Introduction to the Special Series on the Chronification of Headache: Mechanisms, Risk Factors, and Behavioral Strategies Aimed at Primary and Secondary Prevention of Chronic Headache

Chronic Migraine: An Update on Physiology, Imaging, and the Mechanism of Action of Two Available Pharmacologic Therapies

Several lines of research support the hypothesis that migraine is a spectrum of illness, with clinical symptoms that vary along a continuum from episodic migraine to chronic migraine. Physiologic changes may result in episodic migraine evolving into chronic migraine over months to years in susceptible individuals. With chronification, headache frequency increases, becoming more disabling and less responsive to therapy. Neurophysiologic and functional imaging research has reported that chronic migraine may be associated with severity‐specific metabolic, functional, and structural abnormalities in the brainstem. Without longitudinal studies, it is unclear whether these changes may represent a continuum of individual progression and/or are reversible. Furthermore, chronic migraine is associated with larger impairments in cortical processing of sensory stimuli when compared with episodic migraine, possibly caused by more pronounced cortical hyperexcitability.

Progressive changes in nociceptive thresholds and subsequent central sensitization due to recurrent migraine attacks in vulnerable individuals contribute to the chronic migraine state. This may result in changes to baseline neurologic function between headache attacks, evident in both electrophysiological and functional imaging research. Patients experiencing migraine chronification may report increased non‐headache pain, fatigue, psychiatric disorders (eg, depression, anxiety), gastrointestinal complaints, and other somatic conditions associated with their long‐term experience with migraine pain.

Recent research provides a foundation for differentiating episodic and chronic migraine based on neurophysiologic and neuroimaging tools. In this literature review, we consider these findings in the context of models designed to explain the physiology and progression of episodic migraine into chronic migraine, and consider treatment of chronic migraine in susceptible individuals. Advances in pharmacotherapy provide treatment options for chronic migraine. Of the currently available treatment options, only onabotulinumtoxinA and topiramate have received regulatory approval and have demonstrated efficacy in patients with chronic migraine, although the exact mechanisms of action are not fully elucidated.

Mechanism of chronic migraine

Chronic migraine typically evolves from episodic migraine over months to years in susceptible individuals. Headaches increase in frequency over time, becoming less intense but more disabling and less responsive to treatment. Results of electrophysiologic and functional imaging studies indicate that chronic migraine is associated with abnormalities in the brainstem that may be progressive. Additionally, chronic migraine is associated with a greater degree of impairment in cortical processing of sensory stimuli than is episodic migraine, perhaps due to a more pervasive or persistent cortical hyperexcitability. These findings fit with the model of migraine as a spectrum disorder, in which the clinical and pathophysiological features of migraine may progress over time. This progression is postulated to result from changes in nociceptive thresholds and ensuing central sensitization caused by recurrent migraine in susceptible individuals, for whom a variety of risk factors have been described. This may lead to changes in baseline neurologic function between episodes of headache, evident not only in electrophysiologic and functional imaging studies, but also as an increase in depression, anxiety, nonhead pain, fatigue, gastrointestinal disorders, and other somatic complaints that may occur after years of episodic migraine. From the current research and migraine models, a conceptualization of chronic migraine, in which relatively permanent and pervasive central changes that warrant novel and tolerable treatments have occurred, is emerging. This model also implies that prevention of chronic migraine is an important goal in the management of episodic migraine, particularly in individuals who exhibit risk factors for chronic transformation.

Chronic headache: the role of the psychologist

The role of the psychologist in chronic headache needs to be tailored to the patient's presentation. For some patients, psychological issues need to be directly addressed (eg, psychiatric comorbidity, difficulties coping with headache, significant problems with sleep and/or stress, medication overuse, and history of abuse). Other situations (eg, patients' beliefs about their readiness to change ability to actively manage headaches, medication adherence, and managing triggers) involve behavioral/psychological principles even when there is no direct contact with a psychologist. This article reviews the literature on the importance of psychological issues in headache management and provides suggestions for how to address behavioral and cognitive factors and their potential for improved headache care.

Is Chronic Migraine One End of a Spectrum of Migraine or a Separate Entity?

Chronic migraine is associated with abnormalities in the periaqueductal grey that may be progressive. The condition is also associated with a greater degree of impairment in cortical processing of sensory stimuli than episodic migraine, perhaps due to more pervasive or persistent cortical hyperexcitability These findings fit with the model of migraine as a spectrum disorder, in which the clinical and pathophysiological features may progress over time. This progression may result from changes in nociceptive thresholds and ensuing central sensitization caused by recurrent migraine in susceptible individuals. This may lead to changes in baseline neurological function between headaches, evident not only in electrophysiological and functional imaging studies, but also as psychological and somatic complaints that occur after years of episodic migraine. From current research and migraine models, a conceptualization of chronic migraine is emerging in which relatively permanent and pervasive central changes have occurred that warrant novel and tolerable treatments.

Chronic headache and potentially modifiable risk factors: screening and behavioral management of sleep disorders

Sleep-related variables have been identified among risk factors for frequent and severe headache conditions. It has been postulated that migraine, chronic daily headache, and perhaps other forms of chronic headache are progressive disorders. Thus, sleep and other modifiable risk factors may be clinical targets for prevention of headache progression or chronification. The present paper is part of the special series of papers entitled "Chronification of Headache" describing the empirical evidence, future research directions, proposed mechanisms, and risk factors implicated in headache chronification as well as several papers addressing individual risk factors (ie, sleep disorders, medication overuse, psychiatric disorders, stress, obesity). Understanding the link between risk factors and headache may yield novel preventative and therapeutic approaches in the management of headache. The present paper in the special series reviews epidemiological research as a means of quantifying the relationship between chronic headache and sleep disorders (sleep-disordered breathing, insomnia, circadian rhythm disorders, parasomnias) discusses screening for early detection and treatment of more severe and prevalent sleep disorders, and discusses fundamental sleep regulation strategies aimed at headache prevention for at-risk individuals.