Prediction of chronic post-operative pain: Pre-operative DNIC testing identifies patients at risk

A Randomized Sham-Controlled Cross-Over Study on the Short-Term Effect of Non-Invasive Cervical Vagus Nerve Stimulation on Spinal and Supraspinal Nociception in Healthy Subjects

OBJECTIVE

The aim of the present study was to test the effects of vagus nerve stimulation (VNS) on the descending pain inhibition, quantified by the nociceptive flexor (RIII) reflex and the conditioned pain modulation (CPM) paradigm, and on supraspinal nociceptive responses, assessed by pain intensity and unpleasantness ratings and late somatosensory evoked potentials (SEPs), in healthy subjects.

BACKGROUND

Non-invasive vagus nerve stimulation (nVNS) showed promising effects on headache and pain treatment. Underlying mechanisms are only incompletely understood but may include the activation of the descending pain inhibitory system and/or the modification of emotional responses to pain.

METHODS

Twenty-seven adult, healthy, and pain-free subjects participated in this double-blind cross-over study conducted at a university research center. They received 4 minutes of cervical nVNS or sham stimulation in randomized order. RIII reflexes, pain ratings, and SEPs were assessed before, during, and 5, 15, 30, and 60 minutes after nVNS/sham stimulation, followed by CPM testing. The primary outcome was the nVNS effect on the RIII reflex size. Three subjects were excluded after the preparatory session (before randomization), 1 subject was excluded after outlier analysis, leaving 23 for analysis.

RESULTS

RIII reflex areas were 917.1 ± 563.8 µV × ms (mean ± SD) before, 952.4 ± 467.4 µV × ms during and 929.2 ± 484.0 µV × ms immediately after nVNS and 858.4 ± 489.2 µV × ms before, 913.9 ± 539.7 µV × ms during and 862.4 ± 476.0 µV × ms after sham stimulation, revealing no differences between the immediate effects of nVNS and sham stimulation (F _[3,66]  = 0.67, P = .574). There also were no effects of nVNS over sham on RIII reflex areas up to 60 minutes after nVNS (F _[1.7,37.4]  = 1.29, P = .283). Similarly, there was no statistically significant effect of nVNS on pain intensity ratings and thresholds, RIII reflex thresholds, late SEP amplitudes, and the CPM effect, compared to sham. Pain unpleasantness ratings statistically significantly decreased from 4.4 ± 2.4 (NRS 0-10) to 4.1 ± 2.5 during nVNS compared to sham stimulation (F _[1,22]  = 8.74, P = .007), but there were no longer lasting effects (5-60 minutes after stimulation).

CONCLUSIONS

The present study does not support an acute effect of nVNS on descending pain inhibition, pain intensity perception or supraspinal nociception in healthy adults. However, there was a small effect on pain unpleasantness during nVNS, suggesting that nVNS may preferentially act on affective, not somatosensory pain components.

Kappa opioid signaling in the right central amygdala causes hind paw specific loss of diffuse noxious inhibitory controls in experimental neuropathic pain

Diffuse noxious inhibitory controls (DNICs) is a pain-inhibits-pain phenomenon demonstrated in humans and animals. Diffuse noxious inhibitory control is diminished in many chronic pain states, including neuropathic pain. The efficiency of DNIC has been suggested to prospectively predict both the likelihood of pain chronification and treatment response. Little is known as to why DNIC is dysfunctional in neuropathic pain. Here, we evaluated DNIC in the rat L5/L6 spinal nerve ligation (SNL) model of chronic pain using both behavioral and electrophysiological outcomes. For behavior, nociceptive thresholds were determined using response to noxious paw pressure on both hind paws as the test stimulus before, and after, injection of a conditioning stimulus of capsaicin into the left forepaw. Functionally, the spike firing of spinal wide-dynamic-range neuronal activity was evaluated before and during noxious ear pinch, while stimulating the ipsilateral paw with von Frey hairs of increased bending force. In both assays, the DNIC response was significantly diminished in the ipsilateral (ie, injured) paw of SNL animals. However, behavioral loss of DNIC was not observed on the contralateral (ie, uninjured) paw. Systemic application of nor-binaltorphimine, a kappa opioid antagonist, did not ameliorate SNL-induced hyperalgesia but reversed loss of the behavioral DNIC response. Microinjection of nor-binaltorphimine into the right central amygdala (RCeA) of SNL rats did not affect baseline thresholds but restored DNIC both behaviorally and electrophysiologically. Cumulatively, these data suggest that net enhanced descending facilitations may be mediated by kappa opioid receptor signaling from the right central amygdala to promote diminished DNIC after neuropathy.

Reliability of the conditioned pain modulation paradigm across three anatomical sites

Background and aims Conditioned Pain Modulation (CPM) is a measure of pain inhibition-facilitation in humans that may elucidate pain mechanisms and potentially serve as a diagnostic test. In laboratory settings, the difference between two pain measures [painful test stimulus (TS) without and with the conditioning stimulus (CS) application] reflects the CPM magnitude. Before the CPM test can be used as a diagnostic tool, its reliability on the same day (intra-session) and across multiple days (inter-session) needs to be known. Furthermore, it is important to determine the most reliable anatomical sites for both the TS and the CS. This study aimed to measure the intra-session and inter-session reliability of the CPM test paradigm in healthy subjects with the TS (pressure pain threshold-PPT) applied to three test sites: the face, hand, and dorsum of the foot, and the CS (cold pressor test-CPT) applied to the contralateral hand. Methods Sixty healthy participants aged 18-65 were tested by the same examiner on 3 separate days, with an interval of 2-7 days. On each day, testing was comprised of two identical experimental sessions in which the PPT test was performed on each of the three dominant anatomical sites in randomized order followed by the CPM test (repeating the PPT with CPT on the non-dominant hand). CPM magnitude was calculated as the percent change in PPT. The Intraclass Correlation Coefficient (ICC), Coefficient of Variation (CV), and Bland-Altman analyses were used to assess reliability. Results PPT relative reliability ranged from good to excellent at all three sites; the hand showed an intra-session ICC of 0.90 (0.84, 0.94) before CPT and ICC of 0.89 (0.83, 0.92) during CPT. The PPT absolute reliability was also high, showing a low bias and small variability when performed on all three sites; for example, CV of the hand intra-session was 8.0 before CPT and 8.1 during CPT. The relative reliability of the CPM test, although only fair, was most reliable when performed during the intra-session visits on the hand; ICC of 0.57 (0.37, 0.71) vs. 0.20 (0.03, 0.39) for the face, and 0.22 (0.01, 0.46) for the foot. The inter-session reliability was lower in all three anatomical sites, with the best reliability on the hand with an ICC of 0.40 (0.23, 0.55). The pattern of absolute reliability of CPM was similar to the relative reliability findings, with the reliability best on the hand, showing lower intra-session and inter-session variability (CV% = 43.5 and 51.5, vs. 70.1 and 73.1 for the face, and 75.9 and 78.9 for the foot). The CPM test was more reliable in women than in men, and in older vs. younger participants. Discussion The CPM test was most reliable when the TS was applied to the dominant hand and CS performed on the contralateral hand. These data indicate that using the CS and TS in the same but contralateral dermatome in CPM testing may create the most reliable results.

