Pain: understanding and challenges for the rheumatologist

Exposure time as an influencing factor among rheumatoid arthritis patients subjected to traditional Siwan therapy

Rheumatoid arthritis (RA) is a long-term autoimmune disease characterized by intra- and extra-articular manifestations. Sand therapy is traditionally indicated for RA, chronic pain, skin diseases, and musculoskeletal disorders. Many places in the world use sand therapy, including Siwa, which is a famous place in Egypt. This study investigated the exposure time to Siwan traditional therapy as a factor influencing central sensitization, pain severity, pain threshold, and kinesiophobia in RA by measuring the central sensory inventory (CSI), visual analogue scale, pressure algometer, and TAMPA kinesiophobia scale, respectively. Twenty-four patients with RA were recruited from 6 traditional healing centers, 24 RA patients were recruited and randomly assigned to 2 equal groups (GI and GII). The first received Siwan traditional therapy for 3 days, while the second received the same program for 5 days. The results revealed a significant difference in CSI between pre- and posttreatment within the GII (P = .038). The Tampa Scale score improved significantly in both groups (P = .004 and P = .014, respectively). Pain severity and pain threshold at all sites showed significant posttreatment improvements in the GII. Significant posttreatment changes were only found for GI in terms of pain severity and the most painful joint (P = .010 and P = .035, respectively). Significant changes were observed in kinesiophobia, pain severity, and pain threshold in the most painful joint 3 and 5 days after Siwan traditional therapy. Despite the nonsignificant differences in all parameters between the 2 groups, all the measured parameters produced favorable results after 5 days of treatment, suggesting the need for a long-term effect investigation.

The Case for Comorbid Myofascial Pain-A Qualitative Review

Myofascial pain syndrome is widely considered to be among the most prevalent pain conditions, both in the community and in specialized pain clinics. While myofascial pain often arises in otherwise healthy individuals, evidence is mounting that its prevalence may be even higher in individuals with various comorbidities. Comorbid myofascial pain has been observed in a wide variety of medical conditions, including malignant tumors, osteoarthritis, neurological conditions, and mental health conditions. Here, we review the evidence of comorbid myofascial pain and discuss the diagnostic and therapeutic implications of its recognition.

"Asking Too Much?": Randomized N-of-1 Trial Exploring Patient Preferences and Measurement Reactivity to Frequent Use of Remote Multidimensional Pain Assessments in Children and Young People With Juvenile Idiopathic Arthritis

Background

Remote monitoring of pain using multidimensional mobile health (mHealth) assessment tools is increasingly being adopted in research and care. This assessment method is valuable because it is challenging to capture pain histories, particularly in children and young people in diseases where pain patterns can be complex, such as juvenile idiopathic arthritis (JIA). With the growth of mHealth measures and more frequent assessment, it is important to explore patient preferences for the timing and frequency of administration of such tools and consider whether certain administrative patterns can directly impact on children’s pain experiences.

Objective

This study aimed to explore the feasibility and influence (in terms of objective and subjective measurement reactivity) of several time sampling strategies in remote multidimensional pain reporting.

Methods

An N-of-1 trial was conducted in a subset of children and young people with JIA and their parents recruited to a UK cohort study. Children were allocated to 1 of 4 groups. Each group followed a different schedule of completion of MPT for 8 consecutive weeks. Each schedule included 2 blocks, each comprising 4 different randomized time sampling strategies, with each strategy occurring once within each 4-week block. Children completed MPT according to time sampling strategies: once-a-day, twice-a-day, once-a-week, and as-and-when pain was experienced. Adherence to each strategy was calculated. Participants completed the Patient-Reported Outcomes Measurement Information System Pain Interference Scale at the end of each week to explore objective reactivity. Differences in pain interference scores between time sampling strategies were assessed graphically and using Friedman tests. Children and young people and their parents took part in a semistructured interview about their preferences for different time sampling strategies and to explore subjective reactivity.

