[Chronic migraine and headache by medication overuse. Evolution and revision of classification].

The first International classification of headache disorders (ICHD-I) described migraine as a recurring headache disorder manifesting in attacks lasting 4-72 h. The headache frequency was not considered in these first diagnostic criteria of migraine. Thus, a chronic migraine with at least 15 migraine days per month was only included in the ICHD-II in 2004. Meanwhile, the diagnosis of chronic daily headache and transformed migraine had been established in the USA. The term transformed migraine describes the transformation of an episodic migraine into a chronic one, whether medication overuse had been present or not. Up till now a widely accepted definition of chronic migraine and medication overuse headache has not been established due to different views. An overview of the evolution of the diagnostic criteria for both headache disorders in recent years is provided.