The effect of superior canal dehiscence on cochlear potential in response to air-conducted stimuli in chinchilla

A superior semicircular canal dehiscence (SCD) is a break or hole in the bony wall of the superior semicircular canal. Patients with SCD syndrome present with a variety of symptoms: some with vestibular symptoms, others with auditory symptoms (including low-frequency conductive hearing loss) and yet others with both. We are interested in whether or not mechanically altering the superior canal by introducing a dehiscence is sufficient to cause the low-frequency conductive hearing loss associated with SCD syndrome. We evaluated the effect of a surgically introduced dehiscence on auditory responses to air-conducted (AC) stimuli in 11 chinchilla ears. Cochlear potential (CP) was recorded at the round-window before and after a dehiscence was introduced. In each ear, a decrease in CP in response to low frequency (<2 kHz) sound stimuli was observed after the introduction of the dehiscence. The dehiscence was then patched with cyanoacrylate glue leading to a reversal of the dehiscence-induced changes in CP. The reversible decrease in auditory sensitivity observed in chinchilla is consistent with the elevated AC thresholds observed in patients with SCD. According to the ‘third-window’ hypothesis the SCD shunts sound-induced stapes velocity away from the cochlea, resulting in decreased auditory sensitivity to AC sounds. The data collected in this study are consistent with predictions of this hypothesis.     

Hennebert's sign in superior semicircular canal dehiscence syndrome: a video case report

Superior semicircular canal dehiscence (SSCD) syndrome has been called the great otologic mimicker because its presentation overlaps with otosclerosis, Meniere's disease, perilymphatic fistula, and patulous eustachian tube. A valuable examination finding that can help distinguish SSCD syndrome from other pathologic conditions is the presence of Hennebert's sign, in which pressure changes in the external auditory canal evoke stereotyped eye movements that align in the plane of the dehiscent semicircular canal. This video case report demonstrates Hennebert's sign associated with SSCD syndrome and discusses its pathophysiological basis.     

Another etiology for vertigo due to idiopathic lateral semicircular canal bony defect

Superior semicircular canal dehiscence syndrome (SSCDS) is a set of symptoms, related to vestibular and/or auditory, which dues to a dehiscence of bone at the superior semicircular canal. We reported an extremely rare case with idiopathic bony dehiscence at the lateral semicircular canal, which presents the similar symptoms with SSCDS, and performed a pertinent literature review. A 76-year-old male patient complained experiencing vertigo and autophony caused by loud noises, with a sign of horizontal eye movements for the stimuli of loud noise to his right ear. HRCT scanning presented a solitary 2.0 mm bony defect at the right lateral semicircular canal. For a chief complaint of sound-/pressure-induced vertigo and/or oscillopsia and/or auditory symptoms, semicircular canal dehiscence syndrome should be considered. Then the doctor could presume the location of the dehiscence via the way of the oscillopsia. If the dehiscence lying the lateral semicircular canal, in clinical radiology, both axial and coronal images to be produced though the lateral semicircular canal plane in a parallel and perpendicular way should be carefully assessed for making diagnosis.     

Plasticity of the human otolith-ocular reflex.

The eye movement response to earth vertical axis rotation in the dark, a semicircular canal stimulus, can be altered by prior exposure to combined visual-vestibular stimuli. Such plasticity of the vestibulo-ocular reflex has not been described for earth horizontal axis rotation, a dynamic otolith stimulus. Twenty normal human subjects underwent one of two types of adaptation paradigms designed either to attenuate or enhance the gain of the semicircular canal-ocular reflex prior to undergoing otolith-ocular reflex testing with horizontal axis rotation. The adaptation paradigm paired a 0.2 Hz sinusoidal rotation about a vertical axis with a 0.2 Hz optokinetic stripe pattern that was deliberately mismatched in peak velocity. Pre- and post-adaptation horizontal axis rotations were at 60 degrees/s in the dark and produced a modulation in the slow component velocity of nystagmus having a frequency of 0.17 Hz due to putative stimulation of the otolith organs. Results showed that the magnitude of this modulation component response was altered in a manner similar to the alteration in semicircular canal-ocular responses. These results suggest that physiologic alteration of the vestibulo-ocular reflex using deliberately mismatched visual and semicircular canal stimuli induces changes in both canal-ocular and otolith-ocular responses. We postulate, therefore, that central nervous system pathways responsible for controlling the gains of canal-ocular and otolith-ocular reflexes are shared.     

Semicircular canal function before and after surgery for superior canal dehiscence.

