Botulinum toxin A does not alter capsaicin-induced pain perception in human skin

Botulinum toxin A and neuropathic pain: An update

Botulinum toxin type A is a widely used neurotoxin for the treatment of muscle hyperactivity such as dystonia and spasticity. Several clinical trials have also reported an efficacy of subcutaneous or intradermal administrations of botulinum toxin A on various neuropathic pain conditions including idiopathic trigeminal neuralgia and found that specific sensory phenotypes were predictors of the response. This narrative review summarizes the potential mechanisms of action, efficacy and safety of botulinum toxin A in neuropathic pain as well as its place in the therapeutic algorithm of neuropathic pain.

Mechanisms of Botulinum Toxin Type A Action on Pain

Already a well-established treatment for different autonomic and movement disorders, the use of botulinum toxin type A (BoNT/A) in pain conditions is now continuously expanding. Currently, the only approved use of BoNT/A in relation to pain is the treatment of chronic migraines. However, controlled clinical studies show promising results in neuropathic and other chronic pain disorders. In comparison with other conventional and non-conventional analgesic drugs, the greatest advantages of BoNT/A use are its sustained effect after a single application and its safety. Its efficacy in certain therapy-resistant pain conditions is of special importance. Novel results in recent years has led to a better understanding of its actions, although further experimental and clinical research is warranted. Here, we summarize the effects contributing to these advantageous properties of BoNT/A in pain therapy, specific actions along the nociceptive pathway, consequences of its central activities, the molecular mechanisms of actions in neurons, and general pharmacokinetic parameters.

Botulinum Neurotoxins: Biology, Pharmacology, and Toxicology

The study of botulinum neurotoxins (BoNT) is rapidly progressing in many aspects. Novel BoNTs are being discovered owing to next generation sequencing, but their biologic and pharmacological properties remain largely unknown. The molecular structure of the large protein complexes that the toxin forms with accessory proteins, which are included in some BoNT type A1 and B1 pharmacological preparations, have been determined. By far the largest effort has been dedicated to the testing and validation of BoNTs as therapeutic agents in an ever increasing number of applications, including pain therapy. BoNT type A1 has been also exploited in a variety of cosmetic treatments, alone or in combination with other agents, and this specific market has reached the size of the one dedicated to the treatment of medical syndromes. The pharmacological properties and mode of action of BoNTs have shed light on general principles of neuronal transport and protein-protein interactions and are stimulating basic science studies. Moreover, the wide array of BoNTs discovered and to be discovered and the production of recombinant BoNTs endowed with specific properties suggest novel uses in therapeutics with increasing disease/symptom specifity. These recent developments are reviewed here to provide an updated picture of the biologic mechanism of action of BoNTs, of their increasing use in pharmacology and in cosmetics, and of their toxicology.

Differential effect of Incobotulinumtoxin A on pain, neurogenic flare and hyperalgesia in human surrogate models of neurogenic pain

BACKGROUND

The effectiveness of Botulinum-neurotoxin A (BoNT/A) to treat pain in human pain models is very divergent. This study was conducted to clarify if the pain models or the route of BoNT/A application might be responsible for these divergent findings.

METHODS

Sixteen healthy subjects (8 males, mean age 27 ± 5 years) were included in a first set of experiments consisting of three visits: (1) Visit: Quantitative sensory testing (QST) was performed before and after intradermal capsaicin injection (CAPS, 15 μg) on one thigh and electrical current stimulation (ES, 1 Hz) on the contralateral thigh. During stimulation pain and the neurogenic flare response (laser-Doppler imaging) were assessed. (2) Four weeks later, BoNT/A (Xeomin^® , 25 MU) was injected intracutaneously on both sides. (3) Seven days later, the area of BoNT/A application was determined by the iodine-starch staining and the procedure of the (1) visit was exactly repeated. In consequence of these results, 8 healthy subjects (4 males, mean age 26 ± 3 years) were included into a second set of experiments. The experimental setting was exactly the same with the exception that stimulation frequency of ES was increased to 4 Hz and BoNT/A was injected subcutaneously into the thigh, which was stimulated by capsaicin.

