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Botulinum neurotoxins: fundamentals for the facial plastic surgeon
American Journal of Otolaryngology–Head and Neck Medicine and Surgery 28 (2007) 260 – 266 www.elsevier.com/locate/amjoto
Botulinum neurotoxins: fundamentals for the facial plastic surgeon Robert Todd Adelson, MD4 Division of Facial Plastic and Reconstructive Surgery, Department of Otolaryngology, University of Florida, PO Box 100264, Gainesville, FL, USA Received 29 July 2006
Abstract
The most commonly performed nonsurgical cosmetic procedure in the facial plastic surgery armamentarium involves the various commercial preparations of botulinum neurotoxins. These drugs have undergone a transformation from public health scourge to near ubiquitous therapeutic modality across the entire medical spectrum. Herein, the history of botulinum neurotoxins is reviewed, including an exploration of their pharmacology, neuromuscular junction physiology, a description of the commercially available preparations, and the recent research concerning the practicalities of their clinical use. D 2007 Elsevier Inc. All rights reserved.
1. Introduction The role of botulinum neurotoxins (BTX) has undergone one of the most remarkable transformations in the history of medicine. The journey from public health threat to popular therapeutic modality provides a framework by which one can understand how every field from anesthesiology to urology has successfully used this bmost poisonous poison,Q with its greatest notoriety achieved in the hands of facial plastic surgeons [1-3]. In 2005, the last year for which complete statistics are available, both the American Academy of Facial Plastic and Reconstructive Surgery and the American Society of Plastic Surgeons reported Botox injections to be the most common cosmetic procedure (surgical and nonsurgical) performed by their members [4,5]. The reported number of Botox injections performed by members of the American Society of Plastic Surgeons (3.8 million Botox treatments) and American Academy of Facial Plastic and Reconstructive Surgery (315.6 procedures per surgeon), respectively, emphasizes both the popularity of this modality and the importance of providing excellent care as both the patient base and number of providers increase [4,5]. A complete
4 Division of Facial Plastic and Reconstructive Surgery, Department of Otolaryngology, University of Florida, PO Box 100264, Gainesville, FL 32610, USA. Tel.: +1 352 392 4461. E-mail address: [email protected]. 0196-0709/$ – see front matter D 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.amjoto.2006.09.002
understanding of botulinum toxin therapy will allow physicians to maximize results based on the existing knowledge and perhaps further expand the cosmetic and functional indications for this remarkable neurotoxin. 2. History of botulism Pathologic states attributable to the bacterial toxin of interest include infant (intestinal) botulism, wound botulism, inhalational botulism, iatrogenic botulism, and foodborne (classic) botulism [6]. Of these, foodborne botulism prompted scientific investigations that began in the public health sector and, at present, have resulted in therapeutic indications for more than 100 different conditions across the medical spectrum [1,7-10]. Classic botulism was the call to prominence for Justinus Kerner (1786-1862), a medical officer reporting the symptoms resulting from the bconsumption of smoked blood-sausagesQ in his 1820 monograph, describing the time course of the disease, preservation of consciousness, mydriasis, absence of fever, suppression of secretions, and paralysis of the gut and all skeletal muscles culminating in respiratory and cardiac failure [11,12]. Kerner undertook laboratory and animal experimentation to extract the bsausage toxin,Q and although he never succeeded in isolating the toxin, his 1822 monograph proposed that if such an agent could be isolated, it could be used as a remedy for conditions of an overactive nervous system, citing movement disorders like bSt Vitus danceQ (chorea minor)
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and the hypersecretion of sweat, tears, or mucous [12]. By 1870, Muller differentiated between the distinct symptom complex described by Kerner and other assorted cases of food poisoning, applying the name bbotulismQ to this unique disease based on the earlier work of Kerner and the Latin word for sausage: botulus [12,13]. Ironically, it was the consumption of an uncooked ham at a musician’s funeral in December 1895 that provided the biologic material for Van Ermengem to isolate the culprit bacteria of bsausage poisoningQ: Bacillus botulinus [13]. Van Ermengem’s 1897 publication established the entire foundation of research into botulism by isolating and naming the bacteria, recognizing its anaerobic nature, describing its toxin as a heat-labile substance, and subsequently describing measures of food preservation and preparation that prevent foodborne botulism [11-14]. Although Sommer had produced a crude form of botulinum toxin in 1926, widespread interest in BTX was rekindled during World War II, when the threat of biologic weapons spurred US scientists to purify BTX for military purposes [15-17]. In the wake of WWII, highly purified BTX was made available to scientists who subsequently deciphered its pharmacology and began to explore the use of BTX to treat disease states resulting from hyperactivity of both muscles and the autonomic nervous system, as proposed by Kerner 150 years before the first therapeutic studies incorporating BTX [11,16,17]. In 1973, Scott et al [18] published their pioneering animal experiments with BTX injections into primate extraocular muscles, reporting his ability to safely paralyze a given muscle. The human trials that began in 1977 demonstrated extraocular muscle injections of BTX to be an effective alternative or adjunctive treatment for strabismus, eventually earning full US Food and Drug Administration (USFDA) approval for adult strabismus, bleharospasm, and bfacial nerve disordersQ in 1989 [9,11,16,19]. Although full USFDA approval for limited indications was granted in 1989, the medical community had previously reported successful off-label experience with BTX for conditions ranging from spasmodic torticollis and cerebral palsy to the aesthetic treatment of facial rhytides [20-22]. Jean Carruthers and Theodore Tromovich are largely credited with ushering in the cosmetic era of BTX as they recognized the aesthetic improvements in periocular and brow rhytides of patients treated for blepharospasm [11,21]. Carruthers reported a case series in 1992 using BTX injections to address the underlying cause of glabellar wrinkles, and subsequent confirmatory reports have populated the medical literature, paving the way for the April 2002 USFDA approval of BTX type A in the treatment of moderate to severe glabellar rhytides [23-27]. As the collective experience and treatment indications for various BTX formulations continue to expand, facial plastic surgeons will require a firm foundation in the basic science and pharmacology of BTX from which to explore this valuable agent.
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3. Clostridia botulinum neurotoxins Clostridia botulinum is a ubiquitous, Gram-positive, obligate anaerobic bacterium, which is categorized according to the immunologically distinct toxins they secrete: A, B, C1, C2, D, E, F, G [1,9,15,28-32]. Serotype C2, the lone clostridial toxin without activity against nervous tissue, is a unique binary actin-ADP-ribosylating toxin which disrupts elements of the cytoskeleton, resulting in increased permeability to fluids, vasodilation, hypotension, pulmonary edema, and lung hemorrhages [30,32-34]. The 7 remaining serotypes are neurotoxins. Although differing in their biosynthesis, size, structure, pharmacologic properties, and specific cellular site of action, all botulinal neurotoxins exert their effect on the nervous system by inhibition of acetylcholine (Ach) exocytosis at the neuromuscular junction (NMJ), preganglionic sympathetic and parasympathetic neurons, and postganglionic parasympathetic neurons [8,10,11,15,16,29,32,35]. Botulinum neurotoxins are synthesized as a single chain polypeptide protoxin with a molecular weight of approximately 150 kd and a sedimentation coefficient of 7 Svedbergs [15,29,31,32]. The protoxin undergoes a process of activation before it is secreted from the bacterium into the environment as an active neurotoxin. Endogenous bacterial proteases enzymatically cleave (bnickQ) the polypeptide to produce an active, dichain moiety consisting of a heavy chain (100 kd) and a light chain (50 kd) linked by one or more interchain disulfide bonds [11,15,29,32]. An additional endogenous cleavage of the heavy chain component (at the carboxyl terminus) further activates the toxin [32]. The bacterium then coats the active toxin with noncovalently associated surface proteins which are believed to stabilize the toxin in the environment into which it is released [29]. The final weight of these multimeric complexes ranges between 900 kd for type A and 700 kd for type B, and largely reflects the weight of the added surface proteins [15,29,32]. The activation steps dramatically increase the biologic potency of BTX and will be revisited as part of the explanation for difference in potencies between serotypes.
