Serotonin, glutamate and glycerol are released after the injection of hypertonic saline into human masseter muscles - a microdialysis study

Does botulinum toxin have any role in the management of chronic pain associated with bruxism?

OBJECTIVE

To determine the efficacy of injectable botulinum toxin (BTA) in the management of pain associated with nocturnal bruxism.

METHODS

The study subjects consisted of 40 male and female volunteers who complained of chronic pain in the masseter muscles bilaterally due to bruxism. Twenty units of BTA were injected bilaterally into the masseter muscles. Clinical parameters of pain at rest and chewing were assessed at baseline, 2, 4, 8, 12, 16, 18, and 24 weeks using the visual pain scale (VPS).

RESULTS

The study included 24 females and 16 males aged 21 to 52 years (mean 33.9 ± 31.0). The mean VPS score on the first day was 5.75 (SD 1.9), significantly decreasing after two weeks to 0.44 (SD 0.727). The mean difference of VPS from 8 weeks up to 24 weeks gradually increase from 0.69 at 8 weeks to 2.00 at 24 weeks.

CONCLUSION

This study provides evidence that BTA could reduce the pain of nocturnal bruxism in affected patients.

Myofascial Temporomandibular Disorders at a Turning Point: Pragmatic or Evidence-Based Management?

Unvalidated theories have been proposed for the etiopathogenesis of masticatory myofascial temporomandibular disorders (mTMD). Modalities such as cone-beam computed tomography/computed tomography and MRI contributes little to the diagnosis of mTMD. Diagnosing mTMD is based on the recognition of "familiar pain" presentation in the masticatory myofascial tissue. This assessment tool contributes little our understanding of the underlying disease process. Thus, management of mTMD is empirical and arbitrary. Exploring emerging technologies to identify biomarkers and objectively assess myofascial tissue physiology in disease and health may be key in moving the diagnosis of mTMD from the pragmatic paradigm to an evidence-based paradigm.

Effectiveness of Botulinum Toxin Injection on Bruxism: A Systematic Review and Meta-analysis of Randomized Controlled Trials

OBJECTIVE

The present study compiled evidence on the efficacy of botulinum toxin A (BTX) for management of bruxism.

METHODS

A literature review that included randomized control, cohort, as well as observational studies published between January 2000 and November 2022 was conducted. All studies related to BTX injections administered into the masseters of patients with bruxism were included. Primary outcomes were measured by performing a meta-analysis of changes in maximal biting forces and pain severity and meta-regression of the effects of the BTX dose.

RESULTS

Ten studies were included for quantitative analysis. The analysis of the maximal biting force after BTX injections demonstrated a significant reduction at 1 month or less compared with both oral splints (P < 0.000001) and saline injections (P = 0.01). BTX continued to outperform oral splinting (P = 0.001) and saline placebos (P = 0.03) at 3 months. Between 3 and 6 months, a significantly higher maximal biting strength was observed in the BTX group than the oral splinting group (P < 0.00001). No significant differences in the maximal biting force were observed between the BTX and saline placebo groups (P = 0.50). A similar trend was observed in the analysis of pain reduction after botulinum treatment. Additionally, for every unit increase in the BTX dose, pain severity decreased by 0.0831 points (P = 0.0011).

CONCLUSION

BTX is effective in reducing biting strength and pain severity. BTX effects are evident at less than 4 weeks, peak between 5 and 8 weeks, and last for up to 24 weeks. Higher BTX doses result in greater improvement in pain. Although BTX benefits manifest earlier, they gradually diminish, and oral splinting exerts a more enduring effect, especially after 9-12 weeks. BTX injections into masseters are recommended as management options for bruxers, especially for those having difficulties complying with wearing oral splints or those seeking earlier symptom relief. However, future studies should determine BTX effects beyond 24 weeks and after repetitive injections and how bruxers of different ages or genders respond to treatment.

LEVEL OF EVIDENCE III

This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Preclinical models of deep craniofacial nociception and temporomandibular disorder pain

Chronic pain in temporomandibular disorder (TMD) is a common health problem. Cumulating evidence indicates that the etiology of TMD pain is complex with multifactorial experience that could hamper the developments of treatments. Preclinical research is a resource to understand the mechanism for TMD pain, whereas limitations are present as a disease-specific model. It is difficult to incorporate multiple risk factors associated with the etiology that could increase pain responses into a single animal. This article introduces several rodent models which are often employed in the preclinical studies and discusses their validities for TMD pain after the elucidations of the neural mechanisms based on the clinical reports. First, rodent models were classified into two groups with or without inflammation in the deep craniofacial tissues. Next, the characteristics of each model and the procedures to identify deep craniofacial pain were discussed. Emphasis was directed on the findings of the effects of chronic psychological stress, a major risk factor for chronic pain, on the deep craniofacial nociception. Preclinical models have provided clinically relevant information, which could contribute to better understand the basis for TMD pain, while efforts are still required to bridge the gap between animal and human studies.

