The tension headache, migraine headache continuum: a hypothesis.

There is evidence in the literature that tension-type and migraine headaches do not exist as discrete pathophysiologic entities but form a continuum. The mechanisms for tension and migraine headaches are discussed and the evidence for this continuum, both basic scientific and clinical, is reviewed. A model for chronic headaches based on this evidence and based on clinical observations by the author is presented. Clinical and research implications of this model and means of testing the model are discussed.