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Electrocochleography as a Diagnostic and Intraoperative Adjunct in Superior Semicircular Canal Dehiscence Syndrome

Otology & Neurotology
32:1506Y1512 Ó 2011, Otology & Neurotology, Inc.
Electrocochleography as a Diagnostic and
Intraoperative Adjunct in Superior Semicircular
Canal Dehiscence Syndrome
*Meredith E. Adams, †Paul R. Kileny, †Steven A. Telian, †Hussam K. El-Kashlan,
†Katherine D. Heidenreich, †Gregory R. Mannarelli, and †H. Alexander Arts
*Department of OtolaryngologyYHead and Neck Surgery, University of Minnesota, Minneapolis,
Minnesota; and ÞDepartment of OtolaryngologyYHead and Neck Surgery, University of Michigan,
Ann Arbor, Michigan, U.S.A.
Objective: To determine the electrocochleographic characteristics of ears with superior semicircular canal dehiscence (SSCD)
and to examine its use for intraoperative monitoring in canal
occlusion procedures.
Study Design: Case series.
Setting: Academic medical center.
Patients: Thirty-three patients (45 ears) had clinical and computed tomographic evidence of SSCD; 8 patients underwent intraoperative electrocochleography (ECoG) during superior canal
occlusion; 9 patients underwent postoperative ECoG after SSCD
Interventions: Diagnostic, intraoperative, and postoperative extratympanic ECoG; middle fossa or transmastoid occlusion of
the superior semicircular canal.
Main Outcome Measure: Summating potential (SP) to action
potential (AP) ratio, as measured by ECoG, and alterations in
SP/AP during canal exposure and occlusion.
Results: Using computed tomography as the standard, elevation of SP/AP on ECoG demonstrated 89% sensitivity and 70%
specificity for SSCD. The mean SP/AP ratio among ears with
SSCD was significantly higher than that among unaffected ears
(0.62 versus 0.29, p G 0.0001). During occlusion procedures,
SP/AP increased on exposure of the canal lumen (mean change T
standard deviation, 0.48 T 0.30). After occlusion, SP/AP dropped
below the intraoperative baseline in most cases (mean change,
j0.23 T 0.52). All patients experienced symptomatic improvement. All patients who underwent postoperative ECoG 1 to 3
months after SSCD repair maintained SP/AP of 0.4 or lesser.
Conclusion: These findings expand the differential diagnosis of
abnormal ECoG. In conjunction with clinical findings, ECoG may
support a clinical diagnosis of SSCD. Intraoperative ECoG facilitates dehiscence documentation and allows the surgeon to confirm
satisfactory canal occlusion. Key Words: ElectrocochleographyV
Semicircular canal dehiscenceVSummating potential.
Semicircular canal dehiscence syndrome results from
the absence of bone overlying the superior (superior
semicircular canal dehiscence [SSCD]) or posterior semicircular canal. The constellation of associated auditory and
vestibular symptoms includes sound- and pressure-evoked
vertigo and oscillopsia, conductive hearing loss (CHL),
conductive hyperacusis, autophony, pulsatile tinnitus, and
aural fullness (1Y4). Intraoperative visualization of a dehiscence in symptomatic patients provides the greatest degree
of diagnostic certainty. Pending direct surgical confirmation, SSCD is best identified on high-resolution temporal bone computed tomography (CT) reformatted in the
planes parallel and orthogonal to the semicircular canal
(5). Because radiographically apparent dehiscences may
be asymptomatic, it also is important to obtain physiologic confirmation that the SSCD is affecting inner ear
function (6). Clinical findings that have been advocated
for this purpose include the following: 1) nystagmus in
the plane of the affected canal in response to sound and
pressure stimuli (2), 2) reduced thresholds for vestibularevoked myogenic potentials (VEMPs) (7), and 3) CHL
(air-bone gap, Q10 dB) with preserved acoustic reflexes (8).
Surgical plugging of the dehiscent canal can provide
substantial relief to patients with disabling symptoms.
The symptomatic improvement is thought to result from
Otol Neurotol 32:1506Y1512, 2011.
