Tension-type headache is very common, with a lifetime prevalence in the general population ranging in different studies between 30% and 78%. It has a high socio-economic impact. While it was previously considered to be primarily psychogenic, a number of studies since ICHD-I strongly suggest a neurobiological basis.
The division of 2. Tension-type headache into episodic and chronic types, introduced in ICHD-I, has proved extremely useful. In ICHD-II, the episodic type was further divided into an infrequent type, with headache episodes less than once per month, and a frequent type. 2.2 Frequent episodic tension-type headache can be associated with considerable disability, and sometimes warrants treatment with expensive drugs. In contrast, 2.1 Infrequent episodic tension-type headache, which occurs in almost the entire population, usually has very little impact on the individual and, in most instances, requires no attention from the medical profession. The distinction of 2.1 Infrequent episodic tension-type headache from 2.2 Frequent episodic tension-type headache thus separates individuals who typically do not require medical management, and avoids categorizing almost the entire population as having a significant headache disorder, yet allows their headaches to be classified. 2.3 Chronic tension-type headache is a serious disease, causing greatly decreased quality of life and high disability.
The exact mechanisms of 2. Tension-type headache are not known. Peripheral pain mechanisms are most likely to play a role in 2.1 Infrequent episodic tension-type headache and 2.2 Frequent episodic tension-type headache, whereas central pain mechanisms play a more important role in 2.3 Chronic tension-type headache. Increased pericranial tenderness is the most significant abnormal finding in patients with any type of 2. Tension-type headache: it is typically present interracially, is exacerbated during actual headache and increases with the intensity and frequency of headaches. Increased tenderness is very probably of pathophysiological importance. ICHD-II therefore distinguished patients with and without such disorder of the pericranial muscles, a subdivision maintained in ICHD-3 to stimulate further research in this area.
