Vasodilative effect of isosorbide dinitrate ointment in complex regional pain syndrome type 1

Novel Co-crystal of Pentoxifylline and Protocatechuic Acid Relieves Allodynia in Rat Models of Peripheral Neuropathic Pain and CRPS by Alleviating Local Tissue Hypoxia

Local tissue ischemic hypoxia is a peripheral process that can be targeted with topical treatment to alleviate pain under chronic pain conditions such as complex regional pain syndrome (CRPS) and peripheral neuropathic pain. We recently reported three novel salts and a co-crystal composed of vasoactive agents and antioxidant nutraceuticals, all of which produced potent topical anti-allodynic effects in the chronic postischemic pain (CPIP) rat model of CRPS. One of the products, pentx-pca, is a co-crystal synthesized from pentoxifylline (pentx) and protocatechuic acid (pca). Pentx-pca exhibited potent topical anti-allodynic effects in CPIP and rats with chronic constriction injury of the sciatic nerve exceeding effects produced individually by pentx and pca. We hypothesized that the anti-allodynic effects of pentx-pca in CPIP rats were due to its impact on local tissue oxygenation and subsequent oxygen-dependent mitochondrial respiration. Percutaneous tissue oxygen saturation (SaO₂) measurements taken from the hind paw of the CPIP rats revealed that anti-allodynic doses of topical pentx-pca increased local tissue SaO₂. Moreover, assessment of the oxygen-dependent mitochondrial function using a triphenyl tetrazolium chloride assay revealed that mitochondrial dysfunction significantly declined in the plantar muscle collected from CPIP rats topically treated with anti-allodynic doses of pentx-pca as compared to vehicle-treated CPIP rats. Furthermore, time-dependent resolution of plantar muscle mitochondrial dysfunction, that occurred in the CPIP rats at 6-week post procedure, paralleled the loss of the anti-allodynic response to topical treatment with pentx-pca. Our results indicated that pentx-pca produced potent anti-allodynic effects in the CPIP rat model of CRPS by alleviating peripheral tissue ischemia/hypoxia and downstream hypoxia-driven mitochondrial dysfunction.

Peripheral nitric oxide signaling directly blocks inflammatory pain

Pain is a classical sign of inflammation, and sensitization of primary sensory neurons (PSN) is the most important mediating mechanism. This mechanism involves direct action of inflammatory mediators such as prostaglandins and sympathetic amines. Pharmacologic control of inflammatory pain is based on two principal strategies: (i) non-steroidal anti-inflammatory drugs targeting inhibition of prostaglandin production by cyclooxygenases and preventing nociceptor sensitization in humans and animals; (ii) opioids and dipyrone that directly block nociceptor sensitization via activation of the NO signaling pathway. This review summarizes basic concepts of inflammatory pain that are necessary to understand the mechanisms of peripheral NO signaling that promote peripheral analgesia; we also discuss therapeutic perspectives based on the modulation of the NO pathway.

Topical combinations to treat microvascular dysfunction of chronic postischemia pain

BACKGROUND

Growing evidence indicates that patients with complex regional pain syndrome (CRPS) exhibit tissue abnormalities caused by microvascular dysfunction in the blood vessels of skin, muscle, and nerve. We tested whether topical combinations aimed at improving microvascular function would relieve allodynia in an animal model of CRPS. We hypothesized that topical administration of either α2-adrenergic (α2A) receptor agonists or nitric oxide (NO) donors given to increase arterial blood flow, combined with either phosphatidic acid (PA) or phosphodiesterase (PDE) inhibitors to increase capillary blood flow, would effectively reduce allodynia and signs of microvascular dysfunction in the animal model of chronic pain.

METHODS

Mechanical allodynia was induced in the hindpaws of rats with chronic postischemia pain (CPIP). Allodynia was assessed before and after topical application of vehicle, single drugs or combinations of an α2A receptor agonist (apraclonidine) or an NO donor (linsidomine), with PA or PDE inhibitors (lisofylline, pentoxifylline). A topical combination of apraclonidine + lisofylline was also evaluated for its effects on a measure of microvascular function (postocclusive reactive hyperemia) and tissue oxidative capacity (formazan production by tetrazolium reduction) in CPIP rats.

RESULTS

Each of the single topical drugs produced significant dose-dependent antiallodynic effects compared with vehicle in CPIP rats (N = 30), and the antiallodynic dose-response curves of either PA or PDE inhibitors were shifted 5- to 10-fold to the left when combined with nonanalgesic doses of α2A receptor agonists or NO donors (N = 28). The potent antiallodynic effects of ipsilateral treatment with combinations of α2A receptor agonists or NO donors with PA or PDE inhibitors were not reproduced by the same treatment of the contralateral hindpaw (N = 28). Topical combinations produced antiallodynic effects lasting up to 6 hours (N = 15) and were significantly enhanced by low-dose systemic pregabalin in early, but not late, CPIP rats (N = 18). An antiallodynic topical combination of apraclonidine + lisofylline was also found to effectively relieve depressed postocclusive reactive hyperemia in CPIP rats (N = 61) and to increase formazan production in postischemic tissues (skin and muscle) (N = 56).