Address correspondence and reprint requests to Meredith E. Adams,
M.D., Department of OtolaryngologyYHead and Neck Surgery, University of Minnesota, 420 Delaware St SE, MMC 396, Minneapolis,
MN 55455; E-mail: meadams@umn.edu
No sources of support require acknowledgment.
The authors have no financial conflicts of interest to report.
Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.
correction of abnormal flow of endolymph within the
canal (6). VEMP thresholds may normalize after canal
occlusion, potentially confirming successful SSCD closure (7). Postoperative correction of CHL and sound- and
pressure-evoked nystagmus has not been consistently demonstrated (2). None of these tests can be performed while a
patient is under general anesthesia, precluding intraoperative physiologic confirmation of SSCD closure.
Although poorly understood, abnormalities of the cochlear summating potential (SP) have previously been
identified in inner ear disorders such as Ménière’s disease and perilymphatic fistula. These SP abnormalities
presumably reflect altered electrophysiologic and/or hydrodynamic function in the inner ear. As such, it might
be anticipated that similar abnormalities may be found
in SSCD. Preliminary findings of elevated SP to action
potential (AP) ratios that consistently normalized after
canal occlusion in a small cohort of patients with SSCD
have been previously reported (9). The aim of the present
study was to further investigate the use of electrocochleography (ECoG) in the clinical diagnosis, surgical management, and postoperative assessment of SSCD. In this study,
additional experience with ECoG in SSCD patients, as well
as a novel use of intraoperative ECoG, will be described.
A review was performed of existing clinical data of patients
who underwent a comprehensive neurotologic evaluation for
SSCD at a tertiary referral academic medical center from January
2006 to April 2011. Inclusion criteria for the study were as follows: 1) presence of one or more auditory or vestibular symptoms
characteristic of SSCD syndrome (sound- and pressure-evoked
vertigo and oscillopsia, CHL, conductive hyperacusis, autophony,
pulsatile tinnitus, and aural fullness); and 2) SSCD identified in
at least 1 ear on CT. Audiometric testing, cervical VEMP testing,
and ECoG were performed as part of the evaluation. Testing for
tone-evoked nystagmus was not performed. There were 33 patients who qualified for inclusion on the basis of these criteria.
Eleven of these patients were previously described (9). Twelve
of these 33 patients underwent surgical canal occlusion. The
study protocol was approved by the institutional review board.
SSCD was identified in all patients by high-resolution noncontrast computed tomographic imaging. Images were obtained in
the axial plane at 0.625-mm intervals and reformatted in the planes
parallel and orthogonal to the superior semicircular canal (5).
Air- and bone-conduction audiometry was performed in
the standard fashion using calibrated equipment and a soundattenuated booth (ANSI-1969). CHL was defined as an air-bone
gap of 10 dBnHL or greater at 1 frequency. Cervical VEMPs were
obtained using clicks presented to the test ear at a rate of 4.7 per
second (9). The range of normal values for click-evoked cervical
VEMP threshold has been reported to be 80 to 100 dBnHL or
75 to 100 dBnHL, depending on test performance (10). Thresholds less than 80 dBnHL were considered to be abnormal for
purposes of this study.
ECoG was performed using a hydrogel-tipped tympanic
membrane electrode (TM-ECochGtrode, Bio-Logic Systems/
Natus Medical, Mundelein, IL, USA) introduced onto the surface of the tympanic membrane under microscopic visualization and stabilized by the foam tip of the insert audio transducer.
The patients were supine, with head elevated approximately
30 degrees. Stimuli consisted of unfiltered alternating polarity
clicks of 100-Ks duration, presented at an intensity of 85 dBnHL.
Two replications of averaged responses elicited by 1,500 clicks
presented at a rate of 11.7 per second were obtained. Responses
were band-pass filtered (20Y1,500 Hz) and averaged, and the
SP/AP ratio was calculated. SP/AP ratio of greater than 0.4 was
defined as abnormal for purposes of this study, based on commonly used standards for clinical testing (11,12).
