Topical capsaicin treatment of chronic postherpetic neuralgia

Complex regional pain syndrome type I treated with topical capsaicin: a case report

This report describes the case of a multitrauma patient who underwent an amputation of the left arm and had a complicated left crural fracture with a delayed union. He was treated in an inpatient setting for preprosthetic training for a myoelectric prosthesis and to regain walking abilities. After consolidation of the crural fracture, complex regional pain syndrome type I (CRPS I) developed in the left foreleg, which hindered mobilization. Topical capsaicin .075% was prescribed and a stress-loading mobilization schema was instituted. No other treatment modalities directed at CRPS I were added. After 6 weeks, no signs or symptoms of CRPS I were present and capsaicin was discontinued. Capsaicin is a well-accepted and documented treatment modality in neuropathic pain states such as postherpetic neuralgia. However, it has rarely been described in CRPS I. Capsaicin is discussed within the framework of recent insights in the neurobiology of nociception, and it is concluded that it may provide a theory-driven treatment for CRPS I, especially in the acute stage, that facilitates physical therapy and prevents peripheral and spinal sensitization.

The analgesic efficacy of topical capsaicin is enhanced by glyceryl trinitrate in painful osteoarthritis: a randomized, double blind, placebo controlled study

The aim of this study was to assess if the pain of osteoarthritis is reduced by topical capsaicin and to determine whether addition of glyceryl trinitrate has an effect on analgesic efficacy and tolerability of capsaicin. A randomized, double blind, placebo controlled study was carried out on 200 adult patients attending a Pain Clinic with osteoarthritis pain. Patients applied one of four creams topically over the affected joint over a 6 week period. Creams contained either placebo (vehicle), 0.025% capsaicin, 1.33% glyceryl trinitrate or 0.025% capsaicin + 1.33% glyceryl trinitrate. Analgesic efficacy, tolerability of cream and analgesic consumption were assessed. One hundred and sixty-seven of 200 patients completed the study. Baseline visual analogue scores (0-10 scale) for pain were 6.40. There was a significant reduction in pain scores in the glyceryl trinitrate group (mean decrease 0.59, p< 0.05, 95% confidence limits 0.04-1.14), 0.025% capsaicin group (mean decrease 0.5, p< 0.05, 95% confidence limits 0.05-1.05) and the glyceryl trinitrate capsaicin group (mean decrease 1.1, p<0.05, 95% confidence limits 0.22-1.98). Baseline discomfort of application scores were similar for all but the capsaicin groups (they were significantly higher (by 2.1 units, p< 0.001)). The odds ratio in favour of continuing treatment was 2.1 (95% confidence limits 1.0-4.4) for glyceryl trinitrate and 2.4 (95% confidence limits 1.2-5.1) for capsaicin and 5.0 (95% confidence limits 3.8-6.4) for capsaicin GTN combination. The study showed that topical capsaicin and glyceryl trinitrate have an analgesic effect in painful osteoarthritis. When used together this effect is increased with the combination being more tolerable than capsaicin alone. Analgesic consumption is decreased by capsaicin, glyceryl trinitrate and to a greater extent by both combined.

Treatment of prurigo nodularis with topical capsaicin

BACKGROUND

Prurigo nodularis is an eruption of lichenified or excoriated nodules caused by intractable pruritus that is difficult to treat. Therefore the antipruritic efficacy of capsaicin seemed to be of particular interest because this alkaloid, extractable from red pepper, interferes with the perception of pruritus and pain by depletion of neuropeptides in small sensory cutaneous nerves.

OBJECTIVE

The aim of this concentration- and regimen-ranging study was to evaluate the efficacy, safety, and practicability of capsaicin in the topical treatment of prurigo nodularis in a large series of patients.

METHODS

A total of 33 patients with prurigo nodularis of various origins were selected to receive capsaicin (0.025% to 0.3%) 4 to 6 times daily for 2 weeks up to 10 months. The consecutive follow-up period was up to 6 months. In 7 patients, skin biopsy specimens were taken before, during, and after therapy and investigated histologically, immunohistochemically, and ultrastructurally.

RESULTS

All 33 patients could be evaluated for efficacy. After cessation of the symptoms of neurogenic inflammation, such as burning sensations or erythema, all of them experienced a complete elimination of pruritus within 12 days. In addition, capsaicin largely contributed to the gradual healing of the skin lesions. After discontinuation of the therapy, pruritus returned in 16 of 33 patients within 2 months. At the ultrastructural level, no degenerative changes of cutaneous nerves could be found during or after capsaicin therapy. Depletion of substance P was demonstrated by confocal laser scanning microscopy thus confirming the specific effect of capsaicin in vivo.

CONCLUSION

Topical treatment of prurigo nodularis with capsaicin proved to be an effective and safe regimen resulting in clearing of the skin lesions.

