Botox, Dysport and the Pelvic Floor: Why Brand Matters Less Than Mechanism

OnabotulinumtoxinA (Botox) is widely used in colorectal and pelvic floor practice, and institutional protocols are often expressed in Botox units. However, some health services use abobotulinumtoxinA (Dysport), and formulary changes can necessitate brand switching. During colorectal and pelvic floor training in New South Wales, Botox was the routine preparation; following relocation to Cairns, a health service switch to Dysport, largely driven by procurement considerations, highlighted practical questions about unit conversion and clinical equivalence for injections into the internal anal sphincter and pelvic floor. This Perspective reviews the shared mechanism of botulinum toxin type A preparations, the evidence comparing Dysport and Botox for anorectal and pelvic floor indications, and the clinical governance needed when switching brands. Historically, Dysport is not a late arrival. The modern Botox story begins with Alan Scott's development of ‘Oculinum’ for extraocular muscle injection in the 1970s and 1980s, leading to US Food and Drug Administration approval in 1989 and subsequent rebranding as Botox [1]. Over the following decades, its licence expanded across neuromuscular, autonomic and cosmetic indications. In parallel, abobotulinumtoxinA was developed in the United Kingdom by doctors who had trained with Scott and was launched as Dysport; it has now been in routine clinical use for more than 25 years and is licensed in many countries for dystonia, spasticity and aesthetic indications [2]. From a historical perspective, Botox and Dysport are true contemporaries rather than older and newer siblings. The perception that Dysport is ‘new’ reflects local formulary choices and the timing of approvals, not immaturity of the product. Mechanistically, all botulinum toxin type A (BoNT-A) preparations share the same core 150 kDa neurotoxin. After binding to presynaptic terminals of cholinergic nerves, the toxin is internalised, its light chain translocates into the cytosol and cleaves SNAP-25, a SNARE protein required for calcium-dependent synaptic vesicle fusion [3]. This blocks acetylcholine release and produces a reversible chemodenervation of the target muscle. Clinically, the effect emerges over several days, peaks at around 2–4 weeks and gradually wanes over 2–4 months as new synapses form [3]. BoNT-A also modulates autonomic efferents and sensory afferents; experimental data suggest reduced release of noradrenaline, substance P, calcitonin gene-related peptide and glutamate, which is highly relevant to anorectal and pelvic floor pain syndromes [3]. Chronic anal fissure provides a clear example of BoNT-A pharmacomechanics. It is characterised by a hypertonic internal anal sphincter (IAS), elevated resting anal pressure and relative ischaemia at the posterior midline, with a pain–spasm–ischaemia cycle that impairs healing [4]. Lateral internal sphincterotomy reliably heals fissures by permanently dividing part of the IAS, but at the cost of a small risk of continence disturbance, particularly in multiparous women or those with pre-existing sphincter damage [4]. BoNT-A injection into the IAS aims to achieve the same physiological goal without structural division. Ex vivo work on porcine IAS by Jones et al. [5] showed that BoNT-A predominantly disrupted sympathetically mediated, noradrenergic excitatory transmission while preserving nitrergic relaxation, supporting the concept of a pharmacological ‘chemical sphincterotomy’ that lowers resting pressure and improves perfusion. Clinical studies confirm that BoNT-A injection into the IAS improves pain and promotes healing in a substantial proportion of chronic fissures, with adverse events largely limited to transient flatus incontinence [4]. For clinicians asked to switch from Botox to Dysport, the key question is whether comparable clinical effects are achieved once unit potency is accounted for. A randomised trial by Brisinda et al. [6] addressed this directly, allocating 100 patients with chronic anal fissure to receive either 50 units of Botox or 150 units of Dysport injected into the IAS, a 1:3 conversion ratio. At 2 months, fissure healing was 92% in the Botox group and 94% in the Dysport group, with comparable reductions in resting pressure and similar rates of minor, self-limiting incontinence [6]. In a related randomised study, BoNT-A (30 units of Botox or 90 units of Dysport) was compared with 0.2% nitroglycerin ointment. Healing at 2 months was 92% with BoNT-A and 70% with nitroglycerin, and nitroglycerin caused substantially