Vestibular evoked myogenic potential

The vestibular evoked myogenic potential 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

Main article: vestibular system

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).

History

Bickford et al. (1964) ^[1] and subsequently^[2], 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 ^[3] 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) ^[4] 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.^[5]

VsEPA and VSEPL

VsEP assesses the non-auditory portions of the labyrinth and requires kinematic stimuli (i.e. motion) instead of sound stimuli. 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.

Generators and thresholds

Early responses are said to originate from the peripheral vestibular nerve (cranial nerve VIII), and the later responses originate from the brainstem on up to the cortex, but the precise locations are debatable.

The response threshold estimates the sensitivity of the vestibular system and its pathway. The threshold can be defined as the lowest level a response is visible, the highest level no response is visible, or the halfway point between the two.

Stimulus, recording, and subject variables all need to be closely monitored as they can have a significant impact on the response. Stimulus factors that can have an effect on the response are the kinematic components, the stimulus magnitude, the repetition rate, and the axis in which the stimulus is applied. Recording variables that can affect the response are the hardware parameters (i.e. the filter cutoffs and the electrode sites) and the signal averaging parameters (i.e. the sampling rate and the number of responses averaged). The subject variables that can impact the response are the general physiological condition, core temperature, age, and genetic background of the subject. Additionally, the level of anesthesia can affect the later responses, and various pathological and pharmacological conditions have been shows to have an impact on the response.

Application

An early application was in the diagnosis of superior canal dehiscence (previously known as the Tullio phenomenon), a condition in which there are usually 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.^[6] It has diagnostic applications in Ménière's disease, vestibular neuritis, otosclerosis as well as central disorders such as Multiple Sclerosis.

Other methods of activating the vestibular apparatus have been developed, including taps to the head^[7], bone vibration ^[8] and short duration electrical stimulation.^[9] It is likely that both air-conducted and bone-conducted stimuli primarily excite irregularly discharging otolith afferents. ^[10] The two otolith receptors appear to have differing resonances that may also explain their responses.^[11]

In addition to the response in the SCM, similar reflexes can be shown for the masseter^[12] and for eye muscles (oVEMPs or OVEMPs = ocular vestibular evoked myogenic potentials) ^[13].

See also

• Vestibular system
• Electrophysiology
• Evoked potential
• Auditory evoked potential
• Visual evoked potential
• Auditory brainstem potential

References

1. ^ Bickford RG, Jacobson JL, Cody DTR (1964). Nature of average evoked potentials to sound and other stimuli in man. Ann NY Acad Sci 112:204-218.
2. ^ Townsend GL, Cody DTR (1971). The averaged inion response evoked by acoustic stimulation: its relation to the saccule. Ann Otol Rhinol Laryngol 80: 121-131.
3. ^ Colebatch JG, Halmagyi GM(1992). Vestibular evoked potentials in human neck muscles before and after unilateral deafferation. Neurology 42: 1635-1636.
4. ^ Colebatch JG, Halmagyi GM, Skuse NF (1994). Myogenic potentials generated by a click-evoked vestibulocollic reflex. J Neurol Neurosurg Psychiatry 57:190-197.
5. ^ Colebatch JG, Rothwell JC (2004). Motor unit excitability changes mediating vestibulocollic reflexes. Clin Neurophysiol 115(11):2567-2573.
6. ^ Welgampola MS, Myrie OA, Minor LB, Carey JP (2008). Vestibular-evoked myogenic potential thresholds normalize on plugging superior canal dehiscence. Neurology 70:464-472.
7. ^ Halmagyi GM, Yavor RA, Colebatch JG (1995). TApping the head activates the vestibular system: a new use for the clinical reflex hammer. Neurology 45(10); 1927-29.
8. ^ Sheykholeslami K, Murofushi T, Kermany MH, Kaga K (2000). Bone-conducted evoked myogenic potentials from the sternomastoid muscles. Acta Otolaryngol 120(6): 731-4.
9. ^ Watson SRD, Colebatch JG (1998). Vestibulocollic reflexes evoked by short-duration galvanic stimulation in man. J Physiol 513(2):587-97.
10. ^ Curthoys IS, Kim J, McPhedran SK, Camp AJ (2006). Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig. Exp Brain Res 175:256-267.
11. ^ Todd NPM, Rosengren SM, Colebatch JG (2009). A utricular origin of frequency tuning to low-frequency vibration in the human vestibular system?. Neurosci Lett 451:175-180.
12. ^ Deriu F, Rothwell JC. A sound-evoked vesibulomasseteric reflex in healthy humans. J Neurophysiol 93(5): 2739-51.
13. ^ Rosengren SM, Todd NPM, Colebatch JG (2005). Vestibular-evoked extraocular potentials produced by stimulation with bone-conducted sound. Clin Neurophysiol 116(8): 1938-48.