Botulinum Toxin in the Field of Dermatology: Novel Indications

Since its approval by the US Food and Drug Administration in 2002 for glabellar wrinkles, botulinum toxin (BTX) has been widely used to correct facial wrinkles. As a result, many consider BTX synonymous with cosmetic dermatology. Recent studies indicate that BTX elicits biological effects on various skin cell types via the modulation of neurotransmitter release, and it seems that BTX has a wider zone of dermatologic influence than originally understood. Clinicians and researchers are now beginning to explore the potential of BTX beyond the amelioration of facial lines and encouraging results are seen with BTX in a variety of skin conditions. In this paper, we review novel dermatological indications of BTX which includes (but not limited to) scar prevention, facial flushing, post-herpetic neuralgia and itch. These areas show great promise, but there is definite need for larger, double-blinded, randomized control trials against established treatments before BTX becomes a clinical reality.


Introduction
Botulinum toxin (BTX) is a potent neurotoxin produced by the bacterium Clostridium botulinum. Seven distinct isoforms (BTX-A, B, C, D, E, F, and G) have been described, with BTX-A and BTX-B being commercially available. BTX blocks the release of acetylcholine and a number of other neurotransmitters from presynaptic vesicles by deactivating SNARE proteins and has a long history of therapeutic application in neurological conditions with a strong efficacy and safety profile. As widely known, the skin interacts with the nervous system and there is increasing evidence that the neurological system directly participates in cutaneous inflammation and wound healing [1,2]. With that said, BTX has been used experimentally in a number of dermatological conditions which include scar prevention, facial flushing, post-herpetic neuralgia and itch with good results. The general mechanism which underlies these novel indications includes suppression of mast cell activity, and the inhibition of substance P, calcitonin gene-related peptide (CGRP) and glutamate release. In this review, we analyze the possible off-label applications of BTX based on published data.

BTX in Hypertrophic Scar Treatment
Scars are defined as marks that remain after the healing of a wound. They cause significant cosmetic concern, especially when located on conspicuous areas such as the head and neck.
Hypertrophic scars and keloids represent an aberrant response to the wound healing process and are characterized by dysregulated growth and excessive collagen formation [3].
BTX has been reported as a treatment measure for hypertrophic scars and keloids in a number of studies [4][5][6][7] (Table 1). In one study [4], BTX injection (2.5 IU/cm 3 ) was performed once a month for three months, leading to a significant decrease in erythema, itching, and pliability of the scar. In another study [7], 12 keloid patients received BTX injection (70-140 IU per session, every 3 months for a maximum of 9 months) and achieved more than 50% improvement in symptoms, size, height, and induration of the scar. In a randomized controlled trial (RCT) [5], the efficacy of BTX (5 IU/cm 3 , 3 sessions, repeated every 8 weeks) was compared with that of steroid injection (triamcinolone, kenacort 10 mg/cc, 6 sessions, repeated every 4 weeks) in keloids, where BTX led to a more significant reduction of subjective complaints (itch and pain of the scar).
The molecular mechanism of BTX on hypertrophic scars and keloids is not yet perfectly explained, but BTX has been shown to inhibit the proliferation of fibroblasts derived from hypertrophic scar tissues. In addition, BTX is reported to suppress the expression of transforming growth factor (TGF)-β1, collagen I and III, α-smooth muscle actin and myosin II protein in keloid fibroblasts [8][9][10][11].
One particularly favorable aspect of BTX is its ability to control the subjective symptoms of hypertrophic scars. BTX can immobilize the local muscles of a scar and reduce skin tension caused by the muscle pull [12]. This relieves trapped nerve fibers in keloids, neutralizing the itch and pain associated with small-fiber neuropathy [13]. Another advantage of BTX is the absence of skin atrophy and telangiectasia which is often seen after steroid injection.
The limitations of BTX on hypertrophic scars and keloids would be the high cost of the drug (with the dosages mentioned in prior studies) and its potential effect on the surrounding muscles. Due to these limitations, many suggest the use of BTX as an adjuvant rather than first line treatment for hypertrophic scars.