Results

A total of 14 children and young people (aged 7-16 years) and their parents participated. Adherence to pain reporting was higher in less intense time sampling strategies (once-a-week=63% [15/24]) compared with more intense time sampling strategies (twice-a-day=37.8% [127/336]). There were no statistically significant differences in pain interference scores between sampling strategies. Qualitative findings from interviews suggested that children preferred once-a-day (6/14, 43%) and as-and-when pain reporting (6/14, 43%). Creating routine was one of the most important factors for successful reporting, while still ensuring that comprehensive information about recent pain was captured.

Conclusions

Once-a-day pain reporting provides rich contextual information. Although patients were less adherent to this preferred sampling strategy, once-a-day reporting still provides more frequent assessment opportunities compared with other less intense or overburdensome schedules. Important issues for the design of studies and care incorporating momentary assessment techniques were identified. We demonstrate that patient reporting preferences are key to accommodate and are important where data capture quality is key. Our findings support frequent administration of such tools, using daily reporting methods where possible.

"Reluctant to Assess Pain": A Qualitative Study of Health Care Professionals' Beliefs About the Role of Pain in Juvenile Idiopathic Arthritis

OBJECTIVE

Reducing pain is one of the main health priorities for children and young people with juvenile idiopathic arthritis (JIA); however, some studies indicate that pain is not routinely assessed in this patient group. The aim of this study was to explore health care professionals' (HCPs) beliefs about the role of pain and the prioritization of its assessment in children and young people with JIA.

METHODS

Semi-structured interviews were conducted with HCPs who manage children and young people with JIA in the UK (including consultant and trainee pediatric rheumatologists, nurses, physical therapists, and occupational therapists). Data were analyzed qualitatively following a framework analysis approach.

RESULTS

Twenty-one HCPs participated. Analyses of the data identified 6 themes, including lack of training and low confidence in pain assessment, reluctance to engage in pain discussions, low prioritization of pain assessment, specific beliefs about the nature of pain in JIA, treatment of pain in JIA, and undervaluing pain reports. Assessment of pain symptoms was regarded as a low priority and some HCPs actively avoided conversations about pain.

CONCLUSION

These findings indicate that the assessment of pain in children and young people with JIA may be limited by knowledge, skills, and attitudinal factors. HCPs' accounts of their beliefs about pain in JIA and their low prioritization of pain in clinical practice suggest that a shift in perceptions about pain management may be helpful for professionals managing children and young people with this condition.

Risk factors for development and persistence of chronic widespread pain in spondyloarthritis: a population-based two-year follow-up study

Objective: To study chronic widespread pain (CWP) over time in patients with spondyloarthritis (SpA), and to identify risk factors for development and persistence of CWP.Methods: In this cohort study with baseline and 2.5 year follow-up postal surveys, patients with ankylosing spondylitis (AS) and undifferentiated spondyloarthritis (USpA) (47% women) answered questions regarding pain, and were categorized as no chronic pain (NCP), chronic regional pain (CRP), or CWP. For each risk factor candidate (disease duration, body mass index, smoking, and patient-reported outcome measures), logistic regression analyses with CWP as the main outcome were performed separately, together with a basic model including age, gender, and SpA subgroup.Results: Altogether, 644 patients could be categorized at both time-points, yielding similar prevalence estimates at baseline and follow-up, although 38% transitioned between pain groups. Risk factors (odds ratio; 95% confidence interval) for development of CWP included more pain regions (1.36; 1.20‒1.53), higher pain intensity (1.35; 1.20‒1.52), worse fatigue (1.25; 1.13‒1.38), and worse global health (1.35; 1.19‒1.54). Persistent CWP was reported by 72%. In addition to factors predicting development of CWP, higher age (1.02; 1.00‒1.04), female gender (1.82; 1.06‒3.10), and anxiety (1.07; 1.00-1.14) also predicted persistence.Conclusion: The prevalence of CWP remained high over time, but with individual transitions between the pain groups. The development and persistence of CWP were predicted by more pain and worse health, with the addition of female gender and higher age for persistent CWP. Special attention and treatment alternatives for patients with SpA and concomitant CWP are essential in the clinic.

