Evidence of a Vestibular Origin for Crossed-Sternocleidomastoid Muscle Responses to Air-Conducted Sound

Abstract

Objectives: Small, excitatory surface potentials can sometimes be recorded from the contralateral sternocleidomastoid muscle (SCM) following monaural acoustic stimulation. Little is known about the physiological properties of these crossed reflexes. In this study, we sought the properties of crossed SCM responses and through comparison with other cochlear and vestibular myogenic potentials, their likely receptor origin. Design: Surface potentials were recorded from the ipsilateral and contralateral SCM and postauricular (PAM) muscles of 11 healthy volunteers, 4 patients with superior canal dehiscence and 1 with profound hearing loss. Air-conducted clicks of 105 dB nHL and tone bursts (250 to 4000 Hz) of 100 dB nHL were presented monaurally through TDH 49 headphones during head elevation. Click-evoked responses were recorded under two conditions of gaze in random order: gaze straight ahead and rotated hard toward the contralateral recording electrodes. Amplitudes (corrected and uncorrected) and latencies for crossed SCM responses were compared with vestibular (ipsilateral SCM) and cochlear (PAM) responses between groups and across the different recording conditions. Results: Surface waveforms were biphasic; positive-negative for the ipsilateral SCM, and negative-positive for the contralateral SCM and PAM. There were significant differences in the amplitudes and latencies (p = 0.004) for click responses of healthy controls across recording sites. PAM responses had the largest mean-corrected amplitudes (2.3 ± 2.8) and longest latencies (13.0 ± 1.2 msec), compared with ipsilateral (1.6 ± 0.5; 12.0 ± 0.7 msec) and contralateral (0.8 ± 0.3; 10.4 ± 1.0 msec) SCM responses. Uncorrected amplitudes and muscle activation for PAM increased by 104.4% and 46.8% with lateral gaze respectively, whereas SCM responses were not significantly affected. Click responses of patients with superior canal dehiscence followed a similar latency, amplitude, and gaze modulation trend as controls. SCM responses were preserved in the patient with profound hearing loss, yet all PAM were absent. There were significant differences in the frequency tuning of the three reflexes (p < 0.001). Tuning curves of healthy controls were flat for PAM and down sloping for ipsilateral and contralateral SCM responses. For superior canal dehiscence, they were rising for PAM and slightly down sloping for SCM responses. Conclusions: Properties of crossed SCM responses were similar, though not identical, to those of ipsilateral SCM responses and are likely to be predominantly vestibular in origin. They are unlikely to represent volume conduction from the PAM as they were unaffected by lateral gaze, were shorter in latency, and had different tuning properties. The influence of crossed vestibulo-collic pathways should be considered when interpreting cervical vestibular-evoked myogenic potentials recorded under conditions of binaural stimulation.

Age Effects of Bone Conduction Vibration Vestibular-evoked Myogenic Potentials (VEMPs) Using B81 and Impulse Hammer Stimuli

“…Therefore, we chose to maintain the same method to make direct comparisons between stimuli. However, bilateral SCM contraction could be causing a crossed vestibulo-collic pathway with the binaural stimulation, as discussed earlier (Taylor et al 2020).…”

“…of opposite polarity has been noted in 38-53% of normal individuals (Welgampola & Colebatch 2001;McNerney & Burkard 2011;Taylor et al 2020). When completing cVEMP with bilateral stimulation, this contralateral response can potentially reduce the peak-to-peak amplitude (Taylor et al 2020).…”

“…Another source of possible contamination is the presence of the crossed, contralateral response (Welgampola & Colebatch 2001;Murofushi et al 2004;Taylor et al 2020). In response to unilateral stimulation, a low amplitude, contralateral response Fig.…”

Electrode Montage for Bilateral Cervical Vestibular Evoked Myogenic Potential (cVEMP) Testing

“…First, there is no uniform recommendation for electrode montages 19 . While the active/non-inverting electrode is uniformly placed on the belly of the SCM, there are variations in the reference/inverting electrode placement, primarily being placed on the manubrium of the sternum, the clavicle or on the SCM tendon [20][21][22][23][24][25] . Second, BCV placement and SCM contraction method varies, depending on the method of BCV stimulation.…”

“…However, one confounding issue when completing BCV cVEMP testing using bilateral SCM contraction is the presence of contralateral crossed cVEMP responses. The presence of these contralateral crossed responses can be appreciated in studies using monaural ACS and bilateral SCM contraction [20][21][22][23] . These contralateral-crossed responses are most often inverted compared to the ipsilateral response [20][21][22] .…”