Semicircular Canal Function Before and After Surgery for Superior Canal Dehiscence

Superior canal dehiscence syndrome in children--a case report

Initially described in 1998, superior semicircular canal dehiscence syndrome (SCDS) has become a well-studied neurootologic entity in adults by now. Unfortunately, experience with children is limited and a diagnostic and therapeutic algorithm is lacking. The article therefore wants to provide an overview of the existing literature on superior semicircular canal dehiscence syndrome in children. Furthermore a diagnostic algorithm for daily clinical life based on a case report from an eleven-year-old girl is presented.

Radiologic classification of superior canal dehiscence: implications for surgical repair

OBJECTIVE

Surgical access to repair a superior canal dehiscence (SCD) is influenced by the location of the bony defect and its relationship to surrounding tegmen topography as seen on computed tomography. There are currently no agreed-upon methods of characterizing these radiologic findings. We propose a formal radiologic classification system of SCD based on dehiscence location and adjacent tegmen topography.

STUDY DESIGN

Retrospective case review

SETTING

Tertiary, neurotology referral center

PATIENTS

We identified 298 patients with superior canal dehiscence on CT from February 2001 to October 2013. Of these, 251 had symptomatic superior canal dehiscence syndrome and were included in the study.

INTERVENTION

Patients underwent high-resolution temporal bone CT scans with creation of axial, coronal, Pöschl, and Stenver reformatted images to examine the superior semicircular canal. Two residents-in-training and a head and neck radiologist independently read the scans.

MAIN OUTCOME MEASURES

CT scans were assessed for (1) superior canal dehiscence or "near" dehiscence, (2) defect location relative to the skull base, (3) surrounding tegmen defects, (4) geniculate ganglion dehiscence, (5) superior petrosal sinus-associated dehiscence (SPS), (6) low-lying tegmen, and (7) the distance between the outer table of the temporal bone and the arcuate eminence.

Long-term results of middle fossa plugging of superior semicircular canal dehiscences: clinically and instrumentally demonstrated efficiency in a retrospective series of 16 ears

The objective of this study is to report the surgical outcome after middle fossa approach (MFA) plugging in patients suffering from a superior semi-circular canal dehiscence (SCD) syndrome. This is a retrospective case review. Tertiary referral center. Sixteen ears in 13 patients with a SCD syndrome suffering from severe and disabling vestibular symptoms with a bony dehiscence on CT scan >3 mm and decreased threshold of cervical vestibular evoked potentials (cVEMPs). We assessed preoperatively: clinical symptoms, hearing, cVEMPs threshold, size of dehiscence and videonystagmography (VNG) with caloric and 100 Hz vibratory tests. Postoperatively, we noted occurrences of neurosurgical complication, evolution of audiological and vestibular symptoms, and evaluation of cVEMP data. Tullio’s phenomenon was observed in 13 cases (81.3 %) and subjectively reported hearing loss in seven (43.7 %). All patients were so disabled that they had to stop working. No neurosurgical complications were observed in the postoperative course. In three cases (16.6 %), an ipsilateral and transitory immediate postoperative vestibular deficit associated with a sensorineural hearing loss (SNHL) was noted, which totally resolved with steroids and bed rest. All patients were relieved of audiological and vestibular symptoms and could return to normal activity with a mean follow-up of 31.1 months (range 3–95). No patient had residual SNHL. cVEMPs were performed in 14 ears postoperatively and were normalized in 12 (85.7 %). Two of the three patients operated on both sides kept some degree of unsteadiness and oscillopsia. MFA plugging of the superior semi-circular canal is an efficient and non-hearing deteriorating procedure.

A Puzzle of Vestibular Physiology in a Meniere's Disease Acute Attack

The aim of this paper is to present for the first time the functional evaluation of each of the vestibular receptors in the six semicircular canals in a patient diagnosed with Meniere's disease during an acute attack. A 54-year-old lady was diagnosed with left Meniere's disease who during her regular clinic review suffers an acute attack of vertigo, with fullness and an increase of tinnitus in her left ear. Spontaneous nystagmus and the results in the video head-impulse test (vHIT) are shown before, during, and after the attack. Nystagmus was initially left beating and a few minutes later an upbeat component was added. No skew deviation was observed. A decrease in the gain of the vestibuloocular reflex (VOR) and the presence of overt saccades were observed when the stimuli were in the plane of the left superior semicircular canal. At the end of the crisis nystagmus decreased and vestibuloocular reflex returned to almost normal. A review of the different possibilities to explain these findings points to a hypothetical utricular damage.

