Inter-Individual Responses to Experimental Muscle Pain: Baseline Physiological Parameters Do Not Determine Whether Muscle Sympathetic Nerve Activity Increases or Decreases During Pain

Heart Rate Variability and Pain: A Systematic Review

Background and Objective: Heart rate variability (HRV) as an index of the autonomic nervous system appears to be related to reactivity to experimental pain stimuli. HRV could better explain the contributions of sympathetic and parasympathetic activity response to nociceptive stimulation. The aim of this study was to systematically review and synthesize the current evidence on HRV in relation to the experience of pain in experimental tasks. Databases and Data Treatment: Studies indexed in the PubMed, PsycINFO, MEDLINE, WebOfScience, and Scopus databases were reviewed for eligibility. Studies on the autonomic response (i.e., HRV) to experimentally induced pain in healthy adults were included. Different methods of pain induction were considered (e.g., thermal, pressure, and electrical). Data were synthesized considering the association between HRV and both pain induction and subjective measures of pain. Results: Seventy-one studies were included. The results underline significant change in both the sympathetic and parasympathetic autonomic nervous systems during the painful stimulation independent of the pain induction method. The autonomic reaction to pain could be affected by several factors, such as sex, age, body mass index, breathing patterns, the intensity of the stimulation, and the affective state. Moreover, an association between the autonomic nervous system and the subjective experience of pain was found. Higher parasympathetic activity was associated with better self-regulation capacities and, accordingly, a higher pain inhibition capacity. Conclusions: HRV appears to be a helpful marker to evaluate nociceptive response in experimentally induced pain. Future studies are also needed in clinical samples to understand better the interindividual changes of autonomic response due to pain stimuli.

Cutaneous warmth, but not touch, increases muscle sympathetic nerve activity during a muscle fatigue hand-grip task

In homeostasis, somatosensory C fibre afferents are hypothesised to mediate input to the brain about interactions with external stimuli and sympathetic efference provides the output that regulates bodily functions. We aimed to test this hypothesis and whether different types of innocuous somatosensory input have differential effects. Healthy volunteers performed a muscle fatigue (hand-grip) task to exhaustion, which produces increased muscle sympathetic nerve activity (MSNA), as measured through microneurography. Participants completed the muscle fatigue task without concurrent cutaneous sensory stimulation (control) or we applied skin warming (heat pack) as a C fibre stimulation, slow brush stroking as C and Aβ fibre stimulation, or vibration as Aβ fibre stimulation, to the participant’s forearm. We also measured heart rate, the duration of the hand-grip task, and ratings of pain at the end of the task. Concurrent skin warming showed increased MSNA compared to the other conditions. Tactile stimuli (brushing, vibration) were not significantly different to the control (no intervention) condition. Warming increased the pain from the muscle contraction, whereas the tactile stimuli did not. We interpret the effect of warming on MSNA as providing relevant afferent information during muscle contraction, which needed to be counteracted via vasoconstriction to maintain homeostasis. Brushing and vibration were less homeostatically relevant stimuli for the muscle contraction and hence had no significant effect. The findings add sensory specificity to our current understanding of homeostatic regulation through somatosensory afferent and sympathetic efferent pathways.

The intensity of pain in the prehospital setting is most strongly reflected in the respiratory rate among physiological parameters

BACKGROUND

In order to treat pain optimally, the Emergency Medical Service (EMS) clinician needs to be able to make a reasonable estimation of the severity of the pain. It is hypothesised that various physiological parameters will change as a response to pain.

AIM

In a cohort of patients who were seen by EMS clinicians, to relate the patients' estimated intensity of pain to various physiological parameters.

METHODS

Patients who called for EMS due to pain in a part of western Sweden were included. The intensity of pain was assessed according to the visual analogue scale (VAS) or the Numerical Rating Scale (NRS). The following were assessed the same time as pain on EMS arrival: heart rate, systolic and diastolic blood pressure, respiratory rate, moist skin and paleness.

