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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4612_Библиотеки_им_академика_М_И_Перельмана
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Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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OHCs have an active mechanical component to their function as the cells have been shown
to change length and shape in response to depolarizing and hyperpolarizing currents.
As a sound wave travels (traveling wave) through cochlear fluid creating displacement of the
basilar membrane, there is an increase in viscous drag, and subsequent loss of energy, due to
the motion of the wave.
The cochlear amplifier, created by OHC motility, is a necessary to amplify the wave as it
loses energy.
OAEs are a by-product of the cochlear amplification process as there is displacement of the
basilar membrane that creates energy movement back toward the basal end of the cochlea
generating vibration on the oval and round windows and thus vibration of the ossicular
chain and tympanic membrane.
If there is dysfunction with the cochlear amplifier, OAEs cannot be generated.
n
DPOAEs are generated by a mechanical process within the cochlea with intermodulation
between two tones (f
, f2), relatively close in frequency, that will create new frequency-specific
1
components.
Measured based on frequency and dB SPL
These new frequency components are distortion products of the interaction between the
two initial tones on the basilar membrane.
The new frequency components are mathematically related to the two initial tones. The
mathematical relationship that generally provides the largest DPOAE response is 2 f
and is the most common formula used in generation of DPOAEs.
Normative values: no universal standard exists for DPOAEs but a general recommendation
is an SNR of ≥6 dB and additional considerations for emission level (i.e., the emission must
be of a certain level in dB). Normative data are typically specified by the manufacturer of
OAE testing equipment.
Figure 5–10 demonstrates clinical DPOAE measurements.
n
TEOAEs
TEOAEs are generated using a broadband click stimulus.
After the presentation of a brief click stimulus, time-synchronous averaging allows for
appropriate measurement of the response. This averaging method allows for removal of
noise within the recording but maintains the emission response.
The TEOAE response is analyzed using a fast Fourier transform (FFT) technique, which
converts a time-domain response into a frequency-domain response. This is important
because utilizing a broadband stimulus will elicit different portions of the TEOAE response
at different latencies due to the tonotopicity of the cochlea.
− f2
1
The TEOAE recording will feature two waveforms that are averaged over time. During
analysis, those two waveforms are compared to each other with the difference between
them being attributed to noise levels. Therefore, displayed TEOAE results will feature the
TEOAE-level waveform with a second (and hopefully smaller) waveform superimposed
representing the noise level of the response.
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FIGURE 5 –10. DPOAE measurement.
While DP and TEOAE responses represent cochlear function and are widely used clinically, there
are differences between the two methods that offer some clinical utility.
n
DPOAEs can utilize a wide-frequency spectrum for observation and can be seen up to 10 kHz.
n
Due to the continuous tone presentation associated with DPOAEs, they tend to be impacted
less by subtle ear conditions compared to TEOAEs. Therefore, DPOAEs can be recorded
in individuals with up to moderate hearing losses, whereas TEOAEs are typically absent in
individuals with more than a mild hearing loss (20–30 dB HL).
n
Clinically, both tests offer similar reflection of the cochlea’s frequency resolution and are good
indicators of hearing loss.
n
Common utilization of OAEs include newborn hearing screening and part of the test battery
for differential diagnosis of hearing loss (cochlear vs. retrocochlear).
n
It must be remembered the OAEs are not a test of hearing but a test of cochlear physiology.
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The intensity of the OAE response relates to cochlear function but can be influenced by
other nonauditory factors such as the probe fit in the ear canal. Therefore, the focus of
the response should be on the presence of the emission and not necessarily the strength
of the response.
Auditory Brainstem Response (ABR)
Auditory brainstem response (ABR) is an objective tool that has been utilized for decades to evaluate
auditory function. ABR tests the function from the peripheral auditory system to the lower brainstem.
When completing an ABR, a waveform is obtained (Figure 5–11).
n
Each waveform will have a number of peaks (Jewett waves), typically five, that are marked I to
V. Each of the different waves represents different generator sites along the auditory pathway
(Appendix 5–A).
n
ABRs are often utilized in the pediatric population to estimate hearing sensitivity when
behavioral thresholds cannot be obtained or are thought to be inaccurate thresholds.
n
ABRs can still be utilized to estimate hearing in adults who are unable to complete behavioral
testing or may have nonorganic/functional hearing loss.
n
When estimating hearing thresholds via ABR in adults, similar electrode placement and
procedures are used as in pediatrics (refer to Chapter 6).
