Clinically, tension-type headache and migraine seem to fall on a continuum with episodic tension-type headache on one side and migraine on the other. In between are the two (almost) daily headache conditions, chronic tension-type headache with or without coexistent migraine. The continuum is referred to as such because of the many intermediary stages that can be observed clinically between the headache conditions on it. Patients can move along it from one of the episodic to the daily headache conditions and back.
The principle of the headache continuum was confirmed by a study of more than 250 patients with "chronic daily headache." In the study, the presentation, development, and outcome of this particular headache condition were determined. Of the patients, 63% developed the daily headaches gradually out of initially intermittent headaches. The remaining 37% developed the daily headaches more or less abruptly, in one-fifth with a prior history of severe headaches. The transition from initially intermittent to daily headaches took an average of 10.7 years.
Of the patients with "gradual-onset chronic daily headache," 33% initially had mild headaches and 67% severe ones. The severe headaches were significantly more frequently associated with nausea and vomiting than the mild ones. This suggests that the initial, intermittent mild headaches were episodic tension-type headaches and the initial, intermittent severe headaches, migraine headaches. However, the daily headaches that these patients ultimately developed were the same, whether their initial headaches were tension-type or migraine. Of the patients who went back to having intermittent headaches, 77% of those who initially had migraine, also presented with migraine at follow-up. Of the patients who initially had episodic tension-type headache, 43% had episodic tension-type at follow-up.