Effect of sympathetic nerve block on acute inflammatory pain and hyperalgesia

Methodology and applicability of the human contact burn injury model: A systematic review

The contact burn injury model is an experimental contact thermode-based physiological pain model primarily applied in research of drug efficacy in humans. The employment of the contact burn injury model across studies has been inconsistent regarding essential methodological variables, challenging the validity of the model. This systematic review analyzes methodologies, outcomes, and research applications of the contact burn injury model. Based on these results, we propose an improved contact burn injury testing paradigm. A literature search was conducted (15-JUL-2020) using PubMed, EMBASE, Web of Science, and Google Scholar. Sixty-four studies were included. The contact burn injury model induced consistent levels of primary and secondary hyperalgesia. However, the analyses revealed variations in the methodology of the contact burn injury heating paradigm and the post-burn application of test stimuli. The contact burn injury model had limited testing sensitivity in demonstrating analgesic efficacy. There was a weak correlation between experimental and clinical pain intensity variables. The data analysis was limited by the methodological heterogenicity of the different studies and a high risk of bias across the studies. In conclusion, although the contact burn injury model provides robust hyperalgesia, it has limited efficacy in testing analgesic drug response. Recommendations for future use of the model are being provided, but further research is needed to improve the sensitivity of the contact burn injury method. The protocol for this review has been published in PROSPERO (ID: CRD42019133734).

Effect of Stellate Ganglion Block on the Regional Hemodynamics of the Upper Extremity: A Randomized Controlled Trial

BACKGROUND

The success of stellate ganglion block (SGB) is traditionally determined on the basis of findings such as Horner's syndrome, temperature rise in the face, hyperemia of the tympanic membrane, and nasal congestion. However, decreases in vascular resistance and increases in blood flow in the arm may be more meaningful findings. To date, the effect of SGB on the regional hemodynamics of the arm has not been evaluated using pulsed-wave Doppler ultrasound.

METHODS

A total of 52 patients who were to undergo orthopedic surgery of the forearm were randomly assigned to either the mepivacaine group (SGB with 5 mL of 0.5% mepivacaine) or the saline group (SGB with 5 mL of normal saline). Before surgery, a single anesthesiologist performed a SGB under ultrasound guidance. The temperature of the upper extremity and the resistance index and blood flow in the brachial artery were measured before SGB, 15 and 30 minutes after SGB, and 1 hour after surgery. The severity of pain, requirement for rescue analgesics, and side effects of the local anesthetic agent were all documented.

RESULTS

After SGB, the resistance index decreased significantly and the blood flow increased significantly in the brachial artery of members of the mepivacaine group (15 minutes: P = .004 and P < .001, respectively; 30 minutes: P < .001 and P < .001, respectively). However, these values normalized after surgery. The severity of pain, need for rescue analgesics, and incidence of adverse effects were not significantly different between the 2 groups.

CONCLUSIONS

Although SGB did not decrease the pain associated with forearm surgery, ultrasound-guided SGB did increase blood flow and decrease vascular resistance in the arm. Therefore, pulsed-wave Doppler may be used to monitor the success of SGB.

Demarcation of secondary hyperalgesia zones: Punctate stimulation pressure matters

BACKGROUND

Secondary hyperalgesia is increased sensitivity in normal tissue near an injury, and it is a measure of central sensitization reflecting injury-related effects on the CNS. Secondary hyperalgesia areas (SHAs), usually assessed by polyamide monofilaments, are important outcomes in studies of analgesic drug effects in humans. However, since the methods applied in demarcating the secondary hyperalgesia zone seem inconsistent across studies, we examined the effect of a standardized approach upon the measurement of SHA following a first degree burn injury (BI).

NEW METHOD

The study was a two-observer, test-retest study with the two sessions separated by 6wk. An observer-blinded design adjusted to examine day-to-day and observer-to-observer variability in SHA was used. In 23 healthy volunteers (12 females/11 males) a BI was induced by a contact thermode (47.0°C, 420s, 2.5×5.0cm(2)). The SHA, demarcated by polyamide monofilaments (bending force: 0.2, 69 and 2569mN) and a "weighted-pin" stimulator (512mN), were assessed 45 to 75min after each BI.

