Eye movements in patients with superior canal dehiscence syndrome align with the abnormal canal

Third Window Syndrome: An Up-to-Date Systematic Review of Causes, Diagnosis, and Treatment

Third window syndrome (TWS) is an inner ear condition caused by an additional compliant point in the otic capsule that disrupts auditory and vestibular functions. Superior semicircular canal dehiscence is the most common cause, presenting with hearing loss, vertigo, and autophony, significantly impairing quality of life. This study evaluated the pathophysiology, diagnostics, treatments, and recent advancements in TWS while identifying research gaps. Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, 70 studies from Embase, MEDLINE, Cochrane, and UpToDate databases were analyzed. TWS affects inner ear mechanics, enhancing bone conduction and reducing air conduction. Diagnosis involves clinical evaluations, high-resolution imaging, and functional tests such as vestibular evoked myogenic potentials, which are known for their high sensitivity and specificity. Management strategies range from vestibular rehabilitation and pharmacotherapy to surgical interventions, including transmastoid and middle cranial fossa approaches, which achieve over 75% success. Emerging minimally invasive techniques, such as underwater endoscopic ear surgery and round window reinforcement, show promise but carry risks like cerebrospinal fluid leakage and inconsistent symptom relief. Advancements in TWS management have improved outcomes, yet gaps remain, particularly in terms of false-positive imaging and long-term efficacy. Future studies should prioritize predictive models and minimally invasive techniques. A multidisciplinary approach is essential to improve patient care.

Defining Potential Pathomechanisms Behind an Impaired Canal Function at the Video-Head Impulse Test in Canal Dehiscence. Reply to Ionescu et al. Comment on "Castellucci et al. Impaired Vestibulo-Ocular Reflex on Video Head Impulse Test in Superior Canal Dehiscence: "Spontaneous Plugging" or Endolymphatic Flow Dissipation? Audiol. Res. 2023, 13, 802-820"

We read with great interest the comment on our articles of Dr [...].

Model of superior semicircular canal dehiscence: asymmetrical vestibular dysfunction induces reversible balance impairment

Background

Superior semicircular canal dehiscence (SSCD) is a vestibular-cochlear disorder in humans in which a pathological third mobile window of the otic capsule creates changes to the flow of sound pressure energy through the perilymph/endolymph. The primary symptoms include sound-induced dizziness/vertigo, inner ear conductive hearing loss, autophony, headaches, and visual problems. We have developed an animal model of this human condition in the Mongolian Gerbil that uses surgically created SSCD to induce the condition. A feature that is unique in this model is that spontaneous resurfacing of the dehiscence occurs via osteoneogenesis without a subsequent intervention. In this study, we completed our assessment of this model to include reversible asymmetrical vestibular impairments that interfere with balance.

Methods

Adult Mongolian gerbils (N = 6) were trained to complete a balance beam task. They were also trained to perform a Rotarod task. After 10 days of training, preoperative ABR and c+VEMP testing was followed by a surgical fenestration of the left superior semicircular canal. Balance beam testing recommenced at postoperative day 6 and continued through postoperative day 15 at which point final ABR and c+VEMP testing was carried out.

Results

Behavioral comparison of preoperative and postoperative performance show a significant decrease in Rotarod performance, increased rates of falling, and an increase in time to cross the balance beam. Impairments were the most significant at postoperative day 7 with a return toward preoperative performance by postoperative day 14. This behavioral impairment was correlated with residual impairments to auditory thresholds and vestibular myogenic amplitudes at postoperative day 14.

Conclusion

These results confirm that aberrant asymmetric vestibular output in our model of SSCD results in reversible balance impairments. The level of these behavioral impairments is directly correlated with severity of the vestibular dysfunction as we have previously reported for peripheral ear physiology and cognition.

Ocular Movement Examination in Peripheral Vestibular Disorders as a Tool to Improve Diagnosis: A Literature Review

Background and Objectives: This study reviews the current literature on ocular movements, specifically focusing on nystagmus associated with peripheral vestibular disorders, to enhance diagnostic accuracy. The evaluation of ocular movements, particularly nystagmus, provides essential insights into the function and dysfunction of the vestibular system, helping clinicians distinguish between peripheral and central causes of vertigo and imbalance. Materials and Methods: A comprehensive search of PubMed was conducted using key terms such as "ocular movements", "nystagmus", "vestibular nystagmus", and "peripheral vestibular disorders". Results: The search yielded 2739 titles, and after a rigorous selection process, 52 articles were reviewed in full. Discussion: The review highlights different classifications and types of nystagmus, including physiological and pathological forms, and their diagnostic relevance in vestibular disorders such as benign paroxysmal positional vertigo (BPPV), vestibular neuritis, and Meniere's disease. Diagnostic techniques like video/electro-oculography are emphasized for their role in assessing vestibular function and identifying abnormalities. The study underscores the importance of detailed ocular examination in the diagnosis of peripheral vestibular disorders and proposes an algorithm to aid this process. Conclusions: While not a systematic review, this study highlights the importance of detailed ocular examination in diagnosing peripheral vestibular disorders and presents an algorithm to facilitate this process. It also emphasizes the need for continued research and advancements in vestibular medicine to further understand ocular movements and their clinical significance, ultimately contributing to improved patient outcomes.

