Vestibular evoked myogenic potential

The vestibular evoked myogenic potential (VEMP or VsEP) is a neurophysiological assessment technique used to determine the function of the otolithic organs (utricle and saccule) of the inner ear. It complements the information provided by caloric testing and other forms of inner ear (vestibular apparatus) testing.

The vestibular system helps a person maintain: balance, visual fixation, posture, and lower muscular control.

There are six receptor organs located in the inner ear: cochlea, utricle, saccule, and the lateral, anterior, and posterior semicircular canals. The cochlea is a sensory organ with the primary purpose to aid in hearing. The utricle and saccule are sensors for detecting angular or linear acceleration, and the three semicircular canals detect head rotation.

Located within the membranous labyrinthine walls of the vestibular system are approximately 67,000 hair cells in total. This includes ~7,000 hair cells from each of the semicircular canals located within the crista ampullaris, ~30,000 hair cells from the utricle, and ~16,000 hair cells from the saccule. Each hair cell has about 70 stereocilia (short rod-like hair cells) and one kinocilium (long hair cell).

Bickford et al. (1964) and subsequently Townsend and Cody, provided evidence for a short latency response in posterior neck muscles in response to loud clicks that appeared to be mediated by activation of the vestibular apparatus. These authors made the additional important observations that the response was generated from EMG (muscle) activity and that it scaled with the level of tonic activation. Subsequent work led to the suggestion that the saccule was the end organ excited.

In 1992 Colebatch and Halmagyi reported a patient with a short latency response to loud clicks studied using a modified recording site (the sternocleidomastioid muscles: SCM) and which was abolished by selective vestibular nerve section. Colebatch et al. (1994) described the basic properties of the response. These were: the response occurred ipsilateral to the ear stimulated, the click threshold was high, the response did not depend upon hearing (cochlear function) per se, it scaled in direct proportion to the level of tonic neck contraction, the response was small (although large compared to many evoked potentials) and required averaging, and only the initial positive-negative response (p13-n23 by latency) was actually vestibular-dependent. It was subsequently shown to be generated by a brief period of inhibition of motor unit discharge.

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