CONCLUSIONS

The present results support the hypothesis that allodynia in an animal model of CRPS is effectively relieved by topical combinations of α2A receptor agonists or NO donors with PA or PDE inhibitors. This suggests that topical treatments aimed at improving microvascular function by increasing both arterial and capillary blood flow produce effective analgesia for CRPS.

Skin vasodilation and analgesic effect of a topical nitric oxide-releasing hydrogel

New approaches based on topical treatments are needed for treating pain and impaired dermal blood flow. We used a topical Pluronic F127 hydrogel containing S-nitrosoglutathione (GSNO) as a prodrug to generate free NO, an effector molecule that exerts both dermal vasodilation and antinociceptive effects. GSNO-containing hydrogels underwent gelation above 12 °C and released free NO at rates that were directly dependent on the GSNO concentration in the range of 50-150 mM. The topical application of this material led to dose-response dermal vasodilation in healthy volunteers and to a reduction of up to 50 % of the hypernociception intensity in Wistar rats that were subjected to inflammatory pain. Mechanistic investigations indicated that the antinociceptive effect of the topical F127/GSNO hydrogels is produced by the local activation of the cGMP/PKG/KATP channel-signaling pathway, which was stimulated by the free NO that diffused through the skin. These results expand the scope of the biomedical applications of this material and may represent a new approach for the topical treatment of inflammatory pain.

Topical combinations aimed at treating microvascular dysfunction reduce allodynia in rat models of CRPS-I and neuropathic pain

Growing evidence indicates that various chronic pain syndromes exhibit tissue abnormalities caused by microvasculature dysfunction in the blood vessels of skin, muscle, or nerve. We tested whether topical combinations aimed at improving microvascular function would relieve allodynia in animal models of complex regional pain syndrome type I (CRPS-I) and neuropathic pain. We hypothesized that topical administration of either α(2)-adrenergic (α(2)A) receptor agonists or nitric oxide (NO) donors combined with either phosphodiesterase (PDE) or phosphatidic acid (PA) inhibitors would effectively reduce allodynia in these animal models of chronic pain. Single topical agents produced significant dose-dependent antiallodynic effects in rats with chronic postischemia pain, and the antiallodynic dose-response curves of PDE and PA inhibitors were shifted 2.5- to 10-fold leftward when combined with nonanalgesic doses of α(2)A receptor agonists or NO donors. Topical combinations also produced significant antiallodynic effects in rats with sciatic nerve injury, painful diabetic neuropathy, and chemotherapy-induced painful neuropathy. These effects were shown to be produced by a local action, lasted up to 6 hours after acute treatment, and did not produce tolerance over 15 days of chronic daily dosing. The present results support the hypothesis that allodynia in animal models of CRPS-I and neuropathic pain is effectively relieved by topical combinations of α(2)A or NO donors with PDE or PA inhibitors. This suggests that topical treatments aimed at improving microvascular function may reduce allodynia in patients with CRPS-I and neuropathic pain.

PERSPECTIVE

This article presents the synergistic antiallodynic effects of combinations of α(2)A or NO donors with PDE or PA inhibitors in animal models of CRPS-I and neuropathic pain. The data suggest that effective clinical treatment of chronic neuropathic pain may be achieved by therapies that alleviate microvascular dysfunction in affected areas.

Mechanical stimulation enhances endothelin-1 hyperalgesia

When comparing a cumulative dose-response curve for endothelin-1 (ET-1)-induced mechanical hyperalgesia to the effect of individual doses (1 ng, 10 ng, 100 ng, and 1 μg) administered in separate groups of rats, a marked difference was observed in the peak magnitude of hyperalgesia. Hyperalgesia was measured as decrease in the threshold for mechanically-induced withdrawal of the hind paw. The cumulative dosing protocol produced markedly greater maximum hyperalgesia. To determine whether this was due to the cumulative dosing protocol or to the repeated exposure to the mechanical test stimulus, we evaluated the impact of repeated testing on ET-1-induced mechanical hyperalgesia. While ET-1-induced mechanical hyperalgesia was dose- and time-dependent, repeated testing of nociceptive threshold, at 5 min intervals, following a single dose of ET-1, produced further decrease in nociceptive threshold. This mechanical stimulation-induced enhancement of ET-1 hyperalgesia lasted only 3-4 h, while the hyperalgesia lasted in excess of 5 days. The stimulation-enhanced hyperalgesia also occurred after a second injection of ET-1, administered 24 h after the initial dose. That this phenomenon is unique to ET-1 is suggested by the observation that while five additional, direct-acting hyperalgesic agents-prostaglandin E2 (PGE2), nerve growth factor (NGF), glia-derived neurotrophic factor (GDNF), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFα)-induced robust mechanical hyperalgesia, none produced mechanical stimulation-enhanced hyperalgesia.