Audiovestibular Findings in Patients with Concurrent Superior Canal Dehiscence and Vestibular Schwannoma

OBJECTIVE

To describe the clinical-instrumental findings in case of concurrent superior canal dehiscence (SCD) and ipsilateral vestibular schwannoma (VS), aiming to highlight the importance of an extensive instrumental assessment to achieve a correct diagnosis.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

Five patients with concurrent SCD and VS.

INTERVENTION

Clinical-instrumental assessment and imaging.

MAIN OUTCOME MEASURE

Clinical presentation, audiovestibular findings, and imaging.

RESULTS

The chief complaints were hearing loss (HL) and unsteadiness (80%). Other main symptoms included tinnitus (60%) and pressure-induced vertigo (40%). Mixed-HL was identified in three patients and pure sensorineural-HL in 1, including a roll-over curve in speech-audiometry in two cases. Vibration-induced nystagmus was elicited in all cases, whereas vestibular-evoked myogenic potentials showed reduced thresholds and enhanced amplitudes on the affected side in three patients. Ipsilesional weakness on caloric testing was detected in three patients and a bilateral hyporeflexia in one. A global canal impairment was detected by the video-head impulse test in one case, whereas the rest of the cohort exhibited a reduced function for the affected superior canal, together with ipsilateral posterior canal impairment in two cases. All patients performed both temporal bones HRCT scan and brain-MRI showing unilateral SCD and ipsilateral VS, respectively. All patients were submitted to a wait-and-scan approach, requiring VS removal only in one case.

CONCLUSION

Simultaneous SCD and VS might result in subtle clinical presentation with puzzling lesion patterns. When unclear symptoms and signs occur, a complete audiovestibular assessment plays a key role to address imaging and diagnosis.

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 Single Fast Test for Semicircular Canal Dehiscence—oVEMP n10 to 4000 Hz—Depends on Stimulus Rise Time

As previously reported, a single test measuring oVEMP n10 to 4000 Hz stimuli (bone-conducted vibration (BCV) or air-conducted sound (ACS)) provides a definitive diagnosis of semicircular canal dehiscence (SCD) in 22 CT-verified patients, with a sensitivity of 1.0 and specificity of 1.0. This single short screening test has great advantages of speed, minimizing testing time, and the exposure of patients to stimulation. However, a few studies of the 4000 Hz test for SCD have reported sensitivity and specificity values which are slightly less than reported previously. We hypothesized that the rise time of the stimulus is important for detecting the oVEMP n10 to 4000 Hz, similarly to what we had shown for 500 and 750 Hz BCV. We measured oVEMP n10 in 15 patients with CT-verified SCD in response to 4000 Hz ACS or BCV stimuli with rise times of 0, 1, and 2 ms. As a result, increasing the rise time of the stimulus reduced the oVEMP n10 amplitude. This outcome is expected from the physiological evidence of guinea pig primary vestibular afferents, which are activated by sound or vibration. Therefore, for clinical VEMP testing, short rise times are optimal (preferably 0 ms).

Case Report: Fremitus Nystagmus in Superior Canal Dehiscence Syndrome

Superior canal dehiscence syndrome (SCDS) is a structural bony defect of the roof of the superior semi-circular canal into the middle cranial fossa and is responsible for the creation of a third window, which alters the dynamics of the inner ear. During humming, vibratory waves entering the vestibulum and cochlea are re-routed through the dehiscence, leading to stimulation of the otolithic and ampullary vestibular organs. This is responsible for the torsional-vertical nystagmus known as “fremitus nystagmus”. In this case report, we video-document a rare case of fremitus nystagmus and its resolution after plugging of the superior semi-circular canal.

The Supine Superior Semicircular Canal Dehiscence Test

Purpose The purpose of this clinical focus article is to describe a new method for assessment of superior semicircular canal dehiscence by laying the patient supine during Valsalva-induced nystagmus testing. Method The traditional Valsalva-induced nystagmus test is described, followed by a new method for assessment of superior semicircular dehiscence conducted by laying the patient supine during testing. A case study is presented to illustrate this new testing technique known as the Supine Superior Semicircular Canal Dehiscence Test. Results It is hypothesized that during Valsalva-induced nystagmus testing performed in the upright, seated position, the dura mater could potentially seal the superior semicircular canal fistula, thereby concealing a defect in the bony labyrinth and yielding a false-negative test. To circumvent this, the patient should be placed in the supine position during Valsalva-induced nystagmus testing in order to prevent the dura mater from inadvertently sealing itself against the petrous portion of the temporal bone. The Supine Superior Semicircular Canal Dehiscence Test may reveal the defect in the bony labyrinth and improve the sensitivity of the Valsalva-induced nystagmus test. Conclusions The Supine Superior Semicircular Canal Dehiscence Test may be more sensitive for identifying superior semicircular canal dehiscence in patients with traditional symptoms and a negative Valsalva-induced nystagmus test in the seated position. While a case study is presented to illustrate the potential benefits of including the Supine Superior Semicircular Canal Dehiscence Test in the battery of diagnostic tests, further research is needed in larger samples.

