Electrocochleography Flashcards
What is ECochG?
a method for recording the electrical potentials that arise from the cochlea
What are the 3 components of ECochG?
- summating potential
- action potential
- cochlear microphonic
From what structure does the SP arise?
from the cochlea, specifically the inner hair cells
From what structure does the AP arise?
from the auditory nerve
What types of electrodes are used during ECochG?
transtympanic, and extratympanic
Transtympanic electrode description
goes through the TM onto the cochlear promontory or the round window
Extratympanic electrode description
electrode stays within the ear canal
What type of stimulus is used to evoke ECochG?
clicks
Found on slide 4, what do TTracings display?
responses to rarefaction (R) and condensation (C) polarity clicks
The adding of separate R and C responses (middle tracing) enhances what?
the SP and AP
The subtracting of separate R and C responses (bottom tracing) enhances what?
the cochlear microphonic
What type of stimulus are AP and SP evoked by?
a stimulus of alternating polarity (a mix of condensation and rarefaction)
What type of of stimulus is the CM evoked by?
either a condensation or a rarefaction stimulus
How were the earliest ECochG measurements taken?
on patients undergoing ME surgery, or through a non-surgical approach involving a transtympanic electrode
What are the most popular applications of ECochG?
-diagnosis/assessment/monitoring of Ménière’s disease and endolymphatic hydrops
- assessment/monitoring of treatment for the above conditions
- enhancement of wave I of ABR
- intraoperative monitoring of cochlear and auditory nerves
- diagnosis of auditory neuropathy
CM characteristics
- no latency
- most effectively elicited by single polarity stimuli
- arises from the outer hair cells
SP characteristics
- arises from the inner hair cells
- reflects the extracellular activity of hair cells during acoustic stimulation
- a direct current distortion potential
AP characteristics
- arises from the afferent cochlear nerve fibers as they enter the habenula perforata
- represents the synchronous firing of the auditory nerve fibers
The SP/AP amplitude or area ratio is used to determine what?
if the ECochG results are normal or abnormal
The TIPTrode serves as both an … and a …?
electrode, and stimulus deliverer
What is the general procedure for placing TIPTrodes?
- perform otoscopy to ensure no cerumen or dirt and debris are present
- scrub the ear canal with Nu-prep on a Q-tip
- easily squish the gold foil insert and place it into the ear
- make sure the black tubing on the insert is flush with the ear canal entrance
- attach the alligator clip to where it touches the foil on the black tubing
What are the testing parameters for ECochG?
- a loud stimulus of at least 90 dB (can be detected as low as 60 dB)
- use an insert transducer
- use an alternating polarity stimulus, click, with 100k gain
- set high and low pass filters at 10 - 500 Hz, with the artifact filter on
- 5-10 msec time window
- at least 2,000 sweeps
- have an acquisition delay of -1, so there will be a small time before the stimulus onset
What is the general goal of ECochG?
to look for a difference between the SP and the AP
When using ECochG, what type of stimulus would we NOT use for diagnosis of ANSD?
an alternating polarity stimulus
Which is the first component of the ECochG waveform?
the cochlear microphonic
The cochlear microphonic is a…
stimulus-dependent cochlear response, which changes direction with the changing polarity
Which component of ECochG will not be visible if an alternating click stimulus is used?
the cochlear microphonic
The SP can be seen as a … on the AP or Wave I.
leading hump (although it can appear as a completely separate hump)
The last component to look for on the ECochG waveform is the what?
- the AP (which is the same as Wave I on the ABR)
- an alternating current response generated by the cochlear end of the VIIIth nerve
What are the norms for ECochG responses obtained with TIPTrodes?
- SP/AP amplitude ratio of less than 50% is within normal limits (0.5)
- SP/AP area ratio of less than 35% is within normal limits (0.35)
What is the most common electrode montage used for ECochG?
left ear = TIPTrode
right ear = TIPTrode
high forehead = electrode
low forehead = ground electrode
What is the procedure for setting up the electrode montage?
