Epidemiological classification and social impact of chronic headache.

The current prevalent opinion is that a diagnosis of chronic headache should be established in patients who have had a form of primary headache other than cluster headache on at least 15 days a month for at least 3 months. Chronic headache so defined includes four different subtypes: hemicrania continua and new daily persistent headache--two rare forms currently included in the group "Other primary headaches" of the 2004 International Classification of Headache Disorders, second edition (ICHD-II)--chronic tension-type headache, and chronic migraine. Regarding the latter, which has been better known lately as transformed migraine, the current revised diagnostic criteria establish that migraine must be present on at least 8 of the at least 15 days of headache per month and that there must not be any overuse of symptomatic medication. Chronic headache affects about 3-5% of the general population and results in a variety of negative repercussions both on individuals and on society at large.