Pulse-synchronous torsional nystagmus

Purely torsional spontaneous nystagmus almost always has a central vestibular cause. We describe a man with spontaneous pulse-synchronous torsional nystagmus in which the clockwise component corresponded to his pulse upswing, in keeping with a peripheral vestibular cause; following imaging we diagnosed left-sided superior canal dehiscence syndrome. Identifying pulse synchronicity of spontaneous nystagmus may help to distinguish central from peripheral vestibular torsional nystagmus, and is readily confirmed at the bedside using Frenzel's glasses and a pulse oximeter.

Superior semicircular canal dehiscence syndrome: Diagnostic criteria consensus document of the committee for the classification of vestibular disorders of the Bárány Society

This paper describes the diagnostic criteria for superior semicircular canal dehiscence syndrome (SCDS) as put forth by the classification committee of the Bárány Society. In addition to the presence of a dehiscence of the superior semicircular canal on high resolution imaging, patients diagnosed with SCDS must also have symptoms and physiological tests that are both consistent with the pathophysiology of a ‘third mobile window’ syndrome and not better accounted for by another vestibular disease or disorder. The diagnosis of SCDS therefore requires a combination of A) at least one symptom consistent with SCDS and attributable to ‘third mobile window’ pathophysiology including 1) hyperacusis to bone conducted sound, 2) sound-induced vertigo and/or oscillopsia time-locked to the stimulus, 3) pressure-induced vertigo and/or oscillopsia time-locked to the stimulus, or 4) pulsatile tinnitus; B) at least 1 physiologic test or sign indicating that a ‘third mobile window’ is transmitting pressure including 1) eye movements in the plane of the affected superior semicircular canal when sound or pressure is applied to the affected ear, 2) low-frequency negative bone conduction thresholds on pure tone audiometry, or 3) enhanced vestibular-evoked myogenic potential (VEMP) responses (low cervical VEMP thresholds or elevated ocular VEMP amplitudes); and C) high resolution computed tomography (CT) scan with multiplanar reconstruction in the plane of the superior semicircular canal consistent with a dehiscence. Thus, patients who meet at least one criterion in each of the three major diagnostic categories (symptoms, physiologic tests, and imaging) are considered to have SCDS.

Current Trends, Controversies, and Future Directions in the Evaluation and Management of Superior Canal Dehiscence Syndrome

Patients with superior canal dehiscence syndrome (SCDS) can present with a range of auditory and/or vestibular signs and symptoms that are associated with a bony defect of the superior semicircular canal (SSC). Over the past two decades, advances in diagnostic techniques have raised the awareness of SCDS and treatment approaches have been refined to improve patient outcomes. However, a number of challenges remain. First, there is currently no standardized clinical testing algorithm for quantifying the effects of superior canal dehiscence (SCD). SCDS mimics a number of common otologic disorders and established metrics such as supranormal bone conduction thresholds and vestibular evoked myogenic potential (VEMP) measurements; although useful in certain cases, have diagnostic limitations. Second, while high-resolution computed tomography (CT) is the gold standard for the detection of SCD, a bony defect does not always result in signs and symptoms. Third, even when SCD repair is indicated, there is a lack of consensus about nomenclature to describe the SCD, ideal surgical approach, specific repair techniques, and type of materials used. Finally, there is no established algorithm in evaluation of SCDS patients who fail primary repair and may be candidates for revision surgery. Herein, we will discuss both contemporary and emerging diagnostic approaches for patients with SCDS and highlight challenges and controversies in the management of this unique patient cohort.

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.

Perilymphatic Fistula: A Review of Classification, Etiology, Diagnosis, and Treatment

A perilymphatic fistula (PLF) is an abnormal communication between the perilymph-filled inner ear and the middle ear cavity, mastoid, or intracranial cavity. A PLF most commonly forms when the integrity of the oval or round window is compromised, and it may be trauma-induced or may occur with no known cause (idiopathic). Controversy regarding the diagnosis of idiopathic PLF has persisted for decades, and the presenting symptoms may be vague. However, potential exists for this condition to be one of the few etiologies of dizziness, tinnitus, and hearing loss that can be treated surgically. The aim of this review is to provide an update on classification, diagnosis, and treatment of PLF. Particular attention will be paid to idiopathic PLF and conditions that may have a similar presentation, with subsequent information on how best to distinguish them. Novel diagnostic criteria for PLF and management strategy for PLF and PLF-like symptoms is presented.

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.

