Side asymmetry of the jaw jerk in human craniomandibular dysfunction.

The jaw jerk elicited by tapping the chin with a reflex hammer was electromyographically recorded in 14 patients with craniomandibular dysfunction, who were selected because of their strictly unilateral symptoms. Mandibular deviation, as measured by means of a kinesiograph, was on the same side as the pain. Neurological and neurophysiological investigations, including the recording of masseter motor potentials evoked by transcranial stimulation, showed normal function of the sensory and motor trigeminal nerve fibres. Latency and amplitude of the jaw jerk recorded in postural position and intercuspal occlusion were, respectively, longer and smaller on the affected side. In some cases the latency difference exceeded 1 ms, the limit usually considered significant for trigeminal neuropathy or brainstem lesions. Jaw-jerk asymmetry is probably due to facilitation on the side contralateral to mandibular deviation. In intercuspal occlusion, contralateral facilitation might be produced by a stronger input from muscle spindles and periodontal mechanoreceptors. In postural position, other factors probably intervene.