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TMelectrode
Figure 17–4. Tympanic membrane electrode (top trace) versus tiptrode
electrode (bottom trace): significant difference in the resolution of the
and the AP.
These were recorded simultaneously from a normal subject.
of wave V, these two are identical far-field recordings.
As for the earlier components, the tympanic membrane
recording is relatively near-field, as opposed to the tiptrode recording, which is relatively far-field for the SP
and AP generators, and thus the significant differences,
in favor of the tympanic membrane recording when
it comes to the identification of the components that
make up the ECochG. A summary of ECochG recording techniques is presented in Table 17–1.
“Tiptrode”
SP
Table 17–1. ECochG Recording Techniques
Transtympanic Needle electrode
Ball window electrode
Extratympanic
Tympanic Flexible tubing with hydrogel or
Tiptrode
Leaf electrode
cotton tip
Stimulating Parameters
The configuration of the response and its diagnostic
utility also depend significantly on the stimulus choice
and recording parameters. In general, regardless of
the specific stimulus used, it is recommended to use
relatively high stimulus intensities, such as 80 to 95 dB
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nHL in order to promote a good response resolution.
This is not different from stimulus intensities used for
neurodiagnostic ABR applications. The summating
potential, cochlear microphonic, and action potential
can be elicited by both tone-burst stimuli and clicks.
Each has advantages and disadvantages, and in addition to the specific stimulus, the delivery using constant
versus alternating polarity changes the configuration
of the response. When using tone bursts, the dominant
component recorded is the cochlear microphonic. It is
typically difficult to identify or discern the cochlear
nerve action potential. In particular, if the summating
potential is relatively large, it can be identified and it
combines with the cochlear microphonic. As it grows, it
“displaces” the cochlear microphonic baseline, giving
the impression that the cochlear microphonic “rides”
on top of a rectangular ramp (Figure 17–5).
One way to eliminate the cochlear microphonic
and be left with the summating potential is to elicit
responses with alternating polarity tone bursts, or sum
responses obtained with rarefaction and condensation
polarity. This will cancel out the cochlear microphonic
and leave the summating potential, especially in cases
where it is relatively prominent.
When using clicks, constant polarity stimulation will result in a cochlear microphonic followed
by the cochlear nerve action potential, when adding
responses elicited by rarefaction and condensation
clicks, or eliciting a response with alternating polarity
clicks, the cochlear microphonic (which is very brief
due to the nature of the stimulus) will be canceled and
the cochlear nerve AP will be preceded by a summating potential, as illustrated in Figure 17–6. A summary
of ECochG stimulating parameters is presented in
Table 17–2.
figure 17–6. Constant polarity (rarefaction and condensation) click-evoked responses result in the definition
of the corresponding cochlear microphonic preceding
the cochlear nerve action potential (AP) (first and second trace).
using alternating polarity clicks) results in the resolution
of the summating potential (SP), preceding the AP.
Their sum, shown in the bottom trace, (or
figure 17–5. Cochlear microphonic recorded using
a tympanic membrane electrode superimposed on
prominent summating potential. The amplitude of the
summating potential amplitude is defined by the two
parallel lines.
table 17–2. ECochG Stimulating Parameters
Stimuli Broad-band click (100 microseconds)
Tone burst (2 ms rise/fall, 10 ms plateau)
Polarity Alternating (can also use condensation
and rarefaction
Rate 5–11/s
Level 85–95 dB nHL
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Recording Parameters
The recording parameters need to also be adjusted to
optimize the identification of the components of interest. Primarily, it is important to utilize appropriate
bandpass filtering, keeping in mind that the summating potential is very low frequency in nature; in fact,
it is often referred to as a direct-current component.
Therefore, the usual high-pass filters used for neurodiagnostic auditory brainstem response testing are not
adequate for this recording. The high-pass filter needs
to be between 10 and 20 Hz with a low slope, such as
6 dB per octave in order to avoid distortion and ringing
that can alter the response and make it difficult to interpret. The low-pass filter is somewhat less important;
however, if desired to also include the typical components of the ABR, the low-pass filter should not be any
lower than 1500 Hz.