The effect of superior canal dehiscence size and location on audiometric measurements, vestibular-evoked myogenic potentials and video-head impulse testing

PURPOSE

To correlate objective measures of audio-vestibular function with superior canal dehiscence (SCD) size and location in ears with SCD and compare results with literature.

METHODS

We retrospectively evaluated 242 patients exhibiting SCD and/or extremely thinned bone overlying superior canals (SC) on CT scans and selected 73 SCD patients (95 ears with SCD). Data concerning audiometry, impedance audiometry, video-head impulse test (vHIT), cervical vestibular-evoked myogenic potentials (cVEMPs) and ocular VEMPs (oVEMPs) to air- (AC) and bone-conducted (BC) stimuli were collected for each pathologic ear and correlated with dehiscence size and location.

RESULTS

AC pure-tone average (PTA) (p = 0.013), low-frequency air-bone gap (ABG) (p  < 0.001), AC cVEMPs amplitude (p = 0.002), BC cVEMPs amplitude (p < 0.001) and both AC and BC oVEMPs amplitude (p < 0.001) positively correlated with increasing SCD size. An inverse relationship between dehiscence length and both AC cVEMPs and oVEMPs thresholds (p < 0.001) and SC vestibulo-ocular reflex (VOR) gain (p < 0.001) was observed. Dehiscences at the arcuate eminence (AE) exhibited lower SC VOR gains compared to SCD along the ampullary arm (p = 0.008) and less impaired BC thresholds than dehiscences at the superior petrosal sinus (p = 0.04).

CONCLUSION

We confirmed that SCD size affects AC PTA, ABG and both amplitudes and thresholds of cVEMPs and oVEMPs. We also described a tendency for SC function to impair with increasing SCD size and when dehiscence is located at the AE. The latter data may be explained either by a spontaneous canal plugging exerted by middle fossa dura or by a dissipation through the dehiscence of mechanical energy conveyed to the endolymph during high-frequency impulses.

Bilateral superior semicircular canal dehiscence: bilateral conductive hearing loss with subtle vestibular symptoms

Superior semicircular canal dehiscence is caused by a bone defect on the roof of the superior semicircular canal. The estimated prevalence when unilateral varies between 0.4% and 0.7% and is still unknown when bilateral. Patients may present with audiologic and vestibular symptoms that may vary from asymptomatic to disabling. We report a case of a 72-year-old Caucasian woman presented to otolaryngology department reporting imbalance, bilateral pulsatile tinnitus, hypoacusis while being very sensitive to certain sounds. Physical examination was unremarkable, except for the Rinne test that was negative in both sides. The patient underwent an audiometry revealing a mild bilateral conductive hearing loss. A temporal bone CT scan was performed which evidenced bilateral superior semicircular canal dehiscence. Cervical vestibular evoked myogenic potentials and electrocochleography confirmed diagnosis. Although rare, superior semicircular canal dehiscence shall be considered in conductive hearing loss with vestibular symptoms.

Interactions between Auditory and Vestibular Modalities during Stimulation with a Combined Vestibular and Cochlear Prosthesis

BACKGROUND

A combined vestibular and cochlear prosthesis may restore hearing and balance to patients who have lost both. To do so, the device should activate each sensory system independently.

OBJECTIVES

In this study, we quantify auditory and vestibular interactions during interleaved stimulation with a combined 16-channel cochlear and 6-channel vestibular prosthesis in human subjects with both hearing and vestibular loss.

METHODS

Three human subjects were implanted with a combined vestibular and cochlear implant. All subjects had severe-to-profound deafness in the implanted ear. We provided combined stimulation of the cochlear and vestibular arrays and looked for interactions between these separate inputs. Our main outcome measures were electrically evoked slow-phase eye velocities during nystagmus elicited by brief trains of biphasic pulse stimulation of the vestibular end organs with and without concurrent stimulation of the cochlea, and Likert scale assessments of perceived loudness and pitch during stimulation of the cochlea, with and without concurrent stimulation of the vestibular ampullae.

RESULTS

All subjects had no auditory sensation resulting from semicircular canal stimulation alone, and no sensation of motion or slow-phase eye movement resulting from cochlear stimulation alone. However, interleaved cochlear stimulation did produce changes in the slow-phase eye velocities elicited by electrical stimulation. Similarly, interleaved semicircular canal stimulation did elicit changes in the perceived pitch and loudness resulting from stimulation at multiple sites in the cochlea.

CONCLUSIONS

There are significant interactions between different sensory modalities during stimulation with a combined vestibular and cochlear prosthesis. Such interactions present potential challenges for stimulation strategies to simultaneously restore auditory and vestibular function with such an implant.

Clinical and Physiologic Predictors and Postoperative Outcomes of Near Dehiscence Syndrome

OBJECTIVE

To identify predictors of near dehiscence (ND) or thin rather than dehiscent bone overlying the superior semicircular canal in patients with signs and symptoms suggestive of superior semicircular canal dehiscence syndrome (SCDS), as well as postoperative outcomes.