4. Neuromuscular junction physiology The clinical interest of facial plastic surgeons centers on the ability of BTX to inhibit contraction of muscles of the face and neck, and, as such, this requires a basic understanding of that nexus between nervous signal and muscular contraction: the NMJ. Secretion of neurotransmitters is a multi-stage process that requires neurotransmitter synthesis, packaging into secretory vesicles, transport to the nerve terminal, vesicle docking, and, finally, vesicle fusion with resultant release of neurotransmitter into the synaptic cleft [36]. Acetylcholine is synthesized from acetyl coenzyme A and choline in the body of the neuron and then transported in vesicles to the nerve terminal, where these
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vesicles become associated with an intricate protein complex upon which the final stages of exocytosis are dependent [15,16]. The soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) complex is the protein assemblage responsible for vesicle docking (the irreversible binding of vesicles to the cellular membrane), and it is this crucial step of neurotransmission that is disrupted by BTX [15,36]. Calcium flux controls the final stage of neurotransmission. Still as of yet uncharacterized reactions driven by calcium gradients at the presynaptic terminal affect vesicle fusion and exocytosis of Ach into the synaptic cleft, resulting in activation of muscular Ach receptors, subsequent membrane depolarization, and muscle contraction [15,36]. Muscle fibers relax as synaptic cleft acetylcholinesterase degrades Ach to its constituent molecules and the cycle of neurotransmission begins again [15]. 5. Mechanism of botulinum neurotoxin action The active di-chain BTX undergoes a well-described 3step process resulting in the inhibition of neural Ach release at the NMJ. Binding, the first stage of this process, is mediated by the carboxyl terminus of the heavy chain of BTX and serotype-specific receptors at cholinergic nerve terminals [29]. The absence of some varieties of BTX receptors partially explains the various interspecies susceptibilities observed by researchers. For example, the frog (Rana pipiens) lacks receptors for type B BTX and, therefore, is not paralyzed by treatment with this serotype of BTX, just as human neurons lack receptors for type D BTX and remain similarly unaffected [29,37]. The neurotoxin binds irreversibly to these specific surface receptors via its heavy chain, initiating the second stage, internalization [15,16,29,37,38]. Receptor-mediated endocytosis results in sequestration of BTX within an endosome in the axon terminal. The amino terminus of the heavy chain is believed to be responsible for acidification of the endosome and translocation of the lightchain portion of the toxin into the cytosol [15,16,29]. The light chain of BTX exerts the key disruptive effects of the agent on neurotransmission: proteolysis of SNARE complex components [15,16,29,32,37]. The light chain of BTX is a zinc-dependent metalloprotease that enzymatically cleaves specific SNARE proteins before they assemble into the complex responsible for vesicle docking, the penultimate step in the exocytosis pathway [8,10,15,16]. Each serotype cleaves a unique and specific peptide bond in its given target SNARE protein, although there is some overlap between serotypes and target proteins [10,29,32]. Serotypes A, E, and C1 cleave different peptide bonds of their 25-kd target, synaptosome-associated protein (SNAP-25), whereas C1 also digests syntaxin [29,32,39,40]. Botulinum neurotoxin serotypes B, D, F, and G exert their proteolytic effect on synaptobrevin (vesicleassociated membrane protein) [15,16]. The enzymatic cleavage of SNARE proteins ultimately results in the observed
clinical effects of muscle paralysis and at the same time initiates the homeostatic mechanisms of NMJ recovery. 6. Mechanism of recovery from botulinum neurotoxin The recovery phase is a multi-faceted biologic response that, teleologically speaking, attempts to circumvent botulinum’s blockade of the NMJ. Perhaps in response to a growth factor secreted by the paralyzed muscle, accessory terminals bsproutQ from the poisoned presynaptic axon and stimulate development of new NMJs [41,42]. Nascent collateral axons appear within 2 to 10 days and lead to a slow recovery after 28 days, after which time the neuron recovers its ability to synthesize SNARE proteins and the sprouts begin to regress [15,16]. The primary nerve terminal recovers full activity and reestablishes itself as the only functioning NMJ by 90 days after BTX administration, unless the treatment is repeated [6,11]. Additional components of NMJ recovery include a 50% reduction of acetylcholinesterase and an increase in extrajunctional Ach receptors, thereby increasing the effective concentration of synaptic cleft Ach [15]. Although atrophic changes can be documented in histopathologic studies of muscles paralyzed by BTX, a consensus in the literature reports these findings to be fully reversible after cessation of treatment and recovery of muscle function [43]. The interaction between poison and person provides a reliable timetable against which repeat treatments can be scheduled and therapeutic success measured. Each BTX serotype has a unique onset of paralysis, duration of clinical efficacy, and time to recovery of normal muscular function. Although the resultant paralysis achieved by BTX is represented by a relationship between the characteristics of a particular serotype, dose administered, and the mass of muscle receiving the injection, several well-accepted generalities can be made with regard to the most popular serotype, type A. Onset of paralysis can occur as rapidly as 6 hours after administration, with clinical effects first noticeable within 24 to 72 hours [24,27,28]. Paralytic effects peak during the following 2 weeks before establishing a more modest plateau over the ensuing 10 weeks as the various mechanisms of neuromuscular recovery affect a return to normal function approximately 90 days after treatment [9,11,15,22,28]. Although the duration of clinical efficacy differs dramatically between serotypes, most human and animal experience acknowledges durations of action (of equivalent dosages) in the following descending order: serotype A, B, C, F, E [44-48]. 7. Immunologic considerations Botulinum neurotoxin possesses many of the characteristics of those antigens against which the human immune system responds most aggressively. Antigens of large size and aggregated proteins of nonhuman origin evoke avid immune responses, especially in conjunction with larger
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doses and more frequent dosing schedules [32,49]. The resulting antibodies to BTX are described as either neutralizing or nonneutralizing, based on their ability to bind the neurotoxin protein at a site that interrupts its biologic activity [49]. Although development of antibodies to BTX is common (40%-60% of patients), fortunately, the development of clinically important neutralizing antibodies is distinctly uncommon (2%-5%) [49,50]. Similarly, although a small population of patients will develop antibodies that cross-react with both the treated serotype of BTX and a serotype to which the patient has never been exposed, cross-neutralizing antibodies have never been described [29,49]. The clinical importance of the immune response to BTX cannot be overstated, as these considerations figure largely in patients who become refractory to treatment. Prevention of antibody formation is accomplished by using formulations of BTX with the lowest protein load, the smallest effective dose of a given BTX serotype, the longest interval between treatments, and by refraining from bboosterQ injections as the paralytic effects first begin to wane [8,10,32]. The preparation of Botox available since 1997 (BCB2024) contains only 20% of the protein content of previous Botox lots, greatly decreasing the antigenic load, risk of allergic reactions, and development of neutralizing antibodies [11,15,35,51]. Myobloc (a preparation of BTX type B, discussed below) contains approximately 10 to 20 times more protein per unit dose than does Botox [15] Protein load and potential for antibody formation are some considerations physicians must consider when choosing a formulation for BTX therapy. Patients who become unresponsive to a given serotype of BTX as a result of neutralizing antibodies would be expected to benefit from treatment with another serotype, as cross-neutralizing antibodies have not been encountered in clinical practice. 