- clean electrode sites with an alcohol wipe
- exfoliate skin with Nu-Prep; wipe off excess solution
- apply conductive paste to reusable electrodes (disposable electrodes are ready to apply on skin)
- apply electrodes and secure with tape
Cochlear microphonic characteristics
- alternating current response that mirrors the stimulus’ waveform (at low-moderate levels of stimulation)
- arises from the OHCs in the Organ of Corti
- its phase and duration are stimulus dependent
- inhibited in response to alternating polarity stimuli
How can cochlear microphonic recordings be applied in clinical scenarios?
It has received recent attention in the diagnosis of ANSD; other inner ear/auditory nerve disorders hasn’t been established yet.
The SP is dependent on …., and tends to be obscured by…
the duration of the stimulus; the CM of stimulus artifact when recorded in response to stimuli presented in condensation or rarefaction polarity
Some components of the SP are thought to represent what?
the sum of various nonlinearities associated with transduction processes within the cochlea
What does the magnitude of the SP represent?
a reflection of the amount of distortion that accompanies or is produced by the transductive processes of the cochlea (this makes the SP useful for certain clinical scenarios)
An enlarged SP value is indicative of what?
Ménière’s disease and/or end-lymphatic hydrops (this is because the amount of distortion associated with the transduction process increases as an effect of the condition)
When does the AP occur?
at the onset of the stimulus (even for tone bursts)
What is the AP characterized by?
brief, typically negative peaks; independent of stimulus phase and duration
The magnitude of the AP relates to…?
the reflection of the number of nerve fibers firing
The latency of the AP represents what?
the time between the stimulus onset and the peak of N1 (this is analogous to the “absolute latency” for ABR components)
What is the most popular application of the AP in clinical situations?
the measurement of its magnitude in comparison to the SP in patients suspected of having Ménière’s disease and/or end-lymphatic hydrops
An enlarged SP/AP amplitude ratio is indicative of what?
end-lymphatic hydrops
What is the primary advantage of using a TT electrode for ECochG?
- it obtains a very close proximity to the structures that generate the response, thus it gives a favorable SNR
- essentially, we can get large readings without as much signal averaging
What is a main disadvantage of using a TT electrode for ECochG?
it’s invasive and require’s a physician’s assistance, and it can be painful even with local anesthetic
What is a main disadvantage of using ET electrodes during ECochG?
obtained responses require more signal averaging, thus the recorded components tend to be smaller in amplitude that their TT electrode counterparts
What is a main advantage of using ET electrodes during ECochG?
they are generally painless, and can be used by audiologists in non-medical settings (no need for sedation/local anesthesia)
When do ECochG components generally occur on the waveform?
within 10msec (latency) of the stimulus’ onset; thus they’re considered to be “early” or “short-latency” AEPs
Which recording electrode is designed for placement on the TM? (hint: this placement ensures the AP displays as negative on the waveform)
the primary/non-inverting (+) electrode
What is the most popular stimulus to evoke short-latency AEPs?
broadband clicks, BECAUSE they excite synchronous discharge from a large amount of neurons (this produces well-defined components in the recording)
What is the most popular duration of the click stimulus to use during ECochG?
100 microseconds
Which comparison measure do most professionals use when assessing possible pathological causes of abnormal ECochG results?
the amplitude measure (the height of the SP vs AP)
How do you compute the SP/AP amplitude ratio?
SP/AP x 100 to get a percentage (typically the software will do the math for you)
Normative data for ECochG depends on what variable during testing?
the type of electrode used (TIPTrode vs tymptrode)
What are the common accepted norms for the SP/AP amplitude ratio for both electrode types?
TIPTrode = >50%
Tymptrode = >25%
In simple terms, normal ears have a … SP/AP amplitude ratio, while “diseased” ears have a … SP/AP amplitude ratio.
larger; smaller
What is the reported incidence rate for the presence of enlarged SP and SP/AP amplitude ratios in patients with Ménière’s disease?
approximately 60-65%
What other condition has presented with enlarged SP/AP amplitude ratios?
perilymphatic fistulae
How is the use of ECochG helpful for intraoperative monitoring?
- avoiding potential trauma to the nerve (can preserve hearing)
- identifying anatomical landmarks, such as the endolymphatic sac
- help predict post-operative outcomes (especially for end-lymphatic decompression/shunt placement for Ménière’s disease/endolymphatic hydrops)