Treatment options in postherpetic neuralgia

Postherpetic neuralgia (PHN) is a separate disease entity that represents a complication of acute herpes zoster. PHN, involving aberrant somatosensory processing in the peripheral and/or central nervous system, is considered to be a chronic neuropathic pain, frequently unresponsive to all treatment modalities. Despite the clinical trial data demonstrating successful pain relief with several drug regimens, the pharmacologic management of neuropathic pain is difficult, particularly in PHN. Response to therapy is generally inhomogeneous. Some patients experience long-term pain control with either topical or oral monotherapy with antidepressants, anticonvulsants, or opioids. Other PHN patients, such as those suffering pain due to central nervous system lesions, are extraordinarily refractory to all measures. This article will review current treatments--tricyclic antidepressants, anticonvulsants, local anesthetics, clonidine, N-methyl-D-aspartate (NMDA)-antagonists, and opioids and focus on mechanism-based pharmacologic interventions. Pharmacologic approaches can be classified into three groups: 1) drugs that act topically in the affected skin area; 2) drugs that act on nerve excitability and conduction in sensory axons; and 3) drugs that act on neural damage related synaptic changes. This last group is the only pain treatment option related to central denervation. To date, the treatment of PHN has relied on the use of tricyclic antidepressants (TCAs), which represent the most comprehensively studied medications for this pain syndrome. Clinical data indicate that TCAs are effective analgesics in approximately 50% of patients; these drugs have been recommended as first-line agents for all neuropathic pain syndromes except trigeminal neuralgia, but are frequently contraindicated or poorly tolerated in elderly patients with PHN. If monotherapy fails, a mechanism- and/or symptom-based multidrug regimen can be used. There is also consistent support for intravenous and topical lidocaine, intravenous ketamine, carbamazepine, and opioids. Gabapentin, a new anticonvulsant, can be considered a first-line oral medication for PHN based on the efficacy and safety results of a recently completed double-blind trial. In addition to positive effects on PHN, sleep, mood, and overall quality of life were significantly improved.

Topical application of doxepin hydrochloride, capsaicin and a combination of both produces analgesia in chronic human neuropathic pain: a randomized, double-blind, placebo-controlled study

AIMS

To assess the analgesic efficacy of topical administration of 3.3% doxepin hydrochloride, 0.025% capsaicin and a combination of 3. 3% doxepin and 0.025% capsaicin in human chronic neuropathic pain.

METHODS

A randomized, double-blind, placebo-controlled study of 200 consenting adult patients. Patients applied placebo, doxepin, capsaicin or doxepin/capsaicin cream daily for 4 weeks. Patients recorded on a daily basis overall pain, shooting, burning, paraesthesia and numbness using a 0-10 visual analogue scale during the week prior to cream application (baseline levels) and for the 4 week study period. Side-effects and desire to continue treatment were also recorded.

RESULTS

Overall pain was significantly reduced by doxepin, capsaicin and doxepin/capsaicin to a similar extent. The analgesia with doxepin/capsaicin was of more rapid onset. Capsaicin significantly reduced sensitivity and shooting pain. Burning pain was increased by doxepin and by capsaicin and to a lesser extent by doxepin/capsaicin. Side-effects were minor. One patient requested to continue placebo cream, 17 doxepin cream, 13 capsaicin and 9 the combination of doxepin and capsaicin.

CONCLUSIONS

Topical application of 3.3% doxepin, 0.025% capsaicin and 3.3% doxepin/0. 025% capsaicin produces analgesia of similar magnitude. The combination produces more rapid analgesia.

Effects of topically applied capsaicin cream on neurogenic inflammation and thermal sensitivity in rats

The effects of capsaicin cream on neurogenic inflammation and thermal nociceptive threshold were investigated in rats. Firstly, for topical application of capsaicin cream to hind paw, we shaped boots from dental cement to prevent the animals from licking off the drug. Capsaicin cream (1%) led to significant increases in the amounts of Evans blue and substance P (SP) released into the perfusate, and the former response was significantly suppressed by pretreatment with RP67580, an NK1-receptor antagonist, but not by treatment with an NK2-receptor antagonist. Subsequent electrical stimulation of the sciatic nerve resulted in a significant reduction in Evans blue and SP extravasation 24 h after topical application of capsaicin cream. On the other hand, when capsaicin cream was repeatedly applied to both hind paws once a day, withdrawal latency for noxious heat stimulation decreased after 24 h, and this thermal hyperalgesia was reversed 3 days later. These results suggest that capsaicin cream initially affects neurogenic inflammation mechanisms and then blocks the pain transmission mechanism.

Effect of ricinoleic acid in acute and subchronic experimental models of inflammation

Observational studies indicate that topical application of ricinoleic acid (RA), the main component of castor oil, exerts remarkable analgesic and anti-inflammatory effects. Pharmacological characterization has shown similarities between the effects of RA and those of capsaicin, suggesting a potential interaction of this drug on sensory neuropeptide-mediated neurogenic inflammation. The aim of this study was to assess RA anti-inflammatory activities in comparison with capsaicin in several models of acute and subchronic inflammation. The acute inflammation was induced by intradermal injection of carrageenan in the mouse or by histamine in the guinea-pig eyelid. In either experiment, the extent of the oedema thickness was measured. Subchronic oedema was induced by complete Freund's adjuvant injection in the ventral right paw of mice. Tissue substance P (SP) was measured in the carrageenan experiments by radioimmunoassay (RIA). It was found that the acute topical application of RA (0.9 mg/mouse) or capsaicin (0.09 mg/mouse) significantly increased the mouse paw oedema induced by carrageenan, while an 8-day repeated topical treatment with the same doses of both compounds resulted in a marked inhibition of carrageenan-induced paw oedema matched by a reduction in SP tissue levels. Similar effects were found against histamine-induced eyelid oedema in guinea-pigs after acute or repeated application of RA or capsaicin. RA and capsaicin given for 1-3 weeks reduced the established oedema induced by Freund's adjuvant, a subchronic model of inflammation, particularly if given by the intradermal route. Either in mouse paw or in guinea-pig eyelid, capsaicin but not RA by itself produced a slight hyperemia and activation of a behavioural response (e.g. scratching of the eyelids). On the basis of the present results, RA may be seen as a new capsaicin-like, non-pungent anti-inflammatory agent suitable for peripheral application.

