From migraine to chronic daily headache: the biological basis of headache transformation.

Migraine headache carries the potential of transforming into chronic daily headache (CDH) over a period of time. Although several risk factors for migraine progression to CDH have been identified, the biological basis of this transformation is unknown. In this review, the consequences of stressful life events and medication overuse, 2 risk factors associated with the development of CDH, on brain processes involved in headache are examined. The extensive overlap in both neural circuitry and cellular events that occur with stress, medication overuse, and migraine provide insight into potential mechanisms that may lead to CDH. Particular attention is devoted to the effect of stress and medication overuse on peripheral and central neuroimmune interactions that can facilitate pain signaling. These interactions include the degranulation of mast cells in the dura, causing the sensitization of primary afferent neurons, as well as the activation of glial cells in the brain that can lead to central sensitization. It is hypothesized that the biological processes involved in migraine headache are directly impacted by stress, medication overuse, and other risk factors, resulting in a reduced threshold for induction of headache and transformation of episodic migraine to CDH.