Canal plane dynamic visual acuity in superior canal dehiscence

OBJECTIVE

1) To characterize normal, horizontal active dynamic visual acuity (DVA) and passive canal plane head thrust DVA (htDVA) across ages to establish appropriate control data and 2) to determine whether horizontal active DVA and passive canal plane htDVA are significantly different in individuals with superior canal dehiscence syndrome (SCDS) before and after surgical repair in the acute (within 10 d) and nonacute stage (>6 wk).

STUDY DESIGN

Prospective study.

SETTING

Tertiary referral center

PATIENTS

Patients diagnosed with SCDS (n = 32) and healthy control subjects (n = 51).

INTERVENTIONS

Surgical canal plugging on a subset of patients.

MAIN OUTCOME MEASURES

Static visual acuity (SVA), active horizontal DVA, and canal plane htDVA.

RESULTS

Visual acuity (SVA, active DVA, and htDVA) declines with age. In SCDS, SVA and active DVA are not significantly affected in individuals after surgical canal plugging; however, htDVA in the plane of the affected canal is significantly worse after canal plugging.

CONCLUSION

Age-based normative data are necessary for DVA testing. In SCDS, htDVA in the plane of the affected canal is normal before surgery but permanently reduced afterward.

A novel method of 3D image analysis of high-resolution cone beam CT and multi slice CT for the detection of semicircular canal dehiscence

HYPOTHESIS

We investigated if current-generation computed tomographic (CT) scanners have the resolution required to objectively detect bone structure defects as small as 0.1 mm. In addition, we propose that our method is able to predict a possible dehiscence in a semicircular canal.

BACKGROUND

In semicircular canal dehiscence (SCD), the bone overlying the superior canal (SC) is partially absent, causing vertigo, autophony, hyperacusis or hearing loss. Diagnosis of SCD is typically based on multi-slice computed tomography (MSCT) images combined with the consideration of clinical signs and symptoms. Recent studies have shown that MSCT tends to overestimate the size of dehiscences and may skew the diagnosis towards dehiscence when a thin bone layer remains. Evaluations of CT scans for clinical application are typically observer based.

METHODS

We developed a method of objectively evaluating the resolution of CT scanners. We did this for 2 types of computed tomography: MSCT, and cone beam computed tomography (CBCT), which have been reported to have a higher resolution for temporal bone scans. For the evaluation and comparison of image accuracy between different CT scanners and protocols, we built a bone cement phantom containing small, well-defined structural defects (diameter, 0.1-0.4 mm). These small inhomogeneities could reliably be detected by comparing the variances of radiodensities of a region of interest (i.e., a region containing a hole) with a homogenous region. The Fligner-Killeen test was used to predict the presence or absence of a hole (p ≥ 0.05). For our second goal, that is, to see how this technique could be applied to the detection of a possible dehiscence in a SC, a cadaveric head specimen was used to create an anatomic model for a borderline SCD; the SC was drilled to the point of translucency. After semi-automatically fitting the location of the canal, our variance-based approach allowed a clear, significant detection of the thin remaining bone layer.

RESULTS

Our approach of statistical noise analysis on bone cement phantoms allowed us to distinguish real irregularities from measured image noise or reconstruction errors. We have shown that with computed tomography, an approach comparing radiodensity variance in regions of interest is capable of detecting inhomogeneities down to 0.1 mm (p ≤ 0.0001).

CONCLUSION

Our analysis of data from the cadaveric head specimen demonstrates that this approach can be used to objectively detect thin layers of bone overlying an SC. This should provide the basis for using this approach for a semi-automated, objective detection of SCD.

Round window reinforcement for superior semicircular canal dehiscence: a retrospective multi-center case series

PURPOSE

To evaluate the outcome of round window (RW) tissue reinforcement in the management of superior semicircular canal dehiscence (SSCD).

MATERIALS AND METHODS

Twenty-two patients with confirmed diagnosis of SSCD by clinical presentation, imaging, and/or testing were included in the study. Six surgeons at four institutions conducted a multicenter chart review of patients treated for symptomatic superior canal dehiscence using RW tissue reinforcement or complete RW occlusion. A transcanal approach was used to reinforce the RW with various types of tissue. Patients completed a novel postoperative survey, grading preoperative and postoperative symptom severity.

RESULTS

Analysis revealed statistically significant improvement in all symptoms with the exception of hearing loss in 19 patients who underwent RW reinforcement. In contrast, 2 of 3 participants who underwent the alternate treatment of RW niche occlusion experienced worsened symptoms requiring revision surgery.

CONCLUSION

RW tissue reinforcement may reduce the symptoms associated with SSCD. The reinforcement technique may benefit SSCD patients by reducing the "third window" effect created by a dehiscent semicircular canal. Given its low risks compared to middle cranial fossa or transmastoid canal occlusion, RW reinforcement may prove to be a suitable initial procedure for intractable SSCD. In contrast, complete RW occlusion is not advised.

