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Conceptualizing the relationship between chronic migraine and episodic migraine

Migraine is commonly divided into two broad forms, episodic migraine (EM) and chronic migraine (CM). EM occurs less than 15 days per month and is characterized by attacks of pain and associated symptoms, including nausea, photo phobia, phonophobia and aura in various combinations. According to the International Classification of Headache Disorders, Second Edition (ICHD-2), CM, is characterized by headaches that occur on 15 or more days per month; on at least 8 of those days the headaches must either meet diagnostic criteria for migraine without aura or respond to migraine-specific treatment [1]. In ICHD-2, EM and CM are distinguished by a clearly defined, but somewhat arbitrary, boundary of headache frequency. Efforts to conceptualize the relationship between CM and EM are rare. ICHD-2 conceptualizes CM as a complication of EM, viewing it as a form of very high frequency EM. An alternative view suggests that CM and EM are qualitatively distinct, that they differ not just in degree but also in class. This perspective postulates that CM may involve brain mechanisms distinct from those of EM. Herein, we argue that CM and EM should be conceptualized as distinct but intimately related disorders. Delineating the boundary between related conditions is a classic problem in classification research [2]. Ideally, disease classification is based on causes and mechanisms (etiologic classification), but when etiologic classification is not possible symptom profiles are used (symptomatic classification). In the ICHD-2, secondary headache disorders are classified based on their etiology as headaches attributable to an underlying cause, such as an infection or a mass lesion. For primary headache disorders, such as CM and EM, the exact etiology is unknown and so diagnostic boundaries must be defined in other ways. In the absence of an etiologic gold standard, conditions can be distinguished based on a series of features, including symptoms and signs, clinical course/prognosis, epidemio logic profiles, comorbidities, treatment response and family history/ genetics, as well as neuroimaging findings and biomarkers [2]. No single feature can be used to reliably determine whether EM and CM differ simply in degree or also in kind. These judgments rely on the overall pattern of features and ultimately on insights into the etiopathogenesis as they emerge. We will discuss CM and EM taking these features into account (Table 1). Chronic migraine and EM differ in attack frequency and in clinical course. In general, EM is characterized by a relatively stable course with a headache frequency from 1 to 4 days per month ranging up to 14 days per month [3,4]. By contrast, CM is characterized by a phase of headache acceleration followed by a plateau phase with attacks occurring from 15 to 30 days per month [4–6]. CM is far more disabling than EM and more likely to be persistent [6–8]. Chronic migraine and EM also differ in their epidemiologic profiles, risk factors and comorbidities. In the general population, individuals with CM are older, have lower levels of education, lower household income and are less likely to be working full time [9–11]. Risk factors for EM and CM overlap but differ in important ways. For example, female gender is a risk factor for the new onset of EM but it is not a risk factor for progression to CM [9,12,13]. Richard B Lipton, MD