more adverse effects, mainly headache [7]. Berkel and colleagues then randomised patients to 60 units of Dysport versus isosorbide dinitrate 1% ointment and again found superior healing and symptom relief in the Dysport arm [8]. A NICE evidence summary reviewing these and other data concluded that BoNT-A is less effective than lateral internal sphincterotomy for fissure healing, but offers similar or better healing than topical nitrates or calcium channel blockers with a much lower risk of long-term incontinence [4]. Beyond fissures, BoNT-A has been used off-label in dyssynergic defaecation. Here the target is inappropriate contraction or failure of relaxation of the puborectalis and external anal sphincter during attempted defaecation. By reducing striated muscle tone via the same SNAP-25-dependent chemodenervation mechanism, BoNT-A is intended to straighten the anorectal angle and lower outlet resistance. A systematic review by Chaichanavichkij et al. [9] identified 11 studies (including three randomised trials) in adults with dyssynergic defaecation; improvement rates ranged from 29% to 100%, using Botox doses of 12–100 units and Dysport doses of 100–500 units. A more recent systematic review and meta-analysis by Chu et al. [10] reported short-term symptomatic improvement in about two-thirds of patients but diminishing benefit beyond 12 months; BoNT-A improved anorectal physiological measures but did not clearly outperform biofeedback. Overall, the evidence supports BoNT-A as a second-line or salvage therapy in selected patients after failure of conservative management, rather than a first-line treatment. Where brand was specified, both Botox and Dysport were used, with doses again consistent with a 1:2.5–3 conversion ratio and no evidence of clinically meaningful differences in efficacy [9, 10]. Myofascial pelvic floor pain and pelvic floor tension myalgia represent another emerging indication. Women may present with chronic pelvic pain, dyspareunia and bowel or bladder symptoms. On examination, the pelvic floor is hypertonic and tender, with trigger points. BoNT-A injections into the levator ani, puborectalis, or obturator internus are hypothesised to break a cycle of muscular spasm and pain while also reducing peripheral nociceptive input [3]. A systematic review and meta-analysis by Meister et al. [11] found a statistically significant reduction of about 20 points on a 0–100 pain scale at 6 weeks and beyond 12 weeks after BoNT-A injection, with improvements in dyspareunia, dyschezia and quality of life. Adverse events such as transient urinary retention and temporary pelvic floor weakness were reported but usually self-limited [11]. Most included studies used Botox, some used Dysport, and all were small and heterogeneous; nevertheless, there is no evidence that Dysport behaves differently from Botox when dose is adjusted appropriately [9-11]. From a practical standpoint, the key distinction between Botox and Dysport is unit potency, not mechanism. Each manufacturer defines a ‘unit’ using its own bioassay, so units are not numerically interchangeable. A review of comparative data across spasticity, dystonia and cosmetic indications suggests that an onabotulinumtoxinA: abobotulinumtoxinA dose ratio of approximately 1:2.5–3 yields similar efficacy and safety, whereas higher ratios risk overdosing Dysport [12]. The fissure trials that used 50 units of Botox versus 150 units of Dysport and 30 versus 90 units, implicitly adopt this ratio and demonstrated equivalent outcomes [6-8]. For pelvic floor injections, reported Botox and Dysport doses are similarly consistent with a 1:2.5–3 conversion [9-11]. Switching BoNT-A brands can feel unsettling, particularly when injections involve the internal anal sphincter and pelvic floor where complications matter to patients. The historical and pharmacological data are reassuring: Dysport is a mature BoNT-A preparation with decades of use; the molecular mechanism is identical; and randomised fissure data plus broader neurology literature support a roughly 1:2.5–3 conversion with comparable clinical effects. The real challenge is organisational. Brand switching should be accompanied by explicit conversion protocols, opportunities for hands-on training and prospective audit of healing, continence and complications. If these elements are in place, the focus can return to what matters most in anal fissure and pelvic floor practice: selecting the right patients, using sound injection technique and integrating BoNT-A into multidisciplinary care, rather than brand labels. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.