The video Head Impulse Test (vHIT) detects vertical semicircular canal dysfunction

Background

The video head impulse test (vHIT) is a useful clinical tool to detect semicircular canal dysfunction. However vHIT has hitherto been limited to measurement of horizontal canals, while scleral search coils have been the only accepted method to measure head impulses in vertical canals. The goal of this study was to determine whether vHIT can detect vertical semicircular canal dysfunction as identified by scleral search coil recordings.

Methods

Small unpredictable head rotations were delivered by hand diagonally in the plane of the vertical semicircular canals while gaze was directed along the same plane. The planes were oriented along the left-anterior-right-posterior (LARP) canals and right-anterior-left-posterior (RALP) canals. Eye movements were recorded simultaneously in 2D with vHIT (250 Hz) and in 3D with search coils (1000 Hz). Twelve patients with unilateral, bilateral and individual semicircular canal dysfunction were tested and compared to seven normal subjects.

Results

Simultaneous video and search coil recordings were closely comparable. Mean VOR gain difference measured with vHIT and search coils was 0.05 (SD = 0.14) for the LARP plane and −0.04 (SD = 0.14) for the RALP plane. The coefficient of determination R² was 0.98 for the LARP plane and 0.98 for the RALP plane and the results of the two methods were not significantly different. vHIT and search coil measures displayed comparable patterns of covert and overt catch-up saccades.

Conclusions

vHIT detects dysfunction of individual vertical semicircular canals in vestibular patients as accurately as scleral search coils. Unlike search coils, vHIT is non-invasive, easy to use and hence practical in clinics.

Hearing outcomes after surgical plugging of the superior semicircular canal by a middle cranial fossa approach

OBJECTIVE

To determine postoperative hearing outcomes after surgical plugging via middle cranial fossa approach for superior semicircular canal dehiscence syndrome (SCDS).

STUDY DESIGN

Clinical review.

SETTING

Tertiary care medical center.

PATIENTS

Forty-three cases of SCDS based on history, physical examination, vestibular function testing, and computed tomography imaging confirming the presence of a dehiscence. All patients underwent surgical plugging of the superior semicircular canal via middle cranial fossa approach.

INTERVENTION

Pure tone audiometry was performed preoperatively and at 7 days and at least 1 month postoperatively.

MAIN OUTCOME MEASURES

Change in air-bone gap (ABG) and pure tone average (PTA).

RESULTS

Preoperative average ABG across 0.25, 0.5, 1, and 2 kHz was 16.0 dB (standard deviation [SD], 7.5 dB). At 7 days postoperatively, average ABG was 16.5 dB (SD, 11.1; p = 0.42), and at greater than 1 month was 8.1 dB (SD, 8.4; p < 0.001). 53% (95% confidence interval, 33-69) of affected ears had greater than 10 dB increase in their 4-frequency (0.5, 1, 2, and 4 kHz) PTA measured by bone-conduction (BC) threshold 7 days postoperatively and 25% (95% confidence interval, 8-39) at greater than 1 month postoperatively. Mean BC PTA of affected ears was 8.4 dB hearing loss (HL) (SD, 10.4) preoperatively. Compared with baseline, this declined to 19.2 dB HL (SD, 12.6; p < 0.001) at 7 days postoperatively and 16.4 dB HL (SD, 18.8; p = 0.01) at greater than 1 month. No significant differences in speech discrimination score were noted (F = 0.17).

CONCLUSION

Low-frequency air-bone gap decreases after surgical plugging and seems to be due to both increased BC thresholds and decreased AC thresholds. Surgical plugging via a middle cranial fossa approach in SCDS is associated with mild high-frequency sensorineural hearing loss that persists in 25% but no change in speech discrimination.

Balance dysfunction and recovery after surgery for superior canal dehiscence syndrome

OBJECTIVE

To characterize (1) the impairment and recovery of functional balance and (2) the extent of vestibular dysfunction and physiological compensation following superior canal dehiscence syndrome (SCDS) surgical repair.

DESIGN

Prospective study.

SETTING

Tertiary referral center.

PARTICIPANTS

Thirty patients diagnosed as having SCDS.

INTERVENTIONS

Surgical plugging and resurfacing of SCDS.

MAIN OUTCOME MEASURES

Balance measures were assessed in 3 separate groups, each with 10 different patients: presurgery, postoperative short-term (<1 week), and postoperative long-term (≥6 weeks). Vestibular compensation and function, including qualitative head impulse tests (HITs) in all canal planes and audiometric measures, were assessed in a subgroup of 10 patients in both the postoperative short-term and long-term phases.

