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.