Equal pain-Unequal fear response: enhanced susceptibility of tooth pain to fear conditioning

Orofacial musculoskeletal pain: An evidence-based bio-psycho-social matrix model

Pain is a multidimensional experience comprising sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Clinical and research findings have demonstrated a complex interplay between social burdens, individual coping strategies, mood states, psychological disorders, sleep disturbances, masticatory muscle tone, and orofacial musculoskeletal pain. Accordingly, current classification systems for orofacial pain require psychosocial assessments to be an integral part of the multidimensional diagnostic process. Here, we review evidence on how psychosocial and biological factors may generate and perpetuate musculoskeletal orofacial pain. Specifically, we discuss studies investigating a putative causal relationship between stress, bruxism, and pain in the masticatory system. We present findings that attribute brain structures various roles in modulating pain perception and pain-related behavior. We also examine studies investigating how the nervous and immune system on cellular and molecular levels may account for orofacial nociceptive signaling. Furthermore, we review evidence pointing towards associations between orofacial musculoskeletal pain and neuroendocrine imbalances, sleep disturbances, and alterations of the circadian timing system. We conclude with several proposals that may help to alleviate orofacial pain in the future.

Evaluating the Effects of Acupuncture Using a Dental Pain Model in Healthy Subjects - A Randomized, Cross-Over Trial

Acupuncture is a complementary and nonpharmacological intervention that can be effective for the management of chronic pain in addition to or instead of medication. Various animal models for neuropathic pain, inflammatory pain, cancer-related pain, and visceral pain already exist in acupuncture research. We used a newly validated human pain model and examined whether acupuncture can influence experimentally induced dental pain. For this study, we compared the impact of manual acupuncture (real acupuncture), manual stimulation of a needle inserted at nonacupuncture points (sham acupuncture) and no acupuncture on experimentally induced dental pain in 35 healthy men who were randomized to different sequences of all 3 interventions in a within-subject design. BORG CR10 pain ratings and autonomic responses (electrodermal activity and heart rate variability) were investigated. An initial mixed model with repeated measures included preintervention pain ratings and the trial sequence as covariates. The results showed that acupuncture was effective in reducing pain intensity when compared to no acupuncture (β = -.708, P = .002), corresponding to a medium Cohen's d effect size of .56. The comparison to the sham acupuncture revealed no statistically significant difference. No differences in autonomic responses between real and sham acupuncture were found during the intervention procedures. PERSPECTIVE: This study established a dental pain model for acupuncture research and provided evidence that experimentally induced dental pain can be influenced by either real acupuncture or manual stimulation of needles at nonacupuncture points. The data do not support that acupoint specificity is a significant factor in reducing experimental pain.

Dentin hypersensitivity monitored by cold air quantitative sensory testing

BACKGROUND

Quantification of dentin hypersensitivity (DH) is challenging and requires standardised, graded stimulation by natural-like stimuli.

OBJECTIVE

The present study aimed at identifying DH subjects and longitudinally monitoring their pain thresholds by cold air quantitative sensory testing (QST).

METHODS

Subject recruitment started with an online DH questionnaire. Respondents were screened by dental air stimulation. Sensitising and habituating subjects were excluded. A recently developed stimulation device was employed for cold air QST. Single tooth DH was verified by applying an equi-intense stimulus to a control tooth. Descriptive statistics were applied for subject characteristics. Mean values were calculated for the stimulation parameters temperature and air flow. Reliability of temperatures for detecting pain and for evoking moderate pain over multiple time points within a 3-week period was analysed by two-way random single- and average-measure intra-class correlation coefficients.

RESULTS

A total of 353 persons completed the online DH questionnaire of which 117 were screened. Forty-four passed the screening, yet 15 were excluded for various reasons. Twenty-nine subjects were monitored by QST across 3 weeks. Results revealed a high intra-individual stability of the temperature inducing moderate to strong pain intensity (MPI) (single-measure ICC of T_MPI 0.83, P < 0.001). Mean T_MPI was -13.69°C, yet it highly varied among the 29 subjects (SD ± 10.04°C).

