Electrocochleography

Question 1

What is electrocochleography?

Answer 1

Abbreviated ECochG or ECog
Measurement of an evoked response arising from the cochlea and the VIIIth nerve
Response occurs within the first 2 or 3 ms after an abrupt stimulus
Smallest of the evoked potentials

Question 2

What is ECochG mostly used for today?

Answer 2

Meniere’s
VIIIth N viability

Question 3

What does the ECochG response consist of?

Answer 3

Cochlear microphonic (CM)
Summating potential (SP)
Action Potential (AP)

Question 4

What is a cochlear microphonic?

Answer 4

It is a response that originates from the hair cells (mainly the OHCs)
It is an alternating current signal that follows the waveform of the stimulus evoking it - follows the stimulus that is evoking it
Has no latency because it begins with the stimulus - coincides with the stimulus

Question 5

What do you mark on an ECochG?

Answer 5

Base (initiation of stimulus)
SP
AP

Question 6

How does meniere’s show up on an ECochG?

Answer 6

Build up of endolymph and swelling in the inner ear
Increasing the endolymph throws off the endocochlear potential
Increases SP response and decreases ratio between SP and AP

Question 7

What is a summating potential?

Answer 7

A response that is viewed as a direct current (DC) shift in the baseline of an ECochG recording
Usually occurring in the same direction and just prior to the compound AP of the VIIIth nerve
The precise source of the SP within the cochlea is unknown, but it has been attributed to distortion products associated with the basilar membrane and hair cell displacement
Will have more difficulty locating SP with significant hair cell loss

Question 8

What is an action potential?

Answer 8

A compound or whole nerve action potential
Sometimes referred to as N1
Because of its amplitude, it is the easiest to identify
Look for this if you want to know if you have a viable VIIIth nerve

Question 9

What are the limitations for ECochG?

Answer 9

How much hearing loss do they have
How recently have they had a meniere’s attack

Question 10

Can SP and AP be distinct peaks or can they be smashed together?

Answer 10

Yes, much like the wave IV and wave V complex in an ABR

Question 11

What electrodes do you use for ECochG?

Answer 11

Tiptrode
TMtrode
Needle electrode in the TM
*used to get closer to the generator site

Question 12

What anatomic region contributes to CM, SP, and AP?

Answer 12

CM - OHC
SP - hair cells
AP - distal VIIIth nerve (afferent fibers) - synapse between the IHC and auditory fibers, spiral ganglion

Question 13

What is the blood supply for the ECochG?

Answer 13

Vertebral artery
AICA
Interna; auditory artery

Question 14

What are the applications for ECochG?

Answer 14

Operative monitoring or pre-op assessment (to assess viability)
Cochlear hydrops (menieres)
Functional hearing loss or hidden hearing loss
Detecting 3rd window (abnormal pressure gradient - changes the way the basilar and reissers membrane moves)

Question 15

What factors affect ECochG?

Answer 15

Nonpathologic subject factors
Stimulus factors
Acquisition factors
Waveform analysis
*Least affected response to external factors

Question 16

What are nonpathologic subject factors?

Answer 16

Age and Gender
Body Temperature
Attention and State of Arousal
Drugs
Muscular Artifact

Question 17

When is the earliest you can record an AP?

Answer 17

27 weeks conceptional age
In comparison to adult values, latency is prolonged and amplitude is reduced

Question 18

Does advancing age affect ECochG?

Answer 18

Not clear on these effects
No

Question 19

Can changes in body temperature affect ECochG?

Answer 19

Temperatures exceeding ± 1o C from normothermia (normal body temp) must be taken into account a possible factor in recorded results

Question 20

What is hypothermia?

Answer 20

Below normothermia
A decrease in membrane potentials is seen
CM amplitude is reversibly reduced
CM latency show little or no change
Variable findings have been reported for the SP
Basilar membrane traveling wave transit time is increased
*Doesn’t affect it too much

Question 21

Do you need to be awake for ECochG?

Answer 21

No
Most clinical evidence indicates that there is no difference in ECochG waveforms in the awake versus the natural sleep state for moderate-to-high stimulus intensity levels, or
for low-intensity stimuli close to auditory threshold
Attention to the signal stimulus has little or no effect on the ECochG

Question 22

Is ECochG affected by drugs?

Answer 22

Not influenced by sedatives, relaxants, barbiturates, or anesthesia (bc it is a very early sensory response, these drugs typically affect the cortex and not the periphery)
However, abnormal findings have been reported in conjunction with medications such as phenytoin, lidocaine, and diazepam

Question 23

Are ECochGs affected by muscle activity?