BTX in Scar Prevention
Nowadays, many acknowledge the role of active scar prevention important in post-operative scar management. A key factor that determines the final cosmetic appearance of a surgical scar is the tension that acts on the wound edges during the healing phase [14,15]. By blocking acetylcholine neurotransmitter release from peripheral nerves, BTX allows near-complete elimination of dynamic muscle tension on the healing wound. The tension relieving properties, together with the direct inhibitory effects of BTX on fibroblasts and TGF-β1 expression support its usage in surgical scar prevention [16][17][18]. The anti-inflammatory effect of BTX and its action of the cutaneous vasculature calms down the inflammatory phase (immediate to 2-5 days) of the wound healing process which may also contribute to scar prevention.
A number of studies have reported the effectiveness of BTX in scar prevention [19][20][21][22] (Table 2). In a split-scar RCT [19], the safety and efficacy of early postoperative BTX injection was assessed in 15 thyroidectomy scar patients. A single treatment with either BTX (20-65 IU) or 0.9% saline (control) was applied to fresh scars (within 10 days of thyroidectomy), where the BTX-treated halves showed a significantly better outcome in terms of scar scales and patient satisfaction compared to the saline treated sides. In 2006, Gassner [21] tested whether postoperative injection of BTX improved facial scars following forehead lacerations and excisions. BTX  was injected to post-op scars within 24 h after wound closure to produce enhanced wound healing and improved cosmesis compared to placebo (normal saline) injection.
BTX is best used for op scars. It would be optimal to inject BTX intraoperatively or shortly (preferably within days) after the surgery. To note, BTX should be avoided in open wounds as it delays wound closure.  Objective assessment Subjective assessment 12-16 months The outcome was considered highly satisfactory in 36 patients (90%). Thirty patients rated the improvement as marked (75%), six rated it as significant (15%), and four rated it as unchanged (10%). In 3 cases (7.5%), the scars re-widened, while in one case (2.5%), residual scar depression persisted.

BTX in Rosacea and Facial Flushing
Rosacea is a common inflammatory dermatosis characterized by persistent erythema, telangiectasia, papules, pustules, and facial flush. Oral medication, topicals, and laser therapy are routinely performed but often fail to relieve the facial flush. Persistent facial flushing is also a troublesome menopausal symptom.
A number of reports demonstrate the possible action of BTX on rosacea and menopausal hot flashes [23][24][25][26] (Table 3). In a prospective pilot study [23], the effect of BTX on the Dermatology Life Quality Index (DLQI) of patients with facial flushing was examined. BTX was injected once up to a total dose of 30 units on the cheeks which led to a significant decrease in DLQI at 2 months follow-up. Odo et al. [26] reported BTX (6.2 IU of abo-BTX per injection point, 40 points over the face, chest, neck, and scalp) to significantly reduce the mean number of menopausal hot flashes at day 60. The effect of abo-BTX was also investigated in 15 patients with rosacea. 15-45 IU of BTX was injected to the face which resulted in a statistically significant improvement in erythema at 3 months follow-up [24]. Adverse effects were rarely reported in the studies.
One possible mechanism by which BTX improves flushing is the potent blockade of acetylcholine release from autonomic peripheral nerves of the cutaneous vasodilatory system [27,28]. It is also well-known that BTX inhibits the release of inflammatory mediators such as substance P and calcitonin gene-related peptide (CGRP) [29]. The reduction and control of local skin inflammation may allow the erythema to fade out.
Larger, controlled, randomized studies are warranted to determine the optimal dose and duration of BTX activity on rosacea and facial flushing. BTX injection for facial flushing has additional benefits as it also improve the fine lines and wrinkles by diminishing the pull of the facial depressors.