Each waveform will be assessed for the latency, amplitude, morphology (shape), and latencyintensity function of the waves in order to assign a type of peripheral hearing loss (Table 5–12).
Most often, when completing an ABR on an adult, often a neurodiagnostic ABR will be completed
in order to evaluate for potential retrocochlear pathologies.
FIGURE 5–11. ABR waveform.
CHAPTER 5 Adult Assessment and Differential Diagnosis
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TABLE 5 –12 . Waveform Patterns and Correlating Hearing Loss
205
WAVE
MORPHOLOGY WAVE LATENCIES
Normal Hearing Good All within normal
limits
CHL Good Interwave within
normal limits
Waves I–V delayed
Sensory HL Poor Interwave within
normal limits
Waves I–V slightly
delayed
Neural HL Poor Interwave delayed
Waves III and V
delayed
n
Electrode placement is similar to pediatric ABR, but procedures are different since thresholds
AMPLITUDE
Normal Falling within normal
Normal Outside of normative
Waves I–III:
small to absent
Normal High-intensity responses
LATENCY-INTENSITY
FUNCTION
range
range
High-intensity responses
would be within normal
range but all others
would be outside.
would be within normal
range but all others
would be outside.
are not being obtained.
Electrodes are placed with the noninverting electrode at the forehead midline (Fz) or vertex
(Cz) with the inverting electrode at the ipsilateral earlobe or mastoid (A1/A2).
n
In a neurodiagnostic ABR, the amplitude and latencies of the waves are being assessed using a
high-intensity click stimulus (80–85 dB nHL) with different stimulus polarities and rates. The
polarity will be altered from rarefaction and condensation and click rate will be adjusted from
slow (e.g., 21.1 clicks/s) to fast (e.g., 71.1 clicks/s).
Varying the polarity allows one to assess for auditory neuropathy spectrum disorder
(ANSD), which is uncommon to have an adult onset but has been related to genetic causes
(AUNA1). If a cochlear microphonic is the only aspect visible when completing an ABR,
the resultant waveforms will “flip” when polarity is changed. Adding the rarefaction and
condensation tracings will yield an absent or highly abnormal ABR, indicative of ANSD.
OHC are depolarized to rarefacting stimuli and hyperpolarized to condensing stimuli;
this creates a slight latency difference between the polarities. The morphology should not
change drastically between rare and con, but most normative data were collected using
rarefacting clicks.
Increasing the click rate allows for the tester to “stress” the auditory system by increasing
the number of presentations within 1 second. The neural refractory period will often not
allow the neural components to respond as quickly as the presentations are arriving, which
can lead to a poorer waveform morphology as well as an increased latency in wave V.
Abnormally prolonged wave Vs or abnormally poor morphologies can be associated with
demyelinating diseases or neoplasms along the central auditory pathway. Slow presentation
rates allow for the greatest amount of neural synchrony, which is why they can be used to
measure the “best-case” scenario in the ABR.
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n
Suggested filter settings: a high-pass filter of 30 Hz to a low-pass filter of 1500 Hz; note, if
“peakier” waves are desired, the low-pass filter can be increased to 3000 Hz and if muscular
movement interference needs to be minimized, the high-pass filter can be raised to 100 Hz.
Negative consequences of changing the filter settings include altering the waveform
morphology (making it more difficult to peak pick and altering waveform latencies),
ablating the waveform altogether, allowing high-frequency noise into the recording (e.g.,
WiFi), or including frequencies that lead to artifact-contaminated runs.
n
Once these are obtained, the absolute latencies are assessed (Table 5–13).
n
Interaural absolute wave V latency and wave I to V interpeak latencies should be ≤0.4 ms.
With the change in click rate from slow to fast, it is expected to see an increase in wave V
latency of approximately ≤0.5 ms (or 0.1 ms per 10 clicks/s increase in rate).
n
Latency intensity function (LIF) can also be assessed, which is a graphical representation of
wave V latency (y-axis) over different intensities (x-axis). It is well documented that the latency
of wave V increases as stimulus intensity decreases due to reductions in neural synchrony and
neural firing. Here, remember that “low is long.” There is a normal, though nonlinear, range
for this latency shift to occur with relation to the decrease in intensity with an approximate
average of a 0.4-ms increase in latency for every 10-dB decrease in intensity.
In individuals with normal hearing, one would note that as intensity decreases, latency
increases, with all responses falling inside a normative range.
