Acoustic Responses of Vestibular Afferents in a Model of Superior Canal Dehiscence

Skull Vibration-Induced Nystagmus in Superior Semicircular Canal Dehiscence: A New Insight into Vestibular Exploration-A Review

The third window syndrome, often associated with the Tullio phenomenon, is currently most often observed in patients with a superior semicircular-canal dehiscence (SCD) but is not specific to this pathology. Clinical and vestibular tests suggestive of this pathology are not always concomitantly observed and have been recently complemented by the skull-vibration-induced nystagmus test, which constitutes a bone-conducted Tullio phenomenon (BCTP). The aim of this work was to collect from the literature the insights given by this bedside test performed with bone-conducted stimulations in SCD. The PRISMA guidelines were used, and 10 publications were included and analyzed. Skull vibration-induced nystagmus (SVIN), as observed in 55 to 100% of SCD patients, usually signals SCD with greater sensitivity than the air-conducted Tullio phenomenon (ACTP) or the Hennebert sign. The SVIN direction when the test is performed on the vertex location at 100 Hz is most often ipsilaterally beating in 82% of cases for the horizontal and torsional components and down-beating for the vertical component. Vertex stimulations are more efficient than mastoid stimulations at 100 Hz but are equivalent at higher frequencies. SVIN efficiency may depend on stimulus location, order, and duration. In SCD, SVIN frequency sensitivity is extended toward high frequencies, with around 400 Hz being optimal. SVIN direction may depend in 25% on stimulus frequency and in 50% on stimulus location. Mastoid stimulations show frequently diverging results following the side of stimulation. An after-nystagmus observed in 25% of cases can be interpreted in light of recent physiological data showing two modes of activation: (1) cycle-by-cycle phase-locked activation of action potentials in SCC afferents with irregular resting discharge; (2) cupula deflection by fluid streaming caused by the travelling waves of fluid displacement initiated by sound or vibration at the point of the dehiscence. The SVIN direction and intensity may result from these two mechanisms' competition. This instability explains the SVIN variability following stimulus location and frequency observed in some patients but also discrepancies between investigators. SVIN is a recent useful insight among other bedside examination tests for the diagnosis of SCD in clinical practice.

Impaired Vestibulo-Ocular Reflex on Video Head Impulse Test in Superior Canal Dehiscence: "Spontaneous Plugging" or Endolymphatic Flow Dissipation?

Surgical plugging of the superior semicircular canal (SSC) represents an effective procedure to treat disabling symptoms in superior canal dehiscence (SCD), despite resulting in an impaired vestibulo-ocular reflex (VOR) gain for the SSC. On the other hand, SSC hypofunction on video head impulse test (vHIT) represents a common finding in patients with SCD exhibiting sound/pressure-induced vertigo, a low-frequency air-bone gap (ABG), and enhanced vestibular-evoked myogenic potentials (VEMPs). "Spontaneous canal plugging" has been assumed as the underlying process. Nevertheless, missing/mitigated symptoms and/or near-normal instrumental findings would be expected. An endolymphatic flow dissipation has been recently proposed as an alternative pathomechanism for SSC VOR gain reduction in SCD. We aimed to shed light on this debate by comparing instrumental findings from 46 ears of 44 patients with SCD exhibiting SSC hypofunction with post-operative data from 10 ears of 10 patients with SCD who underwent surgical plugging. While no difference in SSC VOR gain values was found between the two groups (p = 0.199), operated ears developed a posterior canal hypofunction (p = 0.002). Moreover, both ABG values (p = 0.012) and cervical/ocular VEMP amplitudes (p < 0.001) were significantly higher and VEMP thresholds were significantly lower (p < 0.001) in ears with SCD compared to operated ears. According to our data, canal VOR gain reduction in SCD should be considered as an additional sign of a third window mechanism, likely due to an endolymphatic flow dissipation.

A bone-conducted Tullio phenomenon-A bridge to understand skull vibration induced nystagmus in superior canal dehiscence

Nystagmus produced in response to air-conducted sound (ACS) stimulation-the Tullio phenomenon-is well known in patients with a semicircular canal (SCC) dehiscence (SCD). Here we consider the evidence that bone-conducted vibration (BCV) is also an effective stimulus for generating the Tullio phenomenon. We relate the clinical evidence based on clinical data extracted from literature to the recent evidence about the physical mechanism by which BCV may cause this nystagmus and the neural evidence confirming the likely mechanism. The hypothetical physical mechanism by which BCV activates SCC afferent neurons in SCD patients is that traveling waves are generated in the endolymph, initiated at the site of the dehiscence. We contend that the nystagmus and symptoms observed after cranial BCV in SCD patients is a variant of Skull Vibration Induced Nystagmus (SVIN) used to identify unilateral vestibular loss (uVL) with the major difference being that in uVL the nystagmus beats away from the affected ear whereas in Tullio to BCV the nystagmus beats usually toward the affected ear with the SCD. We suggest that the cause of this difference is a cycle-by-cycle activation of SCC afferents from the remaining ear, which are not canceled centrally by simultaneous afferent input from the opposite ear, because of its reduced or absent function in uVL. In the Tullio phenomenon, this cycle-by-cycle neural activation is complemented by fluid streaming and thus cupula deflection caused by the repeated compression of each cycle of the stimuli. In this way, the Tullio phenomenon to BCV is a version of skull vibration-induced nystagmus.

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.

Proposal for a Unitary Anatomo-Clinical and Radiological Classification of Third Mobile Window Abnormalities

Introduction: An increased number of otic capsule dehiscence (OCD) variants relying on the third window pathomechanism have been reported lately. Therefore, a characterization of the anatomical structures involved and an accurate radiological description of the third window (TW) interface location have become essential for improving the diagnosis and appropriate therapeutic modalities. The purpose of this article is to propose a classification based on clinical, anatomical, and radiological data of third mobile window abnormalities (TMWA) and to discuss the alleged pathomechanism in lesser-known clinical variants.

Materials and Methods: The imaging records of 259 patients who underwent, over the last 6 years, a high-resolution CT (HRCT) of the petrosal bone for conductive hearing loss were analyzed retrospectively. Patients with degenerative, traumatic, or chronic infectious petrosal bone pathology were excluded. As cases with a clinical presentation similar to those of a TW syndrome have recently been described in the literature but without these being confirmed radiologically, we thought it necessary to be integrated in a separated branch of this classification as “CT - TMWA.” The same goes for certain intralabyrinthine pathologies also recently reported in the literature, which mimic to some extent the symptoms of a TW pathology. Therefore, we suggest to call them intralabyrinthine TW-like abnormalities.

