Successful intravenous regional block with low-dose tumor necrosis factor-alpha antibody infliximab for treatment of complex regional pain syndrome 1

Evidence of a genetic background predisposing to complex regional pain syndrome type 1

BACKGROUND

Complex regional pain syndrome type 1 (CRPS-1) is a rare, disabling and sometimes chronic disorder usually arising after a trauma. This exploratory study examined whether patients with chronic CRPS-1 have a different genetic profile compared with those who do not have the condition.

METHODS

Exome sequencing was performed to seek altered non-synonymous SNP allele frequencies in a discovery cohort of well-characterised patients with chronic CRPS-1 (n=34) compared with population databases. Identified SNP alleles were confirmed by Sanger sequencing and sought in a replication cohort (n=50). Gene expression of peripheral blood macrophages was assessed.

RESULTS

In the discovery cohort, the rare allele frequencies of four non-synonymous SNPs were statistically increased. The replication cohort confirmed this finding. In a chronic pain cohort, these alleles were not overexpressed. In total, 25 out of 84 (29.8%) patients with CRPS-1 expressed a rare allele. The SNPs were rs41289586 in ANO10, rs28360457 in P2RX7, rs1126930 in PRKAG1 and rs80308281 in SLC12A9. Males were more likely than females to have a rare SNP allele, 8 out of 14 (57.1%) vs 17 out of 70 (24.3%) (Fisher's p=0.023). ANO10, P2RX7, PRKAG1 and SLC12A9 were all expressed in macrophages from healthy human controls.

CONCLUSION

A single SNP in each of the genes ANO10, P2RX7, PRKAG1 and SLC12A9 was associated with developing chronic CRPS-1, with more males than females expressing these rare alleles. Our work suggests the possibility that a permissive genetic background is an important factor in the development of CRPS-1.

Perineural treatment with anti-TNF-α antibody ameliorates persistent allodynia and edema in novel mouse models with complex regional pain syndrome

Complex regional pain syndrome (CRPS) is an intractable chronic pain syndrome with various signs and symptoms including allodynia/hyperalgesia, edema, swelling, and skin abnormalities. However, a definitive therapeutic treatment for CRPS has not been established. In CRPS patients, inflammatory cytokines such as TNF-α and IL-1β have been shown to increase in affected areas, suggesting that these molecules may be potential therapeutic targets for CRPS. Here, we first created a novel CRPS mouse model (CRPS-II-like) via sciatic nerve injury and cast immobilization, which was characterized by mechanical allodynia, local edema, and skin abnormalities, to evaluate the pathophysiology and pharmacotherapy of CRPS. When an anti-TNF-α antibody was consecutively administered near the injured sciatic nerve of CRPS model mice, persistent allodynia and CRPS-related signs in the ipsilateral hindpaw were markedly attenuated to control levels. Perineural administration of anti-TNF-α antibody also suppressed the upregulation of inflammatory cytokines as well as the activation of macrophages and Schwann cells in the injured sciatic nerve. These findings indicate that persistent allodynia and CRPS-related signs in CRPS models are primarily associated with TNF-α-mediated immune responses in injured peripheral nerves, suggesting that perineural treatment with anti-TNF-α antibody might be therapeutically useful.

Effectiveness of Infliximab in Patients with Complex Regional Pain Syndrome: A Case Series

Purpose

Complex regional pain syndrome (CRPS) is a multi-mechanism disease, with an exaggerated inflammatory response as an important underlying mechanism. Auto-inflammation can theoretically be combated by anti-inflammatories, such as TNF-α inhibitors. This study's aim was to assess the effectiveness of intravenous infliximab, a TNF-α inhibitor, in patients with CRPS.

Patients and Methods

CRPS patients treated with infliximab between January 2015 and January 2022 were approached to participate in this retrospective study. Medical records were screened for age, gender, medical history, CRPS duration, and CRPS severity score. Additionally, treatment effect, dose and duration, and side effects were extracted from medical records. Patients who still receive infliximab completed a short global perceived effect survey.