BTX in Postherpetic Neuralgia
Postherpetic neuralgia (PHN) is the most frequent chronic complication of herpes zoster and the most common neuropathic pain resulting from infection. It is conventionally defined as dermatomal pain (usually a score of 40 or higher on a Likert scale ranging from 0-no pain to 100-worst possible pain), persisting at least 90 days after the appearance of the acute herpes zoster rash. PHN causes considerable suffering and results in a health care burden at both the individual and societal levels [30].
Treatment approaches include nonsteroidal anti-inflammatory drugs, gabapentin, opioids, and tricyclic antidepressants as well as topical anesthetics and capsaicin cream, but pain can be resistant to all of these drugs.
A number of reports have been made on the efficacy of BTX in PHN [31][32][33] (Table 4). Xiao et al. [33] performed a randomized, double-blind, placebo-controlled study on 60 PHN patients with the following arms: the BTX group, the 0.5% lidocaine group, and the 0.9% saline group. All patients were treated once (as for BTX, a dose of 200 IU at maximum), and were followed-up for 3 months. The BTX treated patients were found to have the most significant improvement in Visual Analog Scale (VAS) and sleep quality compared to those of the other two groups. Apalla et al. [32] also performed a RCT where 30 PHN patients received either BTX (200 IU in total) or placebo. BTX patients showed a significant reduction in VAS pain scores as well as the sleep scores which lasted for approximately 16 weeks. In a prospective study, Ding et al. [31] treated 58 PHN with BTX (50 to 100 IU in total) to find promising results (reduced frequency of pain attacks, lower pain severity, reduction in the quantity of painkillers consumed by patients) with very few adverse reactions.
The mechanism involved in the pain-relieving effect of BTX is still unclear, but it is thought that both the peripheral and central mechanism play a role [33]. The peripheral effects of BTX injection come through the inhibition of neuropeptide release from the peripheral nociceptive nerves [34,35]. In addition, BTX has been suggested to exert central nervous system (CNS) effects through axonal transport to the CNS after peripheral application [36,37].
Although promising, cost would be one of the main considerations to BTX use in PHN. Also, unlike other therapeutic modalities, BTX induces antitoxin antibodies which can limit the clinical effectiveness of the drug after repetitive, long-term use. The sweating and hot flashes were less severe than before abo-BTX treatment, especially at 2 months follow-up; In the control group, there was no significant difference in mean intensity of sweating or in the mean number of hot flashes.
Other menopausal symptoms such as night sweats were found better 2 months after abo-BTX treatment than in the control group (p < 0.001).
BTX: Onabotulinum toxin unless otherwise stated, Abo-BTX: Abobotulinum toxin. Compared with pretreatment, VAS pain scores decreased at day 7 and 3 months posttreatment in all 3 groups. However, the VAS pain scores of the BTX group decreased more significantly compared with lidocaine and placebo groups at day 7 and 3 months posttreatment (p < 0.01). Sleep time improved in all 3 groups but was most significant in the BTX group compared with the lidocaine and placebo groups (p < 0.01). The percentage of subjects using opioids posttreatment in the BTX was the lowest (21.1%), compared with the lidocaine (52.6%) and placebo (66.7%) groups (p < 0.01).

BTX in Pruritus
Pruritus (also known as itch) is an unpleasant sensation of the skin leading to the desire to scratch. Among the 4 subtypes (pruriceptive, neurogenic, neuropathic, and psychogenic itch), the pruriceptive itch is a peripherally induced pruritus arising from the skin and mucosa and is often seen in dermatological disease.
A number of reports have been made on the efficacy of BTX in pruriceptive pruritus [38][39][40] (Table 5). Recalcitrant pruritus is a hallmark of lichen simplex, a localized variant of atopic dermatitis. In an open pilot study [40], BTX (abo-BTX, 20-80 IU) was injected intradermally into 5 circumscribed lichenoid lesions with recalcitrant pruritus. Within a week, all patients reported to have noticeable alleviation of itching and at 12 weeks, all were still free from the uncontrollable urge to scratch. Itch is also a common and well-recognized problem in burns [38]. Nine patients with recalcitrant itching secondary to burns were treated with BTX (dosage not specified) where the burn itch fell to 0 out of 10 in 4 weeks. The average duration of symptom free period was reported as nine months.
As clinical evidence has revealed the antipruritic effect of BTX, Arendt-Nielsen et al. [39], investigated the effect of subcutaneous administration of BTX on experimentally histamine-induced itch in human skin. In this double-blind, placebo-controlled study, 14 healthy men received BTX and isotonic saline on the volar surface of either forearm. Histamine prick tests were performed four times at the treatment sites (before treatment, and days 1, 3, and 7 after treatment) where BTX reduced the histamine-induced itch intensity, and itch area compared with saline at all time points.
Several possible mechanisms can be responsible for the reduction of pruriceptive itch. Acetylcholine mediates itch in pruritic skin conditions such as atopic dermatitis [41] and BTX inhibits the release of acetylcholine from presynaptic vesicles [42]. BTX is also known to interact with molecules associated with itch and flare such as substance P (releases histamine via the activation of mast cells, promotes vasodilation) and CGRP (a potent vasodilator) [36,[43][44][45]. BTX inhibits the release of such mediators, thus reducing the sensation of itch. Lastly, BTX has also been shown to stabilize mast cells and inhibit their degranulation [46].
Pruritogenic pruritus is usually accompanied by skin inflammation. Since BTX is capable of reducing neurogenic inflammation [29], it is natural to expect improvement of the primary skin disease (e.g., atopic dermatitis, psoriasis) as well, which has in fact been reported through animal studies [47,48], human studies [49] and case reports [50,51]. Although promising in pruritogenic itch (and also in inflammatory dermatoses), we feel that BTX would be best used as an adjunct to conventional therapy. It should be applied focally, considering that the product can induce muscle weakening.