Beyond stimulus characteristics and electrode
placement, the specifics of the recording protocol also
play an important role in the response presentation
and resolution. The specific components of the ECochG
occur within an approximately 3 ms poststimulus time
window at stimulus intensities ranging from 80 to 95
dB nHL. The latency of the SP for click stimuli ranges
from less than 1 to no more than 1.5 ms. This value
refers to the peak and not the onset of the SP. The onset
is approximately 0.5 ms poststimulus. The latency of
the AP ranges from about 1.5 to 2.5 ms depending
upon hearing status. It is possible to also record the
later components of the ABR, such as waves III and V.
If this is of interest, a recording epoch of 10 ms is sufficient, as the latency of wave V is between 5.5 and 6.5
ms. A 10 ms window provides sufficient resolution to
the early components of interest such as the SP and AP,
as well as allowing the resolution of waves III and V.
Another important aspect of the recording protocol is
to have a 1 to 2 ms prestimulus baseline. This is necessary as it serves as the reference point for measuring
the amplitudes of the SP and the AP. It is highly desirable for this prestimulus baseline to be as electrically
neutral as possible, so that it appears as a reasonably
flat line. Since the prestimulus baseline occurs prior to
the delivery of the stimulus, in theory it is not affected
by auditory system activation. In reality, depending
on the repetition rate, this prestimulus segment may
be somewhat affected by auditory pathway activation. The recommended stimulus rate for ECochG is
typically lower than the recommended rate for auditory brainstem response testing, and we recommend
a rate ranging from approximately 9 to 11 per second.
The lower the rate the less the prestimulus baseline is
affected by auditory pathway activation and is likely
to remain more neutral. At times movement or muscle
artifact may affect the prestimulus baseline to the point
that averaging is not sufficient to eliminate the artifact
within the time period. If that is the case, my recommendation is to delete the response and begin a new
average closely monitoring the status of the baseline.
This is very important for the accuracy of the components of interest. Some individuals have a tendency to
measure the SP and AP amplitudes from the lowest
“valley” preceding the SP which is actually the onset
of the SP, and thus not electrically neutral. This mode of
amplitude measurement is likely to introduce inaccuracies as it is not an electrically neutral reference point,
as is the prestimulus baseline. Figure 17–7 illustrates
an ECochG recorded as outlined above. The baseline,
which is reasonably flat, serves as the common reference point for the measurement of the SP and AP
amplitudes. This then results in an accurate SP-to-AP
ratio. A summary of ECochG recording techniques in
presented in Table 17–3.
Figure 17–7. Typical ECochG recorded with a tympanic
membrane surface electrode. A prestimulus baseline is
necessary to serve as the reference for the SP and AP
amplitudes.