Results

Eighteen patients received infliximab, and all but two gave consent. Trial treatment with three sessions of 5 mg/kg intravenous infliximab was completed in 15 patients (93.7%). Eleven patients (73.3%) were categorized as responders with a positive treatment effect. Treatment was continued in nine patients, and seven patients are currently treated. Infliximab dose is 5 mg/kg, and frequency is every four to six weeks. Seven patients completed a global perceived effect survey. All patients reported improvement (median 2, IQR 1-2) and treatment satisfaction (median 1, IQR 1-2). One patient described side effects such as itching and rash.

Conclusion

Infliximab proved effective in 11 out of 15 CRPS patients. Seven patients are still being treated. Further research is needed on the role of infliximab in the treatment of CRPS and possible predictors of response to treatment.

Autoantibodies against P29ING4 are associated with complex regional pain syndrome

INTRODUCTION

Complex regional pain syndrome (CRPS) is a complication following trauma or surgery and may be difficult to diagnose since biomarkers are lacking. Using protein array technology, we found antibodies binding to p29ING4, which we further characterized using ELISA.

METHODS

Thirty-six sera of early-stage type 1 CRPS, 66 sera of rheumatoid arthritis (RA), 53 sera of axial spondyloarthritis (axSpA), 29 sera of psoriatic arthritis (PsA), 22 sera of patients after radial fractures (trauma control), and 100 sera of blood donors (BD) were analyzed for anti-p29ING4. We established ELISAs with 7 different antigens and using different secondary antibodies binding to IgG, IgG1, IgG2, IgG3, IgG4, IgA, and IgM, and 2 different tests to detect immune complexes (IC) of p29ING4 and IgG or IgG1.

RESULTS

The highest likelihood ratios versus CRPS and trauma control were observed considering the A1-23 (sensitivity 19%, specificity 100%, LR > 19) using IgG as a secondary antibody, the A120-165 (sensitivity 17%, specificity 100%, LR = 17) using IgG as a secondary antibody and the A120-165 (sensitivity 31%, specificity 95%, LR = 6.2) using IgA as a secondary antibody. IC of p29ING4 and IgG were present in 11/36 (31%) CRPS sera, 17/64 (27%) RA sera, 13/53 (25%) SpA sera, 5/29 (17%) PsA sera, 1/22 (5%) trauma control sera, and 4/100 (4%) sera of BD. IC of p29ING4 and IgG1 were present in 14/36 (39%) CRPS sera, 19/64 (30%) RA sera, 13/53 (25%) SpA, 1/29 (3%) PsA, 2/22 (9%) trauma control, and 4/100 (4%) of the BD sera.

CONCLUSION

Due to the lack of other biomarkers of type 1 CRPS, P29ING4 autoantibodies could be helpful in its diagnostic work-up.

Intravenous Regional Anesthesia: A Historical Overview and Clinical Review

Intravenous regional anesthesia (IVRA) is an established, safe and simple technique, being applicable for various surgeries on the upper and lower limbs. In 1908, IVRA was first described by the Berlin surgeon August Bier, hence the name "Bier's Block". Although his technique was effective, it was cumbersome and fell into disuse when neuroaxial and percutaneous plexus blockades gained widespread popularity in the early 20^th century. In the 1960s, it became widespread, when the New Zealand anesthesiologist Charles McKinnon Holmes praised its use by means of new available local anesthetics. Today, IVRA is still popular in many countries being used in the emergency room, for outpatients and for high-risk patients with contraindications for general anesthesia. IVRA offers a favorable risk-benefit ratio, cost-effectiveness, sufficient muscle relaxation and a fast on- and offset. New upcoming methods for monitoring, specialized personnel and improved emergency equipment made IVRA even safer. Moreover, IVRA may be applied to treat complex regional pain syndromes. Prilocaine and lidocaine are considered as first-choice local anesthetics for IVRA. Also, various adjuvant drugs have been tested to augment the effect of IVRA, and to reduce post-deflation tourniquet pain. Since major adverse events are rare in IVRA, it is regarded as a very safe technique. Nevertheless, systemic neuro- and cardiotoxic side effects may be linked to an uncontrolled systemic flush-in of local anesthetics and must be avoided. This review gives a historical overview of more than 100 years of experience with IVRA and provides a current view of IVRA with relevant key facts for the daily clinical routine.