Selected highly symptomatic patients underwent surgical
repair of their SSCD. In all cases, the semicircular canal was
occluded using either fascia/bone pate or bone wax. One patient
was occluded via a transmastoid approach, and the others were
occluded via a middle cranial fossa (MCF) approach. Patients
were positioned supine, with head turned toward the contralateral ear. Intraoperative tympanic membrane ECoG was used for
monitoring purposes during canal occlusion procedures. Stimuli
consisted of alternating polarity clicks presented at 90 dBnHL.
Responses were recorded using a Nicolet Viasys Endeavor neurodiagnostic system (software version; Madison, WI,
USA), a unit commonly used for multimodal intraoperative
monitoring. SP and AP amplitudes were measured repeatedly
throughout the procedures, with particular attention to measurement at critical time points: 1) after the induction of general
anesthesia before the surgical incision (intraoperative baseline);
2) during dural elevation and dehiscence exposure (or canal
fenestration); and 3) after canal occlusion, before emergence from
anesthesia. Postoperative testing was performed between 4 and
12 weeks after canal occlusion.
All preoperative and postoperative electrophysiologic testing was performed and interpreted by the same clinical auditory
neurophysiologist. No attempt was made to blind the neurophysiologist to the clinical impression, individual CT results, or other
clinical findings. Intraoperative testing was performed by audiologists experienced in intraoperative monitoring supervised by the
same individual.
The averaged SP/AP ratios of affected and unaffected ears
were compared using 2-sample t tests after confirming that
assumptions for pooled variance were met. The averaged VEMP
thresholds of affected and unaffected ears were compared using
2-sample t tests assuming unequal variance (SDSCD/SDnormal = 2).
Statistical analysis was performed with Microsoft Excel 2010
(Microsoft Corporation, Redmond, WA, USA).
Diagnostic ECoG
Thirty-three patients with computed tomographic evidence of SSCD underwent diagnostic ECoG. There were
24 women (73%). The average patient age was 49 T 12
(mean T standard deviation [SD]) years. Of the 33 patients,
12 had bilateral SSCD, and 21 had unilateral SSCD, for a
total of 45 affected ears and 21 unaffected ears. Unilateral
SSCD was found in 13 left ears and 8 right ears. ECoG,
VEMP, and audiometric testing were performed on all
66 ears, with the exception of a single unaffected ear that
did not undergo ECoG.
The mean SP/AP ratio among ears with CT-documented
SSCD was significantly higher than that among unaffected
ears (Table 1). The ranges of SP/AP were 0.29 to 1.48
for affected ears and 0.07 to 0.65 for unaffected ears. The
mean VEMP threshold among ears with SSCD was significantly lower than that among unaffected ears (Table 1).
Otology & Neurotology, Vol. 32, No. 9, 2011
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TABLE 1. Mean summating potential to action potential
ratio on electrocochleography and vestibular-evoked myogenic
potential thresholds in affected and unaffected ears
Summating potential
to action potential ratio
on electrocochleography
myogenic potential
threshold (dB nHL)
canal dehiscence
p value
0.62 T 0.21
0.29 T 0.17
70 T 8
82 T 4
Values are mean T standard deviation. p values are for 2-sample
t tests.
All 21 patients with unilateral SSCD had an elevated
SP/AP (90.4) in the affected ear. All 12 patients with
bilateral SSCD had an elevated SP/AP in at least 1 ear.
Seven of the 12 patients with bilateral SSCD had bilateral SP/AP elevation, with the higher SP/AP value in the
more symptomatic ear or nonlateralizing symptoms. In
the remaining 5 patients with bilateral SSCD, a negative
ECoG (SP/AP, e0.4) was documented in 1 ear. Three of
these 5 patients were asymptomatic with regard to the ear
with the normal SP/AP result; 1 patient had otosclerosis
documented intraoperatively after the ECoG and no other
audiovestibular symptoms in that ear, and 1 patient had
already undergone round- and oval window plugging in
the ‘‘false-negative’’ ear (which was still the more symptomatic ear) before being referred to our center.