BTX in Dermatological Conditions Associated with Hyperhidrosis
A number of skin disease are caused by and/or have symptoms that are exacerbated by hyperhidrosis, a condition that can be treated successfully with BTX.
Pompholyx or dyshidrotic eczema is a common vesiculo-bullous disease of the palms and/or soles. A hallmark of this disease is its tendency to relapse in response to various provoking factors which includes wet work, occlusion and hyperhidrosis. An intra-individual study of 10 patients [52] ( Table 6) investigated the use of BTX (mean dose of 162 IU per palm) for pompholyx, using the using the untreated site as control. 70% of patients reported a marked improvement of both sweating and itching on the treated site after 6 weeks. In another side-by-side trial [53], dyshidrotic hand eczema was treated with BTX (100 IU per palm) as an adjunct to topical steroids. Six patients who completed the study were found to have improved symptoms of pompholyx and reduced number of relapses by BTX injection.
The anhidrotic effect of BTX in pompholyx can be explained by its action on smooth muscles surrounding the sweat glands and through the inhibition of acetylcholine release. Inhibition of substance P release also explains the reduction in pruritus [54,55].
Hidradenitis suppurativa (HS) is a chronic inflammatory dermatosis of the apocrine glands which typically affects the axillae and groin. Patients afflicted by HS have severe discomfort and treatment is extremely challenging. It is well-known that a moist environment in folds, especially in the axilla and groin, provides ideal conditions for the flourishing of bacteria and is a precipitating factor of HS.
In 2005, HS on the axillae was first reported to be successfully treated with BTX (abo-BTX, 250 IU) with 10 months of complete remission [56] (Table 6). Khoo et al. [57] also confirmed the efficacy of BTX in HS where a 46-year-old woman with Hurley stage 2 HS responded well to axillary BTX treatment (50 IU per side) with a remission period of 12 months. The patient had been recalcitrant to conventional treatments and also underwent surgical drainage.
The exact mechanism by which BTX affects the disease process in HS is unclear but it is likely that the effect of BTX on sweat production reduces the population of skin flora and its potential inflammatory effect [56,57]. A second hypothesis is that by inhibiting apocrine secretion, BTX prevents the rupture and spread of follicular material from the pilosebaceous unit [57].
BTX has been studied in inverse psoriasis ( Table 6) which is also thought to be exacerbated by excessive sweating. A pilot study of 15 patients with flexural psoriasis [49] showed that 50-100 IU of BTX improved subjective symptoms and objective photographic evidence of disease in 87% of patients at 2, 4, and 12 weeks follow-up. It is hypothesized that the beneficial effects of BTX in inverse psoriasis is largely due to the reduction of local sweating in folds [49]. Patients with psoriasis are also known to have a higher concentration of substance P receptors in their skin [58,59], meaning that BTX can reduce pruritus and vasodilation by inhibiting neuropeptide liberation (and preventing substance P binding to multiple receptors).
Hailey-Hailey disease is an autosomal dominant acantholytic disorder with mutation of the ATP2C1 gene, clinically manifesting as macerated flexural erythema. Heat and sweat aggravate the disease, worsening the discomfort and pruritic symptoms.
Several case reports [60,61] (Table 6) have evidenced improvement of Hailey-Hailey disease with the use of BTX (50-125 IU per side). In one study, the effect of BTX was found to be comparable to that of laser ablation and dermabrasion [61].
BTX can rationally ameliorate the symptoms of Hailey-Hailey disease via its inhibition of acetylcholine and substance P release from the nerve endings [54,55] (Table 6). Although more clinical evidence is needed to prove effectiveness, BTX may be considered as a possible treatment modality for Hailey-Hailey disease recalcitrant to conventional treatment.

