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Respective potencies of Botox and Dysport: a double blind, randomised, crossover study in cervical dystonia
  1. W Poewe
  1. Universitätsklinik für Neurologie, Universität Innsbruck, Anichstraβe 35, A-6020 Innsbruck, Austria
  1. Correspondence to: 
 Dr W Poewe; 

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Clinically appropriate conversion factor may be less than three

The issue of apparently different potencies of the two available formulations of botulinum toxin type A—Dysport and Botox—has continued to perplex clinicians for more than a decade. Empirically chosen doses expressed in mouse units in different series and different indications reported in the literature seemed to differ by factors of three to six.1,2 To date only two randomised controlled studies have tried to answer the question of what the correct conversion factor yielding bioequivalence should be. One was conducted in previously untreated patients with blepharospasm or hemifacial spasm and found a bioequivalence ratio of Botox to Dysport of 1:4 with duration of effect as the primary outcome variable.3 The second comparative trial randomly assigned patients with cervical dystonia previously treated with Botox to receive either their clinically defined individual dose of Botox or three times that dose as Dysport units4 and found similar effect size, duration of effect, and rates of adverse events. In the paper by Ranoux et al (this issue pp 459–462)5 of this issue, results of another double blind randomised study comparing efficacy and safety of the type A preparations seem to suggest that the clinically appropriate conversion factor may be less than three.

Fifty four patients with cervical dystonia and a satisfactory response to two consecutive injections of Botox at the same dose into identical muscles received three successive treatments of either their usually effective dose of Botox or three or four times that dose of Dysport. Treatments were given in randomised order using identical volumes of injection and muscle patterns. The effect size as assessed by changes in Tsui scores and Toronto Western spasmodic torticollis rating scale (TWSTRS) pain scores was significantly greater with both Dysport treatments and duration of effect was also longer. Threefold or fourfold doses of Dysport produced similar effect sizes but duration tended to be increased with the fourfold dose. Side effects were significantly more frequent with both Dysport doses than with Botox but again not significantly different between the two Dysport doses (17.6% of patients treated with Botox, and 33% and 36% of patients treated with Dysport 1:3 and 1:4, respectively).

In summary, the authors suggest that even lower conversion ratios be used than 3:1 for Dysport to Botox. Should it then be 1:2.5 or even 1:2? If so, should we be using lower doses of Dysport or higher doses of Botox to achieve this? With only three randomised trials available differing in design, target population, and results, it is impossible to give a conclusive answer to this question. For the time being clinicians may be best advised to use the following landmarks for their dosing decisions when treating patients with dystonia. Firstly, the equivalence ratio of Dysport to Botox should not be greater than 3:1 according to the majority of available comparative clinical studies. Secondly, for cervical dystonia, the indication studied by Ranoux et al, a double blind dose ranging study has shown that Dysport doses needed for a satisfactory response are greater than 250 units and that doses greater than 500 units are associated with clear increases in adverse event frequency and severity.

Clinically appropriate conversion factor may be less than three



  • Conflict of interest: WP has been reimbursed by Allergen Inc, the manufacturer of Botox and IPSEN, the manufacturer of Dysport, for speaking at conferences or educational courses. His department has also received grants from both.