Results: Temporal bone HRCT and, in some cases, 3T MRI of 97 patients presenting symptomatic or pauci-symptomatic, single or multiple, unilateral or bilateral OCD were used to develop this classification. According to the topography and anatomical structures involved at the site of the interface of the TW, a third-type classification of OCD is proposed.

Conclusions: A classification reuniting all types of TMWA as the one proposed in this article would allow for a better systematization and understanding of this complex pathology and possibly paves the way for innovative therapeutic approaches. To encompass all clinical and radiological variants of TMWA reported in the literature so far, TMWAs have been conventionally divided into two major subgroups: Extralabyrinthine (or “true” OCD with three subtypes) and Intralabyrinthine (in which an additional mobile window-like mechanism is highly suspected) or TMWA-like subtype. Along these subgroups, clinical forms of OCD with multiple localization (multiple OCD) and those that, despite the fact that they have obvious characteristics of OCD have a negative CT scan (or CT – TMWA), were also included.

Sound-Evoked Responses in the Vestibulo-Ocular Reflex Pathways of Rats

Vestibular evoked myogenic potentials (VEMP) have been used to assess otolith function in clinics worldwide. However, there are accumulating evidence suggesting that the clinically used sound stimuli activate not only the otolith afferents, but also the canal afferents, indicating canal contributions to the VEMPs. To better understand the neural mechanisms underlying the VEMPs and develop discriminative VEMP protocols, we further examined sound-evoked responses of the vestibular nucleus neurons and the abducens neurons, which have the interneurons and motoneurons of the vestibulo-ocular reflex (VOR) pathways. Air-conducted clicks (50–80 dB SL re ABR threshold, 0.1 ms duration) or tone bursts (60–80 dB SL, 125–4,000 Hz, 8 ms plateau, 1 ms rise/fall) were delivered to the ears of Sprague-Dawley or Long-Evans rats. Among 425 vestibular nucleus neurons recorded in anesthetized rats and 18 abducens neurons recorded in awake rats, sound activated 35.9% of the vestibular neurons that increased discharge rates for ipsilateral head rotation (Type I neuron), 15.7% of the vestibular neurons that increased discharge rates for contralateral head rotation (Type II neuron), 57.2% of the vestibular neurons that did not change discharge rates during head rotation (non-canal neuron), and 38.9% of the abducens neurons. Sound sensitive vestibular nucleus neurons and abducens neurons exhibited characteristic tuning curves that reflected convergence of canal and otolith inputs in the VOR pathways. Tone bursts also evoked well-defined eye movements that increased with tone intensity and duration and exhibited peak frequency of ∼1,500 Hz. For the left eye, tone bursts evoked upward/rightward eye movements for ipsilateral stimulation, and downward/leftward eye movements for contralateral stimulation. These results demonstrate that sound stimulation results in activation of the canal and otolith VOR pathways that can be measured by eye tracking devices to develop discriminative tests of vestibular function in animal models and in humans.

A comprehensive finite element model for studying Cochlear-Vestibular interaction

We present a 3-D finite element (FE) model of the chinchilla's inner ear consisting of the entire cochlea structure and the vestibular system. The reaction of the basilar membrane to the head rotation and the reaction of ampulla to the stapes movement were investigated. These results demonstrate the existence of hearing-vestibular system interaction. They provide an explanation to the clinical finding on the coexistence between hearing loss and equilibration dysfunction. It is a preliminary, yet critical step toward the development of a comprehensive FE model of an entire ear for mechano-acoustic analysis.

Superior Semicircular Canal Dehiscence by Superior Petrosal Sinus: Proposal for Classification

OBJECTIVES

This study aimed to present 3 different clinical stages in patients presenting with superior semicircular canal dehiscence (SSCD) by the superior petrosal sinus (SPS). A specific 3-class classification based on clinical, radiological, and audio-vestibular arguments is proposed.

MATERIALS AND METHODS

We retrospectively compared clinical and radiological findings in 3 patients with different degrees of audio-vestibular dysfunction in whom the imagery evocated the diagnosis of SSCD by SPS. Imaging sensitivity was improved by combining inner ear high-resolution computed tomography (HRCT) scan and magnetic resonance imaging in fusion, allowing us to compare and corroborate clinical and audio-vestibular findings in each case with the imagery.

RESULTS

HRCT and 3T inner ear fusion imaging highlighted a direct contact and/or compression between SPS and the membranous superior semicircular canal (SSC). We propose a new classification of SSCD by SPS. Class "A" corresponds to an HRCT image with a "cookie bite" and thin bone still covering the SSC. Class "B" corresponds to a "cookie bite" image with confirmed contact between the SPS wall and the membranous SSC in MRI labyrinthine sequences. Class "C" type corresponds to a "cookie bite" image, contact, and obvious compression of the membranous SSC by SPS on MRI sequences.

CONCLUSION

Anatomical systematization is needed for daily practice. This classification of SSCD by SPS would contribute to a better understanding of the wide variety and variability in the occurrence and onset of symptoms.

Superior Canal Dehiscence Similarly Affects Cochlear Pressures in Temporal Bones and Audiograms in Patients

OBJECTIVES

The diagnosis of superior canal dehiscence (SCD) is challenging and audiograms play an important role in raising clinical suspicion of SCD. The typical audiometric finding in SCD is the combination of increased air conduction (AC) thresholds and decreased bone conduction thresholds at low frequencies. However, this pattern is not always apparent in audiograms of patients with SCD, and some have hearing thresholds that are within the normal reference range despite subjective reports of hearing impairment. In this study, we used a human temporal bone model to measure the differential pressure across the cochlear partition (PDiff) before and after introduction of an SCD. PDiff estimates the cochlear input drive and provides a mechanical audiogram of the temporal bone. We measured PDiff across a wider frequency range than in previous studies and investigated whether the changes in PDiff in the temporal bone model and changes of audiometric thresholds in patients with SCD were similar, as both are thought to reflect the same physical phenomenon.

DESIGN

We measured PDiff across the cochlear partition in fresh human cadaveric temporal bones before and after creating an SCD. Measurements were made for a wide frequency range (20 Hz to 10 kHz), which extends down to lower frequencies than in previous studies and audiograms. PDiff = PSV- PST is calculated from pressures measured simultaneously at the base of the cochlea in scala vestibuli (PSV) and scala tympani (PST) during sound stimulation. The change in PDiff after an SCD is created quantifies the effect of SCD on hearing. We further included an important experimental control-by patching the SCD, to confirm that PDiff was reversed back to the initial state. To provide a comparison of temporal bone data to clinical data, we analyzed AC audiograms (250 Hz to 8kHz) of patients with symptomatic unilateral SCD (radiographically confirmed). To achieve this, we used the unaffected ear to estimate the baseline hearing function for each patient, and determined the influence of SCD by referencing AC hearing thresholds of the SCD-affected ear with the unaffected contralateral ear.

