Prevalence and Cost Analysis of Complex Regional Pain Syndrome (CRPS): A Role for Neuromodulation

Examining the effects of race/ethnicity and other factors on outcomes of care for complex regional pain syndrome type 1 in the United States

Complex regional pain syndrome is a chronic pain disorder marked by symptoms such as swelling, impaired motor function, and sympathetic dysfunction. Our primary objective was to determine the total number of complex regional pain syndrome type 1 (CRPS-1) emergency department (ED) visits and hospitalizations by race/ethnicity, as well as to assess sex and age distributions by race/ethnicity. Secondary objectives were to examine whether race/ethnicity, as well as select characteristics, are associated with hospitalization and longer length of stay. We completed a cross-sectional study of adults (19+ years) using acute and inpatient care data from the 2020 Nationwide Emergency Department Sample and the National Inpatient Sample. The overall rate of CRPS-1 diagnosis among ED visits and hospitalizations was 0.02% and 0.04%, respectively. Most CRPS-1 care was provided to White (ED: 83.1%; inpatient: 82.8%) patients. Within race/ethnicity groups, CRPS-1 ED visits and hospitalizations generally increased with age. Secondary findings included: 1) ED visits by Black individuals (compared with White) were significantly negatively associated with immediate hospitalization (adjusted odds ratio (AOR) 0.74, 95% CI 0.55 to 0.99); 2) hospitalizations by Black patients (compared with White) were independently associated with increased length of stay (odds ratio (OR) 1.45, 95% CI 1.07 to 1.96), though the association diminished with adjustment; and 3) drug abuse was significantly associated with hospitalization (AOR 4.67, 95% CI 3.53 to 6.18) and longer length of stay (AOR 1.81, 95% CI 1.34 to 2.46). Race/ethnicity was minimally associated with studied CRPS-1 outcomes. Additional studies are required to determine the impact of race/ethnicity on seeking care for CRPS-1.

Rehabilitation Interventions for Adults With Complex Regional Pain Syndrome: A Scoping Review Protocol

OBJECTIVES

This scoping review will explore the literature related to rehabilitation interventions for the treatment of adults living with complex regional pain syndrome (CRPS), describe the domains and outcome measures used to assess their effectiveness, and examine the neurophysiological bases of these interventions.

INTRODUCTION

The unremitting symptoms of CRPS, a chronic pain condition, are associated with long-term disability, poor psychological health, decreased emotional and social well-being, and reduced quality of life. Effective treatment for persistent symptoms is notoriously difficult. Therapeutic approaches such as graded motor imagery or pain exposure therapy are recommended for CRPS but show mixed results, insufficient effectiveness, variability in outcome measures, and unclear neurophysiological bases.

INCLUSION CRITERIA

This review will consider studies that include any form of non-invasive rehabilitation intervention delivered by a healthcare professional in any setting for adults with a CRPS diagnosis. Quantitative, qualitative and observational studies, text and opinion papers will be considered.

METHODS

The Joanna Briggs Institute (JBI) methodology will be used to conduct this scoping review. MEDLINE, Embase, Scopus, APA PsycINFO, CINAHL, Cochrane, OpenGrey Google and ProQuest Dissertations and Theses Global (ProQuest) will be searched for studies in English published between 2007 and 2024. Two independent reviewers will screen the titles, abstracts, and full texts of the selected studies. Data collection will be performed using a tool developed by the researchers based on the standardised JBI tool. Data will be presented in a comprehensive narrative summary.

TRAIL REGISTRATION

https://doi.org/10.17605/OSF.IO/P967T.

Complex regional pain syndrome after distal radius fracture: A survey of current practices

INTRODUCTION

Complex regional pain syndrome (CRPS) is a common complication following distal radius fractures that is difficult to diagnose and can lead to permanent disability. While various proposed prophylaxis and treatment modalities exist, high-quality evidence guiding practice is limited. This survey of Orthopaedic Trauma Association (OTA) and Canadian Orthopaedic Association (COA) members was conducted with the primary aim of assessing practice patterns in distal radius fractures complicated with CRPS.

METHODS

An electronic survey was distributed to practicing orthopaedic surgeons in the COA and OTA. Questions assessed practice setting, preference in management of distal radius fractures and CRPS, comfort level in managing CRPS, and identification of gaps in management. Responses were anonymized and collected over 8 months. Response data was analyzed using descriptive statistics; thematic analysis was used on free text response.

RESULTS

134 survey responses were completed. 84% of respondents felt the incidence of CRPS in distal radius fractures was 1-10%, while 15% felt it was closer to 11-20%. 24% of respondents utilized the "Budapest Criteria" to diagnose CRPS. 40% offered prophylaxis in patients felt to be at high risk of developing CRPS. 66% of surgeons felt neutral, uncomfortable, or very uncomfortable managing CRPS in distal radius fractures. When asked to consider adopting a prophylactic therapy, 38% of surgeons indicated that a therapy that reduced the absolute risk of CRPS by 6-10% would change their practice. Gaps in current practice included lack of evidence-based treatment and prevention strategies and diagnostic uncertainty.

CONCLUSION

This study identified that amongst orthopaedic surgeons in the COA and OTA, diagnosis, treatment, and prophylaxis strategies for CRPS in distal radius fractures are heterogeneous. Surgeons are not confident in their treatment of CRPS. Future studies using rigorous research methods are warranted to improve management.

Sex-specific role of RNA-binding protein, pAUF1, on prolonged hypersensitivity after repetitive ischemia with reperfusion injury

ABSTRACT

Repetitive ischemia with reperfusion (I/R) injury is a common cause of myalgia. Ischemia with reperfusion injuries occur in many conditions that differentially affect males and females including complex regional pain syndrome and fibromyalgia. Our preclinical studies have indicated that primary afferent sensitization and behavioral hypersensitivity caused by I/R injury may be due to sex-specific gene expression in the dorsal root ganglia (DRG) and distinct upregulation of growth factors and cytokines in the affected muscles. To determine how these unique gene expression programs may be established in a sex-dependent manner in a model that more closely mimics clinical scenarios, we used a developed prolonged ischemic myalgia model in mice whereby animals experience repeated I/R injuries and compared behavioral results with unbiased and targeted screening strategies in male and female DRG. Several distinct proteins were found to be differentially expressed in male and female DRG, including phosphorylated AU-rich element RNA-binding protein (pAUF1), which is known to regulate gene expression. Nerve-specific siRNA-mediated knockdown of AUF1 inhibited prolonged hypersensitivity in females only, whereas overexpression of AUF1 in male DRG neurons increased pain-like responses. AUF1 knockdown was able to specifically inhibit repeated I/R-induced gene expression in females potentially downstream of prolactin receptor signaling. Data suggest RNA-binding proteins such as pAUF1 may underlie the sex-specific effects on DRG gene expression that modulates behavioral hypersensitivity after repeated I/R injury through prolactin signaling. This study may aid in finding distinct receptor differences related to the evolution of acute to chronic ischemic muscle pain development between sexes.

