An epidemiological approach to the nosography of chronic daily headache.

Chronic daily headache (CDH), a heterogeneous group of headaches, includes different forms that occur daily, or almost daily, over a prolonged period of time. The nosography of this group is still a matter of debate, and in the most recent classification of the IHS (1988) only a few types of CDH are included: chronic tension-type headache, coexisting migraine. This study is an epidemiological approach to identifying the clinical features of CDH and the possible factors involved in changing episodic headache in CDH. Ninety CDH outpatients were investigated using a computerized record chart. The main observed forms were: (a) chronic tension-type headache--migraine with interparoxysmal headache, an evolved form of migraine in which a constant low severity headache develops between attacks; (b) transformed migraine, an evolved form of migraine with progressive worsening of the disease which reaches the level of continuous pain with the disappearance of typical migraine attacks. Interval headaches in migraine with interparoxysmal headache partly fulfil the IHS criteria for chronic tension-type headache. Analgesic drug abuse plays a prominent role in inducing CDH and in determining its clinical features.