8. Preparations and potencies of botulinum toxin A standard unit of measurement for BTX is necessary to evaluate the various preparations and inherent potencies of different serotypes available for clinical use around the world. Although other methods of quantifying the biologic activity of BTX have been proposed, the mouse potency assay is the universally accepted standard through which units of biologic activity (U) are expressed [28,52]. One unit (U) of BTX represents the intraperitoneal dose required to kill 50% of a group (LD50) of 18- to 20-g female Swiss-Webster mice [15,16]. Because of lack of standardization of the mouse potency assay across laboratories, varying sensitivities of each species to the many serotypes of BTX, and intermanufacturer differences in the ratio of surface protein to active toxin in a given formulation, the commercial preparations of BTX are not directly comparable with regard to dosing or biologic activity [15,29]. References to units of BTX used must always describe the brand of BTX used. Commercial preparations of BTX available in the United States include Botox (BTX type A, Allergan, Irvine, Calif)
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and Myobloc (BTX type B, Solstice Neurosciences, Inc, San Francisco, Calif), whereas Dysport (BTX type A, Ipsen Ltd, Maidenhead, UK) is used in many countries and in the near future also may be approved for use in the United States. Botox is packaged in vials containing a lyophilized complex of 100 U of vacuum-dried BTX type A (approximately 4.9 ng of protein), 0.5 mg of human albumin, and 0.9 mg of sodium chloride having a pH of 7 and requiring reconstitution with saline before clinical use [47,53-55]. Freeze-dried Botox is stable for 2 years when refrigerated at 28C to 88C. Reconstituted Botox retains its clinical efficacy for up to 6 weeks when stored at 48C; yet the package insert recommends use of the entire vial within 4 hours when stored at 28C to 88C [53,56]. Myobloc is supplied in aqueous form, in ready-to-use vials of pH 5.6 containing either 2500, 5000, or 10 000 U of type B BTX [57,58]. Myobloc is stable for 9 months when stored at room temperature and for more than 30 months when kept in refrigerated conditions [57-59]. Each vial of Myobloc is overfilled reliably, such that the 2500-U vial contains 4100 U, the 5000-U vial contains 6800 U, and the 10 000-U vial contains 12 650 U of Myobloc [58,59]. Clinicians should adjust their treatments accordingly. Dysport is another formulation of BTX type A which is distributed as a freeze-dried powder containing 2.5 mg of lactose, 125 lg of albumin, and 500 U (12.5 ng protein) of Dysport toxin that requires reconstitution with normal saline as does Botox [60,61]. Dysport has a refrigerated shelf life of 1 year at 28C to 88C, yet must be used within 8 hours of reconstitution and storage at the same temperature [60]. Each of the 3 available formulations of BTX described above has a particular clinical efficacy and the units of one formulation are not directly equivalent to that of another. Botox is the most widely used preparation, and, as such, the clinical efficacy of Dysport and Myobloc is described in reference to the industry leader. One unit of Botox is clinically equivalent to 2.5 to 5 U of Dysport and to 50 to 125 U of Myobloc [28,59,61]. Differences in pharmaceutical processing techniques and potency assays as well as the inherent extent of biologic activation (bnickingQ) for each serotype (BTX type A is 90%-95% nicked, whereas BTX type B is 70% nicked) are largely responsible for the variation in clinical efficacy between drugs [29]. Although studies comparing the cosmetic efficacy of Dysport to Myobloc have not yet been reported, a few authors have examined Botox’s properties with respect to the more recently approved Myobloc. In contrast to Botox, a general consensus finds Myobloc to have a more rapid onset of action, a greater degree of diffusion from the injection site, a more painful injection, higher incidence of anticholinergic side effects, and a shorter duration of clinical paralysis [11,47,48,54,62]. The pharmacologic properties that distinguish one agent from another will allow facial plastic surgeons to better tailor treatment plans to meet the needs of each individual patient.
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9. Basic science and clinical practicalities To further maximize both the safety and efficacy of BTX treatments, the clinical practicalities of reconstitution, patient preparation, injection technique, and pain control have been studied and reviewed by a number of interested authors. Reconstitution of Botox with preserved normal saline is recommended by the manufacturer; however, recent randomized double-blind placebo-controlled trials have demonstrated a significant reduction of patient discomfort during injection when Botox is reconstituted with preservative-containing normal saline (0.9% benzyl alcohol) [62]. Importantly, nearly identical trials evaluated Myobloc and found dilution with preservative-containing saline to significantly reduce patient discomfort during injection [64]. Myobloc’s reputation for painful injections due to its low pH (5.6) and higher volume may be mitigated by judicious use of preservative-containing saline diluent. The choice of diluent has been demonstrated to have no effect on either the clinical efficacy or the stability of Myobloc or Botox [63-65]. Although treatment outcomes are not altered by the nature of the diluent, objective differences in clinical results can be demonstrated based upon the volume and concentration of the injection. Clinical trials documented that low concentration and high injection volumes result in greater diffusion and a larger area of paralytic effect for both Botox and Myobloc [53,66]. Myobloc has an anecdotal reputation for a greater tendency for diffusion than does Botox. In the treatment of dynamic forehead rhytides, 5 U of Botox was shown to result in a smaller area of paralysis than did a comparable dose of 500 U of Myobloc [62]. Confouding this finding are animal studies that report Botox to diffuse a significantly greater distance from an injection site [59]. Facial plastic surgeons should be able to vary the concentration and volume of injection to accommodate a particular aesthetic problem. Low-volume, high-concentration injections can target specific areas and more tightly circumscribe the agent’s paralytic action to a diameter of 2.5 to 3 cm from the injection site [27,35]. Higher volume, lower concentration injections can affect a less complete and perhaps more natural muscle relaxation, although the risk of greater diffusion can paralyze unintended muscles with consequences ranging from ptosis to diplopia [66,67]. Patients are encouraged not to massage the injection sites, as even gentle pressure can cause greater spread of BTX from the carefully selected injection sites. Research concerning every step of the procedure of BTX injection has better delineated those features most likely to improve patient outcomes. Authors still recommend gentle methods of reconstitution designed to prevent degradation of BTX, such as using large bore hypodermic needles and a gentle swirling technique to create a particulate-free solution [27,28], yet no decrement in clinical efficacy is documented in cases when more vigorous methods of reconstitution are used [68]. Most physicians continue to use the more gentle
