Mechanical hyperesthesia of human facial skin induced by tonic painful stimulation of jaw muscles.
The function of the somatosensory system in patients with painful temporomandibular disorders is still a matter of discussion. We wished to determine cutaneous sensitivity to innocuous mechanical stimuli in the orofacial region before, during (3 and 12 min) and after standardized experimental jaw-muscle pain. Twelve healthy subjects were exposed to tonic infusion of hypertonic (5%) and isotonic (0.9%) saline into the masseter muscle. All subjects experienced moderate pain with hypertonic saline, and the area of self-reported pain increased significantly from 3 min after infusion start to 12 min after infusion start (mean +/- SEM: 115+/-49%; P < 0.05). The psychophysical ratings of punctate von Frey hair stimulation were significantly increased 12 min after start of hypertonic saline infusion as compared to baseline and post-baseline ratings at the site of infusion (50+/-10%; P < 0.05) and at two adjacent facial sites (18+/-7%, 37+/-9%; P < 0.05). In contrast, isotonic saline infusion was associated with a significant decrease in ratings at post-baseline as compared to baseline ratings. The psychophysical ratings of a stroking cotton swab stimulation were not significantly affected by infusion of saline. These results in a human model of jaw-muscle pain are comparable to animal studies demonstrating increased size of cutaneous receptive fields and increased responsiveness of brain stem neurons to cutaneous mechanical stimuli. Similar hyperexcitability changes may be part of the pathophysiological mechanisms involved in painful temporomandibular disorders.