Exaggeration of tissue trauma induces signs and symptoms of acute CRPS I, however displays distinct differences to experimental CRPS II

Animal models of complex regional pain syndrome: A scoping review

BACKGROUND

complex regional pain syndrome (CRPS) leads to a debilitating chronic pain condition. The lack of cause, etiology, and treatment for CRPS has been widely explored in animal models.

OBJECTIVE

Provide a comprehensive framework of the animal models used for investigating CRPS.

ELIGIBILITY CRITERIA

Preclinical studies to induce the characteristics of CRPS, with a control group, in any language or publication date.

SOURCES OF EVIDENCE

The search was performed in the Medline (PubMed) and ScienceDirect databases.

RESULTS

93 studies are included. The main objective of the included studies was to understand the CRPS model. Rats, males and adults, exposed to ischemia/reperfusion of the paw or fracture of the tibia were the most common characteristics. Nociceptive evaluation using von Frey monofilaments was the most widely adopted in the studies.

CONCLUSIONS

For the best translational science between the animal models and individuals with CRPS, future studies should include more heterogeneous animals, and multiple assessment tools, in addition to improving the description and performance of measures that reduce the risk of bias.

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.

Effect of a new formulation of micronized and ultramicronized N-palmitoylethanolamine in a tibia fracture mouse model of complex regional pain syndrome

Complex regional pain syndrome type 1 (CRPS-I) is a disabling and frequently chronic condition. It involves the extremities and is a frequent consequence of distal tibia and radius fractures. The inflamed appearance of the affected CRPS-I limb suggests that local production of inflammatory mediators may be implicated in the ensuing etiology. A rodent tibia fracture model, characterized by inflammation, chronic unilateral hindlimb warmth, edema, protein extravasation, allodynia and hyperalgesia resembles the clinical features of patients with acute CRPS-I. N-palmitoylethanolamine (PEA), a member of the family of naturally-occurring N-acylethanolamines, is well-known for its ability to modulate inflammatory processes and regulate pain sensitivity. However, the large particle size and lipidic nature of PEA may limit its bioavailability and solubility when given orally. Micronized formulations are frequently used to enhance the dissolution rate of drug and reduce its variability of absorption when orally administered. The aim of this study was to assess the effects of a formulation of micronized and ultramicronized PEA (PEA-MPS), given orally in a mouse model of CRPS-I. CD-1 male mice were subjected to distal tibia fracture and divided into two groups: control and treated with PEA-MPS (PEA micronized 300 mg/kg and ultramicronized 600 mg/kg). Sensibility to pain was monitored in all mice throughout the course of the experiment. Twenty-eight days after tibia fracture induction animals were sacrificed and biochemical parameters evaluated. The PEA-MPS-treated group showed an improved healing process, fracture recovery and fibrosis score. PEA-MPS administration decreased mast cell density, nerve growth factor, matrix metalloproteinase 9 and cytokine expression. This treatment also reduced (poly-ADP)ribose polymerase activation, peroxynitrite formation and apoptosis. Our results suggest that PEA-MPS may be a new therapeutic strategy in the treatment of CRPS-I.

Complex regional pain syndrome-significant progress in understanding

Research into complex regional pain syndrome (CRPS) has made significant progress. First, there was the implementation of the official IASP "Budapest" diagnostic criteria. It would be desirable to also define exclusion and outcome criteria that should be reported in studies. The next step was to recognize the complex pathophysiology. After trauma, some inflammation is physiological; in acute CRPS, this inflammation persists for months. There is an abundance of inflammatory and a lack of anti-inflammatory mediators. This proinflammatory network (cytokines and probably also other mediators) sensitizes the peripheral and spinal nociceptive system, it facilitates the release of neuropeptides from nociceptors inducing the visible signs of inflammation, and it stimulates bone cell or fibroblast proliferation, and endothelial dysfunction leading to vascular changes. Trauma may also expose nervous system structures to the immune system and triggers autoantibodies binding to adreno- and acetylcholine receptors. In an individual time frame, the pain in this inflammatory phase pushes the transition into "centralized" CRPS, which is dominated by neuronal plasticity and reorganization. Sensory-motor integration becomes disturbed, leading to a loss of motor function; the body representation is distorted leading to numbness and autonomic disturbances. In an attempt to avoid pain, patients neglect their limb and learn maladaptive nonuse. The final step will be to assess large cohorts and to analyze these data together with data from public resources using a bioinformatics approach. We could then develop diagnostic toolboxes for individual pathophysiology and select focused treatments or develop new ones.