Efficacy of pharmacological treatments of neuropathic pain: an update and effect related to mechanism of drug action

Tricyclic antidepressants and carbamazepine have become the mainstay in the treatment of neuropathic pain. Within the last decade, controlled trials have shown that numerous other drugs relieve such pain. We identified all placebo-controlled trials and calculated numbers needed to treat (NNT) to obtain one patient with more than 50% pain relief in order to compare the efficacy with the current treatments, and to search for relations between mechanism of pain and drug action. In diabetic neuropathy, NNT was 1.4 in a study with optimal doses of the tricyclic antidepressant imipramine as compared to 2.4 in other studies on tricyclics. The NNT was 6.7 for selective serotonin reuptake inhibitors, 3.3 for carbamazepine, 10.0 for mexiletine, 3.7 for gabapentin, 1.9 for dextromethorphan, 3.4 for tramadol and levodopa and 5.9 for capsaicin. In postherpetic neuralgia, the NNT was 2.3 for tricyclics, 3.2 for gabapentin, 2.5 for oxycodone and 5.3 for capsaicin, whereas dextromethorphan was inactive. In peripheral nerve injury, NNT was 2.5 for tricyclics and 3.5 for capsaicin. In central pain, NNT was 2.5 for tricyclics and 3. 4 for carbamazepine, whereas selective serotonin reuptake inhibitors, mexiletine and dextromethorphan were inactive. There were no clear relations between mechanism of action of the drugs and the effect in distinct pain conditions or for single drug classes and different pain conditions. It is concluded that tricyclic antidepressants in optimal doses appear to be the most efficient treatment of neuropathic pain, but some of the other treatments may be important due to their better tolerability. Relations between drug and pain mechanisms may be elucidated by studies focusing on specific neuropathic pain phenomena such as pain paroxysms and touch-evoked pain.

Civamide (cis-capsaicin) for treatment of primary or recurrent experimental genital herpes

Neuropharmacologic agents able to disrupt normal virus-neuron interactions may provide an alternative strategy for the treatment of herpes simplex virus (HSV) infections. We have previously shown that prophylactic treatment with capsaicin, a natural compound that alters function in sensory neurons, can protect guinea pigs against cutaneous HSV disease, even though the compound has no direct antiviral activity. Here we have examined the ability of civamide, the cis isomer of capsaicin, to interfere with HSV disease. We show that, even when the onset of treatment was delayed until after intravaginal virus challenge, primary genital skin disease severity was significantly reduced. In addition, animals treated during primary infection subsequently experienced a long-lasting reduction in recurrent disease. Civamide treatment during latent infection also significantly reduced recurrent disease, although for a shorter period. Further a single weekly treatment with civamide during latent infection was sufficient to reduce recurrent disease, indicating that an infrequent suppressive maintenance therapy might be possible.

Postherpetic neuralgia: role of gabapentin and other treatment modalities

Postherpetic neuralgia (PHN) is a chronic and painful condition that may occur after a herpes zoster infection. The frequency of PHN after untreated zoster varies widely. Age is the most important risk factor for development of PHN. The condition occurs in an estimated 50% of patients older than 50 years. The pain of PHN can be severe and debilitating and is frequently associated with allodynia. Although in most patients pain remits within the first year, it may persist for a lifetime. Tricyclic antidepressants (TCAs), topical agents, opioids, and gabapentin, a structural gamma-amino butyric acid (GABA) analogue, are the only agents that have demonstrated efficacy in randomized clinical trials for treatment of both the shooting and the burning form of pain associated with PHN. TCAs are among the most commonly used classes of agents for treating PHN and are effective in a significant proportion of patients. However, various adverse events can limit treatment. These side effects tend to be more acute in the elderly, the population most likely to suffer from PHN. Topical agents have led to mild to moderate improvement in patients with PHN but are usually ineffective as monotherapy for this condition. Until recently, carbamazepine was the only antiepileptic drug evaluated for the treatment of PHN. Over the past few years, however, gabapentin has received increasing attention as a useful treatment for neuropathic pain. Gabapentin lacks significant drug-drug interactions and has a favorable safety profile, which makes it particularly useful for treatment of PHN.

