Effects of experimental and clinical noxious counterirritants on pain perception

Corticospinal excitability and reflex modulation in a contralateral non-stretched muscle following unilateral stretching

PURPOSE

Muscle stretching effect on the range of motion (ROM) and force deficit in non-stretched muscle, and the underlying mechanisms, is an ongoing issue. This study aimed to investigate crossover stretching effects and mechanisms on the plantar flexor muscles.

METHODS

Fourteen recreationally active females (n = 5) and males (n = 9) performed six sets of 45-s static stretching (SS) (15-s recovery) to the point of discomfort of the dominant leg (DL) plantar flexors or control (345-s rest). Participants were tested for a single 5-s pre- and post-test maximal voluntary isometric contraction (MVIC) with each plantar flexor muscle and were tested for DL and non-DL ROM. They were tested pre- and post-test (immediate, 10-s, 30-s) for the Hoffman (H)-reflex and motor-evoked potentials (MEP) from transcranial magnetic stimulation in the contralateral, non-stretched muscle.

RESULTS

Both the DL and non-DL-MVIC force had large magnitude, significant (↓10.87%, p = 0.027, pƞ2 = 0.4) and non-significant (↓9.53%, p = 0.15, pƞ2 = 0.19) decreases respectively with SS. The SS also significantly improved the DL (6.5%, p < 0.001) and non-DL (5.35%, p = 0.002) ROM. The non-DL MEP/MMax and HMax/MMax ratio did not change significantly.

CONCLUSION

Prolonged static stretching improved the stretched muscle's ROM. However, the stretched limb's force was negatively affected following the stretching protocol. The ROM improvement and large magnitude force impairment (statistically non-significant) were transferred to the contralateral muscles. The lack of significant changes in spinal and corticospinal excitability confirms that the afferent excitability of the spinal motoneurons and corticospinal excitability may not play a substantial role in non-local muscle's ROM or force output responses.

Non-local Acute Passive Stretching Effects on Range of Motion in Healthy Adults: A Systematic Review with Meta-analysis

BACKGROUND

Stretching a muscle not only increases the extensibility or range of motion (ROM) of the stretched muscle or joint but there is growing evidence of increased ROM of contralateral and other non-local muscles and joints.

OBJECTIVE

The objective of this meta-analysis was to quantify crossover or non-local changes in passive ROM following an acute bout of unilateral stretching and to examine potential dose-response relations.

METHODS

Eleven studies involving 14 independent measures met the inclusion criteria. The meta-analysis included moderating variables such as sex, trained state, stretching intensity and duration.

RESULTS

The analysis revealed that unilateral passive static stretching induced moderate magnitude (standard mean difference within studies: SMD: 0.86) increases in passive ROM with non-local, non-stretched joints. Moderating variables such as sex, trained state, stretching intensity, and duration did not moderate the results. Although stretching duration did not present statistically significant differences, greater than 240-s of stretching (SMD: 1.24) exhibited large magnitude increases in non-local ROM compared to moderate magnitude improvements with shorter (< 120-s: SMD: 0.72) durations of stretching.

CONCLUSION

Passive static stretching of one muscle group can induce moderate magnitude, global increases in ROM. Stretching durations greater than 240 s may have larger effects compared with shorter stretching durations.

Crossover Effects of Unilateral Static Stretching and Foam Rolling on Contralateral Hamstring Flexibility and Strength

CONTEXT

Static stretching (SS) and self-administered foam rolling (SAFR) are both effective techniques often used in rehabilitation settings to improve one's range of motion (ROM). However, their effects on nonintervened contralateral limb's performance remain equivocal.

OBJECTIVE

To examine the acute effects of unilateral hamstring's SS and SAFR on the contralateral hip-flexion passive ROM and the strength performance.

DESIGN

Randomized crossover trial.

SETTING

Controlled laboratory.

PARTICIPANTS

A total of 23 healthy young adults (13 males and 10 females) participated in this investigation.

INTERVENTIONS

Ten sets of 30-second SS or SAFR were performed on the participants' dominant hamstring muscles.

MAIN OUTCOME MEASURES

Before (pre) and after (post) the interventions, the contralateral hip-flexion passive ROM, the isometric strength of the contralateral hamstrings, and surface electromyography amplitude were measured. Separate 2-way (time × intervention) repeated measures analyses of variance were used to examine the changes in the dependent variables.

RESULTS

Both interventions significantly increased the contralateral hip-flexion passive ROM. In addition, the post-ROM value was significantly greater (P = .03) for the SS (mean ± SE = 73.5° ± 4.7°) than that for the SAFR (mean ± SE = 70.3° ± 4.5°). There were also main effects for time (P = .03) and intervention (P = .02) for the contralateral hamstring strength. However, no significant interaction or main effects were found for the normalized electromyography amplitude of the knee flexor muscles.

CONCLUSIONS

The increased contralateral hip-flexion passive ROM following both interventions was likely due to the enhanced stretch tolerance. However, the differential strength performance responses might be due to different neural mechanisms, which are proposed and discussed.

Transcutaneous electrical nerve stimulation improves fatigue performance of the treated and contralateral knee extensors

PURPOSE

Transcutaneous electrical nerve stimulation (TENS) can reduce acute and chronic pain. Unilateral fatigue can produce discomfort in the affected limb and force and activation deficits in contralateral non-exercised muscles. TENS-induced local pain analgesia effects on non-local fatigue performance are unknown. Hence, the aim of the study was to determine if TENS-induced pain suppression would augment force output during a fatiguing protocol in the treated and contralateral muscles.

METHODS

Three experiments were integrated for this article. Following pre-tests, each experiment involved 20 min of TENS, sham, or a control condition on the dominant quadriceps. Then either the TENS-treated quadriceps (TENS_Treated) or the contralateral quadriceps (TENS_Contra) was tested. In a third experiment, the TENS and sham conditions involved two\; 100-s isometric maximal voluntary contractions (MVC) (30-s recovery) followed by testing of the contralateral quadriceps (TENS_Contra-Fatigue). Testing involved single knee extensors (KE) MVCs (pre- and post-test) and a post-test 30% MVC to task failure.

RESULTS

The TENS-treated study induced greater (p = 0.03; 11.0%) time to KE (treated leg) failure versus control. The TENS_Contra-Fatigue induced significant (p = 0.04; 11.7%) and near-significant (p = 0.1; 7.1%) greater time to contralateral KE failure versus sham and control, respectively. There was a 14.5% (p = 0.02) higher fatigue index with the TENS (36.2 ± 10.1%) versus sham (31.6 ± 10.6%) conditions in the second fatigue intervention set (treated leg). There was no significant post-fatigue KE fatigue interaction with the TENS_Contra.

CONCLUSIONS

Unilateral TENS application to the dominant KE prolonged time to failure in the treated and contralateral KE suggesting a global pain modulatory response.

Neonatal bladder inflammation alters the role of the central amygdala in hypersensitivity produced by Acute Footshock stress in adult female rats

There is increasing evidence that chronic pain may be associated with events that occur during critical periods of development. Recent studies have identified behavioral, spinal neurophysiological and spinal/peripheral neurochemical differences in rats that have experienced neonatal bladder inflammation (NBI): a putative model of the chronically painful bladder disorder, interstitial cystitis. Stress has been shown to exacerbate symptoms of interstitial cystitis and produces bladder hypersensitivity in animal models. We recently reported that Acute Footshock-induced bladder hypersensitivity was eliminated in otherwise normal rats by prior bilateral lesions of the central nucleus of the amygdala. Since the spinal and peripheral nervous systems of NBI-treated rats are known to differ from normal rats, the present experiments sought to determine whether a supraspinal nervous system structure, the central amygdala, is still necessary for the induction of Acute Footshock-induced hypersensitivity. The effect of bilateral amygdala electrolytic lesions on Acute Footshock-induced bladder hypersensitivity in adult female rats was tested in Control rats which underwent a control protocol as neonates and in experimental rats which experienced NBI. Consistent with our previous report, in Control rats, Acute Footshock-induced bladder hypersensitivity was eliminated by bilateral Amygdala Lesions. In contrast, Acute Footshock-induced bladder hypersensitivity in NBI-treated rats was unaffected by bilateral Amygdala Lesions. These findings provide evidence that NBI results in the recruitment of substrates of bladder hypersensitivity that may differ from those of normal rats. This, in turn, suggests that unique therapeutics may be needed for painful bladder disorders like interstitial cystitis.

