Vestibular evoked myogenic potentials in humans: a review

The human vestibule has preserved an ancestral sound sensitivity and it has been suggested that a reflex could originate from this property, thus inducing cervical muscle microcontractions secondary to strong acoustic stimulations. This reflex is assumed to originate in the saccule, the afferent pathways being either the vestibulocochlear nerve or the inferior vestibular nerve, and the efferent pathways the vestibulospinal tract. Averaging these muscular responses allows vestibular evoked myogenic potentials (VEMPs) to be obtained. The responses consist of two alternatively positive and negative successive waves (p13-n23, p33-n43). The characteristics of this reflex are defined in the literature as follows: it has been established that VEMP amplitude depends on muscular tension. All studies give concording evidence that in healthy subjects the first component of VEMP is more consistent than the second. Binaural stimulation is always responsible for responses of greater amplitude than those obtained from monaural stimulation. Following monaural stimulation, however, VEMPs are either of greater amplitude on the muscle ipsilateral to the stimulation or of the same amplitude on both muscles. There is consensus in the literature demonstrating that VEMP amplitude depends on stimulus intensity: the threshold of VEMP occurrence is clearly above auditory level but varies from one individual to the next. In the 1970s, recordings performed in cases of specific audiovestibular defects suggested that the reflex receptor could be the saccule. More recent studies suggest that the cochlea too could be involved in the response. Likewise, while a number of studies tend to demonstrate that VEMPs depend on vestibular integrity, others suggest that afferent pathways could be of both cohlear and vestibular origin. Finally, while it has been suggested that VEMP efferent pathways travel through the vestibulospinal tract, whether it is the lateral or the medial vestibulospinal tract that is concerned remains to be clarified. A few points regarding VEMP receptors and afferent and efferent pathways call for further investigation. They are inaccurate for use in routine vestibular examination. Once precise receptor localization and pathways are clarified, VEMP recording will provide both a straightforward non-invasive exploration of each vestibule independently and an attractive method by which to explore otolithic receptors and vestibulospinal pathways.