscular infusion of hypertonic saline and intramuscular electrical stimulation to induce experimental pain. Visual analog scale (VAS)-score, intramuscular electrical pain thresholds and pain area (local and referred) were measured. Both alfentanil and morphine at their highest doses induced a 6 to 7-fold increase in pain thresholds to single and repetitive (5 stimulations, 2Hz) electrical stimulation. Alfentanil and morphine also reduced VAS score about 4 to 5-fold during suprathreshold electric stimulation and during infusion of hypertonic saline. None of the drugs decreased referred pain. There were no apparent differences between the drugs, in terms of effect or adverse reactions. In conclusion, this is the first study to compare two high doses of alfentanil and morphine on experimental muscle pain in humans. Both alfentanil and morphine reduced experimental muscle pain. There were no indications of any true pharmacodynamic differences between the two drugs.

Referred pain is dependent on sensory input from the periphery: A psychophysical study

Muscle pain can be characterized by local pain and pain referred to distant somatic structures with concomitant cutaneous and deep somatosensory changes. The mechanisms responsible for referred muscle pain are poorly understood. The aim of this study was to study the origin of experimentally-induced referred muscle pain by anaesthetizing the skin overlying the referred pain area and to quantify deep somatosensory changes in the area. Fourteen healthy subjects (mean age = 25.1 years, range 22-34 years) were included in a placebo controlled study consisting of two sessions separated by 1 week. Two stimulation needles were inserted into the right anterior tibial muscle. Electrical stimuli (1Q Hz) were delivered by a computer-controlled constant current stimulator. The intensity required to generate referred pain was determined and the circumference of the referred pain area was marked. At the centre of the area, pressure pain threshold and pinprick perception threshold were determined. Either an anaesthetic cream (EMLA, Astra AB, Sweden) or a placebo cream (Astra AB, Sweden) covered by an occlusive dressing was applied to the marked referred pain area for 90 min. Afterwards, a 600-s stimulation at 150% of the referred pain threshold was induced while the VAS score of referred pain was recorded continuously. Pressure pain threshold and pinprick perception threshold were determined before, during and 5 min after the prolonged stimulation. A significantly lower referred pain visual analogue scale (VAS) score was recorded during the interval from 50 to 150s (p=0.04). The area under the referred pain VAS score vs time curve tended to be lower (22.7%) with the application of skin anaesthetic (p=0.07). The mean referred pain threshold and the mean referred pain area did not differ significantly between the two sessions (p>0.6). No difference was found in pressure pain threshold between the two treatments or between the four recordings during each session (p>0.8). Pinprick perception threshold increased significantly after EML application (p<0.04). Decreased referred pain intensity with application of anaesthetic cream at the referred pain site indicates that referred muscle pain depends on input from the periphery (skin) in humans.

Sensory manifestations in experimental and work-related chronic neck-shoulder pain

Little is known about the mechanisms leading to chronic neck-shoulder musculo-skeletal disorders. The aim of the present study was to investigate and compare the sensory manifestations of experimental or chronic neck-shoulder pain under controlled, low load, repetitive work. Experimental and clinical experiments were carried out. In Experiment 1, experimental muscle pain was induced in healthy subjects by intramuscular injection of hypertonic saline in the trapezius (n = 10) and infraspinatus (n = 10) muscles. Experiment 2 was performed on 18 workers with (n = 12) and without (n = 6) chronic neck-shoulder pain. Sensory assessments were performed before and/or after a session of controlled, low load, repetitive work. The pain intensity was assessed by a visual analogue scale. The pain quality and location(s) were monitored together with pressure-pain thresholds. Moderate-to-strong deep pain intensity was experienced in the experimental and clinical part of the working session (six patients out of 12). The verbal descriptors used to describe the experimental or chronic pain qualities were partially similar, i.e. first of all verbal descriptors from the sensory class. The patterns of local and referred pain characteristics were similar (around the shoulder girdle), but it is difficult to evoke pain in extensive areas with hypertonic saline. Patients exhibited hypersensitivity or tenderness to pressure stimuli in both sides of the upper body, i.e. in the trapezius, supraspinatus, infraspinatus and pectoralis major muscles (p < 0.05). This hypersensitivity was only seen at the referred pain site in the healthy volunteers exposed to infraspinatus experimental muscle pain. In conclusion, the present study demonstrated similar sensory manifestations in the experimental and clinical neck-shoulder pain experiments. The presented experimental model can most likely be used to investigate the underlying pain mechanisms involved in work-related chronic musculoskeletal pain.