RESULTS

PDiff measured in temporal bones (n = 6) and AC thresholds in patients (n = 53) exhibited a similar pattern of SCD-related change. With decreasing frequency, SCD caused a progressive decrease in PDiff at low frequencies for all temporal bones and a progressive increase in AC thresholds at low frequencies. SCD decreases the cochlear input drive by approximately 6 dB per octave at frequencies below ~1 kHz for both PDiff and AC thresholds. Individual data varied in frequency and magnitude of this SCD effect, where some temporal-bone ears had noticeable effects only below 250 Hz.

CONCLUSIONS

We found that with decrease in frequency the progressive decrease in low-frequency PDiff in our temporal bone experiments mirrors the progressive elevation in AC hearing thresholds observed in patients. This hypothesis remains to be tested in the clinical setting, but our findings suggest that that measuring AC thresholds at frequencies below 250 Hz would detect a larger change, thus improving audiograms as a diagnostic tool for SCD.

Rare Disorders of the Vestibular Labyrinth: of Zebras, Chameleons and Wolves in Sheep's Clothing

The differential diagnosis of vertigo syndromes is a challenging issue, as many - and in particular - rare disorders of the vestibular labyrinth can hide behind the very common symptoms of "vertigo" and "dizziness". The following article presents an overview of those rare disorders of the balance organ that are of special interest for the otorhinolaryngologist dealing with vertigo disorders. For a better orientation, these disorders are categorized as acute (AVS), episodic (EVS) and chronic vestibular syndromes (CVS) according to their clinical presentation. The main focus lies on EVS sorted by their duration and the presence/absence of triggering factors (seconds, no triggers: vestibular paroxysmia, Tumarkin attacks; seconds, sound and pressure induced: "third window" syndromes; seconds to minutes, positional: rare variants and differential diagnoses of benign paroxysmal positional vertigo; hours to days, spontaneous: intralabyrinthine schwannomas, endolymphatic sac tumors, autoimmune disorders of the inner ear). Furthermore, rare causes of AVS (inferior vestibular neuritis, otolith organ specific dysfunction, vascular labyrinthine disorders, acute bilateral vestibulopathy) and CVS (chronic bilateral vestibulopathy) are covered. In each case, special emphasis is laid on the decisive diagnostic test for the identification of the rare disease and "red flags" for potentially dangerous disorders (e. g. labyrinthine infarction/hemorrhage). Thus, this chapter may serve as a clinical companion for the otorhinolaryngologist aiding in the efficient diagnosis and treatment of rare disorders of the vestibular labyrinth.

Case Report: Could Hennebert's Sign Be Evoked Despite Global Vestibular Impairment on Video Head Impulse Test? Considerations Upon Pathomechanisms Underlying Pressure-Induced Nystagmus due to Labyrinthine Fistula

We describe a case series of labyrinthine fistula, characterized by Hennebert's sign (HS) elicited by tragal compression despite global hypofunction of semicircular canals (SCs) on a video-head impulse test (vHIT), and review the relevant literature. All three patients presented with different amounts of cochleo-vestibular loss, consistent with labyrinthitis likely induced by labyrinthine fistula due to different temporal bone pathologies (squamous cell carcinoma involving the external auditory canal in one case and middle ear cholesteatoma in two cases). Despite global hypofunction on vHIT proving impaired function for each SC for high accelerations, all patients developed pressure-induced nystagmus, presumably through spared and/or recovered activity for low-velocity canal afferents. In particular, two patients with isolated horizontal SC fistula developed HS with ipsilesional horizontal nystagmus due to resulting excitatory ampullopetal endolymphatic flows within horizontal canals. Conversely, the last patient with bony erosion involving all SCs developed mainly torsional nystagmus directed contralaterally due to additional inhibitory ampullopetal flows within vertical canals. Moreover, despite impaired measurements on vHIT, we found simultaneous direction-changing positional nystagmus likely due to a buoyancy mechanism within the affected horizontal canal in a case and benign paroxysmal positional vertigo involving the dehiscent posterior canal in another case. Based on our findings, we might suggest a functional dissociation between high (impaired) and low (spared/recovered) accelerations for SCs. Therefore, it could be hypothesized that HS in labyrinthine fistula might be due to the activation of regular ampullary fibers encoding low-velocity inputs, as pressure-induced nystagmus is perfectly aligned with the planes of dehiscent SCs in accordance with Ewald's laws, despite global vestibular impairment on vHIT. Moreover, we showed how pressure-induced nystagmus could present in a rare case of labyrinthine fistulas involving all canals simultaneously. Nevertheless, definite conclusions on the genesis of pressure-induced nystagmus in our patients are prevented due to the lack of objective measurements of both low-acceleration canal responses and otolith function.

Bone-Conducted oVEMP Latency Delays Assist in the Differential Diagnosis of Large Air-Conducted oVEMP Amplitudes

Background: A sensitive test for Superior Semicircular Canal Dehiscence (SCD) is the air-conducted, ocular vestibular evoked myogenic potential (AC oVEMP). However, not all patients with large AC oVEMPs have SCD. This retrospective study sought to identify alternate diagnoses also producing enlarged AC oVEMPs and investigated bone-conducted (BC) oVEMP outcome measures that would help differentiate between these, and cases of SCD.

Methods: We reviewed the clinical records and BC oVEMP results of 65 patients (86 ears) presenting with dizziness or balance problems who underwent CT imaging to investigate enlarged 105 dB nHL click AC oVEMP amplitudes. All patients were tested with BC oVEMPs using two different stimuli (1 ms square-wave pulse and 8 ms 125 Hz sine wave). Logistic regression and odds ratios were used to determine the efficacy of BC oVEMP amplitudes and latencies in differentiating between enlarged AC oVEMP amplitudes due to dehiscence from those with an alternate diagnosis.