RESULTS

BoNT/A reduced the 1 Hz ES flare size (p < 0.001) and pain ratings (p < 0.01), but had no effect on 4 Hz ES and capsaicin-induced pain, hyperalgesia, or flare size, regardless of the depth of BoNT/A injection (i.c./s.c). Moreover, i.c. BoNT/A injection significantly increased warm detection and heat pain thresholds in naive skin (WDT, Δ 2.2 °C, p < 0.001; HPT Δ 1.8 °C, p < 0.005).

CONCLUSION

BoNT/A has a moderate inhibitory effect on peptidergic and thermal C-fibers in healthy human skin.

SIGNIFICANCE

The study demonstrates that BoNT/A (Incobotulinumtoxin A) has differential effects in human pain models: It reduces the neurogenic flare and had a moderate analgesic effects in low frequency but not high frequency current stimulation of cutaneous afferent fibers at C-fiber strength; BoNT/A had no effect in capsaicin-induced (CAPS) neurogenic flare or pain, or on hyperalgesia to mechanical or heat stimuli in both pain models. Intracutaneous BoNT/A increases warm and heat pain thresholds on naïve skin.

A literature review on the pharmacological sensitivity of human evoked hyperalgesia pain models

AIMS

Human evoked pain models can be used to determine the efficacy of new and existing analgesics and to aid in the identification of new targets. Aspects of neuropathic pain can be simulated by inducing hyperalgesia resulting from provoked sensitization. The present literature review aimed to provide insight into the sensitivity of different hyperalgesia and allodynia models of pharmacological treatment.

METHODS

A literature search was performed to identify randomized, double-blind, placebo-controlled studies that included human hyperalgesia pain models and investigated the pharmacodynamic effects of different classes of drugs.

RESULTS

Three hyperalgesia models [ultraviolet B (UVB) irradiation, capsaicin and thermode burn] have been used extensively. Assessment of hyperalgesia/allodynia and pharmacological effect are measured using challenge tests, which generally comprise thermal (heat/cold) or mechanical stimulation (pin-prick, stroking or impact). The UVB model was sensitive to the antihyperalgesic effects of nonsteroidal anti-inflammatory drugs (NSAIDs) and opioids. The capsaicin model was partially sensitive to opioids. The burn model did not detect any antihyperalgesic effects when NSAIDs or local anaesthetics were administered but responded to the effects of N-methyl D-aspartate (NMDA) receptor antagonists by moderately reducing mechanical hyperalgesia.

CONCLUSIONS

Based on pharmacological sensitivity, the UVB model adequately reflects inflammatory pain and was sensitive to NSAIDs and opioids. Findings from the capsaicin and burn models raised questions about the translatability of these models to the treatment of neuropathic pain. There is a need for a reproducible and predictive model of neuropathic pain, either in healthy subjects or in patients.

Current status and future directions of botulinum neurotoxins for targeting pain processing

Current evidence suggests that botulinum neurotoxins (BoNTs) A1 and B1, given locally into peripheral tissues such as skin, muscles, and joints, alter nociceptive processing otherwise initiated by inflammation or nerve injury in animal models and humans. Recent data indicate that such locally delivered BoNTs exert not only local action on sensory afferent terminals but undergo transport to central afferent cell bodies (dorsal root ganglia) and spinal dorsal horn terminals, where they cleave SNAREs and block transmitter release. Increasing evidence supports the possibility of a trans-synaptic movement to alter postsynaptic function in neuronal and possibly non-neuronal (glial) cells. The vast majority of these studies have been conducted on BoNT/A1 and BoNT/B1, the only two pharmaceutically developed variants. However, now over 40 different subtypes of botulinum neurotoxins (BoNTs) have been identified. By combining our existing and rapidly growing understanding of BoNT/A1 and /B1 in altering nociceptive processing with explorations of the specific characteristics of the various toxins from this family, we may be able to discover or design novel, effective, and long-lasting pain therapeutics. This review will focus on our current understanding of the molecular mechanisms whereby BoNTs alter pain processing, and future directions in the development of these agents as pain therapeutics.