An acute session of roller massage prolongs voluntary torque development and diminishes evoked pain

INTRODUCTION

Roller massage (RM) has been reported to reduce pain associated with exercise-induced muscle soreness and increase range of motion without force or activation impairments. The objective was to examine RM effects on evoked pain and contractile properties.

METHODS

Twelve men received three sets of 30-s RM at a perceived discomfort level of 7/10 on a visual analogue scale on the ipsilateral (IPSI-R) stimulated plantar flexors (PF), contralateral PF (CONTRA-R), Sham (light rolling on stimulated PF), or Control. At pre-test, post-test, and 5-min post-test, they received evoked maximal twitch, tetanus, and 70% maximal tetanic stimulation, and performed a maximal voluntary isometric contraction (MVIC). Data analysis included perceived pain and contractile properties.

RESULTS

The 70% tetanus illustrated significant 9-10% increases in pain perception with Sham and Control at post- and 5-min post-test, respectively (p < 0.01). There was no pain augmentation with IPSI-R and CONTRA-R. There were no main effects or interactions for most contractile properties. However, MVIC force developed in the first 200 ms showed 9.5% (p = 0.1) and 19.1% (p = 0.03) decreases with IPSI-R at post-test and 5-min post-test.

CONCLUSION

Data suggest that RM-induced neural inhibition decreased MVIC F200 and nullified the testing-induced increase in evoked pain associated with 70% tetanic stimulation.

Pain pressure threshold of a muscle tender spot increases following local and non-local rolling massage

Background

The aim of the present study was to determine the acute effect of rolling massage on pressure pain threshold (PPT) in individuals with tender spots in their plantar flexor muscles.

Methods

In a randomized control trial and single blinded study, tender spots were identified in 150 participants’ plantar flexor muscles (gastrocnemius or soleus). Then participants were randomly assigned to one of five intervention groups (n = 30): 1) heavy rolling massage on the calf that exhibited the higher tenderness (Ipsi-R), 2) heavy rolling massage on the contralateral calf (Contra-R), 3) light stroking of the skin with roller massager on the calf that exhibited the higher tenderness (Sham), 4) manual massage on the calf that exhibited the higher tenderness (Ipsi-M) and 5) no intervention (Control). PPT was measured at 30 s and up to 15 min post-intervention via a pressure algometer.

Results

At 30 s post-intervention, the Ipsi-R (24 %) and Contra-R (21 %) demonstrated higher (p < 0.03) PPT values compared with Control and Sham. During 15 min post-intervention, PPT was higher (p < 0.05) following Ipsi-R (19.2 %), Contra-R (15.9 %) and Ipsi-M (10.9 %) compared with Control. There was no difference between the effects of three deep tissue massages (Ipsi-R, Ipsi-M and Contra-R) on PPT.

Discussion

Whereas the increased PPT following ipsilateral massage (Ipsi-R and Ipsi-M) might be attributed to the release of fibrous adhesions; the non-localized effect of rolling massage on the contralateral limb suggests that other mechanisms such as a central pain-modulatory system play a role in mediation of perceived pain following brief tissue massage.

Conclusion

Overall, rolling massage over a tender spot reduces pain perception.

Trial registration

ClinicalTrials.gov (NCT02528812), August 19^th, 2015.

Fibromyalgia, milnacipran and experimental pain modulation: study protocol for a double blind randomized controlled trial

BACKGROUND

The prevalence of fibromyalgia increases worldwide and is characterized by widespread and chronic pain. Treatment is difficult and includes both drug and non-drug approaches. Milnacipran, an antidepressant, is used for fibromyalgia, with a possible beneficial effect on central pain modulation. Our hypothesis is that the efficacy of milnacipran in fibromyalgia depends on the performance of pain inhibitory controls.

METHODS/DESIGN

A randomized, double blind, clinical trial (NCT01747044) with two parallel groups, in 48 women with fibromyalgia, is planned in the Clinical Pharmacology Center, University Hospital, Clermont-Ferrand, France. Conditioned pain modulation (estimated with thermal stimuli using a numeric pain rating scale), the primary endpoint measure, is evaluated before and one month after treatment with milnacipran or placebo. Secondary outcome measures include the predictability of pain descending pathways performance for milnacipran efficacy, tolerance and cognitive function. Data analysis is performed using mixed models; the tests are two-sided, with a type I error set at alpha = 0.05. Not only will this trial allow estimation of the beneficial effect of milnacipran on pain and on descending pain pathways but it will also evaluate whether the performance of this modulatory system could be predictive of its efficacy in alleviating pain.

DISCUSSION

This method would allow clinicians to take a pro-active attitude by performing a rapid psychophysical test before starting milnacipran treatment and would avoid unnecessary prescription while preventing therapeutic failure in patients who often face this recurrent problem.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01747044 .

Clinical diagnosis of pulp inflammation based on pulp oxygenation rates measured by pulse oximetry

INTRODUCTION

The objective of this study was to investigate correlations between pulp oxygenation rates (%SpO(2)) and clinical diagnoses of reversible pulpitis (RP), irreversible pulpitis (IP), or pulp necrosis (PN).

METHODS

Sixty patients who presented with a tooth with endodontic pathology were grouped according to a clinical diagnosis of either RP (n = 20), IP (n = 20), or PN (n = 20). The clinical diagnosis was based on the patient's dental history, periapical radiographs, clinical inspection, and percussion and thermal sensitivity testing. Pulse oximetry (PO) was used to determine pulp oxygenation rates. For every patient, one additional endodontically treated tooth (negative control [NC], n = 60) and one additional healthy tooth with healthy pulp status (positive control [PC], n = 60) were evaluated. Analysis of variance, the Tukey HSD test, and the Student's t test were used for statistical analysis.

RESULTS

The mean %SpO(2) levels were as follows: RP: 87.4% (standard deviation [SD] ±2.46), IP: 83.1% (SD ±2.29), PN: 74.6% (SD ±1.96), PC: 92.2% (SD ±1.84), and NC: 0% (SD ±0.0). There were statistically significant differences between RP, IP, and PN compared with NC and PC and between RP, IP, and PN (all P ≤ .01).

CONCLUSIONS

The evaluation of pulp oxygenation rates by PO may be a useful tool to determine the different inflammatory stages of the pulp to aid in endodontic diagnosis.

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

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

Heterotopic noxious conditioning stimulation (HNCS) reduced the intensity of spontaneous pain, but not of allodynia in painful peripheral neuropathy

In 15 patients with painful peripheral neuropathy and dynamic mechanical allodynia, the influence of spontaneous ongoing neuropathic pain on pain sensitivity in a remote pain-free area was examined, as was the influence of ischemia-induced heterotopic noxious conditioning stimulation (HNCS) on the intensity of ongoing pain and brush-evoked allodynia. In addition, the modulating effect of HNCS on pain sensitivity in a pain-free area was investigated. Pain thresholds to pressure and heat as well as the sensitivity to suprathreshold pressure- and heat pain were assessed in the pain-free area. Dynamic mechanical allodynia was induced by a recently developed semi-quantitative brushing technique and the patients continuously rated the intensity of the allodynia using a computerized visual analogue scale (VAS). The total brush-evoked pain intensity was calculated as the area under the VAS curve. At baseline, no significant difference in pain sensitivity was found between patients and their healthy controls in the pain-free area, indicating a lack of activation of pain modulatory systems from the spontaneous pain. Compared to baseline, the patients rated the ongoing neuropathic pain intensity significantly lower during the HNCS-procedure (p<0.05). In contrast, there was no influence from HNCS on the total brush-evoked pain intensity. In the pain-free area higher pressure pain thresholds were demonstrated during conditioning stimulation in patients and controls alike (p<0.01). In controls only, a significantly higher heat pain threshold was found during the HNCS-procedure (p<0.01). The main finding of the present study was that HNCS altered differentially spontaneous and brush-provoked pain in patients with painful peripheral neuropathy.

