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 indu cing cervical muscle microcontractions secondary to strong acoustic stimula tions. This reflex is assumed to originate in the saccule, the afferent pat hways being either the vestibulocochlear nerve or the inferior vestibular n erve, and the efferent pathways the vestibulospinal tract. Averaging those muscular responses allows vestibular evoked myogenic potentials (VEMPs) to be obtained. The responses consist of two alternatively positive :Ind negat ive successive a al es (p13-n23. p33-n43). The characteristics of this refl ex are defined in the literature as follows: it has been established that V EMP amplitude depends on muscular tension. All studies give concording evid ence that in healthy subjects the first component of VEMP is more consisten t than the second. Binaural stimulation is always responsible for responses of greater amplitude than those obtained from monaural stimulation. Follow ing 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 ampl itude depends on stimulus intensity: the threshold of VEMP occurrence is cl early above auditory level but varies from one individual to the next. In t he 1970s, recordings performed in cases of specific audiovestibular defects suggested that the reflex receptor could be the saccule. More recent studi es suggest that the cochlea too could be involved in the response. Likewise , while a number of studies tend to demonstrate that VEMPs depend on vestib ular integrity, others suggest that afferent pathways could be of both cohl ear and vestibular origin. Finally, while it has boon suggested that VEMP e fferent pathways travel through the vestibulospinal tract, whether it is th e lateral or the medial vestibulospinal tract that is concerned remains to he clarified, A few points regarding VEMP receptors and afferent and effere nt pathways call for further investigation. They are inaccurate for use in routine vestibular examination. Once precise receptor localization and path ways are clarified. VEMP recording will provide both a straightforward non- invasive exploration of each vestibule independently and an attractive meth od by which to explore otolithic receptors and vestibulospinal pathways.