RESULTS

A random effect, linear mixed model demonstrated a logarithmic correlation between elicited skin pressures (mN/mm(2)) and the SHAs (P<0.0001). No day-to-day or observer-to-observer differences in SHAs were observed. Intraclass correlation coefficients, in the range of 0.51 to 0.84, indicated a moderate to almost perfect reliability between observers.

COMPARISON WITH EXISTING METHODS

No standardized approach in SHA-assessment has hitherto been presented.

CONCLUSIONS

This is the first study to demonstrate that demarcation of secondary hyperalgesia zones depends on the developed pressure of the punctate stimulator used.

Paravertebral blocks

Paravertebral blocks have been demonstrated to represent an interesting alternative to epidural, especially for the management of perioperative and trauma pain. Initially performed mostly as single-shot blocks for breast surgery, thoracotomy, and hernia repairs in adults and children, presently these blocks are also used for placement of a paravertebral catheter, either unilateral or bilateral. Although complications associated with the performance of these blocks are infrequent, the use of ultrasound-guided approaches, which allow performing the block under direct vision, is becoming the standard in most groups performing these blocks routinely.

Procedural pain management for neonates using nonpharmacological strategies: part 2: mother-driven interventions

This is the second of a 2-part series to provide an overview of our current level of knowledge related to nonpharmacological strategies to diminish the pain associated with commonly performed procedures in the NICU. In our first article we discussed the prevalence of repeated pain exposure in the NICU and the importance of nonpharmacological strategies specifically containment or facilitated tucking, swaddling, positioning, nonnutritive sucking, and sweet solutions. These strategies are generally nurse-driven and we believe their importance has been underutilized. In this article we will emphasize the importance of maternal presence as a mediator for pain relief. The efficacy of breastfeeding, maternal skin-to-skin care (often referred to as kangaroo care), and multisensorial stimulation such as auditory and olfactory recognition will be the primary focus of our discussion. In addition, although primarily mother-driven, these strategies are ultimately nurse-enabled, thus the importance of this connection cannot be under appreciated with respect to successful implementation in the NICU.

The sympathetic nervous system and pain

The sympathetic nervous system (SNS) and pain interact on many levels of the neuraxis. In healthy subjects, activation of the SNS in the brain usually suppresses pain mainly by descending inhibition of nociceptive transmission in the spinal cord. Furthermore, some experimental data even suggest that the SNS might control peripheral inflammation and nociceptive activation. However, even subtle changes in pathophysiology can dramatically change the effect of SNS on pain, and vice versa. In the periphery, inflammation or nociceptive activation is enhanced, spinal descending inhibition is reversed to spinal facilitation, and finally the awareness of all these changes will induce anxiety, which furthermore amplifies pain perception, affects pain behavior, and depresses mood. Unraveling the detailed molecular mechanisms of how this interaction of SNS and pain is established in health and disease will help us to treat pain more successfully in the future.

Depletion of noradrenaline inhibits electrically-evoked pain in the skin of the human forearm

Guanethidine displaces noradrenaline from sympathetic varicosities, and blocks sympathetic noradrenergic neurotransmission by inhibiting the release of noradrenaline from depleted neural stores. The aim of this study was to determine whether depletion of noradrenaline with guanethidine would oppose thermal hyperalgesia and/or electrically-evoked pain in mildly-burnt skin. Guanethidine was transferred by iontophoresis into a small patch of skin on the forearm of 35 healthy human subjects. The heat-pain threshold to a temperature gradient that increased at 0.5 degrees C/s was then measured at the guanethidine site, a nearby saline-control iontophoresis site, and in untreated skin. In addition, participants rated pain intensity to a 47 degrees C stimulus that was applied to each site for 7s. Shortly after the iontophoreses, sensitivity to heat was greater at the guanethidine site than the two control sites, suggesting that ejection of noradrenaline from sympathetic varicosities increased sensitivity to heat. One day later, when neural stores of noradrenaline were depleted, sensitivity to heat did not differ between the guanethidine and control sites. The guanethidine pretreatment did not influence thermal hyperalgesia induced by a mild burn, but inhibited pain evoked by electrical stimulation of the skin (0.2 mA direct current for 4 min). These findings indicate that ongoing sympathetic neural discharge does not normally influence thermal hyperalgesia in inflamed skin, because depleting noradrenergic stores had no effect. However, electrically-evoked release of noradrenaline may increase nociceptive sensations. Further clarification of this human pain model could provide insights into the mechanism of adrenergic hyperalgesia in certain neuropathic pain syndromes.