Refractory CRPS pain treated with psilocybin: A case report

Psilocybin shows promise as a treatment for CRPS, offering significant pain relief and functional improvement in a patient with refractory symptoms. This case highlights the need for further research into psilocybin's efficacy and optimal dosing for chronic pain management.

Effect of an interdisciplinary inpatient program for patients with complex regional pain syndrome in reducing disease activity-a single-center prospective cohort study

OBJECTIVE

The aim of this study was to evaluate the benefit of inpatient treatment in reducing disease activity in patients with complex regional pain syndrome (CRPS) who have exhausted outpatient options. Furthermore, the study sought to identify patient-related outcome variables that predict a reduction in disease activity.

METHODS

The primary outcome was disease severity (CRPS Severity Score, range 0-16 points). Secondary outcomes included depression, anxiety, physical function, pain interference, fatigue, sleep disturbance, and the ability to participate in social roles and activities, all of which were assessed using the PROMIS-29. Furthermore, pain catastrophizing, neuropathic pain, quality of life, pain self-efficacy, medication intake, and the patient's global impression of change were examined in accordance with current international agreed recommendations, assessed at discharge, 3-month, and 6-month post-discharge. Mixed-effects models were conducted to identify baseline variables associated with CRPS severity.

RESULTS

Twenty-five patients completed the program (mean age 49.28 [SD 11.23] years, 92% females, mean symptom duration 8.5 [SD 6.5] months). Results showed a significant reduction between baseline and discharge of disease activity (CSS -2.36, P < .0001), pain (PROMIS-29 pain -0.88, P = .005), and emotional function (PROMIS-29 depression -5.05, P < .001; fatigue -4.63, P = .002). Moderate evidence for a reduction between baseline and discharge could be observed for pain interference (+2.27, P = .05), social participation (PROMIS-29 + 1.93, P = .05), anxiety (PROMIS-29 -3.32, P = .02) and physical function (PROMIS-29 + 1.3, P = .03). On discharge, 92% of patients (23 of 25) reported improvement in their overall condition. In the follow-up period, medication intake could be reduced after 3 (MQS -8.22, P = .002) and 6 months (MQS -8.69, P = .001), and there was further improvement in social participation after 3 months (PROMIS-29 + 1.72, 0.03) and sleep after 6 months (PROMIS-29 + 2.38, 0.008). In the mixed models, it was demonstrated that patients experiencing less pain at baseline also exhibited lower disease activity.

CONCLUSION

The results of this study confirm that inpatient interdisciplinary treatment of CRPS patients improves disease activity, pain, physical function, emotional function, and social participation. Most improvements were maintained for up to 6 months after discharge. The majority of patients reported that their overall condition had improved during the study period.

Alpha-lipoic acid reduces nociception by reducing oxidative stress and neuroinflammation in a model of complex regional pain syndrome type I in mice

Complex regional pain syndrome type I (CRPS-I) is a disabling pain condition without adequate treatment. Chronic post-ischemia pain injury (CPIP) is a model of CRPS-I that causes allodynia, spontaneous pain, inflammation, vascular injury, and oxidative stress formation. Antioxidants, such as alpha lipoic acid (ALA), have shown a therapeutic potential for CRPS-I pain control. Thus, we aim to evaluate if ALA repeated treatment modulates neuroinflammation in a model of CRPS-I in mice. We used male C57BL/6 mice to induce the CPIP model (O-ring torniquet for 2 h in the hindlimb). For the treatment with ALA or vehicle (Veh) mice were randomly separated in four groups and received 100 mg/kg orally once daily for 15 days (CPIP-ALA, CPIP-Veh, Control-ALA, and Control-Veh). We evaluated different behavioral tests including von Frey (mechanical stimulus), acetone (cold thermal stimulus), rotarod, open field, hind paw edema determination, and nest-building (spontaneous pain behavior). Also, hydrogen peroxide (H2O2) levels, NADPH oxidase and superoxide dismutase (SOD) activity in the sciatic nerve and spinal cord, and Iba1, Nrf2, and Gfap in spinal cord were evaluated at 16 days after CPIP or sham induction. Repeated ALA treatment reduced CPIP-induced mechanical and cold allodynia and restored nest-building capacity without causing locomotor or body weight alteration. ALA treatment reduced SOD and NADPH oxidase activity, and H2O2 production in the spinal cord and sciatic nerve. CPIP-induced neuroinflammation in the spinal cord was associated with astrocyte activation and elevated Nfr2, which were reduced by ALA. ALA repeated treatment prevents nociception by reducing oxidative stress and neuroinflammation in a model of CRPS-I in mice.

An update on the pharmacotherapeutic options for complex regional pain syndrome

INTRODUCTION

Complex regional pain syndrome (CRPS) is a rare and painful condition that has a wide range of triggering factors, often traumatic, and can present various clinical manifestations. The lack of knowledge about the underlying mechanisms has led to numerous treatment approaches, both conservative and surgical, which work through different mechanisms of action.

AREAS COVERED

In this review, the authors explore the key aspects of CRPS, including definition, diagnostic criteria, pitfalls, pathogenic hypotheses, and treatment strategies with a focus on pharmacotherapy. The review was based on a comprehensive search of the literature using PubMed, while also considering international guidelines for managing CRPS.

EXPERT OPINION

Based on the available evidence, pharmacological interventions appear to be effective in treating CRPS, especially when they target peripheral mechanisms, specifically nociceptive inflammatory pain, and when administered early in the course of the disease. However, there is still a lack of reliable evidence regarding the effects of drugs on central mechanisms of chronic pain in CRPS. In our expert opinion, drug therapy should be initiated as soon as possible, particularly in warm CRPS patient clusters, to prevent significant functional limitations, psychological distress, and negative impacts on individuals' social and economic well-being.