Botulinum Toxin Type a as a Therapeutic Agent against Headache and Related Disorders

Botulinum neurotoxin A (BoNT/A) is a toxin produced by the naturally-occurring Clostridium botulinum that causes botulism. The potential of BoNT/A as a useful medical intervention was discovered by scientists developing a vaccine to protect against botulism. They found that, when injected into a muscle, BoNT/A causes a flaccid paralysis. Following this discovery, BoNT/A has been used for many years in the treatment of conditions of pathological muscle hyperactivity, like dystonias and spasticities. In parallel, the toxin has become a “glamour” drug due to its power to ward off facial wrinkles, particularly frontal, due to the activity of the mimic muscles. After the discovery that the drug also appeared to have a preventive effect on headache, scientists spent many efforts to study the potentially-therapeutic action of BoNT/A against pain. BoNT/A is effective at reducing pain in a number of disease states, including cervical dystonia, neuropathic pain, lower back pain, spasticity, myofascial pain and bladder pain. In 2010, regulatory approval for the treatment of chronic migraine with BoNT/A was given, notwithstanding the fact that the mechanism of action is still not completely elucidated. In the present review, we summarize experimental evidence that may help to clarify the mechanisms of action of BoNT/A in relation to the alleviation of headache pain, with particular emphasis on preclinical studies, both in animals and humans. Moreover, we summarize the latest clinical trials that show evidence on headache conditions that may obtain benefits from therapy with BoNT/A.

The Role of Botulinum Toxin Type A in the Clinical Management of Refractory Anterior Knee Pain

Anterior knee pain is a highly prevalent condition affecting largely young to middle aged adults. Symptoms can recur in more than two thirds of cases, often resulting in activity limitation and reduced participation in employment and recreational pursuits. Persistent anterior knee pain is difficult to treat and many individuals eventually consider a surgical intervention. Evidence for long term benefit of most conservative treatments or surgical approaches is currently lacking. Injection of Botulinum toxin type A to the distal region of vastus lateralis muscle causes a short term functional “denervation” which moderates the influence of vastus lateralis muscle on the knee extensor mechanism and increases the relative contribution of the vastus medialis muscle. Initial data suggest that, compared with other interventions for anterior knee pain, Botulinum toxin type A injection, in combination with an active exercise programme, can lead to sustained relief of symptoms, reduced health care utilisation and increased activity participation. The procedure is less invasive than surgical intervention, relatively easy to perform, and is time- and cost-effective. Further studies, including larger randomized placebo-controlled trials, are required to confirm the effectiveness of Botulinum toxin type A injection for anterior knee pain and to elaborate the possible mechanisms underpinning pain and symptom relief.

The effects of intraplantar and intrathecal botulinum toxin type B on tactile allodynia in mono and polyneuropathy in the mouse

BACKGROUND

Mononeuropathies (MNs: nerve ligation) and polyneuropathies (PNs: cisplatin) produce unilateral and bilateral tactile allodynia, respectively. We examined the effects of intraplantar (IPLT) and intrathecal (IT) botulinum toxin B (BoNT-B) on this allodynia.

METHODS

Mice (male c57Bl/6) were prepared with an L5 nerve ligation. Others received cisplatin (IP 2.3 mg/kg/d, every other day for 6 injections). Saline and BoNT-B were administered through the IPLT or IT route. We examined mechanical allodynia (von Frey hairs) before and at intervals after BoNT. As a control, we injected IPLT BoNT-B treated with dithiothreitol to cleave heavy chain from light chain. We measured motor function using acute thermal escape and sensorimotor tests.