Peripheral glutamate receptor and transient receptor potential channel mechanisms of craniofacial muscle pain

Temporomandibular joint disorder is a common chronic craniofacial pain condition, often involving persistent, widespread craniofacial muscle pain. Although the etiology of chronic muscle pain is not well known, sufficient clinical and preclinical information supports a contribution of trigeminal nociceptors to craniofacial muscle pain processing under various experimental and pathological conditions. Here, we review cellular and molecular mechanisms underlying sensitization of muscle nociceptive afferents. In particular, we summarize findings on pronociceptive roles of peripheral glutamate in humans, and we discuss mechanistic contributions of glutamate receptors, including N-methyl-D-aspartate receptors and metabotropic glutamate receptors, which have considerably increased our understanding of peripheral mechanisms of craniofacial muscle pain. Several members of the transient receptor potential (TRP) family, such as transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1, also play essential roles in the development of spontaneous pain and mechanical hypersensitivity in craniofacial muscles. Furthermore, glutamate receptors and TRP channels functionally and bi-directionally interact to modulate trigeminal nociceptors. Activation of glutamate receptors invokes protein kinase C, which leads to the phosphorylation of TRPV1. Sensitization of TRPV1 by inflammatory mediators and glutamate receptors in combination with endogenous ligands contributes to masseter hyperalgesia. The distinct intracellular signaling pathways through which both receptor systems engage and specific molecular regions of TRPV1 are offered as novel targets for the development of mechanism-based treatment strategies for myogenous craniofacial pain conditions.

Sterile water; a novel and promising human experimental craniofacial muscle pain model

BACKGROUND

The aim of this study was to investigate if intramuscular injection of sterile water can be used as a human experimental pain model that resembles clinical craniofacial muscle pain and to analyse if the effects differ between sexes.

METHODS

This randomised, double-blind, placebo-controlled cross-over study included 30 healthy age-matched women and men (23.6 ± 2.4 years). At three sessions, with at least one week of washout in between, 0.2 mL of either sterile water (test-substance), hypertonic saline (58.5 mg/mL; active control) or isotonic saline (0.9 mg/mL; passive control) was randomly injected into the right masseter muscle. Pain intensity (VAS) was continuously assessed during 5 min whereafter pain duration (s) and pain area (au) were calculated; pressure pain thresholds (PPT;kPa) were recorded every 5 minutes during 30 minutes.

RESULTS

Sterile water evoked pain of similar intensity (74.5 ± 49.9) as hypertonic saline (74.0 ± 50.5); whereas, isotonic saline evoked low-intensity pain (11.4 ± 23.4). The pain induced by sterile water and hypertonic saline had higher intensity (P < 0.001), longer duration (P < 0.001) and larger pain area (P < 0.001) than isotonic saline. There were no significant differences in any pain variable between sterile water and hypertonic saline. The PPT did not change significantly after any substance, except for in women 5 minutes after sterile water injection (P < 0.002). Pain duration was longer in the men for all substances (P < 0.006), while the pain area was larger in women after injection of hypertonic saline (P < 0.003).

CONCLUSION

These results indicate that pain evoked by sterile water resembles clinical muscle pain and may offer a novel and simpler alternative to hypertonic saline injections.

Salivary Alpha-Amylase in Experimentally-Induced Muscle Pain

Salivary alpha-amylase (sAA) is a marker of psychological stress and might also be a potential marker for pain-associated stress due its non-invasive, cost-effective, and stress-free collection. The current study aimed to investigate whether the levels of sAA are influenced by experimentally induced muscle pain. In this study, 26 healthy, pain-free and age-matched participants (23.8 ± 2.6 years) were included, 13 women and 13 men. Prior to the experiment, questionnaires assessing health and anxiety were completed. Muscle pain was then induced through intramuscular injection of 0.4 mL hypertonic saline (56.5 mg/mL) into the masseter muscle and unstimulated whole saliva samples were collected at baseline before injection, 2 min, and 15 min after injection. A commercially available colorimetric assay was used to analyze the sAA. Perceived pain and stress were assessed using a 0–100 Numeric Rating Scale for each sample. There were no significant differences in sAA levels prior and after injection of hypertonic saline (p > 0.05) although sAA levels showed a slight decrease during experimentally-induced muscle pain. However, a strong correlation was observed between self-reported pain and perceived level of stress during experimentally-induced muscle pain (r₂ = 0.744; p < 0.0001). Furthermore, there was a moderate correlation between the levels of sAA at baseline and during experimental pain (r₂ = 0.687; p < 0.0001). In conclusion, this study could not show any association between the levels of sAA and perceived pain and or/stress. However, since a significant strong correlation could be observed between perceived stress and pain intensity, this study indicates that experimentally-induced muscle pain could be used as a stress model.

