Chronic daily headache and the revised international headache society classification.

Chronic daily headache (CDH) is surprisingly common. It is best defined as a primary headache disorder with head pain on most days. There are a number of possible secondary causes of persistent headache, including traumatic, vascular, neoplastic, and infectious processes, all of which must be ruled out when the patient with frequent headache is evaluated. However, most patients with CDH seem to have a primary neurophysiologic disorder. This category of primary CDH does not seem to be a homogeneous disorder, but rather one with important subtypes. Several authors have proposed subdivisions of primary CDH such as chronic migraine, evolved migraine, transformed migraine, chronic tension-type headache, new daily persistent headache, and hemicrania continua. The International Headache Society (IHS) Classification published in 1988 did not address CDH other than to define a category "chronic tension-type headache." The revised IHS Classification (ICHD II) attempts to characterize CDH more thoroughly with the addition of chronic migraine and new daily persistent headache diagnoses, but this complex issue continues to defy easy categorization. This article provides a review of thinking about the nature of primary CDH and how ICHD II attempts to organize this category for much needed research purposes.