Is it all central sensitization? Role of peripheral tissue nociception in chronic musculoskeletal pain

Fibromyalgia syndrome (FM) is a highly prevalent musculoskeletal disorder that is often accompanied by somatic hyperalgesia (enhanced pain from noxious stimuli). Neural mechanisms of somatic hyperalgesia have been analyzed via quantitative sensory testing of FM patients. Results of these studies suggest that FM pain is associated with widespread primary and secondary cutaneous hyperalgesia, which are dynamically maintained by tonic impulse input from deep tissues and likely by brain-to-spinal cord facilitation. Enhanced somatic pains are accompanied by mechanical hyperalgesia and allodynia in FM patients as compared with healthy controls. FM pain is likely to be at least partially maintained by peripheral impulse input from deep tissues. This conclusion is supported by results of several studies showing that injection of local anesthetics into painful muscles normalizes somatic hyperalgesia in FM patients.

Pain variability in fibromyalgia is related to activity and rest: role of peripheral tissue impulse input

Because fibromyalgia (FM) patients frequently report activity-dependent deep tissue pains, impulse input from painful body regions may be relevant for their musculoskeletal complaints. In addition, peripheral impulse input may induce and maintain thermal and mechanical hyperalgesia of FM patients. If so, activity and rest may alternately enhance and diminish intensity of FM pain. However, the effects of exercise on pain are ambiguous in studies of FM. Whereas exercise-only studies demonstrated increased pain and hyperalgesia during and after physical activity, some exercise studies that included rest periods resulted in decreased FM pain and increased function. To further clarify these effects, we examined the effects of alternating exercise with rest on clinical pain and thermal/mechanical hyperalgesia of 34 FM patients and 36 age-matched healthy controls (NC). Using an ergometer, all subjects performed arm exercise to exhaustion twice alternating with 15-minute rest periods. Although strenuous muscle activity was reported as painful by most FM subjects, overall clinical pain consistently decreased during the rest periods. Additionally, FM subjects' pain sensitivity to mechanical pressure decreased after each exercise and rest session.

CONCLUSION

Alternating strenuous exercise with brief rest periods not only decreased overall clinical pain of FM subjects but also their mechanical hyperalgesia. No prolonged worsening of overall FM pain and hyperalgesia occurred despite vigorous muscle activity. Our findings contribute further evidence that FM pain and hyperalgesia are at least partially maintained by muscle impulse input, and that some types of exercises may be beneficial for FM.

PERSPECTIVE

FM is a pain-amplification syndrome that depends at least in part on peripheral tissue impulse input. Whereas muscle activity increased overall pain, short rest periods produced analgesic effects.

Central processing of noxious somatic stimuli in patients with irritable bowel syndrome compared with healthy controls

OBJECTIVE

To compare a central analgesic mechanism known as diffuse noxious inhibitory controls (DNIC) using somatic test stimuli and somatic conditioning stimuli, (CS) in irritable bowel syndrome (IBS) patients and healthy controls.

METHODS

Participants were 48 premenopausal females (27 with IBS), mean age of 29 years. The phasic heat test stimulus (peak temperature, 50 degrees C) was applied to the left palm. The DNIC effect, which measured reductions in average pain ratings (APR) during counter irritation (submersion of the participant's right hand in painful 12 degrees C circulating water) compared with baseline, was compared between groups. In addition, a second, counterbalanced, CS protocol (right hand submerged in nonpainful 32 degrees C circulating water) was performed. Differences in APR between the 2 counterirritation protocols were compared between groups to control for nonspecific effects known to influence DNIC. Psychologic measures and cardiovascular reactivity were also assessed.

RESULTS

IBS patients demonstrated smaller DNIC than controls (P=0.011, repeated measures analysis of variance), and greater state-anxiety, depression, catastrophizing, and anger-out expression (P<0.05). Group differences in DNIC were enhanced after controlling for nonspecific effects occurring during the nonpainful CS, and for psychologic measures (P=0.001, repeated measures analysis of covariance). There were no group differences in age, cardiovascular reactivity, APR, or pain ratings for the 12 degrees C CS.

DISCUSSION

These data demonstrate deficient DNIC in IBS. This is the first study to adequately control for alternative explanations of pain reduction during counterirritation. Only by controlling for nonspecific effects can evidence of deficient DNIC be attributed to dysregulation in endogenous analgesic mechanisms.

Central sensitivity syndromes: mounting pathophysiologic evidence to link fibromyalgia with other common chronic pain disorders

The aim of this study was to review emerging data from the fields of nursing, rheumatology, dentistry, gastroenterology, gynecology, neurology, and orthopedics that support or dispute pathophysiologic similarities in pain syndromes studied by each specialty. A literature search was performed through PubMed and Ovid using the terms fibromyalgia, temporomandibular joint disorder, irritable bowel syndrome, irritable bladder/interstitial cystitis, headache, chronic low back pain, chronic neck pain, functional syndromes, and somatization. Each term was linked with pathophysiology and/or central sensitization. This paper presents a review of relevant articles with a specific goal of identifying pathophysiologic findings related to nociceptive processing. The extant literature presents considerable overlap in the pathophysiology of these diagnoses. Given the psychosomatic lens through which many of these disorders are viewed, demonstration of evidence-based links supporting shared pathophysiology between these disorders could provide direction to clinicians and researchers working to treat these diagnoses. "Central sensitivity syndromes" denotes an emerging nomenclature that could be embraced by researchers investigating each of these disorders. Moreover, a shared paradigm would be useful in promoting cross-fertilization between researchers. Scientists and clinicians could most effectively forward the understanding and treatment of fibromyalgia and other common chronic pain disorders through an appreciation of their shared pathophysiology.

Deficiency in endogenous modulation of prolonged heat pain in patients with Irritable Bowel Syndrome and Temporomandibular Disorder

Females with Irritable Bowel Syndrome (IBS) and Temporomandibular Disorder (TMD) are characterized by enhanced sensitivity to experimental pain. One possible explanation for this observation is deficiencies in pain modulation systems such as Diffuse Noxious Inhibitory Control (DNIC). In a few studies that used brief stimuli, chronic pain patients demonstrate reduced DNIC. The purpose of this study was to compare sensitivity to prolonged heat pain and the efficacy of DNIC in controls to IBS and TMD patients. Heat pain (experimental stimulus; 44.0-49.0 degrees C), which was applied to left palm, was continuously rated during three 30-s trials across three separate testing sessions under the following conditions: without a conditioning stimulus; during concurrent immersion of the right foot in a 23.0 degrees C (control); and during noxious cold immersion in a (DNIC; 8.0-16.0 degrees C) water bath. Compared to controls, IBS and TMD patients reported an increased sensitivity to heat pain and failed to demonstrate pain inhibition due to DNIC. Controls showed a significant reduction in pain during the DNIC session. These findings support the idea that chronic pain patients are not only more pain sensitive but also demonstrate reduced pain inhibition by pain, possibly because of dysfunction of endogenous pain inhibition systems.

Determinants of endogenous analgesia magnitude in a diffuse noxious inhibitory control (DNIC) paradigm: Do conditioning stimulus painfulness, gender and personality variables matter?