RESULTS

Balance measures were significantly impaired immediately but not 6 weeks after SCDS repair. All patients demonstrated deficient vestibulo-ocular reflexes for HITs in the plane of the superior canal following surgical repair. Unexpectedly, spontaneous or post-head-shaking nystagmus beat ipsilesionally in most patients, whereas contrabeating nystagmus was noted only in patients with complete canal paresis (ie, positive HITs in all canal planes). There were no significant deviations in subjective visual vertical following surgical repair (P = .37). The degree of audiometric air-bone gap normalized 6 weeks after surgery.

CONCLUSIONS

All patients undergoing SCDS repair should undergo a postoperative fall risk assessment. Nystagmus direction (spontaneous and post-head-shaking) seems to be a good indicator of the degree of peripheral vestibular system involvement and central compensation. These measures correlate well with the HIT.

Utility of cVEMPs in bilateral superior canal dehiscence syndrome

OBJECTIVES/HYPOTHESIS

To determine the utility of cervical vestibular evoked myogenic potential (cVEMP) thresholds in the surgical management of bilateral superior canal dehiscence syndrome (SCDS).

STUDY DESIGN

Retrospective review.

METHODS

We identified patients who underwent surgical treatment for SCDS from our database of 147 patients diagnosed with superior canal dehiscence (SCD) between 2000 and 2011 at our institution. The diagnosis of SCDS was based on clinical signs and symptoms, audiometric and cVEMP testing, and high-resolution computed tomography.

RESULTS

We identified 38 patients who underwent SCD surgery in 40 ears (2 bilateral). In seven patients with bilateral SCD, the more symptomatic ear had lower cVEMP thresholds, a larger air bone gap and a lateralizing tuning fork. In 13 patients with perioperative cVEMP testing, thresholds increased in 12 patients following primary repair, and no threshold shift was seen in one patient with persistence of symptoms after revision surgery. Audiometric data showed a significant mean decrease of the low-frequency air-bone gap and a mild (high-frequency) bone conduction loss after surgical repair.

CONCLUSIONS

We found that, 1) preoperative cVEMP thresholds, the magnitude of the air-bone gap and tuning-fork testing are important to confirm the worse ear in patients with bilateral SCD, 2) elevation of cVEMP thresholds following surgery correlates with improvement of symptoms and underscores the importance of postoperative testing in patients with bilateral disease or recurrence of symptoms and, 3) SCD plugging is associated with a partial closure of the air-bone gap and a mild (high-frequency) sensorineural hearing loss.

Surgical treatment of posterior semicircular canal dehiscence syndrome caused by jugular diverticulum

Objective:

We report a rare case of posterior semicircular canal dehiscence caused by a jugular diverticulum, and we describe its surgical treatment using a dehiscence resurfacing manoeuvre.

Method:

The clinical findings, surgical procedure and outcomes are presented.

Results:

A 66-year-old man presented with disequilibrium, sound-induced vertigo, a reduced ocular vestibular evoked myogenic potential threshold, and pressure-induced vertical and torsional nystagmus. Computed tomography revealed a right posterior semicircular canal dehiscence caused by a diverticulum of the jugular bulb. The defect in the posterior semicircular canal was localised and resurfaced with bone paté, temporalis muscle fascia and conchal cartilage, under direct visualisation. Post-operatively, the patient's symptoms disappeared and his ocular vestibular evoked myogenic potential threshold normalised.

Conclusion:

This case illustrates that posterior semicircular canal dehiscence can be surgically managed by resurfacing the defect site via a transmastoid approach.

Transmastoid semicircular canal occlusion: a safe and highly effective treatment for benign paroxysmal positional vertigo and superior canal dehiscence

OBJECTIVES/HYPOTHESIS

Transmastoid occlusion of the superior semicircular canal in superior semicircular canal dehiscence (SSCD) syndrome and the posterior semicircular canal in intractable benign paroxysmal positional vertigo (BPPV) will produce resolution of preoperative symptoms.

STUDY DESIGN

Retrospective review, quality assurance.

METHODS

Sixteen patients with SSCD and 61 patients (65 ears) with intractable BPPV who underwent canal occlusion were reviewed. All patients underwent occlusion of the affected semicircular canal through a transmastoid approach.

RESULTS

Preoperative symptoms (vestibular, 13 patients; pulsatile tinnitus, 2 patients; or hyperacusis, 1 patient) were greatly improved or completely resolved in 15 of the 16 SSCD patients who underwent transmastoid occlusion of the superior canal. Hearing was preserved in 14 patients and improved in two patients. Vestibular symptoms were resolved in all intractable BPPV patients who underwent transmastoid occlusion of the posterior canal. One patient had a late recurrence of atypical BPPV. Almost all BPPV patients with normal preoperative hearing have an initial transient postoperative hearing loss, which when tested for is usually a mild to moderate mixed loss. Delayed sensorineural hearing loss was noted in three patients; one loss was profound whereas two were mild.

