Sex differences in the cerebral BOLD signal response to painful heat stimuli

There are limited data addressing the question of sex differences in pain-related cerebral processing. This study examined whether pain-related blood oxygenation level-dependent (BOLD) signal change measured with functional magnetic resonance imaging (fMRI) demonstrated sex differences, under conditions of equivalent pain perception. Twenty-eight healthy volunteers (17 women, 11 men) were subject to a fMRI scan while noxious heat stimuli were applied to the dorsum of the left foot. Significant BOLD signal modulation was observed in several nociceptive processing regions of interest (ROIs) in all subjects. There were no sex differences in the spatial extent of BOLD signal change for any ROI, but the signal amplitude was lower for women in most ROIs and significantly so for the primary somatosensory cortex (S1), the midanterior cingulate cortex, and the dorsolateral prefrontal cortex (DLPFC). The BOLD signal response could be positive or negative, and frequently, both polarities were observed within a single ROI. In most ROIs, women show proportionately more voxels with negative signal change than men, and this difference was statistically significant for the S1 and the DLPFC. The time course of the negative signal change was very similar to that of the positive signal change, suggesting that the latter was not “driving” the former. The location of negative and positive clusters formed distinct patterns in several of the ROIs, and these patterns suggest something other than a local “steal” phenomenon as an explanation for the negative signal changes. Sex differences in baseline cerebral blood flow may contribute to the BOLD signal differences observed in this study

Long-term changes in purposive and reflexive responses to nociceptive stimulation following anterolateral chordotomy

Macaca nemestrina monkeys received unilateral interruption of the spinothalamic tract, producing contralateral hypalgesia and a bilateral decrease in amplitude of the flexion reflex. These effects on operant escape and reflex responses to electrocutaneous stimulation (ES) were monitored for months to evaluate relationships between the extent of each lesion and the presence or absence of recovery from the early postoperative deficits. Before surgery, the animals were trained to perform an operant response that terminated ES to the lateral calf of either leg. The durations of ES tolerated by each monkey were inversely related to stimulus intensities within the pain sensitivity range of human subjects. The vigor of operant escape responses and the frequency of intertrial pulls of the manipulandum were directly related to stimulus intensity. Following anterolateral chordotomy at an upper thoracic level, these measures revealed a contralateral hypalgesia for each animal. Operant responsivity to stimulation contralateral to the lesion recovered to control levels for 7 animals (group R). Sustained contralateral recovery of operant reactivity was not observed for 8 monkeys (group U). Most of the lesions in groups R and U were similar in extent and location, involving the classical distribution of the spinothalamic tract (in the anterolateral and ventral columns). Thus, recovery was not determined solely by lesion configuration. However, when recovery did occur, it was associated with medially extensive lesions. A subgroup of 3 unrecovered animals received superficial lesions that did not substantially involve the gray matter or the ventral columns. For all animals, reflex magnitudes were initially diminished bilaterally and then increased over months of testing. Reflex recovery was greater for the animals that demonstrated recovery of intentional reactions to nociception (group R). An ipsilateral hyperreflexia became apparent for group R. Contralateral recovery to normal levels was observed for group R but not for group U. The time course of recovery for operant and reflex responses clearly differed, indicating that different processes determined changes in these spinal and supraspinal reactions to nociceptive stimulation

Non-invasive Motor Cortex Neuromodulation Reduces Secondary Hyperalgesia and Enhances Activation of the Descending Pain Modulatory Network