A hypothesis for the cause of complex regional pain syndrome-type I (reflex sympathetic dystrophy): pain due to deep-tissue microvascular pathology

Complex regional pain syndrome-type I (CRPS-I; reflex sympathetic dystrophy) is a chronic pain condition that usually follows a deep-tissue injury such as fracture or sprain. The cause of the pain is unknown. We have developed an animal model (chronic post-ischemia pain) that creates CRPS-I-like symptomatology. The model is produced by occluding the blood flow to one hind paw for 3 hours under general anesthesia. Following reperfusion, the treated hind paw exhibits an initial phase of hyperemia and edema. This is followed by mechano-hyperalgesia, mechano-allodynia, and cold-allodynia that lasted for at least 1 month. Light microscopic analyses and electron microscopic analyses of the nerves at the site of the tourniquet show that the majority of these animals have no sign of injury to myelinated or unmyelinated axons. However, electron microscopy shows that the ischemia-reperfusion injury produces a microvascular injury, slow-flow/no-reflow, in the capillaries of the hind paw muscle and digital nerves. We propose that the slow-flow/no-reflow phenomenon initiates and maintains deep-tissue ischemia and inflammation, leading to the activation of muscle nociceptors, and the ectopic activation of sensory afferent axons due to endoneurial ischemia and inflammation. These data, and a large body of clinical evidence, suggest that in at least a subset of CRPS-I patients, the fundamental cause of the abnormal pain sensations is ischemia and inflammation due to microvascular pathology in deep tissues, leading to a combination of inflammatory and neuropathic pain processes. Moreover, we suggest a unifying idea that relates the pathogenesis of CRPS-I to that of CRPS-II. Lastly, our hypothesis suggests that the role of the sympathetic nervous system in CRPS-I is a factor that is not fundamentally causative, but may have an important contributory role in early-stage disease.

Diagnostic criteria in patients with complex regional pain syndrome assessed in an out-patient clinic

BACKGROUND

Specific criteria have been described and accepted worldwide for diagnosing patients with complex regional pain syndrome (CRPS). Nevertheless, a clear-cut diagnosis cannot be confirmed in a number of cases.

AIM

The objective of this study was to investigate the effectiveness of the described diagnostic criteria used by several clinical disciplines.

METHODS

We included 195 patients who were referred to our pain clinic within a period of 1 year. Data were collected on patient characteristics, signs, symptoms, disease-related medication, and the background of the referring clinicians.

RESULTS

The Harden and Bruehl criteria were confirmed in 95 patients (49%). These patients used a higher than average number of analgesics, opiates, and anti-oxidants, and frequently received prescriptions for benzodiazepines instead of anti-depressants. The mean disease duration was 29 +/- 4.6 months and the mean visual analogue score for pain was 8.1 +/- 0.19. A subgroup of patients had a colder temperature in the affected extremity compared with the unaffected extremity. This subgroup showed a longer disease duration and higher visual analogue scale pain.

CONCLUSION

The diagnostic criteria used to determine CRPS should be further improved. A large number of referred patients experienced substantial pain, without receiving adequate medication. Disease-related medication is unrelated to CRPS-specific disease activity. Knowledge of underlying mechanisms is warranted before an adequate pharmaceutical intervention can be considered.

Complex regional pain syndrome: what's in a name?

Within a 2-year period in the 1940s, 2 Boston physicians published dramatically opposing views on the underlying nature of a syndrome now known as complex regional pain syndrome (CRPS). Evans suggested, in several papers in 1946-1947, that sympathetic reflexes maintain pain and dystrophy in affected limbs. Foisie, in 1947, suggested arterial vasospasms were key in the etiology of this pain syndrome. Evans' hypothesis established the nomenclature for this syndrome for 60 years, and his term, "reflex sympathetic dystrophy," guided clinical treatment and research activities over the same period. Foisie's proposed nomenclature was unrecognized, and had virtually no impact on the field. Recent evidence suggests that Evans' contribution to the field may have in fact led clinicians and researchers astray all those years. This focus article on CRPS compares recent observations with these 2 earlier theories and asks the question-what if we had adopted Foisie's nomenclature from the beginning?

PERSPECTIVE

This article discusses 2 opposing historical views on the etiology of what is now known as CRPS, and how they affected nomenclature, research, and clinical therapy in subsequent decades. This focus article may help researchers and clinicians realize the importance of syndrome names, and how they may inadvertently misdirect research and treatment.

