Chronic daily headache.

Chronic daily headache (CDH) is a heterogeneous group of headaches that includes primary and secondary varieties. Primary CDH is a frequent entity that probably affects 4% to 5% of the population. It can be subdivided into headaches of short duration (less than 4 hours per attack), like chronic cluster headache, and disorders of long duration (greater than 4 hours per attack). Primary CDH of long duration includes transformed migraine, chronic tension-type headache, new daily persistent headache, and hemicrania continua. Analgesic or ergot overuse is frequent in all types of CDH. We review recent insights into the proposed classification, epidemiology, pathophysiology, clinical characteristics, and treatment of CDH.