Transformed migraine.

Chronic tension-type headache, which is included in the International Headache Classification, is present in only a minority of patients who present with chronic daily headache. The majority have what is termed transformed migraine, with a history of distinct episodes of migraine in the initial years which progresses into chronic daily headache. These patients with transformed migraine exhibit mixed features of migraine and chronic tension-type headache. Two distinct types of transformed migraine are identifiable, namely those related to excessive intake of medications (drug-induced transformed migraine) and those unrelated to excessive use of medications. The clinical features of transformed migraine and the drug-induced variety are described. The need for revision of the International Classification to include chronic daily headache and the subtypes of transformed migraine is pointed out.