Varicella zoster viral disease

Herpes zoster is cause of considerable morbidity, especially among elderly patients, with a suggestion of a slight increase in incidence among female patients. Substantial research on the biology of the varicella zoster virus has led to advances in our knowledge of the pathophysiology of the disease along with more successful therapy for the acute episodes of herpes zoster. Ophthalmic zoster is more common than zoster in other cranial nerves and is associated with pronounced suffering. This article reviews the epidemiology, biology, and latency of herpes zoster, discusses the pathophysiology of the disease, and describes treatment options with antivirals and corticosteroids. The pathophysiology and treatment options for postherpetic neuralgia are also addressed. The varicella vaccine is now available, and initial results suggest that this may lessen the effect of herpes zoster in the future.

Peripheral and central actions of capsaicin and VR1 receptor

Vanilloid receptor subtype 1 (VR1), a capsaicin receptor, is expressed in primary sensory neurons and vagal nerves. Heat and protons as well as capsaicin activate VR1 to induce the influx of cations, particularly Ca2+ and Na+ ions. Characteristic effects of capsaicin are the induction of a burning sensation after acute administration and the desensitization of sensory neurons after large doses and prolonged administration. The latter feature made capsaicin cream applicable for the treatment of chronic pain and pruritus. Capsaicin alters several visceral functions, which may be mediated by action on vagal nerves and central neurons. Capsaicin affects thermoregulation after intra-hypothalamic injection and releases glutamate from the hypothalamus and cerebral cortex slices, while VR1-like immunoreactivity is not apparent in these regions. These findings taken together suggest the existence of other subtypes of vanilloid receptors in the brain.

Varicella zoster virus

Because of its ability to produce two clinically distinct disease entities (chickenpox and shingles), varicella zoster virus (VZV) is an unusual etiologic agent. Although in the past viral exanthems were mostly only of academic interest to the practitioner, the development of antiviral agents and the newly approved varicella (OKA) vaccine have increased the clinical significance. Also, with the increasing seroprevalence of HIV infection, more patients will be stricken with zoster (at a younger age) and disseminated varicella. In this review, the history, incidence, pathogenesis, clinical manifestations, and treatment options (of VZV infection and postherpetic neuralgia) will be discussed.

Permeability of a capsaicin derivative, [14C]DA-5018 to blood-brain barrier corrected with HPLC method

In the present work, the transport mechanism of a capsaicin derivative, DA-5018, through blood-brain barrier (BBB) has been investigated to evaluate the feasibility of potential drug development. The result of pharmacokinetic parameters obtained from the intravenous injection of plasma volume marker, [3H]RSA and [14C]DA-5018, indicated that both AUC, area under the plasma concentration curve and VD, volume of distribution in brain of [3H]RSA agreed with those reported (1620 +/- 10 percentage injected dose minute per milliliter (%IDmin/ml) and 12.0 +/- 0.1 microliters/g, respectively). Elimination half-life and AUC of [14C]DA-5018 is corrected by the HPLC analysis, 19.6 +/- 1.2 min and 7.69 +/- 0.85% IDmin/ml, respectively. The metabolic rate of [14C] DA-5018 was very rapid. The blood-brain barrier permeability surface area (PS) product of [14C]DA-5018 was calculated to be 0.24 +/- 0.05 microliter/min/g. The result of internal carotid artery perfusion and capillary depletion suggested that [14C]DA-5018 pass through BBB with the time increasingly. Investigation of transport mechanism of [14C]DA-5018 using agonist and antagonist suggested that vanilloid (capsaicin) receptor did not exist in the BBB, and nutrient carrier system in the BBB has no effect on the transport of DA-5018. In conclusion, despite the fact that penetration of DA-5018 through BBB is significant, the intact drug found in the brain tissue is small because of a rapid metabolism. Therefore, for the central analgesic effect of DA-5018, the method to increase the metabolic stability in plasma and the brain permeability should be considered.

Treatment of postherpetic neuralgia

OBJECTIVE

To review treatment options for postherpetic neuralgia (PHN).

DATA SOURCES

Clinical literature selected by the authors accessed via MEDLINE. Search terms included postherpetic neuralgia, capsaicin, antidepressants, anticonvulsants, and lidocaine.

STUDY SELECTION

Controlled trials relevant to PHN.

DATA SYNTHESIS

Traditional analgesics offer little benefit for the treatment of PHN. The best results for pain relief have come from capsaicin and tricyclic antidepressants. Anticonvulsants have also been used, although the number of studies evaluating this is limited. More invasive therapies, such as transcutaneous electrical nerve stimulation and nerve blocks, can be considered if other therapies fail.

CONCLUSION

Early diagnosis and treatment of herpes zoster may offer patients the best chance of preventing the development of PHN. However, if PHN does develop, the patient should seek treatment early for the best chance of pain relief.

