Are Episodic and Chronic Migraine One Disease or Two?

Migraine is a debilitating headache disorder that has a significant impact on the world population, in both economic and sociologic capacities. Migraine has two main categories: (1) chronic migraine (CM), defined as the patient having 15 or more headache days per month, with at least five attacks fulfilling measures for EM with aura or EM without aura, and (2) episodic migraine (EM), defined as less than 15 headache days per month. With this definition, CM can only exist in the presence of EM, and it questions whether the two are separate diseases. Migraine has a significant impact on the population, as each year, about 2.5 % of patients with EM develop new-onset CM (Manack et al., Curr Pain Headache Rep 15:70-78, 2011) (Loder et al. Headache 55:214-228, 2015), with certain risk factors being evident only with CM. In addition, there are comorbid diseases that are only associated with CM, suggesting two separate diseases rather than one. Differentiation in response to treatments, both preventive and abortive, demonstrates both a similarity and a difference in EM versus CM. Also, comparing the two processes based upon functional imaging has been a recent development, beginning to show a physiological difference in regional cortical thickness, cortical surface area, and regional volumes in patients with EM and CM. Evidence regarding whether EM and CM demonstrate one disease with a significant level of complication or if two independent processes is inconclusive, and additional research must be performed to further characterize their relationship.