A pan-caspase inhibitor reduces myocyte apoptosis and neuropathic pain in rats with chronic constriction injury of the sciatic nerve

BACKGROUND

Chronic constriction injury is a widely used model for neuropathic pain in rats. It presents with symptoms resembling human neuropathic pain, such as spontaneous pain, hyperalgesia, and allodynia. Recently, myocyte apoptosis was found in neuropathic rats as a possible promoter of pain and motor dysfunction. Our aim in this study was to demonstrate whether muscle cell apoptosis contributes to neuropathic pain in this animal model.

METHODS

To clarify this issue, we examined pain, nutritive perfusion, and inflammation in muscle tissue as well as myocyte apoptosis in rats with neuropathic pain established by chronic constriction injury of the sciatic nerve. Animals received either the pan-caspase inhibitor zVAD (OMe)-fmk (n = 5) or equivalent volumes of vehicle (n = 6). Sham-operated rats served as controls (n = 6).

RESULTS

At day 4 after nerve ligation, there were no signs of perfusion failure or muscle tissue inflammation in all experimental groups. However, animals treated with the vehicle had marked myocyte apoptosis, which was found almost completely blocked in zVA-Dtreated animals. The zVA-Dtreated animals presented with a significant reduction of pain upon heat, cold, and mechanical stimulation comparable with values found in sham controls.

CONCLUSIONS

Myocyte apoptosis possibly contributes to thermal and mechanical allodynia in this experimental model for neuropathic pain. The development of neuropathic pain symptoms did not depend on disturbances in microcirculation or muscle tissue inflammation.

Surgical treatment of complex regional pain syndrome type II with regional subcutaneous venous sympathectomy

BACKGROUND

The effectiveness of a new surgical technique for the treatment of severe chronic pain stages was evaluated. For the last 140 years, the treatment of complex regional pain syndrome type II (CRPS II) has been an unsolved problem. Therapeutic approaches have included conventional pain medication, physical therapy, sympathetic blocks, transcutaneous or spinal cord stimulation, injections or infusion therapies, and sympathectomy. When used alone or in combination, these therapies often yield unfavorable results. The majority of physicians who treat patients with CRPS are convinced that a surgical treatment of the affected extremity only exacerbates the symptoms, especially its hallmark excruciating pain.

METHODS

Sixteen patients with a CRPS type II of the upper or lower limb were included in the study after ineffective pain therapy for more than 6 months. The most proximal region of pain associated with CRPS was localized, and 2% lidocaine was injected into that area. Once the sympathetic, deep, burning pain had been blocked, the subcutaneous veins in the previously determined area were surgically removed. A visual analog scale, the Nottingham Health Profile, and physical examinations were used to evaluate the outcome of the operation.

RESULTS

Twelve (75%) surgically treated patients showed significant improvement in limb function, the visual analog scale, and the Nottingham Health Profile.

CONCLUSIONS

These data and recent findings in animal models conclude that CRPS type II is maintained by a coupling of newly sprouted sympathetic and sensible fibers. These fibers can be resected with a regional subcutaneous venous sympathectomy.

LEVEL OF EVIDENCE

Therapeutic study, level III.

Magnetic resonance imaging findings of fatty infiltrate in the cervical flexors in chronic whiplash

STUDY DESIGN

Retrospective investigation of muscle changes in patients suffering from chronic whiplash-associated disorders (WAD).

OBJECTIVES

To quantitatively compare the presence of muscle alterations (fatty infiltrate [MFI] and cross-sectional area [CSA]) in the anterior musculature of the cervical spine in a cohort of chronic whiplash patients (WAD II) and healthy control subjects across muscle and cervical segmental level.