Effect of valdecoxib pretreatment on pain and secondary hyperalgesia: a randomized controlled trial in healthy volunteers [ISRCTN05282752, NCT00260325]

Background

Induction of the COX-2 isoenzyme appears to play a major role in the genesis of central sensitization after nociceptive stimulation. This study aimed to investigate the efficacy of a single, oral dose of the specific COX-2 inhibitor-valdecoxib in attenuating the central sensitization – induced secondary hyperalgesia in a heat/capsaicin pain model in healthy volunteers.

Methods

The study was a randomized, double blind, placebo controlled, crossover, single dose efficacy trial using 20 healthy volunteers. Two hours following placebo or 40 mg, PO valdecoxib, participants underwent skin sensitization with heat/capsaicin, as well as supra-threshold pain and re-kindling measurements according to an established, validated pain model. Subjects rated pain intensity and unpleasantness on a visual analog scale and the area of secondary hyperalgesia was serially mapped.

Results

The area of secondary hyperalgesia produced after 40 mg of valdecoxib was no different than that after placebo. Furthermore, there were no significantly relevant differences when volunteers were treated with valdecoxib or placebo in relation to either cold- or hot pain threshold or the intensity of pain after supra-threshold, thermal pain stimulation.

Conclusion

We demonstrated that a single, oral dose of valdecoxib when does not attenuate secondary hyperalgesia induced by heat/capsaicin in a cutaneous sensitization pain model in healthy volunteers.

Neurogenic inflammation of the bladder

Current evidence suggests multiple and redundant pathways through which the nervous system can initiate, amplify, and perpetuate inflammation. Many of the processes initiated by neurogenic inflammation have the capacity to recruit the participation of additional sensory nerves. These observations indicate that effective strategies for prevention or treatment of neurogenic inflammation of the bladder will entail or require intervention at multiple points. It has been observed that pain management in the future will be based on selective intervention tailored to the specific processes modulating pain perception in individual patients. It is exciting to contemplate the same approach to prevention and treatment of neurogenic bladder inflammation.

The effect of sympathetic activity on thermal hyperalgesia in capsaicin-treated skin during body cooling and warming

An adrenergic mechanism is thought to contribute to pain in conditions that sometimes develop during chronic inflammation and after nerve or tissue injury. There is some doubt, however, about whether adrenergic activity influences nociception in acute inflammation. To investigate this issue, the noncompetitive alpha-(alpha) adrenergic antagonist phenoxybenzamine was introduced by iontophoresis into the skin of 16 healthy volunteers either before or after the topical application of capsaicin. When applied before capsaicin, phenoxybenzamine increased thermal hyperalgesia at normal ambient temperatures and during body warming. These findings suggest that phenoxybenzamine blocked an analgesic mechanism when applied before the onset of inflammation. However, this effect disappeared during body cooling. When applied after capsaicin, phenoxybenzamine inhibited thermal hyperalgesia at normal ambient temperatures, and during body warming and cooling. Thus, phenoxybenzamine blocked a hyperalgesic mechanism when applied after the onset of inflammation. It was concluded that the presence of inflammation influences the nociceptive effect of alpha-adrenergic blockage, possibly by increasing access to excitatory adrenergic receptors on nociceptive afferents. An excitatory adrenergic influence on nociception may overcome an inhibitory adrenergic influence during inflammation.

Vascular and nociceptive effects of localized prolonged sympathetic blockade in human skin

Supersensitivity to noradrenaline contributes to certain vascular disorders (e.g., hypertension) and chronic neuropathic pain conditions (e.g., complex regional pain syndrome). We aimed to develop a procedure for inducing adrenergic supersensitivity that could be used to investigate the role of catecholamines in these clinical conditions. In the first study, three doses of guanethidine were administered by iontophoresis to separate small patches of skin in the forearm of healthy human volunteers. Four to five hours later. the vasoconstrictor response to the adrenergic releasing agent tyramine was inhibited in a dose-dependent manner by iontophoretic pretreatment with guanethidine, indicating that guanethidine had depleted endogenous adrenergic stores. In a second study, guanethidine and saline were administered by iontophoresis four times over approximately 2 weeks at separate sites in the forearm. One to two days after the final pretreatment, vasoconstriction to the iontophoresis of a weak dose of noradrenaline was enhanced at sites pretreated with guanethidine. To investigate the effect of guanethidine pretreatment on thermal hyperalgesia. the experimental sites were sensitized to heat by the topical application of 0.6% capsaicin. Both before and after the application of capsaicin, the heat-pain threshold and heat-pain ratings to suprathreshold stimulation were similar at sites pretreated for 2 weeks with guanethidine or saline. However, after the iontophoresis of noradrenaline, thermal hyperalgesia was greater at the guanethidine-pretreated site than the saline pretreated site. These observations indicate that prolonged depletion of adrenergic stores by guanethidine induces adrenergic supersensitivity in cutaneous vessels, and that adrenergic supersensitivity enhances thermal hyperalgesia in the presence of noradrenaline.

