The human jaw jerk: electrophysiologic methods to measure the latency, normal values, and changes in multiple sclerosis.

The reflex contraction of the masseter muscles to the stretch imposed by a tap on the chin is a monosynaptic reflex. This reflex contraction of the master muscles was present in all control subjects but commonly absent in definite multiple sclerosis (MS) patients. In a few MS patients the latency was prolonged. Abnormalities in the jaw jerk, however, were less frequent than with the blink reflex responses to supraorbital nerve stimulation or light stimulation. Nonetheless, there were patients in whom the blink reflexes were normal but the jaw jerk responses was abnormal. The last observation suggests that the jaw jerk may be occasionally helpful in the detection of brainstem lesions in MS.