Descending modulation of pain can be demonstrated psychophysically by dual pain stimulation. This study evaluates in 31 healthy subjects the association between parameters of the conditioning stimulus, gender and personality, and the endogenous analgesia (EA) extent assessed by diffuse noxious inhibitory control (DNIC) paradigm. Contact heat pain was applied as the test stimulus to the non-dominant forearm, with stimulation temperature at a psychophysical intensity score of 60 on a 0-100 numerical pain scale. The conditioning stimulus was a 60s immersion of the dominant hand in cold (12, 15, 18 degrees C), hot (44 and 46.5 degrees C), or skin temperature (33 degrees C) water. The test stimulus was repeated on the non-dominant hand during the last 30s of the conditioning immersion. EA extent was calculated as the difference between pain scores of the two test stimuli. State and trait anxiety and pain catastrophizing scores were assessed prior to stimulation. EA was induced only for the pain-generating conditioning stimuli at 46.5 degrees C (p=0.011) and 12 degrees C (p=0.003). EA was independent of conditioning pain modality, or personality, but a significant gender effect was found, with greater EA response in males. Importantly, pain scores of the conditioning stimuli were not correlated with EA extent. The latter is based on both our study population, and on additional 82 patients, who participated in another study, in which EA was induced by immersion at 46.5 degrees C. DNIC testing, thus, seems to be relatively independent of the stimulation conditions, making it an easy to apply tool, suitable for wide range applications in pain psychophysics.

An investigation into the effect of electrode placement of transcutaneous electrical nerve stimulation (TENS) on experimentally induced ischemic pain in healthy human participants

INTRODUCTION

It is claimed that transcutaneous electrical nerve stimulation (TENS) operates via a segmental mechanism by reducing ongoing transmission and sensitization of nociceptive dorsal horn neurons. Hence, TENS electrodes are usually placed at the site of pain.

OBJECTIVE

This study compared TENS administered at the site of experimentally induced ischemic pain (ipsilateral forearm) with TENS administered at a location not related to pain (contralateral lower leg).

METHODS

Ten healthy, pain free volunteers took part in a cross-over study during which ischemic pain was induced in the nondominant arm using a modified version of submaximal effort tourniquet technique. Pain intensity was taken at 1-minute interval/s for 5 minutes while receiving TENS either at the ipsilateral arm or contralateral leg.

RESULTS

There were no statistically significant differences in pain intensity or McGill Pain Questionnaire ratings between TENS given at the arm compared with the leg.

DISCUSSION

Taken at face value, the findings suggest that TENS effects were nonspecific and that electrode location does not affect outcome. However, this study should be seen as a call for further research rather than a definitive conclusion.

Activation of central 5HT2A receptors reduces the craniofacial nociception of rats

We assessed the contribution of central 5HT2A receptors to the craniofacial tissue nociception in naïve male rats. First, we tested whether activation of central 5HT2A receptors affected nociceptive neural activities recorded from superficial laminae of the trigeminal subnucleus caudalis (Vc)/upper cervical spinal cord junction (Vc/C2) region. Two types of units, such as deep-nociceptive or skin-wide dynamic range (WDR) units were identified from extracellular recordings. Topical administration of 5HT2A receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) onto the Vc/C2 region significantly reduced deep-nociceptive unit discharges evoked by formalin injection into the masseter muscle. Noxious pinch stimulation to the facial skin-evoked skin-WDR unit discharges was significantly reduced by topical administration of 0.1 mg/rat DOI onto the Vc/C2 region. Second, we tested whether i.c.v. administration of DOI affected Fos-like immunoreactivity (-LI) evoked by formalin injection into the masseter muscle. Fos-LI was significantly induced mainly at the ventrolateral (vl) area of trigeminal subnucleus interpolaris (Vi)/Vc junction (vl-Vi/Vc) region and Vc/C2 region in vehicle-treated rats. Formalin-evoked Fos-LI was significantly reduced in laminae I-II of the Vc/C2, but not vl-Vi/Vc region after i.c.v. administration of DOI. Finally, orofacial nocifensive behavioral activities evoked by formalin injection into the masseter muscle were significantly reduced by intracisternal administration of DOI. These results suggest that 5HT2A receptors in the Vc/C2 region mediate antinociceptive effects in the craniofacial nociception.

Psychological intervention in acute dental pain: review

Acute dental pain is an unpleasant experience. This article studies acute dental pain and examines the role of psychological intervention. After identification of the psychological factors associated with dental pain we go on to assess the role of psychological interventions.

Pressure pain threshold changes after repeated mechano-nociceptive stimulation of the trapezius muscle: possible influence of previous pain experience

We examined the relation between repeated noxious pressure over the trapezius muscle and changes in pressure pain thresholds (PPTs) in a before-after trial design. A conditioning series of 30 mechano-nociceptive stimuli was applied manually with a handheld algometer probe, and PPTs were measured over 1 trapezius muscle (skin anaesthetized) in 27 healthy women before and after the intervention. With a mean stimulation rate of 0.40 Hz and a mean nociceptive stimulation intensity of 1.78 x Threshold, subjects were found to systematically react with a change in PPT, either a decrease or an increase. Normalized data, transformed into mean unidirectional PPT differences, showed statistically highly significant changes after intervention. The relative risk of reacting with lowered PPTs on noxious stimulation was 3.7 times higher for subjects who had not given birth to children than for subjects who had given birth to 1 or several children (P<.046). When 11 subjects were tested at a second session, a clear correlation of PPT reactions (r=0.527; P<.001) was found. In summary, repetitive mechano-nociceptive stimulation of the trapezius muscle in healthy females evokes moderate and temporary changes in PPT that last for at least 35 minutes after cessation of stimulation.

PERSPECTIVE

A possible development of the response with transiently decreased PPTs into a model for human muscle pain is an intriguing possibility, since other models usually involve the introduction of chemical or thermal agents in the muscle, but this must await further research.

Mechanisms underlying development of spatially distributed chronic pain (fibromyalgia)

Chronic fibromyalgia (FM) pain is prevalent (estimated as high as 13%), predominantly affects women, and is associated with a variety of focal pain conditions. Ongoing FM pain is referred to deep tissues and is described as widespread but usually is maximally located within a restricted region such as the shoulders. Palpation of deep tissues reveals an enhanced nociceptive sensitivity that is not restricted to regions of clinical pain. Similarly, psychophysical testing reveals allodynia and hyperalgesia for cutaneous stimulation at locations beyond regions of clinical pain referral. The combination of widely distributed clinical pain and generalized hypersensitivity is highly disabling, but no satisfactory treatment is regularly prescribed. A thorough understanding of mechanisms will likely be required to develop and document adequate therapies. The generalized hypersensitivity associated with FM has focused considerable interest on central (CNS) mechanisms for the disorder. These include central sensitization, central disinhibition and a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis. However, the central effects associated with FM can be produced by a peripheral source of pain. Chronic nociceptive input induces central sensitization, magnifying pain, and it activates the HPA and the sympathetic nervous system. Chronic sympathetic activation indirectly sensitizes peripheral nociceptors and sets up a vicious cycle. Thus, it appears that central mechanisms of FM pain are dependent on abnormal peripheral input(s) for development and maintenance of this condition. A substantial literature defines peripheral-CNS-peripheral interactions that are integral to FM pain. These reciprocal actions and related phenomena of relevance to FM pain are reviewed here, leading to suggestions for testing of therapeutic approaches.