technique, as it is unlikely that vigorous shaking of the BTX solution will produce better results. Although BTX may be a more robust protein than previously estimated, the alcohol swab applied to either the patient’s skin or to the vial can denature the toxin if a needle is introduced before the alcohol evaporates completely [27,67]. Patient satisfaction can be improved through measures that decrease the pain of injections without compromising safety and efficacy. Pretreatment application of ice to the planned treatment area has been shown to significantly decrease pain perception as well as the duration required to complete a series of injections [69]. Many physicians favor smaller-gauge needles (30 gauge) and high-concentration injections to decrease the number of painful stimuli a patient receives [53]. Electromyographic (EMG) guidance has been suggested not just as a method of increasing the accuracy of injections, but as a technique to further decrease the quantity of injections and to expeditiously complete a treatment session. The reliable anatomy of the facial muscles in relation to observed rhytides has relegated EMG guidance to those patients with submaximal responses to standard injections [67,70]. No authors have produced data regarding pain perception with EMG guidance in comparison to standard injection techniques. 10. Toxicity, precautions, and contraindications to treatment with BTX The safety profile of BTX is the most obvious concern related to elective intramuscular injection of a potent toxin for the purpose of achieving aesthetic gains. Commercially available preparations of BTX have an excellent safety profile, especially in the hands of facial plastic surgeons. The LD50 of Botox in humans, as derived from primate data, is estimated to be 40 U/kg, or 2800 U in the 70-kg patient [15,28]. Although physicians treating spastic muscle conditions can use several hundred units in a single affected muscle group, a complete course of treatment for every site in the face and neck should not require more than 100 to 125 U of Botox, with the majority of patients receiving much smaller doses to efface more focused areas of rhytides [53,71]. In fact, facial BTX treatments have never been associated with severe, life-threatening complications, but instead with localized adverse consequences of unintended and transient muscle paralysis [16,71]. Should a patient receive an overdose of BTX, an antidote is effective if administered within 21 to 24 hours of exposure [10,16,71]. Just as with the administration of any pharmacologic agent, physicians should have an awareness of a patient’s concomitant medical conditions, medications, and the potential implications for treatment with BTX. Peripheral motor neuropathies or neuromuscular disorders such as Eaton-Lambert syndrome, multiple sclerosis, and myasthenia gravis are absolute contraindications to treatment with BTX, as further chemodenervation may exacerbate muscle
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weakness [15,35,53,71]. Botox is a category C drug and, despite the absence of known teratogenicity or the ability of BTX to be excreted in breast milk, physicians are strongly cautioned to avoid treatment during pregnancy or while breast-feeding [35,53,71]. The BTX dosages indicated for facial aesthetic procedures are not thought to be sufficient to result in clinically significant interactions with other drugs; however, an awareness of the pharmaceuticals most likely to result in untoward consequences will allow the facial plastic surgeon to exert an ounce of prevention in such cases. Aminoglycosides, cyclosporine, muscle relaxants, magnesium sulfate, and lincosamide can exacerbate the inhibitory effect of BTX on neuromuscular transmission and result in states of flaccid paralysis that approximate classic botulism [15,16,35,53]. Diminished responses to BTX could result in patients treated with chloroquine or hydroxychloroquine, as these aminoquinolones interrupt BTX’s toxic actions within the presynaptic nerve terminal [71].
11. Conclusion Botulinal neurotoxins represent an effective adjuvant medical treatment or surgical alternative for those disorders that can be assuaged by the induction of a focal paresis in areas of undesired muscular contraction. The reliable anatomy of facial muscles and their well-defined roles in hyperdynamic rhytides have placed the facial plastic surgeon at the vanguard of the aesthetic applications of BTX. The knowledge surrounding these agents is increasing exponentially as both physicians and patients recognize their remarkable results, enormous spectrum of indications, and impressive safety profile. A review of the pharmacology of BTX, its commercial preparations, and the clinical data regarding its administration will allow facial plastic surgeons to develop treatment plans that carefully consider the serotype and formulation of BTX and its inherent properties, the diluent, reconstitution methods, specific volumes and concentrations to address particular concerns, methods for pain control, and dosing schedules. Knowledge of the basic science of BTX and the most recent literature concerning its administration is a fundamental responsibility of cosmetic surgeons as they endeavor to avoid complications and deliver maximal aesthetic results to their patients.
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[34] Richard JF, Petit L, Gibert M, et al. Bacterial toxins modifying the actin cytoskeleton. Int Microbiol 1999;2(3):185 - 94. [35] Klein AW. Contraindications and complications with the use of botulinum toxin. Clin Dermatol 2004;22:66 - 75. [36] Bajjalieh SM. Synaptic vesicle docking and fusion. Curr Opin Neurobiol 1999;9:321 - 8. [37] Coffield JA, Bakry N, Zhang RD, et al. In vitro characterization of botulinum toxin types A, C and D action on human tissues: combined electrophysiologic, pharmacologic and molecular biologic approaches. J Pharmacol Exp Ther 1997;280(3):1489 - 98. [38] Glogau RG. Review of the use of botulinum toxin for hyperhidrosis and cosmetic purposes. Clin J Pain 2002;18(Suppl):S191- 7. [39] Blasi J, Chapman ER, Link E. Botulinum neurotoxin A selectively cleaves the synaptic protein SNAP25. Nature 1993;365:160 - 3. [40] Foran P, Lawrence GW, Shone CC, et al. Botulinum neurotoxin C1 cleaves both syntaxin and SNAP-25 in intact and permeabilized chromaffin cells: correlation with its blockade of catecholamine release. Biochemistry 1996;35(8):2630 - 6. [41] Angaut-Petit D, Molgo J, Comella JX, Faille L, et al. Terminal sprouting in mouse neuromuscular junctions poisoned with botulinum type A toxin: morphological and electrophysiological features. Neuroscience 1990;37(3):799 - 808. [42] Pamphlett R. Early terminal and nodal sprouting of motor axons after botulinum toxin. J Neurol Sci 1989;92(2-3):181 - 92. [43] Harris CP, Alderson K, Nebeker J, et al. Histologic features of human orbicularis oculi treated with botulinum A toxin. Arch Ophthalmol 1991;109(3):393 - 5. [44] Eleopra R, Tugnoli V, Rosetto O, et al. Different time courses of recovery after poisoning with botulinum neurotoxin serotypes A and E in humans. Neurosci Lett 1998;256:135 - 8. [45] Eleopra R, Tugnoli V, Quatrale R, et al. Different types of botulinum toxin in humans. Mov Disord 2004;19(Suppl 8):S53 - 9. [46] Mezaki T, Kaji R, Kohara N. Comparison of therapeutic efficacies of type A and F botulinum toxins for blepharospasm: a double-blind, controlled study. Neurology 1995;45:506 - 8. [47] Sadick NS, Matarasso SL. Comparison of botulinum toxins A and B in the treatment of facial rhytides. Dermatol Clin 2004;22:221 - 6. [48] Sloop RR, Cole BA, Escutin RO. Human response to botulinum toxin injection: type B compared with type A. Neurology 1997; 49(1):189 - 94. [49] Critchfield J. Considering the immune response to botulinum toxin. Clin J Pain 2002;18(Suppl):S133 - 41. [50] Kessler KR, Skutta M, Benecke R. Long-term treatment of cervical dystonia with botulinum toxin A: efficacy, safety, and antibody frequency. J Neurol 1999;246(4):265 - 74. [51] Carruthers A, Carruthers J. Toxins 99, new information about the botulinum neurotoxins. Dermatol Surg 2000;26:174 - 6. [52] Pearce LB, Borodic GE, Johnson EA, et al. The median paralysis unit: a more pharmacologically relevant unit of biologic activity for botulinum toxin. Toxicon 1995;33(2):217 - 27.