The effects of recovery sleep on pain perception: A systematic review

Experimental studies highlight profound effects of sleep disruptions on pain, showing that sleep deprivation (SD) leads to hyperalgesic pain changes. On the other hand, given that sleep helps normalizing bodily functions, a crucial role of restorative sleep in the overnight restoration of the pain system seems likely. Thus, a systematic review of experimental studies on effects of recovery sleep (RS; subsequently to SD) on pain was performed with the aim to check whether RS resets hyperalgesic pain changes occurring due to SD. Empirical animal and human studies including SD-paradigms, RS and pain assessments were searched in three databases (PubMed, Web of Science, PsycINFO) using a predefined algorithm. 29 studies were included in this review. Most results indicated a reset of enhanced pain sensitivity and vulnerability following RS, especially when total SD was implemented and pressure pain or painful symptoms (human studies) were assessed. Further research should focus on whether and how recovery is altered in chronic pain patients, as this yields implications for pain treatment by enhancing or stabilizing RS.

What goes up must come down: insights from studies on descending controls acting on spinal pain processing

Descending controls link higher processing of noxious signals to modulation of spinal cord responses to their noxious inputs. It has become possible to study one key inhibitory system in animals and humans using one painful stimulus to attenuate another distant response and so eliciting diffuse noxious inhibitory controls (DNIC) or the human counterpart, conditioned pain modulation (CPM). Here, we discuss the neuronal pathways in both species, their pharmacology and examine changes in descending controls with a focus on osteoarthritis. We will also discuss the opposing descending facilitatory system. Strong parallels between DNIC and CPM emphasize the possibility of forward and reverse translation.

Contribution of Baroreceptor Function to Pain Perception and Perioperative Outcomes

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain homeostasis by coordinating physiologic responses to external and internal stimuli. While it is recognized that carotid and cardiopulmonary baroreceptor reflexes modulate autonomic output to mitigate excessive fluctuations in arterial blood pressure and to maintain intravascular volume, increasing evidence suggests that baroreflex pathways also project to key regions of the central nervous system that regulate somatosensory, somatomotor, and central nervous system arousal. In addition to maintaining autonomic homeostasis, baroreceptor activity modulates the perception of pain, as well as neuroimmune, neuroendocrine, and cognitive responses to physical and psychologic stressors. This review summarizes the role that baroreceptor pathways play in modulating acute and chronic pain perception. The contribution of baroreceptor function to postoperative outcomes is also presented. Finally, methods that enhance baroreceptor function, which hold promise in improving postoperative and pain management outcomes, are presented.

Reference values of conditioned pain modulation

Background and aims Endogenous pain modulation can be studied in humans by conditioned pain modulation (CPM): pain induced by a test stimulus is attenuated by a distantly applied noxious conditioning stimulus. The detection of impaired CPM in individual patients is of potential importance to understand the pathophysiology and predict outcomes. However, it requires the availability of reference values. Methods We determined reference values of CPM in 146 pain-free subjects. Pressure and electrical stimulation were the test stimuli. For electrical stimuli, we recorded both pain threshold and threshold for the nociceptive withdrawal reflex. Cold pressor test was the conditioning stimulus. The 5th, 10th and 25th percentiles for the three tests were computed by quantile regression analyses. Results The average thresholds increased after the conditioning stimulus for all three tests. However, a subset of subjects displayed a decrease in thresholds during the conditioning stimulus. This produced negative values for most of the computed percentiles. Conclusions This study determined percentile reference values of CPM that can be used to better phenotype patients for clinical and research purposes. The negative value of percentiles suggests that a slightly negative CPM effect can be observed in pain-free volunteers. Implications Pain facilitation rather than inhibition during the conditioning stimulus occurs in some pain-free volunteers and may not necessarily represent an abnormal finding.

Chemotherapy-induced peripheral neuropathy: where are we now?

Chemotherapy-induced peripheral neuropathy (CIPN) is a major challenge, with increasing impact as oncological treatments, using potentially neurotoxic chemotherapy, improve cancer cure and survival. Acute CIPN occurs during chemotherapy, sometimes requiring dose reduction or cessation, impacting on survival. Around 30% of patients will still have CIPN a year, or more, after finishing chemotherapy. Accurate assessment is essential to improve knowledge around prevalence and incidence of CIPN. Consensus is needed to standardize assessment and diagnosis, with use of well-validated tools, such as the EORTC-CIPN 20. Detailed phenotyping of the clinical syndrome moves toward a precision medicine approach, to individualize treatment. Understanding significant risk factors and pre-existing vulnerability may be used to improve strategies for CIPN prevention, or to use targeted treatment for established CIPN. No preventive therapies have shown significant clinical efficacy, although there are promising novel agents such as histone deacetylase 6 (HDAC6) inhibitors, currently in early phase clinical trials for cancer treatment. Drug repurposing, eg, metformin, may offer an alternative therapeutic avenue. Established treatment for painful CIPN is limited. Following recommendations for general neuropathic pain is logical, but evidence for agents such as gabapentinoids and amitriptyline is weak. The only agent currently recommended by the American Society of Clinical Oncology is duloxetine. Mechanisms are complex with changes in ion channels (sodium, potassium, and calcium), transient receptor potential channels, mitochondrial dysfunction, and immune cell interactions. Improved understanding is essential to advance CIPN management. On a positive note, there are many potential sites for modulation, with novel analgesic approaches.

Impact of Therapeutic Interventions on Pain Intensity and Endogenous Pain Modulation in Knee Osteoarthritis: A Systematic Review and Meta-analysis

OBJECTIVE

To study the impact of therapeutic interventions on pain analgesia and endogenous pain modulation in knee osteoarthritis (KOA).

DESIGN

Systematic review and meta-analysis.

METHODS

We searched for KOA randomized clinical trials and observational studies with data on therapeutic interventions comparing pain intensity, temporal summation (TS), and conditioned pain modulation (CPM) scores relative to control. These data were pooled as Hedge's g. To study the relationship between pain intensity and TS/CPM, we performed metaregression with 10,000 Monte-Carlo permutations.

RESULTS

We reviewed 11 studies (559 participants). On studying all the interventions together, we found no significant changes in pain modulation, TS, or CPM. Our findings show that this lack of difference is likely because surgical and nonsurgical interventions resulted in contrary effects. Metaregression significantly correlated pain reduction with normalization of TS and CPM.

CONCLUSIONS

We demonstrate an association between pain reduction and TS/CPM normalization. Though we cannot directly compare these interventions, the results allow us to draw hypotheses on potential practice schemas. Recovering defective endogenous pain modulation mechanisms may help establish long-term analgesia. However, to validate these paradigms as robust clinical biomarkers, further investigation into their mechanisms would be necessary. The registration number for this review is CRD42017072066.