STUDY DESIGN

Retrospective case-control study.

SETTING

Tertiary referral center.

PATIENTS

All 288 patients who underwent middle cranial fossa approach for repair of SCDS (1998-2018) were reviewed for cases of ND. Demographics, symptoms, and clinical signs including nystagmus, ocular vestibular-evoked myogenic potential (oVEMP) amplitude, cervical vestibular-evoked myogenic potential (cVEMP) thresholds, and low-frequency air-bone gap were compared before and after surgery.

MAIN OUTCOME MEASURE

Presence of preoperative ND and postoperative symptoms and physiologic measures.

RESULTS

Seventeen cases of ND (16 patients, 17 ears) and 34 cases (34 ears) of frank SCDS were identified. ND cases differed from frank dehiscence cases in that they were less likely to have nystagmus in response to ear canal pressure or loud sounds, OR = 0.05 (95% CI 0.01-0.25) and Valsalva, OR = 0.08 (0.01-0.67), smaller peak-to-peak oVEMP amplitudes, OR = 0.84 (0.75-0.95), and higher cVEMP thresholds, OR = 1.21 (1.07-1.37). Patients with ND had similar symptoms to those with frank SCDS before surgery, and after surgery had outcomes similar to patients with frank SCDS.

CONCLUSIONS

In patients with symptoms consistent with SCDS, predictors of ND include absence of nystagmus in response to pressure/loud sounds, greater cVEMP thresholds, and smaller oVEMP amplitudes. We propose ND is on a spectrum of dehiscence that partially accounts for the diversity of clinical presentations of patients with SCDS.

Contribution of Reformatted Multislice Temporal Computed Tomography Images in the Planes of Stenvers and Pöschl to the Diagnosis of Superior Semicircular Canal Dehiscence

OBJECTIVE

In the diagnosis of superior semicircular canal dehiscence (SSCD), computed tomography (CT) is the only imaging method. The aims of the study were to show that reformat images are more accurate than standard planes for diagnosis of SSCD and to determine the prevalence of SSCD.

METHODS

The retrospective review yielded 1309 temporal CTs performed in our radiology department for any reason. Two radiologist interpreted CTs in standard planes collaboratively. Patients with SSCD were reinterpreted in Pöschl and Stenvers planes by 2 radiologists separately.

RESULTS

Statistical analysis was made by accepting that 2 radiologists diagnosis were accurate in Pöschl plane. Coronal plane sensitivity 86%, specificity 64%, Stenvers plane sensitivity 96%, and specificity 52% have been found in the mean result of 2 observers (P < 0.001).

CONCLUSIONS

In the diagnosis of SSCD, standard and Stenvers planes can cause false-negative and false-positive diagnoses. Interpretation in Pöschl plane can significantly increase sensitivity, specificity, negative, and positive predictive values for diagnosing dehiscence.

Continuous vestibular implant stimulation partially restores eye-stabilizing reflexes

BACKGROUNDBilateral loss of vestibular (inner ear inertial) sensation causes chronically blurred vision during head movement, postural instability, and increased fall risk. Individuals who fail to compensate despite rehabilitation therapy have no adequate treatment options. Analogous to hearing restoration via cochlear implants, prosthetic electrical stimulation of vestibular nerve branches to encode head motion has garnered interest as a potential treatment, but prior studies in humans have not included continuous long-term stimulation or 3D binocular vestibulo-ocular reflex (VOR) oculography, without which one cannot determine whether an implant selectively stimulates the implanted ear's 3 semicircular canals.METHODSWe report binocular 3D VOR responses of 4 human subjects with ototoxic bilateral vestibular loss unilaterally implanted with a Labyrinth Devices Multichannel Vestibular Implant System vestibular implant, which provides continuous, long-term, motion-modulated prosthetic stimulation via electrodes in 3 semicircular canals.RESULTSInitiation of prosthetic stimulation evoked nystagmus that decayed within 30 minutes. Stimulation targeting 1 canal produced 3D VOR responses approximately aligned with that canal's anatomic axis. Targeting multiple canals yielded responses aligned with a vector sum of individual responses. Over 350-812 days of continuous 24 h/d use, modulated electrical stimulation produced stable VOR responses that grew with stimulus intensity and aligned approximately with any specified 3D head rotation axis.CONCLUSIONThese results demonstrate that a vestibular implant can selectively, continuously, and chronically provide artificial sensory input to all 3 implanted semicircular canals in individuals disabled by bilateral vestibular loss, driving reflexive VOR eye movements that approximately align in 3D with the head motion axis encoded by the implant.TRIAL REGISTRATIONClinicalTrials.gov: NCT02725463.FUNDINGNIH/National Institute on Deafness and Other Communication Disorders: R01DC013536 and 2T32DC000023; Labyrinth Devices, LLC; and Med-El GmbH.