Central serotonin 3 receptors play an important role in the modulation of nociceptive neural activity of trigeminal subnucleus caudalis and nocifensive orofacial behavior in rats with persistent temporomandibular joint inflammation

The role of central serotonin 3 receptors on neural activities recorded from superficial laminae of trigeminal subnucleus caudalis/upper cervical spinal cord junction region was investigated using rats with (Complete Freund's Adjuvant day 7 group) or without (non-Complete Freund's Adjuvant group) persistent temporomandibular joint inflammation evoked by Complete Freund's Adjuvant for 7 days. We identified two types of units, Deep-wide dynamic range units and Skin-wide dynamic range units from extracellular recordings. Deep-wide dynamic range units have mechanoreceptive fields in the deep craniofacial tissues including masseter muscle but do not have cutaneous mechanoreceptive fields. Deep-wide dynamic range unit discharges evoked by the formalin injection into masseter muscle were significantly enhanced in the late phase in Complete Freund's Adjuvant day 7 group. Discharges of Skin-wide dynamic range units evoked by the noxious pinch stimulation to facial skin in Complete Freund's Adjuvant day 7 group were significantly enhanced compared with those in non-Complete Freund's Adjuvant group. Topical administration of central serotonin 3 receptor antagonist, tropisetron, onto trigeminal subnucleus caudalis/upper cervical spinal cord junction region significantly reduced both formalin-evoked Deep-wide dynamic range unit and pinch-evoked Skin-wide dynamic range unit discharges in non-Complete Freund's Adjuvant and Complete Freund's Adjuvant day 7 groups significantly. The inhibitory effects of tropisetron on pinch-evoked Skin-wide dynamic range unit discharges were prolonged in Complete Freund's Adjuvant day 7 group compared with those in non-Complete Freund's Adjuvant group. The role of central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region was also tested by orofacial formalin test in Complete Freund's Adjuvant day 7 group. Intracisternal administration of tropisetron decreased the orofacial nocifensive behavior in the late phase evoked by the injection of formalin into the masseter muscle. These results suggest that central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region are involved in mediating pronociceptive effects in both superficial and deep craniofacial tissues nociception during persistent temporomandibular joint inflammation.

Differentiation of nociceptive- from stress-induced modulatory influences on human reflexes

This paper describes a new protocol that addresses the question of whether, in human experiments, modulatory effects of remote nociceptive conditioning stimuli on reflex responses are mediated by the stress induced by the conditioning stimuli. The protocol has been illustrated by a study into the effect of a remote nociceptive conditioning stimulus on an inhibitory jaw reflex. Electromyograms were recorded from an active masseter muscle and inhibitory reflexes were evoked by applying electrical stimuli to the upper lip. This protocol utilised the application of discrete electrical conditioning stimuli applied to the sural nerve prior to the test stimulus. A preliminary experiment determined that the optimal interval between the conditioning and test stimuli, which produced modulatory effects was 100 ms. In the definitive study, computer software was used to deliver control and conditioned sweeps in a double-blind randomised sequence. This resulted in a "stress-equal" protocol in which the level of stress would be the same for both control and conditioned sweeps. Therefore any observed modulatory effects on the reflexes could not have been wholly secondary to stress. This protocol could be adapted to the study of the modulation of other reflexes or evoked sensations by nociceptive conditioning stimuli.

Individual differences in diffuse noxious inhibitory controls (DNIC): association with clinical variables

Laboratory pain research has been criticized as being irrelevant to the clinical experience of pain. Previous findings have been inconsistent with some studies suggesting that experimental pain responses may be related to the reported presence or severity of chronic pain, while others report no such associations. However, few of these studies assess a variety of laboratory pain responses, and none has assessed relationships between clinical pain and diffuse noxious inhibitory controls (DNIC) in healthy subjects. We administered questionnaire measures of pain, quality of life, and psychological variables to a sample of healthy adults participating in a laboratory study of age differences in pain responses. DNIC was not related to other laboratory pain responses, psychological variables, or physiological variables measured in the present study. Regression models predicting health-related quality of life (e.g. pain, physical functioning) revealed that age, sex, and DNIC responses explained between 10 and 25% of the variance in these dependent measures. Of the laboratory pain variables, only DNIC was the sole consistent predictor of clinical pain and physical health, with greater DNIC responses related to less pain, better physical functioning, and better self-rated health. In addition, age differences in DNIC appeared to partially mediate age differences in physical functioning. These findings highlight the potential clinical relevance of experimental pain procedures and suggest that DNIC may be the laboratory pain response most closely associated with clinical pain and health-related variables.

Reduction of TMD pain by high-frequency vibration: a spatial and temporal analysis

Under some conditions, vibration delivered to the skin can reduce pain (vibratory analgesia). Previous studies of this phenomenon in a clinical context have been somewhat variable in terms of stimulus control, and have not examined the way in which the spatial distribution of pain is affected. In the present study, we used rigorously controlled conditions to examine vibratory analgesia in participants (N=17) with painful temporomandibular disorders (TMD). Results of 20- and 100-Hz vibration were compared with data from a no-vibration control condition. The results document for the first time that vibratory analgesia occurs in TMD chronic pain conditions. We measured its time course using continuous visual analog scale (VAS) recording, and its spatial aspects by asking subjects to indicate painful regions on standardized drawings. VAS ratings and drawings both showed that pain is reduced by 100-Hz, but not by 20-Hz, vibration. The effectiveness of the high-frequency vibration cannot be attributed to a mechanism involving Pacinian corpuscles, since these receptors are lacking in the skin of the orofacial region. Spatial analyses revealed that ipsilateral and contralateral effects of vibration were statistically equivalent, suggesting that vibratory analgesia relies at least in part on central nervous system processes rather than local mechanisms.

Age-related differences in endogenous pain modulation: a comparison of diffuse noxious inhibitory controls in healthy older and younger adults

Despite decades of research, hundreds of studies, and a number of recent reviews, the effects of aging on the experience of pain remain poorly understood. Many prior investigators have reported increases in persistent pain conditions and diminished tolerance for certain types of laboratory-induced pain among the elderly. While explanations for these effects often propose senescent decrements in endogenous analgesic systems as a possible contributory mechanism, almost no direct empirical evidence for this hypothesis has yet emerged in human studies. The present investigation was designed to evaluate the existence and nature of these putative age-related differences in endogenous pain inhibition. Groups of healthy younger (n=45, mean age=21.6 years, range=18-25) and older (n=48, mean age=63.1 years, range=55-67) adults participated in a controlled, two-session laboratory assessment of diffuse noxious inhibitory controls (DNIC), a measure of endogenous pain inhibition. In this study, we examined age differences in the effects of concurrent cold pain on ratings of heterotopically presented repetitive noxious thermal stimuli. Interestingly, older adults demonstrated facilitation rather than inhibition of thermal pain during concurrent noxious cold stimulation while younger adults demonstrated some expected DNIC effects (i.e. a reduction in thermal pain ratings during heterotopic stimulation with noxious cold). Collectively, the findings of the present study suggest age-associated decrements in at least one form of endogenous analgesic response. If replicated, such findings of reduced pain-modulatory capacity in the elderly may partially explain age-related differences in the prevalence, severity, and impact of chronic pain.

Inhibitory effects do not depend on the subjective experience of pain during heterotopic noxious conditioning stimulation (HNCS): a contribution to the psychophysics of pain inhibition

Heterotopic noxious conditioning stimulation (HNCS) has been thought to give access to the diffuse noxious inhibitory controls (DNIC) in man, which can be activated in wide-dynamic-range neurons by noxious stimulation from remote areas of the body and form the neurophysiological basis of the phenomenon 'pain inhibits pain'. The latter phenomenon suggests that the subjective experience of pain is a prerequisite for an inhibitory action. The necessity of using painful stimuli as conditioning and as test stimuli to produce inhibitory effects was investigated in the present study, using a HNCS paradigm. Twenty young men received conditioning stimuli created by tonic heat at painful and non-painful levels, using either hot water (hand) or thermode (forearm). The test stimuli were phasic heat stimuli (thermode) at painful and non-painful levels applied to the cheek. Only painful but not non-painful heat as conditioning stimulus increased the heat pain threshold and decreased the ability to discriminate between painful heat of different intensities. These two findings are in accord with an inhibitory effect depending on a painful conditioning stimulus. However, the intensity ratings of the test stimuli indicated inhibitory effects of the conditioning stimuli also upon non-painful levels. Furthermore, non-painful heat as conditioning stimulus also appeared to be capable of decreasing the ratings of the test stimuli at painful levels. The latter two findings suggest: (i) that very strong but subjectively still non-painful stimulation can trigger pain inhibitory effects and (ii) that also subjectively non-painful stimuli are affected by inhibitory influences during HNCS.