Use of strong opioids for chronic pain in osteoarthritis: an insight into the Latin American reality

INTRODUCTION

Osteoarthritis is the most common cause of arthritis and one of the main causes of chronic pain. Although opioids are frequently employed for chronic pain treatment, their usage for osteoarthritis pain remains controversial due to the associated adverse effects. Most guidelines reserve their use for refractory pain in patients with hip and knee osteoarthritis. The situation is even more complex in Latin America, where the prevalence of insufficient pain treatment is high because of the limited availability and use of strong opioids. Areas covered: In this article we review the epidemiology of osteoarthritis, its socioeconomic burden, its impact as a chronic pain cause and the pharmacological treatment options, giving emphasis to the role of strong opioids, their safety and efficacy, especially in Latin American countries, where restrictions regulate their usage. Expert commentary: Usage of strong opioids is safe and effective in the short-term management of osteoarthritis with moderate to severe pain, when other pharmacological treatments are inadequate and surgery is contraindicated, provided their use adheres to existing guidelines. Educational programs for patients and physicians and further research on treating chronic pain with opioids should be implemented to reduce adverse effects and improve care quality.

Multidisciplinary treatment for rheumatic pain

Chronic pain experienced by patients with rheumatic conditions is recognized to contribute importantly to suffering. Multidisciplinary pain clinics that adhere to the biopsychosocial concept of pain management provide an effective treatment strategy for many with chronic pain. Other than for low back pain and fibromyalgia, little attention has been given to the specific experience of treating those with rheumatic diseases in such a setting. It is, however, reasonable to suggest that many patients with chronic rheumatic pain could benefit from exposure to a multidisciplinary pain treatment programme that incorporates components of education, exercise and activity, as well as psychological techniques and support. Although the specifics of such a treatment will require defining, rheumatologists can look forward to expanded care for their patients with chronic rheumatic pain, and they should be encouraged to become more involved in these clinics.

TGF-β is a potent inducer of Nerve Growth Factor in articular cartilage via the ALK5-Smad2/3 pathway. Potential role in OA related pain?

OBJECTIVE

Pain is the main problem for patients with osteoarthritis (OA). Pain is linked to inflammation, but in OA a subset of patients suffers from pain without inflammation, indicating an alternative source of pain. Nerve Growth Factor (NGF) inhibition is very efficient in blocking pain during OA, but the source of NGF is unclear. We hypothesize that damaged cartilage in OA releases Transforming Growth Factor-β (TGF-β), which in turn stimulates chondrocytes to produce NGF.

DESIGN

Murine and human chondrocyte cell lines, primary bovine and human chondrocytes, and cartilage explants from bovine metacarpal joints and human OA joints were stimulated with TGF-β1 and/or Interleukin-1 (IL-1)β. We analyzed NGF expression on mRNA level with QPCR and stained human OA cartilage for NGF immunohistochemically. Cultures were additionally pre-incubated with inhibitors for TAK1, Smad2/3 or Smad1/5/8 signaling to identify the TGF-β pathway inducing NGF.

RESULTS

NGF expression was consistently induced in higher levels by TGF-β than IL-1 in all of our experiments: murine, bovine and human origin, in cell lines, primary chondrocytes and explants cultures. TAK1 inhibition consistently reduced TGF-β-induced NGF whereas it fully blocked IL-1β-induced NGF expression. In contrast, ALK5-Smad2/3 inhibition fully blocked TGF-β-induced NGF expression. Despite the large variation in basal NGF in human OA samples (mRNA and histology), TGF-β exposure led to a consistent high level of NGF induction.