Economic evaluation of management strategies for complex regional pain syndrome (CRPS)

Background: The economic impact of Complex Regional Pain Syndrome (CRPS) on both patients and the global healthcare system continues to escalate. However, the economic implications associated with management interventions for CRPS have received limited attention. Therefore, our objective is to perform a thorough examination of published economic assessments of the various management strategies utilized for CRPS. Methods: A thorough search spanning four general medical databases and three health economic databases to identify full economic evaluations on CRPS management strategies from January 1994 to June 2023 were conducted. The quality of these studies were evaluated by employing the Consolidated Health Economic Evaluation Reporting Standards (CHEERS) statement. To enable cross-study comparisons conducted in different countries, we adjusted the costs reported in the selected studies for inflation and converted them into 2023 US dollars. Results: A total of nine economic evaluations, consisting of eight high-quality and one medium-quality, were identified across five nations during a span of 29 years. The findings suggest that the most economically efficient intervention for CRPS are interventional approaches of Spinal Cord Stimulation (SCS) in comparison to conventional management for long periods of time. Furthermore, in situations where there is a limited time period of less than 1 year, rehabilitation therapies, particularly physical therapy, have been demonstrated to be more effective in terms of both cost and clinical outcomes. Conclusion: The interventional management strategies, particularly for severe and persistent CRPS over long periods, may offer the greatest cost efficiency. In conditions with limited timelines, rehabilitation measures, such as rehabilitation therapies, can be cost-effective. However, insufficient data for other common interventions prevents the formation of a definitive conclusion. Similarly, it is crucial to recognize that the results of these interventions might be affected by the selection of comparator and the threshold for willingness to pay.

Gender-Related Issues in the Management of Low-Back Pain: A Current Concepts Review

BACKGROUND

Low back pain (LBP) is an emerging disease. This review aims to investigate the role of gender-related factors in the diagnosis, clinical, and surgical management of LBP.

METHODS

From January 2002 to March 2023, EMBASE, SCOPUS, OVID-MEDLINE, Google Scholar, PubMed, and Web of Science were searched to identify relevant papers for further analysis.

RESULTS

Fifteen papers were included in this review. Sex- and gender-related differences were analyzed regarding the following points: (1) LBP epidemiology; (2) LBP physiopathology; (3) conservative management of LBP; (4) major vertebral surgery for LBP. The conservative treatment of LBP highlights that women claim services later in terms of poorer health status than men. In the postoperative phase, female patients show worse LBP, quality of life, and disability, but equal or greater interval change, compared with male patients complaining of lumbar degenerative disease.

CONCLUSIONS

LBP epidemiology and clinical outcomes, following conservative and surgical management of patients complaining of back pain, might depend on both sex- and gender-related factors. It is mandatory to assess gender-related indicators in patients referred to LBP and address them to improve their clinical outcomes and quality of life.

The Role of Neuroinflammation in Complex Regional Pain Syndrome: A Comprehensive Review

Complex Regional Pain Syndrome (CRPS) is an excess and/or prolonged pain and inflammation condition that follows an injury to a limb. The pathogenesis of CRPS is multifaceted that remains incompletely understood. Neuroinflammation is an inflammatory response in the peripheral and central nervous systems. Dysregulated neuroinflammation plays a crucial role in the initiation and maintenance of pain and nociceptive neuronal sensitization, which may contribute to the transition from acute to chronic pain and the perpetuation of chronic pain in CRPS. The key features of neuroinflammation encompass infiltration and activation of inflammatory cells and the production of inflammatory mediators in both the central and peripheral nervous systems. This article reviews the role of neuroinflammation in the onset and progression of CRPS from six perspectives: neurogenic inflammation, neuropeptides, glial cells, immune cells, cytokines, and keratinocytes. The objective is to provide insights that can inform future research and development of therapeutic targets for CRPS.

Scrambler Therapy for Patients With Complex Regional Pain Syndrome: A Case Series

Complex regional pain syndrome is a chronic debilitating pain disorder that is difficult to manage, in part due to its heterogeneous clinical presentation and lack of clearly defined pathophysiology. Patients usually require a multidisciplinary approach to treatment, which can entail pharmacotherapy, physical therapy, behavioral therapy, and interventional pain procedures, such as sympathetic nerve blocks, spinal cord stimulation, and dorsal root ganglion stimulation. However, many patients continue to experience pain refractory to these multimodal strategies. Scrambler therapy (ST) is a noninvasive method of neuromodulation that is applied through cutaneous electrodes, and can alleviate chronic neuropathic pain by stimulating C-fibers and replacing endogenous pain signals with synthetic non-nociceptive signals. Although the use of ST has been reported for several types of refractory central and peripheral neuropathic pain, there is a paucity of data regarding the use of ST for complex regional pain syndrome. We present two patients with complex regional pain syndrome of the right lower extremity, who each underwent ST and experienced significant pain relief and improvement in function and quality of life.

Incidence and risk factors for complex regional pain syndrome in radius fractures: meta-analysis

PURPOSE

This meta-analysis aimed to evaluate the incidence and risk factors for complex regional pain syndrome (CRPS) in radius fractures.

METHODS

The meta-analysis was performed using the PubMed, Embase, Scopus, and Cochrane Collaboration Library databases. Studies focusing on patients with conservative or surgically treated radius fractures leading to CRPS were included. A control group consisting of patients with radius fractures and no CRPS (-) was included. The outcome measures were incidence and risk factors. Comparative studies were also included. Data were combined using Review Manager 5.4.

RESULTS

Out of 610 studies, nine studies were included. The incidence of CRPS after radius fractures ranged from 0.19 to 13.63% (95% CI: 11.12-16.15%). Open fractures, high-energy mechanisms in radial head fractures, and associations with ulnar fractures were risk factors for CRPS [(RR: 0.98; 95% CI: 0.97-1.00), (RR: 0.18; 95% CI: 0.07-0.47), and (RR: 1.25; 95% CI: 1.17-1.35), respectively]. Other risk factors were female sex and high body mass index [(RR: 1.20; 95% CI: 1.05-1.37) and (MD: 1.17; 95% CI: 0.45-1.88)]. Psychiatric factors also increased the incidence of CRPS (RR: 2.04; 95% CI: 1.83-2.28). On the other hand, the type of surgery (external fixation or open reduction and internal fixation) and manipulations; associated comorbidities (diabetes and hypertension) together with tobacco and alcohol abuse; marital status, educational level, employment status, and socioeconomic status were not risk factors (p > 0.05).

CONCLUSION

The incidence of CRPS in radius fractures was 13.63%. Fractures with greater complexity or greater associated tissue damage, female sex, high BMI, and psychiatric disorders were risk factors for the development of CRPS.