CONCLUSIONS

Using a novel approach, namely dental QST based on cold air stimuli, we present evidence for temporally stable DH perceptions over a 3-week period. The method fulfils international guideline requirements and is recommendable for obtaining valid results when testing various interventions for DH management.

Pain-Related Fear-Dissociable Neural Sources of Different Fear Constructs

Fear of pain demonstrates significant prognostic value regarding the development of persistent musculoskeletal pain and disability. Its assessment often relies on self-report measures of pain-related fear by a variety of questionnaires. However, based either on “fear of movement/(re)injury/kinesiophobia,” “fear avoidance beliefs,” or “pain anxiety,” pain-related fear constructs plausibly differ while it is unclear how specific the questionnaires are in assessing these different constructs. Furthermore, the relationship of pain-related fear to other anxiety measures such as state or trait anxiety remains ambiguous. Advances in neuroimaging such as machine learning on brain activity patterns recorded by functional magnetic resonance imaging might help to dissect commonalities or differences across pain-related fear constructs. We applied a pattern regression approach in 20 human patients with nonspecific chronic low back pain to reveal predictive relationships between fear-related neural pattern information and different pain-related fear questionnaires. More specifically, the applied multiple kernel learning approach allowed the generation of models to predict the questionnaire scores based on a hierarchical ranking of fear-related neural patterns induced by viewing videos of activities potentially harmful for the back. We sought to find evidence for or against overlapping pain-related fear constructs by comparing the questionnaire prediction models according to their predictive abilities and associated neural contributors. By demonstrating evidence of nonoverlapping neural predictors within fear-processing regions, the results underpin the diversity of pain-related fear constructs. This neuroscientific approach might ultimately help to further understand and dissect psychological pain-related fear constructs.

Brain responses to stimuli mimicking dental treatment among non-phobic individuals: A meta-analysis

Numerous neuroimaging studies have attempted to identify how the brain responds to stimuli mimicking dental treatment in normal non-phobic individuals. However, results were sometimes inconsistent due to small sample sizes and methodological variations. This meta-analysis employs standardized procedures to summarize data from previous studies to identify brain regions that were consistently activated across studies, elicited by stimuli such as pictures, sounds, or audiovisual footage mimicking those encountered during dental treatments. A systematic literature search was carried out using PubMed and Scopus. The meta-analysis analyzed data from 120 healthy subjects from seven neuroimaging studies. We assessed the risk of bias among the included studies with the Risk of Bias Assessment Tool for Nonrandomized Studies. One study appeared to have a high risk of selection bias, whereas the others were considered to have a low risk of bias. Results revealed three clusters of activation with cluster sizes ranging from 768 mm³ to 1,424 mm³ . Stimuli mimicking dental treatment consistently activated the bilateral anterior insula; right dorsal anterior cingulate, putamen, and medial prefrontal cortex; and left claustrum. This study confirmed that audio and/or visual stimuli mimicking dental treatment consistently activated the fear-related brain regions among healthy subjects, mostly consistent with activations from general anxiety but without the involvement of the amygdala.

Amplified parabrachial nucleus activity in a rat model of trigeminal neuropathic pain

The parabrachial (PB) complex mediates both ascending nociceptive signaling and descending pain modulatory information in the affective/emotional pain pathway. We hypothesized that PB hyperactivity influences chronic pain behavior after trigeminal nerve injury in rats. Following induction of neuropathic pain using the chronic constriction injury of the infraorbital nerve (CCI-ION) model, rats displayed spontaneous markers of pain and mechanical hyperalgesia extending beyond the receptive field of the injured nerve. PB neurons recorded from rats with CCI-ION displayed amplified activity, manifesting as significantly longer responses to sensory stimuli, compared to shams. These findings suggest that chronic neuropathic pain involves PB hyperactivity.