Remifentanil inhibits muscular more than cutaneous pain in humans

In experimental studies, drug-induced analgesia is usually assessed by cutaneous stimulation. If analgesics act differently on cutaneous and deep nociception, the results of these studies may not be entirely applicable to clinical pain involving deep structures. We tested the hypothesis that opioids have different abilities to inhibit cutaneous and muscular pain. Either the opioid remifentanil or placebo was infused in 12 healthy volunteers in a cross-over fashion. Repeated electrical stimulation (five impulses at 2 Hz) was applied to both skin and muscle. Pain thresholds were recorded. Remifentanil caused a higher increase in the muscular pain thresholds than in the cutaneous pain thresholds (P = 0.035). We conclude that opioids inhibit muscular pain more strongly than cutaneous pain in humans.

Causalgia and reflex sympathetic dystrophy: does the sympathetic nervous system contribute to the generation of pain?

The striking response of causalgia and reflex sympathetic dystrophy (RSD) to sympatholytic procedures together with signs of autonomic nervous system abnormalities suggest that the sympathetic efferent system can generate or enhance pain (sympathetically maintained pain, SMP). This concept is supported by human and animal experiments indicating that sympathetic activity and catecholamines can activate primary afferent nociceptors. Some clinical evidence, however, calls the SMP concept into question and alternative explanations have been advanced. In this review, we describe the clinical features of causalgia and RSD and the evidence for sympatholytic efficacy. The major barrier to proving the SMP concept is that all available sympatholytic procedures are problematic. We conclude that, although the weight of current evidence supports the SMP concept and its relevance to causalgia and RSD, it remains unproven by scientific criteria. More careful adherence to diagnostic criteria and well-controlled trials of sympatholysis are needed to finally settle the issue.

The effect of repeated epidural sympathetic nerve block on "failed back surgery syndrome" associated chronic low back pain

STUDY OBJECTIVE

To assess the therapeutic benefits of repeated epidural local anesthetic administration on pain perception and straight leg raise (SLR) in patients suffering from chronic low back pain.

DESIGN

Prospective, randomized, controlled, single-blind study protocol.

PATIENTS

50 ASA physical status I, II, and III patients at least 18 years of age, who had previously undergone spine surgery.

INTERVENTIONS

All participants were admitted to hospital for the 5-day duration of the study. Following epidural catheterization, fluoroscopy was performed to verify correct placement of the epidural catheter. On the first study day, all patients received depomedrol 80 mg, in 10-ml 1% lidocaine, epidurally. Thereafter, patients were randomized into two equal groups. In Group Bupivacaine (B) 10-ml 0.125% bupivacaine was administered. In Group Saline (S), an equal volume of saline was administered. Epidural injections were performed twice daily (09H00 and 14H00) for 4 days. Sympathetic blockade was confirmed by the presence of peripheral vasodilatation. Sensory blockade was assessed using loss of pin prick and temperature sensation. SLR as well as patient-generated 100-mm visual analog score (VAS) for pain were performed prior to each injection, at 15 minutes after injection, and hourly for 2 hours thereafter. Similar parameters were measured 1 week, 1 month, and 3 months after discharge.