LEVEL OF EVIDENCE

Meta-analysis of cohort and case series studies; II.

Complex Regional Pain Syndrome: Evidence-Based Advances in Concepts and Treatments

PURPOSE OF REVIEW

This review presents the most current information about the epidemiology of complex regional pain syndrome (CRPS), classification and diagnostic criteria, childhood CRPS, subtypes, pathophysiology, conventional and less conventional treatments, and preventive strategies.

RECENT FINDINGS

CRPS is a painful disorder with multifactorial pathophysiology. The data describe sensitization of the central and peripheral nervous systems, inflammation, possible genetic factors, sympatho-afferent coupling, autoimmunity, and mental health factors as contributors to the syndrome. In addition to conventional subtypes (type I and type II), cluster analyses have uncovered other proposed subtypes. Prevalence of CRPS is approximately 1.2%, female gender is consistently associated with a higher risk of development, and substantial physical, emotional, and financial costs can result from the syndrome. Children with CRPS seem to benefit from multifaceted physical therapy leading to a high percentage of symptom-free patients. The best available evidence along with standard clinical practice supports pharmacological agents, physical and occupational therapy, sympathetic blocks for engaging physical restoration, steroids for acute CRPS, neuromodulation, ketamine, and intrathecal baclofen as therapeutic approaches. There are many emerging treatments that can be considered as a part of individualized, patient-centered care. Vitamin C may be preventive. CRPS can lead to progressively painful sensory and vascular changes, edema, limb weakness, and trophic disturbances, all of which substantially erode healthy living. Despite some progress in research, more comprehensive basic science investigation is needed to clarify the molecular mechanisms of the disease so that targeted treatments can be developed for better outcomes. Incorporating a variety of standard therapies with different modes of action may offer the most effective analgesia. Introducing less conventional approaches may also be helpful when traditional treatments fail to provide sufficient improvement.

Randomized controlled study to evaluate the efficacy and safety of soticlestat as adjunctive therapy in adults with complex regional pain syndrome

OBJECTIVE

The objective was to investigate the efficacy and safety of soticlestat as adjunctive therapy in participants with complex regional pain syndrome (CRPS).

DESIGN

A proof-of-concept phase 2a study, comprising a 15-week randomized, double-blind, placebo-controlled, parallel-group study (part A), and an optional 14-week open-label extension (part B).

METHODS

Twenty-four participants (median age 44.5 years [range, 18-62 years]; 70.8% female) with chronic CRPS were randomized (2:1) to receive oral soticlestat or placebo. Soticlestat dosing started at 100 mg twice daily and was titrated up to 300 mg twice daily. In part B, soticlestat dosing started at 200 mg twice daily and was titrated up or down at the investigator's discretion. Pain intensity scores using the 11-point Numeric Pain Scale (NPS) were collected daily. The Patient-Reported Outcomes Measurement Information System (PROMIS)-29, Patients' Global Impression of Change (PGI-C), and CRPS Severity Score (CSS) were completed at screening and weeks 15 and 29.

RESULTS

From baseline to week 15, soticlestat treatment was associated with a mean change in 24-hour pain intensity NPS score (95% confidence interval) of -0.75 (-1.55, 0.05) vs -0.41 (-1.41, 0.59) in the placebo group, resulting in a non-significant placebo-adjusted difference of -0.34 (-1.55, 0.88; P = .570). Statistically non-significant numerical changes were observed for the PROMIS-29, PGI-C, and CSS at weeks 15 and 29.

CONCLUSIONS

Adjunctive soticlestat treatment did not significantly reduce pain intensity in participants with chronic CRPS.

Interventions for treating pain and disability in adults with complex regional pain syndrome- an overview of systematic reviews

BACKGROUND

Complex regional pain syndrome (CRPS) is a chronic pain condition that usually occurs in a limb following trauma or surgery. It is characterised by persisting pain that is disproportionate in magnitude or duration to the typical course of pain after similar injury. There is currently no consensus regarding the optimal management of CRPS, although a broad range of interventions have been described and are commonly used. This is the first update of the original Cochrane review published in Issue 4, 2013.

OBJECTIVES

To summarise the evidence from Cochrane and non-Cochrane systematic reviews of the efficacy, effectiveness, and safety of any intervention used to reduce pain, disability, or both, in adults with CRPS.

METHODS

We identified Cochrane reviews and non-Cochrane reviews through a systematic search of Ovid MEDLINE, Ovid Embase, Cochrane Database of Systematic Reviews, CINAHL, PEDro, LILACS and Epistemonikos from inception to October 2022, with no language restrictions. We included systematic reviews of randomised controlled trials that included adults (≥18 years) diagnosed with CRPS, using any diagnostic criteria.  Two overview authors independently assessed eligibility, extracted data, and assessed the quality of the reviews and certainty of the evidence using the AMSTAR 2 and GRADE tools respectively. We extracted data for the primary outcomes pain, disability and adverse events, and the secondary outcomes quality of life, emotional well-being, and participants' ratings of satisfaction or improvement with treatment.  MAIN RESULTS: We included six Cochrane and 13 non-Cochrane systematic reviews in the previous version of this overview and five Cochrane and 12 non-Cochrane reviews in the current version. Using the AMSTAR 2 tool, we judged Cochrane reviews to have higher methodological quality than non-Cochrane reviews. The studies in the included reviews were typically small and mostly at high risk of bias or of low methodological quality. We found no high-certainty evidence for any comparison.  There was low-certainty evidence that bisphosphonates may reduce pain intensity post-intervention (standardised mean difference (SMD) -2.6, 95% confidence interval (CI) -1.8 to -3.4, P = 0.001; I2 = 81%; 4 trials, n = 181) and moderate-certainty evidence that they are probably associated with increased adverse events of any nature (risk ratio (RR) 2.10, 95% CI 1.27 to 3.47; number needed to treat for an additional harmful outcome (NNTH) 4.6, 95% CI 2.4 to 168.0; 4 trials, n = 181).  There was moderate-certainty evidence that lidocaine local anaesthetic sympathetic blockade probably does not reduce pain intensity compared with placebo, and low-certainty evidence that it may not reduce pain intensity compared with ultrasound of the stellate ganglion. No effect size was reported for either comparison. There was low-certainty evidence that topical dimethyl sulfoxide may not reduce pain intensity compared with oral N-acetylcysteine, but no effect size was reported. There was low-certainty evidence that continuous bupivacaine brachial plexus block may reduce pain intensity compared with continuous bupivacaine stellate ganglion block, but no effect size was reported. For a wide range of other commonly used interventions, the certainty in the evidence was very low and provides insufficient evidence to either support or refute their use. Comparisons with low- and very low-certainty evidence should be treated with substantial caution. We did not identify any RCT evidence for routinely used pharmacological interventions for CRPS such as tricyclic antidepressants or opioids.

