Nociceptive flexion reflex thresholds and pain during rest and computer game play in patients with hypertension and individuals at risk for hypertension

Spinal hyperexcitability in patients with chronic musculoskeletal pain or headache as evidenced by alterations in the nociceptive withdrawal reflex: a systematic review and meta-analysis

ABSTRACT

The nociceptive withdrawal reflex (NWR) is a spinal withdrawal reflex induced by painful stimulation. It is a measure of spinal hyperexcitability, which is believed to contribute to chronic musculoskeletal pain (MSKP) and headache. Previous syntheses of the evidence for alterations in the NWR in patients with chronic MSKP and headache needed a comprehensive update. This systematic review and meta-analysis was performed after the Preferred Items for Systematic reviews and Meta-Analyses guidelines. Studies examining NWR-related outcome measures in patients with chronic MSKP and headache compared to pain-free controls were identified through electronic database searches and included after screening against predefined eligibility criteria. Standardized mean differences or mean differences and 95% confidence intervals (CI) were calculated. Thirty-one studies were included in the systematic review and 25 in the meta-analysis. Moderate-quality evidence was found indicating lower NWR threshold (-3.68; 95% CI, -4.56 to -2.80; P < 0.001), larger NWR area (standardized mean difference = 0.69; 95% CI, 0.37-1.01; P < 0.001), and shorter NWR latency (mean difference = -13.68; 95% CI, -22.69, -4.67; P = 0.003) in patients compared to controls. These findings remained robust when performing meta-regressions based on subgroups (ie, headache, fibromyalgia, whiplash-associated disorder, and osteoarthritis). Low-quality evidence demonstrated facilitated temporal summation of NWR threshold (-2.48; 95% CI, -3.13 to -1.83; P < 0.001) in patients compared to controls. Spinal hyperexcitability as evidenced by lowered NWR threshold values and temporal summation of the NWR is present in patients with chronic MSKP and headache. No evidence was found for alterations in NWR duration and NWR magnitude. Future research is needed to address the gap in research on NWR-related outcome measures other than NWR threshold.

Healthy women show more experimentally induced central sensitization compared with men

ABSTRACT

Women more often experience chronic pain conditions than men. Central sensitization (CS) is one key mechanism in chronic pain that can differ between the sexes. It is unknown whether CS processes are already more pronounced in healthy women than in men. In 66 subjects (33 women), a thermal CS induction protocol was applied to the dorsum of one foot and a sham protocol to the other. Spatial extent [cm 2 ] of secondary mechanical hyperalgesia (SMH) and dynamic mechanical allodynia were assessed as subjective CS proxy measures, relying on verbal feedback. Changes in nociceptive withdrawal reflex magnitude (NWR-M) and response rate (NWR-RR) recorded through surface electromyography at the biceps and rectus femoris muscles were used as objective CS proxies. The effect of the CS induction protocol on SMH was higher in women than in men (effect size 2.11 vs 1.68). Nociceptive withdrawal reflex magnitude results were statistically meaningful for women (effect size 0.31-0.36) but not for men (effect size 0.12-0.29). Differences between men and women were not meaningful. Nociceptive withdrawal reflex response rate at the rectus femoris increased in women after CS induction and was statistically different from NWR-RR in men (median differences of 13.7 and 8.4% for 120 and 140% reflex threshold current). The objective CS proxy differences indicate that dorsal horn CS processes are more pronounced in healthy women. The even larger sex differences in subjective CS proxies potentially reflect greater supraspinal influence in women. This study shows that sex differences are present in experimentally induced CS in healthy subjects, which might contribute to women's vulnerability for chronic pain.

Are changes in nociceptive withdrawal reflex magnitude a viable central sensitization proxy? Implications of a replication attempt

OBJECTIVE

The nociceptive withdrawal reflex (NWR) has been proposed to read-out central sensitization (CS). Replicating a published study, it was assessed if the NWR magnitude reflects sensitization by painful heat. Additionally, NWR response rates were compared for two stimulation, the sural nerve at the lateral malleolus (SU) and the medial plantar nerve on the foot sole (MP), and three recording sites, biceps femoris (BF), rectus femoris (RF), and tibialis anterior (TA) muscles.

METHODS

16 subjects underwent one experiment with six blocks of eight transcutaneous electrical stimulations to elicit the NWR while surface electromyography was collected. Tonic heat was concurrently applied in the same dermatome. Temperatures rose from 32 °C in the first to 46 °C in the last block following the previously published protocol.