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Table 17–3. ECochG Recording Parameters
Amplification (gain) ×50,000–100,000 (extratympanic)
×5,000–25,000 (transtympanic)
Epoch 5–10 milliseconds (with a 20 ms
prestimulus period)
Filter bandpass 1–3000 Hz
Number of accepted samples 1000–2000 (extratympanic)
150–250 (transtympanic)
Number of completed averages 2
Electrode impedances <10,000 Ω, with interelectrode
impedance differences <2000 Ω
Artifact rejection Enabled
RATIONALE AND PRINCIPLE OF
ELECTROCOCHLEOGRAPHY
SUMMATING POTENTIAL TO
AND
ACTION POTENTIAL RATIO
The principles of the SP and AP recording and diagnostic applications are based on extensive experimental animal studies where these components were
recorded either directly from scala tympani or scala
vestibuli or from an electrode in direct contact with the
round window. Early work by Davis and associates in
the 1950s defined the summating potential and demonstrated that static pressure to the scala tympani could
change the amplitude of the summating potential and
change its polarity. Eggermont (1976) summarized the
effects of specific recording site, stimulus frequency,
and stimulus intensity on the polarity of the SP relative
to the AP. He expressed the opinion that the polarity
of the SP reflects an anatomical difference relative to
the specific site of the electrode. In the majority of the
cases recorded from human subjects and patients, he
observed the SP with the same polarity as the AP. Eggermont concluded that while the polarity may be subject to recording site relative to scala tympani or scala
vestibuli, since the promontory SP probably reflects
responses from both scalae, the sign of the SP is not a
reflection of cochlear pathology. However, the magnitude of the SP may be associated with “some form of
cochlear disorder.” Eggermont evaluated input/output
functions for the SP and the AP at various frequencies
for normal hearing controls, Ménière’s patients, and
patients with cochlear hearing loss without hearing
loss and without Ménière’s. He also evaluated SP/AP
ratio for different frequencies and at different intensities and found that the mean value, or overall value,
was slightly higher than in normal ears, and in normal ears, the ratio was dependent on stimulus intensity, increasing from low to high intensity. In Ménière’s
patients there was no correlation between stimulus
intensity and SP/AP ratio, that is, the ratio which was
overall higher than in normal ears was the same at low
and high intensities. The SP/AP ratio difference was
highest between Ménière’s ears and ears with nonMénière’s sensory hearing loss.
There has been extensive literature regarding the
utility of ECochG in the diagnosis of Ménière’s disease
and in monitoring the effects of treatment. The SP/AP
amplitude ratio has been reported to exceed a specificity of 90% (Ferraro, Best, & Arenberg, 1983; Murphy
& Gates, 1999). In terms of sensitivity most reports
agree that approximately 60 to 65% of patients with
Ménière’s disease present with an increased SP/AP
amplitude ratio. In an attempt to improve the sensitivity, Ferraro and his colleagues (Al-Momani, Ferraro,
Gajewski, & Ator, 2009) recommended to augment the
measurement of the SP/AP amplitude ratio with the SP
and AP area under the curve. This addition substantially increased the sensitivity of ECochG for Ménière’s
disease to 92% based on this study.
It is clear from experimental animal studies and
human clinical studies that the position of the basilar
membrane that would be displaced in the presence of
endolymphatic hydrops does have an effect on the amplitude of the SP. It is thought that in the presence endolymphatic hydrops, the basilar membrane is statically
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displaced toward the scala tympani. While it is thought
that under normal conditions the SP is generated by
the inner hair cells, when the basilar membrane is displaced toward scala tympani, there is a larger contribution from the outer hair cells due to a change in their
electrical properties influenced by the abnormal position of the basilar membrane, thus increasing the magnitude of the summating potential.
DIAGNOSTIC APPLICATIONS OF
ELECTROCOCHLEOGRAPHY
ECochG in Ménière’s Disease
It stands to reason that the diagnostic applications
of ECochG will pertain to conditions that alter the
mechanics of the cochlea, which in turn would contribute to the change in the magnitude or nature of
cochlear potentials, such as the summating potential. Ménière’s disease continues to represent a diagnostic challenge for the clinician. Given that effective
treatment depends on accurate diagnosis, clinicians
continue to make every effort to improve diagnostic
accuracy in Ménière’s disease.
Histopathologic studies of temporal bones with
classic Ménière’s disease signs and symptoms have
demonstrated the presence of distortion and dilation of the endolymphatic spaces in the membranous
labyrinth. This phenomenon is typically referred to
as endolymphatic hydrops, which is considered to
be the pathologic basis of Ménière’s disease. It is of
note, however, that these pathologic changes do not
always correspond to clinical manifestations. Some
studies have shown a complete match of pathologic
and clinical findings (Rauch, Merchant, & Thedinger,
1989). Others did not find such close correspondence
(Minor, Schessel, & Carey, 2004). The American Academy of Otolaryngology–Head and Neck Surgery
(AAO-HNS) has provided criteria for the diagnosis of
Ménière’s disease based on the nature of the recurrent
spontaneous vertigo, hearing loss, aural fullness, tinnitus, as well as audiological documentation of hearing
loss (Committee on Hearing and Equilibrium, 1995).
