Sporadic failure of botulinum toxin treatment in usually responsive patients with adductor spasmodic dysphonia

Botulinum toxin (BT) injections into vocalis (thyroarytenoid) muscle is currently considered the first-choice treatment for adductor spasmodic dysphonia, producing improvement for an average period of 3 months. In our experience, sporadic failure of BT efficacy can occur even in patients usually responsive to this therapy. The reasons for these episodes have not been clarified. In a retrospective, open trial, we investigated the effect toxin preparation (Botox or Dysport) and injection monitoring (electromyography or laryngoscopy) on the success rate of BT treatment. We studied 15 patients with adductor dysphonia usually responsive to BT therapy. BT was administered into the vocalis muscle in 112 and 36 injections under electromyographic or laryngoscopic guidance, respectively. Botox and Dysport were used in 106 and 42 sessions, respectively. In 29% of all injections, no subjective or objective changes, nor side effects were observed. Failure rate did not differ using electromyographic (28.6%) or laryngoscopic (30.5%) guidance. Failure rates with Botox and Dysport were 30.2% and 26.2%, respectively, but this difference was not statistically significant. These data suggest that treatment failure may occur regardless of the method of injection and of the drug preparation used, possibly due to mislocalisation of vocal folds.