RESULTS

Tonic heat did not influence NWR magnitude. The highest NWR response rate was obtained for MP-TA combination (79%). Regarding elicitation in all three muscles, SU stimulation outperformed MP (59% vs 57%).

CONCLUSIONS

The replication failed. NWR magnitude as a CS proxy in healthy subjects needs continued investigation. With respect to response rates, MP-TA proved efficient, whereas SU stimulation seemed preferable for multiple muscle recordings.

SIGNIFICANCE

Unclear methodological descriptions in the original study affected CS and NWR replication. The NWR magnitude changes induced by CS may closely depend on the different stimulation methods used.

EFFECT OF HYPERTENSION ON PAIN THRESHOLD IN PATIENTS WITH CHRONIC PAIN

Introduction: Little is known about the relationship between pain and hypertension (HT). This study aimed to analyze the effect of HT on pain sensitivity in patients with chronic musculoskeletal pain complaints. Methods: This analytical, cross-sectional study included 45 patients aged 29–75 years with HT. The control group comprised 44 normo/hypotensive patients aged 19–66 (P = 0.107). Education status, age, gender, height, weight, smoking, presence of diabetes mellitus, physical activity level, blood pressure measurement, anti-HT drug use, vital parameters, including heart rate, respiratory rate, and body temperature, were recorded for all participants. Results: The pain score was significantly higher in the HT group, with a mean of 72.1 ± 30.2 points (P = 0.008). Also, the HT group’s systolic blood pressure and pain score were significantly positively correlated (rho = 0.245, P = 0.02). The pulse rate was found to be significantly related to the pain score and pain threshold (P < 0.001); it was negatively correlated with algometer values (rho = –0.286, P = 0.015). Systolic pressure (P = 0.033) and BMI ( P < 0.001) were significantly different among the groups according to physical activity level. The Spearman correlation analysis showed a positive correlation of diastolic blood pressure with the body mass index (rho = 0.224, P = 0.036) and pain score (rho = 0.456, P < 0.001).  Conclusion: The present study showed that the pain complaint increases as the blood pressure and weight increase. Also, the pain threshold decreases as the heart rate increases. The pain threshold was lower in women than in men, independent of tension.

Stress and pain: modality-specific opioid mediation of stress-induced analgesia

Preclinical research has demonstrated that exposure to acute stress is associated with attenuated pain perception, so called stress-induced analgesia (SIA). Mechanisms of SIA in humans have not been reliably demonstrated. This study examined the role of the endogenous opioid system in the impact of combined interpersonal and mental stressors on evoked pain responses in 84 participants (34 women). Using a within-subject, double-blinded, counterbalanced design, participants were administered either oral placebo or the opioid antagonist naltrexone (50 mg) across two testing sessions. In each session, they experienced two evoked pain stimuli (cold pressor test [CPT], heat pain) after an extended rest period (rest condition) and after exposure to an acute stressor (a combination of public speaking and mental arithmetic challenge; stress condition). Results showed that both stress and opioid blockade produced significant changes in hormonal and cardiovascular measures, consistent with successful induction of acute stress. Stress was associated with attenuated pain perception (SIA) as indicated by significantly increased CPT tolerance. These effects were particularly pronounced in individuals experiencing the stress condition first. More importantly, SIA effects on CPT tolerance were abolished by opioid blockade. There were no significant SIA effects on heat pain responses. This study demonstrates that the endogenous opioid system may mediate effects of acute stress on pain perception, although this effect seems to be qualified by the type of evoked pain stimuli experienced.

Effects of Conditioned Pain Modulation on the Nociceptive Flexion Reflex in Healthy People: A Systematic Review

OBJECTIVES

The nociceptive flexion reflex (NFR) is a spinal reflex induced by painful stimuli resulting in a withdrawal response. Research has shown that the NFR is inhibited through endogenous pain inhibitory mechanisms, which can be assessed by conditioned pain modulation (CPM) paradigms. Although accumulating research suggests that the NFR can be affected by CPM, no clear overview of the current evidence exists. Therefore, the present review aimed at providing such a synthesis of the literature. In addition, the influence of personal factors on the CPM of the NFR was investigated.

MATERIALS AND METHODS

A systematic review was performed and reported following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines. Five electronic databases were searched to identify relevant articles. Retrieved articles were screened on eligibility using predefined inclusion criteria. Risk of bias was investigated according to the modified Newcastle-Ottawa Scale. Levels of evidence and strength of conclusion were assigned following the guidelines of the Dutch Institute for Healthcare Improvement.