The Effect of Microdialysis Catheter Insertion on Glutamate and Serotonin Levels in Masseter Muscle in Patients with Myofascial Temporomandibular Disorders and Healthy Controls

Myofascial temporomandibular disorders (TMD) are the most common cause of chronic pain in the orofacial region. Microdialysis has been used to study metabolic changes in the human masseter muscle. The insertion of the microdialysis probe causes acute tissue trauma that could affect the metabolic milieu and thereby influence the results when comparing healthy subjects to those with TMD. This study aimed to investigate the levels of serotonin and glutamate during the acute tissue trauma period in healthy subjects and in patients with TMD. Microdialysis was carried out in 15 patients with TMD and 15 controls, and samples were collected every 20 min during a period of 140 min. No significant alterations of serotonin or glutamate were observed over the 2 h period for the healthy subjects. For the TMD group, a significant decrease in serotonin was observed over time (p < 0.001), followed by a significant increase between 120 and 140 min (p < 0.001). For glutamate, a significant reduction was observed at 40 min compared to baseline. The results showed that there was a spontaneous increase of serotonin 2 h after the insertion of the catheter in patients with TMD. In conclusion, the results showed that there are differences in the masseter muscle levels of serotonin and glutamate during acute nociception in patients with myofascial TMD compared to healthy subjects.

Increased levels of intramuscular cytokines in patients with jaw muscle pain

BACKGROUND

The aim of this study was to investigate cytokine levels in the masseter muscle, their response to experimental tooth-clenching and their relation to pain, fatigue and psychological distress in patients with temporomandibular disorders (TMD) myalgia.

METHODS

Forty women, 20 with TMD myalgia (Diagnostic Criteria for TMD) and 20 age-matched healthy controls participated. Intramuscular microdialysis was performed to sample masseter muscle cytokines. After 140 min (baseline), a 20-minute tooth-clenching task was performed (50% of maximal voluntary contraction force). Pain (Numeric rating scale 0-10) and fatigue (Borg's Ratings of Perceived Exertion 6-20) were assessed throughout microdialysis, while pressure-pain thresholds (PPT) were assessed before and after microdialysis. Perceived stress (PSS-10) and Trait Anxiety (STAI) were assessed before microdialysis.

RESULTS

The levels of IL-6, IL-7, IL-8 and IL-13 were higher in patients than controls (Mann Whitney U-test; P's < 0.05) during the entire microdialysis. IL-6, IL-8 and IL-13 changed during microdialysis in both groups (Friedman; P's < 0.05), while IL-1β, IL-7 and GM-CSF changed only in patients (P's < 0.01). IL-6 and IL-8 increased in response to tooth-clenching in both groups (Wilcoxon test; P's < 0.05), while IL-7, IL-13 and TNF increased only in patients (P's < 0.05). Patients had higher pain and fatigue than controls before and after tooth-clenching (P < 0.001), and lower PPTs before and after microdialysis (P < 0.05). There were no correlations between cytokine levels, pain or fatigue. Also, there were no differences in stress or anxiety levels between groups.

CONCLUSIONS

In conclusion, the masseter levels of IL-6, IL-7, IL-8 and IL-13 were elevated in patients with TMD myalgia and increased in response to tooth-clenching. Tooth-clenching increased jaw muscle pain and fatigue, but without correlations to cytokine levels. This implies that subclinical muscle inflammation may be involved in TMD myalgia pathophysiology, but that there is no direct cause-relation between inflammation and pain.

Increased Interstitial Concentrations of Glutamate and Pyruvate in Vastus Lateralis of Women with Fibromyalgia Syndrome Are Normalized after an Exercise Intervention - A Case-Control Study

BACKGROUND

Fibromyalgia syndrome (FMS) is associated with central alterations, but controversies exist regarding the presence and role of peripheral factors. Microdialysis (MD) can be used in vivo to study muscle alterations in FMS. Furthermore for chronic pain conditions such as FMS, the mechanisms for the positive effects of exercise are unclear. This study investigates the interstitial concentrations of algesics and metabolites in the vastus lateralis muscle of 29 women with FMS and 28 healthy women before and after an exercise intervention.

METHODS

All the participants went through a clinical examination and completed a questionnaire. In addition, their pressure pain thresholds (PPTs) in their upper and lower extremities were determined. For both groups, MD was conducted in the vastus lateralis muscle before and after a 15-week exercise intervention of mainly resistance training of the lower limbs. Muscle blood flow and interstitial muscle concentrations of lactate, pyruvate, glutamate, glucose, and glycerol were determined.

RESULTS

FMS was associated with significantly increased interstitial concentrations of glutamate, pyruvate, and lactate. After the exercise intervention, the FMS group exhibited significant decreases in pain intensity and in mean interstitial concentrations of glutamate, pyruvate, and glucose. The decrease in pain intensity in FMS correlated significantly with the decreases in pyruvate and glucose. In addition, the FMS group increased their strength and endurance.

CONCLUSION

This study supports the suggestion that peripheral metabolic and algesic muscle alterations are present in FMS patients and that these alterations contribute to pain. After an exercise intervention, alterations normalized, pain intensity decreased (but not abolished), and strength and endurance improved, all findings that suggest the effects of exercise are partially peripheral.