Capsaicin binds to the intracellular domain of the capsaicin-activated ion channel

Capsaicin (CAP) excites small sensory neurons, causing pain, neurogenic inflammation, and other visceral reflexes. These effects have been proposed to be the result of CAP activation of a nonselective cation current. It is generally assumed that CAP binds to an extracellular domain of the membrane receptor. However, the exact binding site is not known because of the lipophilic nature of CAP. To determine whether the binding domain is extracellular or intracellular, we tested the effect of a synthetic water-soluble CAP analog, DA-5018.HCl, on current activation. CAP activated the 45 pS (at -60 mV) nonselective cation channel from either side of the membrane. However, DA-5018.HCl, which had a greater potency and efficacy than CAP, activated the channels only from the cytosolic side of the patch membrane in a capsazepine, a CAP receptor antagonist, reversible manner. When applied extracellularly, DA-5018. HCl did not, but CAP did, activate whole-cell currents in sensory neurons, as well as in oocytes expressing vanilloid receptor 1, a recently cloned CAP receptor. Hydrogen ions, reported as a possible endogenous activator of cation current, failed to elicit any current when acidic medium (pH 5.0-6.0) was applied intracellularly, indicating that H+ does not mediate the CAP effect. These results indicate that CAP and its analog bind to the cytosolic domain of the CAP receptor and suggest that an endogenous CAP-like substance other than H+ may be present in the cell.

Pain caused by herpes zoster infection

Postherpetic neuralgia (PHN) is a neuropathic pain disorder that occurs most often in the elderly. This painful condition is uniquely suited for clinical research, resulting in an emerging understanding of the pathophysiology of the persistent pain. Until recently, only the tricyclic antidepressants proved effective for PHN. Controlled trials of a wide variety of therapeutic strategies are in progress or have been recently completed.

Postherpetic neuralgia: irritable nociceptors and deafferentation

Postherpetic neuralgia (PHN) is a common and often devastatingly painful condition. It is also one of the most extensively investigated of the neuropathic pains. Patients with PHN have been studied using quantitative testing of primary afferent function, skin biopsies, and controlled treatment trials. Together with insights drawn from an extensive and growing literature on experimental models of neuropathic pain these patient studies have provided a preliminary glimpse of the pain-generating mechanisms in PHN. It is clear that both peripheral and central pathophysiological mechanisms contribute to PHN pain. Some PHN patients have abnormal sensitization of unmyelinated cutaneous nociceptors (irritable nociceptors). Such patients characteristically have minimal sensory loss. Other patients have pain associated with small fiber deafferentation. In such patients pain and temperature sensation are profoundly impaired but light moving mechanical stimuli can often produce severe pain (allodynia). In these patients, allodynia may be due to the formation of new connections between nonnociceptive large diameter primary afferents and central pain transmission neurons. Other deafferentation patients have severe spontaneous pain without hyperalgesia or allodynia and presumably have lost both large and small diameter fibers. In this group the pain is likely due to increased spontaneous activity in deafferented central neurons and/or reorganization of central connections. These three types of mechanism may coexist in individual patients and each offers the possibility for developing new therapeutic interventions.

The management of established postherpetic neuralgia: a comparison of the quality and content of traditional vs. systematic reviews

In the face of an exponential increase in published biomedical studies, dermatologists frequently turn to review articles in order to keep abreast of important developments in the treatment of skin diseases. Traditional review articles have recently been criticized on the basis of their incompleteness and susceptibility to bias. Such biases can be minimized by employing a systematic approach to gathering, combining and interpreting the evidence of treatment efficacy. Using eight predetermined quality criteria, we compared the quality of 10 traditional review with one systematic review of treatments for postherpetic neuralgia, which were identified from the Medline database for 1992-96. None of the 10 traditional review articles satisfied all eight criteria: one satisfied five, two satisfied four and the rest satisfied two or fewer criteria. There was a wide variation in the recommendations of the authors for the treatment of postherpetic neuralgia, often based on anecdotal evidence and clinical experience. On the other hand, the systematic review fulfilled seven of the eight quality criteria, failing only to discuss future directives. Furthermore, treatment recommendations were made solely on the basis of randomized controlled trials, which are considered to be the gold standard for measuring the benefits of any intervention. The variation in quality and treatment recommendations of traditional reviews is worrying. Systematic reviews should be encouraged in dermatology because they provide a summary of evidence of the effects of dermatological treatments, which has been derived using explicit methods widely accepted within science.

Review of the effectiveness of capsaicin for painful cutaneous disorders and neural dysfunction

BACKGROUND

Topical capsaicin is known to be a safe and effective pain management adjunct for rheumatoid arthritis, osteoarthritis, neuralgias, and diabetic neuropathy. However, studies and case reports in the literature have indicated that other conditions may also benefit from capsaicin: painful or itching cutaneous disorders from operations, injuries, or tumors; neural dysfunction; or inflammation of the airways and urinary tract.

METHODS

To determine the effectiveness of capsaicin for painful cutaneous disorders and neural dysfunction, the authors analyzed data from 33 reports (MEDLINE search of 1966-96) on the efficacy of capsaicin. Outcome measures consisted of the response rate and degree of pain relief. Results from placebo-controlled trials were pooled when possible; effect of treatment was estimated by the method of DerSimonian and Laird.

RESULTS

Pain relief for postmastectomy syndrome and cluster headache was greater with capsaicin than with placebo; also, psoriasis and pruritus responded better to capsaicin. Uncontrolled studies and case reports have indicated that pain or dysfunction was less at the end of capsaicin therapy for neck pain, loin pain/hematuria syndrome, oral mucositis, rhinopathy, reflex sympathetic dystrophy syndrome, detrusor hyperreflexia, and cutaneous pain due to tumor of the skin.