Ultrasound guided, painful electrical stimulation of lumbar facet joint structures: an experimental model of acute low back pain

Quantitative sensory testing has indicated generalized muscle hyperalgesia in patients with chronic low back pain. The temporal development of such hyperalgesia is not well understood. The aim of the present study was to demonstrate whether generalized muscle hyperalgesia can develop within minutes of acute low back pain using a new experimental model of lumbar facet joint pain. Thirteen healthy volunteers were included and baseline pressure pain thresholds were assessed at eight separate sites, outside the area of evoked low back and referred pain. Using ultrasonography, two electrode needles were placed either side of a lumbar facet joint (right L3-4) and used to induce experimental low back pain for 10 min with continuous stimulation. Thresholds, stimulus-response relationships, distribution and quality of the electrically induced pain were recorded. Electrical facet joint stimulation induced low back pain and pain referral into the anterior leg, ipsilaterally, proximal to the knee, similar to what is observed clinically. Pressure pain thresholds did not change significantly before, during and after facet joint stimulation. In conclusion, we describe a novel model of acute experimental low back pain and demonstrate that generalized hyperalgesia did not develop within minutes of acute low back pain.

Spatial and temporal summation of pain evoked by mechanical pressure stimulation

Chronic pain patients often suffer from widespread and long lasting pain. The integrative effect of combined spatial and temporal summation on pain intensity has not been quantitatively tested. The present study was designed to investigate: (1) if the size of the stimulation area would facilitate the temporal summation of pain to repetitive pressure stimulation, and (2) if temporal summation is effective when stimulating separated sites, repetitively. Twenty healthy male subjects participated in this study. The test sites were located on the bilateral upper trapezius and tibialis anterior muscles. The ten stimuli (each with a duration of 1s) were applied to a single site at three inter-stimulus intervals (ISI: 1, 5, 30s) using five different probe sizes (0.5, 1, 2, 4 and 8cm(2)). The stimulation intensity was equal to the pressure pain threshold (PPT) determined for each probe size. Similar repetitive stimulations at two inter-stimulus intervals (5s and 30s) using two sizes of probes (0.5cm(2) and 2cm(2)) were applied to ten separate sites. The PPT at the trapezius muscle decreased significantly with the increase in stimulus area from 0.5cm(2) to 8cm(2) (P<0.001) due to spatial summation. Temporal summation of pain was evoked by repetitive pressure stimuli on the same site for all ISI and was more pronounced at 5s and 30s ISI with larger probe areas (2, 4, and 8cm(2)) compared to smaller probe areas. There was no temporal summation of pain to stimuli with ISI 5 and 30s when stimulating the separated sites. The current study indicated that spatial summation facilitated the temporal summation of pain for stimuli given at 5s and 30s ISI. The combination of temporal and spatial integration of nociceptive input facilitates the pain intensity, suggesting that temporal summation is clinically relevant in conditions with widespread pain.

Pathophysiological mechanisms in chronic musculoskeletal pain (fibromyalgia): the role of central and peripheral sensitization and pain disinhibition

Chronic musculoskeletal pain has biological, psychological and social components. This review deals with the biological factors, with emphasis on the fibromyalgia syndrome (FMS). Studies on central sensitization of pain-transmitting neurons, changes in endogenous pain modulation that give rise to pain disinhibition, referred pain, pain-related decrease in muscle strength and endurance, and pain generators in deep tissues are reviewed. In FMS there is strong scientific support for the statement that the biological part of the syndrome is a longstanding or permanent change in the function of the nociceptive nervous system that can be equated with a disease. Further research is necessary in order to determine which methods are best for diagnosis of the pain hypersensitivity in clinical practice. FMS may be the far end of a continuum that starts with chronic localized/regional musculoskeletal pain and ends with widespread chronic disabling pain.

Delayed onset muscle soreness at tendon-bone junction and muscle tissue is associated with facilitated referred pain

Delayed onset muscle soreness (DOMS) involves central and peripheral pain mechanisms. Referred pain patterns following stimulation of DOMS affected tissue have not been fully described. Referred pain may provide information on how central mechanisms are involved in DOMS, as referred pain is a central mechanism. Further, tendon tissue involvement in DOMS is not clear. This study assessed pressure pain threshold (PPT) sensitivity at the tendon, tendon-bone junction (TBJ) and muscle belly sites of tibialis anterior pre- and during DOMS in 45 subjects (34 males, 11 females). Furthermore, pain and referred pain areas at these three sites in response to hypertonic saline injection (n = 15 per injection site) were investigated pre- and during DOMS. DOMS was induced using controlled plantarflexion from a platform (bodyweight as resistance) causing eccentric contraction of the tibialis anterior muscle. DOMS induced PPT decrease was found at the TBJ and muscle belly sites only (P < 0.001). No mechanical effect was found in the unexercised limb. Maximal pain intensity induced by hypertonic saline given pre-DOMS was significantly higher for the tendon and TBJ injections compared to intramuscular injections (P < 0.05). Significantly higher referred pain frequency and enlarged pain areas were found at the muscle belly and TBJ sites following injection during DOMS compared to pre-DOMS. The results indicate that muscle belly and TBJ sites are sensitised while tendon tissue per se is unaffected by DOMS. Central sensitivity changes caused by DOMS may explain the increase in referred pain frequency and enlarged pain areas.