CONCLUSIONS

The transmastoid approach to canal plugging is successful in the treatment of symptoms in both SSCD and intractable BPPV, and is a familiar approach for the otologist. This is a viable alternative to the middle fossa approach for SSCD, thereby avoiding a craniotomy. Transmastoid is the definitive approach for posterior canal occlusion.

Second-side surgery in superior canal dehiscence syndrome

OBJECTIVE

Bilateral superior canal (SC) dehiscence syndrome poses a challenge because bilateral SC dehiscence (SCD) plugging might be expected to result in oscillopsia and disability. Our aims were as follows: 1) to evaluate which symptoms prompted patients with bilateral SCD syndrome (SCDS) to seek second-side surgery, and 2) to determine the prevalence of disabling imbalance and oscillopsia after bilateral SC plugging.

STUDY DESIGN

Prospective observational study.

SETTING

Tertiary referral center.

PATIENTS

Five patients with bilateral SCDS based on history, audiometric and physiologic testing, and computed tomographic findings. This includes all of our patients who have had second-side plugging surgery to date.

INTERVENTION(S)

Bilateral sequential middle fossa craniotomy and plugging of SCs.

MAIN OUTCOME MEASURE(S)

Cochleovestibular symptoms, cervical and ocular vestibular-evoked myogenic potential testing, dizziness handicap inventory, short-form 36 Health Survey, dynamic visual acuity testing.

RESULTS

The most common symptoms prompting second-side surgery were sound- and pressure-induced vertigo and autophony. Three of the 5 patients reported that symptoms shifted to the contralateral ear immediately after plugging the first side, whereas in 2 patients, contralateral symptoms developed several years after the first SC plugging. Two of 4 patients experienced ongoing oscillopsia after bilateral SCDS surgery; however, all patients reported relief from their SCD symptoms and were glad that they had pursued bilateral surgery.

CONCLUSION

In patients with bilateral SCDS, sound- and pressure-induced vertigo most commonly prompted second-side surgery. Despite some degree of oscillopsia after bilateral SCDS surgery, patients were very satisfied with second-side surgery, given their relief from other SCDS symptoms.

[Receptor function of the semicircular canals. Part 2: pathophysiology, diseases, clinical findings and treatment aspects]

Perturbation of semicircular canal function may result in a pathological angular vestibulo-ocular reflex (aVOR). The resulting impairment in gaze stabilization is perceived as "vertigo" or "dizziness" and may occur following receptor function impairment of all three semicircular canals. The head impulse test reveals hidden (covert-catchup) or visible (overt-catchup) saccades in disturbances of semicircular function. Most peripheral vestibular disorders can be treated conservatively. There are surgical treatment options for some diseases, such as intractable benign paroxysmal positional vertigo and superior semicircular canal dehiscence. Vestibular training promotes central reorganization of the VOR. Impaired semicircular receptor function, in particular bilateral vestibulopathy, may affect spatial orientation and cognitive processes. Balance prostheses could serve as a replacement for receptors in the future.

Bilateral simultaneous caloric test for superior semicircular canal evaluation

OBJECTIVE

To verify if the superior semicircular canal (SCC) can be stimulated using the modified "bilateral simultaneous caloric test" (BSCT).

STUDY DESIGN

Prospective study in a tertiary care center.

METHODS

We performed a simultaneous irrigation of both ears with cold water at 24°C. Simultaneous irrigation of both ears inhibits the response of the horizontal SSC and avoids vegetative reaction. Modified BSCT was completed on 12 healthy individuals with no previous otologic history and on 12 patients with unilateral superior SCC obliteration for dehiscence syndrome using a middle fossa approach. Caloric response was recorded in 3 different positions (midline, head rotated to the right and then rotated to the left), and results were compared.

RESULTS

Nystagmus was vertical and upbeating. In the healthy population, the median intensity of nystagmus was 14.42 ± 7.32, 11, and 10.5 degrees per second for the 3 head positions. In the population with one obliterated superior SCC, the median intensity of nystagmus was 7.58 ± 3.8 degrees per second when the head was in the midline position, 6.0 degrees per second when the healthy superior SCC was stimulated, and 1.5 degrees per second when the obliterated canal was stimulated. Using a nonparametric statistical test, the mean intensity difference was statistically significant between the 2 populations (p < 0.001) and statistically significant between the obliterated and nonobliterated canals (p = 0.003).

CONCLUSION

This study shows that the superior SCC can be specifically stimulated in humans. Modified BSCT can stimulate the superior canal and can provide a good estimation of its stimulation intensity after the surgery.