CONCLUSIONS

Capsaicin is effective for psoriasis, pruritus, and cluster headache; it is often helpful for the itching and pain of postmastectomy pain syndrome, oral mucositis, cutaneous allergy, loin pain/hematuria syndrome, neck pain, amputation stump pain, and skin tumor; and it may be beneficial for neural dysfunction (detrusor hyperreflexia, reflex sympathetic dystrophy, and rhinopathy). A universal problem for many of the studies analyzed was the absence of a "burning placebo" such as camphor.

Clinical trials of herbs

Consumers, clinicians, and corporations are taking an interest in botanical medicine. Although large gaps remain in research, many clinical trials of herbs exist. This article reviews these trials.

Sensory neuropeptides: their role in inflammation and wound healing

Vasoactive neuropeptides including substance P and calcitonin gene-related peptide (CGRP) are localised in sensory nerves which innervate blood vessels. These are the major vasoactive neuropeptides released from sensory nerve endings and both have been suggested to have roles in inflammatory and cardiovascular disease. The neuropeptides have potent effects on microvascular tone and permeability, which are seen soon after release from perivascular nerves. There is also evidence that neuropeptides can affect various activities of inflammatory cells and that sensory nerves play a role in the recovery of the healthy microcirculation during wound healing phases. This review concentrates on evidence that the neuropeptides substance P, acting via tachykinin NK1 and NK2 receptors, and CGRP, acting via CGRP1 receptors, play a pro-inflammatory role in disease and a beneficial role in wound healing. In addition, results from clinical trials of recently developed neuropeptide antagonists are discussed.

Vanilloid receptors: new insights enhance potential as a therapeutic target

Compounds related to capsaicin and its ultrapotent analog, resiniferatoxin (RTX), collectively referred to as vanilloids, interact at a specific membrane recognition site (vanilloid receptor), expressed almost exclusively by primary sensory neurons involved in nociception and neurogenic inflammation. Desensitization to vanilloids is a promising therapeutic approach to mitigate neuropathic pain and pathological conditions (e.g. vasomotor rhinitis) in which neuropeptides released from primary sensory neurons play a major role. Capsaicin-containing preparations are already commercially available for these purposes. The use of capsaicin, however, is severely limited by its irritancy, and the synthesis of novel vanilloids with an improved pungency/desensitization ratio is an on-going objective. This review highlights the emerging evidence that the vanilloid receptor is not a single receptor but a family of receptors, and that these receptors recognize not simply RTX and capsaicin structural analogs but are broader in their ligand-binding selectivity. We further focus on ligand-induced messenger plasticity, a recently discovered mechanism underlying the analgesic actions of vanilloids. Lastly, we give a brief overview of the current clinical uses of vanilloids and their future therapeutic potential. The possibility is raised that vanilloid receptor subtype-specific drugs may be synthesized, devoid of the undesirable side-effects of capsaicin.

Management of chronic pain. Part I

Chronic pain is associated with substantial psychosocial and economic stress, coupled with functional loss and various levels of vocational dysfunction. The role of a pain center is to focus on chronic pain in a multidisciplinary, comprehensive manner, providing the patient with the most effective opportunity to manage his or her chronic disease syndrome. This article focuses on methods to manage many types of chronic pain and describes a broad range of pharmacologic and nonpharmacologic interventions and options available to the patient. Part I of this two-part monograph describes pharmacotherapeutic interventions and regional nerve blocks. Part II focuses on psychologic assessment and treatment and physical therapy. A multimodal management strategy offers patients the greatest improvement potential for specific chronic pain syndromes. Cognitive and behavioral therapies and physical therapies are described. This combination of therapies may provide patients with the skills and knowledge needed to increase their sense of control over pain. The integration of appropriate pharmacotherapeutic regimens, neural blockades, physical therapy, and psychologic techniques maximizes a patient's effectiveness in dealing with chronic pain. Three case studies are presented in Part II.

Topical capsaicin selectively attenuates heat pain and A delta fiber-mediated laser-evoked potentials

Cutaneous stimulation with CO2 laser pulses activates A delta of nociceptive afferents and evokes late cerebral potentials (LEPs), the amplitude of which correlates parametrically with the perceived magnitude estimation of laser pulses. Capsaicin is known to desensitize the nociceptive terminals of C fibers. In this double-blind, vehicle-controlled experiment, we tested the hypothesis that topical capsaicin would inactivate A delta afferents and lead to an attenuation of the LEPs. Subjects applied capsaicin cream to the dorsum of one hand and vehicle cream to the other 3 times daily for a period of 5 weeks. At weekly intervals before starting, during administration and after discontinuation of capsaicin, LEPs were recorded and psychophysical thresholds and magnitude estimation for several sensory modalities were determined. The results of this study showed that topical capsaicin significantly and reversibly decreased the magnitude estimation of suprathreshold heat pain, laser pulses and amplitude of the LEPs. There was no statistically significant difference in light touch, deep pain and mechanical pain detection thresholds between the capsaicin- and vehicle-treated hands. It indicated that topical capsaicin caused a definite functional and reversible inactivation of A delta nociceptive afferent transmission. The decline in the magnitude estimation of laser pulses concomitantly with the attenuation of LEP amplitudes supports the hypothesis that some A delta afferents mediate noxious heat in humans. These findings demonstrate the usefulness of LEP in the physiological evaluation of nociceptive pathways and its potential usefulness in objectively documenting the effect of pharmacological treatment on pain perception.