RESULTS

MN and PN mice showed a persistent ipsilateral and bilateral allodynia, respectively. IPLT BoNT-B resulted in an ipsilateral dorsal horn reduction in the synaptic protein target of BoNT-B (vesicle-associated membrane protein) and a long-lasting (up to approximately 17 days) reversal of allodynia in PN and MN models. The predominant effect after IPLT delivery was ipsilateral to IPLT BoNT. The effects of IPLT BoNT-B in MN mice were blocked by prior reduction of BoNT-B with dithiothreitol. IT BoNT-B in mice with PN resulted in a bilateral reversal of allodynia. With these dosing parameters, hind paw placing and stepping reflexes were unaltered, and there were no changes in thermal escape latencies. After cisplatin, dorsal root ganglions displayed increases in activation transcription factor 3, which were reduced by IT, but not IPLT BoNT-B.

CONCLUSIONS

BoNT-B given IPLT and IT yields a long-lasting attenuation of the allodynia in mice displaying MN and PN allodynia.

Blockade of glutamate release by botulinum neurotoxin type A in humans: a dermal microdialysis study

BACKGROUND

The analgesic action of botulinum neurotoxin type A (BoNTA) has been linked to the blockade of peripheral release of neuropeptides and neurotransmitters in animal models; however, there is no direct evidence of this in humans.

OBJECTIVES

To investigate the effect of BoNTA on glutamate release in humans, using an experimental model of pain and sensitization provoked by capsaicin plus mild heat.

METHODS

Twelve healthy volunteers (six men, six women) were pretreated with BoNTA (10 U) on the volar forearm and with a saline control on the contralateral side. Dermal microdialysis was applied one week later to collect interstitial samples before and after the application of a capsaicin patch (8%) plus mild heat (40°C⁄60 min) to provoke glutamate release, pain and vasodilation. Samples were collected every hour for 3 h using linear microdialysis probes (10 mm, 100 kD). Dialysate was analyzed for glutamate concentration. Pain intensity and skin vasomotor reactions (temperature and blood flow changes) were also recorded.

RESULTS

BoNTA significantly reduced glutamate release compared with saline (P<0.05). The provoked pain intensity was lower in the BoNTA-pretreated arm (P<0.01). The reduction in pain scores was not correlated with glutamate level. Cutaneous blood flow (P<0.05), but not cutaneous temperature (P≥0.05), was significantly reduced by BoNTA. There was a correlation between glutamate level and skin blood flow (r=0.58⁄P<0.05) but not skin temperature (P≥0.05). No differences according to sex were observed in any response.

CONCLUSIONS

The present study provided the first direct evidence supporting the inhibitory effect of BoNTA on glutamate release in human skin, which is potentially responsible for some of the analgesic action of BoNTA.

Botulinum toxin A, brain and pain

Botulinum neurotoxin type A (BoNT/A) is one of the most potent toxins known and a potential biological threat. At the same time, it is among the most widely used therapeutic proteins used yearly by millions of people, especially for cosmetic purposes. Currently, its clinical use in certain types of pain is increasing, and its long-term duration of effects represents a special clinical value. Efficacy of BoNT/A in different types of pain has been found in numerous clinical trials and case reports, as well as in animal pain models. However, sites and mechanisms of BoNT/A actions involved in nociception are a matter of controversy. In analogy with well known neuroparalytic effects in peripheral cholinergic synapses, presently dominant opinion is that BoNT/A exerts pain reduction by inhibiting peripheral neurotransmitter/inflammatory mediator release from sensory nerves. On the other hand, growing number of behavioral and immunohistochemical studies demonstrated the requirement of axonal transport for BoNT/A's antinociceptive action. In addition, toxin's enzymatic activity in central sensory regions was clearly identified after its peripheral application. Apart from general pharmacology, this review summarizes the clinical and experimental evidence for BoNT/A antinociceptive activity and compares the data in favor of peripheral vs. central site and mechanism of action. Based on literature review and published results from our laboratory we propose that the hypothesis of peripheral site of BoNT/A action is not sufficient to explain the experimental data collected up to now.