[53] Carruthers J, Faigen S, Matarasso SL. Consensus recommendations on the use of botulinum toxin type A in facial aesthetics. Plast Reconstr Surg 2004;114(Suppl):1S - 22S. [54] Yamauchi PS, Lowe NJ. Botulinum toxin types A and B: comparison of efficacy, duration, and dose-ranging studies for the treatment of facial rhytides and hyperhidrosis. Clin Dermatol 2004; 22(1):34 - 9. [55] Botox PackageInsert, Allergan,Inc. www.botox.com/site/professionals/ home.asp [56] Hexsel DM, De Almeida AT, Rutowitsch M, et al. Multicenter, double-blind study of the efficacy of injections with botulinum toxin type A reconstituted up to six consecutive weeks before application. Dermatol Surg 2003;29(5):523 - 9. [57] Myobloc Package Inset. Solstice Neurosciences, Inc. http:// www.myobloc.com [58] Baumann L, Black L. Botulinum toxin type B (Myobloc). Dermatol Surg 2003;29(5):496 - 500. [59] Flynn TC. Myobloc. Dermatol Clin 2004;22:207 - 11. [60] Dysport Package Information, Ipsen Limited; http://www.ipsen.ltd.uk. [61] Markey AC. Dysport. Dermatol Clin 2004;22:213 - 9. [62] Flynn TC, Clark RE. Botulinum toxin type B (MYOBLOC) versus botulinum toxin type A (BOTOX) frontalis study: rate of onset and radius of diffusion. Dermatol Surg 2003;29(5):519 - 22. [63] Alam M, Dover JS, Arndt KA. Pain associated with injection of botulinum toxin A exotoxin reconstituted using isotonic sodium chloride with and without preservative. Arch Dermatol 2002; 138:510 - 4. [64] Van Laborde S, Dover JS, Moore M, et al. Reduction in injection pain with botulinum toxin type B further diluted using saline with preservative: a double-blind, randomized controlled trial. J Am Acad Dermatol 2003;48(6):875 - 7. [65] Callaway JE. Botulinum toxin type B (Myobloc): pharmacology and biochemistry. Clin Dermatol 2004;22:23 - 8. [66] Hsu TS, Dover JS, Arndt KA. Effect of volume and concentration on the diffusion of botulinum exotoxin A. Arch Dermatol 2004; 140(11):1351 - 4. [67] Mantell AM. Dilution, storage and electromyographic guidance in the use of botulinum toxins. Dermatol Clin 2004;22:135 - 6. [68] Almeida De Trindade AR, Kadunc BV, Di Chiacchio N, et al. Foam during reconstitution does not affect the potency of botulinum toxin type A. Dermatol Surg 2003;29(5):530 - 1. [69] Sarifakioglu N, Sarifakioglu E. Evaluating the effects of ice application on the pain felt during botulinum toxin type-A injections: a prospective, randomized, single-blind controlled trial. Ann Plast Surg 2004;53(6):543 - 6. [70] Klein AW, Mantell A. Electromyographic guidance in injecting botulinum toxin. Dermatol Surg 1998;24:1184 - 6. [71] Vartanian AJ, Dayan DH. Complications of botulinum toxin A use in facial rejuvenation. Facial Plast Surg Clin North Am 2003; 11:48 - 492.
Botulinum neurotoxins: fundamentals for the facial plastic surgeon Robert Todd Adelson, MD4 Division of Facial Plastic and Reconstructive Surgery, Department of Otolaryngology, University of Florida, PO Box 100264, Gainesville, FL, USA Received 29 July 2006
Abstract
The most commonly performed nonsurgical cosmetic procedure in the facial plastic surgery armamentarium involves the various commercial preparations of botulinum neurotoxins. These drugs have undergone a transformation from public health scourge to near ubiquitous therapeutic modality across the entire medical spectrum. Herein, the history of botulinum neurotoxins is reviewed, including an exploration of their pharmacology, neuromuscular junction physiology, a description of the commercially available preparations, and the recent research concerning the practicalities of their clinical use. D 2007 Elsevier Inc. All rights reserved.
1. Introduction The role of botulinum neurotoxins (BTX) has undergone one of the most remarkable transformations in the history of medicine. The journey from public health threat to popular therapeutic modality provides a framework by which one can understand how every field from anesthesiology to urology has successfully used this bmost poisonous poison,Q with its greatest notoriety achieved in the hands of facial plastic surgeons [1-3]. In 2005, the last year for which complete statistics are available, both the American Academy of Facial Plastic and Reconstructive Surgery and the American Society of Plastic Surgeons reported Botox injections to be the most common cosmetic procedure (surgical and nonsurgical) performed by their members [4,5]. The reported number of Botox injections performed by members of the American Society of Plastic Surgeons (3.8 million Botox treatments) and American Academy of Facial Plastic and Reconstructive Surgery (315.6 procedures per surgeon), respectively, emphasizes both the popularity of this modality and the importance of providing excellent care as both the patient base and number of providers increase [4,5]. A complete
4 Division of Facial Plastic and Reconstructive Surgery, Department of Otolaryngology, University of Florida, PO Box 100264, Gainesville, FL 32610, USA. Tel.: +1 352 392 4461. E-mail address: [email protected]. 0196-0709/$ – see front matter D 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.amjoto.2006.09.002
understanding of botulinum toxin therapy will allow physicians to maximize results based on the existing knowledge and perhaps further expand the cosmetic and functional indications for this remarkable neurotoxin. 2. History of botulism Pathologic states attributable to the bacterial toxin of interest include infant (intestinal) botulism, wound botulism, inhalational botulism, iatrogenic botulism, and foodborne (classic) botulism [6]. Of these, foodborne botulism prompted scientific investigations that began in the public health sector and, at present, have resulted in therapeutic indications for more than 100 different conditions across the medical spectrum [1,7-10]. Classic botulism was the call to prominence for Justinus Kerner (1786-1862), a medical officer reporting the symptoms resulting from the bconsumption of smoked blood-sausagesQ in his 1820 monograph, describing the time course of the disease, preservation of consciousness, mydriasis, absence of fever, suppression of secretions, and paralysis of the gut and all skeletal muscles culminating in respiratory and cardiac failure [11,12]. Kerner undertook laboratory and animal experimentation to extract the bsausage toxin,Q and although he never succeeded in isolating the toxin, his 1822 monograph proposed that if such an agent could be isolated, it could be used as a remedy for conditions of an overactive nervous system, citing movement disorders like bSt Vitus danceQ (chorea minor)
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and the hypersecretion of sweat, tears, or mucous [12]. By 1870, Muller differentiated between the distinct symptom complex described by Kerner and other assorted cases of food poisoning, applying the name bbotulismQ to this unique disease based on the earlier work of Kerner and the Latin word for sausage: botulus [12,13]. Ironically, it was the consumption of an uncooked ham at a musician’s funeral in December 1895 that provided the biologic material for Van Ermengem to isolate the culprit bacteria of bsausage poisoningQ: Bacillus botulinus [13]. Van Ermengem’s 1897 publication established the entire foundation of research into botulism by isolating and naming the bacteria, recognizing its anaerobic nature, describing its toxin as a heat-labile substance, and subsequently describing measures of food preservation and preparation that prevent foodborne botulism [11-14]. Although Sommer had produced a crude form of botulinum toxin in 1926, widespread interest in BTX was rekindled during World War II, when the threat of biologic weapons spurred US scientists to purify BTX for military purposes [15-17]. In the wake of WWII, highly purified BTX was made available to scientists who subsequently deciphered its pharmacology and began to explore the use of BTX to treat disease states resulting from hyperactivity of both muscles and the autonomic nervous system, as proposed by Kerner 150 years before the first therapeutic studies incorporating BTX [11,16,17]. In 1973, Scott et al [18] published their pioneering animal experiments with BTX injections into primate extraocular muscles, reporting his ability to safely paralyze a given muscle. The human trials that began in 1977 demonstrated extraocular muscle injections of BTX to be an effective alternative or adjunctive treatment for strabismus, eventually earning full US Food and Drug Administration (USFDA) approval for adult strabismus, bleharospasm, and bfacial nerve disordersQ in 1989 [9,11,16,19]. Although full USFDA approval for limited indications was granted in 1989, the medical community had previously reported successful off-label experience with BTX for conditions ranging from spasmodic torticollis and cerebral palsy to the aesthetic treatment of facial rhytides [20-22]. Jean Carruthers and Theodore Tromovich are largely credited with ushering in the cosmetic era of BTX as they recognized the aesthetic improvements in periocular and brow rhytides of patients treated for blepharospasm [11,21]. Carruthers reported a case series in 1992 using BTX injections to address the underlying cause of glabellar wrinkles, and subsequent confirmatory reports have populated the medical literature, paving the way for the April 2002 USFDA approval of BTX type A in the treatment of moderate to severe glabellar rhytides [23-27]. As the collective experience and treatment indications for various BTX formulations continue to expand, facial plastic surgeons will require a firm foundation in the basic science and pharmacology of BTX from which to explore this valuable agent.