The grimace scale reliably assesses chronic pain in a rodent model of trigeminal neuropathic pain

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Facial expressions were analyzed after constriction injury of infraorbital nerve.

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The grimace score reliably assesses ongoing pain in a this model.

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The grimace score can be used in both rats and mice with trigeminal neuropathic pain.

The limited success in translating basic science findings into effective pain management therapies reflects, in part, the difficulty in reliably assessing pain in experimental animals. This shortcoming is particularly acute in the field of chronic, ongoing pain. Quantitative analysis of facial expressions—the grimace score—was introduced as a promising tool, however, it is thought to reliably assess only pain of short or medium duration (minutes to hours). Here, we test the hypothesis that grimace scores are a reliable metric of ongoing neuropathic pain, by testing the prediction that chronic constriction injury of the infraorbital nerve (CCI-ION) will evoke significant increases in grimace scale scores. Mice and rats were subjected to CCI-ION, and tested for changes in mechanical hypersensitivity and in grimace scores, 10 or more days after surgery. Both rats and mice with CCI-ION had significantly higher grimace scores, and significantly lower thresholds for withdrawal from mechanical stimuli applied to the face, compared to sham-operated animals. Fentanyl reversed the changes in rat grimace scale scores, suggesting that these scores reflect pain perception. These findings validate the grimace scale as a reliable and sensitive metric for the assessment of ongoing pain in a rodent model of chronic, trigeminal neuropathic pain.

[Distinctive features of orofacial pain perception]

BACKGROUND

In the widely used German pain questionnaire, qualitative pain characteristics are assessed by the Schmerzbeschreibungsliste (SBL). In clinical practice these adjectives insufficiently cover the spectrum of orofacial pain perceptions (OFP) and have not been proven by data. The aim of this study was the analysis of a questionnaire focussing on OFP (ZZM-FB) in which nine additional pain descriptors were amended to the SBL (SBL-OF).

OBJECTIVES

The objectives of this work were to (1) compare selection frequencies between the items of the SBL and the SBL-OF and (2) test the null hypothesis that the SBL and SBL-OF item selection is not influenced by gender, age, pain features (intensity, duration, time pattern, distribution), hospital anxiety and depression scale (HADS) and diagnosis.

MATERIAL AND METHODS

A total of 224 patients consecutively referred to the interdisciplinary OFP unit between 2010 and 2012 completed a ZZM-FB. The corresponding diagnosis was obtained from the clinical reports. In all, 209 OFP-Q were anonymized and data retrospectively analysed at a significance level of p ≤ 0.01.

RESULTS

The descending rank order of the 12 most frequently selected items was as follow: pressing (SBL), dull (SBL), pulling (SBL), stinging (SBL), dreadful (SBL), miserable (SBL), exhausting (SBL-OF), grueling (SBL-OF), agonizing (SBL-OF), atrocious (SBL), horrible (SBL), pulsating (SBL). The null hypothesis was rejected for all parameters except for age. The selection frequency was significantly influenced by gender, pain intensity, duration and distribution, time pattern, HADS values and diagnosis.

CONCLUSIONS

In this cohort, some ADD descriptors were selected more frequently than some SBL items, indicating that the SBL does not adequately capture the characteristics of OFP. These results will be considered in the construction of a revised ZZM-FB, which is currently in development.