Time course analysis of the effects of botulinum neurotoxin type A on pain and vasomotor responses evoked by glutamate injection into human temporalis muscles

The effect of botulinum neurotoxin type A (BoNTA) on glutamate-evoked temporalis muscle pain and vasomotor responses was investigated in healthy men and women over a 60 day time course. Subjects participated in a pre-BoNTA session where their responses to injection of glutamate (1 M, 0.2 mL) and saline (0.2 mL) into the temporalis muscles were assessed. On Day 1, BoNTA (5 U) was injected into one temporalis muscle and saline into the contralateral temporalis muscle, in a randomized order. Subjects then received intramuscular injections of glutamate (1 M, 0.2 mL) into the left and right temporalis muscles at 3 h and subsequently 7, 30 and 60 days post-injection of BoNTA. Pain intensity, pain area, and neurogenic inflammation (skin temperature and skin blood perfusion) were recorded. Prior to BoNTA treatment, glutamate evoked significantly greater pain and vasomotor reactions (P < 0.001) than saline. BoNTA significantly reduced glutamate-evoked pain intensity (P < 0.05), pain area (P < 0.01), skin blood perfusion (P < 0.05), and skin temperature (P < 0.001). The inhibitory effect of BoNTA was present at 3 h after injection, peaked after 7 days and returned to baseline by 60 days. Findings from the present study demonstrated a rapid action of BoNTA on glutamate-evoked pain and neurogenic inflammation, which is in line with animal studies.

Botulinum toxin B in the sensory afferent: transmitter release, spinal activation, and pain behavior

We addressed the hypothesis that intraplantar botulinum toxin B (rimabotulinumtoxin B: BoNT-B) has an early local effect upon peripheral afferent terminal releasing function and, over time, will be transported to the central terminals of the primary afferent. Once in the terminals it will cleave synaptic protein, block spinal afferent transmitter release, and thereby prevent spinal nociceptive excitation and behavior. In mice, C57Bl/6 males, intraplantar BoNT-B (1 U) given unilaterally into the hind paw had no effect upon survival or motor function, but ipsilaterally decreased: (1) intraplantar formalin-evoked flinching; (2) intraplantar capsaicin-evoked plasma extravasation in the hind paw measured by Evans blue in the paw; (3) intraplantar formalin-evoked dorsal horn substance P (SP) release (neurokinin 1 [NK1] receptor internalization); (4) intraplantar formalin-evoked dorsal horn neuronal activation (c-fos); (5) ipsilateral dorsal root ganglion (DRG) vesicle-associated membrane protein (VAMP); (6) ipsilateral SP release otherwise evoked bilaterally by intrathecal capsaicin; (7) ipsilateral activation of c-fos otherwise evoked bilaterally by intrathecal SP. These results indicate that BoNT-B, after unilateral intraplantar delivery, is taken up by the peripheral terminal, is locally active (blocking plasma extravasation), is transported to the ipsilateral DRG to cleave VAMP, and is acting presynaptically to block release from the spinal peptidergic terminal. The observations following intrathecal SP offer evidence for a possible transsynaptic effect of intraplantar BoNT. These results provide robust evidence that peripheral BoNT-B can alter peripheral and central terminal release from a nociceptor and attenuate downstream nociceptive processing via a presynaptic effect, with further evidence suggesting a possible postsynaptic effect.

A phase II, open-label, non-comparative study of Botulinum toxin in Restless Legs Syndrome

OBJECTIVE

To assess the efficacy of intradermally injected botulinum neurotoxin type A (BoNT/A) in patients with Restless Legs Syndrome (RLS).

