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Abstract
Tension-type headache (TTH) is characterized by bilateral, pressing or tightening pain; pressure or band-like tightness; and/or increased tenderness on palpation of neck and shoulder muscles. These features resemble the descriptions of referred pain originating in myofascial trigger points (TrPs). The present paper deals with the scientific evidence supporting the hypothesis that referred pain elicited by TrPs contributes to pain perception in TTH. Animal and human studies clearly show the convergence of cervical and trigeminal afferents in the trigeminal nerve nucleus caudalis, constituting the anatomical basis for the referred head pain from neck and shoulder muscle TrPs. Referred pain patterns have been described from different TrPs in several head and neck muscles that have the potential to refer to the head. Several human pain models have confirmed some of these clinical pain patterns by injecting algogenic substances into the muscles. Recent studies have demonstrated that chronic TTH was associated with active TrPs in the suboccipital, upper trapezius, sternocleidomastoid, temporalis, and superior oblique muscles. In addition, chronic TTH patients with active TrPs have greater headache intensity and frequency than those with latent TrPs. Active TrPs have also been found on the symptomatic, but not in the contra-lateral side in unilateral migraine patients. Higher levels of algogenic substances have been found in active TrPs as compared to latent TrPs and tender points, which may lead to increased afferent bombardment into the nucleus caudalis. This would result in temporal and spatial summation of neuron signals and may cause central sensitization in chronic TTH. Pain at rest falls in this category if it results from TrP activity. It seems that the pain profile of TTH can include referred pain from TrPs in the posterior cervical, head (including extra-ocular muscles), and shoulder muscles. Further research could delineate more information on the relation between TrPs on TTH.