Desensitization follows excitation of bladder primary afferents by intravesical capsaicin, as shown by c-fos activation in the rat spinal cord

Activation of the protooncogene c-fos at spinal cord segments T12-L2 and L5-S1 was used to study the effects of topical administration of capsaicin on bladder primary afferents coursing in the hypogastric (HGN) or pelvic (PN) nerves of adult rats. Two hours after capsaicin instillation in the bladder numerous Fos cells occurred in lamina I at T12-L2 and in lamina I, intermediolateral gray matter (ILG) and dorsal commissure (DCM) at L5-S1. Twenty-four hours later, the Fos immunoreaction had disappeared from the spinal cord. At this time, instillation of 1% acetic acid into the bladder of capsaicin-treated rats induced considerably fewer Fos cells than in animals that had been instilled only with the vehicle solution for capsaicin. The difference in the average number of Fos cells was statistically significant in lamina I, ILG and DCM at L5-S1 but not in lamina I at T12-L2. Thus, intravesical capsaicin at the doses used excites bladder primary afferents coursing in the HGN and PN, but only desensitizes those coursing in the PN. It is suggested that this may depend on the differential occurrence of capsaicin receptors in the two nerves.

Neuropeptides in skin

Neuropeptides are a heterogeneous group of more than 50 molecules that play a role in various cutaneous functions and diseases; they act as neuromodulators, neurotransmitters, neurohormones, and hormones. In the skin, neuropeptides are synthesized locally (i.e., in keratinocytes and in endothelial cells) and are transported by nerve fibers or immune cells (i.e., lymphocytes, monocytes, and polymorphonuclear cells). Specific receptors and binding sites for neuropeptides have been described in different cell lines in the skin (keratinocytes, endothelial cells, immune cells, fibroblasts). Many different biologic actions of neuropeptides have been demonstrated. Depletion of cutaneous neuropeptides (i.e., with capsaicin cream) or therapeutic use of neuropeptide agonists and/or antagonists may aid in the treatment of skin diseases.

Contact dermatitis associated with capsaicin: Hunan hand syndrome

"Hunan hand" is a contact dermatitis resulting from the direct handling of chili peppers containing capsaicin. Capsaicin also is found in an over-the-counter topical agent for treatment of postherpetic neuralgia, diabetic neuropathy, and arthritis. We present the case of a patient with capsaicin-induced dermatitis and discuss the pathophysiology, therapy, and current uses of capsaicin.

Oral capsaicin provides temporary relief for oral mucositis pain secondary to chemotherapy/radiation therapy

Pain from oral mucositis afflicts from 40% to 70% of patients receiving chemotherapy or radiation therapy. Current methods of clinical pain management (for example, topical anesthetics, systemic analgesics) have limited success. In a pilot study, we examined the ability of oral capsaicin to provide temporary relief of oral mucositis pain. Capsaicin, the active ingredient in chili peppers, desensitizes some neurons and has provided moderate pain relief when applied to the skin surface. Oral capsaicin in a candy (taffy) vehicle produced substantial pain reduction in 11 patients with oral mucositis pain from cancer therapy. However, this pain relief was not complete for most patients and was only temporary. Additional research is needed to fully utilize the properties of capsaicin desensitization and thus optimize analgesia.

Topical capsaicin as an adjuvant analgesic

Topical capsaicin has been studied in a variety of conditions by uncontrolled and controlled trials. It is attractive because it is a simple, safe treatment. Although these studies suggest an analgesic effect, even placebo-controlled trials have been impossible to blind due to the burning sensation induced by the capsaicin. A high placebo response rate in the controlled trials is an interesting observation and may account for the apparent salutary effect reported in the studies lacking a control. A careful scrutiny of the results of these trials to date as well as clinical experience indicate at best a modest effect with the currently available preparations with many patients failing to find relief, finding the relief unsatisfactory, or being unable to tolerate the burning sensation. Occasional patients appear to have a very good result, and these unusual cases may not be reflected by clinical trials. Topical capsaicin is generally not satisfactory as a sole therapy for chronic painful conditions, although it may serve as an adjuvant to other approaches.