OBJECTIVE: To characterize semicircular canal function before and after surgery for superior semicircular canal dehiscence (SCD) syndrome. STUDY DESIGN: Prospective unblinded study of physiologic effect of intervention. SETTING: Tertiary referral center. PATIENTS: Patients with SCD syndrome documented by history, sound- or pressure-evoked eye movements, vestibular-evoked myogenic potential testing, and high-resolution multiplanar computed tomographic scans. INTERVENTION: Nineteen subjects with SCD had quantitative measurements of their angular vestibulo-ocular reflexes (AVOR) in response to rapid rotary head thrusts measured by magnetic search coil technique before and after middle fossa approach and repair of the dehiscence. In 18 subjects, the dehiscence was plugged; and in 1, it was resurfaced. MAIN OUTCOME MEASURES: The AVOR gains (eye velocity/head velocity) for excitation of each of the semicircular canals. RESULTS: Vertigo resulting from pressure or loud sounds resolved in each case. Before surgery, mean AVOR gains were normal for the ipsilateral horizontal (0.94 +/- 0.07) and posterior (0.84 +/- 0.09) canals. For the superior canal to be operated on, AVOR gain was 0.75 +/- 0.13; but this was not significantly lower than the gain for the contralateral superior canal (0.82 +/- 0.11, p = 0.08). Mean AVOR gain decreased by 44% for the operated superior canals (to 0.42 +/- 0.11, p < 0.0001). There was a 13% decrease in gain for the ipsilateral posterior canal (p = 0.02), perhaps because plugging affected the common crus in some cases. There was a 10% decrease in gain for excitation of the contralateral posterior canal (p < 0.0001), which likely reflects the loss of the inhibitory contribution of the plugged superior canal during head thrusts exciting the contralateral posterior canal. Mean AVOR gain did not change for any of the other canals, but two subjects did develop hypofunction of all three ipsilateral canals postoperatively. CONCLUSION: Middle fossa craniotomy and repair of SCD reduce the function of the operated superior canal but typically preserve the function of the other ipsilateral semicircular canals.     

Directional Plasticity Rapidly Improves 3D Vestibulo-Ocular Reflex Alignment in Monkeys Using a Multichannel Vestibular Prosthesis

Bilateral loss of vestibular sensation can be disabling. We have shown that a multichannel vestibular prosthesis (MVP) can partly restore vestibular sensation as evidenced by improvements in the 3-dimensional angular vestibulo-ocular reflex (3D VOR). However, a key challenge is to minimize misalignment between the axes of eye and head rotation, which is apparently caused by current spread beyond each electrode’s targeted nerve branch. We recently reported that rodents wearing a MVP markedly improve 3D VOR alignment during the first week after MVP activation, probably through the same central nervous system adaptive mechanisms that mediate cross-axis adaptation over time in normal individuals wearing prisms that cause visual scene movement about an axis different than the axis of head rotation. We hypothesized that rhesus monkeys would exhibit similar improvements with continuous prosthetic stimulation over time. We created bilateral vestibular deficiency in four rhesus monkeys via intratympanic injection of gentamicin. A MVP was mounted to the cranium, and eye movements in response to whole-body passive rotation in darkness were measured repeatedly over 1 week of continuous head motion-modulated prosthetic electrical stimulation. 3D VOR responses to whole-body rotations about each semicircular canal axis were measured on days 1, 3, and 7 of chronic stimulation. Horizontal VOR gain during 1 Hz, 50 °/s peak whole-body rotations before the prosthesis was turned on was <0.1, which is profoundly below normal (0.94 ± 0.12). On stimulation day 1, VOR gain was 0.4–0.8, but the axis of observed eye movements aligned poorly with head rotation (misalignment range ∼30–40 °). Substantial improvement of axis misalignment was observed after 7 days of continuous motion-modulated prosthetic stimulation under normal diurnal lighting. Similar improvements were noted for all animals, all three axes of rotation tested, for all sinusoidal frequencies tested (0.05–5 Hz), and for high-acceleration transient rotations. VOR asymmetry changes did not reach statistical significance, although they did trend toward slight improvement over time. Prior studies had already shown that directional plasticity reduces misalignment when a subject with normal labyrinths views abnormal visual scene movement. Our results show that the converse is also true: individuals receiving misoriented vestibular sensation under normal viewing conditions rapidly adapt to restore a well-aligned 3D VOR. Considering the similarity of VOR physiology across primate species, similar effects are likely to occur in humans using a MVP to treat bilateral vestibular deficiency.     

Could vestibular evoked myogenic potentials (VEMPs) also be useful in the diagnosis of perilymphatic fistula?