RESULTS

In all, 19,908 patients (≥18 years), were studied (51% women). There were significant associations between intensity of pain and the respiratory rate (r = 0.198; p < 0.0001), heart rate (r = 0.037; p < 0.0001), systolic blood pressure (r = -0.029; p < 0.0001), moist skin (r = 0.143; p < 0.0001) and paleness (r = 0.171; p < 0.0001). The strongest association was found with respiratory rate among patients aged 18-64 years (r = 0.258; p < 0.0001).

CONCLUSION

In the prehospital setting, there were significant but weak correlations between intensity of pain and physiological parameters. The most clinically relevant association was found with an increased respiratory rate and presence of pale and moist skin among patients aged < 65 years. Among younger patients, respiratory rate may support in the clinical evaluation of pain.

The effects of audiovisual distraction on the muscle sympathetic responses to experimental muscle pain

Pain elicited by intramuscular infusion of hypertonic saline solution causes muscle sympathetic nerve activity (MSNA) to increase in some subjects, yet decrease in others. Although the direction of the response is not predictable based on baseline physiological and psychological parameters, we know that it results from sustained functional changes in specific brain regions that are responsible for the behavioral and cardiovascular responses to psychological stressors, as well as those involved in attention. The aim of this study was to investigate whether MSNA responses to experimental muscle pain in humans could be altered with an audiovisual stimulus that served to distract them from the pain. MSNA was recorded from the left common peroneal nerve of 20 young healthy individuals during a 45-min intramuscular infusion of hypertonic saline solution into the ipsilateral tibialis anterior muscle. The distracting stimulus commenced 15 min after the start of the infusion and lasted for 15 min. Fifteen subjects showed an increase in mean burst amplitude of MSNA (to 176.4 ± 7.9% of baseline), while five showed a decrease (to 73.1 ± 5.2% of baseline); distraction had no effect on these profiles. These results indicate that even though the subjects were attending to the audiovisual stimulus, and were presumably distracted from the pain, it failed to alter the MSNA responses to muscle pain.

Muscle sympathetic nerve activity-coupled changes in brain activity during sustained muscle pain

INTRODUCTION

Long-lasting experimental muscle pain elicits divergent muscle sympathetic responses, with some individuals exhibiting a persistent increase in muscle sympathetic nerve activity (MSNA), and others a decrease. These divergent responses are thought to result from sustained functional changes in specific brain regions that modulate the cardiovascular responses to pain.

AIM

The aim of this study was to investigate brain regions that are functionally coupled to the generation of an MSNA burst at rest and to determine their behavior during tonic muscle pain.

METHODS

Functional magnetic resonance imaging of the brain was performed concurrently with microelectrode recording of MSNA from the common peroneal nerve during a 40 min infusion of hypertonic saline into the ipsilateral tibialis anterior muscle of 37 healthy human subjects.

RESULTS

At rest, blood oxygen level-dependent signal intensity coupled to bursts of MSNA increased in the rostral ventrolateral medulla, insula, dorsolateral prefrontal cortex, posterior cingulate cortex, and precuneus and decreased in the region of the midbrain periaqueductal gray. During pain, MSNA-coupled signal intensity was greater in the region of the nucleus tractus solitarius, midbrain periaqueductal gray, dorsolateral prefrontal, medial prefrontal, and anterior cingulate cortices, than at rest. Conversely, MSNA-coupled signal intensity decreased during pain in parts of the prefrontal cortex.

CONCLUSIONS

These results suggest that multiple brain regions are recruited in a burst-to-burst manner, and the magnitude of these signal changes is correlated to the overall change in MSNA amplitude during tonic muscle pain.

In Silico Prediction of the Anti-Depression Mechanism of a Herbal Formula (Tiansi Liquid) Containing Morinda officinalis and Cuscuta chinensis