By nature of the study design, identifying only patients
with SSCD in at least 1 ear, all 6 of the ‘‘false-positive’’
ECoG results were found in unaffected ears of patients
with a contralateral SSCD. Two of the 6 ears had CHL
(Q10 dB), and a different 2 of 6 had a VEMP threshold
less than 80 dB, but there were no lateralizing auditory or
vestibular symptoms attributed to the ears with the falsepositive ECoG results.
Table 2 summarizes the performance statistics for
ECoG, VEMP, and CHL in this series. We were interested
in comparing sensitivity and specificity of ECoG versus
VEMP for the diagnosis and confirmation of SSCD. For
purposes of these calculations, we used the presence of a
dehiscence on CT as the ‘‘gold standard’’ because most
Summary statistics for tests for superior
semicircular canal dehiscence
Sensitivity (%)
Specificity (%)
Table modeled after (14).
CHL indicates conductive hearing loss with air bone gap of 10 dB
or higher; CT, computed tomography; ECoG, electrocochleography
with SP/AP greater than 0.4; FN, false negative; FP, false positive; TN,
true negative; TP, true positive; VEMP, vestibular-evoked myogenic
potential threshold less than 80 dB nHL.
CT is the clinical gold standard for this study, recognizing that rare
false-positive studies are likely.
dehiscences were not confirmed by direct visualization in
the operating room. Although it is well established that
a proportion of dehiscences present on CT are not symptomatic (13,14), in the present study, all dehiscences present on CT also were associated with a CHL of at least
10 dB at 1 frequency and/or an abnormally low VEMP
threshold. These latter 2 findings suggest that all of
the dehiscences seen on CT were physiologically active.
Using these assumptions, ECoG had comparable sensitivity and specificity to VEMP if a VEMP threshold less
than 80 dBnHL is considered abnormal. If VEMP threshold less than 75 dBnHL is considered abnormally low,
VEMP sensitivity decreases to 69%, and specificity increases to 100%. ECoG did not enlarge the patient group,
with objective findings attributable to SSCD, but not all
patients had all 3 objective findings.
Intraoperative ECoG
Twelve patients underwent superior canal occlusion
for symptomatic SSCD. Nine patients were scheduled
FIG. 1. Intraoperative SP/AP amplitude ratios, in response to
click stimuli, are plotted at 3 time points during SSCD occlusion
procedures: at the start of the operative procedure before dural
elevation or canal fenestration (baseline), after the dural elevation
when the dehiscence is exposed or after the canal is fenestrated
(exposure), and after canal occlusion (occlusion). (*) Patient with
bilateral SSCD and preoperative SP/AP of 0.59 in operated ear.
Although the SP/APBASE = SP/APOCC = 1.0, the patient reported
postoperative improvement in lateralizing audiovestibular symptoms and postoperative SP/AP of 0.26. (**) Large SSCD with
preoperative SP/AP of 0.7 and SP/APEXP of 1.0. SP/AP decreased
to below 0.4 with initial canal occlusion with bone wax. Moments
after packing, AP amplitude decreased, and SP increased, prompting repacking, but SP/AP remained at 1.0. Postoperatively, the
patient was acutely vertiginous and was confirmed to have a new
unilateral vestibular paresis without sensorineural hearing loss.
Sound-evoked vestibular symptoms in the operated ear resolved
and postoperative SP/AP was borderline normal at 0.4.
Otology & Neurotology, Vol. 32, No. 9, 2011
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for intraoperative ECoG; however, in 1 case, ECoG could
not be monitored after dural elevation because of fluid
entering the middle ear through multiple large tegmen
dehiscences. Of the 8 fully monitored cases, 7 were performed by the MCF approach, and one by a transmastoid
approach. A definitive dehiscence was identified at the
time of surgery in all cases. Although continuous ECoG
was performed throughout the cases, the value of SP/AP
was specifically established at 3 points during the procedures for this study: 1) intraoperative baseline before
the surgical incision (SP/APBASE); 2) upon exposure of
the dehiscence or fenestration of the canal (SP/APEXP);
and 3) after canal occlusion (SP/APOCC). Data for SP/
APEXP were available for 7 of the 8 cases. Data for SP/
APBASE and SP/POCC were available for all 8 cases.