Large-amplitude 5-HT1A receptor activation: a new mechanism of profound, central analgesia

We report the discovery of F 13640 and evidence suggesting this agent to produce powerful, broad-spectrum analgesia by novel molecular and neuroadaptative mechanisms. F 13640 stimulates G(alphaomicron) protein coupling to 5-HT(1A) receptors to an extent unprecedented by selective, non-native 5-HT(1A) ligands. Fifteen minutes after its injection in normal rats, F 13640 (0.01-2.5 mg/kg) decreases the vocalization threshold to paw pressure; 15 min upon injection in rats that are exposed to formalin-induced tonic nociception, F 13640 inhibits pain behavior. The initial hyperalgesia induced by 0.63 mg/kg F 13640 was followed, 8 hrs later, by paradoxical hypo-algesia; 5 mg/kg of morphine produces the opposite effects (i.e., hypo-algesia followed by hyper-algesia). Repeated F 13640 injections cause an increase in the basal vocalization threshold and a reduction of F 13640-produced hyperalgesia; in these conditions, morphine causes basal hyperalgesia and antinociceptive tolerance. Continuous two-week infusion of F 13640 (0.63 mg/day) exerts little effect on the threshold in normal rats, but markedly reduces analgesic self-administration in arthritic rats. F 13640 infusion also decreases allodynic responses to tactile and thermal stimulations in rats sustaining spinal cord or sciatic nerve injury. In these models of chronic nociceptive and neuropathic pain, the analgesia afforded by F 13640 consistently surpasses that of morphine (5 mg/day), imipramine (2.5 mg/day), ketamine (20 mg/day) and gabapentin (10 mg/day). Very-high-efficacy 5-HT(1A) receptor activation constitutes a novel mechanism of central analgesia that grows rather than decays with chronicity, that is amplified by nociceptive stimulation, and that may uniquely relieve persistent nociceptive and neuropathic pains.

Trigeminal c-Fos expression and behavioral responses to pulpal inflammation in ferrets

Injury to peripheral dental tissues evokes dynamic alternations in central sensory pathways. We have previously reported that transient stimulation of the dental pulp with noxious heat evokes the induction of the immediate early gene product Fos in the transitional region between subnucleus interpolaris and caudalis (Vi/Vc) and subnucleus caudalis (Vc). A question arises as to whether similar changes occur in response to inflammation to the tooth pulp. In this study, the effects of pulpal inflammation produced by bacterial lipopolysaccharide (LPS) on face-grooming behavior and trigeminal Fos expression were examined. Face-grooming behaviors were recorded daily for 3 days pre- and 24, 48 and 72 h post- LPS or saline application. All animals were perfused 72 h post- LPS or saline application. Brainstems were processed for Fos-like immunoreactivity (Fos-LI). Teeth were processed for H&E staining. Histological examination of LPS-treated teeth revealed features of an acute pulpitis. Moreover, LPS-treated animals showed greater face-grooming activity (i.e. tongue protrusions) directed to the injured tooth than the sham-operated group. The number of Fos-positive neurons was greater in the trigeminal subnucleus caudalis (Vc) and the transitional regions (Vi/Vc) in LPS-treated animals compared with sham-operated animals, and greater in the deeper laminae than the superficial laminae of each trigeminal region. LPS treatment did not evoke Fos expression in the rostral trigeminal regions above Vi/Vc. These results demonstrate that LPS-induced pulpal inflammation results in significant alterations in the Vi/Vc and Vc, and such changes may underlie the observed nociceptive behavioral responses and may play an important role in producing a symptomatic pulpitis in humans.

Somatosensory perception and function of diffuse noxious inhibitory controls (DNIC) in patients suffering from rheumatoid arthritis

The purpose was to investigate the influence of ongoing pain from an inflammatory nociceptive pain with two different disease durations on somatosensory functions and the effect of heterotopic noxious conditioning stimulation (HNCS) on 'diffuse noxious inhibitory controls' (DNIC) related mechanisms. Eleven patients with rheumatoid arthritis of a short duration (<1 year) (RA1), and 10 patients with rheumatoid arthritis of longer duration (>5 years) (RA5) as well as 21 age- and sex-matched healthy controls participated. Pressure pain sensitivity, low threshold mechanoreceptive function and thermal sensitivity, including thermal pain, were assessed over a painful and inflamed joint as well as in a pain-free area, i.e. the right thigh before HNCS (cold-pressor test) and repeated at the thigh only during and following HNCS. In RA1 and RA5 allodynia to pressure was seen over the joint (p<0.02 and p<0.001 respectively) in conjunction with hypoaesthesia to light touch (p<0.02) and hyperaesthesia to innocuous cold (p<0.05) in RA5. At the thigh, allodynia to pressure was found in RA5 (p<0.002). During HNCS, the sensitivity to pressure pain decreased in patients and controls alike (p<0.001). In conclusion, over an inflamed joint allodynia to pressure was found in both RA groups, with additional sensory abnormalities in RA5. In a non-painful area, allodynia to pressure was found in RA5, suggesting altered central processing of somatosensory functions in RA5 patients. The response to HNCS was similar in both RA groups and controls, indicating preserved function of DNIC-related mechanisms.

The effect of maximal exercise on temporal summation of second pain (windup) in patients with fibromyalgia syndrome

Exercise activates endogenous opioid and adrenergic systems, but attenuation of experimental pain by exercise has not been shown consistently. In this study, effects of exercise on temporal summation of late pain responses to stimulation of unmyelinated (C) nociceptors were assessed. When a preheated thermode was applied repetitively to glabrous skin of the hand in a series of brief contacts at rates of 0.2 to 0.5 Hz, the perceived intensity of late thermal sensations increased after successive contacts. This summation of pain sensations provides information regarding the status of central opioid and N-methyl-D-aspartate receptor systems. For normal subjects, temporal summation of late pain sensations was substantially attenuated when testing began 1.5 or 10 minutes after exercise. Individuals diagnosed with fibromyalgia syndrome (FMS) report generalized chronic pain that is increased after exercise. Therefore, we hypothesized that strenuous exercise would increase summation of late pain sensations in this cohort. Patients with FMS and control subjects exerted to similarly high metabolic rates, as shown by physiologic monitoring. Ratings of late pain sensations increased for patients with FMS after exercise, an effect opposite to a decrease in ratings for age/sex-matched control subjects. In contrast to this result for experimentally induced pain, clinical pain ratings were not substantially altered after strenuous exercise by patients with FMS.

Sex differences in chest pain in patients with documented coronary artery disease and exercise-induced ischemia: Results from the PIMI study

BACKGROUND

Sex differences in the pathophysiologic course of coronary artery disease (CAD) are widely recognized, yet accurate diagnosis of coronary artery disease in women remains challenging.

METHODS

To determine sex differences in the clinical manifestation of CAD, we studied chest pain reported during daily activities, exercise, and mental stress in 170 men and 26 women. All patients had documented CAD (>50% narrowing in at least 1 major coronary artery or prior myocardial infarction) and all had 1-mm ST-segment depression on treadmill exercise. We collected psychologic test results, serum samples (potassium, epinephrine, norepinephrine, cortisol, b-endorphin, and glucose), and cardiac function, sensory threshold, and autonomic function data at specified times before, during, or after exercise and mental stress tests to assess measures of depression, anxiety, and neurohormonal and thermal pain perception.