The effectiveness of topically applied capsaicin. A meta-analysis

To undertake a quantitative overview of trials of topical capsaicin for the treatment of diabetic neuropathy, osteoarthritis, post-herpetic neuralgia, and psoriasis. A systematic search of the literature using both computerized and manual methods for identifying clinical trials of capsaicin. The trials identified were abstracted for response data, which then were analysed using established meta-analytic methods for both fixed and random effects modelling. The odds ratio of the response rate of subjects receiving topical capsaicin relative to that of subjects on placebo was used as the main outcome measure. The difference in the response rate was used as the response variable under the random effects model. When dropouts were mentioned and unambiguous assignment could not be made, the analysis was made on the basis of intention to treat. Capsaicin cream give more pain relief to patients with diabetic neuropathy than placebo did. The odds ratio (OR) and corresponding 95% confidence interval (95% CI) in favour of capsaicin cream were OR = 2.74 (95% CI = 1.73, 4.32). Using a random effect model the rate difference (RD) in favour of capsaicin cream was RD = 0.25 (95% CI = 0.15, 0.35). Capsaicin cream was also better than placebo in providing pain relief in osteoarthritis: OR = 4.36 (95% CI = 2.77, 6.88) and RD = 0.29 (95% CI = 0.20, 0.37) and in psoriasis: OR = 2.80 (95% CI = 1.69, 4.62) and RD = 0.35 (95% CI = 0.14, 0.56). There was, however, evidence of heterogeneity in the individual RDs in psoriasis, and complete binding was difficulty because of the initial discomfort associated with topical capsaicin.(ABSTRACT TRUNCATED AT 250 WORDS)

A double-blind evaluation of topical capsaicin in pruritic psoriasis

BACKGROUND

Substance P, an undecapeptide neurotransmitter, has been implicated in the pathophysiology of psoriasis and pruritus.

OBJECTIVE

Safety and efficacy of topical capsaicin, a potent substance P depletor, were evaluated in patients with pruritic psoriasis.

METHODS

Patients applied capsaicin 0.025% cream (n = 98) or vehicle (n = 99) four times a day for 6 weeks in this double-blind study. Efficacy was based on a physician's global evaluation and a combined psoriasis severity score including scaling, thickness, erythema, and pruritus.

RESULTS

Capsaicin-treated patients demonstrated significantly greater improvement in global evaluation (p = 0.024 after 4 weeks and p = 0.030 after 6 weeks) and in pruritus relief (p = 0.002 and p = 0.060, respectively), as well as a significantly greater reduction in combined psoriasis severity scores (p = 0.030 and p = 0.036, respectively). The most frequently reported side effect in both treatment groups was a transient burning sensation at application sites.

CONCLUSION

Topically applied capsaicin effectively treats pruritic psoriasis, a finding that supports a role for substance P in this disorder.

Capsaicin: identification, nomenclature, and pharmacotherapy

OBJECTIVE

To provide a brief overview of the chemical history, analysis, nomenclature, biology, pharmacology, and pharmacotherapy of capsaicin.

DATA SOURCES

Chemical Abstracts, Biological Abstracts, and a MEDLINE search were used to identify pertinent literature; selected literature was used in this review.

DATA EXTRACTION

Original articles, reviews, and abstracts of articles were used to select material pertinent to the objectives of the review. The volume of material available prohibits comprehensive data extraction.

CONCLUSIONS

A history of the use of Capsicum spp. and the predominant active ingredient, capsaicin, the parent compound of a group of vanillyl fatty acid amides, is presented. Distinct structural differences are noted between this compound and the capsaicinoids, especially the synthetic analog nonivamide, which has appeared as an adulterant in capsaicin-labeled products. Analysis shows that although some of these synthetic analogs eventually may prove to be true natural products, conclusive evidence based on isolation and structure elucidation is still absent after decades of attempted isolation from several potential natural sources. Although the crude, dark oleoresin extract of capsicum contains over 100 distinct volatile compounds and therefore may function in many ways dissimilar to capsaicin, the oleoresin continues to be marketed in products with a high degree of variability in efficacy. Capsaicin as a pure white crystalline material, however, acts specifically by depleting stores of substance P from sensory neurons, and has been successful in the treatment of several painful conditions (e.g., rheumatoid arthritis, osteoarthritis, peripheral neuropathies.

The actions of capsaicin applied topically to the skin of the rat on C-fibre afferents, antidromic vasodilatation and substance P levels

1. Single applications of solutions of capsaicin were made to the intact skin of anaesthetized rats and the effects on cutaneous blood flow and the firing of C-nociceptor afferents determined. Blood flow was measured by laser-Doppler flowmetry. C-fibre activity was recorded from filaments dissected from the saphenous nerve. 2. Following the application of a capsaicin solution (concentration > or = 1 mM) to rat saphenous skin, low frequency firing occurred in C-polymodal nociceptors that sometimes continued for > 10 min. At the some time, large increases in skin blood flow occurred exceeding 300% in some instances. 3. After the initial excitation, some C-polymodal nociceptors lost their sensitivity to pressure whilst their sensitivity to heat was lost or enhanced depending on the vehicle used. 4. Sensitivity of C-polymodal nociceptors to heat recovered in < 1 day following a single application of 33 mM capsaicin. Thresholds to mechanical pressure, however, were still significantly elevated by 123% on day 1, but had recovered on day 2. 5. Vasodilatation in response to saphenous nerve stimulation ('antidromic vasodilatation') was significantly reduced by 35%, 2 days after a single application of 33 mM capsaicin, but was normal at 4 days. 6. Following a single application of 33 mM capsaicin, skin substance P levels fell to only half the normal value at day 1 and remained at this level throughout the 4 day period examined. 7. It is suggested that the ability of relatively low concentrations of capsaicin to desensitize C-fibre nociceptors may underlie the analgesic action of topical capsaicin in man.