SP/APBASE was elevated in all patients (Fig. 1). During
all 7 procedures for which data for SP/APEXP was available, SP/AP increased from the SP/APBASE upon exposure
of the dehiscence or fenestration of the canal (SP/APEXP,
mean change T SD, 0.48 T 0.30). After canal occlusion,
SP/APOCC dropped below SP/APEXP in all 8 cases (mean
change T SD, j0.66 T 0.34). SP/APOCC dropped below
SP/APBASE in 6 of the 8 cases and dropped to be equal
to SP/APBASE in 1 case (mean change, j0.23 T 0.52).
SP/APOCC was higher than SP/APBASE in 1 case. The
exceptions to the observed pattern are indicated in Figure 1
and discussed further in the legend.
An illustrative ECoG recording from an MCF approach
for occlusion of SSCD is provided in Figure 2. The contribution of SP changes versus AP changes to the alteration of SP/AP varied between patients and could not be
precisely determined because of the test-retest variation
that is inherent to the operative setting.
Postoperative ECoG
Postoperative ECoG was performed 4 to 12 weeks after
SSCD occlusion in 9 of the operated cases. Occlusion of the
semicircular canal effectively relieved the lateralizing preoperative symptoms in all patients who underwent postoperative testing. All cases had preoperative SP/AP greater
than 0.5 and postoperative SP/AP of 0.4 or lesser. The
average decrease from preoperative SP/AP was 0.47 T 0.36.
FIG. 2. Intraoperative ECoG recordings from a patient undergoing
an MCF approach for occlusion of SSCD. The preoperative SP/AP
was 0.66. The intraoperative baseline SP/AP was 0.49. During dural
elevation, upon complete exposure of the SSCD, the SP/AP increased to 1.0. When plugging with bone pate and fascia did not
result in a normal SP/AP, the surgeon suspected incomplete occlusion. The dehiscence was packed with additional bone pate, resulting in a normal SP/AP of 0.17. The patient experienced excellent
postoperative symptom resolution. x axis: time, 1 ms/division. y axis:
amplitude, 0.5 KV/division.
In this study, SP/AP elevation was detected in the
dehiscent ear in every patient with unilateral SSCD and
at least 1 ear of every patient with bilateral SSCD.
Overall, the sensitivity of ECoG for detecting SSCD
was 89%, and specificity was 70% when using CT as the
gold standard for diagnosis. In cases of bilateral SSCD
in which symptoms lateralized to 1 ear, the SP/AP tended
to be higher in the more symptomatic ear. The SP/AP
was found to normalize in those patients who underwent
ECoG after superior canal occlusion. It was possible to
detect physiologic changes in the inner ear in ‘‘real time’’
by using intraoperative ECoG.
Several limitations of the study design necessitate caution when interpreting preoperative ECoG test performance characteristics. First, this is a retrospective study
without a normal control group. Because of the high number of bilateral cases in this series, test performance characteristics were calculated based on a small group of
negative control ears in patients with computed tomographic evidence of SSCD in the contralateral ear. Multiplanar reformatted CT is reported to have a 100% negative
predictive value for SSCD, making it unlikely that the
‘‘negative’’ ears in our study have undiagnosed SSCD
Otology & Neurotology, Vol. 32, No. 9, 2011
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(13,14). A normal control group would allow us to determine if an asymptomatic patient with SSCD would
have SP/AP elevation and would help to establish if the
threshold value of SP/AP (0.4), selected based on experience with endolymphatic hydrops (ELH), is appropriate
for third-window lesions. Additionally, some ears considered to have SSCD in this series may represent false
positives because CT was used as the gold standard. The
radiographic prevalence of SSCD in asymptomatic patients
on CT reformatted in the plane of the superior canal has
been estimated to be 4.0%, a value that exceeds the 0.5%
prevalence on histopathologic examination (13,15). Although not part of the inclusion criteria, all of the patients
in this series were symptomatic and had at least 1 objective
finding of SSCD (CHL, low VEMP threshold), potentially
reducing the likelihood of false positives. Still, the presence of false positives may explain the low specificity for
ECoG that was observed. Additional studies to define the
clinical usefulness of ECoG in the diagnosis of SSCD are
ECoG is the measurement of electrical potentials generated by the cochlea and auditory nerve in response to
acoustic stimulation. The recording electrode is placed
as close to the cochlea as possible, and the epoch of interest is the first 3 ms after stimulus presentation. In this
study, we obtained consistent results in both the outpatient
clinic and the operating room with a commercially available tympanic membrane electrode, obviating the need
for transtympanic needle placement. Depending on the
nature of the stimulus and the mode of stimulus delivery,
the ECoG response may consist of the cochlear microphonic, the SP, and the cochlear nerve compound AP.