RESULTS

Women reported chest pain more often than men during daily activities (P =.04) and during laboratory mental stressors (P =.01) but not during exercise. Men had lower scores than women on measures of depression, trait anxiety, harm avoidance, and reward dependence (P <.05 for all). Women had significantly lower plasma b-endorphin levels at rest (4.2 +/- 3.9 vs 5.0 +/- 2.5 pmol/L for men, P =.005) and at maximal mental stress (6.4 +/- 5.1 vs 7.4 +/- 3.5 pmol/L for men, P <.01). A higher proportion of women than men had marked pain sensitivity to graded heat stimuli applied to skin (hot pain threshold <41 degrees C, 33% vs 10%, P =.001).

CONCLUSIONS

Our results reflect sex differences in the affective and discriminative aspects of pain perception and may help explain sex-related differences in clinical presentations.

Lack of pressure pain modulation by heterotopic noxious conditioning stimulation in patients with painful osteoarthritis before, but not following, surgical pain relief

To investigate the influence of chronic nociceptive pain on endogenous pain modulation, the effect of heterotopic noxious conditioning stimulation (HNCS) on perception of various somatosensory modalities was assessed in 15 patients with painful osteoarthritis of the hip. Thirteen patients were re-assessed when pain-free 6-14 months following surgery. Sex- and age matched healthy subjects assessed at similar time intervals served as controls. The effects of HNCS were tested using the upper extremity submaximal effort tourniquet test. Subjects rated tourniquet-induced pain intensity on a visual analogue scale (VAS). Quantitative sensory testing (QST) was performed contralaterally to the maximally painful area in 13 patients and contralaterally to the second most painful area in two patients (i.e. lateral thigh n = 12, frontal thigh n = 1, lateral calf n = 2). Sensibility was assessed before, during and 45 min following the tourniquet test. Perception thresholds to light touch were assessed using von Frey filaments and pressure pain thresholds by pressure algometry. Perception thresholds to non-painful and painful warmth and cold were determined using a Thermotest. In both sessions, patients rated the tourniquet-induced pain higher than controls at the start (P < 0.003 and P < 0.006, respectively), but not at the end of the tourniquet test. Decreased sensitivity to light touch (P < 0.001) and innocuous cold (P < 0.002) was seen during the tourniquet in patients and controls alike, on both occasions, while perception thresholds to innocuous warmth and heat pain remained unaffected. In the first session, pressure pain thresholds increased during the tourniquet test in controls (P < 0.002), but not in patients. In the second session, pressure pain thresholds increased during the tourniquet test in controls (P < 0.001) and in patients (P < 0.02). In conclusion, no pressure pain modulation was induced by HNCS in patients before surgery, as opposed to controls, suggesting a dysfunction in systems subserving 'diffuse noxious inhibitory controls' (DNIC). Normal pressure pain modulation induced by HNCS was seen when patients were re-assessed in a pain-free state following surgery, indicating that the dysfunction of DNIC had been maintained by chronic nociceptive pain.

Exteroceptive suppression periods and pericranial muscle tenderness in chronic tension-type headache: effects of psychopathology, chronicity and disability

We examined pericranial muscle tenderness and abnormalities in the second exteroceptive suppression period (ES2) of the temporalis muscle in chronic tension-type headache (CTTH; n = 245) utilizing a blind design and methods to standardize the elicitation and scoring of these variables. No ES2 variable differed significantly between CTTH sufferers and controls (all tests, P>0.05). We found no evidence that CTTH sufferers with daily or near daily headaches, a mood or an anxiety disorder, or high levels of disability exhibit abnormal ES2 responses (all tests, P>0.05). CTTH sufferers were significantly more likely than controls to exhibit pervasive tenderness in pericranial muscles examined with standardized (500 g force) manual palpation (P<0.005). Female CTTH sufferers exhibited higher levels of pericranial muscle tenderness than male CTTH sufferers at the same level of headache activity (P<0.0001). Elevated pericranial muscle tenderness was associated with a comorbid anxiety disorder. These findings provide further evidence of pericranial hyperalgesia in CTTH and suggest this phenomenon deserves further study. Basic research that better elucidates the biological significance of the ES2 response and the factors that influence ES2 assessments appears necessary before this measure can be of use in clinical research.

Dental injury models: experimental tools for understanding neuroinflammatory interactions and polymodal nociceptor functions

Recent research has shown that peripheral mechanisms of pain are much more complex than previously thought, and they differ for acutely injured normal tissues compared with chronic inflammation or neuropathic (nerve injury) pain. The purpose of the present review is to describe uses of dental injury models as experimental tools for understanding the normal functions of polymodal nociceptive nerves in healthy tissues, their neuroinflammatory interactions, and their roles in healing. A brief review of normal dental innervation and its interactions with healthy pulp tissue will be presented first, as a framework for understanding the changes that occur after injury. Then, the different types of dental injury that allow gradation of the extent of tissue damage will be described, along with the degree and duration of inflammation, the types of reactions in the trigeminal ganglion and brainstem, and the type of healing. The dental injury models have some unique features compared with neuroinflammation paradigms that affect other peripheral tissues such as skin, viscera, and joints. Peripheral inflammation models can all be contrasted to nerve injury studies that produce a different kind of neuroplasticity and neuropathic pain. Each of these models provides different insights about the normal and pathologic functions of peripheral nerve fibers and their effects on tissue homeostasis, inflammation, and wound healing. The physical confinement of dental pulp and its innervation within the tooth, the high incidence of polymodal A-delta and C-fibers in pulp and dentin, and the somatotopic organization of the trigeminal ganglion provide some special advantages for experimental design when dental injury models are used for the study of neuroinflammatory interactions.

Effects of gender and acute dental pain on thermal pain responses

OBJECTIVE

Considerable research suggests that females exhibit greater sensitivity to laboratory pain procedures than do males; however, whether the presence of acute clinical pain influences this sex difference in pain sensitivity has not been investigated. The present experiment investigated the effects of sex and acute dental pain on laboratory pain responses.

DESIGN

Thermal pain onset and tolerance were determined in 46 dental patients (15 male, 31 female) experiencing pain due to acute irreversible pulpitis and in 33 healthy controls (13 male, 20 female). In addition, measures of mood and coping were obtained in all participants. All subjects participated in two experimental sessions. The first session took place immediately before the patients underwent endodontic treatment for relief of pulpal pain. The second session took place approximately 1-2 weeks later, when pulpitis patients were pain free after treatment. During each session, thermal pain onset and tolerance were assessed with a 1-cm2 contact thermode applied to the right volar forearm using an ascending method of limits.

RESULTS

During both sessions, thermal pain onset and tolerance were lower in control females than in control males; however, male and female pulpitis patients did not differ in their thermal pain responses during either session. Pulpitis patients also showed greater affective distress than controls.

CONCLUSIONS

These data suggest that the sex difference in thermal pain sensitivity frequently reported in pain-free subjects appears to be absent in patients presenting with acute dental pain. However, this effect cannot be explained solely based on the presence of clinical pain because the effect on pain threshold and tolerance persisted into session 2, when pulpitis patients were pain free. Potential explanations for these results are discussed.

Low hot pain threshold predicts shorter time to exercise-induced angina: results from the psychophysiological investigations of myocardial ischemia (PIMI) study

OBJECTIVES

The purpose of this study was to test whether cutaneous thermal pain thresholds are related to anginal pain perception.

BACKGROUND

Few ischemic episodes are associated with angina; symptoms have been related to pain perception thresholds.

METHODS

A total of 196 patients with documented coronary artery disease underwent bicycle exercise testing and thermal pain testing. The Marstock test of cutaneous sensory perception was administered at baseline after 30 min of rest on two days and after exercise and mental stress. Resting hot pain thresholds (HPTs) were averaged for the two baseline visits and divided into two groups: 1) average HPT <41 degrees C, and 2) average HPT > or =41 degrees C, to be clearly indicative of abnormal hypersensitivity to noxious heat.