RESULTS

Forty articles were included. There is some evidence that CPM produced by thermal or mechanical stimuli induces inhibitory effects on the NFR. However, inconclusive evidence exists with regard to the effect of electrical conditioning stimuli. While several personal factors do not affect CPM of the NFR, increased cognitive interference is associated with reduced NFR inhibition.

DISCUSSION

The present review demonstrates that certain types of nociceptive conditioning stimuli have the potential to depress, at the spinal level, nociceptive stimuli elicited from distant body regions. Although CPM of the NFR seems to be robust to the influence of several personal factors, it can be affected by cognitive influences.

Anger Inhibition and Pain Modulation

BACKGROUND

The tendency to inhibit anger (anger-in) is associated with increased pain. This relationship may be explained by the negative affectivity hypothesis (anger-in increases negative affect that increases pain). Alternatively, it may be explained by the cognitive resource hypothesis (inhibiting anger limits attentional resources for pain modulation).

METHODS

A well-validated picture-viewing paradigm was used in 98 healthy, pain-free individuals who were low or high on anger-in to study the effects of anger-in on emotional modulation of pain and attentional modulation of pain. Painful electrocutaneous stimulations were delivered during and in between pictures to evoke pain and the nociceptive flexion reflex (NFR; a physiological correlate of spinal nociception). Subjective and physiological measures of valence (ratings, facial/corrugator electromyogram) and arousal (ratings, skin conductance) were used to assess reactivity to pictures and emotional inhibition in the high anger-in group.

RESULTS

The high anger-in group reported less unpleasantness, showed less facial displays of negative affect in response to unpleasant pictures, and reported greater arousal to the pleasant pictures. Despite this, both groups experienced similar emotional modulation of pain/NFR. By contrast, the high anger-in group did not show attentional modulation of pain.

CONCLUSIONS

These findings support the cognitive resource hypothesis and suggest that overuse of emotional inhibition in high anger-in individuals could contribute to cognitive resource deficits that in turn contribute to pain risk. Moreover, anger-in likely influenced pain processing predominantly via supraspinal (e.g., cortico-cortical) mechanisms because only pain, but not NFR, was associated with anger-in.

Long term reliability of nociceptive withdrawal reflex thresholds

BACKGROUND

The nociceptive withdrawal reflex (NWR) is a polysynaptic spinal reflex protecting the body from harmful stimuli. Two different methods to assess its' threshold (NWR-T) have been part of clinical trials concerning the evaluation of the nociceptive system in the human body. NWR-T's are gathered by stimulation at the sole of the foot and over the sural pathway. Consequently, EMG analyzes the muscle activity over the biceps femoris and tibialis anterior muscle. Past studies favor stimulation at the sole of the foot.

NEW METHOD

The two methods were compared concerning retest-reliability and subjective pain ratings. The retest-reliability was tested over a period of 21 days using an up-down staircase method. Reliability was evaluated with a Bland Altman agreement analysis. Subjective pain ratings were evaluated with a numeric rating scale (NRS).

RESULTS

NWR-T assessment was successful for all subjects. The EMG muscle activity had larger reflex amplitudes for measurements of the tibialis anterior muscle. NWR-T values showed greater variability than NRS values.

COMPARISON WITH EXISTING METHOD

The retest-reliability over a period of 21 days showed stable NWR-T results for both stimulation sites, but superior reliability was gathered with stimulation at the sole of the foot. Subjects rated stimulation over the sural pathway as more painful.

CONCLUSIONS

The NWR-T upholds reliable measurements over a longer period of time and seems to be a stable measure for pain condition. Reliability estimations, EMG recordings, and subject's rating show stimulation at the sole of the foot could be the better choice.

The Influence of Placebo Analgesia Manipulations on Pain Report, the Nociceptive Flexion Reflex, and Autonomic Responses to Pain

Expectations for pain relief and experience/conditioning are psychological factors that contribute to placebo analgesia, yet few studies have studied the physiological mechanisms underlying their effects. This study randomized 133 participants to 4 groups: an expectation only (E-only) group, a conditioning only (C-only) group, an expectation plus conditioning (E+C) group, and a natural history (NH) control group. Painful electric stimulations were delivered before and after an inert cream was applied to the site of stimulation. Pain-related outcomes (pain ratings, nociceptive flexion reflex [NFR], skin conductance response, and heart rate acceleration) were recorded after each stimulation. NFR (a measure of spinal nociception) assessed if placebo analgesia inhibited spinal processing of pain. E+C was the only manipulation that significantly inhibited pain and skin conductance response. Surprisingly, NFR was facilitated in the E+C and E-only groups. No effects were noted for C-only. Mediation analysis suggested 2 descending processes were engaged during E+C that influenced spinal nociception: 1) descending facilitation and 2) descending inhibition that was also responsible for pain reduction. These results suggest that E+C manipulations produce the strongest analgesia and have a complex influence on spinal nociception involving both inhibitory and facilitatory processes. PERSPECTIVE: This study assessed whether placebo analgesia manipulations that include expectations, conditioning, or both modulate the NFR (measure of spinal nociception). Only the manipulation that involved expectations and conditioning inhibited pain, but both expectation manipulations facilitated NFR. This suggests a complex modulation of spinal neurons by placebo manipulations.

