Objective: The purpose of this study was to find the conversion factor, safety, and efficacy of type A to type B toxin for laryngeal muscles.
Methods: Thirty-two patients with adductor spasmodic dysphonia with stable doses of A toxin to manage their symptom were given type B toxin starting at a conversion of 1 U of BTX-A to 50 U of BTX-B. The patients were followed for 1 year, and doses adjusted according to response.
Results: The conversion factor was found to be 52.3 U:1 U. The onset of action of type B was more rapid (2.09 days vs 3.2 days [P = 0.028]), with a shorter duration of benefit (10.8 weeks vs 17 weeks [P = 0.002). The safety profile for A and B toxin appeared the same, with 3 patients receiving Myobloc reporting dry mouth.
Conclusion: This study shows that a conversion factor of 52.3:1 Myobloc (BTX-B) to Botox (BTX-A) and that Myobloc is an effective alternative to Botox (BTX-A) for patients with spasmodic dysphonia.