Results: Fifty-three ears (61.6%) with enlarged AC oVEMP amplitudes were identified as having frank dehiscence on imaging; 33 (38.4%) had alternate diagnoses that included thinning of the bone covering (near dehiscence, n = 13), vestibular migraine (n = 12 ears of 10 patients), enlarged vestibular aqueduct syndrome (n = 2) and other causes of recurrent episodic vertigo (n = 6). BC oVEMP amplitudes of dehiscent and non-dehiscent ears were not significantly different (p > 0.05); distributions of both groups overlapped with the range of healthy controls. There were significant differences in BC oVEMP latencies between dehiscent and non-dehiscent ears for both stimuli (p < 0.001). A prolonged n1 125 Hz latency (>11.5 ms) was the best predictor of dehiscence (odd ratio = 27.8; 95% CI:7.0-111.4); abnormal n1 latencies were identified in 79.2% of ears with dehiscence compared with 9.1% of ears without dehiscence.

Conclusions: A two-step protocol of click AC oVEMP amplitudes and 125 Hz BC oVEMP latency measures optimizes the specificity of VEMP testing in SCD.

Evidence-based diagnostic use of VEMPs : From neurophysiological principles to clinical application

BACKGROUND

Vestibular evoked myogenic potentials (VEMPs) are increasingly being used for testing otolith organ function.

OBJECTIVE

This article provides an overview of the anatomical, biomechanical and neurophysiological principles underlying the evidence-based clinical application of ocular and cervical VEMPs (oVEMPs and cVEMPs).

MATERIAL AND METHODS

Systematic literature search in PubMed until April 2019.

RESULTS

Sound and vibration at a frequency of 500 Hz represent selective vestibular stimuli for the otolith organs. The predominant specificity of oVEMPs for contralateral utricular function and of cVEMPs for ipsilateral saccular function is defined by the different central projections of utricular and saccular afferents. VEMPs are particularly useful in the diagnosis of superior canal dehiscence and otolith organ specific vestibular dysfunction and as an alternative diagnostic approach in situations when video oculography is not possible or useful.

CONCLUSION

The use of VEMPs is a simple, safe, reliable and selective test of dynamic function of otolith organs.

Biomechanics of Third Window Syndrome

Third window syndrome describes a set of vestibular and auditory symptoms that arise when a pathological third mobile window is present in the bony labyrinth of the inner ear. The pathological mobile window (or windows) adds to the oval and round windows, disrupting normal auditory and vestibular function by altering biomechanics of the inner ear. The most commonly occurring third window syndrome arises from superior semicircular canal dehiscence (SSCD), where a section of bone overlying the superior semicircular canal is absent or thinned (near-dehiscence). The presentation of SSCD syndrome is well characterized by clinical audiological and vestibular tests. In this review, we describe how the third compliant window introduced by a SSCD alters the biomechanics of the inner ear and thereby leads to vestibular and auditory symptoms. Understanding the biomechanical origins of SSCD further provides insight into other third window syndromes and the potential of restoring function or reducing symptoms through surgical repair.

Nondestructive and objective assessment of the vestibular function in rodent models: A review

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

Investigating short latency subcortical vestibular projections in humans: what have we learned?

Vestibular evoked myogenic potentials (VEMPs) are now widely used for the noninvasive assessment of vestibular function and diagnosis in humans. This review focuses on the origin, properties, and mechanisms of cervical VEMPs and ocular VEMPs; how these reflexes relate to reports of vestibular projections to brain stem and cervical targets; and the physiological role of (otolithic) cervical and ocular reflexes. The evidence suggests that both VEMPs are likely to represent the effects of excitation of irregularly firing otolith afferents. While the air-conducted cervical VEMP appears to mainly arise from excitation of saccular receptors, the ocular VEMP evoked by bone-conducted stimulation, including impulsive bone-conducted stimuli, mainly arises from utricular afferents. The surface responses are generated by brief changes in motor unit firing. The effects that have been demonstrated are likely to represent otolith-dependent vestibulocollic and vestibulo-ocular reflexes, both linear and torsional. These observations add to previous reports of short latency otolith projections to the target muscles in the neck (sternocleidomastoid and splenius) and extraocular muscles (the inferior oblique). New insights have been provided by the investigation and application of these techniques.

[Evidence-based diagnostic use of VEMPs : From neurophysiological principles to clinical application. German version]

BACKGROUND

Vestibular evoked myogenic potentials (VEMPs) are increasingly being used for testing otolith organ function.

OBJECTIVE

This article provides an overview of the anatomical, biomechanical and neurophysiological principles of an evidence-based clinical application of ocular and cervical VEMPs (oVEMPs and cVEMPs).

MATERIAL AND METHODS

Systematic literature search in PubMed until April 2019.

RESULTS

Sound and vibration at a frequency of 500 Hz represent selective vestibular stimuli for the otolith organs. The predominant specificity of oVEMPs for contralateral utricular function and of cVEMPs for ipsilateral saccular function is defined by the different neuronal projections of the utricle and the saccule. VEMPs are particularly useful in the diagnosis of superior canal dehiscence and otolith organ-specific vestibular dysfunction and as an alternative diagnostic approach in situations when video oculography is not possible or useful.

CONCLUSION

The use of VEMPs is a simple, safe, reliable and selective test of dynamic function of otolith organs.

A review of mechanical and synaptic processes in otolith transduction of sound and vibration for clinical VEMP testing

Older studies of mammalian otolith physiology have focused mainly on sustained responses to low-frequency (<50 Hz) or maintained linear acceleration. So the otoliths have been regarded as accelerometers. Thus evidence of otolithic activation and high-precision phase locking to high-frequency sound and vibration appears to be very unusual. However, those results are exactly in accord with a substantial body of knowledge of otolith function in fish and frogs. It is likely that phase locking of otolith afferents to vibration is a general property of all vertebrates. This review examines the literature about the activation and phase locking of single otolithic neurons to air-conducted sound and bone-conducted vibration, in particular the high precision of phase locking shown by mammalian irregular afferents that synapse on striolar type I hair cells by calyx endings. Potassium in the synaptic cleft between the type I hair cell receptor and the calyx afferent ending may be responsible for the tight phase locking of these afferents even at very high discharge rates. Since frogs and fish do not possess full calyx endings, it is unlikely that they show phase locking with such high precision and to such high frequencies as has been found in mammals. The high-frequency responses have been modeled as the otoliths operating in a seismometer mode rather than an accelerometer mode. These high-frequency otolithic responses constitute the neural basis for clinical vestibular-evoked myogenic potential tests of otolith function.

