Myofascial trigger points and sensitization: an updated pain model for tension-type headache.

Present pain models for tension-type headache suggest that nociceptive inputs from peripheral tender muscles can lead to central sensitization and chronic tension-type headache (CTTH) conditions. Such models support that possible peripheral mechanisms leading to pericranial tenderness include activation or sensitization of nociceptive nerve endings by liberation of chemical mediators (bradikinin, serotonin, substance P). However, a study has found that non-specific tender points in CTTH subjects were not responsible for liberation of algogenic substances in the periphery. Assuming that liberation of algogenic substances is important, the question arising is: if tender muscle points are not the primary sites of on-going neurogenic inflammation, which structure can be responsible for liberation of chemical mediators in the periphery? A recent study has found higher levels of algogenic substances, and lower pH levels, in active myofascial trigger point (TrPs) compared with control tender points. Clinical studies have demonstrated that referred pain elicited by head and neck muscles contribute to head pain patterns in CTTH. Based on available data, an updated pain model for CTTH is proposed in which headache can at least partly be explained by referred pain from TrPs in the posterior cervical, head and shoulder muscles. In this updated pain model, TrPs would be the primary hyperalgesic zones responsible for the development of central sensitization in CTTH.