Signs and symptoms, including hearing loss, tend to
fluctuate, and physical findings in general are lacking in this condition. Appropriately summarized are
the questions confronting the clinicians relative to the
diagnosis of Ménière’s disease. The questions outlined
included whether the patient had Ménière’s disease,
and this can be based on audiologic and vestibular
evaluations as well as on the AAO-HNS criteria. Which
ear is causing the symptoms? This question may be
answered by results of audiologic and vestibular tests,
especially if vestibular and auditory results coincide,
such as low-frequency, fluctuating sensorineural hearing loss and vestibular weakness in the same ear. The
next important question is whether there is bilateral
disease, as this will significantly impact the treatment
plan. Finally, we need to ask whether the treatment
that has been initiated for a given patient is effective.
ECochG can be used to address the questions posed
in the diagnosis of Ménière’s disease. One of the main
clinical advantages of ECochG is that the results are
ear specific, and therefore the question of which ear is
responsible for the symptoms and whether the patient
presents with bilateral disease may be answered by the
results of this test. The typical expected result in an ear
positive for Ménière’s disease is an increase in the summating potential amplitude relative to the action potential amplitude — that is, an increase in the SP/AP ratio
(Coats, 1981; Dauman et al., 1988). As mentioned elsewhere in this chapter, it is believed that the presence of
hydrops affects the resting position of the basilar membrane, displacing it toward the scala tympani, which
in turn results in changes in the electro-anatomy of the
hair cells, increasing the magnitude of the summating
potential. Instead of measuring and reporting the absolute amplitude of the summating potential, the SP/AP
amplitude ratio is used, in order to avoid individual
variability. Most investigators have used a value range
for the SP/AP ratio of 0.3 to 0.5 as the outer limits of the
normal range. Using these values, approximately twothirds of Ménière’s patients presented with abnormally
elevated SP/AP ratios (Aso, Watanabe, & Mizukoshi,
1991). Due in part to differences in measurement, there
is some variation in the literature in terms of normal
values of SP/AP ratio in subjects without Ménière’s
disease. In order to eliminate this variability, Margolis,
Rieks, Fournier, and Levine (1995) published a study
on normative ECochG data from 53 subjects. They
used tympanic surface electrodes and found that the
SP/AP ratios were dependent on stimulus level, ranging from 0.22 at 78 db nHL to 0.29 at 68 db nHL. The
95th percentile for the SP/AP ratio ranged from 0.40 to
0.49. Based on these data, they considered 0.35 or less
to be a normal value and 0.5 or above to be a definitely
abnormal result. Based on our own normative data and
the Margolis publication, the definition of the upper
limit normal range in our clinic is 0.40. Values above
0.40 are considered to be elevated. The following case
study illustrates our diagnostic protocol including the
interpretation of the ECochG.
The patient was a 38-year-old woman with a two-
year history of episodic vertigo lasting 10 to 15 min,
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accompanied by nausea and occasional emesis. These
episodes had been occurring every two weeks and
their severity varied. Each one of these episodes left the
patient fatigued and incapacitated for a few hours. In
between attacks, she was functioning quite well, with
the exception of some disequilibrium she experienced
for a few days after the attacks. She also reported rightsided aural fullness and a midfrequency tinnitus that
was essentially constant, but at times it would become
more noticeable. She was unclear whether the tinnitus
increase coincided with the onset of the vertiginous
episodes. She also noticed hearing loss in her right
ear, but she felt that her hearing in her right ear was at
times better. Her left ear was completely symptom free.
There was no family history of hearing loss or diagnosed Ménière’s disease. She was otherwise in good
health, with no history of hypertension or diabetes.
