Curr Pain Headache Rep (2014) 18:386 DOI 10.1007/s11916-013-0386-z

ANESTHETIC TECHNIQUES IN PAIN MANAGEMENT (D WANG, SECTION EDITOR)

An Update on Botulinum Toxin A Injections of Trigger Points for Myofascial Pain Jon Y. Zhou & Dajie Wang

Published online: 13 December 2013 # Springer Science+Business Media New York 2013

Abstract Myofascial pain syndrome (MPS) is a common chronic pain condition that is characterized by distinct “trigger points.” Despite current treatments with physical therapy, analgesics, anti-depressants and trigger-point injections, myofascial pain remains a challenging chronic pain condition in clinical practice. Botulinum toxin A (BTX-A) can cause prolonged muscle relaxation through inhibition of acetylcholine release. It may offer some advantages over the current treatments for MPS by providing a longer sustained period of pain relief. Despite numerous clinical trials, the efficacy of BTX-A in alleviating MPS is not well-established due to mixed results from recent clinical trials. Active trigger points are associated with referred pain and greatly impact many aspects of activities of daily living, mood, and health status. This review is designed to analyze the clinical trials regarding the efficacy of BTX-A injection of active trigger points as a treatment for MPS. The literature referenced was obtained via a computer search with Google Scholar, Pubmed, Medline and EMbase. Our search terms included “Botulinum toxin,” “myofascial pain,” “trigger points,” “myofascial trigger points,” “chronic pain.” Additional references were retrieved from the reference list of the reports found via this search. Studies were considered eligible for inclusion if they were double-blinded, randomized, controlled trials evaluating the

This article is part of the Topical Collection on Anesthetic Techniques in Pain Management J. Y. Zhou (*) Department of Anesthesiology, Thomas Jefferson University Hospital, 111. S 11th Street, Suite 8290 Gibbon, Philadelphia, PA, USA e-mail: [email protected] D. Wang Department of Anesthesiology, Jefferson Medical College, 834 Chestnut Street, Suite T-150, Philadelphia, PA, USA e-mail: [email protected]

efficacy of BTX-A injections into trigger points for pain reduction, and if the trigger point selection in the trial included referred pain and/or local twitch response. Open-label studies, case reports, and other non-randomized studies were excluded. Eight trials were found according to the above criteria and are summarized in Table 1. There are well-designed clinical trials to support the efficacy of trigger-point injections with BTX-A for MPS. However, further clinical trials with considerations of minimizing placebo effect, repeated dosing, adequate coverage of trigger points, and using ultrasound confirmation and guidance are required to provide conclusive evidence for BTX-A in the treatment of myofascial pain. Keywords Myofascial pain syndrome . Botulinum toxin . Chronic pain . Trigger point

Introduction Myofascial pain syndrome is characterized by acute or chronic pain arising from “trigger points” located within palpable tight muscle bands of the affected regions. Although the exact prevalence is unknown in the general population, it is one of the common chronic pain conditions that physicians encounter [1]. Myofascial pain syndrome is most common between ages 27 to 50 years, with preference in sedentary patient populations [2]. The key to understanding myofascial pain syndrome is that the pain arises from trigger points. Trigger points are extremely sensitive, palpable taut bands in muscles, and can produce specific referred pain patterns upon palpation. Myofascial pain syndrome often presents itself as hypersensitivity and allodynia at the tenderness site [3]. Despite the advances in clinical research on myofascial pain syndrome, the exact pathophysiology of MPS is still unclear. The hypotheses of the pathogenesis include ischemic muscle spasm, muscle spindle overactivity, peripheral sensitization, and