METHODS

We conducted an optimal two-stage, phase II exploratory, open label, non-comparative clinical trial. The primary outcome measure was the efficacy of BoNT/A defined by the proportion of patients (responders) with ⩾50% improvement of their RLS severity score at week 2 following injections compared to baseline score at inclusion. Twenty-seven patients were to be included in the first stage of the trial, which was to be stopped if less than nine responders were documented. Selected patients had a minimum score of 21 on the International RLS Rating Scale. They all received a series of 20 intradermal injections of 0.05 ml of BoNT/A in symptomatic areas in their lower limbs. Change of RLS severity was evaluated over a 6 months period.

RESULTS

Of the 27 selected patients, only six achieved the primary endpoint at week 2. In these six patients, the median duration (Inter-Quartile Range) of the IRLSRS score improvement of at least 50% was 46 days (42-126).

CONCLUSIONS

Considering the proportion of responders as the primary endpoint of this trial, BoNT/A showed no efficacy in alleviating RLS sensory symptoms.

[Botulinum toxin and painful peripheral neuropathies: what should be expected?]

Botulinum toxin type A (BTX-A) is a potent neurotoxin that blocks acetylcholine release from presynaptic nerve terminals by cleaving the SNARE complex. BTX-A has been reported to have analgesic effects independent of its action on muscle tone. The most robust results have been observed in patients with neuropathic pain. Neuropathic pain due to peripheral lesions has been the most widely studied. BTX-A has shown its efficacy on pain and allodynia in various animal models of inflammatory neuropathic pain. The only randomized, double-blind, placebo-controlled trial in patients with focal painful neuropathies due to nerve trauma or postherpetic neuralgia demonstrated significant effects on average pain intensity from 2 weeks after the injections to 14 weeks. Most patients reported pain during the injections, but there were no further local or systemic side effects. The efficacy of BTX-A in painful peripheral neuropathies has been more recently studied. Results were positive in the only study in an animal model of peripheral neuropathy. One study in patients with diabetic painful peripheral neuropathy demonstrated a significant decrease in Visual Analog Scale. In conclusion, one session of multiple intradermal injection of BTX-A produces long-lasting analgesic effects in patients with focal painful neuropathies and diabetic neuropathic pain, and is particularly well tolerated. The findings are consistent with a reduction of peripheral sensitisation, the place of a possible central effect remaining to define. Further studies are needed to assess some important issues, i.e. BTX-A efficacy in patients with small fiber neuropathies and the relevance of early and repeated injections. Future studies could also provide valuable insights into pathophysiology of neuropathic pain.

The role of botulinum toxin in management of pain: an evidence-based review

PURPOSE OF REVIEW

In the present review we discuss the role of botulinum neurotoxins (BoNTs) in the management of different pain conditions, with evidence-based data on the toxins' efficacy on pain and its mechanisms.

RECENT FINDINGS

Experimental in-vitro studies have reported promising results of a novel recombinant chimera of BoNT A and E that inhibits the calcitonin gene-related peptide exocytosis from brainstem sensory neurons. Animal studies in neuropathic pain rat models have reported an analgesic effect of BoNT A given after the neuropathic procedure and a bilateral antinociceptive effect to the unilateral noxious stimuli. There is a growing body of evidence that BoNTs are effective in myofascial pain syndrome, neuropathic pain, and joint pain. The pre-existing evidence that BoNTs are ineffective in migraine or other headache disorders has not yet been challenged. In other pain syndromes, studies published in the last review year have not contributed significantly in either demonstrating or invalidating the research that has so far proved inconclusive.

SUMMARY

The role of BoNTs in management of pain is not yet well established. Larger studies in neuropathic pain, joint pain, and myofascial pain syndrome are needed to fully ascertain the role for BoNT therapy in those areas.

Botulinum toxin A for treatment of allodynia of complex regional pain syndrome: a pilot study

OBJECTIVE

To investigate the efficacy and tolerability of Botulinum toxin A (BoNT-A) in allodynia of patients with complex regional pain syndrome.

