Chronically evolving headaches: classification and terminology.

Chronic migraine appeared for the first time in the second edition of the International Classification of Headache Disorders (ICHD-II) in 2004, listed among the complications of migraine. Unfortunately, the diagnostic criteria of ICHD-II for this headache form tend to equate it with a migraine with a high frequency of attacks, rather than with an unfavourable evolution of migraine with loss of symptom-free intervals between attacks. On the other hand, the latter occurrence has increasingly been described in the last few years with the term "transformed migraine". Therefore, it seems advisable to carry out a revision of the ICHD-II in order to: (a) subdivide migraine at the three-digit level into infrequent episodic, frequent episodic and chronic migraine; and (b) introduce transformed migraine among the complications of migraine.