Physiology, clinical evidence and diagnostic relevance of sound-induced and vibration-induced vestibular stimulation

Purpose of review: To examine the recent literature concerning the neural basis and clinical evidence for the response of the labyrinth to sound and vibration: vestibular-evoked myogenic potentials (VEMPs) and vibration-induced nystagmus (VIN). Recent findings: There are two streams of information from each otolith - a sustained stream (afferents with regular resting activity, signalling gravity and low-frequency linear accelerations) and a transient stream (afferents with irregular resting activity) signalling onset of linear acceleration, and sound and vibration. These irregular neurons are synchronized to each cycle of the stimulus. Neurons in the transient stream are tested by presenting sounds or vibration (500 Hz) and using surface electrodes to measure myogenic potentials from muscles activated by otolithic stimuli (VEMPs). 100 Hz vibration activates irregular canal afferents and causes a stimulus-locked VIN in patients with asymmetric canal function. These new tests of the transient system have one big advantage over older tests of the sustained system - they reliably show the effect of long-term unilateral vestibular loss. Summary: The new physiological and anatomical evidence shows how sound and vibration activate otolith and canal receptors and so provides the scientific foundation for VEMPs and VIN, which are important tools for diagnosing vestibular disorders. VIDEO ABSTRACT: http://links.lww.com/CONR/A47

Plasticity during Vestibular Compensation: The Role of Saccades

This paper is focused on one major aspect of compensation: the recent measures of saccadic responses to high acceleration head turns during human vestibular compensation and their possible implications for recovery after unilateral vestibular loss (UVL). New measurement techniques have provided additional insights into how patients recover after UVL and have given clues for vestibular rehabilitation. Prior to this it has not been possible to quantify the level of function of all the peripheral vestibular sense organs. Now it is. By using vestibular-evoked myogenic potentials to measure utricular and saccular function and by new video head impulse testing to measure semicircular canal function to natural values of head accelerations. With these new video procedures it is now possible to measure both slow phase eye velocity and also saccades during head movements with natural values of angular acceleration. The present evidence is that after UVL there is little or no restoration/compensation of slow phase eye velocity responses to natural head accelerations. It is doubtful as to whether the modest changes in slow phase eye velocity to small angular accelerations are functionally effective during compensation. On the other hand it is now clear that saccades can play a very important role in helping patients compensate and return to a normal lifestyle. Preliminary evidence suggests that different patterns of saccadic response may predict how well patients recover. Furthermore it may be possible to train patients to produce more effective saccadic patterns in the first days after their unilateral loss and possibly improve their compensation process. Some patients do learn new strategies, new behaviors, to conceal their inadequate vestibulo-ocular response but when those strategies are prevented from operating by using passive, unpredictable, high acceleration natural head movements, as in the head impulse test, the vestibular loss can be demonstrated. It is those very strategies which the tests exclude, which may be the cause of their successful compensation

The Effect of Ocular Torsional Position on Perception of the Roll-tilt of Visual Stimuli

AbstractPerceived postural orientation during whole-body roll-tilt is commonly inferred from settings of a visual line to the perceived gravitational horizontal or vertical. This inference assumes that the change in ocular torsional position (ocular counterrolling) which occurs during roll-tilt has no effect on the perceived orientation of the visual stimulus. We investigated this assumption by measuring, during whole body roll-tilt stimulation, settings of a visual line and a somatosensory bar to the perceived gravitational horizontal and comparing the difference in these settings to the objectively measured ocular torsional position for each subject. Two stimulus paradigms were used: one where the subject was given a roll-tilt stimulus and the eye torted, the other where there was eye torsion without a roll-tilt stimulus. In both paradigms there was a very close relationship in magnitude and direction between the difference in the settings of the two perceptual indicators to gravitational horizontal and the objectively measured ocular torsion. We conclude that change in ocular torsional position alone changes the perceived orientation of a visual line. The corollary is that settings of a visual line cannot be used to infer perceived postural orientation directly. © 1997 Elsevier Science Ltd. All rights reserved

RE: Letter to the editor by Dr. S. J. Xie, concerning our manuscript Manzari L, Burgess AM, Curthoys IS: "Dissociation between cVEMP and oVEMP responses: different vestibular origins of each VEMP?"

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A Review of Neural Data and Modelling to Explain How a Semicircular Canal Dehiscence (SCD) Causes Enhanced VEMPs, Skull Vibration Induced Nystagmus (SVIN), and the Tullio Phenomenon

Angular acceleration stimulation of a semicircular canal causes an increased firing rate in primary canal afferent neurons that result in nystagmus in healthy adult animals. However, increased firing rate in canal afferent neurons can also be caused by sound or vibration in patients after a semicircular canal dehiscence, and so these unusual stimuli will also cause nystagmus. The recent data and model by Iversen and Rabbitt show that sound or vibration may increase firing rate either by neural activation locked to the individual cycles of the stimulus or by slow changes in firing rate due to fluid pumping (“acoustic streaming”), which causes cupula deflection. Both mechanisms will act to increase the primary afferent firing rate and so trigger nystagmus. The primary afferent data in guinea pigs indicate that in some situations, these two mechanisms may oppose each other. This review has shown how these three clinical phenomena—skull vibration-induced nystagmus, enhanced vestibular evoked myogenic potentials, and the Tullio phenomenon—have a common tie: they are caused by the new response of semicircular canal afferent neurons to sound and vibration after a semicircular canal dehiscence