Semicircular canal biomechanics in health and disease

The semicircular canals are responsible for sensing angular head motion in three-dimensional space and for providing neural inputs to the central nervous system (CNS) essential for agile mobility, stable vision, and autonomic control of the cardiovascular and other gravity-sensitive systems. Sensation relies on fluid mechanics within the labyrinth to selectively convert angular head acceleration into sensory hair bundle displacements in each of three inner ear sensory organs. Canal afferent neurons encode the direction and time course of head movements over a broad range of movement frequencies and amplitudes. Disorders altering canal mechanics result in pathological inputs to the CNS, often leading to debilitating symptoms. Vestibular disorders and conditions with mechanical substrates include benign paroxysmal positional nystagmus, direction-changing positional nystagmus, alcohol positional nystagmus, caloric nystagmus, Tullio phenomena, and others. Here, the mechanics of angular motion transduction and how it contributes to neural encoding by the semicircular canals is reviewed in both health and disease.

Sound abnormally stimulates the vestibular system in canal dehiscence syndrome by generating pathological fluid-mechanical waves

Individuals suffering from Tullio phenomena experience dizziness, vertigo, and reflexive eye movements (nystagmus) when exposed to seemingly benign acoustic stimuli. The most common cause is a defect in the bone enclosing the vestibular semicircular canals of the inner ear. Surgical repair often corrects the problem, but the precise mechanisms underlying Tullio phenomenon are not known. In the present work we quantified the phenomenon in an animal model of the condition by recording fluid motion in the semicircular canals and neural activity evoked by auditory-frequency stimulation. Results demonstrate short-latency phase-locked afferent neural responses, slowly developing sustained changes in neural discharge rate, and nonlinear fluid pumping in the affected semicircular canal. Experimental data compare favorably to predictions of a nonlinear computational model. Results identify the biophysical origin of Tullio phenomenon in pathological sound-evoked fluid-mechanical waves in the inner ear. Sound energy entering the inner ear at the oval window excites fluid motion at the location of the defect, giving rise to traveling waves that subsequently excite mechano-electrical transduction in the vestibular sensory organs by vibration and nonlinear fluid pumping.

Animal Models of Vestibular Evoked Myogenic Potentials: The Past, Present, and Future

Vestibular-evoked myogenic potentials (VEMPs) provide a simple and cost-effective means to assess the patency of vestibular reflexes. VEMP testing constitutes a core screening method in a clinical battery that probes vestibular function. The confidence one has in interpreting the results arising from VEMP testing is linked to a fundamental understanding of the underlying functional anatomy and physiology. In this review, we will summarize the key role that studies across a range of animal models have fulfilled in contributing to this understanding, covering key findings regarding the mechanisms of excitation in the sensory periphery, the processing of sensory information in central networks, and the distribution of reflexive output to the motor periphery. Although VEMPs are often touted for their simplicity, work in animals models have emphasized how vestibular reflexes operate within a broader behavioral and functional context, and as such vestibular reflexes are influenced by multisensory integration, governed by task demands, and follow principles of muscle recruitment. We will conclude with considerations of future questions, and the ways in which studies in current and emerging animal models can contribute to further use and refinement of this test for both basic and clinical research purposes.

Ocular Vestibular Evoked Myogenic Potentials: Where Are We Now?

OBJECTIVE

Over the last decade, ocular vestibular evoked myogenic potentials (oVEMPs) have evolved as a new clinical test for dynamic otolith (predominantly utricular) function. The aim of this review is to give an update on the neurophysiological foundations of oVEMPs and their implications for recording and interpreting oVEMP responses in clinical practice.

CONCLUSION

Different lines of anatomical, neurophysiological, and clinical evidence support the notion that oVEMPs measure predominantly contralateral utricular function, while cervical cVEMPs are an indicator of ipsilateral saccular function. Bone-conducted vibration (BCV) in the midline of the forehead at the hairline (Fz) or unilateral air-conducted sound (ACS) are commonly used as stimuli for oVEMPs. It is recommended to apply short stimuli with short rise times for obtaining optimal oVEMP responses. Finally, this review summarizes the clinical application and interpretation of oVEMPs, particularly for vestibular neuritis, Ménière's disease, superior canal dehiscence and "challenging" patients.

Otolithic Receptor Mechanisms for Vestibular-Evoked Myogenic Potentials: A Review

Air-conducted sound and bone-conduced vibration activate otolithic receptors and afferent neurons in both the utricular and saccular maculae, and trigger small electromyographic (EMG) responses [called vestibular-evoked myogenic potentials (VEMPs)] in various muscle groups throughout the body. The use of these VEMPs for clinical assessment of human otolithic function is built on the following logical steps: (1) that high-frequency sound and vibration at clinically effective stimulus levels activate otolithic receptors and afferents, rather than semicircular canal afferents, (2) that there is differential anatomical projection of otolith afferents to eye muscles and neck muscles, and (3) that isolated stimulation of the utricular macula induces short latency responses in eye muscles, and that isolated stimulation of the saccular macula induces short latency responses in neck motoneurons. Evidence supports these logical steps, and so VEMPs are increasingly being used for clinical assessment of otolith function, even differential evaluation of utricular and saccular function. The proposal, originally put forward by Curthoys in 2010, is now accepted: that the ocular vestibular-evoked myogenic potential reflects predominantly contralateral utricular function and the cervical vestibular-evoked myogenic potential reflects predominantly ipsilateral saccular function. So VEMPs can provide differential tests of utricular and saccular function, not because of stimulus selectivity for either of the two maculae, but by measuring responses which are predominantly determined by the differential neural projection of utricular as opposed to saccular neural information to various muscle groups. The major question which this review addresses is how the otolithic sensory system, with such a high density otoconial layer, can be activated by individual cycles of sound and vibration and show such tight locking of the timing of action potentials of single primary otolithic afferents to a particular phase angle of the stimulus cycle even at frequencies far above 1,000 Hz. The new explanation is that it is due to the otoliths acting as seismometers at high frequencies and accelerometers at low frequencies. VEMPs are an otolith-dominated response, but in a particular clinical condition, semicircular canal dehiscence, semicircular canal receptors are also activated by sound and vibration, and act to enhance the otolith-dominated VEMP responses.

