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

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.

Orthodontic Treatment, Genetic Factors and Risk of Temporomandibular Disorder

Traditionally, four groups of factors have been identified in the etiology of temporomandibular disorder (TMD): anatomical variation in the masticatory system; psychosocial characteristics; pain in other body regions; and demographics. Orthodontic treatment has been variously cited both as a protective and harmful factor in TMD etiology. Recently, a search has begun for a genetic influence on TMD etiology. Genetic markers can be of additional value in identifying gene-environment interactions, that is, isolating population sub-groups, defined by genotype in which environmental influences play a relatively greater or lesser etiological role. This paper reviews concepts and study design requirements for epidemiological investigations into TMD etiology. Findings are presented from a prospective cohort study of 186 females that illustrate an example of gene-environment interaction in TMD onset. Among people with a variant of the gene encoding catechol-O-methyl-transferase, an enzyme associated with pain responsiveness, risk of developing TMD was significantly greater for subjects who reported a history of orthodontic treatment compared with subjects who did not (P=0.04). While further studies are needed to investigate TMD etiology, this genetic variant potentially could help to identify patients whose risk of developing TMD is heightened following orthodontic treatment, hence serving as a risk marker useful in planning orthodontic care.

Genetic and environmental determinants of postthoracotomy pain syndrome

PURPOSE OF REVIEW

Pain after thoracic surgery may persist for up to a year or longer in as many as 50% of patients undergoing lung resection. There is currently no specific therapy, and our ability to predict who will develop a persistent pain syndrome is poor at best. Persistent pain after thoracotomy is not an acute somatic pain, rather it is a complex syndrome with many of the characteristics of neuropathic, dysesthetic pain.

RECENT FINDINGS

The pain genetics field has been dominated by reports of single variants leading to severe phenotypes. These (Mendelian) diseases are not representative of the more common, complex phenotype that is characterized by the lay term 'pain threshold'. Recently, work describing the association of genetic variants with idiopathic pain disorders has appeared in the literature, and here the authors suggest that these concepts are applicable to postthoracotomy pain syndrome.

SUMMARY

Postthoracotomy pain syndrome likely arises as a direct result of an environmental stress (surgery) occurring on a landscape of susceptibility that is determined by an individual's behavioral, clinical and genetic characteristics.

The influence of expectation on spinal manipulation induced hypoalgesia: an experimental study in normal subjects

Background

The mechanisms thorough which spinal manipulative therapy (SMT) exerts clinical effects are not established. A prior study has suggested a dorsal horn modulated effect; however, the role of subject expectation was not considered. The purpose of the current study was to determine the effect of subject expectation on hypoalgesia associated with SMT.

Methods

Sixty healthy subjects agreed to participate and underwent quantitative sensory testing (QST) to their leg and low back. Next, participants were randomly assigned to receive a positive, negative, or neutral expectation instructional set regarding the effects of a specific SMT technique on pain perception. Following the instructional set, all subjects received SMT and underwent repeat QST.

Results

No interaction (p = 0.38) between group assignment and pain response was present in the lower extremity following SMT; however, a main effect (p < 0.01) for hypoalgesia was present. A significant interaction was present between change in pain perception and group assignment in the low back (p = 0.01) with participants receiving a negative expectation instructional set demonstrating significant hyperalgesia (p < 0.01).

Conclusion

The current study replicates prior findings of c- fiber mediated hypoalgesia in the lower extremity following SMT and this occurred regardless of expectation. A significant increase in pain perception occurred following SMT in the low back of participants receiving negative expectation suggesting a potential influence of expectation on SMT induced hypoalgesia in the body area to which the expectation is directed.

Evaluation of pain perception in patients with temporomandibular disorders

PURPOSE

The aim of this study was to assess the pain perception of patients with temporomandibular disorders and to investigate a possible association between pain sensitivity and temporomandibular disorders.

PATIENTS AND METHODS

Fifty patients with temporomandibular disorders who were diagnosed according to research diagnostic criteria for temporomandibular disorders and 45 healthy volunteers were conducted. Visual analog scale, designed as 0 being no pain and 100 being worst pain ever experienced, was used to determine subjective pain expression during maximum mouth opening in patients and control subjects. All subjects' pressure pain thresholds were measured 3 times on the hypothenar region of the left hand with a mechanical algometer. Mean value of measured scores was assumed to be the general pressure pain threshold of the individual.

RESULTS

Patients with temporomandibular disorder had significantly lower general pressure pain threshold values than pain-free subjects. Women had lower general pressure pain threshold values than men within groups as well. In the patient group, those with limited mouth opening (</=40 mm) had significantly lower general pressure pain threshold values. Patients with general pressure pain threshold scores equal or less than 6 kg/cm(2) had higher visual analog scale scores (>50 mm). However this result was statistically insignificant.

CONCLUSIONS

According to the results of this study temporomandibular disorder patients were significantly different in terms of general pain perception. Systemic pain regulation mechanisms might be involved in temporomandibular disorder development. Some symptoms of temporomandibular disorder patients such as movement limitation and subjective pain expression might be originated from pain perception discrepancies besides severity of disease.

Influence of psychological factors on risk of temporomandibular disorders

Psychological characteristics potentially may be a cause or consequence of temporomandibular disorder (TMD). We hypothesized that psychological characteristics associated with pain sensitivity would influence risk of first-onset TMD, but the effect could be attributed to variation in the gene encoding catechol-O-methyltransferase (COMT). We undertook a prospective cohort study of healthy female volunteers aged 18-34 yrs. At baseline, participants were genotyped, they completed psychological questionnaires, and underwent quantitative sensory testing to determine pain sensitivity. We followed 171 participants for up to three years, and 8.8% of them were diagnosed with first-onset TMD. Depression, perceived stress, and mood were associated with pain sensitivity and were predictive of 2- to 3-fold increases in risk of TMD (P < 0.05). However, the magnitude of increased TMD risk due to psychological factors remained unchanged after adjustment for the COMT haplotype. Psychological factors linked to pain sensitivity influenced TMD risk independently of the effects of the COMT haplotype on TMD risk.

Studying sex and gender differences in pain and analgesia: a consensus report

In September 2006, members of the Sex, Gender and Pain Special Interest Group of the International Association for the Study of Pain met to discuss the following: (1) what is known about sex and gender differences in pain and analgesia; (2) what are the "best practice" guidelines for pain research with respect to sex and gender; and (3) what are the crucial questions to address in the near future? The resulting consensus presented herein includes input from basic science, clinical and psychosocial pain researchers, as well as from recognized experts in sexual differentiation and reproductive endocrinology. We intend this document to serve as a utilitarian and thought-provoking guide for future research on sex and gender differences in pain and analgesia, both for those currently working in this field as well as those still wondering, "Do I really need to study females?"