SUMMARY OF BACKGROUND DATA

Magnetic resonance imaging can be regarded as the gold standard for muscle imaging. There is little knowledge about in vivo features of anterior neck muscles in patients suffering from chronic WAD and how muscle structure differs across the factors of muscle, vertebral level, age, self-reported pain and disability, body mass index, and duration of symptoms.

METHODS

Reliable magnetic resonance imaging measures for MFI and CSA were performed for the anterior cervical muscles bilaterally in 109 female subjects (78 WAD, 31 healthy control; 18-45 years, 3 months to 3 years postinjury). The measures were performed on all subjects for the longus capitis and colli and the sternocleidomastoid muscles.

RESULTS

The WAD subjects had significantly larger MFI and CSA for the anterior muscles compared to healthy control subjects (all P < 0.0001). In addition, the amount of MFI varied by both cervical level and muscle, with the longus capitis/colli having the largest amount of fatty infiltrates at the C2-C3 level (P < 0.0001). MFI was inversely related to age, self-reported pain/disability, and body mass index but directly proportional to duration of symptoms.

CONCLUSION

There is significantly greater MFI and CSA in the anterior neck muscles, especially in the deeper longus capitis/colli muscles, in subjects with chronic WAD when compared to healthy controls. Future studies are required to investigate the relationships between muscular morphometry and symptoms in patients suffering from acute and chronic WAD.

Plasma vitamin C is lower in postherpetic neuralgia patients and administration of vitamin C reduces spontaneous pain but not brush-evoked pain

OBJECTIVES

Plasma vitamin C concentrations have been suggested to be related to pain modulation in postherpetic neuralgia (PHN), an intractable neuropathic pain syndrome. In this study, we first compared plasma concentrations of vitamin C between healthy volunteers and PHN patients and then designed a symptom-based and mechanism-based approach to assess the analgesic effect of intravenous vitamin C on spontaneous and brush-evoked pain.

METHODS

Study 1 was cross-sectional that enrolled 39 healthy volunteers and 38 PHN patients. Study 2 was a double-blinded, placebo-controlled intervention study, which comprised 41 patients randomly allocated into the ascorbate group and placebo. Each patient received normal saline infusion with or without ascorbate on days 1, 3, and 5 and answered questionnaires that included side effects; numeric rating pain scale (NRS) on spontaneous and brush-evoked pain on days 1, 3, 5, and 7; and patient global impression of change on spontaneous and brush-evoked pain on day 7.

RESULTS

Study 1 revealed that plasma concentrations of vitamin C were significantly lower in patients with PHN than in healthy volunteers (P<0.001). Study 2 showed that ascorbate treatment effectively restored plasma vitamin C concentrations in the patients and decreased spontaneous pain by 3.1 in NRS from baseline to day 7, as compared with a decrease of 0.85 in NRS by placebo treatment (P<0.001). Conversely, ascorbate treatment did not significantly affect brush-evoked pain. Ascorbate treatment also resulted in a better efficacy than placebo in patient global impression of change on spontaneous pain (P<0.001) on day 7 and did not affect brush-evoked pain. No side effects were observed.

CONCLUSIONS

Plasma vitamin C status plays a role in PHN, and intravenous ascorbate helps relieve spontaneous pain in PHN.

Characterization of acute and chronic whiplash-associated disorders

SYNOPSIS

The development of chronic pain and disability following whiplash injury is common and contributes substantially to personal and economic costs related with this condition. Emerging evidence demonstrates the clinical presence of alterations in the sensory and motor systems, including psychological distress in all individuals with a whiplash injury, regardless of recovery. However, individuals who transition to the chronic state present with a more complex clinical picture characterized by the presence of widespread sensory hypersensitivity, as well as significant posttraumatic stress reactions. Based on the diversity of the signs and symptoms experienced by individuals with a whiplash condition, clinicians must take into account the more readily observable/measurable differences in motor, sensory, and psychological dysfunction. The implications for the assessment and management of this condition are discussed. Further review into the pathomechanical, pathoanatomical, and pathophysiological features of the condition also will be discussed.