Emotional Modulation of Pain and Spinal Nociception in Sexual Assault Survivors

OBJECTIVE

Sexual assault (SA) is associated with an increased risk for chronic pain and affective distress. Given that emotional processes modulate pain (e.g., negative emotions enhance pain, positive emotions inhibit pain), increased pain risk in SA survivors could stem from a disruption of emotional modulation processes.

METHODS

A well-validated affective picture-viewing paradigm was used to study emotional modulation of pain in 33 healthy, pain-free SA survivors and a control group of 33 healthy, pain-free individuals with no reported history of SA (matched on age, sex, race, and number of non-SA traumas). Unpleasant (mutilation), neutral, and pleasant (erotic) pictures were presented, while painful electrocutaneous stimulations were delivered at the ankle. Pain intensity ratings and nociceptive flexion reflex (NFR) magnitudes (a physiologic measure of spinal nociception) were recorded in response to electric stimuli. Multilevel models were used to analyze the data with group (SA versus non-SA) and content (mutilation, neutral, erotic) as independent variables.

RESULTS

Both groups demonstrated similar emotional modulation of pain (FGroupbyContent(2,646.52) = 0.44, p = .65), but a main effect of group (FGroup(1,65.42) = 4.24, p = .043) indicated the SA group experienced more overall pain from electric stimuli (hyperalgesia). A significant group by content interaction for NFR (p = .035) indicated that emotional modulation of NFR was present for the non-SA group (FContentSimpleEffect(2,684.55) = 12.43, p < .001), but not the SA group (FContentSimpleEffect(2,683.38) = 1.71, p = .18).

CONCLUSIONS

These findings suggest that SA survivors have difficulty emotionally engaging brain-to-spinal cord mechanisms to modulate spinal nociception. A disruption of descending inhibition plus hyperalgesia could contribute to comorbidity between sexual trauma and chronic pain.

Stress and Addiction: When a Robust Stress Response Indicates Resiliency

OBJECTIVE

Stress reactivity research has traditionally focused on the idea that exaggerated responses to stress may have adverse effects on health. Accumulating evidence suggests that attenuated responses to stress and delayed recovery may also be problematic.

METHODS

This review focuses on the role of the stress response of the hypothalamic-pituitary-adrenocortical axis, the endogenous opioid system, and the cardiovascular system in hypertension, pain perception, and addictive behaviors. Results from multiple methods of assessment and stress paradigms conducted in our laboratory over the past two decades are integrated with research from other investigators and with existing theories.

RESULTS

Research indicates that exaggerated biological and physiological responses to stress and attenuated pain perception are associated with hypertension and risk for cardiovascular diseases. This research complements work linking reduced stress responses with enhanced pain sensitivity and discomfort. Multiple studies have also demonstrated that an attenuated stress response is linked to exacerbation of withdrawal symptoms and relapse in nicotine addiction. Evidence indicates important moderators (i.e., sex, personality traits, and early life adversity) and hypothalamic-pituitary-adrenocortical- and endogenous opioid system-related mechanisms in the altered response to stress. I integrate these findings in a conceptual model emphasizing that robust stress responses in the context of addiction and relapse should be considered as a marker of resiliency.

CONCLUSIONS

A blunted stress response may indicate long-term physiological dysregulation that could usher harmful consequences for cardiovascular disease, pain perception, and addictive disorders. The impact of dysregulation is influenced by multiple individual and situational factors that should be considered in evaluating the clinical significance of stress response dysregulation.