Differential effects of estradiol on encoding properties of TMJ units in laminae I and V at the spinomedullary junction in female rats

To determine whether estrogen status modulated dorsal horn neural activity relevant to temporomandibular joint (TMJ) processing single units were recorded in superficial and deep laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) junction of ovariectomized (OvX) female rats under barbiturate anesthesia after 17beta-estradiol (E2) treatment for 2 days. E2 dose-dependently enhanced the response to intra-TMJ stimulation by adenosine triphosphate (ATP) of neurons classified as nociceptive specific (NS), but not wide dynamic range (WDR), in superficial laminae. ATP caused similar responses among NS and WDR neurons from deep laminae in all groups. By contrast, the cutaneous receptive field areas of WDR, but not NS, units in superficial and deep laminae were enlarged in high E2-treated (HE2) compared with low E2-treated (LE2) females. Units from untreated or vehicle-treated male rats displayed responses similar to those of LE2 females. TMJ units in superficial laminae from females were more likely to receive convergent cutaneous input and respond to jaw movement than males, independent of E2 treatment. Western blot analysis revealed similar levels of P2X2 and P2X3 receptor protein in Vc/C1-2 or trigeminal ganglion samples in all groups. Immunohistochemistry revealed dense terminal labeling for P2X3 receptors in superficial laminae and moderate labeling in deep laminae at the Vc/C1-2 junction. These data indicated a significant linkage between estrogen status and the magnitude of articular input evoked by ATP from TMJ neurons in the superficial laminae at the Vc/C1-2 junction, whereas estrogenic modulation of TMJ neurons in deep laminae affected only the convergent input from overlying facial skin.

Back pain in relation to musculoskeletal disorders in the jaw-face: A matched case–control study

Back pain and temporomandibular disorders are both common conditions in the population with influence on the human motor system, but a possible co-morbidity between these conditions has not been fully investigated. The aim of this study was to test the hypothesis of an association between long-term back pain and pain and/or dysfunction in the jaw-face region. Back pain was defined as pain in the neck, shoulders and/or low back. The study-population comprised 96 cases with long-term back pain and 192 controls without back pain. We used a screening procedure, a questionnaire and a clinical examination of the jaw function. The questionnaire focused on location, frequency, duration, intensity and impact on daily life of symptoms in the jaw-face and back regions. The analysis was conducted on 16 strata, matched by age and sex for case vs. control, using Mantel-Haenszel estimates of matched odds ratio (OR) and 95% confidence interval (CI) as well as the corrected Mantel-Haenszel chi(2) test. The overall prevalence of frequent symptoms in the jaw-face region, as reported in the questionnaire, was 47% among cases and 12% among controls. The difference was statistically significant (P<0.0001) with a sevenfold odds ratio (CI: 3.9-13.7). Moderate to severe signs from the jaw region were clinically registered among 49% of the cases and 17% of the controls (P<0.0001, OR: 5.2, CI: 2.9-9.2). The results showed statistically significant associations between long-term back pain and musculoskeletal disorders in the jaw-face and indicate co-morbidity between these two conditions.

Muscle pain in the head: overlap between temporomandibular disorders and tension-type headaches

PURPOSE OF REVIEW

A variety of painful problems can affect the muscles in the head and face. Both temporomandibular disorders and tension-type headaches are believed to have a significant contribution from the skeletal muscles and have several clinical features in common. It still unclear, however, to what extent these two prevalent disorders are separate entities or have similar pathophysiological background.

RECENT FINDINGS

There is now reasonably good evidence that myofascial temporomandibular disorder patients are more likely to have a tension-type headache problem and vice versa, but the overlap is not complete. Studies have documented similarities regarding sensitization of the nociceptive pathways, dysfunction of the endogenous pain modulatory systems as well as contributing genetic factors, but there are also a number of distinct differences between temporomandibular disorders and tension-type headaches that need to be considered.

SUMMARY

Using the current classification systems, myofascial temporomandibular disorder pain and tension-type headache disorders do overlap and appear to share many of the same pathophysiological mechanisms, but it would be premature to consider them as identical entities since the importance of, for example, the affected muscles and associated function and genetic background needs to be established. Orofacial pain and headache specialists should collaborate to further develop diagnostic procedures and management strategies of temporomandibular disorders and tension-type headaches.

Role of central sensitization in symptoms beyond muscle pain, and the evaluation of a patient with widespread pain

Patients with widespread pain or fibromyalgia syndrome have many symptoms besides musculoskeletal pain: e.g. fatigue, sleep difficulties, a swollen feeling in tissues, paresthesia, cognitive dysfunction, dizziness, and symptoms of overlapping conditions such as irritable bowel syndrome, headaches and restless legs syndrome. There is evidence for central sensitization in these conditions, but further studies are needed. Anxiety, stress and depression are also present in 30-45% of patients. Other factors that may contribute to symptoms include endocrine dysfunction, psychosocial distress, trauma, and disrupted sleep. Evaluation of a patient presenting with widespread pain includes history and physical examination to diagnose both fibromyalgia and associated or concomitant conditions. Fibromyalgia should be diagnosed by its own characteristic features. Some patients with otherwise typical symptoms of fibromyalgia may have as few as four to six tender points in clinical practice. Patients with rheumatoid arthritis and systemic lupus erythematosus should be evaluated for fibromyalgia, since 20-30% of them have associated fibromyalgia, requiring a different treatment approach.

Fibromyalgia and Overlapping Disorders: The Unifying Concept of Central Sensitivity Syndromes

OBJECTIVES

To discuss fibromyalgia syndrome (FMS) and overlapping conditions, eg, irritable bowel syndrome, headaches, and chronic fatigue syndrome, within the concept of central sensitivity syndromes (CSS).

METHODS

A critical overview of the literature and incorporation of the author's own views.

RESULTS

The concept of CSS seems viable. It is based on mutual associations among the CSS conditions as well as the evidence for central sensitization (CS) among several CSS members. However, such evidence is weak or not available in other members at this time, requiring further studies. The biology of CSS is based on neuroendocrine aberrations, including CS, that interact with psychosocial factors to cause a number of symptoms.

CONCLUSIONS

CSS is an important new concept that embraces the biopsychosocial model of disease. Further critical studies are warranted to fully test this concept. However, it seems to have important significance for new directions for research and patient care involving physician and patient education. Each patient, irrespective of diagnosis, should be treated as an individual considering both the biological and psychosocial contributions to his or her symptoms and suffering.

The emerging role of botulinum toxin in the treatment of temporomandibular disorders

The objective of this review was to discuss the emerging role of botulinum toxin in the treatment of temporomandibular disorders (TMD), to review the current literature, recent clinical trials, as well as preliminary data from our own clinical study, and to formulate an algorithm for the work-up and treatment of TMD.

Overview of orofacial pain: epidemiology and gender differences in orofacial pain

Chronic orofacial pain is a prevalent problem that encompasses numerous disorders with diverse causes and presenting symptoms. Compared with men, women of reproductive age seek treatment for orofacial pain conditions, as well as other chronic pain disorders more frequently. Important issues have been raised regarding gender and sex differences in genetic, neurophysiologic, and psychosocial aspects of pain sensitivity and analgesia. Efforts to improve our understanding of qualitative sex differences in pain modulation signify a promising step toward developing more tailored approaches to pain management.

Tinnitus and pain

Tinnitus has many similarities with the symptoms of neurological disorders such as paresthesia and central neuropathic pain. There is considerable evidence that the symptoms and signs of some forms of tinnitus and central neuropathic pain are caused by functional changes in specific parts of the central nervous system and that these changes are caused by expression of neural plasticity. The changes in the auditory nervous system that cause tinnitus and the changes in the somatosensory systems that cause central neuropathic pain may have been initiated from the periphery, i.e. the ear or the auditory nerve for tinnitus and receptors and peripheral nerves in the body for pain. In the chronic condition of tinnitus and pain, abnormalities in the periphery may no longer play a role in the pathology, but the tinnitus is still referred to the ear and central neuropathic pain is still referred to the location on the body of the original pathology. In this chapter we will discuss specific similarities between tinnitus and pain, and compare tinnitus with other phantom disorders. Since much more is known about pain than about tinnitus, it is valuable to take advantage of the knowledge about pain in efforts to understand the pathophysiology of tinnitus and find treatments for tinnitus.

