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Number of migraine attacks per month (mean ± SEM). Abbreviations: CM chronic migraine, EM episodic migraine, nVNS non-invasive vagus nerve stimulation, SEM standard error of the mean. The number of migraine attacks per month at baseline (blue bars) and after 3 months of treatment with nVNS (orange bars) in all patients, patients with EM, and patients with CM. Baseline measures were determined on the basis of patient reporting and medical history. Data are shown as mean ± SEM