Incidence of contralateral neurosensitive changes and persistent postoperative pain 6 months after mastectomy: A prospective, observational investigation

Mirror image sensory dysfunction (MISD) after breast surgery has not yet been studied. This prospective observational study aimed to determine the incidence of MISD, persistent postoperative pain (PPP) and mirror image pain (MIP) during 6 months after total unilateral mastectomy.Visual analogue scale (VAS), Neuropathic Pain Symptom Inventory (NPSI), Pain Catastrophizing Scale (PCS), Hospital Anxiety and Depression Scale (HADS), Life orientation test (LOT) and Quantitative Sensory Testing (QST) (in ipsi and contralateral breast, axilla and thigh) were recorded. VAS > 3 at 1, 3, and 6 postoperative months was considered PPP. Contralateral changes of QST at any time was considered indicative of MISD and spontaneous contralateral VAS ≥ 1 as MIP.Sixty-four patients were included. PPP at 1, 3 and 6 months was 18.8%, 56.2%, and 21.3%, respectively Ten patients presented MIP. MISD was detected in 79.7% patients in contralateral breast and 62.5% in contralateral axilla. Furthermore, changes in QST were present in 39.1% of patients in thigh. Electronic Von Frey (EVF) changes in both contralateral breast and axilla, and in thigh significantly diminished at all postoperative times. Changes of postoperative EVF ≥ 20% in contralateral breast were associated to higher VAS values. NPSI scores were significantly higher at all postoperative times. At 1 month, PCS, depression HADS subscale and LOT scores were significantly worse than all the other periods.MISD incidence was almost 80%, and 15.6% of patients showed spontaneous contralateral VAS ≥ 1. At 6 months 21.3% of patients manifested PPP. The worst alteration of factors related to PPP occurred at 1 postoperative month. Most consistent QST was EVF.

Impaired hemodynamic activity in the right dorsolateral prefrontal cortex is associated with impairment of placebo analgesia and clinical symptoms in postherpetic neuralgia

The dorsolateral prefrontal cortex (dlPFC) is functionally linked to the descending pain modulation system and has been implicated in top down pain inhibition, including placebo analgesia. Therefore, functions of the dlPFC may be impaired in patients with chronic pain. Postherpetic neuralgia (PHN) is one of several syndromes with chronic neuropathic pain. In the present study, we investigated possible dysfunction of the dlPFC in chronic pain using patients with PHN. In a conditioning phase, heathy controls (n = 15) and patients with PHN (n = 7) were exposed to low (LF) and high (HF) frequency tones associated with noxious stimuli: weak (WS) and strong (SS) electrical stimulation, respectively. After the conditioning, cerebral hemodynamic activity was recorded from the bilateral dlPFC while the subjects were subjected to the cue tone-noxious electrical stimulation paradigm, in which incorrectly cued noxious stimuli were sometimes delivered to induce placebo and nocebo effects. The results indicated that hemodynamic responses to the LF tone in the right dlPFC was significantly lower in patients with PHN compared to the healthy controls. Furthermore, the same hemodynamic responses in the right dlPFC were correlated with placebo effects. In addition, clinical symptoms of PHN were negatively correlated to cerebral hemodynamic responses in the right dlPFC and magnitudes of the placebo effects. The results suggest that the right dlPFC, which is closely associated with the descending pain modulation system, is disturbed in PHN.

Spinal Activation of Tropomyosin Receptor Kinase-B Recovers the Impaired Endogenous Analgesia in Neuropathic Pain Rats

BACKGROUND

Although endogenous analgesia plays an important role in controlling pain states, chronic pain patients exhibit decreased endogenous analgesia compared to healthy individuals. In rats, noxious stimulus-induced analgesia (NSIA), which is an indicator of endogenous analgesia, diminished 6 weeks after spinal nerve ligation (SNL6W). A recent study in rats with deleted noradrenergic fibers demonstrated that the noradrenergic fibers were essential to NSIA. It has also been reported that brain-derived neurotrophic factor increased spinal noradrenergic fibers. Therefore, this study examined the effect of TrkB activation, which is the receptor for brain-derived neurotrophic factor, on impaired NSIA in SNL6W rats. In addition, we also examined the effect of endogenous analgesia on acute incisional pain.

METHODS

After 5 daily intraperitoneal injections of 7,8-dihydroxyflavone (7,8-DHF, TrkB agonist, 5 mg/kg), NSIA was examined by measuring the withdrawal threshold increment in the left (contralateral to nerve ligation) hindpaw at 30 minutes after capsaicin injection (250 μg) in the forepaw. K252a (TrkB antagonist, 2 μg) was administrated intrathecally for 5 days. Idazoxan (α2 adrenoceptor antagonist, 30 μg), atropine (muscarinic antagonist, 30 μg), and propranolol (nonselective β adrenoceptor antagonist, 30 μg) were administered intrathecally for 15 minutes before capsaicin injection. Microdialysis and immunohistochemistry were performed to examine the noradrenergic plasticity in the spinal dorsal horn. A hindpaw incision was performed on the left (contralateral to nerve ligation) hindpaw. Data were analyzed by 1-way analyses of variance or 2-way repeated-measures 1-way analysis of variance followed by a Student t test with Bonferroni correction.

RESULTS

Five daily intraperitoneal injections of 7,8-DHF restored the attenuated NSIA in SNL6W rats (n = 7, P = .002; estimated treatment effect [95% CI]: 62.9 [27.0-98.7] g), with this effect blocked by 5 daily intrathecal coadministrations of K252a (n = 6, P < .001; -57.8 [-78.3 to -37.2] g). This effect was also inhibited by a single intrathecal administration of idazoxan (n = 8, P < .001; -61.6 [-92.4 to -30.9] g) and atropine (n = 8, P = .003; -52.6 [-73.3 to -31.9] g), but not by propranolol. Furthermore, 7,8-DHF increased the noradrenergic fiber in the spinal dorsal horn and the noradrenaline release in response to the capsaicin injection in the forepaw in SNL6W rats. In addition, repeated injections of 7,8-DHF prevented delayed recovery from incisional pain in SNL6W rats.

CONCLUSIONS

Spinal activation of TrkB may recover the attenuated endogenous analgesia by improving the adrenergic plasticity, thereby leading to prevention of pain prolongation after surgery.

Cannabinoid effects on responses to quantitative sensory testing among individuals with and without clinical pain: a systematic review

There has been an explosion of interest in the utility of cannabinoids as potential analgesics. This systematic review critically synthesizes the evidence for cannabinoid analgesic effects on quantitative sensory testing outcomes in both healthy adults and patients with chronic noncancer pain. Our systematic review protocol is preregistered on PROSPERO (CRD42018117367). An electronic search was made in PsycINFO, Cochrane, Google Scholar, Embase, and Pubmed of all literature published until August 2018. Of the 1217 studies found from the search, a total 39 placebo-controlled studies that met the eligibility criteria were synthesized for this study. Because of substantial heterogeneity of study designs, populations, cannabinoid compounds, and quantitative sensory testing outcomes, meta-analysis was not conducted. More consistent evidence of cannabinoid analgesia was observed for inhaled cannabis than synthetic cannabinoids. Analgesic effects were most commonly observed in tests of cold pain sensitivity, and hyperalgesic effects were most commonly observed in tests of electrical stimulation. Patterns of findings from studies with healthy subjects did not substantively differ from those with chronic noncancer pain. However, these observations are qualified by the high degree of inconsistency across studies and methodological heterogeneity. We offer recommendations for future studies to improve study rigor and reproducibility.