Central sensitization is a driving mechanism in many chronic pain patients, and manifests as hyperalgesia and allodynia beyond any apparent injury. Recent studies have demonstrated analgesic effects of motor cortex (M1) stimulation in several chronic pain disorders, yet its neural mechanisms remain uncertain. We evaluated whether anodal M1 transcranial direct current stimulation (tDCS) would mitigate central sensitization as measured by indices of secondary hyperalgesia. We used a capsaicin-heat pain model to elicit secondary mechanical hyperalgesia in 27 healthy subjects. In an assessor and subject-blind randomized, sham-controlled, crossover trial, anodal M1 tDCS decreased the intensity of pinprick hyperalgesia more than cathodal or sham tDCS. To elucidate the mechanism driving analgesia, subjects underwent fMRI of painful mechanical stimuli prior to and following induction of the pain model, after receiving M1 tDCS. We hypothesized that anodal M1 tDCS would enhance engagement of a descending pain modulatory (DPM) network in response to mechanical stimuli. Anodal tDCS normalized the effects of central sensitization on neurophysiological responses to mechanical pain in the medial prefrontal cortex, pregenual anterior cingulate cortex, and periaqueductal gray, important regions in the DPM network. Taken together, these results provide support for the hypothesis that anodal M1-tDCS reduces central sensitization-induced hyperalgesia through the DPM network in humans

Subjective Sleep Quality Deteriorates Before Development of Painful Temporomandibular Disorder

There is good evidence that poor sleep quality increases risk of painful temporomandibular disorder (TMD). However little is known about the course of sleep quality in the months preceding TMD onset, and whether the relationship is mediated by heightened sensitivity to pain. The Pittsburgh Sleep Quality Index was administered at enrollment into the OPPERA prospective cohort study. Thereafter the Sleep Quality Numeric Rating Scale was administered every three months to 2,453 participants. Sensitivity to experimental pressure pain and pinprick pain stimuli was measured at baseline and repeated during follow-up of incident TMD cases (n=220) and matched TMD-free controls (n=193). Subjective sleep quality deteriorated progressively, but only in those who subsequently developed TMD. A Cox proportional hazards model showed that risk of TMD was greater among participants whose sleep quality worsened during follow-up (adjusted hazard ratio=1.73, 95% confidence limits: 1.29, 2.32). This association was independent of baseline measures of sleep quality, psychological stress, somatic awareness, comorbid conditions, non-pain facial symptoms and demographics. Poor baseline sleep quality was not significantly associated with baseline pain sensitivity or with subsequent change in pain sensitivity. Furthermore the relationship between sleep quality and TMD incidence was not mediated via baseline pain sensitivity nor change in pain sensitivity

Identification of clusters of individuals relevant to temporomandibular disorders and other chronic pain conditions: the OPPERA study

The classification of most chronic pain disorders gives emphasis to anatomical location of the pain to distinguish one disorder from the other (eg, back pain vs temporomandibular disorder [TMD]) or to define subtypes (eg, TMD myalgia vs arthralgia). However, anatomical criteria overlook etiology, potentially hampering treatment decisions. This study identified clusters of individuals using a comprehensive array of biopsychosocial measures. Data were collected from a case–control study of 1031 chronic TMD cases and 3247 TMD-free controls. Three subgroups were identified using supervised cluster analysis (referred to as the adaptive, pain-sensitive, and global symptoms clusters). Compared with the adaptive cluster, participants in the pain-sensitive cluster showed heightened sensitivity to experimental pain, and participants in the global symptoms cluster showed both greater pain sensitivity and greater psychological distress. Cluster membership was strongly associated with chronic TMD: 91.5% of TMD cases belonged to the pain-sensitive and global symptoms clusters, whereas 41.2% of controls belonged to the adaptive cluster. Temporomandibular disorder cases in the pain-sensitive and global symptoms clusters also showed greater pain intensity, jaw functional limitation, and more comorbid pain conditions. Similar results were obtained when the same methodology was applied to a smaller case–control study consisting of 199 chronic TMD cases and 201 TMD-free controls. During a median 3-year follow-up period of TMD-free individuals, participants in the global symptoms cluster had greater risk of developing first-onset TMD (hazard ratio = 2.8) compared with participants in the other 2 clusters. Cross-cohort predictive modeling was used to demonstrate the reliability of the clusters

Temporal change in headache and its contribution to the risk of developing first-onset temporomandibular disorder in the Orofacial Pain: Prospective Evaluation and Risk Assessment (OPPERA) study