Transmastoid resurfacing of superior semicircular canal dehiscence

OBJECTIVES/HYPOTHESIS

To describe a new and fast surgical technique in treating superior semicircular canal dehiscence syndrome by resurfacing the canal defect via the transmastoid approach without retraction of the whole temporal lobe and to demonstrate the clinical and audiologic results of the superior canal dehiscence repair. Superior semicircular canal dehiscence syndrome is a well-described pathology. Surgical procedures through the middle fossa approach to resurface the superior canal and transmastoid plugging are considered the main surgical therapeutic options for patients with debilitating symptoms. Both have drawbacks; plugging is invasive to the inner ear, and resurfacing requires a middle fossa approach.

STUDY DESIGN

Retrospective review.

METHODS

Four patients presented with classic symptomatic semicircular canal dehiscence syndrome with radiographic confirmation of their dehiscence. The patients underwent the resurfacing procedure with a transmastoid approach.

RESULTS

All four patients reported resolution of their symptoms. Audiograms documented some improvement in three subjects.

CONCLUSIONS

The transmastoid approach for resurfacing superior semicircular canal dehiscence is a safe and less-invasive technique than the standard middle fossa approach, which has many potential complications and requires much longer hospitalization. In our study, the surgeries were completed within 90 minutes, and patients stayed in the hospital only overnight.

Unidirectional rotations produce asymmetric changes in horizontal VOR gain before and after unilateral labyrinthectomy in macaques

Unilateral vestibular lesions cause marked asymmetry in the horizontal vestibulo-ocular reflex (VOR) during rapid head rotations, with VOR gain being lower for head rotations toward the lesion than for rotations in the opposite direction. Reducing this gain asymmetry by enhancing ipsilesional responses would be an important step toward improving gaze stability following vestibular lesions. To that end, there were two goals in this study. First, we wanted to determine whether we could selectively increase VOR gain in only one rotational direction in normal monkeys by exposing them to a training session comprised of a 3-h series of rotations in only one direction (1,000°/s² acceleration to a plateau of 150°/s for 1 s) while they wore 1.7 × magnifying spectacles. Second, in monkeys with unilateral vestibular lesions, we designed a paradigm intended to reduce the gain asymmetry by rotating the monkeys toward the side of the lesion in the same way as above but without spectacles. There were three main findings (1) unidirectional rotations with magnifying spectacles result in gain asymmetry in normal monkeys, (2) gain asymmetry is reduced when animals are rotated towards the side of the labyrinthectomy via the ipsilesional rotation paradigm, and (3) repeated training causes lasting reduction in VOR gain asymmetry.

Surgical management of common peripheral vestibular diseases

PURPOSE OF REVIEW

To review the latest developments in the surgical management of common peripheral vestibular disorders.

RECENT FINDINGS

Although surgery is indicated mainly for patients with Meniere's disease, other less common disorders such as benign paroxysmal positional vertigo, semicircular canal dehiscence, perilymphatic fistulae, fistulae of semicircular canals, VIII nerve vascular compression, chronic vestibular neuronitis, and dizziness following sudden sensorineural hearing loss may require surgical intervention when conservative management has failed.

SUMMARY

Surgery for vestibular disorders is less often indicated at present. Office-administered intratympanic gentamicin and steroid treatment have been found to be effective for control of vertigo in Meniere's disease. Side-effects of this treatment are overall acceptable. Surgery may be considered for Meniere's disease patients with good hearing who have failed intratympanic steroid treatment. Surgery is indicated for patients with debilitating dizziness associated with benign paroxysmal positional vertigo, semicircular canal dehiscence, vascular loop compression, and perilymphatic fistulae.

Effects of canal plugging on the vestibuloocular reflex and vestibular nerve discharge during passive and active head rotations

Mechanical occlusion (plugging) of the slender ducts of semicircular canals has been used in the clinic as well as in basic vestibular research. Here, we investigated the effect of canal plugging in two macaque monkeys on the horizontal vestibuloocular reflex (VOR) and the responses of vestibular-nerve afferents during passive head rotations. Afferent responses to active head movements were also studied. The horizontal VOR gain decreased after plugging to <0.1 for frequencies <2 Hz but rose to about 0.6 as frequency was increased to 15 Hz. Afferents innervating plugged horizontal canals had response sensitivities that increased with the frequency of passive rotations from <0.01 (spikes/s)/( degrees/s) at 0.5 Hz to values of about 0.2 and 0.5 (spikes/s)/( degrees/s) at 8 Hz for regular and irregular afferents, respectively (<50% of responses in controls). An increase in phase lead was also noted following plugging in afferent discharge, but not in the VOR. Because the phase discrepancy between the VOR and afferent discharge is much larger than that seen in control animals, this suggests that central adaptation shapes VOR dynamics following plugging. The effect of canal plugging on afferent responses can be modeled as an increase in stiffness and a reduction in the dominant time constant and gain in the transfer function describing canal dynamics. Responses were also evident during active head rotations, consistent with the frequency content of these movements. We conclude that canal plugging in macaques is effective only at frequencies <2 Hz. At higher frequencies, afferents show significant responses, with a nearly 90 degrees phase lead, such that they encode near-rotational acceleration. Our results demonstrate that afferents innervating plugged canals respond robustly during voluntary movements, a finding that has implications for understanding the effects of canal plugging in clinical practice.

