Characteristics of a jaw reflex in humans with temporomandibular disorders: a preliminary report.

Electromyography was used to compare characteristics of an inhibitory jaw reflex in 10 temporomandibular disorder patients and 10 age- and sex-matched healthy controls. The methodology was novel in that the reflex was that evoked in the active masseter muscle, by electrical stimulation of perioral skin. This response has advantages over those previously studied as it avoids problems associated with stimulating in the moist intra-oral environment and it is monophasic, thus permitting easy quantification. The results have shown that (i) with the stimulation parameters employed, the reflex was present in all 10 control subjects, but in only eight of 10 temporomandibular disorder patients. (ii) When stimulation intensities were expressed as multiples of sensory threshold, there was no significant difference in the minimum level of stimulation required to evoke the reflex between the groups, although there was a trend for the patients with temporomandibular disorders to require higher intensities. (iii) Comparison of data from subjects giving responses at the same stimulus intensity (6 x sensory threshold: seven temporomandibular disorder patients, eight controls), showed no significant differences in the latencies or magnitudes of the reflex between the groups. However, the overall duration of the reflex was significantly shorter for the patients with temporomandibular disorders, with the reflex finishing significantly earlier. Thus even within the limitations of this study, it appears that an inhibitory jaw reflex evoked from stimulation around the mouth, may be weaker in temporomandibular disorder patients. This conclusion is consistent with previous studies on more complex jaw reflexes evoked by intra-oral stimuli.