Sensitivity of patients with painful temporomandibular disorders to experimentally evoked 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.

The influence of pain intensity on somatosensory perception in patients suffering from subacute/chronic lateral epicondylalgia

A confounding factor in the analysis of chronic pain patients is the finding of signs of somatosensory disturbances not only in neuropathic pain patients but also in a subgroup of patients with musculoskeletal pain. The purpose was to investigate if patients suffering from subacute/chronic lateral epicondylalgia demonstrated altered sensibility, and if this was affected by pain intensity. At the start of the experiment, quantitative sensory testing (QST) (thermal, pressure pain, touch) was performed in the local pain area and in the area of pain referral. QST was repeated following pain provocation (weight lifting). A local anaesthetic was then injected into the lateral epicondyle and QST was repeated in the area of pain referral. The contralateral arm was assessed, treated and injected in the same way. At the baseline assessment there was no difference in sensibility between sides, with the exception of a significantly lowered threshold to noxious heat (p<0.04) in the area of pain referral, present during the whole experiment. In the affected arm only, weight lifting resulted in significantly increased pain intensity in the local (p<0.01) and referred (p<0.01) pain areas, respectively. Repeated muscle contractions resulted in altered somatosensory functions in both the affected arm and the unaffected arm, consequently not dependent on ongoing pain in the assessed area. Tactile perception thresholds increased significantly following pain provocation in the area of pain referral (p<0.04) only and normalized following injection of local anaesthetic (p<0.02), indicating that the sensitivity to light touch was altered by the nociceptive input from the affected arm.

Morphine and NMDA receptor antagonism reduce c-fos expression in spinal trigeminal nucleus produced by acute injury to the TMJ region

Pain management in temporomandibular disorders (TMDs) often involves pharmacotherapy; however, the site of action for drugs that reduce TMD pain is not known. To determine possible central neural targets of analgesic drugs relevant in TMD pain, morphine or the N-methyl-D-aspartate receptor antagonist, MK-801, was given alone or in combination prior to TMJ injury. The number of neurons expressing the immediate early gene, c-fos, was quantified in the lower brainstem and upper cervical spinal cord as an index of neural activation. It was hypothesized that those neuronal groups most necessary for the sensory-discriminative aspects of acute TMJ injury should display the greatest reduction in c-fos expression after drug treatment. Barbiturate-anesthetized male rats were given morphine or MK-801 15 min prior to injection of mustard oil into the TMJ region. Morphine given centrally (i.c.v.) or peripherally (i.v.) caused a marked dose-related reduction in Fos-like immunoreactivity (Fos-LI) in laminae I-II at the middle portions of subnucleus caudalis (mid-Vc) and at the subnucleus caudalis/upper cervical spinal cord (Vc/C2) transition. Higher doses of morphine also reduced Fos-LI in the dorsal paratrigeminal region (dPa5) and at the subnucleus interpolaris/subnucleus caudalis (Vi/Vc-vl) transition. MK-801 given i.v. reduced Fos-LI only in laminae I-II at the Vc/C2 transition. Combined subthreshold doses of morphine and MK-801 reduced c-fos expression in the dPa5, mid-Vc, and the Vc/C2 transition region, below that predicted from the effects of either drug alone. These results suggest that neurons in laminae I-II of the mid-Vc and Vc/C2 transition and, to a lesser extent, in the dPa5 region play a critical role in mediating the sensory and/or reflex aspects of pain after acute injury to the TMJ region.

Dynamic assessment of abnormalities in central pain transmission and modulation in tension-type headache sufferers

OBJECTIVE

To examine and compare central pain processing and modulation in young tension-type headache sufferers with that of matched healthy controls using an induced headache "challenge" paradigm.

BACKGROUND

Recent research has suggested that abnormalities in central pain processing and descending pain modulation may contribute to chronic tension-type headache. These abnormalities, if they contribute to headache pathogenesis, should be present in young adult tension-type headache sufferers. Recent research using static measures of physiological variables, such as muscle tenderness and exteroceptive suppression, has identified chronic muscle tenderness as a characteristic of young tension-type headache sufferers, but other central nervous system functional abnormalities may require a dynamic "challenge" to be observed.