Classification of vestibular signs and examination techniques: Nystagmus and nystagmus-like movements

This paper presents a classification and definitions for types of nystagmus and other oscillatory eye movements relevant to evaluation of patients with vestibular and neurological disorders, formulated by the Classification Committee of the Bárány Society, to facilitate identification and communication for research and clinical care. Terminology surrounding the numerous attributes and influencing factors necessary to characterize nystagmus are outlined and defined. The classification first organizes the complex nomenclature of nystagmus around phenomenology, while also considering knowledge of anatomy, pathophysiology, and etiology. Nystagmus is distinguished from various other nystagmus-like movements including saccadic intrusions and oscillations.

View accompanying videos at http://www.jvr-web.org/ICVD.html

[Superior canal dehiscence syndrome : Diagnosis with vestibular evoked myogenic potentials and fremitus nystagmus. German version]

BACKGROUND

Superior canal dehiscence syndrome (SCDS) is a relatively rare neurotological disorder that is characterized by a heterogeneous clinical picture. Recently, vestibular evoked myogenic potential (VEMP) measurementwas established for the diagnosis of SCDS. In the present study, a case series of patients with SCDS were analyzed, with a focus on VEMP.

METHODS

Four patients with SCDS were prospectively examined with ocular VEMP (oVEMP) and cervical VEMP (cVEMP). The clinical features and the standard audiovestibular test battery results are summarized and analyzed. The diagnostic accuracy of VEMP testing is evaluated.

RESULTS

The increased oVEMP amplitudes had a specificity of 100% in this patient population. All patients had normal caloric function and head impulse testing. The Tullio sign was observed in two patients. Three patients had autophony. The airbone gap was not greater than 10 dB in any of the patients. Two patients had marked fremitus nystagmus. All patients had a bony dehiscence of the superior semicircular canal on computed tomography imaging.

CONCLUSION

The subjective and clinical features in this case series of SCDS patients were heterogeneous. However, objective oVEMP testing had the highest diagnostic value. Furthermore, we describe a new diagnostic clinical sign: fremitus nystagmus.

Virtual Rhesus Labyrinth Model Predicts Responses to Electrical Stimulation Delivered by a Vestibular Prosthesis

To better understand the spread of prosthetic current in the inner ear and to facilitate design of electrode arrays and stimulation protocols for a vestibular implant system intended to restore sensation after loss of vestibular hair cell function, we created a model of the primate labyrinth. Because the geometry of the implanted ear is complex, accurately modeling effects of prosthetic stimuli on vestibular afferent activity required a detailed representation of labyrinthine anatomy. Model geometry was therefore generated from three-dimensional (3D) reconstructions of a normal rhesus temporal bone imaged using micro-MRI and micro-CT. For systematically varied combinations of active and return electrode location, the extracellular potential field during a biphasic current pulse was computed using finite element methods. Potential field values served as inputs to stochastic, nonlinear dynamic models for each of 2415 vestibular afferent axons, each with unique origin on the neuroepithelium and spiking dynamics based on a modified Smith and Goldberg model. We tested the model by comparing predicted and actual 3D vestibulo-ocular reflex (VOR) responses for eye rotation elicited by prosthetic stimuli. The model was individualized for each implanted animal by placing model electrodes in the standard labyrinth geometry based on CT localization of actual implanted electrodes. Eye rotation 3D axes were predicted from relative proportions of model axons excited within each of the three ampullary nerves, and predictions were compared to archival eye movement response data measured in three alert rhesus monkeys using 3D scleral coil oculography. Multiple empirically observed features emerged as properties of the model, including effects of changing active and return electrode position. The model predicts improved prosthesis performance when the reference electrode is in the labyrinth's common crus (CC) rather than outside the temporal bone, especially if the reference electrode is inserted nearly to the junction of the CC with the vestibule. Extension of the model to human anatomy should facilitate optimal design of electrode arrays for clinical application.

CT evaluation of normal bone thickness overlying the superior semicircular canal

Superior semicircular canal dehiscence (SSCD) is a rare inner ear disorder with variable amounts of auditory and vestibular dysfunction. In addition to the absence of bone overlying the superior semicircular canal, thinning of bone in this area can also initiate the vestibulocochlear symptoms of SSCD. We evaluated normal bone thickness overlying the course of the semicircular canal using computed tomography (CT) scans and assessed correlations between bone thickness and age, gender, and location of the thinnest bone. A single-institution retrospective chart review was conducted on 133 high-resolution CT scans from 76 healthy, asymptomatic patients between ages 9 and 96 years. These CT scans of the temporal bone were obtained between January 2012 and August 2017. The superior semicircular canal dome thickness at the apex was reported with a mean of 1.25 mm for all 76 patients; the 10th percentile was 0.60 mm, and the 90th percentile was 2.08 mm. The thinnest area of bone at any location yielded a mean of 0.86 mm. The normal bone thickness overlying the superior semicircular canal does not depend on gender or age. The thinnest location was evenly distributed across the superior semicircular canal. A bone thickness of 0.40 mm or greater was present in 90% of normal patients based on CT scan measurements at the thinnest location.