Amino acid release at the spinomedullary junction after inflammation of the TMJ region in male and female rats

Temporomandibular joint (TMJ) disorders are painful conditions that are more prevalent in women than men. This study tested the hypothesis that acute inflammation of the TMJ region evoked sex-related changes in amino acid transmitter concentrations at the trigeminal subnucleus/upper cervical cord (Vc/C2) junction, the major terminal zone for TMJ sensory afferents. Microdialysis samples were collected in male, intact and ovariectomized (OvX) female rats after injection of mustard oil into the TMJ region (TMJ-MO) under barbiturate anesthesia. Males displayed increases in glutamate, aspartate and serine at 5 min and secondary increases 40-45 min after TMJ-MO. Intact and OvX females given low dose estrogen (LE2) displayed increases in glutamate, aspartate and serine at 5 min but no secondary increase at 40 min, while OvX females given high dose estrogen (HE2) revealed no increases after TMJ-MO. Glycine increased 20 min after TMJ-MO in males and cycling females, but not in OvX rats. Perfusion of high potassium through the probe evoked similar increases in glutamate, aspartate and glycine in all groups. In separate experiments, perfusion of the glutamate-aspartate reuptake inhibitor, L-trans-2,4-pyrrolidine dicarboxylate (PDC), through the probe caused a prompt elevation in glutamate that was significantly greater in HE2 than LE2 females or males. These results suggested sex hormone status affects glutamatergic neurotransmission at the Vc/C2 junction by acting, in part, through modulation of glutamate reuptake. Altered amino acid transmitter release and/or availability at the Vc/C2 junction may contribute to differential processing of sensory input from the TMJ region in males and females.

Contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus produced by acute injury to the masseter muscle during persistent temporomandibular joint inflammation in rats

We investigated the contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus (VSP) following acute masseter muscle injury in male rats with or without temporomandibular joint (TMJ) inflammation persisting for 7 days. TMJ inflammation was evoked by an injection of complete Freund's adjuvant (CFA). Two hours after formalin injection into the masseter muscle produced Fos-like immunoreactivity (Fos-LI) in several regions of the VSP and upper cervical spinal cord (C2), such as ventrolateral (vl) area of the trigeminal subnucleus caudalis (Vc)/subnucleus interpolaris (Vi) transition (vl-Vi/Vc), paratrigeminal nucleus (dPa5), middle portion of the Vc (mid-Vc) and Vc/C2 transition (Vc/C2) regions in both groups. Significant increases in the number of Fos-LI were observed in these areas in CFA group compared with non-CFA group. TMJ inflammation alone did not induce a significant level of Fos-LI in the VSP. In order to assess the effect of antagonizing 5-HT2A or 5-HT3 receptors on formalin-induced Fos-LI, rats were pre-treated with local (masseter muscle) administration of ketanserin or tropisetron (0.01, 0.1 mg/rat) 20 min prior to formalin injection. In CFA group, these antagonists given locally reduced the Fos-LI response in the laminae I-II at the mid-Vc and Vc/C2 regions. These antagonists reduced the Fos-LI response in the dPa5, but not in the vl-Vi/Vc region. The Fos-LI response was not affected by i.v. administration of ketanserin (0.01, 0.1 mg/rat) or tropisetron (0.01 mg/rat). In non-CFA group, these antagonists given locally did not reduce the Fos-LI response. These results suggest that peripheral 5-HT2A and 5-HT3 receptors contribute to nociceptive processing in the masseter muscle in TMJ inflammatory conditions.

Pain-related Catastrophizing in Healthy Women Is Associated With Greater Temporal Summation of and Reduced Habituation to Thermal Pain

OBJECTIVE

Pain-related coping strategies, especially catastrophizing, play an influential role in shaping pain responses. However, although numerous studies have examined the impact of catastrophizing on chronic pain outcomes, relatively few have evaluated relationships between individual differences in pain-related catastrophizing and pain perception, with most of those studies examining only pain threshold or pain tolerance. We assessed, for the first time, catastrophizing's association with the magnitude of temporal summation of pain, a primary marker for central nervous system sensitizability.

METHODS

Thirty-eight healthy young women underwent standardized experimental pain testing, followed by administration of a brief questionnaire that assessed catastrophizing cognitions during the pain-testing session.

RESULTS

Higher levels of pain-related catastrophizing were related to higher suprathreshold pain ratings and greater temporal summation of thermal pain, suggesting that catastrophizing may play a facilitatory role in the processing of pain-related information, though the specific pathways underlying this facilitation are not clear.

CONCLUSIONS

These preliminary findings highlight the importance of coping in shaping individuals' responses to noxious stimuli, and suggest that interventions that decrease pain catastrophizing may reduce the burden of acute and chronic pain.

TMJ inflammation increases Fos expression in the nucleus raphe magnus induced by subsequent formalin injection of the masseter or hindpaw of rats

The study was designed to examine the effect of persistent temporomandibular joint (TMJ) inflammation on neuronal activation in the descending pain modulatory system in response to noxious stimulus. Formalin was injected into the left masseter muscle or hindpaw of rats 10 days after injection of the left TMJ with saline or complete Freund's adjuvant (CFA). The results showed that 10-day persistent TMJ inflammation (induced by CFA) alone did not induce a significant increase in Fos-like immunoreactive (Fos-LI) neurons in the rostral ventromedial medulla (RVM) or locus coeruleus (LC), but that formalin injection of the masseter muscle or hindpaw induced a significant increase in Fos-LI neurons in the RVM and LC of rats with and without TMJ inflammation (P < 0.05). However, persistent TMJ inflammation significantly increased Fos-LI neurons in the nucleus raphe magnus (NRM) induced by subsequent formalin injection of the masseter muscle and hindpaw (70.2% increase and 53.8% increase, respectively, over the control TMJ-saline-injected rats; P < 0.05). The results suggest that persistent TMJ inflammation increases neuronal activity, in particularly in the NRM, by the plastic change of the descending pain modulatory system after ipsilateral application of a noxious stimulus to either orofacial area or a spatially remote body area.

Quantitative sensory testing in children with migraine: Preliminary evidence for enhanced sensitivity to painful stimuli especially in girls

Recent studies showed an enhanced general sensitivity to painful stimuli in adult migraineurs during as well as between attacks. Yet, the influence of a prolonged pain history and potential sex differences has not been studied. We used quantitative sensory testing to examine 25 children with migraine between attacks and 28 controls (age 9-15). The assessment included the measurement of heat and mechanical pain thresholds as well as measures of perceptual sensitization in response to repetitive (mechanical) or tonic (thermal) noxious stimulation at both trigeminal and thenar sites. In addition, the mother was either present or absent during the measurements. Heat pain thresholds were not significantly different between the two groups. However, the child migraineurs showed significantly lower mechanical pain thresholds. Children and especially girls with migraine displayed significantly more sensitization to a tonic heat stimulus at the trigeminal site when the mother was present. The migraineurs also showed a trend towards higher sensitization ratings for mechanical stimuli. Overall, heat pain thresholds were significantly higher in the presence of the mother. In the migraine group only, mechanical pain thresholds were significantly higher when the mother was present. To summarize, an enhanced sensitivity to painful stimuli can already be observed in children suffering from migraine for an average duration of 4.4 years. This may be the result of sensitization in nociceptive pain pathways caused by frequent pain experiences. Girls with migraine were more prone to such sensitization, which may increase their risk for continuing to suffer from migraine throughout adulthood.