The Rodent Tibia Fracture Model: A Critical Review and Comparison With the Complex Regional Pain Syndrome Literature

Distal limb fracture is the most common cause of complex regional pain syndrome (CRPS), thus the rodent tibia fracture model (TFM) was developed to study CRPS pathogenesis. This comprehensive review summarizes the published TFM research and compares these experimental results with the CRPS literature. The TFM generated spontaneous and evoked pain behaviors, inflammatory symptoms (edema, warmth), and trophic changes (skin thickening, osteoporosis) resembling symptoms in early CRPS. Neuropeptides, inflammatory cytokines, and nerve growth factor (NGF) have been linked to pain behaviors, inflammation, and trophic changes in the TFM model and proliferating keratinocytes were identified as the primary source of cutaneous cytokines and NGF. Tibia fracture also activated spinal glia and upregulated spinal neuropeptide, cytokine, and NGF expression, and in the brain it changed dendritic architecture. B cell-expressed immunoglobulin M antibodies also contributed to pain behavior, indicating a role for adaptive immunity. These results modeled many findings in early CRPS, but significant differences were also noted.

PERSPECTIVE

Multiple neuroimmune signaling mechanisms contribute to the pain, inflammation, and trophic changes observed in the injured limb of the rodent TFM. This model replicates many of the symptoms, signs, and pathophysiology of early CRPS, but most post-fracture changes resolve within 5 months and may not contribute to perpetuating chronic CRPS.

Adult Complex Regional Pain Syndrome Type I: A Narrative Review

Complex regional pain syndrome type I (CRPS I) is a multifactorial painful disorder with a complex pathogenesis. Both peripheral and central mechanisms are involved. Acute CRPS I is considered to be an exaggerated inflammatory disorder; however, over time, because of altered function of the sympathetic nervous system and maladaptive neuroplasticity, CRPS I evolves into a neurological disorder. This review thoroughly describes the pathophysiological aspects of CRPS I and summarizes the potential therapeutic options. The mechanisms and targets of the treatment are different in the early and late stages of the disease. This current review builds on a previous review by this author group by deepening the role of the peripheral classic and neuronal inflammatory component in the acute stage of this painful disorder.

LEVEL OF EVIDENCE

Not applicable.

Complex regional pain syndrome type I: a comprehensive review

BACKGROUND

Complex regional pain syndrome type I (CRPS I), formerly known as reflex sympathetic dystrophy (RSD), is a chronic painful disorder that usually develops after a minor injury to a limb. This topical review gives a synopsis of CRPS I and discusses the current concepts of our understanding of CRPS I in adults, the diagnosis, and treatment options based on the limited evidence found in medical literature. CRPS I is a multifactorial disorder. Possible pathophysiological mechanisms of CRPS I are classic and neurogenic inflammation, and maladaptive neuroplasticity. At the level of the central nervous system, it has been suggested that an increased input from peripheral nociceptors alters the central processing mechanisms.

METHODS

A literature search was conducted using, as electronic bibliographic database, Medline from 1980 until 2014.

RESULTS

An early diagnosis and multidisciplinary treatment are necessary to prevent permanent disability.

CONCLUSIONS

The pharmacological treatment of CRPS I is empirical and insufficiently effective. Further research is needed regarding the therapeutic modalities discussed in the guidelines. Physical therapy is widely recommended as a first-line treatment. The efficacy of local anesthetic sympathetic blockade as treatment for CRPS I is questionable.

Cytokine-related and histological biomarkers for neuropathic pain assessment

SUMMARY Neuropathic pain (NP) is a disabling condition that may occur following a disease or a lesion of the somatosensory nervous system. With an estimated prevalence of up to 8.2% in the general population, NP is common, and robust and objective diagnostic tools are warranted for pain assessment and follow-up. In the last years research has focused on defining biochemical and histological markers for this purpose, and possible systemic (blood, cerebrospinal fluid) and local (skin and nerve) targets have been investigated in a number of different NP disorders. This article focuses on an update giving an overview over some potential biomarkers for the diagnosis of NP, and will discuss their clinical relevance.