METHODS

Twenty-four young women meeting the International Headache Society diagnostic criteria for tension-type headache (headache-prone) and a matched group of 24 healthy women who reported fewer than 10 problem headaches per year (control) participated in a double-blind, placebo-controlled, crossover study. Subjects completed jaw clenching and a placebo condition on different days in counterbalanced order. Pericranial muscle tenderness, pressure-pain thresholds on the temporalis, and exteroceptive suppression periods were assessed before and after each procedure. Head pain was recorded for 12 to16 hours following each condition.

RESULTS

Headache-prone subjects were more likely than controls to experience headaches after both the jaw clenching and placebo procedures, but neither group was significantly more likely to experience headaches following jaw clenching than placebo. In pretreatment measurements, headache-prone subjects exhibited greater muscle tenderness than controls, but pressure-pain detection thresholds and exteroceptive suppression periods did not differ in the two groups. Control subjects showed increases in muscle tenderness and exteroceptive suppression periods following both the clenching and placebo procedures, whereas headache-prone subjects exhibited no significant changes in any of the physiological measures following either experimental manipulation.

CONCLUSIONS

These results confirm previous findings indicating abnormally high pericranial muscle tenderness in young tension headache sufferers even in the headache-free state. In addition, the results suggest that the development of headaches following noxious stimulation is more strongly related to headache proneness and associated abnormalities in central pain transmission or modulation (indexed by pericranial muscle tenderness and exteroceptive suppression responses) than muscle strain induced by jaw clenching.

Theoretical review: altered pain regulatory systems in chronic pain

This review synthesizes the existing literature regarding the relationship between resting blood pressure and pain sensitivity, and the literature indicating possible endogenous opioid dysfunction in chronic pain. Adaptive interactions between the cardiovascular and pain regulatory systems occur in healthy individuals, with greater blood pressure associated with decreased acute pain sensitivity. Endogenous opioids appear necessary for full expression of this relationship. There is ample evidence indicating diminished endogenous opioid CSF/plasma levels in chronic pain patients, yet little is known about the functional effects of these opioid changes. A theoretical model is proposed based upon the literature reviewed suggesting progressive dysfunction in endogenous opioid systems with increasing chronic pain duration. This dysfunction is hypothesized to result in dysregulation of normally adaptive relationships between the cardiovascular and pain regulatory systems, resulting in increased chronic pain intensity and increased acute pain sensitivity among chronic pain patients. Preliminary data are consistent with the hypothesis of progressive opioid changes resulting in dysfunctional alterations in the adaptive blood pressure-pain relationship. Clinical implications of this theory are discussed.

The relationship of sex and clinical pain to experimental pain responses

Considerable research indicates increased experience of clinical pain among females relative to males, and females also demonstrate enhanced responses to experimentally-induced pain. However, previous research has not investigated the relationship between clinical and experimental pain responses in healthy females and males. This experiment examined recent clinical pain as well as thermal pain thresholds and tolerances in 209 (117 female, 92 male) healthy young adults. All subjects completed questionnaires concerning pain-related symptoms over the previous month and subsequently underwent thermal pain assessment. Females reported a larger number of pain sites and greater health care utilization over the month preceding the experimental session, and females also exhibited greater sensitivity to thermal stimuli. In addition, females above the median on the number of pain episodes demonstrated greater thermal pain sensitivity compared to females below the median, but thermal pain responses did not differ as a function of clinical pain among males. The differences remained significant after correcting for psychological variables including hypervigilance and sex role expectancies. These results indicate that experimental pain responses may be more clinically relevant for females than males. Potential explanations and implications for this pattern of results are discussed.