AUTHORS' CONCLUSIONS

Despite a considerable increase in included evidence compared with the previous version of this overview, we identified no high-certainty evidence for the effectiveness of any therapy for CRPS. Until larger, high-quality trials are undertaken, formulating an evidence-based approach to managing CRPS will remain difficult. Current non-Cochrane systematic reviews of interventions for CRPS are of low methodological quality and should not be relied upon to provide an accurate and comprehensive summary of the evidence.

Sex specific role of RNA-binding protein, AUF1, on prolonged hypersensitivity after repetitive ischemia with reperfusion injury

Repetitive ischemia with reperfusion (I/R) injury is a common cause of myalgia. I/R injuries occur in many conditions that differentially affect males and females including complex regional pain syndrome and fibromyalgia. Our preclinical studies have indicated that primary afferent sensitization and behavioral hypersensitivity due to I/R may be due to sex specific gene expression in the DRGs and distinct upregulation of growth factors and cytokines in the affected muscles. In order to determine how these unique gene expression programs may be established in a sex dependent manner in a model that more closely mimics clinical scenarios, we utilized a newly developed prolonged ischemic myalgia model in mice whereby animals experience repeated I/R injuries to the forelimb and compared behavioral results to unbiased and targeted screening strategies in male and female DRGs. Several distinct proteins were found to be differentially expressed in male and female DRGs, including AU-rich element RNA binding protein (AUF1), which is known to regulate gene expression. Nerve specific siRNA-mediated knockdown of AUF1 inhibited prolonged hypersensitivity in females only, while overexpression of AUF1 in male DRG neurons increased some pain-like responses. Further, AUF1 knockdown was able to specifically inhibit repeated I/R induced gene expression in females but not males. Data suggests that RNA binding proteins like AUF1 may underlie the sex specific effects on DRG gene expression that modulate behavioral hypersensitivity after repeated I/R injury. This study may aid in finding distinct receptor differences related to the evolution of acute to chronic ischemic muscle pain development between sexes.

A predictive model based on random forest for shoulder-hand syndrome

Objectives

The shoulder-hand syndrome (SHS) severely impedes the function recovery process of patients after stroke. It is incapable to identify the factors at high risk for its occurrence, and there is no effective treatment. This study intends to apply the random forest (RF) algorithm in ensemble learning to establish a predictive model for the occurrence of SHS after stroke, aiming to identify high-risk SHS in the first-stroke onset population and discuss possible therapeutic methods.

Methods

We retrospectively studied all the first-onset stroke patients with one-side hemiplegia, then 36 patients that met the criteria were included. The patients’ data concerning a wide spectrum of demographic, clinical, and laboratory data were analyzed. RF algorithms were built to predict the SHS occurrence, and the model’s reliability was measured with a confusion matrix and the area under the receiver operating curves (ROC).

Results

A binary classification model was trained based on 25 handpicked features. The area under the ROC curve of the prediction model was 0.8 and the out-of-bag accuracy rate was 72.73%. The confusion matrix indicated a sensitivity of 0.8 and a specificity of 0.5, respectively. And the feature importance scored the weights (top 3 from large to small) in the classification were D-dimer, C-reactive protein, and hemoglobin.

Conclusion

A reliable predictive model can be established based on post-stroke patients’ demographic, clinical, and laboratory data. Combining the results of RF and traditional statistical methods, our model found that D-dimer, CRP, and hemoglobin affected the occurrence of the SHS after stroke in a relatively small sample of data with tightly controlled inclusion criteria.

Healthcare Costs, Time to Fitness for Work, and Related Factors in Chronic Complex Regional Pain Syndrome: A Comparative and Longitudinal Study of 5-Year Follow-Up

Purpose

To assess and compare the healthcare costs, time to fitness for work (TFW) between chronic complex regional pain syndrome (CRPS) and non-CRPS; and identify factors associated with these outcomes in a comparative longitudinal study.

Patients and Methods

148 patients with chronic CRPS of the hand and 273 patients with chronic hand impairments but without CRPS (non-CRPS) were admitted at a Swiss rehabilitation clinic between 2007 and 2016. Healthcare costs and TFW were retrieved from insurance data over 5 years after the accident. Socio-demographic factors, biopsychosocial complexity measured by means of the INTERMED questionnaire, pain intensity and DASH disability scores were collected during rehabilitation. Generalized estimation equations and Cox proportional-hazards models were used to identify factors associated with outcomes.

Results

Healthcare costs were increased by 20% for the CRPS versus non-CRPS group (coefficient = 1.20, 95% CI = 1.08-1.35, p<0.001). The median TFW was longer for CRPS than non-CRPS patients (816 vs 672 days, p = 0.02). After adjusting for covariates, TFW did not differ between the two groups (hazard ratio = 0.94, 95% CI = 0.73-1.21, p=0.61). For CRPS patients, higher healthcare costs were associated with severe or moderate initial injury, high INTERMED or DASH disability scores. Longer TFW were associated with severe initial injury, low educational level, no work contract, and high INTERMED or DASH disability scores.

Conclusion

Overall, the healthcare costs were higher for CRPS than non-CRPS patients, but the TFW was comparable. We demonstrated also the significant associations of disability and biopsychosocial factors with the healthcare costs and TFW in CRPS patients.

Development of an Exoskeleton Platform of the Finger for Objective Patient Monitoring in Rehabilitation

This paper presents the application of an adaptive exoskeleton for finger rehabilitation. The system consists of a force-controlled exoskeleton of the finger and wireless coupling to a mobile application for the rehabilitation of complex regional pain syndrome (CRPS) patients. The exoskeleton has sensors for motion detection and force control as well as a wireless communication module. The proposed mobile application allows to interactively control the exoskeleton, store collected patient-specific data, and motivate the patient for therapy by means of gamification. The exoskeleton was applied to three CRPS patients over a period of six weeks. We present the design of the exoskeleton, the mobile application with its game content, and the results of the performed preliminary patient study. The exoskeleton system showed good applicability; recorded data can be used for objective therapy evaluation.