Purpose: Depression is a sickening psychiatric condition that is prevalent worldwide. To manage depression, the underlying modes of antidepressant effect of herbals are important to be explored for the development of natural drugs. Tiansi Liquid is a traditional Chinese medicine (TCM) that is prescribed for the management of depression, however its underlying mechanism of action is still uncertain. The purpose of this study was to systematically investigate the pharmacological mode of action of a herbal formula used in TCM for the treatment of depression. Methods: Based on literature search, an ingredients-targets database was developed for Tiansi Liquid, followed by the identification of targets related to depression. The interaction between these targets was evaluated on the basis of protein-protein interaction network constructed by STITCH and gene ontology (GO) enrichment analysis using ClueGO plugin. Results: As a result of literature search, 57 components in Tiansi Liquid formula and 106 potential targets of these ingredients were retrieved. A careful screening of these targets led to the identification of 42 potential targets associated with depression. Ultimately, 327 GO terms were found by analysis of gene functional annotation clusters and abundance value of these targets. Most of these terms were found to be closely related to depression. A significant number of protein targets such as IL10, MAPK1, PTGS2, AKT1, APOE, PPARA, MAPK1, MIF, NOS3 and TNF-α were found to be involved in the functioning of Tiansi Liquid against depression. Conclusions: The findings elaborate that Tiansi Liquid can be utilized to manage depression, however, multiple molecular mechanisms of action could be proposed for this effect. The observed core mechanisms could be the sensory perception of pain, regulation of lipid transport and lipopolysaccharide-mediated signaling pathway.

Cardiovascular autonomic regulation, inflammation and pain in rheumatoid arthritis

Background

Rheumatoid arthritis (RA) is a chronic inflammatory condition characterised by reduced heart rate variability (HRV) of unknown cause. We tested the hypothesis that low HRV, indicative of cardiac autonomic cardiovascular dysfunction, was associated with systemic inflammation and pain. Given the high prevalence of hypertension (HTN) in RA, a condition itself associated with low HRV, we also assessed whether the presence of hypertension further reduced HRV in RA.

Methods

In RA-normotensive (n = 13), RA-HTN (n = 17), normotensive controls (NC; n = 17) and HTN (n = 16) controls, blood pressure and heart rate were recorded. Time and frequency domain measures of HRV along with serological markers of inflammation (high sensitivity C-reactive protein [hs-CRP], tumour necrosis factor-α [TNF-α] and interleukins [IL]) were determined. Reported pain was assessed using a visual analogue scale.

Results

Time (rMSSD, pNN50%) and frequency (high frequency power, low frequency power, total power) domain measures of HRV were lower in the RA, RA-HTN and HTN groups, compared to NC (p = 0.001). However, no significant differences in HRV were noted between the RA, RA-HTN and HTN groups. Inverse associations were found between time and frequency measures of HRV and inflammatory cytokines (IL-6 and IL-10), but were not independent after multivariable analysis. hs-CRP and pain were independently and inversely associated with time domain (rMMSD, pNN50%) parameters of HRV.

Conclusions

These findings suggest that lower HRV is associated with increased inflammation and independently associated with increased reported pain, but not compounded by the presence of HTN in patients with RA.

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Rheumatoid arthritis (RA) is a chronic inflammatory condition accompanied by low heart rate variability (HRV).

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Important autonomic-immune interactions are suggested, but have not been thoroughly examined in RA.

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We show that low HRV in RA is associated with increased serum inflammatory cytokine levels and patient-reported pain.

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In our patients with RA, reductions in HRV were not compounded by the presence of hypertension.

Central circuitry responsible for the divergent sympathetic responses to tonic muscle pain in humans

Experimentally induced tonic muscle pain evokes divergent muscle vasoconstrictor responses, with some individuals exhibiting a sustained increase in muscle sympathetic nerve activity (MSNA), and others a sustained decrease. These patterns cannot be predicted from an individual's baseline physiological or psychological measures. The aim of this study was to investigate whether the different muscle sympathetic responses to tonic muscle pain were associated with differential changes in regional brain activity. Functional magnetic resonance imaging (fMRI) of the brain was performed concurrently with microelectrode recording of MSNA from the peroneal nerve during a 40-min infusion of hypertonic saline into the ipsilateral tibialis anterior muscle. MSNA increased in 26 and decreased in 11 of 37 subjects during tonic muscle pain. Within the prefrontal and cingulate cortices, precuneus, nucleus accumbens, caudate nucleus, and dorsomedial hypothalamus, blood oxygen level dependent (BOLD) signal intensity increased in the increasing-MSNA group and remained at baseline or decreased in the decreasing-MSNA group. Similar responses occurred in the dorsolateral pons and in the region of the rostral ventrolateral medulla. By contrast, within the region of the dorsolateral periaqueductal gray (dlPAG) signal intensity initially increased in both groups but returned to baseline levels only in the increasing-MSNA group. These results suggest that the divergent sympathetic responses to muscle pain result from activation of a neural pathway that includes the dlPAG, an area thought to be responsible for the behavioral and cardiovascular responses to psychological rather than physical stressors. Hum Brain Mapp 38:869-881, 2017. © 2016 Wiley Periodicals, Inc.