Increased pain sensitivity of the upper extremities of TMD patients with myalgia to experimentally-evoked noxious stimulation: possibility of worsened endogenous opioid systems

The purpose of this study was to examine whether or not temporomandibular disorder (TMD) patients with chronic masticatory myalgia have increased pain sensitivity at remote sites outside of the head and neck region, and to evaluate whether the endogenous pain inhibitory systems triggered ischemic pain functions favorably in those patients. Twenty female TMD patients with chronic myalgia and 20 controls participated in this study. Ischemic pain was produced to activate endogenous opioids. The pain threshold time, pain tolerance time, pain intensity and pain unpleasantness were compared between the TMD patients and controls. The pressure pain thresholds in the hand were also compared before, between, and immediately after the ischemic pain. The TMD patients showed higher severe pain intensity and unpleasantness values and had lower pressure pain thresholds in the hand. Although both groups showed an increase in the pressure pain threshold, there was less of an increase in the pressure pain threshold in the TMD patients than in the controls. These findings indicate that TMD patients have increased pain sensitivity at remote sites, and also indicate additional evidence that the endogenous opioid systems may become impaired in TMD patients with chronic masticatory myalgia.

Temporomandibular disorders: a review of current understanding

OBJECTIVE

The purpose of this article is to conduct a narrative review of current evidence regarding the understanding, evaluation, management, and treatment of temporomandibular disorders to provide a broad perspective and updated introduction to an important and controversial subject with rapidly changing developments and limited well-designed research.

DATA SOURCES

Studies were identified through a search of MEDLINE for 3 topics (temporomandibular disorder, temporomandibular joint, and chronic pain) over a 10-year period (January 1988 to August 1998) and of bibliographies of identified studies and review articles.

STUDY SELECTION

More than 5000 articles were produced. In-depth review of all of this literature was beyond the scope of the present article, which is intended to provide an overview. The amount and diversity of the literature and the limitations of covering such a broad topic being recognized, the papers selected were those that reviewed limited topics or studied focused areas. This report is not a systematic (qualitative) or meta-analysis (quantitative) review. An acknowledged limitation of this narrative review method lies in the potential for bias in selection. The referenced works do not include all papers reviewed; only pertinent literature and reviews with comprehensive references were selectively included.

CONCLUSIONS

Advances in basic and clinical science have resulted in important changes in the understanding and management of temporomandibular disorders. Many treatments are not supported by research, and the role of dentistry is changing to a more diagnostic and management-based model from the hands-on treatment procedures of the past. The present science-based understand-ing of a biopsychosocial disorder is important in properly and responsibly dealing with patients with temporomandibular disorders.

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.

The relationship between chronic facial pain and a history of trauma and surgery

OBJECTIVE

Because pain is the most commonly reported symptom of patients presenting to temporomandibular disorders clinics, it is important to identify factors that modify the perception or reality of such pain. The purpose of this study was to investigate the hypothesis that a patient with a history of trauma and/or non-temporomandibular joint surgery might be sensitized to pain and might report increased pain levels if a temporomandibular disorder later developed.

STUDY DESIGN

This was a retrospective study of 778 consecutive patients seen over a 1-year period in an Orofacial Pain Clinic. Study parameters included gender, lifetime number of self-reported traumas, lifetime number of non-temporomandibular joint operations, and location, intensity, frequency, and type of temporomandibular disorder-related pain.

RESULTS

There were significantly more women than men in the study (609 to 169). There was no relationship between numbers of previous traumas and non-temporomandibular joint operations and types of temporomandibular disorder. However, there were statistically significant relationships between the severity of facial pain and the frequency of facial pain as well as between the severity and frequency of joint pain and the number of traumas. There were also statistically significant associations between the severity and frequency of facial pain and the number of non-temporomandibular joint-related surgical procedures that the patient had undergone.

CONCLUSIONS

There is a relationship between a patient's reported history of trauma and/or non-temporomandibular joint-related operations and the severity and frequency of facial and temporomandibular joint pain, should it develop. It is possible that such traumas and operations sensitize the patient in such a way that the pain of subsequent temporomandibular joint disorders is heightened.