Wave Mechanics of the Vestibular Semicircular Canals

The semicircular canals are biomechanical sensors responsible for detecting and encoding angular motion of the head in 3D space. Canal afferent neurons provide essential inputs to neural circuits responsible for representation of self-position/orientation in space, and to compensatory circuits including the vestibulo-ocular and vestibulo-collic reflex arcs. In this work we derive, to our knowledge, a new 1D mathematical model quantifying canal biomechanics based on the morphology, dynamics of the inner ear fluids, and membranous labyrinth deformability. The model takes the form of a dispersive wave equation and predicts canal responses to angular motion, sound, and mechanical stimulation. Numerical simulations were carried out for the morphology of the human lateral canal using known physical properties of the endolymph and perilymph in three diverse conditions: surgical plugging, rotation, and mechanical indentation. The model reproduces frequency-dependent attenuation and phase shift in cases of canal plugging. During rotation, duct deformability extends the frequency bandwidth and enhances the high frequency gain. Mechanical indentation of the membranous duct at high frequencies evokes traveling waves that move away from the location of indentation and at low frequencies compels endolymph displacement along the canal. These results demonstrate the importance of the conformal perilymph-filled bony labyrinth to pressure changes and to high frequency sound and vibration.

Superior Canal Dehiscence Syndrome: Lessons from the First 20 Years

Superior semicircular canal dehiscence syndrome was first reported by Lloyd Minor and colleagues in 1998. Patients with a dehiscence in the bone overlying the superior semicircular canal experience symptoms of pressure or sound-induced vertigo, bone conduction hyperacusis, and pulsatile tinnitus. The initial series of patients were diagnosed based on common symptoms, a physical examination finding of eye movements in the plane of the superior semicircular canal when ear canal pressure or loud tones were applied to the ear, and high-resolution computed tomography imaging demonstrating a dehiscence in the bone over the superior semicircular canal. Research productivity directed at understanding better methods for diagnosing and treating this condition has substantially increased over the last two decades. We now have a sound understanding of the pathophysiology of third mobile window syndromes, higher resolution imaging protocols, and several sensitive and specific diagnostic tests. Furthermore, we have a treatment (surgical occlusion of the superior semicircular canal) that has demonstrated efficacy. This review will highlight some of the fundamental insights gained in SCDS, propose diagnostic criteria, and discuss future research directions.

Sustained and Transient Vestibular Systems: A Physiological Basis for Interpreting Vestibular Function

Otolithic afferents with regular resting discharge respond to gravity or low-frequency linear accelerations, and we term these the static or sustained otolithic system. However, in the otolithic sense organs, there is anatomical differentiation across the maculae and corresponding physiological differentiation. A specialized band of receptors called the striola consists of mainly type I receptors whose hair bundles are weakly tethered to the overlying otolithic membrane. The afferent neurons, which form calyx synapses on type I striolar receptors, have irregular resting discharge and have low thresholds to high frequency (e.g., 500 Hz) bone-conducted vibration and air-conducted sound. High-frequency sound and vibration likely causes fluid displacement which deflects the weakly tethered hair bundles of the very fast type I receptors. Irregular vestibular afferents show phase locking, similar to cochlear afferents, up to stimulus frequencies of kilohertz. We term these irregular afferents the transient system signaling dynamic otolithic stimulation. A 500-Hz vibration preferentially activates the otolith irregular afferents, since regular afferents are not activated at intensities used in clinical testing, whereas irregular afferents have low thresholds. We show how this sustained and transient distinction applies at the vestibular nuclei. The two systems have differential responses to vibration and sound, to ototoxic antibiotics, to galvanic stimulation, and to natural linear acceleration, and such differential sensitivity allows probing of the two systems. A 500-Hz vibration that selectively activates irregular otolithic afferents results in stimulus-locked eye movements in animals and humans. The preparatory myogenic potentials for these eye movements are measured in the new clinical test of otolith function—ocular vestibular-evoked myogenic potentials. We suggest 500-Hz vibration may identify the contribution of the transient system to vestibular controlled responses, such as vestibulo-ocular, vestibulo-spinal, and vestibulo-sympathetic responses. The prospect of particular treatments targeting one or the other of the transient or sustained systems is now being realized in the clinic by the use of intratympanic gentamicin which preferentially attacks type I receptors. We suggest that it is valuable to view vestibular responses by this sustained-transient distinction.

Superior semicircular canal dehiscence syndrome: a new aetiology

Objective:

We report what we believe to be a unique aetiology of the superior semicircular canal dehiscence syndrome, a recently described condition in which vestibular imbalance and/or hearing loss results from the loss of continuity of the bone overlying the superior semicircular canals.

Case report:

A 58-year-old woman presented with autophony in the right ear and momentary imbalance when shouting (Tullio phenomenon). Temporal bone computed tomography revealed a defect of the right superior semicircular canal caused by an enlarged superior petrosal sinus receiving drainage from a large cerebellar developmental venous anomaly.

Conclusions:

We review superior semicircular canal dehiscence syndrome and its management, and we discuss common aetiologies, contrasting these with the unusual aetiology presented here. We conclude that superior semicircular canal dehiscence syndrome may present with a solely developmental aetiology, despite presenting late in life.

The new vestibular stimuli: sound and vibration-anatomical, physiological and clinical evidence

The classical view of the otoliths-as flat plates of fairly uniform receptors activated by linear acceleration dragging on otoconia and so deflecting the receptor hair bundles-has been replaced by new anatomical and physiological evidence which shows that the maculae are much more complex. There is anatomical spatial differentiation across the macula in terms of receptor types, hair bundle heights, stiffness and attachment to the overlying otolithic membrane. This anatomical spatial differentiation corresponds to the neural spatial differentiation of response dynamics from the receptors and afferents from different regions of the otolithic maculae. Specifically, receptors in a specialized band of cells, the striola, are predominantly type I receptors, with short, stiff hair bundles and looser attachment to the overlying otoconial membrane than extrastriolar receptors. At the striola the hair bundles project into holes in the otolithic membrane, allowing for fluid displacement to deflect the hair bundles and activate the cell. This review shows the anatomical and physiological evidence supporting the hypothesis that fluid displacement, generated by sound or vibration, deflects the short stiff hair bundles of type I receptors at the striola, resulting in neural activation of the irregular afferents innervating them. So these afferents are activated by sound or vibration and show phase-locking to individual cycles of the sound or vibration stimulus up to frequencies above 2000 Hz, underpinning the use of sound and vibration for clinical tests of vestibular function.

Superior semicircular canal dehiscence syndrome

Superior semicircular canal dehiscence (SSCD) syndrome is an increasingly recognized cause of vestibular and/or auditory symptoms in both adults and children. These symptoms are believed to result from the presence of a pathological mobile "third window" into the labyrinth due to deficiency in the osseous shell, leading to inadvertent hydroacoustic transmissions through the cochlea and labyrinth. The most common bony defect of the superior canal is found over the arcuate eminence, with rare cases involving the posteromedial limb of the superior canal associated with the superior petrosal sinus. Operative intervention is indicated for intractable or debilitating symptoms that persist despite conservative management and vestibular sedation. Surgical repair can be accomplished by reconstruction or plugging of the bony defect or reinforcement of the round window through a variety of operative approaches. The authors review the etiology, pathophysiology, presentation, diagnosis, surgical options, and outcomes in the treatment of this entity, with a focus on potential pitfalls that may be encountered during clinical management.