BTX in Oily Skin
Sebum contributes to the delivery of fat-soluble antioxidants to the skin surface and has antimicrobial activity, thereby functioning as a skin barrier. However, excess sebum blocks the pores, provides nourishment to bacteria, and can result in skin inflammation (e.g., acne, seborrheic dermatitis).
Recently, insights into the effect of BTX on sebum production have been published [62,63] (Table 7). Min et al. [62] randomly assigned 42 volunteers with forehead wrinkles to receive 10 or 20 units of BTX, which was administered in five standard injection sites. Treatment with BTX exhibited significant sebum reduction at the injection site of both groups, with a sebum gradient surrounding the injection point. The efficacy did not improve significantly with higher injection doses and the sebum production recovered to normal levels at 16-week follow-up for both treatment groups. Rose and Goldberg [63] also evaluated the safety and efficacy of BTX on the oily skin of 25 subjects. A 10-point injection was made with BTX (abo-BTX, total amount of 30-45 IU) on the forehead to find significantly lower sebum production and high patient satisfaction.
The mechanism by which intradermal BTX injection results in decreased sebum production is not entirely clear because the role of the nervous system and acetylcholine on sebaceous glands is not well defined. However, it is most likely that the arrector pili muscles and the local muscarinic receptors in the sebaceous glands are targets for the neuro-modulatory effects of BTX. Li et al. [64] demonstrated that nicotinic acetylcholine receptor α7 (nAchRα7) is expressed in human sebaceous glands in vivo, and acetylcholine signal increased lipid synthesis in vitro in a dose-dependent manner. Further study is needed to determine the best candidates, optimal injection techniques and doses.  There was no evidence of active inflammation on follow-up at a fortnight after administration. The patient had complete remission of symptoms until approximately 10 months later, when the first symptoms of mild inflammation re-appeared. The location of the psoriasis was as follows: armpits (7 patients), sub-mammary sulcus (6), intergluteal folds (7), inguinal folds (5)   Treatment with BTX exhibited significant sebum alteration at the injection site of both groups (10 IU, 20 IU), with a sebum gradient surrounding the injection point. The efficacy did not improve at higher injection doses, with the four-unit regimen generally not being more potent than the two-unit regimen. The sebum production recovered to normal levels at the 16-week follow-up for both treatment groups, indicating that a higher dosage (4 units) did not results in a longer duration until relapse compared with the two-unit dose.

months
Treatment with BTX resulted in significantly lower sebum production at 1 week and 1, 2, and 3 months after injection (p < 0.001). Twenty-one patients (91%) reported that they were satisfied (50-75% improvement) with intradermal BTX as a treatment for oily skin.

Conclusions
In this review, we highlighted the promising outcomes of BTX in several off-label indications of interest for dermatologists. There is overwhelming evidence that BTX exhibits biological effects on many human cell types, but much is yet to be learned about the drug and its mechanism of action. Knowing that the skin closely interacts with the nervous system, future studies should investigate the link between BTX and the cutaneous neuroimmune system to better understand its therapeutic potential in dermatology. A consensus on the dose regimen and injection technique is also desirable for standardized treatment. Generally, high doses of BTX were applied, with an average total of 300 IU for hypertrophic scars, 50 IU for scar prevention, 50-100 IU for facial flush/rosacea, 100 IU for PHN, 150 IU for pompholyx, 100 IU for HS, 75 IU for inverse psoriasis and 250 IU for Hailey-Hailey disease. Lastly, with the limitations of BTX treatment (high cost, muscle weakening, risk of tachyphylaxis and production of antibodies), BTX may be optimally used as an adjunct in recalcitrant cases to conventional therapy.