MEASUREMENTS AND MAIN RESULTS

46 patients completed the study. VAS for pain was marginally lower in Group B. However, statistical significance was not demonstrated. During the hospitalization period, the SLR in both study groups significantly (0.008) improved with time. However, no difference between the groups was demonstrated. In both groups, 1 week, 1 month, and 3 months after discharge, the SLR was comparable to prestudy recordings. In Group B, at 1 week, 1 month, and 3 months after discharge, patient-generated VAS for pain were significantly (p = 0.002) higher when compared to pain scores at the time of patient discharge.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Secondary hyperalgesia to heat stimuli after burn injury in man

The aim of the study was to examine the presence of hyperalgesia to heat stimuli within the zone of secondary hyperalgesia to punctate mechanical stimuli. A burn was produced on the medial part of the non-dominant crus in 15 healthy volunteers with a 50 x 25 mm thermode (47 degrees C, 7 min), and assessments were made 70 min and 40 min before, and 0, 1, and 2 h after the burn injury. Hyperalgesia to mechanical and heat stimuli were examined by von Frey hairs and contact thermodes (3.75 and 12.5 cm2), and pain responses were rated with a visual analog scale (0-100). The area of secondary hyperalgesia to punctate stimuli was assessed with a rigid von Frey hair (462 mN). The heat pain responses to 45 degrees C in 5 s (3.75 cm2) were tested in the area just outside the burn, where the subjects developed secondary hyperalgesia, and on the lateral crus where no subject developed secondary hyperalgesia (control area). The burns decreased pain thresholds and increased pain responses to both thermal and mechanical stimuli within the burn (P < 10(-5)). Further, the burns induced secondary hyperalgesia (mean 89 cm2) to punctate mechanical stimuli (P < 10(-5)), and increased the pain response to mechanical stimuli in the areas of secondary hyperalgesia (P < 10(-5)). The pain response to heat stimuli increased over time in the area of secondary hyperalgesia (P < 10(-5)), and so did the pain response to heat on the lateral part of the crus (P < 10(-3)). However, the heat pain response increased more (P = 0.006) and was more intense (P = 0.001) within the zone of secondary hyperalgesia than on the lateral part of the crus. Further, the heat pain response was more intense in the zone of primary hyperalgesia than in the zone of secondary hyperalgesia (P = 0.004), in contrast to the mechanical pain response, which was not significantly different between the two zones of hyperalgesia. In conclusion, secondary hyperalgesia in man is not restricted to mechanical stimuli, as significant hyperalgesia to heat developed within the zone of secondary hyperalgesia to punctate mechanical stimuli. The data, combined with other evidence, suggest differences in the mechanisms accounting for primary hyperalgesia to heat and mechanical stimuli, whereas secondary hyperalgesia to heat and mechanical stimuli may be explained by a common central mechanism.

Hyperalgesia in a human model of acute inflammatory pain: a methodological study

The aim of the study was to examine reproducibility of primary and secondary hyperalgesia in a psychophysical model of human inflammatory pain. Mild burns were produced on the crura of 12 volunteers with a 50 x 25 mm thermode (47 degrees C, 7 min). Assessments of (i) cold and warm detection thresholds, (ii) mechanical and heat pain thresholds, (iii) pain to heat (43 degrees C and 45 degrees C, 5 s), (iv) secondary hyperalgesia, and (v) skin erythema were made 1.75 and 0.5 h before, and 0, 1, 2, 4, and 6 h after a burn injury. Sensory thresholds and hyperalgesia to heat and mechanical stimuli were examined by contact thermodes and von Frey hairs, and pain intensity was rated with a visual analog scale (0-100). To describe between-day reproducibility, the subjects were examined three times at intervals of 21 days. Within-day comparisons showed that a 20% change could be detected as significant for all variables with fewer than 12 subjects in a cross-over design (2alpha = 5% and power = 80%). Between-day comparisons demanded up to 25 subjects to detect changes of the same magnitude. The burns caused mild to moderate pain (VAS: mean 29, SD 14) and the subjects (all right-handed) were more sensitive to heat pain on their left side (P < 0.03). Hyperalgesia was induced instantaneously by the burn and outlasted the study period (6 h). However, no spontaneous pain was observed after the injury, and a brief period of hypoesthesia to warm and cold stimuli was induced by the burn. The painful measurements themselves evoked hyperalgesia to heat and mechanical stimuli on the arm, but only to mechanical stimuli on the legs. including secondary hyperalgesia. Hyperalgesia evoked by the measurements was significantly less intense than that induced by injury. Habituation to the painful stimuli was demonstrated by significantly higher pain thresholds and lower pain responses on the second and third day of the study. The burn model is a sensitive psychophysical model of acute inflammatory pain, when cross-over designs and within-day comparisons are used, and the model is suitable for double-blind, placebo-controlled studies of analgesics. In similar models, we recommend that analgesic and placebo are evenly divided between right and left sides and study days.