Unfortunately, there has not been a U.S. telephone random digit dial survey of tension type headache like there has been carried out for migraine. There have been many European studies that put the lifetime chance of developing a period of time of having tension type headaches at 2 out of 3 in men and 9 out of 10 in women. In any given year perhaps 40% of Americans suffer at least one tension type headache. Therefore this is clearly the most common type of primary headache. A primary headache is one not caused by some other systemic illness.

Tension-type headaches as a group are less severe than migraine but because of their larger numbers, do cause a significant societal cost due to lost productivity and health care costs. This is only recently coming to light and with future revisions to the I.H.S. classification system, I feel that improved studies and medical treatment for these patients will come during the next decade, just as improved migraine therapy came during the 1990s. The data, however, could be in error as demonstrated by one study of patients including those with severe episodic tension headaches. In this study patients were classified by headache experts as having I.H.S. migraine, I.H.S. migrainous headaches, and I.H.S. tension type headaches. Only patients with severe tension type headaches were allowed in the study. The patients were then given diaries to fill out during their actual headaches. About 1 in 3 patients had been improperly classified. One thing this tells us is that epidemiological studies (or frequency) studies that depend on patient recall can have a high error rate at least in the area of severe tension type headache.

Chronic tension type headache is an even more difficult area to determine. Many migraine experts feel that a high percentage of patients that meet the definition of chronic tension type headache were potentially migraine patients that transformed to a chronic daily headache that lost much of its migraine qualities. How much of this is part of migraine itself and how much is due to headache medicine overuse (i.e. rebound headache) is not clear. It does appear that up to 4% of Americans suffer from chronic daily headache or near daily headache.