Pain perception: predictive value of sex, depression, anxiety, somatosensory amplification, obesity, and age

OBJECTIVE

Factors affecting pain sensation are still being investigated. In this study, we aimed to examine the effects of sex, age, body mass index (BMI), somatosensory amplification, anxiety, and depression on the perception of pain.

METHODS

Venipuncture was performed on 140 healthy individuals. All the cases completed a sociodemographic data form, visual analog scale (VAS), Beck Anxiety Inventory (BAI), Beck Depression Inventory, and Somatosensory Amplification Scale. Height and weight were also measured.

RESULTS

When both the sexes were compared, there was no difference in terms of VAS, BMI, age, and Beck Depression Inventory, but Somatosensory Amplification Scale and BAI were found to be higher in females. A correlation was found among VAS points, BAI, and BMI. The results of a regression analysis show that the BAI score is a predictor for the VAS score.

CONCLUSION

These results indicate that anxiety may be a predictor of pain, whereas sex, depression, somatosensory amplification, age, and weight do not appear to influence the perception of pain.

Sympathetic Responses to Noxious Stimulation of Muscle and Skin

Acute pain triggers adaptive physiological responses that serve as protective mechanisms that prevent continuing damage to tissues and cause the individual to react to remove or escape the painful stimulus. However, an extension of the pain response beyond signaling tissue damage and healing, such as in chronic pain states, serves no particular biological function; it is maladaptive. The increasing number of chronic pain sufferers is concerning, and the associated disease burden is putting healthcare systems around the world under significant pressure. The incapacitating effects of long-lasting pain are not just psychological – reflexes driven by nociceptors during the establishment of chronic pain may cause serious physiological consequences on regulation of other body systems. The sympathetic nervous system is inherently involved in a host of physiological responses evoked by noxious stimulation. Experimental animal and human models demonstrate a diverse array of heterogeneous reactions to nociception. The purpose of this review is to understand how pain affects the sympathetic nervous system by investigating the reflex cardiovascular and neural responses to acute pain and the long-lasting physiological responses to prolonged (tonic) pain. By observing the sympathetic responses to long-lasting pain, we can begin to understand the physiological consequences of long-term pain on cardiovascular regulation.

Inter-individual responses to experimental muscle pain: Baseline anxiety ratings and attitudes to pain do not determine the direction of the sympathetic response to tonic muscle pain in humans

We have recently shown that intramuscular infusion of hypertonic saline, causing pain lasting ~60min, increases muscle sympathetic nerve activity (MSNA) in one group of subjects, yet decreases it in another. Across subjects these divergent sympathetic responses to long-lasting muscle pain are consistent over time and cannot be foreseen on the basis of baseline MSNA, blood pressure, heart rate or sex. We predicted that differences in anxiety or attitudes to pain may account for these differences. Psychometric measures were assessed prior to the induction of pain using the State and Trait Anxiety Inventory (STAI), Pain Vigilance and Awareness Questionnaire (PVAQ), Pain Anxiety Symptoms Scale (PASS) and Pain Catastrophising Scale (PCS); PCS was also administered after the experiment. MSNA was recorded from the common peroneal nerve, before and during a 45-minute intramuscular infusion of hypertonic saline solution into the tibialis anterior muscle of 66 awake human subjects. Forty-one subjects showed an increase in mean burst amplitude of MSNA (172.8±10.6%) while 25 showed a decrease (69.9±3.8%). None of the measured psychological parameters showed significant differences between the increasing and the decreasing groups. We conclude that inter-individual anxiety or pain attitudes do not determine whether MSNA increases or decreases during long-lasting experimental muscle pain in healthy human subjects.