[Effects of craniofacial pain on jaw jerk in man]

Jaw jerk was evoked in 12 normal subjects (N group) and 12 patients with unilateral craniofacial pain who had neither mandibular deviation nor organic changes in the temporomandibular joint (P group), about every ca. 2 sec by tapping the center of the chin with a reflex hammer, in which a microswitch was embedded to indicate the time of its contact with the chin. The surface electromyogram of the masseter muscle was recorded bilaterally. It was found that (1) in the N group, there was no difference in the onset latency between the right (6.30 +/- 1.06 ms; n = 192) and left (6.32 +/- 1.04 ms; n = 192) sides (P > 0.05); (2) in the P group, the latency was shorter on the painful side than on the pain-free side; and (3) after pain was relieved by treatment, the bilateral difference in the latency of jaw jerk disappeared, and the latencies fell to that in normal subjects. It was concluded that the bilateral difference of the latency of jaw jerk can be used as an investigative criterion for the existence of pain.