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motor endplate overactivity [4]. Recently, in vivo biochemical analysis of trigger points by microdialysis revealed elevated levels of inflammatory mediators at these sites [5]. MPS is a complex medical condition. The successful management of this medical condition requires a multimodal approach. Pharmacological agents including NSAIDS, antidepressants, anti-epileptics, muscle relaxants, and vasodilators are commonly used for myofascial pain. Injections of local anesthetics and steroids, as well as dry needling have also been used to treat MPS with success. Unfortunately, these traditional treatments do not reliably provide long-term relief for patients with MPS. Recent studies demonstrated efficacy of the use of botulinum toxin to alleviate the spasticity [6, 7]. Botulinum toxin injection has been implemented by many physicians to provide patients with long-term pain relief. However, the efficacy of this treatment remains obscure. Botulinum toxin type A (BTX-A) is produced by the bacterium Clostridium Botulinum. It is the potent toxin that causes the lethal paralytic disease, botulism. The toxin affects presynaptic cholinergic nerve terminals and inhibits the release of the neurotransmitter acetylcholine (Ach), which in turn causes muscle relaxation. The toxin works by first binding to the Ach receptors, where it is internalized, and once internalized, BTX-A prevents the release of Ach from the cholinergic vesicles [8]. BTX-A has been used for a number of musculoskeletal conditions since the 1990s, including spasticity, cervical dystonia, and blepharospasm [9]. By increasing muscle relaxation, BTX-A has been shown to decrease the tension of localized muscles that may be encroaching on entrapped nerves causing MPS pain. BTX-A causes localized muscle-spindle relaxation, thereby decreasing firing to the CNS [10]. BTX-A may also cause a direct inhibitory effect on the CNS and the spinal cord [11]. Through a combination of peripheral and central nervous system effects, BTX-A is believed to alleviate the pain from MPS. However, the clinical efficacy of BTX-A injections is still controversial. BTX-A, by causing prolonged muscle relaxation through inhibition of acetylcholine release, may offer some advantages over the current treatment regimen for MPS by providing a longer-lasting pain relief [12]. Sustained muscle contraction may lead to blood vessel and tissue ischemia, causing the distinct trigger-point pain of MPS. It is believed that BTX-A may relieve MPS via the prolonged relaxation of distinct points of muscle tension [13]. One study showed that BTXA works more centrally than the site of injection, and hypothesized that BTX-A may reduce nociceptive transmission to the CNS [14]. In animal studies, BTX-A has been shown to decrease the level of glutamate [15]. In the peripheral nervous system, glutamate mediates the release of Substance P, a proinflammatory neurotransmitter. It has been hypothesized that by inhibiting the glutamate level, BTX-A inhibits peripheral and central sensitization of pain [16]. BTX-A also blocks the release of CGRP (a vasoactive intestinal enzyme), which

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may decrease the level of vasoconstriction-induced muscle ischemia in MPS [17]. The hallmark of MPS is the existence of trigger points. Cakit et al. described the difference between a trigger point and a tender point. One of the characteristics of a trigger point is referred pain, which is typically present in a predictable pattern upon palpation. In contrast, a tender point only causes pain in the area of palpation; it is not associated with referred pain [18, 19]. The other characteristic of a trigger point is a local twitch response that can be elicited by mechanical stimulation. It is hypothesized that there are sensitive loci in the trigger points. These loci are responsible for generating referred pain and local twitches [20]. Trigger points are classified as active and latent. An active trigger point is defined as a palpable, discrete, taut band of skeletal muscle. It causes pain spontaneously and palpation can produce referred pain. An active trigger point is associated with a greater negative impact of many aspects of life, including activity of daily living, mood, and health status [21]. Local anesthetic injection is a widely-used treatment for trigger points. It was found that the pain intensity was significantly reduced after lidocaine injection, if twitch responses were elicited during the procedure [22]. This suggests that outcome may be improved by treatment targeting the active trigger point. Despite numerous clinical trials attempted to confirm the effectiveness of BTX-A in alleviating MPS, its efficacy is not well-established due to mixed results from these studies. Various factors including study designs, dosage of BTX-A injections and selection of trigger points may contribute to these inconsistent clinical outcomes. This review is designed to analyze the current clinical trials regarding the efficacy of BTX-A injection at active trigger points as a treatment for MPS. Methods: The literature referenced was obtained via a computer search with Google Scholar, Pubmed, Medline and EMbase. The search terms used included “Botulinum,” “myofascial pain,” “trigger points,” “myofascial trigger points,” and “chronic pain.” Additional references were retrieved from the reference lists in the reports found via this search. Studies were considered eligible for inclusion if they were double-blinded, randomized, controlled trials evaluating the efficacy of BTX-A injections into trigger points for pain reduction, and if trigger point selection in the trial included referred pain and/or local twitch response. Open-label studies, case reports, and other non-randomized studies were excluded. Eight trials were found according to the above criteria, and are summarized in Table 1.