Translational issues in precision medicine in neuropathic pain

Neuropathic pain remains poorly treated, with most new drugs falling through the translational gap. The traditional model of bench-to-bedside research has relied on identifying new mechanisms/targets in animal models and then developing clinical applications. Several have advocated bridging the translational gap by beginning with clinical observations and back-translating to animal models for further investigation of mechanisms. There is good evidence that phenotyping of patients through quantitative sensory testing can lead to improved treatment selection and hence improved patient outcomes. This practice has been widely adopted in clinical investigations, but its application in preclinical research is not mainstream. In this review, we retrospectively examine our historical rodent data sets with the aim of reconsidering drug effects on sensory neuronal endpoints, their alignment with clinical observations, and how these might guide future clinical studies.

No Differences in the Prevalence and Intensity of Chronic Postsurgical Pain Between Laparoscopic Hysterectomy and Abdominal Hysterectomy: A Prospective Study

Objective

To compare the prevalence and characteristics of chronic postsurgical pain (CPSP) between laparoscopic hysterectomy (LH) and abdominal hysterectomy (AH) groups 3, 6, and 12 months after surgery, and to assess the impact of pain on the activities of daily living (ADL) of patients.

Methods

The demographic characteristics, intraoperative clinical factors, and postoperative pain score were collected prospectively in patients scheduled for elective LH or AH for benign disease at our institution from July 2014 to June 2015. Patients were interviewed by telephone and followed up for pain assessment 3, 6, and 12 months after surgery. The prevalence, intensity, and specific locations of pain, as well as analgesic administration and impact on the ADL, were included in the questionnaire.

Results

The results from 406 patients (225 patients in the LH group and 181 patients in the AH group) were obtained. Three months after surgery, the prevalence of CPSP was 20.9% in the LH group and 20.4% in the AH group. At 6 months, the prevalence of pain declined to 11.6% in the LH group and 9.4% in the AH group. At 12 months after surgery, only 13 (5.8%) patients in the LH group and 11 (6.1%) patients in the AH group complained about persistent pain. The prevalence of CPSP, as well as the average numerical rating scale pain scores at rest and during movement, during 12 months after surgery were not significantly different between the groups. CPSP after hysterectomy exhibited a negative impact on the ADL.

Conclusion

The prevalence and intensity of CPSP were not significantly different between patients undergoing LH or AH within 12 months after surgery. A tendency towards a reduction in chronic pain over time was documented. Chronic post-hysterectomy pain exhibited a negative impact on the ADL.

Effects of chronic pain history on perceptual and cognitive inhibition

Measures of sensory and cognitive inhibition were obtained from university students with and without a history of chronic pain. The form of sensory inhibition measured was diffuse noxious inhibitory controls (DNIC), the capacity of a painful stimulus to reduce the subjective intensity of a second stimulus delivered to a remote body site. To measure cognitive inhibition, the Stroop effect was used. Participants with a history of chronic pain showed less DNIC (i.e., less sensory inhibition) than the healthy controls, but had a smaller Stroop effect (indicating greater cognitive inhibition). The fact that chronic pain history is associated with opposite changes in these two measures casts doubt on the view that the two inhibitory processes are related. Scores on each experimental measure were equivalent in pain-history subjects with ongoing chronic pain and those whose chronic pain had resolved. This equivalence suggests that chronic pain in childhood or adolescence may have lingering effects on sensory and cognitive inhibition.

Human experimental pain models

Pain is an unpleasant sensory experience, associated with existing or potential tissue damage. It has also strong cognitive and emotional components. Stimuli that causes pain goes through process of nociception, which includes transduction, transmission, modulation and perception of said stimuli. Depending on the type of stimuli, we can classify human experimental pain models into mechanical, electrical, thermal and chemical. Information about pain mechanisms can be obtained from the following: 1) in vitro studies, 2) animal experiments, 3) human experimental pain studies and 4) clinical studies. Chosing the appropriate method for pain evaluation is a key step in the design of pain studies. Combining it with different electro-physiological and imaging methods, it can provide better objectivity and quantification of pain mechanisms. Focus in experimental pain studies is slowly shifting from static parameters of pain, such as pain threshold and maximum tolerance, to dynamic parameters, which can give us valuable insight in function of endogenous analgesic systems. This can be done using conditioned pain modulation. Using experimental pain on healthy voulenteers is key step in switching from animal models to clinical studies, foremost for validization of data from animals, making them important in translational research. Results from experimental pain studies can help us in understanding nociceptive mechanisms of acute and chronic pain, alongside development of new therapeutic modalities.

Determining Real Change in Conditioned Pain Modulation: A Repeated Measures Study in Healthy Volunteers

Conditioned pain modulation (CPM) is a potentially useful biomarker in pain populations; however, a statistically robust interpretation of change scores is required. Currently, reporting of CPM does not consider measurement error. Hence, the magnitude of change representing a "true" CPM effect is unknown. This study determined the standard error of measurement (SEM) and proportion of healthy participants showing a "true" CPM effect with a standard CPM paradigm. Fifty healthy volunteers participated in an intersession reliability study using pressure pain threshold (PPT) test stimulus and contact heat, cold water, and sham conditioning stimuli. Baseline PPTs were used to calculate SEM and >±2 × SEM to determine CPM effect. SEM for PPT was .21 kg/cm². An inhibitory CPM effect (>+2 SEM) was elicited in 59% of subjects in response to cold stimulus; in 44% to heat. Intrasession and intersession reliability of within-subject CPM response was poor (kappa coefficient <.36). Measurement error is important in determining CPM effect and change over time. Even when using reliable test stimuli, and incorporating measures to limit bias and error, CPM intersession reliability was fair and demonstrated a large degree of within-subject variation. Determining "true" change in CPM will underpin future interrogations of intraindividual differences in CPM. PERSPECTIVE: This study used a distribution-based statistical approach to identify real change in CPM, based on the SEM for the test stimulus. Healthy volunteers demonstrate substantial within-subject variation; CPM effect was paradigm dependent at intrasession testing and unstable to the same paradigm at intersession testing.

Modified Biofeedback (Conditioned Biofeedback) Promotes Antinociception by Increasing the Nociceptive Flexion Reflex Threshold and Reducing Temporal Summation of Pain: A Controlled Trial

This study examined whether a modified version of biofeedback (ie, Conditioned Biofeedback) that incorporated placebo analgesia-like manipulations could promote antinociception in healthy, pain-free participants. During Conditioned Biofeedback (n = 28), sympathetic arousal level was displayed visually and participants were asked to reduce it while they received painful electric stimulations that were surreptitiously controlled by their arousal level. Thus, electric pain decreased as arousal decreased to associate successful arousal-reduction/relaxation with pain relief, and to promote expectations for future pain relief. A Biofeedback Only group (n = 24) controlled for the general effects of biofeedback/relaxation. A Biofeedback+Shock group (n = 21) controlled for the effects of practicing biofeedback during painful shocks. Nociceptive flexion reflex (NFR) threshold and temporal summation of pain (TS-pain) were used to assess changes in spinal nociception and pain facilitation, respectively. Results indicated all groups showed pre- to postbiofeedback increases in NFR threshold, but only the Conditioned Biofeedback group showed pre- to postbiofeedback reductions in TS-pain. Moreover, Conditioned Biofeedback resulted in a persistent (prebiofeedback) increase in NFR threshold across sessions, whereas Biofeedback Only resulted in a persistent (prebiofeedback) decrease in TS-pain. In sum, Conditioned Biofeedback may promote antinociception in healthy participants thus reducing risk for chronic pain. The study was registered prospectively on ClinicalTrials.gov (TU1560). PERSPECTIVE: A modified version of biofeedback that employs placebo analgesia manipulations was successful in increasing descending inhibition and reducing pain facilitation in healthy volunteers. As a result, it may be an effective means of reducing risk of future chronic pain onset by promoting an antinociceptive pain profile.