Determinants of complex regional pain syndrome type I in patients with scaphoid waist fracture- a multicenter prospective observational study

Background

The aim of this prospective study was to assess the incidence of complex regional pain syndrome type I (CRPS I) in patients with scaphoid waist fracture and to explore associated factors.

Methods

This was a multicenter, prospective observational study. Demographic, imaging indicators and clinical data were collected before the conservative treatment of scaphoid waist fracture patients. The occurrence of CRPS I and pain condition were the main outcomes. To explore the factors associated with CRPS I, multivariate logistic regression model was used.

Results

A total of 493 scaphoid waist fracture participants undergoing conservative treatment were recruited for this study. The incidence of CRPS I was 20% (n = 87). The average time between injury and the onset of CRPS I was 6.7 ± 2.1 weeks. Multivariable logistic regression analysis revealed that female sex (odds ratio (OR): 1.669; 95% confidence interval (CI): 1.189–2.338), diabetes mellitus (OR: 3.206; 95% CI: 2.284–4.492), and severe pain condition before treatment (visual analog scale (VAS) score more than 4 cm) (OR: 27.966; 95% CI: 19.924–39.187) were independently associated with CRPS I.

Conclusions

Patients suffering from scaphoid waist fracture may be at a higher risk of CRPS I, especially in women with diabetes mellitus who report severe pain before treatment. Early screening and regular follow up evaluation are recommended in these patients.

Long-Term Effects of Dexmedetomidine versus Propofol During the Implantation of a Neurostimulator-A Post-Trial Follow-Up Analysis

Objective

The success of neurostimulation depends partly on the amount of coverage of the neurostimulation-induced paresthesia of the painful area. This is often achieved by asking feedback from patients intraoperatively. If sedation analgesia is used, it is important that the patient is comfortable during sedation and easily arousable. If the patient is not well sedated or experiences residual effects of the sedation during testing, this can directly influence the ideal placement of the leads and indirectly the long-term effect of the treatment. It is our hypothesis that the quality of the sedation is directly coupled to the adequacy of lead placement and in this way in the result of the treatment. Dexmedetomidine is known for its easy production of arousable sedation. The aim of the present study was to compare the long-term effect of using dexmedetomidine versus propofol during the implantation of a neurostimulator.

Materials and Methods

This is a post-trial follow-up analysis of the DexMedPro cohort. The primary outcome was global perceived effect (GPE). The secondary outcomes were the course of pain intensity, the emotional and physical functioning at the time of follow-up, and the course of neurostimulation treatment. In this study, we used the patient satisfaction with sedation as a measure for sedation quality.

Results

Regarding the GPE, no statistically significant differences were found between the experimental groups in either subscale (ie, recovery (p=0.82) and satisfaction with the neurostimulation treatment at follow-up (p=0.06)). The same was found regarding the secondary parameters. A correlation was found between patient satisfaction with sedation during the lead implantation (side effects and procedural recall) and satisfaction at follow-up.

Conclusion

Regarding the long-term efficacy of neurostimulation treatment, no statistically significant differences were found between the dexmedetomidine and the propofol group. We observed a trend towards greater satisfaction with the neurostimulation treatment at follow-up in the dexmedetomidine group, compared to the propofol group.

Complex Regional Pain Syndrome: A Comprehensive Review

Complex regional pain syndrome (CRPS) is a chronic pain condition often involving hyperalgesia and allodynia of the extremities. CRPS is divided into CRPS-I and CRPS-II. Type I occurs when there is no confirmed nerve injury. Type II is when there is known associated nerve injury. Female gender is a risk factor for developing CRPS. Other risk factors include fibromyalgia and rheumatoid arthritis. Unfortunately, the pathogenesis of CRPS is not yet clarified. Some studies have demonstrated different potential pathways. Neuropathic inflammation, specifically activation of peripheral nociceptors of C-fibers, has been shown to play a critical role in developing CRPS. The autonomic nervous system (ANS) is involved. Depending on whether it is acute or chronic CRPS, norepinephrine levels are either decreased or increased, respectively. Some studies have suggested the importance of genetics in developing CRPS. More consideration is being given to the role of psychological factors. Some association between a history of depression and/or post-traumatic stress disorder (PTSD) and the diagnosis of CRPS has been demonstrated. Treatment modalities available range from physical therapy, pharmacotherapy, and interventional techniques. Physical and occupational therapies include mirror therapy and graded motor imagery. Medical management with non-steroidal anti-inflammatory drugs (NSAIDs) has not shown significant improvement. There have been supporting findings in the use of short-course steroids, bisphosphonates, gabapentin, and ketamine. Antioxidant treatment has also shown some promise. Other pharmacotherapies include low-dose naltrexone and Botulinum toxin A (BTX-A). Sympathetic blocks are routinely used, even if their short- and long-term effects are not clear. Finally, spinal cord stimulation (SCS) has been used for decades. In conclusion, CRPS is a multifactorial condition that still requires further studying to better understand its pathogenesis, epidemiology, genetic involvement, psychological implications, and treatment options. Future studies are warranted to better understand this syndrome. This will provide an opportunity for better prevention, diagnosis, and treatment of CRPS.

Use of Ketamine Infusions for Treatment of Complex Regional Pain Syndrome: A Systematic Review

This systematic review aims to review clinical studies on the use of ketamine infusion for patients with treatment-resistant complex regional pain syndrome (CRPS).

The following systematic review was registered on the International Prospective Register of Systematic Reviews (PROSPERO) (CRD42021228470). Studies for the systematic review were identified through three databases: PubMed, Cumulative Index of Nursing and Allied Health Literature (CINAHL), and Cochrane Reviews. Inclusion criteria for studies consisted of randomized clinical trials or cohort studies that conducted trials on the use of ketamine infusion for pain relief in patients with CRPS. Exclusion criteria for studies included any studies that were systematic reviews, meta-analyses, case reports, literature reviews, or animal studies. In the included studies, the primary outcome of interest was the post-drug administration pain score.

In this systematic review, 14 studies met the inclusion criteria and were reviewed. In these studies, the dosage of ketamine infusion used ranged from 0.15 mg/kg to 7 mg/kg with the primary indication being the treatment of CRPS. In 13 of the studies, ketamine infusion resulted in a decrease in pain scores and relief of symptoms.

Patients who received ketamine infusion for treatment-resistant CRPS self-reported adequate pain relief with treatment. This suggests that ketamine infusion may be a useful form of treatment for patients with no significant pain relief with other conservative measures. Future large-scale studies, including randomized double-blind placebo-controlled trials on the use of ketamine infusion for CRPS, must be conducted in a large-scale population to further assess the effectiveness of ketamine infusion in these populations.