Psychological and physiological parameters of masticatory muscle pain

The objective of this research was to identify the psychological and physiological variables that differentiate persons reporting masticatory muscle pain (MMP) from normal controls (NC). This study examined the characteristics of 35 MMP patients in comparison to 35 age-, sex-, and weight-matched NCs. All subjects completed a series of standardized questionnaires prior to undergoing a laboratory evaluation consisting of a psychosocial stressor and pressure pain stimulation at multiple body sites. During the evaluation, subjects' emotional and physiological responses (heart rate, blood pressure, respiration, skin temperature, and muscle activity) were monitored. Results indicated that persons with MMP reported greater fatigue, disturbed sleep, depression, anxiety, menstrual symptoms, and less self-deception (P's < 0.05) than matched controls. At rest, MMPs had lower end tidal carbon dioxide levels (P < 0.04) and lower diastolic blood pressures than the NCs (P < 0.02). During laboratory challenge, both groups responded to the standard stressor with significant physiological activity and emotional responding consistent with an acute stress response (P < 0.01), but there were no differences between the MMPs and NCs. Muscle pain patients reported lower pressure pain thresholds than did NCs at the right/left masseter and right temporalis sites (P's < 0.05); there were no differences in pressure pain thresholds between MMPs and NCs for the left temporalis (P < 0.07) and right/left middle finger sites (P's > 0.93). These results are discussed in terms of the psychological and physiological processes that may account for the development of muscle pain in the masticatory system.

Experimentally induced muscle pain induces hypoalgesia in heterotopic deep tissues, but not in homotopic deep tissues

The ability of muscle pain to generate somatosensory sensibility changes is controversial. Thus, in the present study, tonic infusion of hypertonic saline (5%, 7.1 ml administered over 15 min) into the tibialis anterior (TA) muscle was used as an experimental model to induce local and referred pain. The sensibility to high-intensity pressure stimuli applied to the local pain area, referred pain area and an arm was assessed in 14 healthy volunteers. Infusion of isotonic (0.9%) saline into the other leg served as control. The subject continuously scored the pain intensity on an electronic visual analogue scale (VAS). Pressure pain threshold (PPT) was determined on the TA muscle (2 cm and 10 cm from the infusion site), at the frontal aspect of the ankle (area of referred pain) and on the arm. To minimise the skin component of the PPT, the skin covering the assessment sites was anaesthetised with an anaesthetic creme. The PPTs were obtained before and after cutaneous analgesia, 1 min and 10 min after infusion start and 10 min after the pain had disappeared. Infusion of hypertonic saline caused significantly (P<0. 05) higher VAS scores than infusion of isotonic saline. A significant (P<0.04) increase of the PPT (i.e., decreased sensibility) was found at the ankle and on the arm during muscle pain compared to the control condition. No significant differences in PPTs on the TA muscle were found during saline-induced muscle pain compared to the infusion of isotonic saline. The decrease in deep sensibility at the heterotopic sites (referred pain area and arm), but not at homotopic sites (TA muscle), probably reflected the phenomenon of diffuse noxious inhibitory control (DNIC). The inhibitory mechanism during muscle pain was shown to be effective for the deep tissue sensibility in healthy subjects. Thus, a pathologically disturbed inhibitory mechanism may result in widespread deep hyperalgesia in muscle pain patients.

Sex differences in temporal summation but not sensory-discriminative processing of thermal pain

Gender differences in experimental pain sensitivity have been widely investigated, and the results generally indicate that females exhibit greater sensitivity to noxious stimuli than males. However, results using thermal pain procedures have been inconsistent, with some studies reporting greater responses among females and other studies reporting no gender differences. The present study investigated gender differences in thermal pain perception using several different psychophysical procedures. Twenty-seven females and 22 males underwent thermal testing, including: determination of thermal pain threshold and tolerance, a thermal discrimination procedure, real-time magnitude estimates of heat pulses, and temporal summation of thermal pain. The results indicated lower thermal pain threshold and tolerance and greater temporal summation of thermal pain among females, but no gender differences in thermal discrimination or real-time magnitude estimates of discrete heat pulses. These findings suggest that gender differences in thermal pain perception may be more robust for sustained, temporally dynamic thermal stimuli with a strong C-fiber component.

