Migraine is a debilitating headache disorder. Including both episodic and chronic forms, it affects 14% of the population, and up to 18% of women [1
]. Migraine is currently ranked by the World Health Organization (WHO) as 19th among causes for years lived with disability [1
]. Given the current barriers, improving diagnosis and optimizing treatment paradigms could substantially reduce this global burden.
Because there are no biological markers for migraine, diagnosis is based on clinical history and the exclusion of other headache disorders. Health care professionals apply clinical criteria to guide diagnoses and subsequent treatment. The definition of migraine without aura from the second edition of the International Classification of Headache Disorders (ICHD-2) requires all of the following symptoms: a) recurrent headaches (at least 5 lifetime attacks); b) untreated or unsuccessfully treated headache duration of 4 to 72 h; and c) at least two of the following pain characteristics: unilateral, pulsating, moderate or severe intensity, or aggravated by routine physical activity. In addition, the migraine attacks are associated with at least one of nausea/vomiting, photophobia, or phonophobia. Finally, other causes of headache must be excluded [3
Episodic migraine (EM) is characterized by those with migraine who have 0 to 14 headache days per month, while chronic migraine (CM) is characterized by 15 or more headache days per month. Specifically, revised ICHD-2 (ICHD-2R) criteria define CM as headache on 15 or more days per month for 3 or more months, of which 8 or more days meet criteria for migraine without aura and/or respond to migraine-specific treatment, occurring in a patient with a lifetime history of at least five prior migraine attacks not attributed to another causative disorder and no medication overuse [4
The relationship between EM and CM is complex. EM progresses to CM at the rate of 2.5% per year [5
], and CM often remits to EM (2-year transition rate of 26%) [6
]. The use of a frequency score of 15 or more days per month to classify CM is admittedly somewhat arbitrary. Nonetheless, these clinical definitions identify groups that differ in epidemiologic and symptom profiles, functional consequences and disabilities, indirect and direct costs, patterns of consultation and treatment, and rates of comorbidities. In addition, the patterns of treatment response for EM and CM differ, raising the possibility of both overlapping and distinct biological mechanisms.
Large observational studies have provided valuable information on the distinct clinical characteristics observed in CM and EM [7
••]. Much of the recently published data that highlight the epidemiological distinction between CM and EM have been generated by three large observational studies: the International Burden of Migraine Study (IBMS), the American Migraine Prevalence and Prevention (AMPP) study, and the German Headache Consortium (GHC) study. IBMS is a web-based, cross-sectional, multinational survey that identified and evaluated persons with CM and persons with EM [7
••]. The AMPP study is a large United States (US) population–based, mail-based, longitudinal survey that identified 24,000 respondents with headache and followed them annually for 5 years (2004–2009) [8
••]. The GHC study is a German population–based longitudinal survey where respondents completed questionnaires via mail (n
4642) or phone (n
4708) and were identified as either CM, high-frequency EM (9–14 headache d/mo), or low-frequency EM (0–8 headache d/mo) and then evaluated on an annual basis [10
Herein and with an emphasis on recent key findings, this article provides an update on the similarities and differences between CM and EM in their epidemiologic and symptom profiles, functional consequences and disabilities, indirect and direct costs, patterns of consultation and treatment, and rates of comorbidities.