Efficacy and safety of long-term botulinum toxin treatment in craniocervical dystonia: a systematic review.

Botulinum neurotoxins have been shown to be a safe and effective therapeutic option for most forms of focal dystonia, and are now considered to provide the best symptomatic treatment in these disorders. However, only a few papers addressed the long-term efficacy and safety of repeated treatments with this drug. This article reviews the data from clinical trials that have assessed the long-term results of botulinum neurotoxin type A (BoNT-A) and type B in the treatment of the different forms of focal craniocervical dystonia, cervical dystonia (CD), blepharospasm, oromandibular, and laryngeal dystonia. Studies on the long-term effects of BoNT-A therapy have demonstrated that the majority of patients comply with this repeated treatment because they experience a positive and stable effect over time. It is still unclear whether in patients with focal dystonia the mean dose of BoNT-A changes over time. In spite of the wide spectrum of side effects reported to be associated with BoNT-A treatment, there is no evidence of specific side effects due exclusively to the long-term use of such drugs. The only exception to these positive long-term findings is the occurrence of a subgroup of patients with CD who fail to maintain a sustained response after the first or second effective treatment, partly owing to the development of neutralizing antibodies against the toxin. Longitudinal studies aimed at defining the risk factors for this abnormal pattern of response to botulinum toxin treatment are currently being conducted.