Activation of cutaneous immune responses in complex regional pain syndrome

The pathogenesis of complex regional pain syndrome (CRPS) is unresolved, but tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) are elevated in experimental skin blister fluid from CRPS-affected limbs, as is tryptase, a marker for mast cells. In the rat fracture model of CRPS, exaggerated sensory and sympathetic neural signaling stimulate keratinocyte and mast cell proliferation, causing the local production of high levels of inflammatory cytokines leading to pain behavior. The current investigation used CRPS patient skin biopsies to determine whether keratinocyte and mast cell proliferation occur in CRPS skin and to identify the cellular source of the up-regulated TNF-α, IL-6, and tryptase observed in CRPS experimental skin blister fluid. Skin biopsies were collected from the affected skin and the contralateral mirror site in 55 CRPS patients and the biopsy sections were immunostained for keratinocyte, cell proliferation, mast cell markers, TNF-α, and IL-6. In early CRPS, keratinocytes were activated in the affected skin, resulting in proliferation, epidermal thickening, and up-regulated TNF-α and IL-6 expression. In chronic CRPS, there was reduced keratinocyte proliferation, leading to epidermal thinning in the affected skin. Acute CRPS patients also had increased mast cell accumulation in the affected skin, but there was no increase in mast cell numbers in chronic CRPS.

PERSPECTIVE

The results of this study support the hypotheses that CRPS involves activation of the innate immune system, with keratinocyte and mast cell activation and proliferation, inflammatory mediator release, and pain.

Mast cells: a new target in the treatment of complex regional pain syndrome?

There is convincing evidence that inflammation plays a pivotal role in the pathophysiology of complex regional pain syndrome (CRPS). Besides inflammation, central sensitization is also an important phenomenon. Mast cells are known to be involved in the inflammatory process of CRPS and also play a role (at least partially) in the process of central sensitization. In the development of a more mechanism-based treatment, influencing the activity of mast cells might be important in the treatment of CRPS. We describe the rationale for using medication that counteracts the effects of mast cells in the treatment of CRPS.

Targeting oxidative injury and cytokines' activity in the treatment with anti-tumor necrosis factor-α antibody for complex regional pain syndrome 1

Cytokines and oxygen free radicals have been implicated in the potential pathogenic development of complex regional pain syndrome (CRPS). We aimed to analyze the relationship between clinical status, circulating levels of cytokines, and markers of oxidative damage during the treatment with anti-TNFα antibodies. The patient chosen for treatment had not had improvement through a number of conventional therapies and fulfilled the current diagnostic criteria for CRPS-1. We investigated the clinical variables before and after systemic administration of 1.4 mg/kg anti-TNFα antibody (infliximab), repeated after 1 month in a dose of 3 mg/kg. Blood samples were collected before and after anti-TNFα antibodies administration, and plasma was analyzed for 8-isoprostane-prostaglandin F2α (8-iso-PGF2α, a marker of oxidative injury) and cytokines (TNF-α, IL-4, IL-6, IL-7, IL-8, IL-10, IL-17A). Plasma concentrations of 8-iso-PGF2α were measured with radioimmunoassay (RIA), and the kinetics of cytokines were detected in plasma by antibody-based proximity ligation (PLA). Pathologically high levels of 8-iso-PGF2α were found in the patient. Immediately after each administration of infliximab, the levels of 8-iso-PGF2α decreased. Although the patient showed an improvement of the cutaneous dystrophic symptoms and diminished pain associated with these lesions, the levels of circulating TNFα increased after the administration of anti-TNFα antibodies. In a patient with CRPS-1 treated with anti-TNFα antibodies, we report increased levels of circulating TNFα and a temporary mitigation of oxidative stress as measured by plasma F2 -isoprostane. This case report provides evidence 2 supporting the indication of monitoring the oxidative stress biomarkers during treatment with anti-TNFα antibodies in CRPS 1.