Gender differences in pain and secondary hyperalgesia after heat/capsaicin sensitization in healthy volunteers

In most published studies women are more sensitive to experimental pain than men. Enhanced central pain processing in women has been suggested, but psychosocial factors might also have affected the findings. Data from five completed healthy volunteer studies were analyzed to investigate gender differences in development of secondary hyperalgesia. Cutaneous hyperalgesia was induced with the heat/capsaicin sensitization model. Outcome measures were areas of secondary hyperalgesia to brush and von Frey hair stimulation after heat and capsaicin sensitization, rating of pain during heat/capsaicin sensitization, and heat pain detection thresholds. There was a trend toward smaller areas of secondary hyperalgesia in women. After adjusting for estimated gender differences in forearm surface area, areas to brush but not von Frey hair stimulation after capsaicin sensitization were larger in women. Peak pain, but not total pain, during prolonged noxious thermal stimulation was higher in women. There was no gender difference in pain ratings during capsaicin sensitization or in heat pain detection thresholds. The results provided only limited support to the hypothesis that gender differences in clinical pain syndromes can be explained by enhanced central sensitization in women.

PERSPECTIVE

Our findings suggest that gender differences in nociceptive transmission and neuronal sensitization are small and provide only limited support to the hypothesis that gender differences in acute and chronic pain syndromes can be explained by enhanced central sensitization in women.

Persistent monoarthritis of the temporomandibular joint region enhances nocifensive behavior and lumbar spinal Fos expression after noxious stimulation to the hindpaw in rats

Effects of persistent temporomandibular joint (TMJ) inflammation on nociceptive responses of remote bodily areas of the rat were investigated. Monoarthritis of the TMJ region was evoked by the injection of complete Freund's adjuvant (CFA) into the left TMJ region. Rats without injection of CFA into the TMJ region served as controls (non-CFA group). Time spent on licking behavior evoked by the injection of formalin into the left hindpaw and withdrawal thresholds of mechanical stimulation to both sides of the hindpaw were measured during TMJ inflammation for 3 weeks. Furthermore, expression of Fos protein in the lumbar dorsal horn was immunohistochemically investigated following the injection of formalin into the hindpaw during TMJ inflammation. Formalin-evoked nocifensive behavioral activities were significantly enhanced at 10 and 14 days after CFA injection in the late phase, while the withdrawal threshold to mechanical stimulation was significantly decreased bilaterally at 8, 10 and 14 days after CFA injection. Both formalin-evoked licking behavior and mechanical withdrawal thresholds to bilateral hindpaw at 21 days after CFA injection were similar to those in the non-CFA group. The number of Fos-positive neurons in the lumbar dorsal horn ipsilateral to the formalin injection at 1 and 7 days after CFA injection into the TMJ were similar to those in the non-CFA group; however, those were significantly increased in the laminae I-II and V-VI of the lumbar dorsal horn at 14 days after CFA injection. TMJ inflammation for 7 and 14 days alone produced a small number of Fos-expressing neurons in the lumbar dorsal horn. These results provide evidence that persistent unilateral inflammation of the TMJ region causes an increase in behavioral hyperalgesia of the hindpaw, which is attributed to the modulation of neural activities, in part, in the lumbar dorsal horn, likely mediated by supraspinal neural mechanisms.

Isometric exercise has opposite effects on central pain mechanisms in fibromyalgia patients compared to normal controls

Aerobic exercise has been shown to activate endogenous opioid and adrenergic systems and attenuate experimental pain in normal control subjects (NC). In contrast, fibromyalgia (FM) subjects' experimental pain ratings increase after aerobic exercise, suggestive of abnormal pain modulation. In order to determine whether central or peripheral mechanisms are predominantly involved in the abnormal pain modulation of FM patients, the effects of handgrip exercise on thermal (cutaneous) and mechanical (somatic) experimental pain was tested in local as well as remote body areas of FM and NC subjects. Supra-threshold thermal pain ratings and pressure pain thresholds over both forearms were obtained before and during 90 s of sustained 30% maximal voluntary contraction (MVC). This isometric exercise resulted in substantially decreased thermal pain ratings and increased mechanical thresholds in local as well as remote body areas in NC. Opposite effects were detected in FM patients. Thus, sustained local muscular contraction induced widespread pain inhibitory effects in NC. In contrast, the widespread hyperalgesic effects of exercise on FM patients clearly indicate altered central pain mechanisms. However, whether these exercise effects of FM patients result from abnormal descending inhibition or excessive activation of muscle nociceptive afferents needs to be addressed in future studies.

Tooth contact in patients with temporomandibular disorders

Both experimental and retrospective studies suggest a link between parafunctions and pain in temporomandibular disorder (TMD) patients. To investigate the role of parafunctions in TMD, experience sampling methodology was used as a prospective test of the hypothesis that patients with TMD have higher levels of tooth contact and tension than non-TMD controls. Three groups of TMD patients and a group of normal controls carried pagers for one week, were contacted approximately every two hours by an automated calling system, and completed questionnaires assessing tooth contact, tension, and pain at each contact. Results showed that tooth contact was much more frequent among normal controls than is commonly presumed. Patients with myofascial pain with/without arthralgia reported more frequent contact, higher intensity contact, and more tension than patients with disk displacement or normal controls. Increased masticatory muscle activity responsible for tooth contact and tension may be an important mechanism in the etiology and maintenance of the myofascial pain and arthralgia of TMD.

Effect of persistent monoarthritis of the temporomandibular joint region on acute mustard oil-induced excitation of trigeminal subnucleus caudalis neurons in male and female rats

The effect of persistent inflammation of the temporomandibular (TMJ) region on Fos-like immunoreactivity (Fos-LI) evoked by acute noxious stimulation of the same or opposite TMJ was assessed in male and cycling female rats. Two weeks after inflammation of the TMJ by complete Freund's adjuvant (CFA, 25 microg) the selective small fiber excitant, mustard oil (MO, 20%), was injected into the arthritic or opposite TMJ under barbiturate anesthesia. MO stimulation of the arthritic TMJ increased Fos-LI ipsilateral, but not contralateral, to MO compared to naïve subjects in superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C2) junction independent of sex hormone status. Unexpectedly, MO stimulation of the opposite TMJ in arthritic rats also produced a greater Fos-LI response ipsilateral to MO than naïve animals. Fos-LI produced in the dorsal paratrigeminal region (dPa5) and Vc/C2 junction after MO stimulation of the normal TMJ was significantly greater in proestrous than diestrous females or male monoarthritic rats. In contrast to naïve animals, Fos-LI was produced in deep laminae at the Vc/C2 junction ipsilateral to MO in CFA-treated animals independent of the site of prior CFA inflammation or sex hormone status. These results indicated that persistent monoarthritis of the TMJ region enhanced the excitability of trigeminal brainstem neurons to subsequent TMJ injury that occurred bilaterally in multiple regions of the lower trigeminal brainstem complex and depended on sex hormone status.

Why Look in the Brain for Answers to Temporomandibular Disorder Pain?