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3. Clostridia botulinum neurotoxins Clostridia botulinum is a ubiquitous, Gram-positive, obligate anaerobic bacterium, which is categorized according to the immunologically distinct toxins they secrete: A, B, C1, C2, D, E, F, G [1,9,15,28-32]. Serotype C2, the lone clostridial toxin without activity against nervous tissue, is a unique binary actin-ADP-ribosylating toxin which disrupts elements of the cytoskeleton, resulting in increased permeability to fluids, vasodilation, hypotension, pulmonary edema, and lung hemorrhages [30,32-34]. The 7 remaining serotypes are neurotoxins. Although differing in their biosynthesis, size, structure, pharmacologic properties, and specific cellular site of action, all botulinal neurotoxins exert their effect on the nervous system by inhibition of acetylcholine (Ach) exocytosis at the neuromuscular junction (NMJ), preganglionic sympathetic and parasympathetic neurons, and postganglionic parasympathetic neurons [8,10,11,15,16,29,32,35]. Botulinum neurotoxins are synthesized as a single chain polypeptide protoxin with a molecular weight of approximately 150 kd and a sedimentation coefficient of 7 Svedbergs [15,29,31,32]. The protoxin undergoes a process of activation before it is secreted from the bacterium into the environment as an active neurotoxin. Endogenous bacterial proteases enzymatically cleave (bnickQ) the polypeptide to produce an active, dichain moiety consisting of a heavy chain (100 kd) and a light chain (50 kd) linked by one or more interchain disulfide bonds [11,15,29,32]. An additional endogenous cleavage of the heavy chain component (at the carboxyl terminus) further activates the toxin [32]. The bacterium then coats the active toxin with noncovalently associated surface proteins which are believed to stabilize the toxin in the environment into which it is released [29]. The final weight of these multimeric complexes ranges between 900 kd for type A and 700 kd for type B, and largely reflects the weight of the added surface proteins [15,29,32]. The activation steps dramatically increase the biologic potency of BTX and will be revisited as part of the explanation for difference in potencies between serotypes.
4. Neuromuscular junction physiology The clinical interest of facial plastic surgeons centers on the ability of BTX to inhibit contraction of muscles of the face and neck, and, as such, this requires a basic understanding of that nexus between nervous signal and muscular contraction: the NMJ. Secretion of neurotransmitters is a multi-stage process that requires neurotransmitter synthesis, packaging into secretory vesicles, transport to the nerve terminal, vesicle docking, and, finally, vesicle fusion with resultant release of neurotransmitter into the synaptic cleft [36]. Acetylcholine is synthesized from acetyl coenzyme A and choline in the body of the neuron and then transported in vesicles to the nerve terminal, where these
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vesicles become associated with an intricate protein complex upon which the final stages of exocytosis are dependent [15,16]. The soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) complex is the protein assemblage responsible for vesicle docking (the irreversible binding of vesicles to the cellular membrane), and it is this crucial step of neurotransmission that is disrupted by BTX [15,36]. Calcium flux controls the final stage of neurotransmission. Still as of yet uncharacterized reactions driven by calcium gradients at the presynaptic terminal affect vesicle fusion and exocytosis of Ach into the synaptic cleft, resulting in activation of muscular Ach receptors, subsequent membrane depolarization, and muscle contraction [15,36]. Muscle fibers relax as synaptic cleft acetylcholinesterase degrades Ach to its constituent molecules and the cycle of neurotransmission begins again [15]. 5. Mechanism of botulinum neurotoxin action The active di-chain BTX undergoes a well-described 3step process resulting in the inhibition of neural Ach release at the NMJ. Binding, the first stage of this process, is mediated by the carboxyl terminus of the heavy chain of BTX and serotype-specific receptors at cholinergic nerve terminals [29]. The absence of some varieties of BTX receptors partially explains the various interspecies susceptibilities observed by researchers. For example, the frog (Rana pipiens) lacks receptors for type B BTX and, therefore, is not paralyzed by treatment with this serotype of BTX, just as human neurons lack receptors for type D BTX and remain similarly unaffected [29,37]. The neurotoxin binds irreversibly to these specific surface receptors via its heavy chain, initiating the second stage, internalization [15,16,29,37,38]. Receptor-mediated endocytosis results in sequestration of BTX within an endosome in the axon terminal. The amino terminus of the heavy chain is believed to be responsible for acidification of the endosome and translocation of the lightchain portion of the toxin into the cytosol [15,16,29]. The light chain of BTX exerts the key disruptive effects of the agent on neurotransmission: proteolysis of SNARE complex components [15,16,29,32,37]. The light chain of BTX is a zinc-dependent metalloprotease that enzymatically cleaves specific SNARE proteins before they assemble into the complex responsible for vesicle docking, the penultimate step in the exocytosis pathway [8,10,15,16]. Each serotype cleaves a unique and specific peptide bond in its given target SNARE protein, although there is some overlap between serotypes and target proteins [10,29,32]. Serotypes A, E, and C1 cleave different peptide bonds of their 25-kd target, synaptosome-associated protein (SNAP-25), whereas C1 also digests syntaxin [29,32,39,40]. Botulinum neurotoxin serotypes B, D, F, and G exert their proteolytic effect on synaptobrevin (vesicleassociated membrane protein) [15,16]. The enzymatic cleavage of SNARE proteins ultimately results in the observed
clinical effects of muscle paralysis and at the same time initiates the homeostatic mechanisms of NMJ recovery. 6. Mechanism of recovery from botulinum neurotoxin The recovery phase is a multi-faceted biologic response that, teleologically speaking, attempts to circumvent botulinum’s blockade of the NMJ. Perhaps in response to a growth factor secreted by the paralyzed muscle, accessory terminals bsproutQ from the poisoned presynaptic axon and stimulate development of new NMJs [41,42]. Nascent collateral axons appear within 2 to 10 days and lead to a slow recovery after 28 days, after which time the neuron recovers its ability to synthesize SNARE proteins and the sprouts begin to regress [15,16]. The primary nerve terminal recovers full activity and reestablishes itself as the only functioning NMJ by 90 days after BTX administration, unless the treatment is repeated [6,11]. Additional components of NMJ recovery include a 50% reduction of acetylcholinesterase and an increase in extrajunctional Ach receptors, thereby increasing the effective concentration of synaptic cleft Ach [15]. Although atrophic changes can be documented in histopathologic studies of muscles paralyzed by BTX, a consensus in the literature reports these findings to be fully reversible after cessation of treatment and recovery of muscle function [43]. The interaction between poison and person provides a reliable timetable against which repeat treatments can be scheduled and therapeutic success measured. Each BTX serotype has a unique onset of paralysis, duration of clinical efficacy, and time to recovery of normal muscular function. Although the resultant paralysis achieved by BTX is represented by a relationship between the characteristics of a particular serotype, dose administered, and the mass of muscle receiving the injection, several well-accepted generalities can be made with regard to the most popular serotype, type A. Onset of paralysis can occur as rapidly as 6 hours after administration, with clinical effects first noticeable within 24 to 72 hours [24,27,28]. Paralytic effects peak during the following 2 weeks before establishing a more modest plateau over the ensuing 10 weeks as the various mechanisms of neuromuscular recovery affect a return to normal function approximately 90 days after treatment [9,11,15,22,28]. Although the duration of clinical efficacy differs dramatically between serotypes, most human and animal experience acknowledges durations of action (of equivalent dosages) in the following descending order: serotype A, B, C, F, E [44-48]. 