Inflaming the brain: CRPS a model disease to understand neuroimmune interactions in chronic pain

We review current concepts in CRPS from a neuroimaging perspective and point out topics and potential mechanisms that are suitable to be investigated in the next step towards understanding the pathophysiology of CRPS. We have outlined functional aspects of the syndrome, from initiating lesion via inflammatory mechanisms to CNS change and associated sickness behavior, with current evidence for up-regulation of immunological factors in CRPS, neuroimaging of systemic inflammation, and neuroimaging findings in CRPS. The initiation, maintenances and CNS targets implicated in CRPS and in the neuro-inflammatory reflex are discussed in terms of CRPS symptoms and recent preclinical studies. Potential avenues for investigating CRPS with PET and fMRI are described, along with roles of inflammation, treatment and behavior in CRPS. It is our hope that this outline will provoke discussion and promote further empirical studies on the interactions between central and peripheral inflammatory pathways manifest in CRPS.

Effect of immunomodulating medications in complex regional pain syndrome: a systematic review

BACKGROUND

Different mechanisms are involved in a complex network of interactions resulting in the painful and impairing disorder, complex regional pain syndrome (CRPS). There is convincing evidence that inflammation plays a pivotal role in the pathophysiology of CRPS. Immunomodulating medication reduces the manifestation of inflammation by acting on the mediators of inflammation. Therefore, as inflammation is involved in the pathophysiology of CRPS, immunomodulating medication in CRPS patients may prove beneficial.

OBJECTIVES

To describe the current empirical evidence for the efficacy of administering the most commonly used immunomodulating medication (ie, glucocorticoids, tumor necrosis factor-α antagonists, thalidomide, bisphosphonates, and immunoglobulins) in CRPS patients.

METHODS

PubMed was searched for original articles that investigated CRPS and the use of one of the abovementioned immunomodulating agents.

RESULTS

The search yielded 39 relevant articles: from these, information on study design, sample size, duration of disease, type and route of medication, primary outcome measures, and results was examined.

DISCUSSION

Theoretically, the use of immunomodulating medication could counteract the ongoing inflammation and might be an important step in improving a disabled hand or foot, leading to further recovery. However, more high-quality intervention studies are needed.

Changes in plasma cytokines and their soluble receptors in complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is a chronic and often disabling pain disorder. There is evidence demonstrating that neurogenic inflammation and activation of the immune system play a significant role in the pathophysiology of CRPS. This study evaluated the plasma levels of cytokines, chemokines, and their soluble receptors in 148 subjects afflicted with CRPS and in 60 gender- and age-matched healthy controls. Significant changes in plasma cytokines, chemokines, and their soluble receptors were found in subjects with CRPS as compared with healthy controls. For most analytes, these changes resulted from a distinct subset of the CRPS subjects. When the plasma data from the CRPS subjects was subjected to cluster analysis, it revealed 2 clusters within the CRPS population. The category identified as most important for cluster separation by the clustering algorithm was TNFα. Cluster 1 consisted of 64% of CRPS subjects and demonstrated analyte values similar to the healthy control individuals. Cluster 2 consisted of 36% of the CRPS subjects and demonstrated significantly elevated levels of most analytes and in addition, it showed that the increased plasma analyte levels in this cluster were correlated with disease duration and severity.

PERSPECTIVE

The identification of biomarkers that define disease subgroups can be of great value in the design of specific therapies and of great benefit to the design of clinical trials. It may also aid in advancing our understanding of the mechanisms involved in the pathophysiology of CRPS, which may lead to novel treatments for this very severe condition.

Complex regional pain syndrome type I: neuropathic or not?

Complex regional pain syndrome (CRPS) is clinically characterized by pain, abnormal regulation of blood flow and sweating, edema of skin and subcutaneous tissues, active and passive movement disorders, and trophic changes. It is classified as type I (reflex sympathetic dystrophy) and type II (causalgia). CRPS cannot be reduced to one system or to one mechanism only. In the past decades, there has been absolutely no doubt that complex regional pain syndromes have to be classified as neuropathic pain disorders. This situation changed when a proposal to redefine neuropathic pain states was recently published, which resulted in an exclusion of CRPS from neuropathic pain disorders. We analyzed the strength of the scientific evidence that supports the neuropathic nature of complex regional pain syndromes.