Vestibular hypofunction in the initial postoperative period after surgical treatment of superior semicircular canal dehiscence

OBJECTIVES

1) Determine the prevalence of vestibular hypofunction in the immediate postoperative period after surgical treatment of superior semicircular canal dehiscence syndrome. 2) Evaluate whether dehiscence length is associated with risk of postoperative vestibular hypofunction. 3) Compare the prevalences of immediate and late postoperative vestibular hypofunction.

STUDY DESIGN

Clinical review.

SETTING

Tertiary referral center.

PATIENTS

Subjects with superior canal dehiscence syndrome (n = 42) based on history, physiologic testing, and computed tomography findings, who underwent middle fossa craniotomy and superior canal dehiscence plugging.

INTERVENTION

Dehiscence length was measured intraoperatively. Bedside horizontal head thrust testing (hHTT) was administered between postoperative days 1 to 7 to diagnose immediate postoperative vestibular hypofunction. Both hHTT and quantitative vestibulo-ocular reflex testing were administered 6 to 29 weeks postoperatively to detect late vestibular hypofunction.

MAIN OUTCOME MEASURES

Dehiscence length and hypofunction in response to hHTT.

RESULTS

Thirty-eight percent of the subjects (95% confidence interval, 25-54) had hypofunction in response to hHTT within 1 week after surgery. Mean dehiscence lengths were 4.9 (range, 2.0-10.5 mm) and 3.4 mm (range, 1.0-5.5 mm) in subjects with and without postoperative hypofunction, respectively (p = 0.0018). Each 1-mm increase in dehiscence length increased the odds of immediate postoperative hypofunction 2.6-fold (95% confidence interval, 1.3-5.1). The prevalence of vestibular hypofunction was significantly higher in the early compared with the late postoperative period.

CONCLUSION

Immediate postoperative vestibular hypofunction is common, particularly with larger dehiscences. This hypofunction may typically resolve, given that the prevalence of vestibular hypofunction 6 weeks postoperatively is low. Possible mechanisms include intraoperative loss of perilymph, which may be more likely with larger dehiscences.

Complete round Window Niche Occlusion for Superior Semicircular Canal Dehiscence Syndrome: A Minimally Invasive Approach

Superior semicircular canal dehiscence (SCD) syndrome is a recognized condition associated with varying degrees of vestibular and auditory dysfunction. The authors present a case study of disabling SCD syndrome in a 37-year-old man who was successfully treated with a complete round window niche occlusion via a transcanal approach. This case represents the first report of a transcanal complete round window niche occlusion for the treatment of SCD syndrome. A brief literature review and discussion of surgical techniques follow.

Tonic and phasic contributions to the pathways mediating compensation and adaptation of the vestibulo-ocular reflex

Processes of vestibular compensation mediate recovery of many aspects of vestibular dysfunction following unilateral vestibular injury. The VOR in response to high-frequency, high-acceleration head movements, however, retains an enduring asymmetry. Head movements that are inhibitory with respect to semicircular canals on the intact side lead to a diminished VOR whereas head movements that are excitatory for semicircular canals on the intact side lead to a VOR that returns close to normal. We review our work directed toward understanding the processes of VOR compensation to high-frequency, high-acceleration head movements and the related topic of adaptation to changes in the visual requirements for a compensatory VOR. Our work has shown that the processes of both compensation and adaptation to these stimuli can be described by a mathematical model with inputs from tonic and phasic components. We have further shown that the dynamics of regular afferents have close resemblance to the tonic pathway whereas the dynamics of irregular afferents match those of the phasic pathway.

SUPERIOR SEMICIRCULAR CANAL DEHISCENCE SYNDROME: SUCCESSFUL TREATMENT WITH REPAIR OF THE MIDDLE FOSSA FLOOR

OBJECTIVE

Superior semicircular canal dehiscence syndrome has recently been reported as a cause of pressure- or sound-induced oscillopsia (Tullio phenomenon). We report the presentation and successful treatment of 3 patients with superior semicircular dehiscence syndrome by a joint neurosurgical/neuro-otology team.