The role of vestibular evoked myogenic potentials (VEMPs) is at this time indisputable in the study of vestibular disorders. Furthermore, VEMPs are widely accepted as a diagnostic tool when a superior semicircular canal dehiscence (SCD) is suspected, presenting in such cases a lowering of threshold values able to raise a recordable response due to increased inner ear immittance. According to the same principle, the possibility of another kind of alteration having the same effect on the inner ear might be considered when high-resolution computed tomography has excluded the presence of an SCD. In this paper four cases are described in which high-resolution computed tomography showed normal features without any labyrinthine dehiscence and VEMP threshold values were lowered; the appropriateness of suspecting a perilymphatic fistula in such cases and resorting to VEMPs in detecting a perilymphatic fistula is discussed.     

Off-vertical axis rotation: a test of the otolith-ocular reflex.

The vestibulo-ocular reflex was studied via off-vertical axis rotation (OVAR) in the dark. The axis of the turntable could be tilted from vertical by up to 30 degrees. Eye movements were measured with electro-oculography. Results from healthy asymptomatic subjects indicated that 1) a reliable otolith-induced response could be obtained during constant velocity OVAR using a velocity of 60 degrees/s with a tilt of 30 degrees; 2) constant velocity OVAR rotation was nausea-producing and, especially if subjects were rotated in the dark about an earth-vertical axis prior to being tilted, disorienting; and 3) sinusoidal OVAR produced minimal nausea; the eye movement response appeared to be the result of a combination of semicircular canal and otolith components. We conclude that OVAR has the potential of becoming a useful method for clinically assessing both the otolith-ocular reflex and semicircular canal-otolith interaction.     

Operative management of a posterior semicircular canal dehiscence

OBJECTIVE: The objective of this study was to describe the operative management of posterior canal dehiscence. METHODS: A transmastoid approach to and plugging of the posterior canal was performed for posterior semicircular canal dehiscence (PSCD). RESULTS: Postoperatively, the patient exhibited improvement in conductive hearing loss and vestibular symptoms. CONCLUSIONS: PSCD can cause symptoms identical to that of superior semicircular canal dehiscence. Successful PSCD plugging can be performed without visualization of the actual area of dehiscence.     

Use of the loud sound stimulation test in diagnosis of semicircular canal dehiscence syndrome

Semicircular canal dehiscence (SCD) syndrome is rare, and its diagnosis is a significant challenge in clinical practice. Our aim was to explore application of the loud sound stimulation test for diagnosing SCD syndrome. Eight cases of superior semicircular canal dehiscence (SSCD), among them two patients had bilateral dehiscences and one case of lateral semicircular canal dehiscence (LSCD). A total of 11 dehiscences were studied retrospectively. Loud sounds (pure tones, 100 dB, 110 dB nHL) at frequencies of 500, 1,000, and 2,000 Hz were used to stimulate both ears for 5 s. A temporal bone computed tomography (CT) scan with semicircular canal reconstruction was performed in all patients. Vertigo was present in seven of nine cases following loud sound stimulation. In addition, the patient with LSCD demonstrated horizontal eye movement following loud sound stimulation, whereas six patients with SSCD showed rotational eye movement. Among them, two patients with bilateral superior canal dehiscence showed a positive response to the loud sound stimulation in only one ear. The diagnoses of all patients were confirmed with a high-resolution temporal bone CT with corresponding multi-planar reconstruction of the affected semicircular canals with various size dehiscences. We conclude that the characteristic eye movement following loud sound stimulation is valuable for diagnosing SCD syndrome. In addition, the loud sound stimulation test has unique advantages, especially for confirming the affected ear and the corresponding semicircular canal.     

Prospective radiological study concerning a series of patients suffering from conductive or mixed hearing loss due to superior semicircular canal dehiscence

The aim of this study is to appreciate the incidence of patients with isolated conductive hearing loss with normal drum due to superior semicircular canal dehiscence (SCD). It is a prospective radiological study. Two hundred and seventy-two patients with a normal drum suffering from isolated unilateral or bilateral conductive or mixed hearing loss were included in a prospective radiological study. A high resolution computerized tomography (HRCT) was performed in all the patients. Those who were found to have a unilateral or bilateral SCD underwent further etiological, clinical, audiologic evaluation. Ten patients with conductive or mixed hearing loss were found to have a unilateral or bilateral SCD. The disease was bilateral in five cases, and most often associated with a dehiscence of the tegmen tympani on both sides, supporting the theory of the congenital nature of the disease. There was no clear correlation between symptoms and the size of the SCD. Because patients were not suffering from incapacitating vestibular symptoms, they were not operated for surgical occlusion of the SCD, and were referred to a hearing aid specialist to improve hearing. Conductive or mixed hearing loss due to SCD is relatively frequent, justifying in our opinion that a systematic HRCT be carried out before surgery of any patient with conductive hearing loss.     

