Technical pitfalls of vestibular-evoked myogenic potentials

Abstract

In humans, intense sound evokes ipsilateral myogenic potentials in the sternocleidomastoid muscle (SCM) with a short latency positive-negative (p13-n23) waveform. This sound-evoked vestibulocollic reflex is termed the vestibular-evoked myogenic potential (VEMP). In the present study, the technical and diagnostic pitfalls of the VEMP are introduced.An electromyograph (EMG) is used to record signals from a pair of surface electrodes mounted on the upper-half (active electrode) and the sternal head (reference electrode) of the SCM. The positions of the electrodes influence the p13-n23 amplitude and the VEMP waveform. Head rotation around the yaw axis brings the proximal end of the unilateral SCM muscle close to the distal end, making the outline of the SCM muscle more visible on the body surface. This rotation is useful for the precise positioning of the electrodes.The muscle background activity during the test influences the p13-n23 amplitude of the VEMP. Subjects are instructed to rotate their heads around the yaw axis contralateral to the sound stimulation so as to contract their unilateral SCM muscles. During the VEMP test, the background EMG activity of the tested SCM is continuously monitored, and the peak-to-peak amplitude in the interference pattern of the background EMG should be maintained at more than 200 μV. When the attenuation of the amplitude in the interference pattern begins, the subjects should be instructed to rotate their heads once again so as to recover tonic contraction.The muscle background activity during the test is electromyographically estimated based on the integral value of the rectified EMG activity level for 20 milliseconds preceding the stimulus onset. When the integral value of the rectified EMG activity is less than 1000 μV×ms, the background EMG activity of the tested SCM is objectively evaluated as being too low to induce the evoked response.To correct the amplitude according to the muscle background activity, the p13-n23 amplitude is divided by the integral value of the rectified EMG activity level. This correction is important for the test-retest reproductivity of the VEMP amplitude. To diagnose a right-left asymmetry in the VEMP amplitude, the clinical use of the corrected amplitude is recommended.