7. Immunologic considerations Botulinum neurotoxin possesses many of the characteristics of those antigens against which the human immune system responds most aggressively. Antigens of large size and aggregated proteins of nonhuman origin evoke avid immune responses, especially in conjunction with larger
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doses and more frequent dosing schedules [32,49]. The resulting antibodies to BTX are described as either neutralizing or nonneutralizing, based on their ability to bind the neurotoxin protein at a site that interrupts its biologic activity [49]. Although development of antibodies to BTX is common (40%-60% of patients), fortunately, the development of clinically important neutralizing antibodies is distinctly uncommon (2%-5%) [49,50]. Similarly, although a small population of patients will develop antibodies that cross-react with both the treated serotype of BTX and a serotype to which the patient has never been exposed, cross-neutralizing antibodies have never been described [29,49]. The clinical importance of the immune response to BTX cannot be overstated, as these considerations figure largely in patients who become refractory to treatment. Prevention of antibody formation is accomplished by using formulations of BTX with the lowest protein load, the smallest effective dose of a given BTX serotype, the longest interval between treatments, and by refraining from bboosterQ injections as the paralytic effects first begin to wane [8,10,32]. The preparation of Botox available since 1997 (BCB2024) contains only 20% of the protein content of previous Botox lots, greatly decreasing the antigenic load, risk of allergic reactions, and development of neutralizing antibodies [11,15,35,51]. Myobloc (a preparation of BTX type B, discussed below) contains approximately 10 to 20 times more protein per unit dose than does Botox [15] Protein load and potential for antibody formation are some considerations physicians must consider when choosing a formulation for BTX therapy. Patients who become unresponsive to a given serotype of BTX as a result of neutralizing antibodies would be expected to benefit from treatment with another serotype, as cross-neutralizing antibodies have not been encountered in clinical practice. 8. Preparations and potencies of botulinum toxin A standard unit of measurement for BTX is necessary to evaluate the various preparations and inherent potencies of different serotypes available for clinical use around the world. Although other methods of quantifying the biologic activity of BTX have been proposed, the mouse potency assay is the universally accepted standard through which units of biologic activity (U) are expressed [28,52]. One unit (U) of BTX represents the intraperitoneal dose required to kill 50% of a group (LD50) of 18- to 20-g female Swiss-Webster mice [15,16]. Because of lack of standardization of the mouse potency assay across laboratories, varying sensitivities of each species to the many serotypes of BTX, and intermanufacturer differences in the ratio of surface protein to active toxin in a given formulation, the commercial preparations of BTX are not directly comparable with regard to dosing or biologic activity [15,29]. References to units of BTX used must always describe the brand of BTX used. Commercial preparations of BTX available in the United States include Botox (BTX type A, Allergan, Irvine, Calif)
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and Myobloc (BTX type B, Solstice Neurosciences, Inc, San Francisco, Calif), whereas Dysport (BTX type A, Ipsen Ltd, Maidenhead, UK) is used in many countries and in the near future also may be approved for use in the United States. Botox is packaged in vials containing a lyophilized complex of 100 U of vacuum-dried BTX type A (approximately 4.9 ng of protein), 0.5 mg of human albumin, and 0.9 mg of sodium chloride having a pH of 7 and requiring reconstitution with saline before clinical use [47,53-55]. Freeze-dried Botox is stable for 2 years when refrigerated at 28C to 88C. Reconstituted Botox retains its clinical efficacy for up to 6 weeks when stored at 48C; yet the package insert recommends use of the entire vial within 4 hours when stored at 28C to 88C [53,56]. Myobloc is supplied in aqueous form, in ready-to-use vials of pH 5.6 containing either 2500, 5000, or 10 000 U of type B BTX [57,58]. Myobloc is stable for 9 months when stored at room temperature and for more than 30 months when kept in refrigerated conditions [57-59]. Each vial of Myobloc is overfilled reliably, such that the 2500-U vial contains 4100 U, the 5000-U vial contains 6800 U, and the 10 000-U vial contains 12 650 U of Myobloc [58,59]. Clinicians should adjust their treatments accordingly. Dysport is another formulation of BTX type A which is distributed as a freeze-dried powder containing 2.5 mg of lactose, 125 lg of albumin, and 500 U (12.5 ng protein) of Dysport toxin that requires reconstitution with normal saline as does Botox [60,61]. Dysport has a refrigerated shelf life of 1 year at 28C to 88C, yet must be used within 8 hours of reconstitution and storage at the same temperature [60]. Each of the 3 available formulations of BTX described above has a particular clinical efficacy and the units of one formulation are not directly equivalent to that of another. Botox is the most widely used preparation, and, as such, the clinical efficacy of Dysport and Myobloc is described in reference to the industry leader. One unit of Botox is clinically equivalent to 2.5 to 5 U of Dysport and to 50 to 125 U of Myobloc [28,59,61]. Differences in pharmaceutical processing techniques and potency assays as well as the inherent extent of biologic activation (bnickingQ) for each serotype (BTX type A is 90%-95% nicked, whereas BTX type B is 70% nicked) are largely responsible for the variation in clinical efficacy between drugs [29]. Although studies comparing the cosmetic efficacy of Dysport to Myobloc have not yet been reported, a few authors have examined Botox’s properties with respect to the more recently approved Myobloc. In contrast to Botox, a general consensus finds Myobloc to have a more rapid onset of action, a greater degree of diffusion from the injection site, a more painful injection, higher incidence of anticholinergic side effects, and a shorter duration of clinical paralysis [11,47,48,54,62]. The pharmacologic properties that distinguish one agent from another will allow facial plastic surgeons to better tailor treatment plans to meet the needs of each individual patient.
