Glutamate-evoked jaw muscle pain as a model of persistent myofascial TMD pain?

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Abstract

Objective

Compare pain-related measures and psychosocial variables between glutamate-evoked jaw muscle pain in healthy subjects (HS) and patients with persistent myofascial temporomandibular disorder (TMD) pain.

Design

Forty-seven female HS and 10 female patients with persistent myofascial TMD pain participated. The HS received an injection of glutamate into the masseter muscle to model persistent myofascial TMD pain. Participants filled out a coping strategies questionnaire (CSQ), the symptom checklist 90 (SCL-90) and McGill pain questionnaire (MPQ). Pain intensity was assessed on an electronic visual analogue scale (VAS). Pain-drawing areas, numerical rating scale (NRS) scores of unpleasantness, pressure pain thresholds (PPTs) and pressure pain tolerance (PPTOL) were measured. Unpaired t-tests and correlation tests were used for analyses.

Results

The groups were significantly different when comparing the CSQ scores of control, decrease, diverting attention, increase of behavioural activities and somatization. The peak VAS pain, NRS of unpleasantness and MPQ scores were not significantly different between groups, but PPT and PPTOL were significantly lower in the TMD patients. Significant positive correlations were found in the TMD patients between peak VAS pain and CSQ catastrophizing score and SCL-90 somatization. The scores of PPTs and PPTOLs, in patients showed positive correlations with CSQ reinterpreting pain sensations scores and PPTs correlated with CSQ praying/hoping scores.

Conclusions

Glutamate-evoked pain responses in HS and persistent myofascial TMD pain have similar sensory-discriminative and affective-unpleasantness components but differ in psychosocial features. This study suggests that experimental designs based on glutamate injection into muscle can provide an appropriate model for elucidating persistent myofascial pain conditions.

Introduction

The etiology and pathogenesis of persistent myofascial temporomandibular disorders (TMDs), characterized by symptoms of localized ongoing and activity-provoked masticatory muscle pain, remains unclear.1, 2, 3, 4 Previous studies have shown that experimentally increased levels of glutamate in the human masseter muscle produced pain, decreased pressure pain thresholds (PPTs) and increased the amplitude of the jaw-stretch reflex which suggest that injection of glutamate into the masseter muscle can produce muscle pain and changes in physiological response similar to those reported for myofascial TMD patients.5, 6, 7, 8, 9, 10

Nevertheless, it has to be considered that although human experimental pain models applied to the orofacial area can provide valuable and clinically relevant information, the characteristics of the persistent (months or years) pain experienced by most myofascial TMD patients is more complex than acute (minutes or hours) experimental pain in healthy subjects, for example socio-economic and behavioural factors may differ substantially. Such differences could be related to the natural fluctuation of pain, the chronicity of the pain, psychosocial distress, functional disabilities, and concomitant pain conditions that may all influence the expression of pain.11 Pain is, indeed, influenced by a multitude of factors including psychological factors that have been shown to be important determinants of pain experience.12, 13 Thus, it has been reported that patients suffering from myofascial pain dysfunction or atypical facial pain are more likely to show elevations in psychometric scales for hypochondriasis and depression.13 It has also been shown that psychosocial variables, such as coping strategies, may have implications for the underlying physiology of pain, e.g. to predict important clinical outcomes, including pain severity and disability.14 For example, catastrophizing, which has been defined as a negative emotional and cognitive response to pain involving elements of magnification, helplessness and pessimism,15 is considered as the most robust predictor of pain outcomes.16, 17, 18 Thus, catastrophizers may have a tendency to magnify or exaggerate the threat value or seriousness of the pain sensation16 and catastrophizing has been significantly associated with patient-reported pain-related activity, interference, and depression.19 Furthermore, it has been suggested 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 behavioural measures of pain.20 All these factors suggest awareness and caution when comparing results in experiments involving persistent myofascial TMD pain patients with those obtained from human experimental pain models.

Based on the available literature we decided to complement our previous results10 with a post hoc analysis in order to compare different pain-related measures and psychosocial variables between glutamate-evoked jaw muscle pain in healthy subjects and patients with persistent myofascial TMD pain.

Section snippets

Materials and methods

To perform this post hoc analysis, a portion of the data obtained from two previous studies was used.10, 11

Coping strategies questionnaire (CSQ)

Prior to the experiment, all participants filled out a Danish version of CSQ.23 From the CSQ nine scores were obtained: pain control ability, pain decrease ability, diverting attention, reinterpreting pain sensations, coping safe statements, ignoring sensations, praying and hoping, increase behavioural activities, and catastrophizing.

Symptom checklist 90 (SCL-90)

Before starting the experiment, the volunteers filled out a Danish version of the RDC/TMD questionnaire.21 One of the parts of this instrument is the Axis II that

Baseline clinical characteristics

The baseline clinical characteristics of the two groups are shown in Table 1. The TMD patients showed significantly lower range of jaw movements as well as more painful myofascial or TMJ sites on palpation. Another significant difference was that the age of the TMD pain patients was slightly older than the healthy controls.

CSQ

There were significant differences between healthy controls and myofascial TMD pain patients. The healthy controls had higher scores in sections that assessed ability to

Discussion

The present study has shown that glutamate-evoked jaw muscle pain in healthy subjects shares many of the characteristics of persistent pain in the TMD patients (Table 3; Fig. 2) but that in particular the psychosocial scores differ between the two groups (Table 2). Furthermore, significant associations between measures of pain intensity and psychosocial scores were noted predominately in the TMD pain patients. The findings are discussed from the perspective that experimental myofascial pain

Conclusions

Healthy subjects exposed to glutamate-evoked jaw muscle pain and persistent myofascial TMD pain patients have many similarities in terms of the sensory-discriminative and affective-unpleasantness component of pain but clearly differ in terms of psychosocial measures such as e.g. catastrophizing and somatization. This study supports the view that experimental myofascial pain can appropriately model certain characteristics of persistent myofascial pain conditions.

Acknowledgments

This research was supported by the National Institutes of Health Grant DE 15420. The skillful help of Bente Haugsted is greatly appreciated.

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