Acute and chronic pain in orofacial trauma patients

Trauma or injury to the dentition and supporting tissues is associated with pain and discomfort, as expected, that may present immediately, shortly afterwards, or within a few days. Pain is an essential response to injury because it allows the organism to develop avoidance behavior to potential threats and helps the organism to avoid usage of the injured organ during the healing process. Not only does external trauma induce pain, but also essential invasive dental procedures such as extractions, dental implant insertions, root canal treatments, and oral surgeries are accompanied by similar post-surgical (post-traumatic) pain. The pain intensity after trauma varies and does not always correlate with the extent of injury. Trauma to the orofacial region or the teeth may also indirectly affect and induce pain in other orofacial structures such as the masticatory muscles, the temporomandibular joint, and even the cervical spine. In most cases, the pain will resolve as soon as healing of the affected tissue occurs or after dental and routine palliative treatment. In a limited number of cases, the pain persists beyond healing and evolves into a chronic pain state. Chronic pain in the orofacial region presents diagnostic and management challenges. Misdiagnosis or delayed diagnosis of the oral chronic pain condition may lead to unnecessary dental treatment. This article will discuss diagnosis and treatment for acute and chronic pain as well as potential mechanisms involved in the undesirable transition from acute to chronic pain.

The effect of spinal manipulative therapy on heart rate variability and pain in patients with chronic neck pain: a randomized controlled trial

Background

Recent experimental research has suggested that spinal manipulative therapy (SMT) may reduce pain through modulation of the ascending pain signals and/or the central pain-regulating mechanisms. People with persistent neck pain (NP) have also been found to have disturbances in autonomic nervous system (ANS) regulation. A common way to study the ANS is to measure heart rate variability (HRV). It is not known whether deviations in HRV are related to changes in pain perception or to the treatment response to SMT.

Commonly, an individual in pain will experience pain reduction when exposed to a second pain stimulus, a mechanism known as conditioned pain modulation (CPM). Patients with persistent pain have been found to have a reduced CPM reaction. It is not known whether this is predictive of treatment response to SMT.

The aim of the study is to examine the effects of SMT on HRV and pain. Further, a secondary aim is to test whether a CPM test can be used to predict treatment response in a population of patients with recurrent and persistent NP.

Method/design

A multicentre randomized controlled clinical trial will be carried out in multidisciplinary primary care clinics. This setting is chosen to minimize bias resulting from patient preference for the treatment modality and provider.

The subjects are either self-referred or referred from other health care practitioners locally. The treatment modalities are two well-known interventions for NP; SMT and stretching exercises compared to stretching exercises alone.

HRV will be measured using a portable heart monitor. The subjective pain experience will be investigated by assessing pain intensity and the affective quality of pain. CPM will be measured with a standardized cold pressor test. Measurements will be performed three times during a 2-week treatment series.

Discussion

The study will utilize normal clinical procedures, which should aid the transferability and external validity of the results. The study will provide knowledge regarding the underlying mechanisms of the effects of SMT. Furthermore, the study will examine whether a CPM test is predictive of treatment outcome in a population of patients with recurrent and persistent NP.

Trial registration

ClinicalTrials.gov, NCT03576846. Registered on 3 July 2018.

Electronic supplementary material

The online version of this article (10.1186/s13063-019-3678-8) contains supplementary material, which is available to authorized users.

Association Between the 2011 Fibromyalgia Survey Criteria and Multisite Pain Sensitivity in Knee Osteoarthritis

OBJECTIVES

The present study evaluated the relationship between the 2011 American College of Rheumatology fibromyalgia (FM) survey criteria and quantitative sensory testing (QST).

MATERIALS AND METHODS

Patients with knee osteoarthritis scheduled to undergo knee arthroplasty completed the FM survey criteria and self-report measures assessing clinical symptoms. Patients also underwent a battery of QST procedures at the surgical knee and remote body sites, including pressure algometry, conditioned pain modulation, and temporal summation. All assessments were completed before surgery. FM survey criteria were used to calculate a continuous FM score indicating FM severity.

RESULTS

A total of 129 patients were analyzed. Of these, 52.7% were female, 93.8% were Caucasian, and 3.8% met the FM survey criteria for FM classification. Mean age for females (63.6 y) and males (64.7 y) was similar. Females and males differed significantly in nearly every outcome, including FM severity, clinical pain, anxiety, depression, and pressure pain sensitivity. In females, FM scores significantly correlated with pressure pain sensitivity, but not conditioned pain modulation or temporal summation, such that increased sensitivity was associated with greater FM severity at all body sites examined. In addition, as FM scores increased, the association between pain sensitivity at the surgical knee and pain sensitivity at remote body sites also increased. No relationship between FM score and QST was observed in males.

DISCUSSION

We demonstrated an association between diffuse hyperalgesia as measured by QST and FM severity in females with knee osteoarthritis. These results suggest that the FM survey criteria may represent a marker of pain centralization in females with potential utility in clinical decision making.

A study of cortical and brainstem mechanisms of diffuse noxious inhibitory controls in anaesthetised normal and neuropathic rats

Diffuse noxious inhibitory controls (DNIC) are a mechanism of endogenous descending pain modulation and are deficient in a large proportion of chronic pain patients. However, the pathways involved remain only partially determined with several cortical and brainstem structures implicated. This study examined the role of the dorsal reticular nucleus (DRt) and infralimbic (ILC) region of the medial prefrontal cortex in DNIC. In vivo electrophysiology was performed to record from dorsal horn lamina V/VI wide dynamic range neurones with left hind paw receptive fields in anaesthetised sham‐operated and L5/L6 spinal nerve‐ligated (SNL) rats. Evoked neuronal responses were quantified in the presence and absence of a conditioning stimulus (left ear clamp). In sham rats, DNIC were reproducibly recruited by a heterotopically applied conditioning stimulus, an effect that was absent in neuropathic rats. Intra‐DRt naloxone had no effect on spinal neuronal responses to dynamic brush, punctate mechanical, evaporative cooling and heat stimuli in sham and SNL rats. In addition, intra‐DRt naloxone blocked DNIC in sham rats, but had no effect in SNL rats. Intra‐ILC lidocaine had no effect on spinal neuronal responses to dynamic brush, punctate mechanical, evaporative cooling and heat stimuli in sham and SNL rats. However, differential effects were observed in relation to the expression of DNIC; intra‐ILC lidocaine blocked activation of DNIC in sham rats but restored DNIC in SNL rats. These data suggest that the ILC is not directly involved in mediating DNIC but can modulate its activation and that DRt involvement in DNIC requires opioidergic signalling.

