Analysis of stimulus-evoked pain in patients with myofascial temporomandibular pain disorders

The pathophysiological mechanisms of myofascial temporomandibular disorders (TMD) are still under investigation. The hypothesis that TMD pain is cause d by a generalized sensitization of higher order neurons in the nociceptive pathways combined with a decreased efficacy of endogenous inhibitory syste ms has recently gained support in the literature. This study was designed t o further investigate the somatosensory sensibility within and outside the craniofacial region. Twenty-two patients fulfilled the research diagnostic criteria for TMD for myofascial pain (Dworkin and LeResche, J Craniomandib Disord Facial Oral Pain 6 (1992) 301) and 21 age- and sex-matched subjects served as a control group. The somatosensory sensibility to a deep tonic in put was tested by standardized infusions of hypertonic saline into the mass eter and anterior tibialis muscle. Furthermore, pressure pain thresholds (P PTs) and heat pain thresholds (HPTs) were assessed with phasic stimuli at t he same sites before and following the infusions. Myofascial TMD patients r eported infusion of hypertonic saline to be more painful on 10 cm visual an alogue scales (peak pain 8.8 +/- 0.4 cm) than control subjects (6.8 +/- 0.5 cm, t-test: P = 0.003) in the masseter but not in the anterior tibialis (7 .4 +/- 0.5 vs. 6.6 +/- 0.5 cm, P = 0.181). The perceived area of experiment al masseter pain measured on drawings was marginally larger in TMD patients (2.6 +/- 0.5 arbitrary units (a.u.)) than in control subjects (1.4 +/- 0.2 a.u., Mann-Whitney: P = 0.048) but no differences were observed for the an terior tibialis (P = 0.771). The PPTs were lower in the myofascial TMD pati ents compared to the control group, both in the masseter (analysis of varia nce (ANOVA): P = 0.002) and in the anterior tibialis (P = 0.005), whereas t here were no significant differences in HPT (ANOVAs: P = 0.357, P = 0.101). There were no significant correlations between measures of somatosensory s ensibility and measures of clinical pain intensity, pain duration, graded c hronic pain scores or somatization or depression scores (Pearson: R < 0.304 , P > 0.172). The present study in a well-defined group of myofascial TMD p atients found that the responsiveness to both tonic and phasic deep stimuli , but not to phasic superficial inputs at the pain threshold level, in the craniofacial region was higher compared with a control group. These finding s suggest that myofascial TMD pain is associated with a facilitation of sti mulus-evoked pain primarily, but not exclusively related to the painful reg ion. (C) 2001 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.