CONCLUSION

We show for the first time that TGF-β induces NGF expression in chondrocytes, in a ALK5-Smad2/3 dependent manner. This reveals a potential alternative non-inflammatory source of pain in OA.

Use of the painDETECT tool in rheumatoid arthritis suggests neuropathic and sensitization components in pain reporting

Rheumatoid arthritis (RA) is an inflammatory autoimmune condition typified by systemic inflammation targeted toward synovial joints. Inhibition of proinflammatory networks by disease-modifying antirheumatic drugs, eg, methotrexate and biologic therapies, including tumor necrosis factor-α inhibitors, often leads to suppression of disease activity observed at the clinical level. However, despite the era of widespread use of disease-modifying treatments, there remain significant groups of patients who continue to experience pain. Our study formulated a pain assessment tool in the arthritis clinic to assess feasibility of measurements including the visual analog scale (VAS) and painDETECT to assess multimodal features of pain in people with established RA (n=100). Clinical measures of disease activity (Disease Activity Score in 28 Joints [DAS28]) were also recorded. Our data showed that despite the majority of subjects on at least one disease-modifying agent, the majority of patients reported severe pain (54%) by VAS, despite well-controlled clinical disease, with mean DAS28 2.07±0.9. Using the painDETECT questionnaire, 67% of patients had unlikely neuropathic pain. A significant proportion of subjects (28%) had possible neuropathic pain and 5% had features of likely neuropathic pain by painDETECT scoring. We found a positive correlation between VAS and painDETECT (R (2)=0.757). Of note, the group who had likely or probable neuropathic pain also showed significantly increased pain reporting by VAS (P<0.01). Subjects who were clinically obese (body mass index >30) also had statistically higher proportions of pain reporting (VAS 89.0±0.7 mm) compared with subjects who had a normal body mass index (VAS 45.2±21.8 mm), P<0.05. Our findings suggest that multimodal features of pain perception exist in RA, including neuropathic and sensitization elements, perhaps explaining why a subgroup of people with RA continue to experience ongoing pain, despite their apparent suppression of inflammation.

Rheumatologists lack confidence in their knowledge of cannabinoids pertaining to the management of rheumatic complaints

Background

Arthritis pain is reported as one of the most common reasons for persons using medical herbal cannabis in North America. “Severe arthritis” is the condition justifying legal use of cannabis in over half of all authorizations in Canada, where cannabis remains a controlled substance. As champions for the care of persons with arthritis, rheumatologists must be knowledgeable of treatment modalities both traditional and non-traditional, used by their patients. As study of cannabinoid molecules in medicine is recent, we have examined the confidence in the knowledge of cannabinoids expressed by Canadian rheumatologists.

Methods

The confidence of rheumatologists in their knowledge of cannabinoid molecules and mechanisms relevant to rheumatology, and their ability to advise patients about cannabinoid treatments was recorded by an online questionnaire circulated via email to the entire Canadian Rheumatology Association membership.

Results

Over three quarters of the 128 respondents lacked confidence in their knowledge of cannabinoid molecules. While 45% of respondents believed there was no current role for cannabinoids in rheumatology patient care, only 25% supported any use of herbal cannabis. With 70% never having previously prescribed or recommended any cannabinoid treatment, uncertainty regarding good prescribing practices was prevalent. Concerns about risks of cannabis use were in line with the current literature.

Conclusions

Rheumatologists lacked confidence in their knowledge of cannabinoid molecules in general and in their competence to prescribe any cannabinoid for rheumatic complaints. In line with this uncertainty, there is reticence to prescribe cannabinoid preparations for rheumatology patients. Guidance is required to inform rheumatologists on the evidence regarding cannabinoids.