Resting blood pressure modulates chest pain intensity in patients with acute myocardial infarction

Introduction and Objectives

Animal models and human studies show that resting blood pressure (BP) is inversely associated with pain sensitivity. The phenomenon of "hypertension-associated hypoalgesia" was proposed as a possible explanation for the intervariability in pain perception. Given that a portion of patients with acute myocardial infarction (AMI) do not experience significant pain, we used the model of severe cardiac ischemia to explore whether BP affects the intensity of chest pain.

Methods

Patients with AMI admitted to the cardiac intensive care unit with coronary catheterization-proven completely occluded coronary artery were included (n = 67). Resting BP at admission and 5 days after AMI was obtained. Participants reported chest pain intensity and underwent psychophysical evaluation including pain ratings for pressure, heat, and pinprick stimuli as well as temporal summation and conditioned pain modulation paradigms.

Results

Patients with lower systolic BP (≤120 mm Hg) vs higher (≥140 mm Hg) reported higher chest pain scores at symptom onset (82.3 vs 61.7, P = 0.048) and during peak AMI (82.8 vs 57.5, P = 0.019). Higher pain ratings in response to pinprick stimulus were associated with lower BP at admission (analysis of variance P = 0.036). Patients with hypertension demonstrated lower pain sensitivity in response to pressure stimulation (531.7 ± 158.9 kPa/s vs 429.1 ± 197.4). No significant associations were observed between BP and the other assessed psychophysical measures.

Conclusion

Study findings reinforce the phenomenon of hypertension-associated hypoalgesia through characterization of the association between BP and clinical pain experiences at onset and during AMI in a model of acute clinical pain.

Cerebral peak alpha frequency reflects average pain severity in a human model of sustained, musculoskeletal pain

Heightened pain sensitivity, the amount of pain experienced in response to a noxious event, is a known risk factor for development of chronic pain. We have previously reported that pain-free, sensorimotor peak alpha frequency (PAF) is a reliable biomarker of pain sensitivity for thermal, prolonged pains lasting tens of minutes. To test whether PAF can provide information about pain sensitivity occurring over clinically relevant timescales (i.e., weeks), EEG was recorded before and while participants experienced a long-lasting pain model, repeated intramuscular injection of nerve growth factor (NGF), that produces progressively developing muscle pain for up to 21 days. We demonstrate that pain-free, sensorimotor PAF is negatively correlated with NGF pain sensitivity; increasingly slower PAF is associated with increasingly greater pain sensitivity. Furthermore, PAF remained stable following NGF injection, indicating that the presence of NGF pain for multiple weeks is not sufficient to induce the PAF slowing reported in chronic pain. In total, our results demonstrate that slower pain-free, sensorimotor PAF is associated with heightened sensitivity to a long-lasting musculoskeletal pain and also suggest that the apparent slowing of PAF in chronic pain may reflect predisease pain sensitivity.NEW & NOTEWORTHY Pain sensitivity, the intensity of pain experienced after injury, has been identified as an important risk factor in the development of chronic pain. Biomarkers of pain sensitivity have the potential to ease chronic pain burdens by preventing disease emergence. In the current study, we demonstrate that the speed of pain-free, sensorimotor peak alpha frequency recorded during resting-state EEG predicts pain sensitivity to a clinically-relevant, human model of prolonged pain that persists for weeks.

The Decline of Endogenous Pain Modulation With Aging: A Meta-Analysis of Temporal Summation and Conditioned Pain Modulation

The purpose of this article was to examine age-related changes in conditioned pain modulation (CPM) and temporal summation (TS) of pain using meta-analytic techniques. Five electronic databases were searched for studies, which compared measures of CPM and TS among healthy, chronic pain-free younger, middle-aged, and older adults. Eleven studies were included in the final review for TS and 11 studies were included in the review of CPM. The results suggested a moderate magnitude of difference in TS among younger and middle-aged/older adults, with the older cohorts exhibiting enhanced TS of pain. Considerable variability existed in the magnitude of the effect sizes, which was likely due to the different experimental methodologies used across studies (ie, interstimulus interval, stimulus type, and body location). In regards to CPM, the data revealed a large magnitude of difference between younger and older adults, with younger adults exhibiting more efficient pain inhibition. Differences in CPM between middle-aged and older adults were minimal. The magnitude of pain inhibition during CPM in older adults may depend on the use of concurrent versus nonconcurrent protocols. In summary, the data provided strong quantitative evidence of a general age-related decline in endogenous pain modulatory function as measured by TS and CPM. PERSPECTIVE: This review compared CPM and TS of pain among younger, middle-aged, and older adults. These findings enhance our understanding of the decline in endogenous pain modulatory function associated with normal aging.

Predicting the outcome of persistent sciatica using conditioned pain modulation: 1-year results from a prospective cohort study

Background and aims

Recovery in patients hospitalised with severe sciatica is unpredictable. Prognostic tools to aid clinicians in the early identification of patients at risk of developing chronic sciatic pain are warranted. Conditioned pain modulation (CPM) is a psychophysical measure of the endogenous pain modulatory pathways. Several studies have suggested CPM as a potentially important predictive biomarker for the development of chronic pain. The aim of the study was to determine whether CPM effect in patients still suffering from leg pain 6 weeks after hospital discharge for severe sciatica is associated with persistent leg pain at 12 months. A potential association would suggest that measuring CPM effect could be a valuable prognostic tool in the hospital management of sciatica.

Methods

A prospective cohort study in which CPM effect was measured 6 weeks after hospital discharge following an acute admission with sciatica as the main complaint. The impact of CPM effect on the outcome was analysed using logistic regression. The outcome measured was self-reported leg pain score of ≥1 in the past week on a 0–10 numeric rating scale (NRS) at 12 months post discharge.

Results

A total of 111 patients completed the entire study, 51 of whom received non-randomised surgical treatment. Crude and confounder adjusted analyses showed no significant association between CPM effect and leg-pain measured at 12 months, crude Odds Ratio 0.87, 95% CI 0.7–1.1, p = 0.23.

Conclusions

Our results suggest that CPM assessment has limited prognostic value for the long-term outcome in severe sciatica when measured 6 weeks after hospital discharge.

Implications

The present study adds important knowledge concerning the limited clinical use of late CPM testing in sciatica patients. The heterogeneity in patients, the wide range of treatments received and a generally favourable outcome are factors that may affect CPM’s clinical value as a prognostic factor for severe sciatica.

