61 Defining migraine days 3 INTRODUCTION Determining the number of monthly migraine days (MMD) is essential for clinical practice and clinical trials or other research purposes. According to the current clinical trial guidelines, the number of MMD should be used as primary endpoint.1, 2 However, no clear consensus for the definition of a migraine day (MD) exists in the current clinical trial guidelines for migraine, for example about the minimal headache duration or how to take acute migraine-specific treatment intake into account.1, 2 In general, the number of MMD is often determined in retrospect through clinical interviews, paper diaries or commercial applications (apps), mostly by patients self-reporting and deciding whether a day was a migraine day.3-5 However, recall bias substantially reduces the reliability of retrospective reports during clinical consultation.6 Some currently used diaries lack specificity, because patients are asked to self-report migraine days without further symptom specifications. Moreover, these diaries are not time-locked, making it possible to fill out for a longer period of time in retrospect. The Leiden Headache Center uses an electronic diary (E-diary) developed in collaboration with Interactive Studios (The Netherlands): the Patient Journey App (www.patientjourneyapp.com). With this E-diary reliable information on frequency of headache and associated characteristics and features can be obtained, and it showed to be a validated and useful tool in both clinical and research settings.6-9 With the emergence of more advanced headache E-diaries aforementioned issues are resolved, but in our opinion there is a need for consensus on the definition of a migraine day based on detailed criteria. The International Classification of Headache Disorders (ICHD-3) was designed to diagnose migraine and its (sub)types on a patient level, but not on the level of every individual headache day. However, in practice, these criteria are often integrated in headache diaries to be able to define individual migraine days. This seems a very practical approach, but we make note of two important issues. Firstly, is it appropriate that days with use of migraine-specific acute medication are regarded as migraine days? It is likely that patients are instructed by their treating physicians to take acute medication before a complete migraine attack develops, and thus a migraine day may not develop into all required headache characteristics and associated symptoms. To avoid this problem, the ICHD-3 criteria currently state that headache attacks that meet the criteria for probable migraine should be counted as migraine. However, this remains a sub-optimal way to define migraine days as the ICHD-3 is primarily defined to classify patients and not individual days. Secondly, the required minimum headache duration used for defining a migraine day varies from at least 30 minutes
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