Temporomandibular disorder (TMD) patients often exhibit widespread clinical pain, as well as greater sensitivity to experimental pain than pain-free controls, suggesting a role of central pathophysiologic mechanisms in TMD. Moreover, TMD is more prevalent among women, which may be related to the higher sensitivity of women to experimental pain. Women also exhibit greater temporal summation of heat pain compared to men. Temporal summation, the increase in pain intensity upon repetitive noxious stimulation of constant intensity, at a high frequency is centrally mediated. Thus, greater temporal summation in women indicates that their central nociceptive processing is upregulated compared to men. Recent studies in our research center sought further evidence for upregulation of central nociceptive processing in females compared to males and in TMD patients compared to healthy controls, assessing group differences in temporal summation of mechanically evoked pain, and aftersensations following repetitive noxious stimulation. Sixteen series of 10 repetitive, sharp, mechanical stimuli were applied to the fingers of 25 female TMD patients, 25 healthy women, and 25 healthy men, with a computer-controlled small probe. All subjects rated the pain intensity or the unpleasantness evoked by the 1st, 5th and 10th stimulus in the series, and the aftersensations 15 s and 1 min after the last stimulus on visual-analog scales. TMD patients exhibited greater temporal summation of pain and unpleasantness, stronger aftersensations, and more frequent painful aftersensations than controls. Healthy females showed greater temporal summation of pain intensity and unpleasantness, higher intensity and unpleasantness of aftersensations, and more frequent painful aftersensations than males. Greater temporal summation of pain and aftersensations from digital stimulation of TMD patients than controls suggest a generalized hyperexcitability of the central nociceptive system in this patient group. Such hyperexcitability may contribute to the development and/or maintenance of chronic TMD pain. Moreover, greater temporal summation of pain and aftersensations in healthy females than males indicate that their central processing of nociceptive input may be more easily upregulated into pathological hyperexcitability, possibly accounting for the predominance of TMD among women.

Orofacial Pain—Part I

Orofacial pain is a common complaint, affecting the lives of millions of people around the world. Chronic orofacial pain often constitutes a challenging diagnostic problem that can be complicated by psychosocial factors and typically requires multidisciplinary treatment approaches. The fundamental prerequisite for successful management of orofacial pain is an accurate diagnosis. Generating a differential diagnosis, which will ultimately lead to a definite diagnosis, requires thorough knowledge of the diagnostic range of orofacial pain. There is a vast array of orofacial pain categories including: (1) musculoskeletal, (2) neuropathic, (3) vascular, (4) neurovascular, (5) idiopathic, (6) pain caused by local, distant, or systemic pathology, and (7) psychogenic. This article presents the salient clinical features and the therapeutic approaches for the various subtypes of musculoskeletal and neuropathic pain. Musculoskeletal pain is the most prevalent orofacial pain, with temporomandibular disorders and tension-type headache being the main examples. Neuropathic pain develops secondary to neural injury and/or irritation and can be distinguished into episodic, including trigeminal neuralgia and glossopharyngeal neuralgia, as well as continuous, such as herpetic and postherpetic neuralgia, traumatic neuralgia, and Eagle's syndrome.

Behavioral assessment of temporal summation in the rat: sensitivity to sex, opioids and modulation by NMDA receptor antagonists

RATIONALE

Temporal summation of pain reflects a perceived increase in nociceptive sensitivity following repeated noxious stimulation that can last for approximately 15 s-2 min. This short-lasting change in nociceptive sensitivity has been used as a model to examine factors that influence the central processing of pain and the mechanisms underlying some chronic pain conditions.

OBJECTIVE

The purpose of this study was to develop a behavioral procedure to induce temporal summation in rats and determine the sensitivity of temporal summation (i.e., decrease tail-withdrawal latency following repeated presentations of a nociceptive thermal stimulus) to various parametric manipulations, sex, modulation by the N-methyl-D-aspartate (NMDA) receptor system, and sensitivity to reversal by opioids.

RESULTS

The magnitude of temporal summation generally decreased with increases in the inter-nociceptive stimulus interval, and increased with increases in both the nociceptive stimulus intensity and the number of nociceptive stimulus presentations. Temporal summation was short-lived, evident 3.0 s after the final nociceptive stimulus presentation, but not after 30 s. Males displayed slightly higher levels of temporal summation than females. The non-competitive NMDA antagonists ketamine (3.0-30 mg/kg), dizocilpine (0.03-0.1 mg/kg) and dextromethorphan (10-30 mg/kg) attenuated the level of temporal summation at doses that failed to produce antinociceptive effects (warm water tail-withdrawal procedure). In an antinociception procedure, the opioids morphine (3.0-10 mg/kg), buprenorphine (0.3-3.0 mg/kg), butorphanol (3.0-30 mg/kg) and spiradoline (10-30 mg/kg) were more potent in males, whereas these opioids were equally potent and effective in reducing the level of temporal summation in males and females.

CONCLUSIONS

These findings suggest a number of similarities in the characteristics and receptor modulation of temporal summation in humans and rats, and that in this model of chronic pain there are no sex differences in opioid potency.

Are female patients with orofacial pain medically compromised?

BACKGROUND

Chronic pain has been linked to various medical conditions. The authors assessed whether certain medical conditions are more prevalent in female patients with orofacial pain than in age-matched controls.

METHODS

The study sample consisted of 87 female adult subjects from a university-based orofacial pain center (OPC) and 87 age-matched female subjects from a university-based undergraduate dental clinic (UDC). Subjects were evaluated between February 2003 and July 2003, and they completed a standardized, 78-question medical history questionnaire as part of routine clinical protocol. Using the subjects' medical histories, the authors compared 11 major medical categories and 77 individual conditions for both groups. For statistical analyses, the authors used nonparametric Kolmogorov-Smirnov Z tests and chi2 tests and calculated odds ratios (ORs).

RESULTS

The medical histories of subjects in the OPC group indicated a significantly greater number of medical conditions than did those of the subjects in the UDC group (Z = -4.411, P < .0001). Specifically, subjects in the OPC group reported having significantly more neurological (Z = -5.304, P < .0001), gastrointestinal (Z = -2.897, P = .004), pulmonary (Z = -2.298, P = .022), dermatologic (Z = -2.984, P = .003) and other conditions (Z = -2.885, P = .004) than did subjects in the UDC group. Subjects in the OPC group reported having 12 individual medical conditions significantly more often (P < .05, ORs ranged from 2.5 to 9.7) than did subjects in the UDC group.

CONCLUSIONS

Female patients with orofacial pain complaints appear to have more systemic problems than do female patients seeking routine dental care.

CLINICAL IMPLICATIONS

The presence of multiple medical conditions can influence orofacial pain management options and treatment outcomes. Patients with more medically complicated orofacial pain may require treatment on a multidisciplinary basis.

The role of peripheral 5HT2A and 5HT1A receptors on the orofacial formalin test in rats with persistent temporomandibular joint inflammation

The role of peripheral serotonin (5HT) 2A and 5HT1A receptors on the orofacial nocifensive behavioral activities evoked by the injection of formalin into the masseter muscle was evaluated in the rats with persistent temporomandibular joint (TMJ) inflammation evoked by Complete Freund's Adjuvant (CFA). The orofacial nocifensive behavioral activities evoked by the injection of formalin into masseter muscle were significantly enhanced at 1 day (CFA day 1 group) or 7 days (CFA day 7 group) during TMJ inflammation. Pretreatment with local administration of 5HT2A receptor antagonist, ketanserin (0.01, 0.1 mg/rat) into the masseter muscle or systemic administration of ketanserin via i.p. injection (1 mg/kg) reduced the orofacial nocifensive behavioral activities of the late phase evoked by formalin injection into masseter muscle on the side of TMJ inflammation (CFA day 7 group). However, local (0.001-0.1 mg/rat) or systemic (1 mg/kg) administration of 5HT1A receptor antagonist, propranolol, into masseter muscle did not produce the antinociceptive effect in CFA day 7 group. Moreover, local administration of ketanserin (0.1 mg) or propranolol (0.1 mg) into masseter muscle did not inhibit nocifensive orofacial behavior in rats without TMJ inflammation. These data suggest that persistent TMJ inflammation causes the elevation of the orofacial nocifensive behavior, and peripheral 5HT2A receptors play an important role in mediating the deep craniofacial tissue nociception in rats with TMJ inflammation.