The SP is a direct current potential that consists of a shift,
positive or negative in polarity, from the baseline upon
which the cochlear microphonic is superimposed (16).
It is thought to reflect nonlinear properties of cochlear
hydrodynamics and basilar membrane mechanics (17).
Both inner and outer hair cells likely contribute to the
production of the SP (18). In response to a click stimulus
alternated in polarity, as used in this study, the SP appears
as a deflection, or a shoulder preceding the onset of the
AP, with the same polarity. The AP is the sum of the
individual APs of synchronously firing auditory nerve
fibers and is equivalent to Wave I of the auditory brainstem
Although we can only speculate about the mechanism
of SP/AP elevation in SSCD, we think that insights can
be gleaned from studies of ECoG in presumed hydropic
conditions of the inner ear, most notably Ménière’s disease and perilymph fistula (PLF), in which secondary
ELH is thought to occur. An increase in the SP amplitude
relative to the AP amplitude is widely thought to be the
most useful ECoG measure for identifying ELH. Significant increases in SP amplitude result from experimental
displacement of the basilar membrane toward the scala
tympani, either by increasing hydrostatic pressure above
the basilar membrane (19) or by low-frequency biasing
(20). Clinical SP elevation may reflect hydromechanical
changes within the scala media caused by a relative in-
crease in endolymphatic fluid pressure (17), explaining
the elevated SP/AP observed in two-thirds of patients with
Ménière’s disease (21) and in animal models of hydrops
(22). Moreover, elevation in SP/AP has been acutely induced by suctioning perilymph from surgically generated round window fistulas (PLFs) in guinea pigs. After
the fistula healed, the SP/AP returned to normal (23).
This finding has been replicated in other animal models
and in humans undergoing surgical exploration for PLF.
Arenberg et al. (24) found that 14 of 27 patients with
surgically confirmed PLFs had SP/AP greater than 0.5 and
that animals with an induced ‘‘active’’ PLF (i.e., leaking
perilymph) had ECoG findings and histologic evidence of
ELH. Gibson (25) monitored intraoperative SP/AP during
stapedectomies and cochleosacculotomies. He found that
suctioning perilymph from the vestibule or round window
could induce a significant increase in the SP and decrease
in the AP and that the changes could be reversed if the
vestibule or basal turn refilled with perilymph by increasing intrathoracic pressure.
A SSCD may induce hydrostatic changes similar to
those described for PLF and ELH. The third-mobile
window may reduce the pressure in the scala vestibuli
and scala tympani as pressures equalize around the helicotrema, thus inducing an endolymphatic pressure differential. In the operative setting, exposure of the dehiscence
or canal lumen, thus opening the perilymph compartment,
would acutely lower the perilymphatic pressure. The pressure differential would lead to a temporary relative increase
in endolymphatic pressure, driving the basilar membrane
toward the scala tympani, resulting in an increase in the
SP/AP. Satisfactory plugging of the dehiscence would be
expected to promptly restore physiologic perilymph pressures. Further investigations will be needed to investigate
this appealing but potentially oversimplified model.
