Chronic daily headache. How should it be included in the IHS classification?

A number of patients attending specialty headache centers complain of very frequent, almost continuous or continuous headaches, which are usually grou ped together under the term chronic daily headache, a category which does n ot appear in the International Headache Society (IHS) classification. On th e basis of the IHS criteria, these patients can only be classified as havin g a chronic tension-type headache with the possible addition of migraine, i f migrainous attacks are superimposed on the "background" headache. However , several studies have demonstrated that most patients with chronic daily h eadache originally suffered from migraine and that their migraine has trans formed, in the course of time, into a chronic headache picture in which iso lated migraine attacks may or may not persist. Despite some differences in the personal opinions of authors involved in the care of patients with chro nic daily headache, some views seem to be generally accepted: (1) the great majority of chronic daily headaches are transformations of an original epi sodic migraine and cannot be included in the chronic tension-type headache category, (2) the current IHS classification does not allow many patients p resenting with chronic daily headache to be classified correctly, (3) an im portant noscological category (transformed migraine) has emerged from all t he studies on this subject, (4) it is impossible to diagnose transformed mi graine merely by "photographing" the picture of single attacks. Although so me theoretical problems remain unresolved, it seems to us that the next rev ision of the IHS classification can no longer ignore the existence of chron ic daily headache.