While cross-sectional studies have demonstrated an association between headache and temporomandibular disorder (TMD), whether headache can predict the onset of TMD is unknown. The aims of this study were to evaluate the contribution of headache to the risk of developing TMD and describe patterns of change in headache types over time. An initially TMD-free cohort of 2410 persons with low frequency of headache completed quarterly questionnaires assessing TMD and headache symptoms over a median 3.0-year follow-up period. First-onset TMD was confirmed by clinical examination in 199 participants. Baseline reports of migraine (hazard ratio [HR] = 1.67, 95% confidence interval [CI]: 1.06-2.62) or mixed headache types (HR = 4.11, 95% CI: 1.47-11.46), or headache frequency (HR = 2.13, 95% CI: 1.31-3.48) predicted increased risk of developing TMD. In addition, headache dynamics across the follow-up period before the TMD onset were evaluated in a nested case-control study where 248 incident TMD cases were matched to 191 TMD-free controls. Both headache prevalence and frequency increased across the observation period among those who developed TMD but not among controls. Patients with TMD were more likely to experience worsening in the headache type compared with that by controls, eg, prevalence of definite migraine among TMD cases increased 10-fold. Among all headache types experienced by patients with TMD before the TMD onset, migraine had the highest odds of progression relative to remission (odds ratio = 2.8, 95% CI: 1.6-4.8), whereas for controls this ratio was significant only for the tension-type headache (odds ratio = 2.1, 95% CI: 1.2-3.9). The important clinical implication of these findings is that adequate treatment of migraine may reduce the risk for developing TMD

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\u3epain 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\u27s 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

Modification of COMT-dependent pain sensitivity by psychological stress and sex

Catecholamine-O-methyltransferase (COMT) is a polymorphic gene whose variants affect enzymatic activity and pain sensitivity via adrenergic pathways. Although COMT represents one of the most studied genes in human pain genetics, findings regarding its association with pain phenotypes are not always replicated. Here, we investigated if interactions among functional COMT haplotypes, stress, and sex can modify the effect of COMT genetic variants on pain sensitivity. We tested these interactions in a cross-sectional study, including 2 cohorts, one of 2972 subjects tested for thermal pain sensitivity (Orofacial Pain: Prospective Evaluation and Risk Assessment) and one of 948 subjects with clinical acute pain after motor vehicle collision (post-motor vehicle collision). In both cohorts, the COMT high-pain sensitivity (HPS) haplotype showed robust interaction with stress and number of copies of the HPS haplotype was positively associated with pain sensitivity in nonstressed individuals, but not in stressed individuals. In the post-motor vehicle collision cohort, there was additional modification by sex: the HPS-stress interaction was apparent in males, but not in females. In summary, our findings indicate that stress and sex should be evaluated in association studies aiming to investigate the effect of COMT genetic variants on pain sensitivity

Signs and Symptoms of First-Onset TMD and Sociodemographic Predictors of Its Development: The OPPERA Prospective Cohort Study

Although cross-sectional studies of temporomandibular disorders (TMDs) often report elevated prevalence in young women, they do not address the risk of its development. Here we evaluate sociodemographic predictors of TMD incidence in a community-based prospective cohort study of U.S. adults. Symptoms and pain-related disability in TMD cases are also described. People aged 18 to 44 years with no history of TMD were enrolled at 4 study sites when they completed questionnaires about sociodemographic characteristics. During the median 2.8-year follow-up period, 2,737 participants completed quarterly screening questionnaires. Those reporting symptoms were examined clinically and 260 had first-onset TMD. Additional questionnaires asked about severity and impact of their symptoms. Univariate and multivariate Cox regression models quantified associations between sociodemographic characteristics and TMD incidence. First-onset TMD developed in 3.9% of participants per annum, typically producing mild to moderate levels of pain and disability in cases. TMD incidence was positively associated with age, whereas females had only slightly greater incidence than males. Compared to whites, Asians had lower TMD incidence whereas African Americans had greater incidence, although the latter was attenuated somewhat after adjusting for satisfaction with socioeconomic circumstances