Chronic daily headache: risk factors and pathogenetic considerations.

Chronic daily headache (CDH) is a major clinical concern, although it is still plagued by difficulties with classification and definitions. CDH usually evolves from an episodic headache, mainly migraine. Drug overuse and other somatic or psychological traits are considered risk factors for CDH. The neurobiology underlying this clinical evolution is incompletely understood. There is evidence of a progressive dysfunction of central pain systems in some individuals probably genetically predisposed.