LEVEL OF EVIDENCE

Level 5.J

The efficacy of manual lymphatic drainage therapy in the management of limb edema secondary to reflex sympathetic dystrophy

The objective of this study is to investigate the efficacy of manual lymphatic drainage (MLD) therapy in edema secondary to the reflex sympathetic dystrophy (RSD). A total of 34 patients were allocated randomly into two groups. All of the patients undertook nonstreoidal anti-inflammatory drug, physical therapy and therapeutic exercise program for 3 weeks. Patients in study group undertook MLD therapy additionally. Then the patients continued 2-month maintenance period with recommended home programs. Volumetric measurements pain scores and functional measurements were assessed at baseline, after treatment and 2 months after the treatment. After treatment, improvement in edema was statistically significant in the study group but not in the control group. At follow-up, with respect to baseline, improvements were not significant in both of the groups. Between the groups, difference of the percentage improvements in edema was statistically significant with superiority of MLD group after treatment, but not significant at follow-up. In this pilot study, MLD therapy was found to be beneficial in the management of edema resulted from RSD. Although the long-term results showed tendency towards improvement, the difference was not significant.

Interaction of hyperalgesia and sensory loss in complex regional pain syndrome type I (CRPS I)

Background

Sensory abnormalities are a key feature of Complex Regional Pain Syndrome (CRPS). In order to characterise these changes in patients suffering from acute or chronic CRPS I, we used Quantitative Sensory Testing (QST) in comparison to an age and gender matched control group.

Methods

61 patients presenting with CRPS I of the upper extremity and 56 healthy subjects were prospectively assessed using QST. The patients' warm and cold detection thresholds (WDT; CDT), the heat and cold pain thresholds (HPT; CPT) and the occurrence of paradoxical heat sensation (PHS) were observed.

Results

In acute CRPS I, patients showed warm and cold hyperalgesia, indicated by significant changes in HPT and CPT. WDT and CDT were significantly increased as well, indicating warm and cold hypoaesthesia. In chronic CRPS, thermal hyperalgesia declined, but CDT as well as WDT further deteriorated. Solely patients with acute CRPS displayed PHS. To a minor degree, all QST changes were also present on the contralateral limb.

Conclusions

We propose three pathomechanisms of CRPS I, which follow a distinct time course: Thermal hyperalgesia, observed in acute CRPS, indicates an ongoing aseptic peripheral inflammation. Thermal hypoaesthesia, as detected in acute and chronic CRPS, signals a degeneration of A-delta and C-fibres, which further deteriorates in chronic CRPS. PHS in acute CRPS I indicates that both inflammation and degeneration are present, whilst in chronic CRPS I, the pathomechanism of degeneration dominates, signalled by the absence of PHS. The contralateral changes observed strongly suggest the involvement of the central nervous system.

Assessment of endothelial function in complex regional pain syndrome type I

The objective of this study is to investigate the endothelial function in complex regional pain syndrome. A total of 21 patients with CRPS and 15 healthy controls were enrolled. The mean age of patients was 22 +/- 4.9 years and the mean duration of the disease was 5.9 +/- 2.5 months. Flow-mediated vasodilatation (FMD) technique was performed for evaluating the endothelial function. Parameters were the waveforms obtained and the increase in diameter and blood flow of the brachial artery, which were expressed as the percent change from baseline. The differences of the waveforms obtained in the affected limbs in comparison to non-affected limbs and to controls were statistically significant. We observed a trend of greater percent dilating responses in the affected limbs; however, the differences showed no statistical significance. In macrovascular evaluation, the endothelial function seems to be impaired in the earlier stage of the disease.