Effects of high intensity noise on the vestibular system in rats

Some individuals with noise-induced hearing loss (NIHL) also report balance problems. These accompanying vestibular complaints are not well understood. The present study used a rat model to examine the effects of noise exposure on the vestibular system. Rats were exposed to continuous broadband white noise (0-24 kHz) at an intensity of 116 dB sound pressure level (SPL) via insert ear phones in one ear for three hours under isoflurane anesthesia. Seven days after the exposure, a significant increase in ABR threshold (43.3 ± 1.9 dB) was observed in the noise-exposed ears, indicating hearing loss. Effects of noise exposure on vestibular function were assessed by three approaches. First, fluorescein-conjugated phalloidin staining was used to assess vestibular stereocilia following noise exposure. This analysis revealed substantial sensory stereocilia bundle loss in the saccular and utricular maculae as well as in the anterior and horizontal semicircular canal cristae, but not in the posterior semicircular canal cristae. Second, single unit recording of vestibular afferent activity was performed under pentobarbital anesthesia. A total of 548 afferents were recorded from 10 noise-treated rats and 12 control rats. Noise exposure produced a moderate reduction in baseline firing rates of regular otolith afferents and anterior semicircular canal afferents. Also a moderate change was noted in the gain and phase of the horizontal and anterior semicircular canal afferent's response to sinusoidal head rotation (1 and 2 Hz, 45°/s peak velocity). Third, noise exposure did not result in significant changes in gain or phase of the horizontal rotational and translational vestibulo-ocular reflex (VOR). These results suggest that noise exposure not only causes hearing loss, but also causes substantial damage in the peripheral vestibular system in the absence of immediate clinically measurable vestibular signs. These peripheral deficits, however, may lead to vestibular disorders over time.

Identifying Mechanisms Behind the Tullio Phenomenon: a Computational Study Based on First Principles

Patients with superior canal dehiscence (SCD) suffer from events of dizziness and vertigo in response to sound, also known as Tullio phenomenon (TP). The present work seeks to explain the fluid-dynamical mechanisms behind TP. In accordance with the so-called third window theory, we developed a computational model for the vestibular signal pathway between stapes and SCD. It is based on first principles and accounts for fluid-structure interactions arising between endolymph, perilymph, and membranous labyrinth. The simulation results reveal a wave propagation phenomenon in the membranous canal, leading to two flow phenomena within the endolymph which are in close interaction. First, the periodic deformation of the membranous labyrinth causes oscillating endolymph flow which forces the cupula to oscillate in phase with the sound stimulus. Second, these primary oscillations of the endolymph induce a steady flow component by a phenomenon known as steady streaming. We find that this steady flow of the endolymph is typically in ampullofugal direction. This flow leads to a quasi-steady deflection of the cupula which increases until the driving forces of the steady streaming are balanced by the elastic reaction forces of the cupula, such that the cupula attains a constant deflection amplitude which lasts as long as the sound stimulus. Both response types have been observed in the literature. In a sensitivity study, we obtain an analytical fit which very well matches our simulation results in a relevant parameter range. Finally, we correlate the corresponding eye response (vestibulo-ocular reflex) with the fluid dynamics by a simplified model of lumped system constants. The results reveal a "sweet spot" for TP within the audible sound spectrum. We find that the underlying mechanisms which lead to TP originate primarily from Reynolds stresses in the fluid, which are weaker at lower sound frequencies.

Vestibular-evoked myogenic potentials

The vestibular-evoked myogenic potential (VEMP) is a short-latency potential evoked through activation of vestibular receptors using sound or vibration. It is generated by modulated electromyographic signals either from the sternocleidomastoid muscle for the cervical VEMP (cVEMP) or the inferior oblique muscle for the ocular VEMP (oVEMP). These reflexes appear to originate from the otolith organs and thus complement existing methods of vestibular assessment, which are mainly based upon canal function. This review considers the basis, methodology, and current applications of the cVEMP and oVEMP in the assessment and diagnosis of vestibular disorders, both peripheral and central.

Vestibular evoked myogenic potentials (VEMPs) evoked by air- and bone-conducted stimuli in vestibular neuritis

OBJECTIVE

To compare and characterise abnormalities for short latency vestibular evoked myogenic potentials (VEMPs) elicited by air- (AC) and two differing types of bone-conducted (BC) stimuli during vestibular neuritis (VN).

METHODS

AC (500Hz short tone bursts) and two BC stimuli (500Hz at the forehead and impulses at the mastoids) were used to evoke cervical and ocular potentials (cVEMPs and oVEMPs) in VN patients (n=22) and healthy subjects.

RESULTS

More abnormalities were observed for the oVEMP than the cVEMP when using either AC 500Hz or BC 500Hz. The AC stimulus showed slightly more abnormalities than the BC 500Hz stimulus. In contrast, BC impulses produced frequent abnormalities for both oVEMPs and cVEMPs. The findings were modelled, based upon presumed selective lesions of the superior nerve.

CONCLUSIONS

AC 500Hz stimulation was slightly better than BC 500Hz in demonstrating abnormalities in patients with VN. BC impulses behave as expected for a predominantly utricular stimulus. The relative contributions of saccular and utricular fibres differ for stimulus type and target reflex.

SIGNIFICANCE

AC 500Hz is as effective as BC 500Hz for investigating VN. BC impulses act most strongly on utricular afferents.

How does high-frequency sound or vibration activate vestibular receptors?

The mechanism by which vestibular neural phase locking occurs and how it relates to classical otolith mechanics is unclear. Here, we put forward the hypothesis that sound and vibration both cause fluid pressure waves in the inner ear and that it is these pressure waves which displace the hair bundles on vestibular receptor hair cells and result in activation of type I receptor hair cells and phase locking of the action potentials in the irregular vestibular afferents, which synapse on type I receptors. This idea has been suggested since the early neural recordings and recent results give it greater credibility.