Experimental endotoxemia as a model to study neuroimmune mechanisms in human visceral pain

The administration of bacterial endotoxin (i.e., lipopolysaccharide, LPS) constitutes a well-established experimental approach to study the effects of an acute and transient immune activation on physiological, behavioral, and emotional aspects of sickness behavior in animals and healthy humans. However, little is known about possible effects of experimental endotoxemia on pain in humans. This knowledge gap is particularly striking in the context of visceral pain in functional as well as chronic-inflammatory gastrointestinal disorders. Although inflammatory processes have been implicated in the pathophysiology of visceral pain, it remains incompletely understood how inflammatory mediators interact with bottom-up (i.e., increased afferent input) and top-down (i.e., altered central pain processing) mechanisms of visceral hyperalgesia. Considering the recent findings of visceral hyperalgesia after LPS application in humans, in this review, we propose that experimental endotoxemia with its complex peripheral and central effects constitutes an experimental model to study neuroimmune communication in human pain research. We summarize and attempt to integrate relevant animal and human studies concerning neuroimmune communication in visceral and somatic pain, discuss putative mechanisms, and conclude with future research directions.

Incidence and predictors of secondary fibromyalgia in an early arthritis cohort

OBJECTIVES

Secondary fibromyalgia (FM) is common among patients with inflammatory arthritis, but little is known about its incidence and the factors leading to its development. The authors examined the incidence of secondary FM in an early inflammatory arthritis cohort, and assessed the association between pain, inflammation, psychosocial variables and the clinical diagnosis of FM.

METHODS

Data from 1487 patients in the Canadian Early Arthritis Cohort, a prospective, observational Canadian cohort of early inflammatory arthritis patients were analysed. Diagnoses of FM were determined by rheumatologists. Incidence rates were calculated, and Cox regression models were used to determine HRs for FM risk.

RESULTS

The cumulative incidence rate was 6.77 (95% CI 5.19 to 8.64) per 100 person-years during the first 12 months after inflammatory arthritis diagnosis, and decreased to 3.58 (95% CI 1.86 to 6.17) per 100 person-years 12-24 months after arthritis diagnosis. Pain severity (HR 2.01, 95% CI 1.17 to 3.46) and poor mental health (HR 1.99, 95% CI 1.09 to 3.62) predicted FM risk. Citrullinated peptide positivity (HR 0.48, 95% CI 0.26 to 0.88) was associated with decreased FM risk. Serum inflammatory markers and swollen joint count were not significantly associated with FM risk.

CONCLUSIONS

The incidence of FM was from 3.58 to 6.77 cases per 100 person-years, and was highest during the first 12 months after diagnosis of inflammatory arthritis. Although inflammation was not associated with the clinical diagnosis of FM, pain severity and poor mental health were associated with the clinical diagnosis of FM. Seropositivity was inversely associated with the clinical diagnosis of FM.

Pain and daily activities in rheumatoid arthritis

AIM

The aim of this study was to describe experiences of pain and its relationship to daily activities in people with rheumatoid arthritis (RA).

METHOD

Seven semi-structured focus group discussions were conducted with 33 men and women of different ages with RA. Data were analysed with content analysis.

RESULTS

Pain affected everyday life and may be a barrier to perform valued activities. Regarding the impact of pain on participation and independence, personal factors and the social environment were found to be important. It could be a struggle to find the right activity balance, since it was easy to be overactive, triggering subsequent elevation of pain levels. However, the participants also described activities as a mediator of pain and a distraction from it.

CONCLUSION

The relationship between pain and daily activities in RA was complex. Pain as an impairment was expressed to be related to activity limitations and participation restrictions, as well as to contextual factors. These findings highlight the clinical importance of paying attention to the complexity of pain and its relation to daily activities and participation.

Imaging studies in Freund's complete adjuvant model of regional polyarthritis, a model suitable for the study of pain mechanisms, in the rat

OBJECTIVE

In view of the extensive distribution in the spinal cord of primary afferent nociceptive nerve fibers from the rat hind paw, a model of regional inflammation would be useful in the study of arthritis pain. However, the subcutaneous intraplantar injection of a single low dose of Freund's complete adjuvant (CFA) has been used mostly as an inflammation model. We undertook this study to characterize this model by means of conventional radiography, microfocal computed tomography, and bone densitometry and by examining changes in pain-related behavior.