Discussion Myofascial pain remains a challenging chronic pain condition in clinical practice. The fundamental feature of myofascial

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Table 1 Summary of Clinical Trials Study

Design

Dosage

Identification of TP

Outcome

Cheshire et al. 1994 [26]

Single-center BTX-A vs. Saline six patients Single-center BTX-A vs. Saline 50 patients

50 units for 2–3 sites per patient

Palpation that reproduced typical pattern of radiating pain Palpation of a tender taut band with reproduction of patient’s pain locally and by regional referred pain

Statistically significant reduction in pain scores

Wheeler and Gretz 2001 [28]

231 units with standard deviation of 50 units per patient

Graboski et al. 2005 [29]

Single-center BTX-A vs. Bupivacaine 17 patients

25 units per TP. Up to eight TP injections per patient

Ferrante et al. 2005 P [27]

Single-center BTX-A vs. Saline 132 patients

10, 25, or 50 units per TP, up to five TPs per patient

Wheeler et al. 1998 [30]

Single-center Low dose BTX-A vs. high-dose vs. saline 33 patients

Göbel et al. 2006 [24••]

Multicenter BTX-A vs. Saline 145 Patients Single-center BTX-A vs. Saline 30 patients

BTX-A low-dose (50 units) vs. BTX-A high-dose (100 units) vs. saline. 1 TP per patient 400 units 40 unit per TP 10 TPs per patient 50 units per TP. 1 TP per patient

Qerama, et al. 2006 [31]

Freund and Schwartz 2000 [25]

Single-center BTX-A vs. saline 30 patients

100 units per patient. 20 units per TP

pain is the trigger point. These trigger points are taut bands in the muscle that are responsible for generating persistent pain [23]. It is conceivable that relaxation of the taut band via BTXA’s inhibition of acetylcholine release from the motor nerve terminal at the neuromuscular junction can potentially lead to the decreased action of trigger points and decreased pain. Göbel et al. conducted a randomized, double-blinded, placebo-controlled multicenter trial in 2006 that showed statistically significant improvement in pain relief with BTX-A injections. This was a well-designed study and the largest trial yet, with 145 patients enrolled [24••]. There is strong evidence from this trial and others by Freund [25] and Cheshire [26] to support this hypothesis. However, not all studies support the hypothesis that BTX-A improves MPS. Ferrante et al. completed a randomized, controlled, double-blinded study in 2005, which concluded that the injection of BTX-A directly into trigger points did not improve myofascial pain [27]. This is also a well-designed trial that undertook steps to control

TP was identified by palpation. Bupivacaine injection of TP with 50 % pain reduction for at least 8 h, but no more than 1 month Reproduction of the patient’s pain complaint upon palpation Presence of “jump sign” and referred pain.

Identification was based on the operational definition of Simons [35] A painful spot in the taut band, local twitch response evoked by snapping palpation of the painful spot Five most tender TPs injected. At least one TP with referred pain.

Both treatment and control groups showed a significant decline in pain and disability. No statistical difference between BTX-A and placebo “Both treatments were effective in reducing pain when compared to baseline. No significant difference between the BTX-A and 0.5 % bupivacaine group.” “No significant differences occurred between placebo and BTX-A groups with respect to visual analog pain scores.” “No statistical significant benefit of BTX-A over placebo.”

“Significantly improved pain levels 4– 6 weeks after treatment.” “No effect on either pain (spontaneous or referred) or pain thresholds compared with isotonic saline.” “The treatment group showed a significant improvement in pain and range of motion from pre-injection levels. The placebo group demonstrated no statistically significant changes at any posttreatment time.”

various confounding factors that could potentially affect the outcomes. In this trial, patients were weaned from all pain medications (NSAIDS, antidepressants, muscle relaxants, and opioids) two weeks before injection. A standardized medication regimen including amitriptyline, ibuprofen and propoxyphene–acetaminophen was started for all patients. Despite these efforts, no significant difference was found between placebo and BTX-A groups with respect to pain [27]. Wheeler 2001, et al. published a well-designed study (33 patients) examining BTX-A vs. placebo but had two arms in the treatment group, a high-dose (100-unit BTX-A) and a lowdose (50-unit BTX-A) group. Patients were re-evaluated over a 4-month period for their pain and disability. Patients were offered a second injection of 100 units BTX-A if they did not feel adequate pain relief from the first injection. All three arms had decreased VAS pain scores, but the differences between the groups were not statistically significant. However, patients that received a second injection had a high incidence of pain