Superior semicircular canal dehiscence: positive predictive value of high-resolution CT scanning

Patients with superior dehiscence (SCD) syndrome present with vertigo and oscillopsia evoked by loud sounds and changes in middle ear or intracranial pressure. The first objective of this retrospective cohort study is to demonstrate that thin-section computed tomography (CT) scans reformatted in the plane of the superior semicircular canal (SSC) overestimate this anomaly compared to pathologic studies. The second objective of this study is to re-evaluate the positive predictive value of temporal bone scanning. All temporal bone CT scans with 0.55-mm collimation and reconstruction in the SSC plane performed over a 1-year period were analysed at a tertiary referral centre. CT-positive cases had their clinical data reviewed and patients were re-examined, if available. A total of 581 temporal bone CT-scans were analysed. A dehiscent-appearing superior canal was seen in 4.0% of studies while published pathologic studies report that only 0.5% of temporal bones SSCs have a dehiscence (P < 0.001). Of the 21 patients with positive temporal bone CTs, only 1 presented with sufficient clinical dues to identify the syndrome. Three additional patients did not have symptoms consistent with the diagnosis, but had surgery for a dehiscence of the tegmen mastoideum. When our findings are added to published data, the positive predictive value of temporal bone CT-scanning drops from 93 to 57%. The prevalence of dehiscent-appearing superior canal on thin-section temporal bone scanning with reformation in the SSC plane is much higher than anticipated by pathologic studies. Even with 0.55 mm-collimated helical CT and reformation in the SSC plane, the risk of overdiagnosis is present.     

Incapacitating hypersensitivity to one’s own body sounds due to a dehiscence of bone overlying the superior semicircular canal. A case report

We present a case study of a 49-year-old patient with an 8-year history of hypersensitivity to sound produced by intrinsic but not extrinsic sources. Findings that indicated an organic problem were: a supranormal bone conduction threshold of –25 to –15 dB HL from 0.25 to 1 kHz with an air-bone gap of 15 to 45 dB HL, a lower threshold and larger amplitude for vestibular-evoked myogenic potentials, eye movement reactions to sound and trunk pitch sway in response to sound. Results of immitance audiometry and otoacoustic emission testing were within normal limits and indicative of intact middle ear conductance. A high-resolution CT scan of the temporal bone demonstrated a dehiscence of bone overlying the superior semicircular canal. These findings support previous research indicating that auditory energy reaches the cochleo-vestibular receptor systems more easily via transmission through cerebrospinal fluid than through bone. Therefore, a dehiscence of the bone overlying the superior semicircular canal may lead to hypersensitivity to intrinsic sound. We recommend that similar findings in other patients be followed up with an evaluation of middle ear function and the temporal bone with high-resolution CT scan.     

Dynamic Visual Acuity during Passive Head Thrusts in Canal Planes

We sought to determine whether the dynamic visual acuity (DVA) test, which has been used to measure the function of the two horizontal semicircular canals (SCCs), could be adapted to measure the individual function of all six SCCs using transient, rapid, unpredictable head rotation stimuli (head thrusts) in the direction of maximum sensitivity of each SCC. We examined head-thrust DVA (htDVA) performance in 19 healthy control subjects, five patients before and six patients after plugging of one superior SCC for treatment of superior canal dehiscence, and two subjects with unilateral vestibular deafferentation (UVD) by vestibular neurectomy. We compared htDVA results for each SCC to vestibulo-ocular reflex gains measured using 3-D scleral coil recordings during a passive head-thrust-test paradigm. Individuals with normal vestibular function had similar htDVA scores for each of the six directions (canals) tested (mean 0.058 ± 0.050 LogMAR). Individuals tested after surgical plugging of one superior SCC were similar to normal for all SCCs except the plugged SCC, which had significantly worse htDVA scores (mean 0.270 ± 0.08 LogMAR). Individuals with UVD had significantly worse htDVA scores for head rotations maximally exciting any of the ipsilesional SCC (mean 0.317 ± 0.129 LogMAR) and scores similar to normal subjects for contralesional rotations (0.063 ± 0.051 LogMAR). These findings suggest that the htDVA test, which does not require scleral coil placement, magnetic field coils, or expensive oculography equipment, can provide a useful quantitative measure of individual SCC function.     

Vestibulo-ocular reflex in eccentric rotation in squirrel monkeys.