Evidence based guidelines for complex regional pain syndrome type 1

Background

Treatment of complex regional pain syndrome type I (CRPS-I) is subject to discussion. The purpose of this study was to develop multidisciplinary guidelines for treatment of CRPS-I.

Method

A multidisciplinary task force graded literature evaluating treatment effects for CRPS-I according to their strength of evidence, published between 1980 to June 2005. Treatment recommendations based on the literature findings were formulated and formally approved by all Dutch professional associations involved in CRPS-I treatment.

Results

For pain treatment, the WHO analgesic ladder is advised with the exception of strong opioids. For neuropathic pain, anticonvulsants and tricyclic antidepressants may be considered. For inflammatory symptoms, free-radical scavengers (dimethylsulphoxide or acetylcysteine) are advised. To promote peripheral blood flow, vasodilatory medication may be considered. Percutaneous sympathetic blockades may be used to increase blood flow in case vasodilatory medication has insufficient effect. To decrease functional limitations, standardised physiotherapy and occupational therapy are advised. To prevent the occurrence of CRPS-I after wrist fractures, vitamin C is recommended. Adequate perioperative analgesia, limitation of operating time, limited use of tourniquet, and use of regional anaesthetic techniques are recommended for secondary prevention of CRPS-I.

Conclusions

Based on the literature identified and the extent of evidence found for therapeutic interventions for CRPS-I, we conclude that further research is needed into each of the therapeutic modalities discussed in the guidelines.

Regulation of peripheral blood flow in complex regional pain syndrome: clinical implication for symptomatic relief and pain management

BACKGROUND

During the chronic stage of Complex Regional Pain Syndrome (CRPS), impaired microcirculation is related to increased vasoconstriction, tissue hypoxia, and metabolic tissue acidosis in the affected limb. Several mechanisms may be responsible for the ischemia and pain in chronic cold CPRS.

DISCUSSION

The diminished blood flow may be caused by either sympathetic dysfunction, hypersensitivity to circulating catecholamines, or endothelial dysfunction. The pain may be of neuropathic, inflammatory, nociceptive, or functional nature, or of mixed origin.

SUMMARY

The origin of the pain should be the basis of the symptomatic therapy. Since the difference in temperature between both hands fluctuates over time in cold CRPS, when in doubt, the clinician should prioritize the patient's report of a persistent cold extremity over clinical tests that show no difference. Future research should focus on developing easily applied methods for clinical use to differentiate between central and peripheral blood flow regulation disorders in individual patients.

No recovery of cold complex regional pain syndrome after transdermal isosorbide dinitrate: a small controlled trial

The microcirculation appears to be impaired in cold chronic complex regional pain syndrome (CRPS). This double-blind, placebo-controlled, randomized trial investigated the effect of the nitric oxide (NO) donor isosorbide dinitrate (ISDN) on the peripheral blood flow in patients with chronic CRPS. Twenty-four patients received 1% ISDN in Vaseline or a placebo ointment applied to the dorsum of the affected hand four times daily for 10 weeks. The patients participated in a physical therapy program to improve activity. The primary outcome measure was blood distribution in the affected extremity, which was determined by measuring the skin temperature using videothermography. We also measured NO and endothelin-1 concentrations in blister fluid, pain using the visual analog scale, and activity limitations using an upper limb activity monitor and the Disabilities of Arm Shoulder and Hand Questionnaire. ISDN failed to produce a significant improvement in temperature asymmetry in chronic cold CRPS patients, and it did not result in the expected reduction in pain and increase in activity compared with placebo either. There may be other central or peripheral factors contributing to the disturbed vasodynamics in cold chronic CRPS that are not influenced by NO substitution. This study does not show an improvement of the regional blood distribution by ISDN in the involved extremity of patients with cold-type CRPS.

Expression of endothelial nitric oxide synthase and endothelin-1 in skin tissue from amputated limbs of patients with complex regional pain syndrome

BACKGROUND AND OBJECTIVES

Impaired microcirculation during the chronic stage of complex regional pain syndrome (CRPS) is related to increased vasoconstriction, tissue hypoxia, and metabolic tissue acidosis in the affected limb. Endothelial dysfunction is suggested to be the main cause of diminished blood flow. The aim of this study was to examine the distribution of endothelial nitric oxide synthase (eNOS) and endothelin-1(ET-1) relative to vascular density represented by the endothelial marker CD31-immunoreactivity in the skin tissue of patients with chronic CRPS.

METHODS

We performed immunohistochemical staining on sections of skin specimens obtained from the amputated limbs (one arm and one leg) of two patients with CRPS.

RESULTS

In comparison to proximal specimens we found an increased number of migrated endothelial cells as well as an increase of eNOS activity in distal dermis specimens.

CONCLUSIONS

We found indications that endothelial dysfunction plays a role in chronic CRPS.