Mechanical hyperesthesia of human facial skin induced by tonic painful stimulation of jaw muscles

The function of the somatosensory system in patients with painful temporomandibular disorders is still a matter of discussion. We wished to determine cutaneous sensitivity to innocuous mechanical stimuli in the orofacial region before, during (3 and 12 min) and after standardized experimental jaw-muscle pain. Twelve healthy subjects were exposed to tonic infusion of hypertonic (5%) and isotonic (0.9%) saline into the masseter muscle. All subjects experienced moderate pain with hypertonic saline, and the area of self-reported pain increased significantly from 3 min after infusion start to 12 min after infusion start (mean +/- SEM: 115+/-49%; P < 0.05). The psychophysical ratings of punctate von Frey hair stimulation were significantly increased 12 min after start of hypertonic saline infusion as compared to baseline and post-baseline ratings at the site of infusion (50+/-10%; P < 0.05) and at two adjacent facial sites (18+/-7%, 37+/-9%; P < 0.05). In contrast, isotonic saline infusion was associated with a significant decrease in ratings at post-baseline as compared to baseline ratings. The psychophysical ratings of a stroking cotton swab stimulation were not significantly affected by infusion of saline. These results in a human model of jaw-muscle pain are comparable to animal studies demonstrating increased size of cutaneous receptive fields and increased responsiveness of brain stem neurons to cutaneous mechanical stimuli. Similar hyperexcitability changes may be part of the pathophysiological mechanisms involved in painful temporomandibular disorders.

Excitability of the central masticatory pathways in patients with painful temporomandibular disorders

Much is unclear about the pathophysiological mechanisms underlying painful temporomandibular disorders. In addition to various other theories, masticatory muscle dysfunction and pain have also been attributed to primary central nervous system hyperactivity. We assessed this possibility in a study using recent neurophysiological techniques. From among outpatients whose diagnosis of temporomandibular disorders had been obtained in stomatognathic facilities, we studied 10 patients with bilateral pain and 15 patients with unilateral pain, in whom electromyographic examination of the trigeminal reflexes disclosed normal findings except for absence or amplitude asymmetry of the jaw jerk. Transcranial magnetic stimulation yielded masseter motor evoked potentials of normal latency and amplitude, but five patients had to exert a near-maximum contraction to obtain their responses. The masseter silent periods elicited by the double-shock technique recovered normally. Because these tests measure the excitability of the masticatory system (including motor cortex, corticobulbar and corticoreticular connections, reticular interneurones and lower motoneurones), the lack of facilitation in these patients' responses excluded central hyperactivity as the primary cause of their masticatory dysfunction and pain.

Ischemic but not thermal pain sensitivity varies across the menstrual cycle

OBJECTIVE AND METHOD

Findings from both animal and human research suggest that pain sensitivity changes across the menstrual cycle; however, among humans the nature of these menstrual cycle effects remains unclear. The present study used a repeated-measures design to evaluate changes in thermal and ischemic pain responses during three phases of the menstrual cycle, midfollicular (postmenstrual), ovulatory, and mid-to-late luteal (premenstrual), in 11 healthy women. The cycle phase during which subjects began their participation was determined randomly. Plasma levels of estrogen, progesterone, luteinizing hormone (LH), testosterone, and beta-endorphin were determined at each experimental session. Participants also completed a daily diary of physical and emotional symptoms for two complete menstrual cycles before the experimental sessions.

RESULTS

The results indicated that women showed less ischemic pain sensitivity during the midfollicular compared with the ovulatory and mid-to-late luteal phases, but thermal pain responses did not vary significantly across menstrual cycle phases. Physical and emotional symptoms were minimal and did not change significantly across the menstrual cycle.

CONCLUSIONS

These findings indicate greater ischemic but not thermal pain sensitivity among women after the midcycle LH surge. The practical relevance and potential mechanisms of these findings are discussed.

Relationship between pain sensitivity and resting arterial blood pressure in patients with painful temporomandibular disorders

OBJECTIVE

Patients experiencing temporomandibular disorders (TMD) show greater sensitivity to painful stimuli than age- and gender-matched control subjects. This enhanced pain sensitivity may result, at least in part, from an alteration in pain regulatory systems that are influenced by resting arterial blood pressure. In this study, we examined the relationship between resting systolic blood pressure and pain perception in 64 female TMD and 23 age-matched pain-free female subjects.