Functional changes in brain activity after hypnosis in patients with dental phobia

Visiting the dentist is often accompanied by apprehension or anxiety. People, who suffer from specific dental phobia (a disproportional fear of dental) procedures show psychological and physiological symptoms which make dental treatments difficult or impossible. For such purposes, hypnosis is often used in dental practice as an alternative for a number of treatments adjuvant or instead of sedation or general anaesthetics, as medication is often associated with risks and side effects. This is the first study to address the effects of a brief dental hypnosis on the fear processing structures of the brain in dental phobics using functional magnetic resonance imaging (fMRI). 12 dental phobics (DP; mean 34.9years) and 12 healthy controls (CO; mean 33.2years) were scanned with a 3T MRI whole body-scanner observing brain activity changes after a brief hypnotic invervention. An fMRI event-related design symptom provocation task applying animated audio-visual pseudorandomized strong phobic stimuli was presented in order to maximize the fearful reactions during scanning. Control videos showed the use of familiar electronic household equipment. In DP group, main effects of fear condition were found in the left amygdala and bilaterally in the anterior cingulate cortex (ACC), insula and hippocampu (R<L). During hypnosis DP showed a significantly reduced activation in all of these areas. Reduced neural activity patterns were also found in the control group. No amygdala activation was detected in healthy subjects in the two experimental conditions. Compared to DP, CO showed less bilateral activation in the insula and ACC in the awake condition. Findings show that anxiety-provoking stimuli such as undergoing dental surgery, endodontic treatments or insufficient anaesthetics, can be effectively reduced under hypnosis. The present study gives scientific evidence that hypnosis is a powerful and successful method for inhibiting the reaction of the fear circuitry structures.

Tracking local anesthetic effects using a novel perceptual reference approach

Drug effects of loco-regional anesthetics are commonly measured by unidimensional pain rating scales. These scales require subjects to transform their perceptual correlates of stimulus intensities onto a visual, verbal, or numerical construct that uses a unitless cognitive reference frame. The conceptual understanding and execution of this magnitude estimation task may vary among individuals and populations. To circumvent inherent shortcomings of conventional experimental pain scales, this study used a novel perceptual reference approach to track subjective sensory perceptions during onset of an analgesic nerve block. In 34 male subjects, nociceptive electric stimuli of 1-ms duration were repetitively applied to left (target) and right (reference) mandibular canines every 5 s for 600 s, with a side latency of 1 ms. Stimulus strength to the target canine was programmed to evoke a tolerable pain intensity perception and remained constant at this level throughout the experiment. A dose of 0.6 ml of articaine 4% was submucosally injected at the left mental foramen. Subjects then reported drug effects by adjusting the stimulus strength (in milliamperes) to the reference tooth, so that the perceived intensity in the reference tooth was equi-intense to the target tooth. Pain and stimulus perception offsets were indicated by subjects. Thus, the current approach for matching the sensory experience in one anatomic location after regional anesthesia allows detailed tracking of evolving perceptual changes in another location. This novel perceptual reference approach facilitates direct and accurate quantification of analgesic effects with high temporal resolution. We propose using this method for future experimental investigations of analgesic/anesthetic drug efficacy.

The human brain response to dental pain relief

Local anesthesia has made dental treatment more comfortable since 1884, but little is known about associated brain mechanisms. Functional magnetic resonance imaging is a modern neuroimaging tool widely used for investigating human brain activity related to sensory perceptions, including pain. Most brain regions that respond to experimental noxious stimuli have recently been found to react not only to nociception alone, but also to visual, auditory, and other stimuli. Thus, presumed functional attributions have come under scrutiny regarding selective pain processing in the brain. Evidently, innovative approaches are warranted to identify cerebral regions that are nociceptive specific. In this study, we aimed at circumventing known methodological confounders by applying a novel paradigm in 14 volunteers: rather than varying the intensity and thus the salience of painful stimuli, we applied repetitive noxious dental stimuli at constant intensity to the left mandibular canine. During the functional magnetic resonance imaging paradigm, we suppressed the nociceptive barrage by a mental nerve block. Brain activity before and after injection of 4% articaine was compared intraindividually on a group level. Dental pain extinction was observed to correspond to activity reduction in a discrete region of the left posterior insular cortex. These results confirm previous reports demonstrating that direct electrical stimulation of this brain region-but not of others-evokes bodily pain sensations. Hence, our investigation adds further evidence to the notion that the posterior insula plays a unique role in nociceptive processing.