Continuous intra-arterial application of substance P induces signs and symptoms of experimental complex regional pain syndrome (CRPS) such as edema, inflammation and mechanical pain but no thermal pain

Substance P is involved in nociception in both the peripheral nervous system and the CNS and has been documented to play a crucial role in the complex regional pain syndrome (CRPS). So far, however, most experimental animal models are restricted to the effect of neurokinin-1 receptor blockers to inhibit substance P and do not directly evaluate its action. Thus, this study was conducted to test the hypothesis that local application of substance P causes signs and symptoms of CRPS. For this purpose rats received a continuous infusion of either substance P or saline over 24 h delivered by a mini-osmotic pump connected to an intrafemoral catheter. Animals were analyzed at either day 1 (n=6, each group) or day 4 (n=5, each group) after start of infusion. Substance P application caused a significant and long-lasting decrease in paw withdrawal thresholds upon mechanical stimulation, while animals did not present with thermal allodynia at days 1 and 4 after onset of infusion. In addition, severe s.c. edema was observed in all animals receiving substance P. In vivo fluorescence microscopy of the extensor digitorum longus muscle of the affected hind paw revealed enhanced leukocyte-endothelial cell interaction with a significant rise in the number of leukocytes both rolling along and firmly adhering to the wall of postcapillary venules, while saline-exposed animals were free of this local inflammatory response. Muscle cell apoptosis, as assessed by in vivo bisbenzimide staining, terminal deoxynucleotidyl transferase nick end labeling analysis and caspase 3-cleavage, could not be observed in either of the animals. In summary, the present study indicates that substance P is responsible for neurogenic inflammation, including local cell response, edema formation and mechanical pain, while it seems not to contribute to the generation of thermal allodynia.

Activated protein C reduces tissue hypoxia, inflammation, and apoptosis in traumatized skeletal muscle during endotoxemia

OBJECTIVE

Extensive surgical trauma leads to activation of the coagulation cascade and is often complicated by systemic inflammation and infection. Activated protein C, a natural coagulatory inhibitor, was recently shown to reduce mortality in septic patients. We herein report on the actions of activated protein C on skeletal muscle injury in experimental endotoxemia.

DESIGN

Prospective controlled animal study.

SETTING

University animal research facility.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Closed soft tissue trauma was applied on the left hind limb of pentobarbital-anesthetized rats. Six hours later endotoxemia was induced by intraperitoneal injection of Escherichia coli lipopolysaccharide. An equivalent volume of physiologic saline was given in controls. At the same time point, treatment of animals was started by continuous intravenous application of activated protein C (24 microg/kg.hr) or vehicle solution over 18 hrs. Twenty-four hours after trauma, the extensor digitorum longus muscle was microsurgically exposed and analyzed by means of high-resolution multifluorescence microscopy.

MEASUREMENTS AND MAIN RESULTS

Endotoxemia aggravated traumatized muscle injury, as evidenced by reduced nutritive perfusion, increased tissue hypoxia, enhanced leukocyte-endothelial cell interaction, and apoptotic myocyte cells (249 +/- 17 cm/cm vs. 298 +/- 22 cm/cm; reduced nicotinamide adenine dinucleotide [NADH], 149 +/- 15 arbitrary units [AU] vs. 130 +/- 13 AU; 417 +/- 79 cells/mm vs. 344 +/- 77 cells/mm and 62 +/- 9 cells/mm vs. 31 +/- 5 cells/mm). Therapeutic intervention with activated protein C 6 hrs after trama protected nutritive perfusion and tissue oxygenation (341 +/- 24 cm/cm and 115 +/- 8 AU) and reduced inflammatory leukocyte adherence (185 +/- 60 cells/mm) and cellular apoptosis (15 +/- 4 cells/mm). Of note, the protection of traumatized muscle tissue by activated protein C was also maintained during endotoxemia, as indicated by a functional capillary density of 379 +/- 10 cm/cm, a NADH-fluorescence of 102 +/- 6 AU, a leukocyte adherence of 82 +/- 12 cells/mm, and a myocyte apoptosis of 28 +/- 4 cells/mm.

CONCLUSIONS

Microcirculatory injury of traumatized skeletal muscle tissue is enhanced by intravenous endotoxin application in this model of soft tissue trauma. Activated protein C ameliorates microcirculatory dysfunction and tissue injury, in particular in traumatized animals during endotoxemia.