Botulinum toxin type A is an effective preventive therapy for chronic migraine. Although the guidelines suggest a 50U/ml dilution of OnabotulinumtoxinA (BoNT/A), many clinicians use more concentrated solutions. However, there are no studies regarding the effect and safety of 100U/ml BoNT/A dilution with the saline solution following the PREEMPT paradigm. Our primary goal was to evaluate the efficacy, in reducing migraine frequency, and safety of two different BoNT/A dilutions (100U/ml vs 50U/ml) in the treatment of Chronic migraine. Our secondary goal was to determine the predictors of BoNT/A response. We retrospectively collected data from 113 chronic migraine patients treated with 3 rounds of BoNT/A according to the PREEMPT protocol as a preventive therapy. Patients were divided into two groups, based on BoNT/A dilution: 50U/ml (49 patients) vs. 100U/ml (64 patients) of sodium chloride 0.9%. We compared the migraine days/month, intensity, and intake of symptomatic medications at the baseline with the data obtained after the treatment; moreover, we evaluated the occurrence of adverse effects observed in the two groups. There was no difference regarding efficacy and safety between the two groups except for eyelid ptosis, which was more common in the 50U/ml BoNT/A group (p 0.018). Unilateral localization of migraine was associated with a more favorable outcome (OR 5.593, C.I. 2.358–13.268; p < 0.001) while Major Depressive Disorder predicted a less favorable response (OR 0.213, C.I. 0.087–0.523; p < 0.001). In our study, BoNT/A dilution did not influence the response to the therapy, but 100U/ml dilution could reduce the risk of eyelid ptosis. Unilateral localization of migraine pain might predict a more favorable response to the therapy, while the presence of a Major Depressive Disorder might predict a less favorable response.