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relief. This suggests that repeated injections might be required to show a significant benefit of BTX-A over placebo [28]. There are other trials that showed no statistically significant difference between the BTX-A group and placebo group. But in some of these trials [28, 29], both the treatment group and placebo group had improvement, which suggests that there is a significant placebo effect in these studies. Given the significant placebo effect, it is difficult to interpret these results and to conclude that BTX-A is not clinically effective. Based on the results of these trials, further studies with an effort to minimize the placebo effect is warranted to clarify the role of BTX-A for the treatment of myofascial pain syndrome. Dosage and the number of trigger points injected for each patient varies in clinical practice depending on the physician’s experience and expertise. It is intuitive that these factors may affect the clinical outcomes. In Ferrante’s study, subjects were given either BTX-A (10, 25, or 50 units per trigger point) for doses that ranged up to 250 units, or saline in up to five trigger points [27]. Wheeler et al. [30] identified the most tender trigger point, and injected it with 50 units of BTX-A, 100 units of BTX-A, or normal saline, but there was no significant difference among the three groups. In the two trials that showed significant benefit of injections, one used 40 units per trigger point [26] and the other [24••] used 20 units. From the available data, it appears that the amount of BTX-A injected into each trigger point did not have an impact on the outcome. As to the effect of total number of trigger points injected per patient, the largest trial by Göbel with ten trigger points injected per patient demonstrated significant benefit for pain relief. The studies [27, 30, 31] that showed no significant improvement over placebo involved injections in no more than five trigger points per patient. In Graboski’s trial [29], up to eight trigger points were injected per patient. Although no significant difference was demonstrated between the BTXA and 0.5 % bupivacaine group, both BTX-A and bupivacaine treatments were effective in reducing pain when compared to baseline. Another randomized, double-blinded, placebocontrolled trial by Benecke et al. [32•] used a set of standardized trigger-point sites that included ten predetermined, fixed trigger-point injections in the head, neck, and shoulder for myofascial pain. This study found significant improvement at eight weeks after treatment with BTX-A in patients with upper-back myofascial pain syndrome. A more recent study by Qerama et al. with 30 patients compared motor endplate activity with EMG and pain (spontaneous and referred) thresholds between two groups that received only one injection of either 50 units BTX-A or 0.25 ml isotonic saline [31]. The BTX-A group had decreased motor endplate activity on EMG readings, but had no effect on either referred pain or spontaneous pain compared to placebo group. These trials suggest that the total number of trigger points injected may play a role in determining the efficacy of BTX-A injections.

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Inadequate coverage of trigger points is a potential reason for poor clinical outcomes in some of the trials. Although all the trials in this review adopt a diagnostic criterion to identify active trigger points, the results produced from this review are still mixed. Another factor that may potentially influence the outcome is that various diagnostic criteria exist for myofascial pain, and the diagnosis of myofascial pain is based mainly on history and physical examination. The essential part of the physical examination is to locate the trigger points by palpation of the taut band in the musculature. Proper identification of trigger points requires precision of the pressure applied during examination and the subjective experience of the examiners. Prior studies have confirmed poor consistency among the examiners in identifying a trigger point [23, 33]. This inconsistency can adversely impact the study results. In order to improve clinical outcome, complementary testing should be considered to increase the accuracy of the diagnosis. Based on recent investigations, ultrasound has become a potential tool to confirm the diagnosis of trigger points. Recent ultrasound studies described trigger points as focal, hypoechoic regions of elliptical shape with a size of approximately 0.16 cm. Sikdar et al. conducted a study that differentiated active trigger points from latent trigger points with Doppler spectral waveform [34•]. Furthermore, direct visualization of trigger points, surrounding soft tissues, and vessels can improve the accuracy of injections and reduce the risk of intravascular/intraneural injections.

Conclusion BTX-A is being used increasingly in the treatment of myofascial pain syndrome, and many experienced and skilled clinicians use it effectively in their daily practice. Despite our effort to review well-designed trials that treated active trigger points to ascertain the role of BTX-A for MPS pain, the results are mixed. Just as several studies showed statistically significant pain relief from BTX-A injection, others showed no pain relief compared to placebo saline injections. Further clinical trials with considerations of minimizing placebo effect, repeated dosing, adequate coverage of the trigger points, and using ultrasound confirmation and guidance may provide more conclusive evidence for BTX-A in the treatment of myofascial pain. Compliance with Ethics Guidelines Conflict of Interest Dr. Jon Y. Zhou and Dr. Dajie Wang reported no potential conflicts of interest relevant to this manuscript. Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.

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An update on botulinum toxin A injections of trigger points for myofascial pain.

Myofascial pain syndrome (MPS) is a common chronic pain condition that is characterized by distinct "trigger points." Despite current treatments with ...
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