An audiogram obtained in our clinic showed normal hearing in the left ear across the frequency range
with a word discrimination score of 100% and a speech
reception threshold (SRT) of 5 dB. In the right ear, she
presented with an up-sloping, moderate, low-frequency
sensorineural hearing loss with normal thresholds, but
in the 3000 to 4000 Hz range. The speech reception
threshold in the right ear was 30 dB and the word recognition score was 92% when obtained at 70 dB HL. On
the day she had her audiogram, she indicated that this
was a day when her hearing was subjectively relatively
good in the right ear. We carried out an ECochG evaluation using a tympanic surface electrode as described
elsewhere in this chapter. We used both clicks and tone
bursts as stimuli in an attempt to determine the presence or absence of an elevated summating potential in
the right ear to support the diagnosis of Ménière’s disease. With left ear stimulation using clicks, the SP/AP
ratio was within normal limits based on our criteria,
with a value of 0.3. With right ear stimulation, the SP/
AP ratio was elevated with a value of 0.6, as illustrated
in Figure 17–8.
Both the summating potential and the action
potential amplitudes were measured referenced to a
prestimulus baseline, which was quite flat, and as such
it was an ideal reference for the respective amplitudes
of the two potentials of interest. Measurements were
also carried out using a 1000 Hz tone burst to elicit a
cochlear microphonic and to further evaluate the presence or absence of an elevated summating potential.
With left ear stimulation, the summating potential was
difficult to identify as the cochlear microphonic cycles
were symmetrically displaced relative to baseline. In
contrast, with right ear stimulation, the entire cochlear
microphonic waveform appeared elevated above the
baseline as though displaced on a ramp. This ramp
represents the summating potential, which was much
more prominent with right ear stimulation than with
left ear stimulation. This finding coincides with the
elevated SP/AP ratio obtained with click stimulation
in the right ear. It is of note that in this case, vestibular
testing identified some left-beating positional nystagmus, and the caloric test results consisted of increased
slow-phase velocity with warm water stimulation in
the right ear. This of course may be interpreted as an
irritative lesion at this time in the right ear; however,
the presence of an increased summating potential
amplitude in the right ear, coinciding with the lowfrequency sensorineural hearing loss, made a significant contribution to a definitive diagnosis of Ménière’s
disease in this patient’s right ear.
Note the following about possible effects of hearing loss on the clinical utility of ECochG: Hearing loss
exceeding 40 to 50 dB HL at mid and high frequencies
is considered by some to be a contraindication for ECochG due to poor resolution of the SP and AP and the
possibility of an altered amplitude relationship between
those two components (Mori, Asai, & Sakagami, 1993;
Ferraro, 2010). The latter could result in inaccurate SP/
AP ratios, thus decreasing the measure’s clinical utility.
In this author’s experience, reliable SP, AP, and SP/AP
ratios can be obtained up to 60 to 70 dB HL hearing loss
across the frequency range with tympanic membrane
recording. Very likely as a consequence of recruitment,
in my practice I have experienced highly resolved SP
and AP components elicited by 95 dB nHL clicks in
patients with 70 dB HL flat or high-frequency sloping
pure tone thresholds. Therefore, I consider patients
with up to 70 dB cochlear hearing loss as appropriate
candidates for ECochG.
Electrocochleography in Superior
Semicircular Canal Dehiscence
It is important to note that changes in the magnitude of the summating potential and the elevation of
the SP/AP ratio are not specific to Ménière’s disease.
Given that other conditions can result in a static displacement of the basilar membrane, it stands to reason that an elevated SP/AP ratio can be associated
with other cochleo-vestibular conditions. Recently, it
has been established that the SP/AP ratio is elevated
in patients with confirmed superior semicircular canal
dehiscence (SSCD) as well as other third-window conditions. Given that this condition has been recognized
relatively recently and that much of the literature on
ECochG and Ménière’s disease precedes the recognition and identification of SSCD, it is quite possible
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A
AP
SP
SP/AP=0.3
L
SP/AP=0.6
B
figure 17–8. Audiogram (A) and ECochG (B) from patient with right-sided Ménière’s disease.
that some patients with an elevated SP/AP ratio and a
clinical presentation atypical for Ménière’s disease may
actually have SSCD.