In addition to angular acceleration, eccentric rotation (ECR) imparts linear acceleration to the head positioned eccentric to the axis of rotation. Using ECR in squirrel monkeys, the effects of otolith organ stimulation by linear acceleration on vestibulo-ocular reflex (VOR) gain were investigated. With the animal's head facing away from the rotation axis, ECR significantly enhanced VOR gain over that seen in centric rotation (CR) at 1.0 Hz, but not at 0.5 Hz. However, no enhancement of VOR gain at 1.0 Hz was observed in eccentriclateral rotation when the animal faced tangentially. After bilateral ablation of the otolith organs (sacculectomy and utricular neurectomy), the ECR did not increase VOR gain, even at 1.0 Hz. In animals in which the lateral and posterior semicircular canals were plugged bilaterally, horizontal sinusoidal eye movements were induced by ECR at 1.0 Hz; no clear compensatory eye movement occurred during CR at 1.0 Hz. These findings demonstrate that during ECR, tangential acceleration along the interaural axis stimulates the utricular maculae, inducing horizontal eye movements in addition to those induced by the semicircular canal, thus resulting in an enhancement of VOR gain. Our results also suggest synergistic interactions of the otolith organs and semicircular canals. We conclude that ECR is a useful clinical test of the function of the otolith organs.     

Anterior Canal benign paroxysmal positional Vertigo following surgical Management of superior canal dehiscence

Objective

Benign paroxysmal positional vertigo (BPPV) is a common post-surgical finding in patients managed for superior semicircular canal dehiscence (SSCD). The posterior semicircular canal has been reported as the involved canal in the majority of cases of post-surgical BPPV, with only two cases reported of lateral canal involvement. The objective of this report is to present a case in which an anterior semicircular canal BPPV response was identified in a patient following surgical management for SSCD.

Posterior canal dehiscence syndrome caused by an apex cholesteatoma.

OBJECTIVE: To compare audio-vestibular findings caused by a dehiscence of the posterior semicircular canal with those found in the superior canal dehiscence syndrome. STUDY DESIGN: Case report. SETTING: University hospital, tertiary referral center. PATIENT: The 44-year-old woman suffered from a gradual hearing loss with pulse-synchronous tinnitus as well as sound and pressure-induced vertigo. INTERVENTION: Audio-vestibular testing and high-resolution computed tomography. MAIN OUTCOME MEASURE: The superior canal dehiscence syndrome is caused by failure of normal postnatal bone development in the middle cranial fossa leading to absence of bone at the most superior part of the superior semicircular canal. The typical features for this syndrome are sound- and pressure-induced vertigo with torsional eye movements, pulse synchronous tinnitus and apparent conductive hearing loss in spite of normal middle-ear function. We present a patient with very similar symptoms and findings who, instead, had a posterior semicircular canal dehiscence caused by an apex cholesteatoma. CONCLUSION: Patients with semicircular canal dehiscence have common auditory-vestibular features regardless of which of the two vertical semicircular canals is affected. The only obvious difference between the two is the vertical component of the sound and pressure-induced eye movements (which beats in opposite directions).     

Vestibular-optokinetic interactions in normal subjects and in patients with peripheral vestibular dysfunction.

The vestibulo-ocular reflex (VOR) during rotation, optokinetic nystagmus (OKN), and interactions between the vestibulo-ocular and optokinetic systems were studied quantitatively in 10 normal human subjects, 15 patients with unilateral horizontal semicircular canal paralysis (UP), and 11 patients with bilateral horizontal semicircular canal paralysis (BP). The OKN gain (eye velocity/drum velocity) was not significantly different between the normal subjects and the patient groups. During tests in which rotatory and visual stimuli were presented simultaneously, the contribution to the observed eye movements by the VOR was only one-fourth to one-third of its gain during rotation in the dark in the normal subjects and UP patients. These interactive tests did not differentiate UP patients from normal subjects but did separate BP patients from normal subjects.     

Another cause for conductive hearing loss with present acoustic reflexes

There are numerous potential causes of conductive hearing loss (HL). It is important to obtain a thorough history and perform a complete examination, including audiometric testing and radiographic evaluation when necessary. In this report, we present a patient with an intact tympanic membrane, no history of ear disease or trauma who as an adult developed progressive, conductive HL because of an anomalous course of a dehiscent facial nerve. In the patient with a conductive HL and at least partially intact reflexes, superior semicircular canal dehiscence, fracture of the stapes superstructure proximal to the tendon, other third window phenomena, and now dehiscence of the facial nerve resulting in decreased mobility of the ossicular chain must be considered.