Input-output functions of vestibular afferent responses to air-conducted clicks in rats

Sound-evoked vestibular myogenic potentials recorded from the sternocleidomastoid muscles (the cervical vestibular-evoked myogenic potential or cVEMP) and the extraocular muscles (the ocular VEMP or oVEMP) have proven useful in clinical assessment of vestibular function. VEMPs are commonly interpreted as a test of saccular function, based on neurophysiological evidence showing activation of saccular afferents by intense acoustic click stimuli. However, recent neurophysiological studies suggest that the clicks used in clinical VEMP tests activate vestibular end organs other than the saccule. To provide the neural basis for interpreting clinical VEMP testing results, the present study examined the extent to which air-conducted clicks differentially activate the various vestibular end organs at several intensities and durations in Sprague-Dawley rats. Single unit recordings were made from 562 vestibular afferents that innervated the otoliths [inferior branch otolith (IO) and superior branch otolith (SO)], the anterior canal (AC), the horizontal canal (HC), and the posterior canal (PC). Clicks higher than 60 dB SL (re-auditory brainstem response threshold) activated both semicircular canal and otolith organ afferents. Clicks at or below 60 dB SL, however, activated only otolith organ afferents. Longer duration clicks evoked larger responses in AC, HC, and SO afferents, but not in IO afferents. Intra-axonal recording and labeling confirmed that sound sensitive vestibular afferents innervated the horizontal and anterior canal cristae as well as the saccular and utricular maculae. Interestingly, all sound sensitive afferents are calyx-bearing fibers. These results demonstrate stimulus-dependent acoustic activation of both semicircular canals and otolith organs, and suggest that sound activation of vestibular end organs other than the saccule should not be ruled out when designing and interpreting clinical VEMP tests.

Superior canal dehiscence length and location influences clinical presentation and audiometric and cervical vestibular-evoked myogenic potential testing

Superior canal dehiscence (SCD) is caused by an absence of bony covering of the arcuate eminence or posteromedial aspect of the superior semicircular canal. However, the clinical presentation of SCD syndrome varies considerably, as some SCD patients are asymptomatic and others have auditory and/or vestibular complaints. In order to determine the basis for these observations, we examined the association between SCD length and location with: (1) auditory and vestibular signs and symptoms; (2) air conduction (AC) loss and air-bone gap (ABG) measured by pure-tone audiometric testing, and (3) cervical vestibular-evoked myogenic potential (cVEMP) thresholds. 104 patients (147 ears) underwent SCD length and location measurements using a novel method of measuring bone density along 0.2-mm radial CT sections. We found that patients with auditory symptoms have a larger dehiscence (median length: 4.5 vs. 2.7 mm) with a beginning closer to the ampulla (median location: 4.8 vs. 6.4 mm from ampulla) than patients with no auditory symptoms (only vestibular symptoms). An increase in AC threshold was found as the SCD length increased at 250 Hz (95% CI: 1.7-4.7), 500 Hz (95% CI: 0.7-3.5) and 1,000 Hz (95% CI: 0.0-2.5), and an increase in ABG as the SCD length increased at 250 Hz (95% CI: 2.0-5.3), 500 Hz (95% CI: 1.6-4.6) and 1,000 Hz (95% CI: 1.3-3.3) was also seen. Finally, a larger dehiscence was associated with lowered cVEMP thresholds at 250 Hz (95% CI: -4.4 to -0.3), 500 Hz (95% CI: -4.1 to -1.0), 750 Hz (95% CI: -4.2 to -0.7) and 1,000 Hz (95% CI: -3.6 to -0.5) and a starting location closer to the ampulla at 250 Hz (95% CI: 1.3-5.1), 750 Hz (95% CI: 0.2-3.3) and 1,000 Hz (95% CI: 0.6-3.5). These findings may help to explain the variation of signs and symptoms seen in patients with SCD syndrome.

Frequency tuning of the cervical vestibular-evoked myogenic potential (cVEMP) recorded from multiple sites along the sternocleidomastoid muscle in normal human subjects

Frequency tuning of tone burst-evoked myogenic potentials recorded from the sternocleidomastoid muscle (cervical VEMP or cVEMP) is used clinically to assess vestibular function. Understanding the characteristics of cVEMP is important for improving the specificity of cVEMP testing in diagnosing vestibular deficits. In the present study, we analyzed the frequency tuning properties of the cVEMPs by constructing detailed tuning curves and examining their morphology and dependence on SCM tonic level, sound intensity, and recording site along the SCM. Here we report two main findings. First, by employing nine tone frequencies between 125 and 4,000 Hz, some tuning curves exhibited two distinct peaks, which cannot be modeled by a single mass spring system as previously suggested. Instead, the observed tuning is better modeled as linear summation of two mass spring systems, with resonance frequencies at ~300 and ~1,000 Hz. Peak frequency of cVEMP tuning curves was not affected by SCM tonic level, sound intensity, and location of recording site on the SCM. However, sharpness of cVEMP tuning was increased at lower sound intensities. Second, polarity of cVEMP responses recorded from the lower quarter of the SCM was reversed as compared to that at the two upper sites. While more studies are needed, these results suggest that cVEMP tuning is mediated through multiple generators with different resonance frequencies. Future studies are needed to explore implications of these results on development of selective VEMP tests and determine the nature of polarity inversion at the lower quarter of SCM.

Click-evoked responses in vestibular afferents in rats

Sound activates not only the cochlea but also the vestibular end organs. Research on this phenomenon led to the discovery of the sound-evoked vestibular myogenic potentials recorded from the sternocleidomastoid muscles (cervical VEMP, or cVEMP). Since the cVEMP offers simplicity and the ability to stimulate each labyrinth separately, its values as a test of human vestibular function are widely recognized. Currently, the cVEMP is interpreted as a test of saccule function based on the assumption that clicks primarily activate the saccule. However, sound activation of vestibular end organs other than the saccule has been reported. To provide the neural basis for interpreting clinical VEMP testing, we employed the broadband clicks used in clinical VEMP testing to examine the sound-evoked responses in a large sample of vestibular afferents in Sprague-Dawley rats. Recordings were made from 924 vestibular afferents from 106 rats: 255 from the anterior canal (AC), 202 from the horizontal canal (HC), 177 from the posterior canal (PC), 207 from the superior vestibular nerve otolith (SO), and 83 from the inferior nerve otolith (IO). Sound sensitivity of each afferent was quantified by computing the cumulative probability of evoking a spike (CPE). We found that clicks activated irregular afferents (normalized coefficient of variation of interspike intervals >0.2) from both the otoliths (81%) and the canals (43%). The order of end organ sound sensitivity was SO = IO > AC > HC > PC. Since the sternocleidomastoid motoneurons receive inputs from both the otoliths and the canals, these results provide evidence of a possible contribution from both of them to the click-evoked cVEMP.