METHOD

Resting arterial blood pressure and measures of thermal and ischemic pain threshold and tolerance were determined for each participant. Subjective ratings of thermal pain evoked by suprathreshold noxious thermal stimuli (45-49 degrees C) using a magnitude matching procedure were also obtained for both groups.

RESULTS

TMD patients had lower thermal and ischemic pain thresholds and tolerances than pain-free subjects (ps < .05). Both groups provided equivalent intensity ratings to suprathreshold noxious thermal stimuli. A median split of each group based on resting systolic blood pressure revealed an influence of blood pressure on both thermal and ischemic pain perception for the Pain-Free group. The Pain-Free high resting blood pressure subgroup had higher thermal pain tolerances, higher ischemic pain thresholds, and provided lower magnitude estimates of the intensity of graded heat pulses compared with the Pain-Free low blood pressure subgroup. A trend toward a significant effect of blood pressure level on ischemic pain tolerance was also observed for the Pain-Free group. In contrast to the Pain-Free group, blood pressure level did not influence ischemic or thermal pain perception for TMD patients. Similar to the lack of effect of resting blood pressure on experimental pain perception in TMD patients, resting blood pressure was not related to measures of clinical orofacial pain in TMD patients.

CONCLUSIONS

These findings confirm our previous findings that TMD patients are more sensitive to noxious stimuli and suggest that painful TMD may result, at least in part, from an impairment in central pain regulatory systems that are influenced by resting arterial blood pressure.

Pain sensitivity in patients with temporomandibular disorders: relationship to clinical and psychosocial factors

OBJECTIVE

We have previously reported that patients with temporomandibular disorders (TMD) exhibit enhanced sensitivity to experimentally evoked pain (1); however, the clinical relevance of this increased pain sensitivity remains unclear. The purpose of this study was to investigate the relationship of experimental pain sensitivity to clinical and psychosocial variables among patients with TMD.

DESIGN

Thirty-six TMD patients were studied, half of whom were pain sensitive (PS) and the other half pain tolerant (PT), based on their ability to tolerate an ischemic pain task. Responses to painful thermal and nonpainful visual stimuli as well as clinical/diagnostic symptoms and psychosocial variables were compared for the two groups (i.e., PS vs. PT).

RESULTS

Results indicated that, compared with PT patients, the PS group exhibited greater sensitivity to thermal pain and rated innocuous visual stimuli as more intense. PS patients also reported greater clinical pain, but in general the groups did not differ on diagnostic and psychosocial measures.

CONCLUSIONS

The results suggest that ischemic pain tolerance is a clinically relevant marker of pain sensitivity in TMD patients. These findings are consistent with the hypothesis that impairments in CNS inhibitory pathways may contribute to the pain associated with TMD.

Vibrotactile threshold is elevated in temporomandibular disorders

Experimental pain can elevate vibrotactile threshold, a phenomenon attributed in the literature to the operation of a 'touch gate.' It is not known, however, whether clinical pain produces similar effects. To explore this possibility, we measured vibrotactile threshold in patients with temporomandibular disorders (TMD) whose pain had a prominent myalgic component. Two-interval forced-choice tracking was used to determine threshold for a 25-Hz vibratory stimulus presented on the cheek. Threshold was found to be significantly elevated in the TMD group, compared to an age- and gender-matched control group of pain-free individuals. Within the TMD group, those with a supra-median level of muscle tenderness (corrected for background levels of spontaneous pain) had significantly higher threshold than those with lower levels of palpation pain. These findings are consistent with the idea of a touch gate, and suggest the usefulness of further research in this area with clinical pain populations. The effects of an adapting stimulus (25 Hz, 20 dB SL) were also studied, and found to produce parallel elevations in vibrotactile threshold in the TMD and pain-free groups. This result indicates that at least some adaptation occurs at a higher (subsequent) level of somatosensory information processing than does the touch gating implied by the unadapted thresholds.