Answer 23

Minimally, especially since it occurs within a 2 to 3 ms period after the stimulus
However, a quiet patient state contributes to less background noise and facilitates detection of even a small amplitude response
Random movement-related artifact may confound ECochG interpretations, especially identification of the SP component

Question 24

What is the preferred stimulus for ECochG?

Answer 24

Click (gives us the biggest response bc its broadband)
The CM, SP, and AP are differentially affected by the stimulus frequency
Only do not use a click with someone with a precipitous loss in the HF (bc most of the energy in a click is in the HF range) - then try with tone bursts

Question 25

Will using an alternating polarity cancel out the CM?

Answer 25

Yes, because it follows the polarity of a click stimulus It is not needed and we don't want to mistake a CM for a SP or AP Also gets rid of more stimulus artifact Can also pinch tube to ensure that you're not misidentifying the CM

Question 26

Why would you not use alternating?

Answer 26

If you wanted to assess ANSD

Question 27

What is the analysis time for ECochG?

Answer 27

Epoch or time window Don't want it super short or too large Around 5-10 ms

Question 28

What is the electrode montage for ECochG?

Answer 28

Transtympanic - through the TM Extratympanic - outside of the TM

Question 29

What is an example of transtympanic electrodes?

Answer 29

Needle electrode inserted through the TM Placed on the promontory *Near field recording

Question 30

What is an example of an extratympanic electrode?

Answer 30

Canal electrode No perforation of the TM Placed on the TM *Far field recording - more common method for us to use One of the most invasive things we do as audiologists Watch EEG activity as you are placing it, it will be very noisy before making contact with the TM and will become quiet when it makes contact

Question 31

What filter settings should we use?

Answer 31

Because the CM component reflects stimulus polarity and frequency, it contains energy in the region of the stimulus Filter setting should be sufficiently wide enough to encompass these frequencies and avoid any distortion of CM phase Most of the time we won't touch these settings - it's the last resort if you are getting a bunch of rejects, but turn off artifact reject first There is clinical evidence that a distinct SP component can be recorded with high-pass filter setting in the range of 10 – 100Hz

Question 32

What are the two outcomes of an ECochG?

Answer 32

Little or no response under typical clinical measurement conditions - the most common cause of this response pattern clinically is cochlear pathology , with associated sensory hearing impairment, particularly affecting the higher frequency region (above 1000Hz) Clear SP and AP components by AP amplitude is atypically large (reduced SP/AP ratio) - active menieres or patients with 3rd windows

Question 33

Is amplitude diminished with far field measurements vs near field?

Answer 33

Yes

Question 34

Is ECochG a very site specific test?

Answer 34

Yes, the potential generators are well defined Helps localize the auditory dysfunction as pre-synaptic (outer or inner hair cells) or post-synaptic

Question 35

How long have ECochG's been around?

Answer 35

Since the 1930s Glen Wever discovered them (neurophysiologist) in a cat

Question 36

What are the clinical contributions of ECochG?

Answer 36

Assessment of hearing Identify Wave I in Neurodiagnostic ABR Confirmation and diagnosis of ANSD Diagnosis of Meniere’s Disease (Endolymphatic Hydrops) Intraopreative monitoring

Question 37

Prior to the discovery of ABRs in the 1970s, were ECochGs used in young children to determine "hearing thresholds"?

Answer 37

Yes Recording technique utilized in ECochG yields increased amplitude wave I; sometimes more robust than wave V as intensity decreased to threshold Go down in intensity until wave I is no longer visible

Question 38

Can ECochGs be useful with patients with SNHL?

Answer 38

Yes Especially HF to distinguish cochlea vs neural auditory dysfunction AP amplitude decreases with hearing loss in the 2kHz-8kHz range; no affect on SP Permits calculation of interpeak latencies for neurodiagnostic ABR to rule out retrocochlear or brainstem involvement Good for patients with neurological disorders (retrocochlear) where waves III or V are not identifiable

Question 39

Can ECochG be useful in diagnosing ANSD and monitoring cochlear function in patients with suspected ANSD?

Answer 39

Yes Estimated up to 10% of permanent hearing loss in infants associated w/ ANSD Combined w/ other electrophysiological tests, distinct patterns have emerged OAEs are typically present but disappear over time

Question 40

What are the classic symptoms of Menieres?

Answer 40

Vertigo Hearing loss (typically starts with low frequency rising pattern and fluctuation though may change as the disease progresses, becomes more flat) Tinnitus (typically lower frequency) Aural fullness

Question 41

What is the typical pattern of ECochG in those with menieres disease?

Answer 41

Large SP amplitude relative to AP amplitude Best seen when patients are symptomatic especially experiencing aural fullness and hearing loss Diagnostic specificity of this test is poor from 20%-65% in the literature Variability exists due to episodic nature of the disease, differences in protocols and especially recording electrode locations

Question 42

How do you calculate SP ad AP ratio?

