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Abstract
Retrospective analysis of trigeminal nerve evoked potentials in 40 consecutive patients, most of them with traumatic nerve lesions, showed that in 12 cases no trigemial nerve SEP were obtainable, and 11 of the remaining 28 patients had normal trigeminal nerve SEP. Therefore the jaw-opening reflex was investigated as a potential tool for electrophysiologic analysis of facial sensory disturbances. The jaw-opening reflex was investigated in 60 healthy subjects (31 female, 29 male) aged 23-82 years. It was elicited by electrical 0.1 ms square wave pulses delivered to the lower and upper lips and to the infraorbital region on either side at a rate below 1 per 5 s. The EMG responses were recorded from the bilateral masseter and temporalis muscles at a moderate voluntary activation. Under these conditions, the jaw-opening reflex reveals itself as two inhibitory pauses of the ongoing EMG on both sides, the onset latency of the first EMG-suppression being 10-15 ms, and of the second 35-50 ms. Particular attention was paid to the stimulus strength at threshold (TR) to evoke the jaw-opening reflex. We found that the jaw-opening reflex was constantly evoked by weak stimuli applied to the 2nd and 3rd trigeminal branches. Bilateral reflex responses with unilateral stimulation were a regular finding. The reflex responses increase with increasing stimulus strength (Fig. 1). Moderate to forcible activation of the jaw closing muscles is a prerequisite for optimum recordings of the jaw-opening reflex (Fig. 2). TR may range between 8 and 26 mA, but in one individual is almost symmetric on both sides as well as between the trigeminal branches on one side (Tab. 1, Fig. 3), the mean differences are around 1 mA and 2 mA, respectively. TR somewhat increases with age, and ranges around the threefold of the strength liminal for detection of the stimulus (Fig. 4).
Published Version
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