Based on our findings, we propose that SSCD and
other third-window lesions be added to the differential
diagnosis of an elevated SP/AP and that ECoG may be
used to confirm a physiologically active SSCD. We found
that the sensitivity of ECoG was comparable to VEMP in
our series. Crane et al. (14) reported similar sensitivity
and specificity values for VEMP (G85 dB defined as
abnormal) in their series comparing diagnostic tests and
3-dimensional CT. The mean VEMP thresholds for both
affected and unaffected ears were 10 to 20 dB lower in
this study than in other reported series (2), suggesting
institutional differences in test performance. Regardless
of technique, VEMP requires active participation and continuous muscle contraction on the part of the patient. On
the other hand, ECoG requires a potentially uncomfortable, albeit safe, tympanic membrane surface electrode
placement. Institutional experience with more than 800
tympanic ECoG procedures suggests that the procedure
is well tolerated by patients as young as 10 years old. In
bilateral cases, Welgampola et al. (7) observed that VEMP
thresholds were concordant with symptoms and may be
useful in determining the more severely affected side.
Similarly, this study found that the ECoG tended to be
higher in the symptomatic ear. Further investigation is
Otology & Neurotology, Vol. 32, No. 9, 2011
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necessary to determine if the SP/AP may provide physiologic evidence of the more affected ear when selecting the
ear for surgery in ambiguous bilateral cases.
The greatest use of ECoG in SSCD may be realized
in the operating room. Unlike VEMP, CHL, and soundevoked nystagmus, changes in the SP/AP can be detected
in a patient under general anesthesia. The ECoG surface
electrode can be easily placed and secured before beginning the surgery. The potentials are monitored with standard clinical equipment and software already used during
neurotologic surgery to monitor the auditory brainstem
response (6). It seems that the monitoring audiologist is
able to pinpoint the moment of dehiscence exposure without notification by the surgeon, simply by observing an
increase in the SP/AP. We have used the same technology
intraoperatively when treating posterior semicircular canal
disease, with similar findings. Interestingly, the phenomenon of SP elevation at the moment of canal fenestration
has been described once previously. Gibson (26) published
ECoG tracings, documenting a sudden marked increase
in SP amplitude when the posterior semicircular canal
was inadvertently fenestrated during an endolymphatic sac
decompression. He sealed the opening, rather than plugging the canal, and the SP remained abnormally elevated
at the end of the procedure.
Physiologic documentation of SSCD exposure may
be particularly helpful in patients with multiple tegmen
dehiscences, when the exact location of the semicircular
canal lumen may not be immediately apparent. In such
cases, it should be possible to avoid suction trauma to the
membranous labyrinth while exploring the middle fossa
floor, taking special precautions once the SP/AP rises.
In addition, a decrease in the SP/AP after canal occlusion
confirms satisfactory plugging of the true SSCD. Image
guidance has been used at some centers for similar reasons
(6). Although it does not allow for direct correlation to
imaging findings, ECoG may be considered a simpler and
more physiologic navigation system.
In surgery for SSCD, the operating surgeon is challenged by balancing the need to protect the delicate membranous labyrinth, while sufficiently packing a microscopic
canal lumen. With experience, surgeons may come to rely
on the intraoperative normalization or decrease in SP/AP
to assure that the canal is sufficiently plugged without
overpacking. Normalization of SP/AP can provide useful
feedback to the surgeon that the canal is occluded and that
sensorineural hearing has been maintained. If the SP/AP
fails to decrease after initial plugging, additional packing
may be needed. In this study, all SSCDs were repaired
with canal occlusion techniques. Pure resurfacing techniques may result in different SP/AP findings but would be
expected to follow suit.
Potential limitations of intraoperative ECoG include
an inability to monitor potentials if fluid percolates from
exposed air cells into the middle ear or if a patient
with CSF otorrhea is undergoing concurrent encephalocele repair. In either case, the resultant CHL may prevent
adequate ECoG monitoring. Likewise, patients with preexisting CHL may present special problems.
In summary, results of this study suggest that an elevated SP/AP (90.4) on ECoG may be an additional electrophysiologic indicator of SSCD and that canal plugging
results in postoperative normalization of SP/AP. Thus,
the presence of an undiagnosed SSCD should be added
to the differential diagnosis of an elevated SP/AP. SP/AP
elevation occurs intraoperatively upon exposure of the
canal lumen, either at the time of dural elevation overlying
a dehiscence or at the time of a transmastoid semicircular
canal fenestration. Thus, ECoG may be used in the operating room to document both dehiscence exposure and
satisfactory canal occlusion.
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