Superior semicircular canal dehiscence syndrome
was described by Minor, Solomon, Zinreigh, and
Zee (1998). This condition is associated with several
auditory and vestibular signs and symptoms. In the
auditory domain, these include hypersensitive bone
conduction thresholds, meaning a subzero dB bone
conduction threshold, resulting in what appears on the
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audiogram to be an air–bone gap, overall sensitivity to
sounds including one’s own footsteps and one’s own
voice (autophony), audible eye movements, and some
degree of hyperacusis. The air–bone gap appearing on
the audiogram coexists with a typically normal, type A
tympanogram and intact acoustic reflexes. Among vestibular symptoms, patients complain of overall disequilibrium, hypersensitivity to stimulation, sound- and
pressure-evoked vertigo or imbalance, and sensation
of a bouncing horizon when walking, but no evidence
of oscillopsia.
Several years ago, we made the initial observation in a few patients with confirmed superior canal
dehiscence that they all presented with elevated SP/
AP ratio values (Arts, Adams, Telian, El-Kashlan, &
Kileny, 2008). Subsequently, we evaluated the ECochG
results of 11 patients, 7 with unilateral SSCD and 4 with
bilateral SSCD (15 ears with SSCD). In all these patients
we included ECochG in the preoperative evaluation.
Additionally, all these patients underwent an audiologic evaluation as well as vestibular-evoked myogenic
potential (VEMP) testing and high resolution temporal
bone computed tomography (CT), reformatted to optimally view the semicircular canal. In this series, 14 of
the 15 ears with confirmed SSCD on CT were found to
have an elevated SP/AP ratio (exceeding 0.40). Four of
these patients underwent surgery to repair the dehiscence, and in all, the SP/AP ratio normalized postoperatively. In one of these 4 patients, we continuously
monitored ECochG during the dehiscence repair and
were able to document an immediate intraoperative
resolution of the abnormalities. Subsequently, we
began monitoring intraoperatively all SSCD repair procedures. We concluded following this initial study that
the SP/AP ratio was highly sensitive to SSCD and that
it was advisable to include ECochG in the diagnostic
evaluation of patients suspected of SSCD.
The following cases illustrate ECochG findings in
SSCD. This is the case of a 38-year-old female patient
with a 1- to 2-year history of autophony localized to
her right ear, right-sided pulsatile tinnitus and fullness sensation, sensitivity to loud sounds, and a sense
of disequilibrium, in particular when walking along
the aisle of a store flanked by large shelves on either
side. Figure 17–9A shows her audiogram, which demonstrates normal hearing and no air–bone gap in the
left ear, and an apparent low-frequency mild conductive hearing loss in the right ear with air–bone gaps
from 250 to 1000 Hz and subzero dB bone conduction
thresholds for those same frequencies. Mid-frequency
and high-frequency hearing is in the normal limits,
with absent air–bone gap in the right ear. Tympanic
ECochG was carried out in this patient as described
elsewhere in this chapter. An elevated SP/AP ratio of
0.53 was obtained with right ear stimulation, while the
SP/AP value for left ear stimulation was within normal
limits, with a value of 0.34, as shown in Figure 17–9B.
Auditory brainstem response interpeak latencies were
also evaluated and these were normal and symmetrical bilaterally. VEMP measurements were carried out in
this patient, resulting in a 65 dB threshold for the right
ear, a 75 dB threshold for the left ear, and overall larger
VEMP amplitudes for right ear responses. The patient
was referred for CT imaging for a study of the temporal bones with reformatting to bring the superior canal
in the viewing plane. As illustrated in Figure 17–9C,
there is a clear dehiscence on the right side, while the
left superior canal was covered with bone, albeit somewhat thin.