Characteristics of rectal perception are altered in irritable bowel syndrome

A hallmark symptom of irritable bowel syndrome (IBS) is a lower pain threshold during rectal distension, but the mechanism underlying this disorder remains unclear. Examining the relationship between physiological and perceptual responses to rectal distension can provide insight into the underlying peripheral or central dysfunction in IBS. Therefore, we carried out a study of the rectal sensations of urge to defecate, pain and unpleasantness in relation to the varying states of the rectum. Ten IBS patients and 13 healthy controls underwent six sets of isobaric rectal distensions. The first set was ascending stepwise distensions terminating upon report of moderate pain where verbal ratings of urge, pain, and unpleasantness were acquired. The remaining sets were phasic or tonic distensions at a single pressure eliciting either moderate urge or moderate pain intensity where subjects gave continuous ratings of urge or pain percepts. The McGill Pain Questionnaire (MPQ) was used to assess different qualities of the pain experience during single pressure distensions. Abnormalities in IBS patients included: (1) higher incidence of distensions where unpleasantness is greater than pain intensity, (2) significantly longer persistence of ratings after stimulus termination during phasic distensions eliciting either moderate urge or moderate pain, (3) significantly smaller ratings fluctuations during tonic distensions, and (4) significantly higher MPQ scores for painful tonic distensions. Our study demonstrates that IBS patients have abnormal temporal and intensity properties of rectal sensation. These can be accounted for by either altered peripheral neuromuscular processing and/or processing of ascending rectal input in the central nervous system.

The role of parafunctions, emotions and stress in predicting facial pain

BACKGROUND

The authors conducted a study to examine the degree to which parafunctions and emotional states predicted jaw pain in subjects with temporomandibular disorder (TMD) and control subjects.

METHODS

Ninety-six subjects diagnosed with myofascial pain, myofascial pain and arthralgia, disk displacement or no TMD symptoms participated. The authors used experience sampling methodology to collect data on pain, behaviors and emotions. They paged subjects approximately every two hours, but not during sleep. When paged, subjects completed a brief questionnaire containing rating scales of jaw pain, masticatory muscle tension, time and intensity of tooth contact, mood and stress level.

RESULTS

Analyses of variance showed that groups differed significantly (P < .05) in terms of pain; masticatory muscle tension; and a composite variable measuring time and intensity of contact; mood; and stress. The two myofascial pain groups scored higher on these measures than did the group with disk displacement and the control group. The authors used masticatory muscle tension, the composite variable, mood and stress to predict jaw pain using linear regression. The model was significant and accounted for 69 percent of the variance in jaw pain. Because tension was so highly correlated with jaw pain, the authors removed this variable and re-ran the analysis. The second model also was significant and accounted for 46 percent of the variance in jaw pain.

CONCLUSIONS

Parafunctional behaviors, especially those that increase muscle tension, and emotional states are good predictors of jaw pain levels in patients with TMD and healthy control subjects.

CLINICAL IMPLICATIONS

Treatment that helps patients reduce parafunctions, excess masticatory muscle tension, stress and emotional distress should be effective in reducing TMD pain.

Psychiatric comorbidity in fibromyalgia

This review examines the current literature regarding psychiatric comorbidities associated with fibromyalgia. The aim of this review is to enhance understanding of psychiatric disorders that, alone or in combination with other physiologic (eg, neuroendocrine dysfunction) and psychosocial factors (eg, poor coping skills), may contribute to abnormal pain sensitivity and other illness behaviors of individuals with fibromyalgia. The review first identifies the psychiatric comorbidities that are associated most often with fibromyalgia and tend to aggregate within families of individuals with this disorder. It then examines the literature regarding the extent to which psychiatric illness, environmental stressors, or other psychosocial factors may contribute to the development of fibromyalgia. The review also presents recent findings concerning the extent to which psychosocial factors may contribute to treatment-related outcomes in pain and other health status variables among patients with fibromyalgia.

Sensory and motor effects of experimental muscle pain in patients with lateral epicondylalgia and controls with delayed onset muscle soreness

This study compares the effect of experimental muscle pain on deep tissue sensitivity and force attenuation in the wrist extensors of patients with lateral epicondylalgia (n=20), and healthy controls (n=20) with experimentally induced sensori-motor characteristics simulating lateral epicondylalgia. Delayed onset muscle soreness (DOMS) in wrist extensors of healthy controls was induced by eccentric exercise in one arm 24h prior to injection (Day 0). Saline-induced pain intensity (visual analogue scale, VAS), distribution, and quality were assessed quantitatively in both arms for both groups. Pressure pain thresholds (PPT) were assessed at three different sites in the wrist extensors. Maximal grip force and wrist extension force were recorded. In response to saline-induced pain in the extensor carpi radialis brevis, regardless of arm, the patient group demonstrated a significantly quicker pain onset (P<0.01), mapped larger pain areas and more referred pain areas, compared to healthy controls (P<0.03). Pain persisted significantly longer in the sore arm of the patient group, compared with all other arms (P<0.02). Patients demonstrated significant bilateral hyperalgesia at extensor carpi radialis brevis during and post saline-induced pain compared to pre-injection and healthy controls (P<0.04). The sore arm in patients and the DOMS arms in healthy subjects showed significantly reduced maximal force (P<0.0001), at all Day 1 times compared with the control arms. In patients, the bilateral increase in deep tissue sensitivity and enlarged referred pain areas during saline-induced pain might suggest involvement of central sensitisation.

Differential modulation of TMJ neurons in superficial laminae of trigeminal subnucleus caudalis/upper cervical cord junction region of male and cycling female rats by morphine

Sex differences in the cellular responses to morphine were examined in an animal model of temporomandibular joint (TMJ) pain. TMJ-responsive neurons were recorded in the superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C(2)) junction region, the initial site of synaptic integration for TMJ afferents, in male and cycling female rats under barbiturate anesthesia. Unit activity was evoked by local injection of bradykinin into the TMJ capsule at 30 min intervals and the effects of morphine sulfate (0.03-3 mg/kg, i.v.) were assessed by a cumulative dose regimen. Morphine caused a dose-related inhibition of bradykinin-evoked unit activity in males and diestrous females in a naloxone-reversible manner, while evoked unit activity in proestrous females was not reduced. The apparent sex hormone-related aspect of morphine analgesia was selective for evoked unit activity, since the spontaneous activity of TMJ units was reduced similarly in all groups, while the convergent cutaneous receptive field area of TMJ units did not change in any group. These results were consistent with the hypothesis that sex hormone status interacts with pain control systems to modify neural activity at the level of the Vc/C(2) junction region relevant for TMD pain.