METHODS

Male rats were injected unilaterally with CFA or saline subcutaneously in the hind paw and killed 7, 15, and 30 days after injection. Pain-related behavior was studied using the Hargreaves, von Frey, and acetone tests.

RESULTS

CFA-injected animals developed soft tissue inflammation and polyarthritis restricted to the joints of the injected hind paw. No signs of joint involvement were observed 7 days after CFA injection. On day 15 after CFA injection, there was widening of joint space indicative of joint effusion. By day 30 after CFA injection, there was evidence of joint damage with joint space narrowing, erosions, osteophyte formation, and joint deformity. There were no changes contralaterally or in saline-injected rats. Mechanical hyperalgesia and cold allodynia were present in the affected hind paw from day 1 through day 30.

CONCLUSION

Signs of arthritis were strictly unilateral and started only 2 weeks after injection. Since the affected area has a broad representation in the spinal cord, this model has advantages over monarthritis models for the study of plastic changes in the dorsal horn of the spinal cord.

Mindfulness meditation: a primer for rheumatologists

Over the past decade, there has been an increasing interest in meditation as a mind-body approach, given its potential to alleviate emotional distress and promote improved well being in a variety of populations. The overall purpose of this review is to provide the practicing rheumatologist with an overview of mindfulness and how it can be applied to Western medical treatment plans to enhance both the medical and psychological care of patients.

A randomized double blind comparison of short-term duodenally administrated whale and seal blubber oils in patients with inflammatory bowel disease and joint pain

Compared with soy oil, 10 days treatment with seal oil (SO), 10mLx3 daily, self-administrated through a nasoduodenal feeding tube, relieves joint pain in patients with inflammatory bowel disease (IBD). This randomized, controlled, double blind pilot trial compares SO and whale oil (WO) administered similarly by duodenal tube, for 10 days in 18 patients with IBD-related joint pain (n=9 per group). Other long chain n-3 polyunsaturated fatty acids were prohibited 7-days prior to and during study. Significant changes from baseline to study end were observed in both groups: reduced plasma arachidonic acid to eicosapentaenoic acid ratio and prostaglandin E(2) (PGE(2)) levels (tendency in WO group), decreased IBD-related joint pain and IBD-disease activity, and improved quality of life. These changes were not significantly different between SO and WO groups. Inhibition of cyclooxygenase is consistent with amelioration of IBD-related joint pain, but, as active control was used, effects need confirmation.

Antinociceptive effects of tumor necrosis factor alpha neutralization in a rat model of antigen-induced arthritis: evidence of a neuronal target

OBJECTIVE

The reduction of pain in the course of antiinflammatory therapy can result from an attenuation of the inflammatory process and/or from the neutralization of endogenous mediators of inflammation that act directly on nociceptive neurons. The purpose of this study was to investigate whether analgesic effects of the neutralization of tumor necrosis factor alpha (TNFalpha) are due to an attenuation of inflammation or whether direct neuronal effects significantly contribute to pain relief in the course of therapy.

METHODS

Locomotor and pain-related behavior and histology were assessed in rats with chronic antigen-induced arthritis (AIA) in the knee joint, and the rats were treated with systemic saline, etanercept, or infliximab. The expression of TNF receptors (TNFRs) in dorsal root ganglia was measured using immunohistochemical analysis and polymerase chain reaction. Action potentials were recorded from afferent Adelta fibers and C fibers of the medial knee joint nerve, and etanercept and infliximab were injected intraarticularly into normal or inflamed knee joints (AIA or kaolin/carrageenan-induced inflammation).

