Abstract
Tension-type headache is the most prevalent form of headache with a life-time prevalence of 78%; 30% of sufferers are affected on more than 14 days per year (1) and 3% of a general population are chronically affected (1, 2). Although tension-type headache is, as far as socio-economic impact is concerned, the most important type of headache, remarkably little is known about its pathophysiological background. At present, the best documented abnormality in patients with tension-type headache is increased pericranial myofascial tenderness recorded by palpation. In 1988, diagnostic criteria for tension-type headache were created by the IHS committee (3) and although the scientific evidence was very limited two subdivisions of tension-type headache, one with and one without association with muscular tenderness were created. Of these diagnostic criteria, only tenderness recorded by manual palpation was useful as a diagnostic test, whereas EMG and pressure algometry provided only very limited diagnostic information (4). The pericranial tenderness is positively related to the frequency and to the intensity of tension-type headache, whereas no such relation can be detected in the other diagnostic tests (5). Subjects with chronic tension-type headache had slightly increased EMG levels during resting conditions (6, 7) and decreased levels during maximal voluntary contraction compared with headache-free subjects, indicating insufficient relaxation, or reflex contraction, at rest and impaired recruitment at maximal activity (6).
Is the increased tenderness in pericranial muscles a cause or a consequence of the pain? Tenderness was studied in 28 patients both during and outside a spontaneous episode of tension-type headache. During the actual episode of pain the degree of tenderness is significantly increased compared to values outside headache, and perhaps therefore considered more as a consequence of actual pain rather than its cause (8). To study the cause-effect relationship further, experimental models of tension-type headache are much needed. In 1985, Jensen et al. reported that sustained tooth clenching produced a mild tension-type-like headache in migraineurs but not in healthy controls (9). A similar study of sustained clenching was carried out in 58 patients with frequent episodic or chronic tension-type headache (10). Within 24 h, 69% of patients and 17% of controls developed a tension-type headache. Shortly after clenching, tenderness was increased in the group that subsequently developed headache, whereas tenderness was stable in the group of patients that remained headache-free indicating that tenderness might be a causative factor to the headache (10). To further study the importance of muscular tenderness, 28 patients with episodic tension-type headache, 28 with chronic tension-type headache, and 30 healthy controls were examined. The main result was significantly lower pressure pain detection thresholds and tolerances in all the examined locations in patients with chronic tension-type headache associated with muscular tenderness compared to those without muscular tenderness (11). There were no such differences in the two subgroups of patients with episodic tension-type headache. Thermal pain sensitivity did not differ between patients with and those without muscular tenderness, while EMG levels were significantly higher in patients with chronic tension-type headache with muscle tenderness than in those without it (11). A recent study by Bendtsen et al. demonstrates for the first time that chronic tension-type headache has a physiological basis and is caused at least partly by qualitative changes in the central processing of sensory information (12).
It can therefore be concluded that a peripheral mechanism of tension-type headache is most likely in the episodic subform, whereas a secondary, segmental central sensitization and/or an impaired supraspinal modulation of incoming stimuli seems to be involved in subjects with chronic tension-type headache. Prolonged nociceptive stimuli from myofascial tissue may therefore be of importance for the conversion of the episodic into the chronic tension-type headache. Tension-type headache is probably a multifactorial disorder with several concurrent pathophysiological mechanisms, and extracranial myofascial nociception may constitute only one of them. It is, however, extremely important to study and understand the balance between peripheral and central components in tension-type headache. This may, hopefully, lead us to a better prevention and treatment of the most prevalent type of headache.