Novel insights on diagnosis, cause and treatment of diabetic neuropathy: focus on painful diabetic neuropathy

Diabetic neuropathy is common, under or misdiagnosed, and causes substantial morbidity with increased mortality. Defining and developing sensitive diagnostic tests for diabetic neuropathy is not only key to implementing earlier interventions but also to ensure that the most appropriate endpoints are employed in clinical intervention trials. This is critical as many potentially effective therapies may never progress to the clinic, not due to a lack of therapeutic effect, but because the endpoints were not sufficiently sensitive or robust to identify benefit. Apart from improving glycaemic control, there is no licensed treatment for diabetic neuropathy, however, a number of pathogenetic pathways remain under active study. Painful diabetic neuropathy is a cause of considerable morbidity and whilst many pharmacological and nonpharmacological interventions are currently used, only two are approved by the US Food and Drug Administration. We address the important issue of the 'placebo effect' and also consider potential new pharmacological therapies as well as nonpharmacological interventions in the treatment of painful diabetic neuropathy.

Complex regional pain syndromes: new pathophysiological concepts and therapies

Complex regional pain syndrome (CRPS), formerly known as Sudeck's dystrophy and causalgia, is a disabling and distressing pain syndrome. We here provide a review based on the current literature concerning the epidemiology, etiology, pathophysiology, diagnosis, and therapy of CRPS. CRPS may develop following fractures, limb trauma or lesions of the peripheral or CNS. The clinical picture comprises a characteristic clinical triad of symptoms including autonomic (disturbances of skin temperature, color, presence of sweating abnormalities), sensory (pain and hyperalgesia), and motor (paresis, tremor, dystonia) disturbances. Diagnosis is mainly based on clinical signs. Several pathophysiological concepts have been proposed to explain the complex symptoms of CRPS: (i) facilitated neurogenic inflammation; (ii) pathological sympatho-afferent coupling; and (iii) neuroplastic changes within the CNS. Furthermore, there is accumulating evidence that genetic factors may predispose for CRPS. Therapy is based on a multidisciplinary approach. Non-pharmacological approaches include physiotherapy and occupational therapy. Pharmacotherapy is based on individual symptoms and includes steroids, free radical scavengers, treatment of neuropathic pain, and finally agents interfering with bone metabolism (calcitonin, biphosphonates). Invasive therapeutic concepts include implantation of spinal cord stimulators. This review covers new aspects of pathophysiology and therapy of CRPS.

Mechanical but not painful electrical stimuli trigger TNF alpha release in human skin

Pro-inflammatory cytokines-in particular tumor necrosis factor (TNF)-alpha-play an important role in pain and hyperalgesia. The stimuli inducing TNF-alpha release in humans and the time course of this release are largely unknown. We performed dermal microdialysis in healthy subjects (n=36) during three experimental conditions: The first condition (control) was microdialysis without stimulation, the second condition was 30 min of electrical current stimulation (1 Hz, 20 mA, moderately painful), the third condition was 30 min of repetitive mechanical stimulation via an impact stimulator (bullet 0.5 g; velocity 11 m/s, minimally painful). TNF-alpha was quantified in the samples collected at the end of the baseline perfusion (about 1 h of saline perfusion), at the end of stimulation period (exactly 30 min after stimulation commenced) and at the end of the experiment (exactly 90 min after stimulation commenced) using a commercial enzyme-linked immunosorbent assay. The C-fiber-related flare was quantified with a laser-Doppler imager. ANOVA revealed that TNF-alpha levels increased during the eluate sampling period. At 90 min TNF-alpha in the eluate of the mechanical stimulation condition was significantly increased as compared to electrical current or control condition. Flare intensity was highest in the electrical current stimulation condition and only marginally different from control in mechanical stimulation. Our results show that minimal mechanical trauma is sufficient to induce significant TNF-alpha release in the skin. These results may be relevant to the treatment of posttraumatic pain disorders.

