Cosensitization of pain and psychiatric comorbidity in chronic daily headache.

Chronic migraine occurs in approximately 20% of migraineurs, typically developing over a period of many years. The pathophysiology of this transformation is unknown. However, experts have associated chronic headache with analgesic overuse, physical injury, and psychologic trauma. Research in post-traumatic stress disorder has found that hippocampal sensitivity to stress alters and often amplifies future pain behaviors. Although the most obvious difference between migraine and chronic migraine is the frequency of headaches, this article discusses chronic migraine as a more pervasive neurologic disease in which the patient's neurologic and psychologic function fails to return to a normal baseline. The sensory and affective components of pain are cosensitized, producing other neurologic and psychologic symptoms during and between episodes of headache. A staging paradigm is suggested that defines patients and assesses their overall neurologic function. The goal of this classification is to identify cosensitization early and pinpoint migraine patients who are at risk of developing chronic migraine.