Click Evoked Myogenic Potentials in Vestibulocollic Reflex

Chung, Woon Kyo; Lee, Won Sang; Kwon, Ho Hwi; Park, Sang Il

Abstract

Click sound can evoke myogenic potentials in active sternocleidomastoid(SCM) muscle. These are due to activity of vestibular afferent rather than cochlear, which produce very short latency inhibition of ipsilateral motor unit activity. We investigated the click-evoked vestibulocollic reflexes in neck muscles to find out the significance and relationship of latency and amplitude of myogenic potentials between two different methods of muscles activation. EMGs were recorded from surface electrode evoked by sound stimuli(95dB SPL, 5/sec repetitive rate, 512 times) over SCM muscles on each side. Myogenic potentials have two biphasic responses which were termed P1, N1, P2, N2 based on the polarity of their components. The subjects were 24 normal volunteers. EMG recordings were obtained from 12 subjects(Group 1) who keeped head 10 cm above ground in supine position to activate their neck flexors to the degree required through the averaging runs(bilateral SCM muscles activation method). EMGs from another 12 subjects(Group 2) were recorded by rotating head to the opposite side to activate SCM muscle throughout the procedure(unilateral SCM muscle activation method). The latencies and amplitudes of myogenic potentials in the SCM muscle after stimulation were analysed and compared between the two methods. The latencies(msec) of P1 and N1 were 11.2+/-2.4, 21.7+/-1.1 respectively in Group 1 and 13.3+/-2.4, 23.0+/-1.1 respectively in Group 2. The amplitude(uV) of P1-N1 were 38.78.1 in Group 1 and 33.28.1 in Group 2. There were no significant differences in latencies and amplitudes of P1 and N1 between the groups(p<0.05). Unilateral muscle activation is easier and comfortable than the bilateral muscle activation and there were no significant difference in latency and amplitude of P1. The myogenic potential of unilateral vestibular function loss disappeared in a patient who was labyrinthectomized but there was normal response of potential in a patient who is profound sensory neural hearing loss. Click evoked myogenic potentials is a simple method of examining vestibulocollic reflex. Click evoked myogenic potential is related to vestibular status.