Quantitative sensory testing and predicting outcomes for musculoskeletal pain, disability, and negative affect: a systematic review and meta-analysis

Hypersensitivity due to central pain mechanisms can influence recovery and lead to worse clinical outcomes, but the ability of quantitative sensory testing (QST), an index of sensitisation, to predict outcomes in chronic musculoskeletal disorders remains unclear. We systematically reviewed the evidence for ability of QST to predict pain, disability, and negative affect using searches of CENTRAL, MEDLINE, EMBASE, AMED, CINAHL, and PubMed databases up to April 2018. Title screening, data extraction, and methodological quality assessments were performed independently by 2 reviewers. Associations were reported between baseline QST and outcomes using adjusted (β) and unadjusted (r) correlations. Of the 37 eligible studies (n = 3860 participants), 32 were prospective cohort studies and 5 randomised controlled trials. Pain was an outcome in 30 studies, disability in 11, and negative affect in 3. Meta-analysis revealed that baseline QST predicted musculoskeletal pain (mean r = 0.31, 95% confidence interval [CI]: 0.23-0.38, n = 1057 participants) and disability (mean r = 0.30, 95% CI: 0.19-0.40, n = 290 participants). Baseline modalities quantifying central mechanisms such as temporal summation and conditioned pain modulation were associated with follow-up pain (temporal summation: mean r = 0.37, 95% CI: 0.17-0.54; conditioned pain modulation: mean r = 0.36, 95% CI: 0.20-0.50), whereas baseline mechanical threshold modalities were predictive of follow-up disability (mean r = 0.25, 95% CI: 0.03-0.45). Quantitative sensory testing indices of pain hypersensitivity might help develop targeted interventions aiming to improve outcomes across a range of musculoskeletal conditions.

Impairment of Inhibition of Trigeminal Nociception via Conditioned Pain Modulation in Persons with Migraine Headaches

OBJECTIVE

To assess conditioned pain modulation efficiency in persons with and without migraine headaches.

DESIGN

Cross-sectional assessment of experimental pain.

SETTING

University campus and surrounding community in a large Midwestern US city.

SUBJECTS

Twenty-three adults with and 32 without a history of migraine headaches participated in the study. Participants were mostly female (N = 40) with an average age of 23 years.

METHODS

Four electrocutaneous stimulations of the supraorbital branch of the left trigeminal nerve were delivered at 150% of an individually determined pain threshold. Conditioned pain modulation was assessed by applying a noxious counterstimulus (forearm ischemia) and delivering four more electrocutaneous stimulations. After each stimulation, pain and the nociceptive blink reflex were assessed. Depression and pain catastrophizing were assessed to control for the potential influence of these variables on pain modulation.

RESULTS

Participants with and without migraine headaches had similar baseline pain responsivity, without significant differences in pain report or nociceptive blink reflexes. Pain report was inhibited by conditioned pain modulation in both the migraine and control groups. However, unlike nonmigraine controls, participants with migraines did not exhibit an inhibition of nociceptive blink reflexes during the ischemia task. This pattern persisted after controlling for level of pain catastrophizing and depression.

CONCLUSIONS

Migraine sufferers exhibited impaired conditioned pain modulation of the nociceptive blink reflex, suggesting a deficiency in inhibition of trigeminal nociception, which may contribute to the development of migraine headaches.

Psychophysical or spinal reflex measures when assessing conditioned pain modulation?

BACKGROUND

Assessing conditioning pain modulation (CPM) with spinal reflex measures may produce more objective and stable CPM effects than using psychophysical measures. The aim of the study was to compare the CPM effect and test-retest reliability between a psychophysical protocol with thermal test-stimulus and a spinal reflex protocol with electrical test-stimulus.

METHODS

Twenty-five healthy volunteers participated in two identical experiments separated by minimum 1 week. The thermal test-stimulus was a constant heat stimulation of 120 s on the subjects' forearm with continuous ratings of pain intensity on a 10 cm visual analogue scale. The electrical test-stimulus was repeated electrical stimulation on the arch of the foot for 120 s, which elicited a nociceptive withdrawal reflex recorded from the anterior tibial muscle. Conditioning stimulus was a 7°C water bath. Differences in the magnitude and test-retest reliability were investigated with repeated-measures analysis of variance and by relative and absolute reliability indices.

RESULTS

The CPM effect was -46% and 4.5% during the thermal and electrical test-stimulus (p < 0.001) respectively. Intraclass correlation coefficient of 0.5 and 0.4 was found with the electrical and thermal test-stimulus respectively. Wide limits of agreement were found for both the electrical (-3.4 to 3.8 mA) and the thermal test-stimulus (-3.2 to 3.6 cm).

CONCLUSIONS

More pronounced CPM effect was demonstrated when using a psychophysical protocol with thermal test-stimulus compared to a spinal reflex protocol with electrical test-stimulus. Fair relative reliability and poor absolute reliability (due to high intraindividual variability) was found in both protocols.

SIGNIFICANCE

The large difference in CPM effect between the two protocols suggests that the CPM effect relates to pain perception rather than nociception on the spinal level. Due to poor absolute intrarater reliability, we recommend caution and further research before using any of the investigated CPM protocols in clinical decision making on an individual level.

Neuropathic Pain in Taxane-Induced Peripheral Neuropathy: Evidence for Exercise in Treatment

One in 2 Canadians is expected to acquire cancer in their lifetime. Many cancers, including breast, ovarian, and lung cancer, are treated using taxane chemotherapy with curative intent. A major adverse effect with the use of taxane chemotherapeutic agents is taxane-induced peripheral neuropathy (TIPN). Both positive (spontaneous pain, heightened sensitivity with light touch, tingling, itching, burning) and negative (loss of touch, loss of hot/cold sensations, and loss of pain) sensory symptoms can be experienced in the hands and feet and worsen with increasing dose and treatment duration. The pathophysiology of TIPN is still unknown but likely involves multiple mechanisms, including microtubule impairment, neuroimmune and inflammatory changes, ion channel remodeling, impaired mitochondrial function, and genetic predisposition. This review highlights current theories on the pathophysiology for TIPN, the cellular responses thought to maintain neuropathic pain, and the growing support for exercise in the treatment and prevention of peripheral neuropathy and neuropathic pain in both animal and human models.

Sensory Modulation Disorder (SMD) and Pain: A New Perspective

Sensory modulation disorder (SMD) affects sensory processing across single or multiple sensory systems. The sensory over-responsivity (SOR) subtype of SMD is manifested clinically as a condition in which non-painful stimuli are perceived as abnormally irritating, unpleasant, or even painful. Moreover, SOR interferes with participation in daily routines and activities (Dunn, 2007; Bar-Shalita et al., 2008; Chien et al., 2016), co-occurs with daily pain hyper-sensitivity, and reduces quality of life due to bodily pain. Laboratory behavioral studies have confirmed abnormal pain perception, as demonstrated by hyperalgesia and an enhanced lingering painful sensation, in children and adults with SMD. Advanced quantitative sensory testing (QST) has revealed the mechanisms of altered pain processing in SOR whereby despite the existence of normal peripheral sensory processing, there is enhanced facilitation of pain-transmitting pathways along with preserved but delayed inhibitory pain modulation. These findings point to central nervous system (CNS) involvement as the underlying mechanism of pain hypersensitivity in SOR. Based on the mutual central processing of both non-painful and painful sensory stimuli, we suggest shared mechanisms such as cortical hyper-excitation, an excitatory-inhibitory neuronal imbalance, and sensory modulation alterations. This is supported by novel findings indicating that SOR is a risk factor and comorbidity of chronic non-neuropathic pain disorders. This is the first review to summarize current empirical knowledge investigating SMD and pain, a sensory modality not yet part of the official SMD realm. We propose a neurophysiological mechanism-based model for the interrelation between pain and SMD. Embracing the pain domain could significantly contribute to the understanding of this condition’s pathogenesis and how it manifests in daily life, as well as suggesting the basis for future potential mechanism-based therapies.

