Baseline resting-state functional connectivity determines subsequent pain ratings to a tonic ecologically valid experimental model of orofacial pain

Biological and psychological factors affecting the sensory and jaw motor responses to orthodontic tooth movement

Orthodontic tooth movement (OTM) is associated with an inflammatory response, tooth pain (i.e. orthodontic pain) and changes in dental occlusion. Clinical realms and research evidence suggest that the sensory and jaw motor responses to OTM vary significantly among individuals. While some adjust well to orthodontic procedures, others may not and can experience significant pain or not adjust to occlusal changes. This is of concern, as clinicians cannot anticipate an individual's sensorimotor response to OTM. Converging evidence shows that some psychological states and traits significantly affect the sensorimotor response to OTM and may considerably affect an individual's adaptation to orthodontic or other dental procedures. We performed a topical review to synthesize the available knowledge about the behavioural mechanisms regulating the sensorimotor response to OTM, with the intent of informing orthodontic practitioners and researchers about specific psychological states and traits that should be considered while planning orthodontic treatment. We report on studies focusing on the role of anxiety, pain catastrophising, and somatosensory amplification (i.e. bodily hypervigilance), on sensory and jaw motor responses. Psychological states and traits can significantly affect sensory and jaw motor responses and a patient's adaptation to orthodontic procedures, although large interindividual variability exists. Clinicians can use validated instruments (checklists or questionnaires) to collect information about patients' psychological traits, which can assist in identifying those individuals who may not adjust well to orthodontic procedures. The information included in this manuscript also assists researchers investigating the effect of orthodontic procedures and or/appliances on orthodontic pain.

Improvement in post-orthodontic chronic musculoskeletal pain after local anesthetic injections in the trigeminal area: a case series

Orthodontic treatment has been associated with chronic extraoral pain that is often resistant to common treatments such as drugs or physiotherapy, adversely affecting patients' quality of life. In this case series, we discuss the potential impact of orthodontics on chronic cervical spine pain or gonalgia and explore the long-term effect of local anesthetic injections as a possible therapeutic intervention. Six orthodontic patients with chronic cervical spine pain or gonalgia that substantially affected their quality of life were treated with injections of 0.5% procaine into individual lesions and at palpable points of tissue tension in the oral mucosa and extraoral myofascial areas. All patients in this case series reported significant improvement in their chronic pain, with no residual pain recorded at the 6-month follow-up. Injecting local anesthetic at stress points in the oral mucosal and extraoral myofascial regions may be an effective treatment for post-orthodontic neck and knee pain. Further research is required to better understand the potential benefits of this intervention for patients experiencing orthodontic-related musculoskeletal pain.

Brain Connectivity Predicts Chronic Pain in Acute Mild Traumatic Brain Injury

OBJECTIVES

Previous studies have established the role of the cortico-mesolimbic and descending pain modulation systems in chronic pain prediction. Mild traumatic brain injury (mTBI) is an acute pain model where chronic pain is prevalent and complicated for prediction. In this study, we set out to study whether functional connectivity (FC) of the nucleus accumbens (NAc) and the periaqueductal gray matter (PAG) is predictive of pain chronification in early-acute mTBI.

METHODS

To estimate FC, resting-state functional magnetic resonance imaging (fMRI) of 105 participants with mTBI following a motor vehicle collision was acquired within 72 hours post-accident. Participants were classified according to pain ratings provided at 12-months post-collision into chronic pain (head/neck pain ≥30/100, n = 44) and recovery (n = 61) groups, and their FC maps were compared.

RESULTS

The chronic pain group exhibited reduced negative FC between NAc and a region within the primary motor cortex corresponding with the expected representation of the area of injury. A complementary pattern was also demonstrated between PAG and the primary somatosensory cortex. PAG and NAc also shared increased FC to the rostral anterior cingulate cortex (rACC) within the recovery group. Brain connectivity further shows high classification accuracy (area under the curve [AUC] = .86) for future chronic pain, when combined with an acute pain intensity report.

INTERPRETATION

FC features obtained shortly after mTBI predict its transition to long-term chronic pain, and may reflect an underlying interaction of injury-related primary sensorimotor cortical areas with the mesolimbic and pain modulation systems. Our findings indicate a potential predictive biomarker and highlight targets for future early preventive interventions. ANN NEUROL 2022;92:819-833.

Tonic pain alters functional connectivity of the descending pain modulatory network involving amygdala, periaqueductal gray, parabrachial nucleus and anterior cingulate cortex

INTRODUCTION

Resting state functional connectivity (FC) is widely used to assess functional brain alterations in patients with chronic pain. However, reports of FC accompanying tonic pain in pain-free persons are rare. A network we term the Descending Pain Modulatory Network (DPMN) is implicated in healthy and pathologic pain modulation. Here, we evaluate the effect of tonic pain on FC of specific nodes of this network: anterior cingulate cortex (ACC), amygdala (AMYG), periaqueductal gray (PAG), and parabrachial nuclei (PBN).

METHODS

In 50 pain-free participants (30F), we induced tonic pain using a capsaicin-heat pain model. functional MRI measured resting BOLD signal during pain-free rest with a 32°C thermode and then tonic pain where participants experienced a previously warm temperature combined with capsaicin. We evaluated FC from ACC, AMYG, PAG, and PBN with correlation of self-report pain intensity during both states. We hypothesized tonic pain would diminish FC dyads within the DPMN.

RESULTS

Of all hypothesized FC dyads, only PAG and subgenual ACC was weakly altered during pain (F=3.34; p=0.074; pain-free>pain d=0.25). After pain induction sACC-PAG FC became positively correlated with pain intensity (R=0.38; t=2.81; p=0.007). Right PBN-PAG FC during pain-free rest positively correlated with subsequently experienced pain (R=0.44; t=3.43; p=0.001). During pain, this connection's FC was diminished (paired t=-3.17; p=0.0026). In whole-brain analyses, during pain-free rest, FC between left AMYG and right superior parietal lobule and caudate nucleus were positively correlated with subsequent pain. During pain, FC between left AMYG and right inferior temporal gyrus negatively correlated with pain. Subsequent pain positively correlated with right AMYG FC with right claustrum; right primary visual cortex and right temporo-occipitoparietal junction Conclusion: We demonstrate sACC-PAG tonic pain FC positively correlates with experienced pain and resting right PBN-PAG FC correlates with subsequent pain and is diminished during tonic pain. Finally, we reveal PAG- and right AMYG-anchored networks which correlate with subsequently experienced pain intensity. Our findings suggest specific connectivity patterns within the DPMN at rest are associated with subsequently experienced pain and modulated by tonic pain. These nodes and their functional modulation may reveal new therapeutic targets for neuromodulation or biomarkers to guide interventions.