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.

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.

Disorders of the inner-ear balance organs and their pathways

Disorders of the inner-ear balance organs can be grouped by their manner of presentation into acute, episodic, or chronic vestibular syndromes. A sudden unilateral vestibular injury produces severe vertigo, nausea, and imbalance lasting days, known as the acute vestibular syndrome (AVS). A bedside head impulse and oculomotor examination helps separate vestibular neuritis, the more common and innocuous cause of AVS, from stroke. Benign positional vertigo, a common cause of episodic positional vertigo, occurs when otoconia overlying the otolith membrane falls into the semicircular canals, producing brief spells of spinning vertigo triggered by head movement. Benign positional vertigo is diagnosed by a positional test, which triggers paroxysmal positional nystagmus in the plane of the affected semicircular canal. Episodic spontaneous vertigo caused by vestibular migraine and Ménière's disease can sometimes prove hard to separate. Typically, Ménière's disease is associated with spinning vertigo lasting hours, aural fullness, tinnitus, and fluctuating hearing loss while VM can produce spinning, rocking, or tilting sensations and light-headedness lasting minutes to days, sometimes but not always associated with migraine headaches or photophobia. Injury to both vestibular end-organs results in ataxia and oscillopsia rather than vertigo. Head impulse testing, dynamic visual acuity, and matted Romberg tests are abnormal while conventional neurologic assessments are normal. A defect in the bony roof overlying the superior semicircular canal produces vertigo and oscillopsia provoked by loud sound and pressure (when coughing or sneezing). Three-dimensional temporal bone computed tomography scan and vestibular evoked myogenic potential testing help confirm the diagnosis of superior canal dehiscence. Collectively, these clinical syndromes account for a large proportion of dizzy and unbalanced patients.

Superior Semicircular Canal Ampullae Dehiscence As Part of the Spectrum of the Third Window Abnormalities: A Case Study

A 60-year-old man was referred to the ENT department for intense episodic vertigo triggered by loud sounds. Pure tone audiometry and otoneurological assessment, including videonystagmography using auditory stimulation and cervical vestibular evoked myogenic potential measures, conducted to the hypothesis of a third window syndrome in the left ear. Results from the high-resolution computed tomography of the petrous bone confirmed the hypothesis and revealed the presence of a submillimeter semicircular canal dehiscence, located between the left lateral and superior semicircular canal ampullae on the left side.

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 associated with scuba diving

A 28-year-old female diver presented with dizziness and difficulty clearing her left ear whilst scuba diving. Her pure-tone audiometry and tympanometry were normal. Testing of Eustachian tube function revealed tubal stenosis. Video-oculography revealed a predominantly torsional nystagmus while the patient was in the lordotic position. Fistula signs were positive. High-resolution computed tomography (HRCT) of the temporal bone revealed a diagnosis of bilateral superior semicircular canal dehiscence (SCDS). Cervical vestibular-evoked myogenic potential (cVEMP) testing showed that the amplitude of the cVEMP measured from her left ear was larger than that from the right. In electronystagmography (ENG), nose-pinched Valsalva manoeuvres caused eye movements to be mainly directed counterclockwise with a vertical component. Tullio phenomenon was also positive for both ears. SCDS patients tend to be misdiagnosed and misunderstood; common misdiagnoses in these cases are alternobaric vertigo (AV), inner ear barotrauma, and inner-ear decompression sickness. It is difficult to diagnose vertigo attacks after scuba diving as SCDS; however, when the patient develops sound- and/or pressure-induced vertical-torsional nystagmus, HRCT should be conducted to confirm a diagnosis of SCDS.

Contrasting results of tests of peripheral vestibular function in patients with bilateral large vestibular aqueduct syndrome

OBJECTIVE

To analyze and summarize the effect of bilateral large vestibular aqueducts in peripheral vestibular organ function.

METHODS

Eighteen patients with bilateral large vestibular aqueduct syndrome (LVAS; Study Group) and 18 healthy volunteers (Control Group) were investigated using audiometry, caloric test, sensory organization test (SOT), and vestibular-evoked myogenic potential (VEMP) tests.

RESULTS

All 18 patients (36 ears) exhibited sensorineural hearing loss. For cervical VEMP (cVEMP), the Study Group showed lower thresholds (Study Group vs.

CONTROL GROUP

71.4vs. 75.3dBnHL; p=0.006), N1 latencies (24.1vs. 25.2ms; p=0.026) and shorter P1 (15.3vs. 16.6ms; p=0.003), and higher amplitudes (400.7vs. 247.2µV; p<0.001) than the Control Group. For ocular VEMP (oVEMP), the Study Group had lower thresholds (79.3vs. 81.8dBnHL; p=0.046) and higher amplitudes (40.6vs. 14.4µV; p<0.001) than the Control Group. Fourteen of 16 patients (87.5%) who completed caloric tests had abnormal results, and 10 of 18 patients (55.6%) exhibited abnormal results in SOTs.