Calcitonin gene-related peptide immunoreactive neurons with dichotomizing axons projecting to the lumbar muscle and knee in rats

Dorsal root ganglion (DRG) neurons with dichotomizing axons have been reported in several species and are thought to be related to referred pain. However, these neurons, which have dichotomizing axons to the lumbar muscles and to the knee, have not been investigated. Clinically, pain from the lumbar muscles is sometimes referred to the lower extremities. Two kinds of neurotracers [1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) and fluoro-gold (FG)] were used in the present double-labelling study. DiI crystals were placed in the left lower back muscle, and FG was applied to the medial side of the knee. Bilateral DRGs from L1 through L6 were immunoreacted with calcitonin gene-related peptide (CGRP) antibodies and observed under a fluorescence microscope. DRG neurons double-labelled with DiI and FG were recognized only in the ipsilateral DRGs from levels L1 to L6. Approximately 1% of DRG neurons innervating the low back muscles had other axons to the medial side of the knee. In double-labelled neurons, the ratio of CGRP-immunoreactive DRG neurons was 60%. This finding provides a possible neuroanatomical explanation for referred knee pain from the lower back since CGRP is a marker of sensory neurons typically involved with pain perception. However, these neurons are rare, and mechanisms of referred pain may be explained by the convergence-projection hypothesis.

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.

Central sensitization in fibromyalgia and other musculoskeletal disorders

Muscle hyperalgesia and referred pain play an important role in chronic musculoskeletal pain. New knowledge on the involved basic mechanisms and better methods to assess muscle pain in the clinic are needed to revise and optimize treatment regimens. Increased muscle sensitivity is manifested as pain evoked by a normally non-nociceptive stimulus (allodynia), increased pain intensity evoked by nociceptive stimuli (hyperalgesia), or increased referred pain areas with associated somatosensory changes. Some manifestations of sensitization, such as expanded referred muscle pain areas in patients with chronic musculoskeletal pain, can be explained from animal experiments showing extrasegmental spread of sensitization. An important part of the pain manifestations (eg, tenderness and referred pain) related to chronic musculoskeletal disorders may result from peripheral and central sensitization, which may play a role in the transition from acute to chronic pain.

Experimental muscle pain and tenderness following infusion of endogenous substances in humans

Several human models of myofascial pain exist, but none are similar to clinical pain. The aim of the present study was to develop a clinically relevant model of prolonged human myofascial pain using infusion of the naturally occurring endogenous substances. Initially, bradykinin (Bk), serotonin (5-hydroxytryptamine (5-HT)), histamine (His), prostaglandin E(2) (PGE(2)), adenosine-tri-phosphate (ATP), and their combinations were infused into the trapezius muscle of 36 healthy subjects in a total of 67 sessions to identify substances, which could induce a moderate muscle pain. PGE(2), ATP, and a combination of Bk, 5-HT, His, and PGE(2) produced the intended moderate pain. These substances were further examined in a randomised, blinded, placebo-controlled dose-finding design in 15 healthy subjects in 68 sessions. PGE(2) (3, 6, and 12 nmol/ml) induced mild pain and tenderness not different from placebo. ATP (9000, 18,000, and 36,000 nmol/ml) induced pain of moderate to strong intensity (P=0.04) and the dose of 18,000 nmol/ml furthermore produced moderate local tenderness (P=0.04). Because of unacceptable side effects in subsequent examinations, further studies of ATP in humans were suspended. Infusion of the combination of Bk (92 nmol), 5-HT (156 nmol), His (140 nmol), and PGE(2) (1.95 nmol) produced a moderate pain intensity (P=0.04) and mild tenderness (P=0.04) without inducing unacceptable side effects. Intramuscular infusion of a combination of Bk, 5-HT, His, and PGE(2) induced a prolonged moderate pain and tenderness in healthy humans, and this model may be a valuable tool in future studies of the pathophysiological mechanisms of myofascial pain.

Central hypersensitivity in chronic pain after whiplash injury

OBJECTIVE

The mechanisms underlying chronic pain after whiplash injury are usually unclear. Injuries may cause sensitization of spinal cord neurons in animals (central hypersensitivity), which results in increased responsiveness to peripheral stimuli. In humans, the responsiveness of the central nervous system to peripheral stimulation may be explored by applying sensory tests to healthy tissues. The hypotheses of this study were: (1) chronic whiplash pain is associated with central hypersensitivity; (2) central hypersensitivity is maintained by nociception arising from the painful or tender muscles in the neck.

