A Cochrane review «Botulinum toxin type A therapy for blepharospasm»1 «Costa J, Espírito-Santo C, Borges A et al. Botulinum toxin type A therapy for blepharospasm. Cochrane Database Syst Rev 2005;(1):CD004900. .»1 identified no suitable studies to be included in the systematic review. Few controlled trials were found but these were excluded mainly because of methodological problems and/or the type of patients included. The excluded 13 trials found BtA to be superior to placebo as did large case-control and cohort studies, with around 90% of patients benefiting. The most common adverse effects affected the eyes and were short lived.
The authors conclude, that the effect size seen in open studies makes it very difficult and probably unethical to perform new placebo-controlled trials of efficacy of BtA for blepharospasm.