CONCLUSIONS

The hyperfunction of vestibular test in otolithic organs and the hypofunction of vestibular test in semicircular canals, as well as the dysfunction in the balance test were demonstrated in patients with LVAS.

SIGNIFICANCE

Our findings can help clinicians gain a better understanding of the characteristics of vestibular organ function in patients with LVAS, which can facilitate optimal targeted treatment.

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.

Superior Semicircular Canal Dehiscence Syndrome - Diagnosis and Surgical Management

Introduction Superior semicircular canal dehiscence syndrome was described by Minor et al in 1998. It is a troublesome syndrome that results in vertigo and oscillopsia induced by loud sounds or changes in the pressure of the external auditory canal or middle ear. Patients may present with autophony, hyperacusis, pulsatile tinnitus and hearing loss. When symptoms are mild, they are usually managed conservatively, but surgical intervention may be needed for patients with debilitating symptoms.

Objective The aim of this manuscript is to review the different surgical techniques used to repair the superior semicircular canal dehiscence.

Data Sources PubMed and Ovid-SP databases.

Data Synthesis The different approaches are described and discussed, as well as their limitations. We also review the advantages and disadvantages of the plugging, capping and resurfacing techniques to repair the dehiscence.

Conclusions Each of the surgical approaches has advantages and disadvantages. The middle fossa approach gives a better view of the dehiscence, but comes with a higher morbidity than the transmastoid approach. Endoscopic assistance may be advantageous during the middle cranial fossa approach for better visualization. The plugging and capping techniques are associated with higher success rates than resurfacing, with no added risk of hearing loss.

Síndrome da deiscência do canal semicircular superior: relato de dois casos

RESUMO A Síndrome da Deiscência do Canal Semicircular Superior (SDCSS) é caracterizada pelo desgaste da camada óssea que recobre o canal semicircular superior. São sintomas comuns da SDCSS a presença de vertigem associada à nistagmos induzidos por estímulos sonoros intensos ou por modificações das pressões intracraniana ou da orelha média. O objetivo deste trabalho é descrever os achados audiólogicos e vestibulares de dois pacientes com diagnóstico de Síndrome da Deiscência do Canal Semicircular Superior, com diagnóstico confirmado por meio de tomografia computadorizada. Foram realizadas meatoscopia, anamnese, audiometria tonal e vocal seguida das medidas de imitância acústica, Weber audiométrico, pesquisa do fenômeno de Túlio e manobra de Valsalva, realizados pela mesma pesquisadora em uma única sessão. Foram observados gap aéreo-ósseo, curva timpanométrica tipo A e reflexos acústicos presentes. O gap aéreo-ósseo apresenta-se com maior amplitude nas frequências baixas. As queixas auditivas não foram relatadas pelas pacientes como os primeiros sintomas. O Weber mostrou lateralização, nos dois casos, confirmando a presença de gap. O fenômeno de Túlio apresentou-se positivo para vertigem em ambos os casos. A manobra de Valsalva apresentou alteração em apenas um caso.

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.

Responses of non-eye-movement central vestibular neurons to sinusoidal yaw rotation in compensated macaques after unilateral semicircular canal plugging

After vestibular labyrinth injury, behavioral measures of vestibular performance recover to variable degrees (vestibular compensation). Central neuronal responses after unilateral labyrinthectomy (UL), which eliminates both afferent resting activity and sensitivity to movement, have been well-studied. However, unilateral semicircular canal plugging (UCP), which attenuates angular-velocity detection while leaving afferent resting activity intact, has not been extensively studied. The current study reports response properties of yaw-sensitive non-eye-movement rhesus macaque vestibular neurons after compensation from UCP. The responses at a series of frequencies (0.1-2 Hz) and peak velocities (15-210°/s) were compared between neurons recorded before and at least 6 wk after UCP. The gain (sp/s/°/s) of central type I neurons (responding to ipsilateral yaw rotation) on the side of UCP was reduced relative to normal controls at 0.5 Hz, ±60°/s [0.48 ± 0.30 (SD) normal, 0.32 ± 0.15 ipsilesion; 0.44 ± 0.2 contralesion]. Type II neurons (responding to contralateral yaw rotation) after UCP have reduced gain (0.40 ± 0.27 normal, 0.35 ± 0.25 ipsilesion; 0.25 ± 0.18 contralesion). The difference between responses after UCP and after UL is primarily the distribution of type I and type II neurons in the vestibular nuclei (type I neurons comprise 66% in vestibular nuclei normally; 51% ipsilesion UCP; 59% contralesion UCP; 38% ipsilesion UL; 65% contralesion UL) and the magnitude of the responses of type II neurons ipsilateral to the lesion. These differences suggest that the need to compensate for unilateral loss of resting vestibular nerve activity after UL necessitates a different strategy for recovery of dynamic vestibular responses compared to after UCP.

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.