RESULTS

In rats with AIA, both etanercept and infliximab significantly decreased inflammation-induced locomotor and pain-related behavior, while joint swelling was only weakly attenuated and histomorphology still revealed pronounced inflammation. A large proportion of dorsal root ganglion neurons showed TNFRI- and TNFRII-like immunoreactivity. Intraarticular injection of etanercept reduced the responses of joint afferents to mechanical stimulation of the inflamed joint starting 30 minutes after injection, but had no effect on responses to mechanical stimulation of the uninflamed joint.

CONCLUSION

Overall, these data show the pronounced antinociceptive effects of TNFalpha neutralization, thus suggesting that reduction of the effects of TNFalpha on pain fibers themselves significantly contributes to pain relief.

Generalized and symptom-specific sensitization of chronic itch and pain

BACKGROUND

Physicians are frequently confronted with patients reporting severe itch and pain. Particularly in patients suffering from persistent itch and pain, central and peripheral sensitization processes are assumed to be involved in the long-term maintenance and aggravation of the symptoms. The present study explores generalized and symptom-specific sensitization processes in patients suffering from persistent itch and pain. Specifically, it examines whether patients with chronic itch and pain are more sensitive to somatosensory stimuli (generalized sensitization) and simultaneously perceive somatosensory stimuli as a symptom of their main physical complaint, e.g. pain in chronic pain patients (symptom-specific sensitization).

METHODS

Thresholds for different mechanical and electrical sensory stimuli of Quantitative Sensory Testing were determined in 15 female patients suffering from chronic itch associated with atopic dermatitis, 15 female chronic pain patients diagnosed with fibromyalgia, and 19 female healthy controls. Intensities of itch and pain sensations were rated on a visual analogue scale.

RESULTS

As expected, the patient groups had significantly lower tolerance thresholds for the somatosensory stimuli applied than the healthy controls, supporting generalized sensitization. Moreover, patients with chronic itch consistently reported more itch, while patients with chronic pain partly reported more pain in response to analogous somatosensory stimuli than the healthy controls and the other patient group, indicating symptom-specific sensitization.

CONCLUSION

The present study provides preliminary support that both generalized and symptom-specific sensitization processes play a role in the regulation and processing of somatosensory stimulation of patients with chronic itch and pain.

Sensitization of unmyelinated sensory fibers of the joint nerve to mechanical stimuli by interleukin-6 in the rat: an inflammatory mechanism of joint pain

OBJECTIVE

Pain during mechanical stimulation of the joint and spontaneous pain are major symptoms of arthritis. An important neuronal process of mechanical hypersensitivity of the joint is the sensitization of thin myelinated Adelta fibers and unmyelinated C fibers innervating the joint. Because interleukin-6 (IL-6) is a major inflammatory mediator, we investigated whether this cytokine has the potential to sensitize joint afferents to mechanical stimuli.

METHODS

In electrophysiologic experiments conducted on anesthetized rats, action potentials were recorded from afferent fibers supplying the knee joint. Responses to innocuous and noxious rotation of the tibia against the femur in the knee joint were monitored before and 1-2 hours after injection of test compounds into the joint cavity.

RESULTS

Injection of IL-6 and coinjection of IL-6 plus soluble IL-6 receptor (sIL-6R) caused a gradual increase in the responses of C fibers to innocuous and noxious rotation within 1 hour. The increase in responses to IL-6 and IL-6 plus sIL-6R was prevented by coadministration of soluble glycoprotein 130 (sgp130), but sgp130 did not reverse established mechanical hyperexcitability. Responses of Adelta fibers were not altered by the compounds. While injection of sIL-6R alone into the normal knee joint did not influence responses to mechanical stimulation, injection of sIL-6R into the acutely inflamed knee joint caused an increase in responses.

CONCLUSION

IL-6 has the potential to sensitize C fibers in the joint to mechanical stimulation. Thus, IL-6 contributes to mechanical hypersensitivity, most likely due to an action of IL-6 on nerve fibers themselves.