Résumé
Evolution of episodic headaches, especially migraine to chronic daily or near-daily headache is an interesting phenomenon. Although its pathogenesis is still unclear the derangement of the brainstem modulatory system is a possible explanation. Recent evidences indicate the involvement of rostral brainstem activation and the attack of migraine. Several nuclei located in this area namely periaquiductal grey, nucleus raphe, locus ceruleus are known to be pivotal in the modulation of sensory information. Therefore, derangement of this complex network can result in abnormal sensory perception, e.g. throbbing headache, photophobia, phonophobia, etc as seen during the attacks of migraine. Chronic alteration of this system can lead to an increase in headache frequency. Evidences from animal experiments indicate that dysfunction of the sensory modulation system in the brainstem also promote the development of central sensitization, a condition in which central nociceptive neurons are more responsive to stimuli. Particular symptoms of chronic daily headache, namely increased headache frequency, expansion of headache area and cutaneous allodynia, imply the sensitization of central nociceptive neurons in the trigeminal pathway. A number of animal experiments have confirmed that chronic analgesic exposure lead to changes in serotonin as well as its receptors in the central nervous system. The plasticity of serotonin-dependent pain control system may accelerate the process of sensitization and results in the development of chronic daily headache secondary to analgesic overuse.