RESULTS

Patients with HPT <41 degrees C had significantly shorter time to angina onset on exercise testing than patients with HPT > or =41 degrees C (p < 0.04, log-rank test). Heart rates, systolic blood pressure and rate-pressure product at peak exercise were not different for the two groups. Resting plasma beta-endorphin levels were significantly higher in the HPT <41 degrees C group (5.9+/-3.7 pmol/liter vs. 4.7+/-2.8 pmol/liter, p = 0.02). Using a Cox proportional hazards model, patients with HPT <41 degrees C had an increased risk of angina (p = 0.03, rate ratio = 2.0). These differences persisted after adjustment for age, gender, depression, anxiety and history of diabetes or hypertension (p < 0.01).

CONCLUSIONS

Occurrence of angina and timing of angina onset on an exercise test are related to overall hot pain sensory perception. The mechanism of this relationship requires further study.

NMDA receptor mechanisms contribute to neuroplasticity induced in caudalis nociceptive neurons by tooth pulp stimulation

We recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant and prolonged increases in jaw muscle electromyographic (EMG) activity that are suggestive of central neuroplasticity. Because small-fiber afferents, including pulp afferents, access nociceptive neurons in trigeminal (V) subnucleus caudalis, this study examined whether pulpal application of MO induces neuroplastic changes in caudalis nociceptive neurons (wide dynamic range and nociceptive specific) and whether central N-methyl--aspartate (NMDA) receptor mechanisms are involved in these MO-induced neuroplastic changes. After pretreatment with vehicle (saline, 10 microliter i.t.) to the surface of the medulla, the pulpal application of MO to the maxillary molar tooth pulp produced a significant increase in neuronal spontaneous activity, a significant expansion of the pinch and/or tactile mechanoreceptive field (RF), a significant decrease in mechanical threshold, and significant increases in neuronal responses to graded pinch stimuli. Compared with vehicle-treated rats, pretreatment with the NMDA receptor antagonist MK-801 (10 microgram/10 microliter i.t.) followed by MO application to the pulp in another group of rats significantly reduced or abolished these MO-induced neuroplastic changes in nociceptive neurons. In another group of rats pretreated with saline (intrathecally), mineral oil application to the pulp did not show any significant changes in spontaneous activity or RF properties over the 40-min observation period. The pulpal application of MO in other rats (pretreated with saline, intrathecally) did not produce any significant neuroplastic changes in caudalis low-threshold mechanoreceptive neurons. These results indicate that the MO-induced activation of molar pulpal afferents can produce profound NMDA receptor-related neuroplastic changes in caudalis nociceptive neurons. Such neuroplastic changes may contribute to the hyperalgesia and spread of pain that can be associated with pulpal inflammation.

Pain threshold values during periodontal probing: assessment of maxillary incisor and molar sites

Probing pain threshold (PPT) assessments were conducted in the facial and oral sulci of maxillary central incisors and first molars of 10 periodontally healthy adults. All subjects were systemically healthy, free of pain, and reported no current medication usage. A computer-linked electronic probe, modified to deliver steadily increasing forces up to 200 grams, was used to collect the data. The system contained a subject operated "off-switch" which, upon activation, signaled the computer to record the subject's PPT. Assessments of each subject's PPTs were conducted on 3 separate occasions at 7-day intervals. Results indicated that the facial sulci of the incisors were the most pain sensitive. They displayed a mean PPT of 50.9 +/- 26.6 grams. The oral sulci of the incisors exhibited a mean PPT of 76.5 +/- 45.2 grams. Facial and oral sulci of the molars evidenced mean PPT values of 102.6 +/- 52.1 grams and 113.5 +/- 51.3 grams, respectively. These data suggest that sulci associated with incisor teeth are nearly twice as pain sensitive as sulci associated with molar teeth. In addition, facial sulci are significantly more pain sensitive than oral sulci. Data did not indicate a visit effect nor a side-of-mouth effect on PPT values.

Sensitivity of patients with painful temporomandibular disorders to experimentally evoked pain: evidence for altered temporal summation of pain

Temporomandibular disorders (TMD) represent a group of chronic painful conditions involving the muscles of mastication and the temporomandibular joint. Several studies have reported that TMD is associated with enhanced sensitivity to experimental pain. Twenty-three TMD subjects and 24 pain-free matched control subjects participated in a set of studies which were designed to evaluate whether the temporal integrative aspects of thermal pain perception are altered in TMD patients compared with control subjects. Specifically, we have examined in both TMD patients and in age- and gender-matched control subjects: (1) the time-course and magnitude of perceived pain evoked by the application of sustained 7-s noxious thermal stimuli (45-48 degrees C) to the face and forearm, (2) the central summation of C-fiber-mediated pain produced by applying brief trains of noxious heat pulses to the skin overlying the ventral aspect of the right palm and (3) the ability to discriminate small increments in noxious heat applied to facial and volar forearm skin. Data collected from these studies indicate that TMD patients show enhanced temporal integration of thermal pain compared with control subjects. TMD patients show greater thermal C-fiber-mediated temporal summation than pain-free subjects and they report a greater magnitude of sustained noxious heat pulses applied to either the face or the forearm than control subjects. In contrast to these findings, TMD and pain-free subjects are equally able to discriminate and detect small increments of heat applied to noxious adapting temperatures. These findings suggest that the augmented temporal integration of noxious stimuli may result from alterations in central nervous system processes which contribute to the enhanced pain sensitivity observed in TMD patients.

The Psychophysiological Investigations of Myocardial Ischemia (PIMI) study: objective, methods, and variability of measures

OBJECTIVE

This study evaluated physiological, neuroendocrine, and psychological status and functioning of patients with coronary artery disease in order to clarify their role in the expression of symptoms during myocardial ischemia (MI), and to establish repeatability of responses to mental stress. Design and methods of the study are presented.

METHODS

One hundred ninety-six coronary artery disease patients were examined during physical and mental stress tests in four hospitals. Eligibility criteria included narrowing of at least 50% in the diameter of at least one major coronary artery or verified history of myocardial infarction, and evidence of ischemia on an exercise treadmill test. Psychological, biochemical, and autonomic function data were obtained before, during, and after exposure to mental and exercise stressors during 2 or 3 half-days of testing. Ventricular function was assessed by radionuclide ventriculography, and daily ischemia by ambulatory electrocardiography. Sixty patients returned for a short-term mental stress repeatability study. Twenty-nine individuals presumed to be free of coronary disease were also examined to establish reference values for cardiac responses to mental stress.

RESULTS

Study participants were 41 to 80 years of age; 83 (42%) had a history of MI, 6 (3%) of congestive heart failure, and 163 (83%) of chest pain; 170 (87%) were men; and 90 (46%) had ischemia accompanied by angina during exercise treadmill testing. Ischemia during ambulatory monitoring was found in 35 of 90 (39%) patients with and 48 of 106 (45%) patients without angina during exercise-provoked ischemia. Intraobserver variability of ejection fraction changes during bicycle exercise and two mental stress tests (Speech and Stroop) was good (kappa = 1.0, .90, and .76, respectively; percent agreement = 100, 97.5, and 93.8%, respectively). Variability of assessed wall motion abnormalities during bicycle exercise was better (kappa, agreement = 85%) than during Speech or Stroop kappa and .57, percent agreement = 70% and 82.5%, respectively).

CONCLUSIONS

Study design, quality control data, and baseline characteristics of patients enrolled for a clinical study of symptomatic and asymptomatic myocardial ischemia are described. Lower repeatability of reading wall motion abnormalities during mental stress than during exercise may be due to smaller effects on wall motion and lack of an indicator for peak mental stress.