Answer 42

(Base-SP)/(Base-AP) *compared to the other ear and normative data Can also measure the duration of the SP/AP complex, but this technique has not gained wide support Also can measure the AP latency recorded with rarefaction click and compare to the AP latency form condensation click (in normal ears, AP latency from rarefaction stimuli has shorter latency than condensation) Also can look for the area under the response curve

Question 43

What variables can affect the recording?

Answer 43

The choice of electrodes Near field vs far field Montage used

Question 44

Can you use tiptrodes for EEGs?

Answer 44

Yes Far field Insert surrounded in gold foil and inserted into the canal

Question 45

What is the preferred montage for ECochG?

Answer 45

Ear to ear (horizontal) If stimulating the left ear the tiptrode or TM electrode would be placed in the inverting (-) jack in the amplifier and the contralateral ear would be placed in the non-inverting (+) jack This results in positive deflection of response

Question 46

Have ECochG and ABR often been combined during long surgeries where the ear and auditory system are at risk?

Answer 46

Yes Many modern EP systems are not equipped to monitor those types of surgeries where multiple modalities are of interest to the surgeon Can be recorded in the OR on sedated infants or difficult to test children when behavior testing is not possible Anesthesia (inhalation agents – isoflurane, sevoflurane, or similar) and body temperature can affect ABR but NOT ECochG

Question 47

Can ECochG also be used to assess outcomes of menieres treatments?

Answer 47

Yes, both surgical (endolymphatic sac decompression, shunt) and non-surgical (drugs glycerol and mannitor) pre and post

Question 48

What are the recording parameters?

Answer 48

Transducer: Insert earphones (tiptrodes/TM electrode) Stimulus: Click Duration: 0.1ms Polarity: Alternative (Rarefaction 1st average, Condensation 2nd average) Rate: 8.1/sec Intensity: 95-100 dBnHL Masking: None Filters: 5-1500 Hz Epoch: 5 msec Pre-stimulus baseline: -1 msec Averages: 1000-1500 Sensitivity: 50 uV

Question 49

What is the montage?

Answer 49

Left Ear Stim A1 (-) to A2 (+) Amp 1 Right Ear Stim A2 (-) to A1 (+) Amp 2 Ground forehead

Question 50

What is the normative data?

Answer 50

SP/AP ratios (Hall): Tiptrodes > 50% = abnormal Tympanic membrane electrode > 35% = abnormal Transtympanic needle electrode > 30% = abnormal AP latency condensation - rarefaction: > 0.38 ms Vanderbilt SP/AP ratios > 0.42 = abnormalSP/AP area under curve > 1.6 = abnormal (range 0.8-1.6 normal) > 1.9 = abnormal (other reference) AP condensation – rarefaction latency difference = 0.38 1kHz SP amplitude 1.78 uV 2 kHz SP amplitude 2.25 uV

Question 51

What are some tips and tricks for recording ECochG?

Answer 51

With moderate to severe HFSNHL, record ECochG with TM electrode (tiptrode adequate for detection of AP-wave I if attempting to enhance for neuro evaluation) Use horizontal montage to enhance wave I Impedance values should be below 5 kOhms and within 3 kOhms of each other Always record pre-stimulus (10% of timebase) to determine level of background noise, will see CM immediate w/ stimulation SP is more prominent using HF tonebursts (1 or 2 kHz) High pass filtering important for recording SP Increase intensity to max level to ensure greatest opportunity of recording CM, SP, and AP (SPs cannot be recorded below 50 dB) SP can be affected by intensity Add rarefaction and condensation averages together - CM and stimulus artifact will disappear; SP and AP are easier to identify SNHL negates SP/AP ratio rules If the SP is not identifiable, increase the stimulation rate to 91.1; AP disappears but SP remains the same

Question 52

Do you need a pretty high intensity to see SPs?

Answer 52

Yes, because they come from the inner hair cells Don't go less than 70 dB

Question 53

What is the ideal set up?

Answer 53

High intensity (90 dB nHL) Low rate (11.1)

Question 54

Does the SP/AP area ratio method have a higher sensitivity than amplitude ratio?

Answer 54

Yes, 92% compared to 60%

Question 55

What time is the SP typically present?

Answer 55

0.7 ms *labeling waveforms for sp/ap ratios video

Question 56

What is AP1, AP2, and AP Peak?

Answer 56

AP1 - start of AP after the SP AP peak - the peak of the AP response AP2 - the end of the response

Question 57

What is the CPT code for ECochG?

Answer 57

92584 *only one code