64 Chapter 3 definition, each migraine day in which a headache was present was also considered a headache day. An aura day without headache was considered a migraine day, but not a headache day. Patients with migraine were classified as CM according to the ICHD-3 criteria of ≥ 15 headache days per month, with ≥ 8 migraine days per month during 3 subsequent months.12 Because of important differences in clinical impact a further distinction was made between high frequent episodic migraine (HFEM) and low frequent episodic migraine (LFEM).13-15 Here, non-CM patients who had at least one month with ≥ 8 migraine days were included in the HFEM group. LFEM was determined as patients who had < 8 MMD during all registered months. For each group, MOH patients were defined according to the ICHD-3 criteria of ≥ 15 headache days per month with ≥ 15 analgesic days, or ≥ 10 triptan days, or ≥ 10 mixed acute medication days per month during 3 subsequent months. Number of MMD, number of monthly non-migrainous headache days and number of monthly days with use of acute medication were determined based on the E-diary entries. Missing days were considered headache free and acute medication free. In the E-diary, a month was set to a patient specific fixed period of 28 days.6 Data from months with less than 80% compliance were eliminated from the dataset as the data quality was considered insufficient. Data analysis and statistics Baseline characteristics of the entire cohort and subgroups were determined. For each characteristic, the group mean and standard deviation (SD) was calculated from the patients’ means for normally distributed variables, otherwise the median and interquartile range (IQR) was determined. Differences between the groups CM, HFEM and LFEM regarding nominal variables including sex and MOH diagnosis were evaluated using Chi-squared tests. Differences between groups regarding continuous variables such as monthly acute medication days were tested using one-way analysis of variance (ANOVA) tests. We used the migraine characteristics with a headache duration of ≥4 hours, in combination with triptan intake (effective or not) as the baseline definition for our analyses. We did not include other acute migraine-specific treatments as these were not used by our group of patients (e.g. ergotamines, ditans or gepants). We firstly evaluated the adjustment of the minimum headache duration by comparing the number of migraine days in our data defined by the baseline criteria (headache duration ≥4 hours) to the same criteria with a duration of ≥30 minutes. We investigated
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