Genetic basis for individual variations in pain perception and the development of a chronic pain condition

Pain sensitivity varies substantially among humans. A significant part of the human population develops chronic pain conditions that are characterized by heightened pain sensitivity. We identified three genetic variants (haplotypes) of the gene encoding catecholamine-O-methyltransferase (COMT) that we designated as low pain sensitivity (LPS), average pain sensitivity (APS) and high pain sensitivity (HPS). We show that these haplotypes encompass 96% of the human population, and five combinations of these haplotypes are strongly associated (P=0.0004) with variation in the sensitivity to experimental pain. The presence of even a single LPS haplotype diminishes, by as much as 2.3 times, the risk of developing myogenous temporomandibular joint disorder (TMD), a common musculoskeletal pain condition. The LPS haplotype produces much higher levels of COMT enzymatic activity when compared with the APS or HPS haplotypes. Inhibition of COMT in the rat results in a profound increase in pain sensitivity. Thus, COMT activity substantially influences pain sensitivity, and the three major haplotypes determine COMT activity in humans that inversely correlates with pain sensitivity and the risk of developing TMD.

The relationship of somatization and depression to experimental pain response in women with temporomandibular disorders

OBJECTIVE

Patients with temporomandibular pain disorders (TMD) have greater experimental pain perception when compared with pain-free controls. Common psychological features of TMD include somatization and depression. The impact of depression on experimental pain perception has received considerable attention. However, the role of somatization on experimental pain in a chronic pain population has not been explored.

METHODS

Fifty-six women with TMD and 59 pain-free controls underwent three experimental pain procedures, including palpation at fixed amounts of pressure, pressure pain thresholds, and an ischemic pain task. Levels of depression and somatization were assessed using the Research Diagnostic Criteria for TMD. Multiple regression analyses were performed to determine the extent to which depression and somatization were associated with experimental pain response.

RESULTS

After controlling for characteristic pain intensity and depression, somatization explained a significant proportion of variance in numbers of masticatory sites rated as painful (R2 change = 6.7%, p = .046) with the full model explaining 16.4% of the variance (p = .024). This did not meet an adjusted level of statistical significance (p = .008). After controlling for characteristic pain, only depression added significantly to the model predicting ischemic pain threshold and tolerance. The full models including characteristic pain and depression explained 49% and 20% of the variance in ischemic pain threshold and tolerance, respectively.

CONCLUSIONS

These findings suggest that depression and somatization are associated with different measures of experimental pain. Somatization may be related to more attentional and perceptual measures of clinically relevant pain while depression may be related to more behavioral measures of pain.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Sex differences in temporal summation of pain and aftersensations following repetitive noxious mechanical stimulation

Several studies demonstrate that women are more sensitive to experimental pain than men. In addition, women exhibit greater temporal summation of heat and mechanically evoked pain. Since temporal summation of pain is centrally mediated, its greater expression in women suggests a central nociceptive hyperexcitability relative to men. The purpose of this study was to pursue this theory, by further assessing sex differences in (1) temporal summation of mechanically evoked pain, and (2) aftersensations following repetitive noxious stimulation. Sixteen series of 10 repetitive, mildly noxious, mechanical stimuli were applied to the fingers of 25 women and 25 age-matched men. The subjects rated the pain intensity and unpleasantness caused by the first, fifth and tenth stimulus in the series, as well as their aftersensations 15 s and 1 min following the end of stimulation. Data were analyzed by three-way repeated-measures analysis of variance. Pain and unpleasantness ratings increased with repetition of stimulation (P<0.0001). Temporal summation of pain intensity and unpleasantness ratings were more pronounced in women than men (P<0.0001). In addition, significant temporal summation occurred only with 2 s interstimulus interval for men (P<0.0005) but with 2 and 5 s interstimulus interval for women (P<0.0001). Moreover, women provided greater ratings for the intensity and the unpleasantness of aftersensations (P<0.0005) and reported painful aftersensations at greater frequency (P<0.05) Greater temporal summation of pain and aftersensations in women suggests that their central processing of nociceptive input may be more easily upregulated into pathological hyperexcitability, possibly accounting for the higher prevalence of various chronic pain conditions among women.

Central representation of muscle pain and mechanical hyperesthesia in the orofacial region: a positron emission tomography study

Functional neuroimaging studies of the human brain have revealed a network of brain regions involved in the processing of nociceptive information. However, little is known of the cerebral processing of pain originating from muscles. The aim of this study was to investigate the cerebral activation pattern evoked by experimental jaw-muscle pain and its interference by simultaneous mechanical stimuli, which has been shown to evoke hyperesthesia. Ten healthy subjects participated in a PET study and jaw-muscle pain was induced by bolus injections of 5% hypertonic saline into the right masseter muscle. Repeated von Frey hair stimulation (0.5 Hz) of the skin above the masseter muscle was used as the mechanical stimulus. Hypertonic saline injections caused strong muscle pain spreading to adjacent areas. von Frey stimulation was rated as non-painful but produced hyperesthesia during jaw-muscle pain. Jaw-muscle pain was associated with significant increases in regional cerebral blood flow (rCBF) in the dorsal-posterior insula (bilaterally), anterior cingulate and prefrontal cortices, right posterior parietal cortex, brainstem, cavernous sinus and cerebellum. No rCBF changes occurred in primary or secondary somatosensory cortices. In contrast, von Frey stimulation produced a significant rCBF increase in the contralateral SI face representation. Mechanical hyperesthesia was associated with significant rCBF increases in the subgenual cingulate and the ventroposteromedial and dorsomedial thalamus. These results suggest that the cerebral processing of jaw-muscle pain may differ from the processing of cutaneous pain and that mechanical hyperesthesia, which often is encountered in clinical cases, has a unique representation in the brain.

Influences of gender role and anxiety on sex differences in temporal summation of pain

Previous research has consistently shown moderate to large differences between pain reports of men and women undergoing experimental pain testing. These differences have been shown for a variety of types of stimulation. However, only recently have sex differences been demonstrated for temporal summation of second pain. This study examined sex differences in response to temporal summation of second pain elicited by thermal stimulation of the skin. The relative influences of state anxiety and gender role expectations on temporal summation were investigated. Asymptomatic undergraduates (37 women and 30 men) underwent thermal testing of the thenar surface of the hand in a temporal summation protocol. Our results replicated those of Fillingim et al indicating that women showed increased temporal summation compared to men. We extended those findings to demonstrate that temporal summation is influenced by anxiety and gender role stereotypes about pain responding. When anxiety and gender role stereotypes are taken into account, sex is no longer a significant predictor of temporal summation. These findings highlight the contribution of social learning factors in the differences between sexes' pain perception.

PERSPECTIVE

Results of this study demonstrate that psychosocial variables influence pain mechanisms. Temporal summation was related to gender role expectations of pain and anxiety. These variables explain a significant portion of the differences between men and women's pain processing, and may be related to differences in clinical presentation.

Spatial summation of pressure pain: effect of body region

The characteristics of spatial summation of pressure pain are not clear. Pressure pain threshold (PPT) and perceived pressure pain intensity were measured in the hand, painfree back and myofascial trigger points (MTPs) in the back, using three different stimulus areas (0.5, 1 and 2 cm(2)). PPT decreased and perceived pain increased significantly with an increase in stimulation area in all the regions (e.g. PPT in the back, from 406+/-168 to 205+/-102kPa, P<0.0001). The magnitude of spatial summation of pressure pain was not significantly different between the regions. However, PPT in the back was significantly higher compared to the hand and MTPs (e.g. for 2 cm(2): mean of 205+/-102 vs 175+/-75 and 159+/-72kPa, P<0.01, respectively). Irrespective of body region, the quality of pain evoked with the large areas (1 and 2 cm(2)) was of pressure whereas in the small area (0.5 cm(2)) it was perceived as a prick. In conclusion, both PPT and perceived pressure pain intensity are subject to a considerable spatial summation in all the regions tested. The quality of pressure-evoked pain is probably determined by this spatial summation. Body region significantly affects the PPT level for a fixed stimulation area but not the magnitude of its spatial summation for areas up to 2 cm(2), which are probably within the receptive field of single spinal nociceptive neurons.