DESIGN

A total of 14 patients were studied. Eight patients were participants of a randomized, prospective, double-blind, placebo-controlled protocol. Six patients were studied prospectively in an open-label protocol. Patients were rated at baseline and at 3 weeks and 2 months after BoNT-A administration. Ratings included brief pain inventory, McGill pain questionnaire, clinical pain impact questionnaire, quantitative skin sensory test, sleep satisfaction scale, and patient global satisfaction scale. BoNT-A was injected intradermally and subcutaneously, five units/site into the allodynic area (total dose 40-200 units).

RESULTS

None of the patients with allodynia showed a significant response after treatment. The treatment was painful and poorly tolerated.

CONCLUSION

Intrademal and subcutaneous administration of BoNT-A into the allodynic skin of the patients with complex regional pain syndrome (CRPS) failed to improve pain and was poorly tolerated.

Is botulinum toxin useful in treating headache? No

The discrepancy between the widespread use of botulinum neurotoxin (BoNT) in managing headache and the supporting clinical evidence is unprecedented. No substance seems to have inspired more physicians and patients to undertake spirited treatment attempts. Tremendous treatment success in small, uncontrolled clinical trials has been repeatedly reported, but no substance that has been studied to an equal extent has so utterly failed to provide proof of effect in controlled clinical trials. Nevertheless, even though most randomized, controlled clinical trials have not met their defined primary outcome criterion, BoNT is still considered a promising treatment alternative for primary headache disorders. Experimental approaches to the pathophysiologic impact of BoNT on the perception of pain have been equally unsuccessful. Although most studies have been unable to find a direct antinociceptive effect in humans, some researchers continue to seek specific injection sites or injection techniques that may promise more successful results. Others look for a positive effect by narrowing the indications for BoNT to more homogenous symptoms or special patient subgroups. The results of randomized, controlled studies involving a total of 3552 patients indicate that BoNT injection is probably ineffective for patients with migraine and chronic tension-type headache regardless of injection site, dosage, or injection regimen, and there is insufficient evidence to draw a conclusion about its effectiveness for the treatment of chronic daily headache or subforms. The lack of direct experimental or clinical trial evidence that BoNT has a direct antinociceptive effect in humans must be addressed before more trials are conducted, involving even more patients. Additional pathophysiologically oriented research is also needed to unravel the mechanisms of action of BoNT in human pain perception or, alternatively, to bring it all down to the placebo effect.

Use of botulinum toxin A in adult neurological disorders: efficacy, tolerability and safety

The protein botulinum neurotoxin A (BoNT/A) is one of seven distinct neurotoxins produced by Clostridium botulinum. BoNT/A blocks cholinergic synapses with an extremely high specificity and potency. Appropriately purified and diluted, BoNT/A serves as a reliable and well tolerated drug that is applied by local injection.The efficacy of BoNT/A is evident in the symptomatic therapy of disorders in which muscular hyperactivity plays a prominent role, such as focal dystonias and hemifacial spasm; in these disorders, BoNT/A is considered first-line therapy. BoNT/A is also beneficial in the treatment of both adults and children with spasticity of various causes. The pain that frequently accompanies these conditions is effectively reduced by BoNT/A. A genuine analgesic effect for BoNT/A unrelated to skeletal muscle spasmolysis has been suggested on the basis of in vitro and in vivo (animal) data. However, studies in humans designed to detect such an effect were negative, as were controlled studies of BoNT/A in patients with primary headache disorders.BoNT/A also acts on cholinergic synapses of the autonomic nervous system, and injection of BoNT/A into salivary glands significantly decreases the production of saliva. This may be beneficial for patients with Parkinson's disease, in whom the excessive production of saliva may be problematic.Overall, BoNT/A has been confirmed as an efficacious, predictable and well tolerated drug in an ever-increasing number of neurological disorders.