A Meta-Analysis and Meta-Regression of Frequency and Risk Factors for Poststroke Complex Regional Pain Syndrome

Background and Objectives: This article aimed to investigate the risk factors for poststroke complex regional pain syndrome (CRPS). Materials and Methods: We searched electronic databases including PubMed, Medline, Web of Science, Cochrane Library, and Embase up to 27 October 2021. We enrolled analytical epidemiological studies comprising cohort, case-control, and cross-sectional studies. A quality assessment was performed using the Newcastle–Ottawa Quality Assessment Scale for cohort and case-control studies and the Joanna Briggs Institute critical appraisal checklist for analytical cross-sectional studies. Binary outcomes were reported as odds ratios (ORs), and continuous outcomes were described as standardized mean differences (SMDs) with 95% confidence intervals. For the meta-regression, beta coefficient and p value were adopted. Results: We included 21 articles comprising 2225 participants. Individuals with shoulder subluxation and spasticity were found to have higher risks for poststroke CRPS. Spasticity with higher modified Ashworth scale score, lower Brunnstrom hand stage, and inferior Barthel index scores were observed in patients with poststroke CRPS. The pooled incidence proportion in nine articles was 31.7%, and a correlation was found between effect sizes and the ratio of women and the proportion of left hemiparesis. The summarized prevalence in nine cross-sectional studies was 33.1%, and a correlation was observed between prevalence and the subluxation ratio and Brunnstrom stage. Conclusions: Based on our meta-analysis, being female, left hemiparesis, shoulder subluxation, spasticity, a lower Brunnstrom stage of distal upper limb, and an inferior Barthel index are all features for poststroke CRPS. Larger studies with greater statistical power may confirm our findings and clarify some other unknown risk factors for poststroke CRPS.

An Adaptive Mechatronic Exoskeleton for Force-Controlled Finger Rehabilitation

This paper presents a novel mechatronic exoskeleton architecture for finger rehabilitation. The system consists of an underactuated kinematic structure that enables the exoskeleton to act as an adaptive finger stimulator. The exoskeleton has sensors for motion detection and control. The proposed architecture offers three main advantages. First, the exoskeleton enables accurate quantification of subject-specific finger dynamics. The configuration of the exoskeleton can be fully reconstructed using measurements from three angular position sensors placed on the kinematic structure. In addition, the actuation force acting on the exoskeleton is recorded. Thus, the range of motion (ROM) and the force and torque trajectories of each finger joint can be determined. Second, the adaptive kinematic structure allows the patient to perform various functional tasks. The force control of the exoskeleton acts like a safeguard and limits the maximum possible joint torques during finger movement. Last, the system is compact, lightweight and does not require extensive peripherals. Due to its safety features, it is easy to use in the home. Applicability was tested in three healthy subjects.

Long-Term Outcome of Spinal Cord Stimulation in Complex Regional Pain Syndrome

BACKGROUND

Spinal cord stimulation (SCS) is an effective treatment in chronic neuropathic pain, but its efficacy in complex regional pain syndrome (CRPS) needs to be proven.

OBJECTIVE

To study the outcome of SCS in CRPS as measured by trial success, explantation rate, complications, and changes in opioid and neuropathic pain medication use over a 4-yr follow-up.

METHODS

We retrospectively reviewed all medical records of 35 consecutive CRPS patients who underwent SCS trials at 2 hospitals during January 1998 to December 2016. The purchase data of opioids and neuropathic pain medication during January 1995 to March 2016 were retrieved from national registries.

RESULTS

Based on a 1-wk trial, permanent SCS was implanted in 27 (77%) patients. During the median follow-up of 8 yr, 8 (30%) SCS devices were explanted, of which 7 were because of inefficient pain relief. Complications leading to revision occurred in 17 (63%) patients: 8 electrode migrations or stimulation to the wrong area, 1 deep infection, 9 hardware malfunctions, 2 pulse generator discomforts, and 2 SCS replacements. None of the 6 patients using strong opioids discontinued their use during the 2-yr follow-up. The mean opioid dose increased nonsignificantly both in patients with SCS in permanent use (53 ± 150 morphine milligram equivalents morphine milligram equivalent (MME)/day to 120 ± 240 MME/day) and in patients who had SCS explanted (27 ± 72 MME/day to 57 ± 66 MME/day).

CONCLUSION

Despite the fact that CRPS patients were not able to discontinue or reduce their strong opioid or neuropathic pain medication use, 70% continued to use their SCS device during a median 8-yr follow-up.

Possible Therapeutic Options for Complex Regional Pain Syndrome

Complex regional pain syndrome (CRPS) describes an array of painful conditions that are characterized by continuing regional pain. CRPS comprises severe and inappropriate pain in cases of complete recovery after trauma. Research on the pharmacological treatment of CRPS, however, has not been well investigated. In this study, we compared the pain relief effects of different drugs (URB597, pyrrolidine dithiocarbamate, and hydralazine) in a rat model of chronic post-ischemic pain-induced CRPS. After drug injection, CRPS-induced mechanical allodynia was significantly recovered. After three repetitive drug injections, mechanical sensitivity generally improved as hyper-nociception subsided. Reduced Nav1.7 expression at dorsal root ganglions (DRGs) was observed in the drug treatment groups. Neural imaging analysis revealed decreased neural activity for each drug treatment, compared to vehicle. In addition, treatments significantly reduced IL-1β, IL-6, and TNFα expression in DRGs. These results indicated that drugs could reduce the expression of inflammatory factors and alleviate the symptoms of chronic post-ischemic pain-induced CRPS.

Direct Health Care Cost and Work Incapacity Related to Complex Regional Pain Syndrome in Switzerland: A Retrospective Analysis from 2008 to 2015

OBJECTIVE

First, to determine the number of accident-related complex regional pain syndrome (CRPS) cases from 2008 to 2015 and to identify factors associated with an increased risk for developing CRPS. Second, to analyze the duration of work incapacity and direct health care costs over follow-up periods of two and five years, respectively.

DESIGN

Retrospective data analysis.

SETTING

Database from the Statistical Service for the Swiss National Accident Insurances covering all accidents insured under the compulsory Swiss Accident Insurance Law.

SUBJECTS

Subjects were registered after an accident between 2008 and 2015.