Introduction and validation of a less painful algorithm to estimate the nociceptive flexion reflex threshold

The nociceptive flexion reflex (NFR) is a widely used tool to investigate spinal nociception for scientific and diagnostic purposes, but its clinical use is currently limited due to the painful measurement procedure, especially restricting its applicability for patients suffering from chronic pain disorders. Here we introduce a less painful algorithm to assess the NFR threshold. Application of this new algorithm leads to a reduction of subjective pain ratings by over 30% compared to the standard algorithm. We show that the reflex threshold estimates resulting from application of the new algorithm can be used interchangeably with those of the standard algorithm after adjusting for the constant difference between the algorithms. Furthermore, we show that the new algorithm can be applied at shorter interstimulus intervals than are commonly used with the standard algorithm, since reflex threshold values remain unchanged and no habituation effects occur when reducing the interstimulus interval for the new algorithm down to 3s. Finally we demonstrate the utility of the new algorithm to investigate the modulation of nociception through different states of attention. Taken together, the here presented new algorithm could increase the utility of the NFR for investigation of nociception in subjects who were previously not able to endure the measurement procedure, such as chronic pain patients.

Thermal sensitivity and cardiovascular reactivity to stress in healthy males

This paper examines the association of cardiovascular reactivity with thermal thresholds (detection and unpleasantness). Heart period (HP), systolic (SBP) and diastolic (DBP) blood pressure of 42 health young males were recorded during a cardiovascular reactivity task (a videogame based upon Sidman's avoidance paradigm). Thermal sensitivity, assessing detection and unpleasantness thresholds with radiant heat in the forearm was also estimated for participants. Participants with differential scores in the cardiovascular variables from base line to task > or = P65 were considered as reactors and those how have differential scores < or = P35 were considered as non-reactors. Significant differences were observed between groups in the unpleasantness thresholds in blood pressure (BP) but not in HP. Reactors exhibited significant higher unpleasantness thresholds than non-reactors. No significant differences were obtained in detection thresholds between groups.

The modulation of pain by attention and emotion: a dissociation of perceptual and spinal nociceptive processes

Emotions and attention have been shown to influence the perception of pain and several psychophysiological studies have suggested an implication of descending modulatory mechanisms to explain these effects. However, the specificity of the neurophysiological mechanisms underlying the emotional and attentional modulation of pain still remains unclear. In order to differentiate the supra-spinal and spinal mechanisms involved in emotional and attentional modulation of pain, we measured pain perception (self-ratings) and the RIII reflex in healthy volunteers during the presentation of pleasant, unpleasant and neutral pictures, as well as during a baseline condition with no visual distractor (Experiment 1). In a second experiment, we manipulated the emotional arousal induced by pleasant and unpleasant pictures in order to compare more directly the effects of distraction and arousal. Whereas emotional valence influenced pain and the amplitude of the RIII reflex in the same direction (negative > positive), distraction by neutral pictures reduced pain but increased the RIII reflex relative to baseline. Increased arousal further potentiated the effects of negative valence on both pain and the RIII reflex and the effects of positive emotions on pain, as previously reported. However, arousal did not potentiate the inhibitory effect of positive pictures on the RIII and seems insufficient to account for the effect of distraction on the RIII. Overall, these data provide further evidence that attention and emotion modulate pain through partially dissociable neurophysiological mechanisms.

Effects of opioid blockade on nociceptive flexion reflex thresholds and nociceptive responding in hypertensive and normotensive individuals

Hypertension and risk for hypertension have been associated with reduced pain sensitivity. It has been hypothesised that endogenous opioids contribute to this hypertensive hypoalgesia. The nociceptive flexion reflex can be used as a tool to investigate modulation of nociceptive transmission at spinal level. The current study employed a double-blind placebo-controlled design to compare the effects of naltrexone, an opioid antagonist, and placebo on nociceptive flexion reflex thresholds and nociceptive responding in unmedicated patients with essential hypertension and normotensive individuals. Neither nociceptive flexion reflex thresholds nor nociceptive responding differed between hypertensives and normotensives during placebo or naltrexone. These data provide no support for the hypothesis that essential hypertension is characterised by higher levels of endogenous opioids in the central nervous system and reveal no association between blood pressure status and nociceptive flexion reflex responses.

Effects of naltrexone on electrocutaneous pain in patients with hypertension compared to normotensive individuals

An opioid mechanism may help explain hypertensive hypoalgesia. A double-blind placebo-controlled design compared the effects of opioid blockade (naltrexone) and placebo on electrocutaneous pain threshold, pain tolerance, and retrospective McGill Pain Questionnaire ratings in 35 unmedicated patients with essential hypertension and 28 normotensive individuals. The hypertensives experienced less pain than normotensives during the assessment of their pain tolerance; however, this manifestation of hypertensive hypoalgesia was not moderated by naltrexone. These findings fail to support the hypothesis that essential hypertension is characterised by relative opioid insensitivity.