The vestibular evoked myogenic potential (VEMP or VsEP) is a

neurophysiological Neurophysiology is a branch of physiology and neuroscience that studies nervous system function rather than nervous system architecture. This area aids in the diagnosis and monitoring of neurological diseases. Historically, it has been dominated b ...

assessment technique used to determine the function of the

otolithic organs An otolith ( grc-gre, ὠτο-, ' ear + , ', a stone), also called statoconium or otoconium or statolith, is a calcium carbonate structure in the saccule or utricle (ear), utricle of the inner ear, specifically in the vestibular system of verteb ...

( utricle and

saccule The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves verticall ...

) of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

. It complements the information provided by caloric testing and other forms of inner ear (

vestibular apparatus The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating motor coordination, movement with balance. Together with the cochlea, a part of the auditory syst ...

) testing. There are two different types of VEMPs. One is the oVEMP and another is the cVEMP. The oVEMP measures integrity of the utricule and superior vestibular nerve and the cVemp measures the saccule and the inferior vestibular nerve.

The vestibular system

The vestibular system helps a person maintain:

balance Balance or balancing may refer to: Common meanings * Balance (ability) in biomechanics * Balance (accounting) * Balance or weighing scale * Balance as in equality or equilibrium Arts and entertainment Film * ''Balance'' (1983 film), a Bulgarian ...

, visual fixation,

posture Posture or posturing may refer to: Medicine * Human position ** Abnormal posturing, in neurotrauma **Spinal posture ** List of human positions * Posturography, in neurology Other uses * Posture (psychology) In humans, posture can provide a s ...

, 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 otolith organs (utricle and saccule) are sensors for detecting linear acceleration in their respective planes (utrical=horizontal plane (forward/backward; up/down); saccule=sagital plane (up/down)), and the three semicircular canals (anterior/superior, posterior, and horizontal) detect head rotation or angular acceleration in their respective planes of orientation (anterior/superior=pitch (nodding head), posterior=roll (moving head from one shoulder to other), and horizontal=yaw (shaking head left to right). 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).

History

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 sternocleidomastoid 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 (

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

r 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.

VsEPA and VSEPL

VsEP assesses the non-auditory portions of the labyrinth and requires kinematic stimuli (i.e. motion) instead of sound stimuli and bear only a loose relationship to VEMPs. This kinematic stimuli needs to be well characterized, precisely controlled, consistent in amplitude, and consistent in kinematic makeup. An electromechanical shaker is a stimuli generator that is widely available. This shaker provides a transient stimuli, can generate angular or linear acceleration, and can couple to the skull directly (with skull screws) or via a stimulus platform. The VsEP is commonly divided into two sections: angular vestibular evoked potentials (VsEPA) and linear vestibular evoked potentials (VsEPL).

VsEPA

VsEPA stimuli needs to be a brief or transient, high amplitude, angular acceleration pulse. Currently, the most effective stimuli for the best results have not yet been identified or agreed upon by researchers. The major downfall of the VsEPA response is that it also elicits a VsEPL response.

VsEPL

In contrast to VsEPA, researchers have standardized the VsEPL stimuli but many variants of this standard are being used in research laboratories today. The stimulus needs to be a transient, rapidly changing pulse (i.e. linear jerk stimulus). A rectangular jerk step/pulse is generated by an electromechanical shaker. The main downfall of the VsEPL response is the presence of electrical artifacts due to movement and touching of the wires/electrodes during testing.

Application of VEMPs

An early application was in the diagnosis of

superior canal dehiscence Superior semicircular canal dehiscence syndrome is a set of hearing and balance symptoms, related to a rare medical condition of the inner ear, known as ''superior canal dehiscence''. The symptoms are caused by a thinning or complete absence of ...

a condition in which there can be clinical symptoms and signs of vestibular activation by loud sounds. Such cases have a pathologically lowered threshold for the sound-evoked VEMP. The test is also of use in demonstrating successful treatment. It has diagnostic applications in

Ménière's disease Ménière's disease (MD) is a disease of the inner ear that is characterized by potentially severe and incapacitating episodes of vertigo, tinnitus, hearing loss, and a feeling of fullness in the ear. Typically, only one ear is affected initial ...

vestibular neuritis Labyrinthitis is inflammation of the labyrinth – a maze of fluid-filled channels in the inner ear. Vestibular neuritis is inflammation of the vestibular nerve – the nerve in the inner ear that sends messages related to motion and position t ...

otosclerosis Otosclerosis is a condition of the middle ear where portions of the dense enchondral layer of the bony labyrinth remodel into one or more lesions of irregularly-laid spongy bone. As the lesions reach the stapes the bone is resorbed, then hardene ...

as well as central disorders such as

multiple sclerosis Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...

. Other methods of activating the vestibular apparatus have been developed, including taps to the head, bone vibration and short duration electrical stimulation. It is likely that both air-conducted and bone-conducted stimuli primarily excite irregularly discharging otolith afferents. The two otolith receptors appear to have differing resonances that may also explain their responses. In addition to the response in the SCM, similar reflexes can be shown for the masseter and for eye muscles (oVEMPs or OVEMPs = ocular vestibular evoked myogenic potentials).Rosengren SM, Todd NPM, Colebatch JG (2005). Vestibular-evoked extraocular potentials produced by stimulation with bone-conducted sound. Clin Neurophysiol 116(8): 1938-48.

References

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