Innocuous jaw movements increase c-fos expression in trigeminal sensory nuclei produced by masseter muscle inflammation

Muscle tenderness and pain during movements are prominent symptoms associated with persistent jaw muscle pain. However, there is virtually no information on how trigeminal neurons respond to jaw movements (JM) or muscle palpation in the presence of muscle tissue injury or myositis. In this study, we investigated the effects of innocuous JM in the presence of acute masseteric inflammation on postsynaptic responses in the trigeminal brainstem nuclei by examining the expression of c-fos. In one group of rats, unilateral injections of an inflammatory substance, mustard oil (MO: 20%, 25 microl) were made into a masseter muscle. In another group, controlled and systematic JM were provided following MO injection. Three additional groups of rats were used to control for anesthetic, JM, and injection procedure. MO injected in the masseter muscle induced a high level of Fos protein expression in four principal trigeminal regions: the subnucleus caudalis (Vc), the ventral and dorsal regions of the Vc/Vi (subnucleus interpolaris) transition zone, and the paratrigeminal nucleus (PTN). Movements following MO injection consistently produced a significantly greater level of Fos expression in all these areas, especially in the Vc/Vi transition region and caudal Vc on the ipsilateral side. Importantly, movements also induced a significantly greater level of Fos expression in the caudal Vc on the contralateral side. The present results provide the first documentation that innocuous JM in the presence of muscle inflammation significantly increase the MO-induced c-fos expression in the trigeminal brainstem nuclei, which may explain the greater pain experienced during movement of inflamed or injured muscles.

Assessment of nociceptive trigeminal pathways by laser-evoked potentials and laser silent periods in patients with painful temporomandibular disorders

We assessed the trigeminal nociceptive pathways in patients with painful temporomandibular disorders (TMD) and control subjects using a CO(2)-laser stimulator which provides a predominant activation of the nociceptive system. Fifteen patients with unilateral pain were examined in accordance with the Research Diagnostic Criteria for TMD and 30 gender- and age-matched individuals were included as a control group. Laser-evoked potentials (LEPs) and laser silent periods (LSPs) after stimulation of the perioral region (V2/V3) on the painful and non-painful sides were recorded in all subjects. LEPs were evoked by low-intensity pulses (1.5 x perception threshold (PTh)) and recorded from scalp electrodes at the vertex. LSPs were evoked by high-intensity pulses (4 x PTh) and recorded bilaterally from masseter muscles with surface electromyogram (EMG) electrodes. Subjects also assessed the stimulus intensity on a 0-10 rating scale. LEPs had normal latency but smaller amplitude in TMD patients compared to the control group (P<0.001). Side-to-side comparison within patients showed that LEP amplitude was even more reduced after stimulation on the painful than the non-painful side (P<0.001). TMD patients showed a significant side-asymmetry of the pre-stimulus EMG activity, with a smaller value in the muscle on the painful side (P<0.001). LSPs were completely absent bilaterally in 12 TMD patients and unilaterally in two patients; only one patient had normal and bilateral LSPs. TMD patients perceived the laser stimulus less intense on the painful than the non-painful side (P<0.05). We found suppression of cortical responses and brainstem reflexes elicited by a predominantly nociceptive input in TMD patients. These findings are consistent with recent experimental pain studies and suggest that chronic craniofacial pain in TMD patients may be associated with a dysfunction of the trigeminal nociceptive system.

Response properties of TMJ units in superficial laminae at the spinomedullary junction of female rats vary over the estrous cycle

Neurons responsive to stimulation of the temporomandibular joint (TMJ) region were recorded from superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C(2)) junction region of cycling female rats under barbiturate anesthesia. To determine if receptive field (RF) properties or sensitivity to algesic chemicals of TMJ units vary over the estrous cycle, animals were selected from proestrous (high estrogen) or early diestrous (low estrogen) stages. More than 90% of TMJ units from each group received convergent nociceptive input [wide dynamic range (WDR) or nociceptive specific (NS)-like] from facial skin. The cutaneous high-threshold RF areas of WDR units from proestrous rats were 30% larger than diestrous units, while RF areas of NS units were similar. Bradykinin (BK, 0.1-10 microM) injection into the TMJ region excited a high percentage of units (>80% of total) from both groups in a dose-related manner. However, BK-evoked response magnitude (R(mag), +140%) and duration (+64%) were greater for proestrous than diestrous units. Both WDR and NS-like TMJ units of proestrous females displayed enhanced BK-evoked R(mag) values and response duration. Glutamate or mustard oil excitation of TMJ units was not affected by stage of the estrous cycle. Several TMJ units from proestrous and diestrous females were activated antidromically from the contralateral posterior thalamus, indicating that projection and nonprojection units were included in the sample population. These results were consistent with the hypothesis that factors related to stage of the estrous cycle modify the processing of deep craniofacial inputs by superficial dorsal horn neurons at the spinomedullary junction, a key region for the initial integration of sensory signals from the TMJ.

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.

Reactivity to superficial and deep stimuli in patients with chronic musculoskeletal pain

In this study, we evaluated pain sensitivity in patients with fibromyalgia or other types of chronic, diffuse musculoskeletal pain to establish whether fibromyalgia represents the end of a continuum of dysfunction in the nociceptive system. One hundred and forty five patients and 22 healthy subjects (HS) completed an epidemiological questionnaire to provide information about fatigue, stiffness, sleep, the intensity of pain (VAS 0-100) and its extent both at onset and at present. Algometry was performed at all American College of Rheumatology (ACR) tender points and at ten control points. Patients were divided into five main groups: fibromyalgia (FS) patients, secondary-concomitant fibromyalgia (SCFS) patients, patients with widespread pain (WP) but not reaching the ACR criterion of 11 tender points, patients with diffuse multiregional pain (MP) not reaching the ACR criteria (widespread pain, tender point counts), and patients with multiregional pain associated with at least 11 tender points (MPTE). von Frey monofilaments were used to assess superficial punctate pressure pain thresholds. Heat and cold pain thresholds were determined with a thermal stimulator. Ischemic pain was assessed by the cold pressure test and the submaximal effort tourniquet test. The scores for stiffness and present pain intensity gradually increased concomitantly with the increase in tender point count and pain extent. The pressure pain thresholds for positive tender and positive control points were significantly lower in the SCFS, FS and MPTE groups than in HS, MP and WP groups, the latter three groups displaying similar values. In all groups, there were no differences in pain thresholds between positive tender and positive control points. The heat pain threshold and the pain threshold in the cold pressure test were lower in the FS and SCFS groups than in HS. The cold pressure tolerance was lower in patients with widespread pain than in HS. In the von Frey test, all patient groups except MP had similar values, which were significantly lower than in HS. Finally, all patient groups displayed lower tourniquet tolerance than HS. In each psychophysical test, patients with widespread pain and patients with multiregional pain showed similar thresholds; however, the thresholds in the MP or MPTE groups differed from those in the FS and SCFS groups. In the FS group, pain thresholds and pain tolerance did not differ according to the presence of ongoing pain at the stimulated site and were not correlated to ongoing pain. The results indicate that dysfunction in the nociceptive system is already present in patients with multiregional pain with a low tender point count; it becomes more and more severe as the positive tender point count and pain extent increase and it is maximal in fibromyalgia patients.