Electrical vestibular stimulation in humans. A narrative review

Background: In patients with bilateral vestibulopathy, the regular treatment options, such as medication, surgery, and/ or vestibular rehabilitation, do not always suffice. Therefore, the focus in this field of vestibular research shifted to electri- cal vestibular stimulation (EVS) and the development of a system capable of artificially restoring the vestibular func- tion. Key Message: Currently, three approaches are being investigated: vestibular co-stimulation with a cochlear im- plant (CI), EVS with a vestibular implant (VI), and galvanic vestibular stimulation (GVS). All three applications show promising results but due to conceptual differences and the experimental state, a consensus on which application is the most ideal for which type of patient is still missing. Summa- ry: Vestibular co-stimulation with a CI is based on “spread of excitation,” which is a phenomenon that occurs when the currents from the CI spread to the surrounding structures and stimulate them. It has been shown that CI activation can indeed result in stimulation of the vestibular structures. Therefore, the question was raised whether vestibular co- stimulation can be functionally used in patients with bilat- eral vestibulopathy. A more direct vestibular stimulation method can be accomplished by implantation and activa- tion of a VI. The concept of the VI is based on the technology and principles of the CI. Different VI prototypes are currently being evaluated regarding feasibility and functionality. So far, all of them were capable of activating different types of vestibular reflexes. A third stimulation method is GVS, which requires the use of surface electrodes instead of an implant- ed electrode array. However, as the currents are sent through the skull from one mastoid to the other, GVS is rather unspe- cific. It should be mentioned though, that the reported spread of excitation in both CI and VI use also seems to in- duce a more unspecific stimulation. Although all three ap- plications of EVS were shown to be effective, it has yet to be defined which option is more desirable based on applicabil- ity and efficiency. It is possible and even likely that there is a place for all three approaches, given the diversity of the pa- tient population who serves to gain from such technologies

The feasibility of testing otoliths and semicircular canals function using VEMPs and vHIT in Malaysian children

Early identification of any vestibular dysfunction and balance problem in children is crucial for their general well-being. However the identification process, could be challenging and difficult as compared to adults. We conducted a preliminary study to review our initial experience with ocular and cervical vestibular evoked myogenic potentials (oVEMPs and cVEMPs), video head impulse test (vHIT) and Bruininks Oseretsky Test of Motor Proficiency II (BOT-2) on healthy children and also to determine the feasibility of these tests in this population. Twenty one normal healthy children (12 boys and 9 girls), aged between 6 and 15 years old (mean age, 11.15 ± 2.54 years) participated in the study. They underwent oVEMPs and cVEMPs elicited with bone conduction stimulus via minishaker and air conduction stimulus respectively. All six semicircular canals were assessed using the vHIT. Bilateral coordination, balance, running, speed and agility which are the three subsets of BOT-2 gross motor assessment were conducted for balance assessment. All subjects completed the vestibular and balance assessment except for 1 subject who did not complete the vHIT vertical component. The response rate was 100% for oVEMPs, cVEMPs, and BOT-2, and 95.24% for vHIT. The mean latency and mean amplitude for n10 oVEMPs were 8.88 ± 0.92 and 2.71 ± 1.29, respectively. The mean latency for cVEMPs p13, and n23 were 13.4 ± 1.35 and 21.76 ± 3.71, respectively with interamplitude mean of 97.57 ± 42.69. The vHIT mean for vestibular ocular reflex (VOR) gain were >0.85 for lateral canals and > 0.65 for vertical canals. The mean scale score for bilateral coordination, balance, running, speed, and agility for BOT-2 were 17.52 ± 3.40, 15.14 ± 3.65 and 13.9 ± 5.46, respectively. This study suggest that VEMPs, vHIT, and BOT-2 are feasible test for vestibular and balance assessment in children. Apart from the tests findings, it is hoped that the described experienced and adjustment made in assessing this young population could also be applied by other relevant professionals

A geometric basis for measurement of three-dimensional eye position using image processing

AbstractPolar cross correlation is commonly used for determination of ocular torsion from video images, but breaks down at eccentric positions if the spherical geometry of the eye is not considered. We have extended this method to allow three-dimensional eye position measurement over a range of ±20 deg by determining the correct projection of the eye onto the image plane of the camera. We also determine the orientation of the camera with respect to the eye, allowing eye position to be represented in appropriate head-fixed coordinates. These algorithms have been validated using both in vitro and in vivo measures of eye position

Enhanced Vestibulo-Ocular Reflex Responses on vHIT. Is It a Casual Finding or a Sign of Vestibular Dysfunction?

In current clinical practice, when in response to vHIT testing the observed slow-phase eye-velocity responses are significantly higher than head velocity, the most probable cause is considered to be an inadequate collection method or a recording artifact. We present two cases with clinical diagnoses of Menière's Disease: for both cases, enhanced eye velocity responses were measured with a rigorous vHIT testing protocol. In the first case we measured these enhanced responses on each test performed during a 5 year time series; in the second case multiple measurements were obtained from a patient after the radiologic diagnosis of vestibulo-cochlear hydrops. The two cases presented and the new evidence reported by other researchers suggest that owing to the low probability of artifact and the high consistency of the vHIT measurements, we should consider the hypothesis of a physio-pathologic cause for the enhanced eye responses to vHIT testing of some patients with vestibular dysfunction