CLINICAL PRESENTATION

Patient 1 is a 37-year-old man who presented with complaints of disequilibrium, fullness in the left ear, hearing loss, and oscillopsia when pressure was applied to the left external auditory canal. Patient 2 is a 46-year-old man who presented with complaints of disequilibrium, fullness in the left ear, and blurred vision associated with heavy lifting or straining. On examination, pneumatic otoscopy produced a sense of motion. Patient 3 is a 29-year-old woman who presented with chronic disequilibrium that resulted in frequent falls. She had a positive fistula test on the left, and vertical nystagmus was elicited when pressure was applied to the left ear. In each patient, high-resolution computed tomographic scanning through the temporal bone revealed dehiscence of the superior semicircular canal on the symptomatic side.

INTERVENTION

In all 3 cases, a subtemporal, extradural approach was performed with repair of the middle fossa floor using calcium phosphate BoneSource (Howmedica Leibinger, Inc., Dallas, TX). All patients recovered well, with resolution of their symptoms.

CONCLUSION

Superior semicircular canal dehiscence syndrome is a cause of disequilibrium associated with sound or pressure stimuli. The workup includes a detailed history, electronystagmography including Valsalva maneuvers, and a high-resolution computed tomographic scan though the temporal bone. An extradural repair of the middle fossa floor with BoneSource can successfully treat this condition.

Transmastoid superior semicircular canal occlusion

OBJECTIVE

The traditional surgical repair for superior semicircular canal dehiscence (SSCD) involves either canal plugging or resurfacing via the middle cranial fossa approach. We describe a novel transmastoid occlusion technique.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

Three patients with symptomatic computed tomography-proven SSCD.

INTERVENTION

Transmastoid superior semicircular canal occlusion using bone pate in 2 fenestrations, with 1 placed on either side of the dehiscence.

MAIN OUTCOME MEASURES

Hearing and vestibular symptoms.

RESULTS

Two patients were primary cases of SSCD, and a third patient had failed a previous middle fossa occlusion using fascia at an outside institution. In all 3 cases, the 2 sides of the superior semicircular canal adjacent to the dehiscence were occluded using bone pate, formed from a mix of bone dust and fibrin sealant. This allowed for a permanent bony partition to be achieved between the dehiscence and the remainder of the labyrinth. In all cases, hearing was either preserved or improved, and the procedure was successful in controlling vestibular symptoms.

CONCLUSION

Transmastoid superior semicircular canal occlusion is a viable alternative to the customary middle fossa approach for superior canal dehiscence. Meticulous technique and the use of bone pate may help maximize auditory and vestibular results. Advantages of this technique include obviating a craniotomy, preclusion of temporal lobe retraction, familiarity of the approach for experienced otologists, and the ability to occlude the canal without manipulating the defect. The transmastoid approach for superior canal occlusion may not be possible when the dura is low hanging or when there is extensive cranial base dehiscence requiring reconstruction.

Vergence-mediated modulation of the human angular vestibulo-ocular reflex is unaffected by canal plugging

The angular vestibulo-ocular reflex (AVOR) normally has an increased response during vergence on a near target. Some lines of evidence suggest that different vestibular afferent classes may contribute differentially to the vergence effect. For example, lesions that selectively affect those afferents sensitive to acceleration, i.e. irregular afferents, (galvanic ablation, intratympanic gentamicin) have been found to markedly reduce the vergence-mediated modulation of the AVOR. We hypothesized that a nonspecific and incomplete reduction in the AVOR response caused by canal plugging should have minimal effect on vergence-mediated modulation of the AVOR. The AVOR response to passive head impulses in canal planes (horizontal canals, left anterior-right posterior canals, right anterior-left posterior canals) while viewing a far (124 cm) or near (15 cm) target was measured in seven human subjects before and after anterior canal (AC) plugging to treat vertigo caused by dehiscence of the AC (i.e. superior canal dehiscence). The impulses were low amplitude (approximately 20 degrees ), high velocity ( approximately 150 degrees /s), high-acceleration (approximately 3,000 degrees /s(2)) head rotations administered manually by the investigator. Binocular eye and head velocity were recorded using the scleral search coil technique. The AVOR gain was defined as inverted eye velocity divided by head velocity. Before plugging, AVOR gain for the dehiscent AC went from 0.87 +/- 0.10 for far targets to 1.04 +/- 0.13 for near targets (+19.1 +/- 7.3%). After plugging, the AC AVOR gain went from 0.50 +/- 0.10 for far targets to 0.59 +/- 0.11 for near targets (+19.7 +/- 6.1%). There was no difference in the vergence-mediated gain increase between pre- and post-plugged conditions (multi-way analysis of variance: P = 0.66). AC plugging also did not change the latency of the AVOR for either AC. We hypothesize that canal plugging, unlike gentamicin or galvanic ablation, has no effect on vergence-mediated modulation of the AVOR because plugging does not preferentially affect irregular afferents.