Craniofacial muscle pain: Review of mechanisms and clinical manifestations

Epidemiologic surveys of temporomandibular disorders (TMD) have demonstrate d that a considerable proportion of the population-up to 5% or 6%-will expe rience persistent pain severe enough to seek treatment. Unfortunately, the current diagnostic classification of craniofacial muscle pain is based on d escriptions of signs and symptoms rather than on knowledge of pain mechanis ms. Furthermore, the pathophysiology adn etiology of craniofacial muscle pa in are not known in sufficient detail to allow causal treatment. Many hypot heses have been proposed to explain cause-effect relationships; however, it is still uncertain what may be the cause of muscle pain and what is the ef fect of muscle pain. This article reviews the literature in which craniofac ial muscle pain has been induced by experimental techniques in animals and human volunteers and in which the effects on somatosensory and motor functi on have been assessed under standardized conditions. This information is co mpared to the clinical correlates, which can be derived from the numerous c ross-sectional studies in patients with craniofacial muscle pain. The experimental literature clearly indicates that muscle pain has signific ant effects on both somatosensory and craniofacial motor function. Typical somatosensory manifestations of experimental muscle pain are referred pin a nd increased sensitivity of homotopic areas. The craniofacial motor functio ns is inhibited mainly during experimental muscle pain, but phase-dependent excitation is also found during mastication to reduce the amplitude and ve locity of jaw movements. The underlying neurobiologic mechanisms probably i nvolve varying combinations of sensitization of peripheral afferents, hyper excitability of central neurons, and imbalance in descending pain modulator y systems. Reflex circuits in the brain stem seem important for the adjustm ent of sensorimotor function in the presence of craniofacial pain. Changes in somatosensory and motor function may therefore be viewed as consequences of pain and not factors leading to pain. Implications for the diagnosis an d management of persistent muscle pain are discussed from this perspective.