The first scintigraphic detection of tumor necrosis factor-alpha in patients with complex regional pain syndrome type 1

Tumor necrosis factor (TNF)-alpha has been identified as a pathogenic factor in many immunologically based diseases and complex regional pain syndrome (CRPS). In this case series, we used radiolabeled technetium anti-TNF-alpha antibody to scintigraphically image TNF-alpha in 3 patients with type 1 CRPS. The results show that TNF-alpha was localized only in affected hands of patients with early-stage CRPS. No uptake was seen in clinically unaffected hands and late-stage CRPS. Our findings support the growing evidence for neuroimmune disturbance in patients with CRPS and may have important further implications for specific anticytokine treatment in patients with CRPS.

Status of immune mediators in painful neuropathies

Peripheral neuropathies of identical etiology can be painful or painless. The reason for this difference in clinical presentation is as yet unknown; however, immune mediators, particularly cytokines, may play a role. Cytokines are proteins that are produced by immune and nonimmune cells and are categorized as pro- and anti-inflammatory. The role of cytokines in the induction and maintenance of pain has been well established in animal models. Proinflammatory cytokines are mostly algesic, whereas anti-inflammatory cytokines have analgesic properties. Clinical research also gives evidence for the involvement of cytokines in painful and painless neuropathies. A proinflammatory cytokine profile seems to be associated with pain in peripheral neuropathies of different etiologies and in other painful disorders such as the complex regional pain syndrome. Specifically, an imbalance between pro- and anti-inflammatory cytokines may contribute to pain generation. A better understanding of the underlying pathophysiology may open new opportunities for the treatment of pain.

Mode of action of cytokines on nociceptive neurons

Cytokines are pluripotent soluble proteins secreted by immune and glial cells and are key elements in the induction and maintenance of pain. They are categorized as pro-inflammatory cytokines, which are mostly algesic, and anti-inflammatory cytokines, which have analgesic properties. Progress has been made in understanding the mechanisms underlying the action of cytokines in pain. To date, several direct and indirect pathways are known that link cytokines with nociception or hyperalgesia. Cytokines may act via specific cytokine receptors inducing downstream signal transduction cascades, which then modulate the function of other receptors like the ionotropic glutamate receptor, the transient vanilloid receptors, or sodium channels. This receptor activation, either through amplification of the inflammatory reaction, or through direct modulation of ion channel currents, then results in pain sensation. Following up on results from animal experiments, cytokine profiles have recently been investigated in human pain states. An imbalance of pro- and anti-inflammatory cytokine expression may be of importance for individual pain susceptibility. Individual cytokine profiles may be of diagnostic importance in chronic pain states, and, in the future, might guide the choice of treatment.

[Cytokine regulation and pain. Results of experimental and clinical research]

Cytokines are soluble peptides and proteins that are predominantly produced and secreted by immune cells. In numerous animal experiments the endogenous increase or application of exogenous pro-inflammatory cytokines is associated with pain behavior. In turn, cytokine inhibitors reduce such pain behavior in inflammatory and neuropathic pain models. Several clinical studies point out that cytokines are also important in different human pain states. Several chronic pain syndromes are associated with systemic pro-inflammatory cytokine profiles. In some pain syndromes these pro-inflammatory profiles are accompanied by a lack of analgesic anti-inflammatory cytokines. Numerous case reports and open clinical studies, but also some controlled trials show successful analgesic treatment using cytokine inhibitors. The following review article summarizes the main data of animal experiments and clinical trials concerning the role of cytokines in pain and the potential importance of cytokine modulation in pain treatment.

Cytokine regulation in animal models of neuropathic pain and in human diseases

Cytokines are soluble proteins secreted mainly by immune cells and are key players in the induction and maintenance of pain. Pro-inflammatory cytokines are mostly algesic, while anti-inflammatory cytokines have analgesic properties. After the role of cytokines was shown in diverse animal models of pain, interest arose in the systemic and local regulation of cytokines in human pain states. Most clinical studies give evidence for an imbalance between pro- and anti-inflammatory cytokines in neuropathic and other pain states with pronounced pro-inflammatory cytokine profiles. Anti-cytokine treatment gives encouraging preliminary results and supports the notion of a crucial role of cytokines also in human pain states. Further research is needed for a better understanding of the mechanisms linking altered cytokine profiles to the sensation of pain.