METHODS

Cases were retrospectively retrieved from the Statistical Service for the Swiss National Accident Insurances. Cases were identified using the appropriate International Classification of Diseases, 10th Revision, codes.

RESULTS

CRPS accounted for 0.15% of all accident cases. Age, female gender (odds ratio [OR] = 1.53, 95% confidence interval [CI] = 1.47-1.60), and fracture of the forearm (OR = 38, 95% CI = 35-42) were related to an increased risk of developing CRPS. Over five years, one CRPS case accumulated average insurance costs of $86,900 USD and treatment costs of $23,300 USD. Insurance costs were 19 times and treatment costs 13 times the average costs of accidents without CPRS. Within the first two years after the accident, the number of days lost at work was 20 times higher in patients with CRPS (330 ± 7 days) than in patients without CRPS (16.1 ± 0.1 days). Two-thirds of all CRPS cases developed long-term work incapacity of more than 90 days.

CONCLUSION

CRPS is a relatively rare condition but is associated with high direct health care costs and work incapacity.

Low vitamin D levels in post-menopausal women are associated with complex regional pain syndrome type I in surgically treated distal radius fractures

BACKGROUND

Complex regional pain syndrome type I (CRPS I) is a chronic devastating condition and a relatively common complication of distal radius fractures (DRF). The purpose of this study was to investigate the relationship of vitamin D levels in surgically treated post-menopausal women with CRPS I occurrence in DRF.

METHODS

From February 2016 to March 2017, 158 surgically treated post-menopausal patients with DRF were enrolled. Exclusion criteria were (1) patients who had been taking vitamin D or osteoporosis medication at the time of injury; (2) patients with medical factors that may affect vitamin D levels; (3) patients who were reluctant to enroll in the study; and (4) patient with additional fractures, ligamentous injuries, or neuropathy. A total of 107 patients were available for final analysis. We compared the serum vitamin D levels in post-menopausal women with DRF with CRPS I (group 1) and without CRPS I (group 2). Bone mineral density (BMD) of the femur and spine, osteocalcin, alkaline phosphatase (ALP), body mass index (BMI) were also measured.

RESULTS

The average age at the time of surgery was 66.5 years (range, 39-86 years). The mean follow-up period was 16.3 months after surgery. Among the 107 surgically treated DRF patients, 19 (18%) met the Budapest criteria for CRPS I during the follow-up period. The mean serum vitamin D level in group 1 (15.2 ng/ml) was significantly lower than that in group 2 (20.5 ng/ml, p = 0.027). The mean values of osteocalcin, ALP, BMI, and BMD were not significantly different between the groups.

CONCLUSION

Lower vitamin D levels in post-menopausal women can increase CRPS I occurrence in distal radius fractures.

Spinal Cord Stimulation vs Conventional Therapies for the Treatment of Chronic Low Back and Leg Pain: A Systematic Review of Health Care Resource Utilization and Outcomes in the Last Decade

Objective

The purpose of this review is to critically appraise the literature for evidence supporting the health care resource utilization and cost-effectiveness of spinal cord stimulation (SCS) compared with conventional therapies (CTs) for chronic low back and leg pain.

Methods

The PubMed, MEDLINE, Embase, CINAHL, and Rehabilitation & Sports Medicine databases were searched for studies published from January 2008 through October 2018, using the following MeSH terms: “spinal cord stimulation,” “chronic pain,” “back pain,” “patient readmission,” “economics,” and “costs and cost analysis.” Additional sources were added based on bibliographies and consultation with experts. The following data were extracted and analyzed: demographic information, study design, objectives, sample sizes, outcome measures, SCS indications, complications, costs, readmissions, and resource utilization data.

Results

Of 204 studies screened, 11 studies met inclusion criteria, representing 31,439 SCS patients and 299,182 CT patients. The mean age was 53.5 years for SCS and 55.6 years for CT. In eight of 11 studies, SCS was associated with favorable outcomes and found to be more cost-effective than CT for chronic low back pain. Compared with CT, SCS resulted in shorter hospital stays and lower complication rates and health care costs at 90 days. SCS was associated with significant improvement in health-related quality of life, health status, and quality-adjusted life-years.

Conclusions

For the treatment of chronic low back and leg pain, the majority of studies are of fair quality, with level 3 or 4 evidence in support of SCS as potentially more cost-effective than CT, with less resource expenditure but higher complication rates. SCS therapy may yet play a role in mitigating the financial burden associated with chronic low back and leg pain.

Pharmacotherapeutic options for complex regional pain syndrome

INTRODUCTION

Complex regional pain syndromes (CRPS) are rare painful conditions characterized by considerable variability in possible triggering factors, usually traumatic, and in the clinical scenario. The limited knowledge of the pathophysiological mechanisms has led to countless treatment attempts with multiple conservative and surgical options that act by different mechanisms of action.

AREAS COVERED

In this narrative review, the authors discuss key points about CRPS definitions, diagnostic criteria and pitfalls, pathophysiological hypotheses, and treatment strategies with particular reference to pharmacotherapy. The article was based on a literature search using PubMed while the available guidelines for the management of CRPS were also examined.

EXPERT OPINION

According to the quality of evidence, pharmacological interventions for CRPS seem to be more effective all the more so when they act on peripheral mechanisms, particularly on nociceptive pain, and when applied early in the disease, while reliable evidence about central mechanisms of chronic pain in CRPS is lacking. In our opinion, drug therapy should be preferred as early as possible, particularly in warm forms of CRPS to prevent significant functional limitation, psychological distress, and social and economic fallout.

Sex differences and mechanisms of muscle pain

Clinical conditions resulting in musculoskeletal pain show important sex differences in both prevalence and degree of functional disability. The underlying mechanisms for these distinctions in pain manifestation are not fully known. However, recent preclinical studies have shown at the primary afferent level that males and females present fundamental differences in their peripheral response properties and injury-related gene expression patterns that may underlie observed afferent sensitization. At the spinal cord level, studies in various models of pain suggest important roles for the immune system, glutamate signaling and hormones in modulating sex differences. While preclinical studies have been able to characterize some of the basic underlying molecular mechanisms of sex differences in muscle pain, human studies have relied mainly on functional brain imaging studies to explain differences. Further complicating our understanding of how sex influences muscle pain is the notion that the type of injury sustained, or clinical condition may differentially activate distinct mechanisms of muscle pain development in males versus females. More research is necessary to better understand how the sexes differ in their perception of muscle pain. This review highlights recent advances in both human and animal studies of sex differences in muscle pain.