Subcutaneous Botulinum toxin type A reduces capsaicin-induced trigeminal pain and vasomotor reactions in human skin

The present human study aimed at investigating the effect of subcutaneous administration of Botulinum toxin type A (BoNT/A) on capsaicin-induced trigeminal pain, neurogenic inflammation and experimentally induced cutaneous pain modalities. Fourteen healthy males (26.3+/-2.6 years) were included in this double-blind and placebo-controlled trial. The subjects received subcutaneous BoNT/A (22.5U) and isotonic saline in the mirror sides of their forehead. Pain and neurogenic inflammation was induced by four intradermal injections of capsaicin (100mug/muL) (before, and days 1, 3 and 7 after treatments). The capsaicin-induced pain intensity, pain area, the area of secondary hyperalgesia, the area of visible flare and vasomotor reactions were recorded together with cutaneous heat, electrical and pressure pain thresholds. BoNT/A reduced the capsaicin-induced trigeminal pain intensity compared to saline (F=37.9, P<0.001). The perceived pain area was smaller for the BoNT/A-treated side compared to saline (F=7.8, P<0.05). BoNT/A reduced the capsaicin-induced secondary hyperalgesia (F=5.3, P<0.05) and flare area (F=10.3, P<0.01) compared to saline. BoNT/A reduced blood flow (F(1,26)=109.5, P<0.001) and skin temperature (F(1,26)=63.1, P<0.001) at the capsaicin injection sites compared to saline and its suppressive effect was maximal at days 3 and 7 (P<0.05, post hoc test). BoNT/A elevated cutaneous heat pain thresholds (F=17.1, P<0.001) compared to saline; however, no alteration was recorded for electrical or pressure pain thresholds (P>0.05). Findings from the present study suggest that BoNT/A appears to preferentially target Cfibers and probably TRPV1-receptors, block neurotransmitter release and subsequently reduce pain, neurogenic inflammation and cutaneous heat pain threshold.

Lack of anti-inflammatory effect of botulinum toxin type A in experimental models of inflammation

Botulinum toxin type A (BTX-A) has a long-lasting antinociceptive activity and less clear effect on inflammation. It was proposed that these two effects share the same mechanism--the inhibition of neurotransmitter exocytosis from peripheral nerve endings. However, till now possible anti-inflammatory action of BTX-A did not evoke much attention. In the present paper, we investigate possible anti-inflammatory action of the toxin in carrageenan and capsaicin models of inflammation in rats. BTX-A (5 and 10 U/kg) was injected into the plantar surface of the rat right hind-paw pad 5 days before the injection of the carrageenan (1%) or capsaicin (0.1%) at the same site. Carrageenan-induced paw oedema and capsaicin-induced protein extravasation were measured. Control, inflamed and BTX-A pretreated inflamed paws were photographed and histopathological analysis (haematoxylin & eosin) was performed. Pretreatment with BTX-A had no effect on the size of carrageenan-induced paw oedema, measured as paw volume and weight or capsaicin-induced plasma extravasations, measured by Evans blue as a marker of protein leakage. Neither macroscopic nor microscopic analysis showed a significant difference between BTX-A pretreated and control inflamed tissue. Results show dissociation between the effect of BTX-A on pain and inflammation thus questioning the validity of the suggested assumption about the common peripheral mechanism of action.

Evidence based medicine on the use of botulinum toxin for headache disorders

Botulinum toxin blocks the release of acetylcholine from motor nerve terminals and other cholinergic synapses. In animal studies botulinum toxin also reduces the release of neuropeptides involved in pain perception. The implications of these observations are not clear. Based on the personal experiences of headache therapists, botulinum toxin injections have been studied in patients with primary headaches, namely tension-type headache (TTH), chronic migraine (CM) and chronic daily headache (CDH). So far, the results of randomized, double-blind, placebo controlled trials on botulinum toxin in a total of 1117 patients with CDH, 1495 patients with CM, and 533 patients with TTH have been published. Botulinum toxin and placebo injections have been equally effective in these studies. In some of the studies, the magnitude of this effect was similar to that of established oral pharmacotherapy. This finding may help to explain the enthusiasm that followed the first open-label use of botulinum toxin in patients with headache. However, research is continuing to determine the efficacy of botulinum toxin in certain subgroups of patients with CM or CDH.