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9. Basic science and clinical practicalities To further maximize both the safety and efficacy of BTX treatments, the clinical practicalities of reconstitution, patient preparation, injection technique, and pain control have been studied and reviewed by a number of interested authors. Reconstitution of Botox with preserved normal saline is recommended by the manufacturer; however, recent randomized double-blind placebo-controlled trials have demonstrated a significant reduction of patient discomfort during injection when Botox is reconstituted with preservative-containing normal saline (0.9% benzyl alcohol) [62]. Importantly, nearly identical trials evaluated Myobloc and found dilution with preservative-containing saline to significantly reduce patient discomfort during injection [64]. Myobloc’s reputation for painful injections due to its low pH (5.6) and higher volume may be mitigated by judicious use of preservative-containing saline diluent. The choice of diluent has been demonstrated to have no effect on either the clinical efficacy or the stability of Myobloc or Botox [63-65]. Although treatment outcomes are not altered by the nature of the diluent, objective differences in clinical results can be demonstrated based upon the volume and concentration of the injection. Clinical trials documented that low concentration and high injection volumes result in greater diffusion and a larger area of paralytic effect for both Botox and Myobloc [53,66]. Myobloc has an anecdotal reputation for a greater tendency for diffusion than does Botox. In the treatment of dynamic forehead rhytides, 5 U of Botox was shown to result in a smaller area of paralysis than did a comparable dose of 500 U of Myobloc [62]. Confouding this finding are animal studies that report Botox to diffuse a significantly greater distance from an injection site [59]. Facial plastic surgeons should be able to vary the concentration and volume of injection to accommodate a particular aesthetic problem. Low-volume, high-concentration injections can target specific areas and more tightly circumscribe the agent’s paralytic action to a diameter of 2.5 to 3 cm from the injection site [27,35]. Higher volume, lower concentration injections can affect a less complete and perhaps more natural muscle relaxation, although the risk of greater diffusion can paralyze unintended muscles with consequences ranging from ptosis to diplopia [66,67]. Patients are encouraged not to massage the injection sites, as even gentle pressure can cause greater spread of BTX from the carefully selected injection sites. Research concerning every step of the procedure of BTX injection has better delineated those features most likely to improve patient outcomes. Authors still recommend gentle methods of reconstitution designed to prevent degradation of BTX, such as using large bore hypodermic needles and a gentle swirling technique to create a particulate-free solution [27,28], yet no decrement in clinical efficacy is documented in cases when more vigorous methods of reconstitution are used [68]. Most physicians continue to use the more gentle
technique, as it is unlikely that vigorous shaking of the BTX solution will produce better results. Although BTX may be a more robust protein than previously estimated, the alcohol swab applied to either the patient’s skin or to the vial can denature the toxin if a needle is introduced before the alcohol evaporates completely [27,67]. Patient satisfaction can be improved through measures that decrease the pain of injections without compromising safety and efficacy. Pretreatment application of ice to the planned treatment area has been shown to significantly decrease pain perception as well as the duration required to complete a series of injections [69]. Many physicians favor smaller-gauge needles (30 gauge) and high-concentration injections to decrease the number of painful stimuli a patient receives [53]. Electromyographic (EMG) guidance has been suggested not just as a method of increasing the accuracy of injections, but as a technique to further decrease the quantity of injections and to expeditiously complete a treatment session. The reliable anatomy of the facial muscles in relation to observed rhytides has relegated EMG guidance to those patients with submaximal responses to standard injections [67,70]. No authors have produced data regarding pain perception with EMG guidance in comparison to standard injection techniques. 10. Toxicity, precautions, and contraindications to treatment with BTX The safety profile of BTX is the most obvious concern related to elective intramuscular injection of a potent toxin for the purpose of achieving aesthetic gains. Commercially available preparations of BTX have an excellent safety profile, especially in the hands of facial plastic surgeons. The LD50 of Botox in humans, as derived from primate data, is estimated to be 40 U/kg, or 2800 U in the 70-kg patient [15,28]. Although physicians treating spastic muscle conditions can use several hundred units in a single affected muscle group, a complete course of treatment for every site in the face and neck should not require more than 100 to 125 U of Botox, with the majority of patients receiving much smaller doses to efface more focused areas of rhytides [53,71]. In fact, facial BTX treatments have never been associated with severe, life-threatening complications, but instead with localized adverse consequences of unintended and transient muscle paralysis [16,71]. Should a patient receive an overdose of BTX, an antidote is effective if administered within 21 to 24 hours of exposure [10,16,71]. Just as with the administration of any pharmacologic agent, physicians should have an awareness of a patient’s concomitant medical conditions, medications, and the potential implications for treatment with BTX. Peripheral motor neuropathies or neuromuscular disorders such as Eaton-Lambert syndrome, multiple sclerosis, and myasthenia gravis are absolute contraindications to treatment with BTX, as further chemodenervation may exacerbate muscle
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weakness [15,35,53,71]. Botox is a category C drug and, despite the absence of known teratogenicity or the ability of BTX to be excreted in breast milk, physicians are strongly cautioned to avoid treatment during pregnancy or while breast-feeding [35,53,71]. The BTX dosages indicated for facial aesthetic procedures are not thought to be sufficient to result in clinically significant interactions with other drugs; however, an awareness of the pharmaceuticals most likely to result in untoward consequences will allow the facial plastic surgeon to exert an ounce of prevention in such cases. Aminoglycosides, cyclosporine, muscle relaxants, magnesium sulfate, and lincosamide can exacerbate the inhibitory effect of BTX on neuromuscular transmission and result in states of flaccid paralysis that approximate classic botulism [15,16,35,53]. Diminished responses to BTX could result in patients treated with chloroquine or hydroxychloroquine, as these aminoquinolones interrupt BTX’s toxic actions within the presynaptic nerve terminal [71].
11. Conclusion Botulinal neurotoxins represent an effective adjuvant medical treatment or surgical alternative for those disorders that can be assuaged by the induction of a focal paresis in areas of undesired muscular contraction. The reliable anatomy of facial muscles and their well-defined roles in hyperdynamic rhytides have placed the facial plastic surgeon at the vanguard of the aesthetic applications of BTX. The knowledge surrounding these agents is increasing exponentially as both physicians and patients recognize their remarkable results, enormous spectrum of indications, and impressive safety profile. A review of the pharmacology of BTX, its commercial preparations, and the clinical data regarding its administration will allow facial plastic surgeons to develop treatment plans that carefully consider the serotype and formulation of BTX and its inherent properties, the diluent, reconstitution methods, specific volumes and concentrations to address particular concerns, methods for pain control, and dosing schedules. Knowledge of the basic science of BTX and the most recent literature concerning its administration is a fundamental responsibility of cosmetic surgeons as they endeavor to avoid complications and deliver maximal aesthetic results to their patients.
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