Prolonged Perioperative Use of Pregabalin and Ketamine to Prevent Persistent Pain after Cardiac Surgery

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Persistent postsurgical pain is common and affects quality of life. The hypothesis was that use of pregabalin and ketamine would prevent persistent pain after cardiac surgery.

Methods

This randomized, double-blind, placebo-controlled trial was undertaken at two cardiac surgery centers in the United Kingdom. Adults without chronic pain and undergoing any elective cardiac surgery patients via sternotomy were randomly assigned to receive either usual care, pregabalin (150 mg preoperatively and twice daily for 14 postoperative days) alone, or pregabalin in combination with a 48-h postoperative infusion of intravenous ketamine at 0.1 mg · kg−1 · h−1. The primary endpoints were prevalence of clinically significant pain at 3 and 6 months after surgery, defined as a pain score on the numeric rating scale of 4 or higher (out of 10) after a functional assessment of three maximal coughs. The secondary outcomes included acute pain, opioid use, and safety measures, as well as long-term neuropathic pain, analgesic requirement, and quality of life.

Results

In total, 150 patients were randomized, with 17 withdrawals from treatment and 2 losses to follow-up but with data analyzed for all participants on an intention-to-treat basis. The prevalence of pain was lower at 3 postoperative months for pregabalin alone (6% [3 of 50]) and in combination with ketamine (2% [1 of 50]) compared to the control group (34% [17 of 50]; odds ratio = 0.126 [0.022 to 0.5], P = 0.0008; and 0.041 [0.0009 to 0.28], P &lt; 0.0001, respectively) and at 6 months for pregabalin alone (6% [3 of 50]) and in combination with ketamine 0% (0 of 5) compared to the control group (28% [14 of 50]; odds ratio = 0.167 [0.029 to 0.7], P = 0.006; and 0.000 [0 to 0.24], P &lt; 0.0001). Diplopia was more common in both active arms.

Conclusions

Preoperative administration of 150 mg of pregabalin and postoperative continuation twice daily for 14 days significantly lowered the prevalence of persistent pain after cardiac surgery.

PER3 variable number tandem repeat (VNTR) polymorphism modulates the circadian variation of the descending pain modulatory system in healthy subjects

We evaluated the circadian pattern of variation of the descending pain modulatory system (DPMS) using a conditioned pain modulation (CPM) paradigm according to the variable-number tandem-repeat (VNTR) of the clock gene PER3 polymorphism. We assessed the relationship between the genotypes PER3^4/4 and PER3^5/5 and the temporal pattern of variation across the day using the following measures: the heat pain threshold (HPT), the cold pressure test (CPT), and the serum levels of BDNF and S100-B protein. The ∆-values (from afternoon to morning) of these measures were used for the analysis. The circadian phenotype was according to the mid-point sleep time established by the Munich ChronoType Questionnaire (MCTQ). We included 18 healthy volunteers (15 women) ages 18 to 30. A Generalized Linear Model (GLM) revealed a significant difference in the ∆-CPM-task between Per3^4/4 and Per3^5/5 genotypes, with means (SDs) of -0.41 (0.78) vs. 0.67 (0.90) (χ² = 7.256; df = 1' P = 0.007), respectively. Both sleep deprivation of at least 2 h/day (B = -0.96, 95% confidence interval (CI) = -1.86 to -0.11)) and the ∆-S100-B protein (-0.03, 95% CI = -0.06 to -0.02) were negatively correlated with the ∆-CPM-task, while the ∆-BDNF was positively correlated with the ∆-CPM-task (0.015, 95% CI = 0.01 to 0.03). We observed a difference in the ∆-CPT between PER3^4/4 and PER3^5/5 (0.11 (4.51) vs. 4.00 (2.60), respectively) (χ² = 22.251; df = 1 P = 0.001). These findings suggest that the polymorphism of PER3^5/5 is associated with a decrease in the inhibitory function of the DPMS over the course of the day. However, sleep deprivation is an independent factor that also reduces the inhibitory function of the DPMS, regardless of the PER3 VNTR polymorphism.

The relationship between catastrophizing and altered pain sensitivity in patients with chronic low-back pain

Changes in central pain processing have been shown in patients with chronic low-back pain (cLBP). We used quantitative sensory testing methods to identify differences in pain sensitization between patients with cLBP (N = 167) and healthy controls (N = 33). Results indicated that, compared with healthy pain-free controls, cLBP patients showed increased sensitivity and greater painful aftersensations for mechanical pressure and pin-prick stimuli and lower tactile spatial acuity in the 2-point discrimination task (ps < 0.05). Then, we examined the role of pain catastrophizing as a mediator of the group differences in pain sensitization. We found that catastrophizing partially accounted for group differences in pressure required to produce moderate pain. Finally, we examined the relationship between pain sensitization, catastrophizing, and clinical pain among patients with cLBP. We found that catastrophizing and deep-tissue pressure pain were associated with greater pain intensity in the past month, week, and at the visit as well as low-back pain bothersomeness. Furthermore, deep-tissue pressure pain mediated the associations between catastrophizing and both pain in the past month and low-back pain severity. Taken together, these results indicate that not only do patients with cLBP demonstrate increased pain sensitization and decreased sensitivity to innocuous stimuli, but these changes are also linked with increased catastrophizing. Furthermore, both catastrophizing and sensitization are associated with increased clinical pain among cLBP patients.

The relationship between sensitivity to pain and conditioned pain modulation in healthy people

BACKGROUND

The relationship between sensitivity to pain and conditioned pain modulation (CPM) - a paradigm reflecting the activity of the endogenous descending analgesic system - is still unclear. This study aimed at investigating CPM magnitude in two distinct subgroups of healthy subjects, presenting low vs. high sensitivity to pain (LSP vs. HSP, respectively), by employing two different thermal paradigms of CPM.

METHOD

Ninety-five healthy subjects (out of 293 tested) were identified as LSP (n = 48) or HSP (n = 47) according to their tolerance time to noxious cold stimulation (Cold Pressor Test, 1 °C). All subjects were exposed to two different paradigms of CPM: 1) Fixed temperature 'test-pain' (TP) where phasic, fixed painful heat stimuli of 47 °C were administered before and during a prolonged 'conditioning stimulus' (cold water at 12 °C for 30 s); and 2) Individually based 'pain-60' where TP was determined as the temperature that induced pain at a magnitude of 60 on a 0-100 rating scale (with the same conditioning stimulus).

RESULT

Using both thermal paradigms, LSP subjects showed decreased CPM magnitudes in comparison to HSP (p < 0.0001 in both paradigms). Within each group, no differences in the magnitudes of CPM were found between the two paradigms.

CONCLUSION

These findings show that regardless of the thermal CPM paradigm employed, healthy individuals exhibiting low sensitivity to pain have a low pain inhibition profile and vice-versa. It is suggested that in healthy subjects, pain sensitivity predisposes the magnitude of CPM and not the other way around.