Sensitivity of patients with painful temporomandibular disorders to experimentally evoked pain: evidence for altered temporal summation of pain William Maixner a,b,c, *, Roger Fillingim c , Asgeir Sigurdsson a,c , Shelley Kincaid c , Stefanie Silva a Department of Endodontics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599–7455 USA b Department of Pharmacology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599–7455 USA c Dental Research Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599–7455 USA Received 14 February 1997; received in revised form 7 January 1998; accepted 27 January 1998 Abstract Temporomandibular disorders (TMD) represent a group of chronic painful conditions involving the muscles of mastication and the temporomandibular joint. Several studies have reported that TMD is associated with enhanced sensitivity to experimental pain. Twenty- three TMD subjects and 24 pain-free matched control subjects participated in a set of studies which were designed to evaluate whether the temporal integrative aspects of thermal pain perception are altered in TMD patients compared with control subjects. Specifically, we have examined in both TMD patients and in age- and gender-matched control subjects: (1) the time-course and magnitude of perceived pain evoked by the application of sustained 7-s noxious thermal stimuli (45–48°C) to the face and forearm, (2) the central summation of C-fiber- mediated pain produced by applying brief trains of noxious heat pulses to the skin overlying the ventral aspect of the right palm and (3) the ability to discriminate small increments in noxious heat applied to facial and volar forearm skin. Data collected from these studies indicate that TMD patients show enhanced temporal integration of thermal pain compared with control subjects. TMD patients show greater thermal C-fiber-mediated temporal summation than pain-free subjects and they report a greater magnitude of sustained noxious heat pulses applied to either the face or the forearm than control subjects. In contrast to these findings, TMD and pain-free subjects are equally able to discriminate and detect small increments of heat applied to noxious adapting temperatures. These findings suggest that the augmented temporal integration of noxious stimuli may result from alterations in central nervous system processes which contribute to the enhanced pain sensitivity observed in TMD patients.  1998 International Association for the Study of Pain. Published by Elsevier Science B.V. Keywords: Pain sensitivity; Temporomandibular disorders; Myofascial pain; Thermal pain; Temporal summation; Muscle pain 1. Introduction Temporomandibular disorders (TMD), a heterogeneous family of musculoskeletal disorders, represent the most common chronic orofacial pain condition (Dworkin et al., 1992; Okeson et al., 1996). Although there are several forms of TMD, the most common and debilitating forms are asso- ciated with persistent pain in the region of the temporoman- dibular joint, the periauricular region, and muscles of the head and neck (Dworkin et al., 1992; Okeson et al., 1996). Both peripheral and central mechanisms have been pro- posed to account for the painful pericranial myofascial pain that is so commonly associated with TMD and another headache condition – tension-type headache, which is also associated with persistent pericranial myofascial pain. In general, most investigators have shown that both conditions are associated with cranial myofascial tenderness and lower cranial muscle pain thresholds (Brase, 1986; Schoenen et al., 1991; Jensen et al., 1993; Jensen, 1996; Bendtsen et al., 1996). Recent studies from our laboratory have provided evi- dence that the painful myalgia seen in TMD patients may also result from an impairment in central pain processing Pain 76 (1998) 71–81 0304-3959/98/$19.00  1998 International Association for the Study of Pain. Published by Elsevier Science B.V. PII S0304-3959(98)00028-1 * Corresponding author. Tel.: +1 919 9663756; fax: +1 919 9663683.