DESIGN

Comparison of patients with healthy controls.

SETTING

Pain clinic and laboratory for pain research, university hospital.

PATIENTS

Fourteen patients with chronic neck pain after whiplash injury (car accident) and 14 healthy volunteers.

OUTCOME MEASURES

Pain thresholds to: single electrical stimulus (intramuscular), repeated electrical stimulation (intramuscular and transcutaneous), and heat (transcutaneous). Each threshold was measured at neck and lower limb, before and after local anesthesia of the painful and tender muscles of the neck.

RESULTS

The whiplash group had significantly lower pain thresholds for all tests. except heat, at both neck and lower limb. Local anesthesia of the painful and tender points affected neither intensity of neck pain nor pain thresholds.

CONCLUSIONS

The authors found a hypersensitivity to peripheral stimulation in whiplash patients. Hypersensitivity was observed after cutaneous and muscular stimulation, at both neck and lower limb. Because hypersensitivity was observed in healthy tissues, it resulted from alterations in the central processing of sensory stimuli (central hypersensitivity). Central hypersensitivity was not dependent on a nociceptive input arising from the painful and tender muscles.

Remifentanil inhibits muscular more than cutaneous pain in humans

In experimental studies, drug-induced analgesia is usually assessed by cutaneous stimulation. If analgesics act differently on cutaneous and deep nociception, the results of these studies may not be entirely applicable to clinical pain involving deep structures. We tested the hypothesis that opioids have different abilities to inhibit cutaneous and muscular pain. Either the opioid remifentanil or placebo was infused in 12 healthy volunteers in a cross-over fashion. Repeated electrical stimulation (five impulses at 2 Hz) was applied to both skin and muscle. Pain thresholds were recorded. Remifentanil caused a higher increase in the muscular pain thresholds than in the cutaneous pain thresholds (P = 0.035). We conclude that opioids inhibit muscular pain more strongly than cutaneous pain in humans.

Site- and modality-specific modulation of experimental muscle pain in humans

The neurophysiological mechanisms involved in diffuse noxious inhibitory controls (DNIC) have been investigated extensively, but information is lacking about the effect of different stimulus modalities and somatic locations on the effectiveness of DNIC. This study is the first to examine the hypoalgesic effects on a deep, tonic and painful test stimulus (TS) of both painful and non-painful conditioning stimuli (CS) applied to different sites of the body. Two separate experiments were performed using painful electrical stimulation of the left anterior tibialis muscle as the TS. In the first experiment (n = 9), injection of 5% hypertonic saline was used as a painful CS into one of four muscles: anterior tibialis of each leg and brachioradialis of each arm. In the second experiment (n = 5), a non-painful vibratory stimulus was used as the CS at the same four sites. Compared with TS alone, the perceived pain intensity of the TS increased (4.5 +/- 1.8%; P = 0.019) in combination with the painful CS applied to the same muscle (ipsilateral homotopic site), but decreased (-25.3 +/- 1.4%; P < 0.001) in combination with non-painful CS at the same site. Both painful and non-painful CS applied at the three heterotopic sites caused significant and site-dependent decreases in the perceived pain intensity of the TS (range 15%-37%; P < 0.05). We conclude that a hypoalgesic DNIC-like effect on muscle pain is not produced exclusively by painful stimuli, and that the valence and magnitude of the modulation depend on the nature of the CS and its location relative to the applied TS.

The effect of differential and complete nerve block on experimental muscle pain in humans

We have attempted to examine the nerve fiber population mediating experimentally induced muscle pain in humans. Two established methods induced muscle pain: continuous, intramuscular, electrical stimulation and intramuscular infusion of hypertonic saline. A progressive nerve block was achieved by a combination of compression nerve block and intravenous regional anesthesia. Regular tests of muscle pain intensity were performed during the blocking period of 60 min. At the same time, the blocking of thick and thin afferents was monitored by assessment of proprioception and cutaneous touch, pin-prick, pressure pain, and heat-detection thresholds. Electrically induced muscle pain was inhibited (P < 0.0001) in parallel with proprioception, touch, and pin-prick, which were mediated by thick and thin myelinated nerve fibers. Saline-induced muscle pain was inhibited (P < 0.002) synchronously with heat detection and pressure pain, which are mediated by unmyelinated nerve fibers. Based on the present psychophysical experiments, it is suggested that: (1) myelinated afferents mediated mainly electrically induced muscle pain, and (2) unmyelinated afferents mediated mainly saline-induced muscle pain.