The second case is that of a 45-year-old woman
presenting with left ear symptoms consistent with
SSCD. These included autophony localized to the left
ear, left-sided fullness sensation, and an overall sense
of disequilibrium exacerbated when exercising. This
patient also reported two or three episodes when loud
sounds elicited a sense of disequilibrium. Her audiogram was normal for her right ear and was characterized by bone conduction hypersensitivity in the left ear
for frequencies ranging from 250 to 1000 kHz. Her bone
conduction thresholds ranged from −5 to −10 dB. This
was not the case for the right ear.
ECochG resulted in an SP/AP ratio of 0.78 for the
left, symptomatic ear and a normal SP/AP ratio of 0.36
for the asymptomatic right ear, as illustrated in Figure 17–10A. Figures 17–10B and C illustrate CT imaging studies clearly demonstrating a dehiscent left-sided
superior canal and a right-sided canal that was covered
by bone. This patient underwent repair of her left-sided
superior canal dehiscence and her SP/AP ratio diminished intraoperatively from 0.78 to 0.34. This was subsequently confirmed two months postoperatively with
an SP/AP ratio of 0.3 for the left ear. The right ear continued to have a normal SP/AP ratio.
We subsequently reviewed ECochG findings for
45 affected ears. The mean SP/AP ratio for these 45
ears was 0.62 with a standard deviation of 0.21. We
also evaluated SP/AP ratios in 21 unaffected ears of the
same patients, resulting in a mean SP/AP ratio of 0.29
with a standard deviation of 0.17. A statistical analysis
indicated that the difference in SP/AP ratio between
the affected and unaffected ears was significant, with a
P-value of 0.0001.
What is the pathophysiological phenomenon
underlying the elevated SP in SSCD? Rosowski et al.
(2004) offered the following explanation: As a result of
the dehiscence, impedance may be higher on the scala
vestibuli side of the cochlea than on the scala tympani
side. This may be more prominent at certain stimulus
A
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B C
figure 17–9. A. Audiogram from patient with right-sided SSCD. B. ECochG from patient with right-sided SSCD. C.CT
scans from patient with right-sided SSCD (top, left side; bottom, right side). The absence of bone over the right
superior canal is evident.
452
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SP/APR=0.36
R
SP/APL=0.76
A C
Figure 17–10. A. ECochG from patient with left SSCD. B–C. CT scans from patient with left SSCD.
frequencies. If this is the case, it may result in a basilar
membrane bias toward scala tympani, and a resultant
increase in the SP. It is possible that the presence of a
third window contributes to the alteration of hydrodynamic forces within the cochlea, resulting in the aforementioned basilar membrane bias.
The following example serves as a proof of prin-
ciple for an elevated SP in the presence of a third win-
B
canal occlusion was complete and the bony defect was
restored, the SP returned to its normal, preoperative
value. This example does not explain the mechanism
of SP elevation any further, but it does confirm that the
presence of a third window exposing the membranous
labyrinth contributes to an elevation of the summating
potential and that this phenomenon is reversible when
closing the third window.
dow. We monitored ECochG continuously during a
canal occlusion procedure of the right posterior semicircular canal for benign paroxysmal positional vertigo. Preoperatively, in the absence of endolymphatic
hydrops or a third window, this patient presented with
a normal SP/AP ratio. Canal occlusion involved an ini-
INTRAOPERATIVE APPLICATIONS OF
ELECTROCOCHLEOGRAPHY IN SUPERIOR
SEMICIRCULAR CANAL DEHISCENCE REPAIR
tial exposure of the membranous canal by drilling an
opening of the bony posterior semicircular canal, thus
creating an artificial third window. As shown in Figure 17–11, as soon as the membranous labyrinth was
exposed, the summating potential increased. When the
Soon after recognizing that superior canal dehiscence was characterized by an SP/AP elevation, we
began systematically and consistently monitoring the
ECochG intraoperatively during SSCD repair. In our
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