For individuals with CHL, all latency values for every intensity are outside of normal (i.e.,
pushed to the right of the normal range).
In the presence of SHNL, wave V at high intensities will fall in the normal range, but
as threshold is approached, the latencies prolong abnormally (moving to the right of the
normal range). The slope in SNHL is often steeper than for normal hearing or CHL.
In retrocochlear hearing loss, there is no predictable pattern of the LIF.
TABLE 5–13. Normal Latencies for ABR
WAVE AND
INTERPEAK WAVES
I 1.54
III 3.70
V 5.60
I–III 2.20
III–V 1.84
MEAN LATENCIES
(MS)
I–V 4.04
Interaural Absolute V ≤0.4
Interaural Interpeak
I–V
Source: Hall and Mueller (1998).
≤0.4
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Note that the LIF is not used as often any more since there are frequency-specific tests
and bone-conduction measures that are utilized. However, some hardware such as the
Interacoustics Eclipse will still plot the LIF.
Comparison between latencies and morphologies will assist in determining the site of lesion. In
retrocochlear pathologies, one might see a complete absence of waves, poor morphology, interaural
differences, abnormal prolongation in latency of wave V at a faster rate, prolongation of waves III and
V, and prolongation of interpeak waves I to V.
CASE EXAMPLE: RETROCOCHLEAR PATHOLOGY
Patient is a 53-year-old female who reported progressive right-sided hearing loss and tinnitus.
Patient stated that hearing has declined over the last 7 years. She stated that she has difficulty with
speech understanding. Patient does note mild imbalance but denied aural fullness, otalgia, and
noise exposure.
Otoscopy revealed normal ear canals and tympanic membranes bilaterally. Audiometric
findings demonstrate normal to mild hearing sensitivity in the left ear and a mild sloping to severe
SNHL in the right ear. WRS were excellent (100%) in the left ear and poor (52%) in the right ear.
The patient audiogram can be seen in Figure 5–12.
Acoustic reflexes were present in the left conditions and absent in the right conditions and
can be seen in Table 5–14.
n
Based on the findings of an asymmetrical SNHL, asymmetrical word recognition scores,
and acoustic reflex pattern, an ENT referral was made.
n
ENT requested a neurodiagnostic ABR testing to assess for further pathologies.
FIGURE 5–12. Retrocochlear case audiogram.
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n
ABR testing revealed normal absolute and interpeak latencies for a high-intensity
click in the left ear. Testing in the right ear revealed normal absolute wave I latency
with a prolonged absolute wave V latency. Additionally, wave I to V interpeak latency
was prolonged in the right ear. There were significant interaural latency differences.
Poor morphology was noted in the right ear. The patient ABR tracings can be seen in
Figure5–13.
n
The overall clinical impression is consistent with a right acoustic neuroma.
TABLE 5–14 . Retrocochlear Case MEMRs
ACTIVATOR
RIGHT
IPSILATERAL
(dB HL)
RIGHT
CONTRALATERAL
(dB HL)
LEFT
IPSILATERAL
(dB HL)
LEFT
CONTRALATERAL
(dB HL)
500 Hz Absent Absent Present Present
1000 Hz Absent Absent Present Present
2000 Hz Absent Absent Present Present
4000 Hz Absent Absent Present Present
V
+)
0
Amplitude (microvolt)
(–
Right ABR
I
Left ABR
(+)
V
III
0
Amplitude (microvolt)
(–)
III
I
FIGURE 5–13. Retrocochlear case ABR.
Electrocochleography (ECochG)
During neurodiagnostic ABR testing, if wave I is not present, it may be useful to complete electrocochleography (ECochG) testing, which is a variation of ABR testing.
n
One purpose of ECochG testing is to assist in identifying Ménière’s disease or endolymphatic
hydrops.
012345678910
Latency (msec)
012345678910
Latency (msec)
CHAPTER 5 Adult Assessment and Differential Diagnosis
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n
ECochG testing can assist in increasing the amplitude of wave I due to different click rates,
intensity levels, and transducers utilized.
n
The 100-µs click rate is typically at 7.1 clicks/s, intensity is at 90/95 dB nHL, and transducer
is a tiptrode, tymptrode, or transtympanic needle.
The active electrode is usually placed on the tympanic membrane (tymptrode) or the skin
of the outer ear canal (tiptrode). The active electrode can be placed at the niche of the
round window or the promontory of the cochlea (transtympanic needle electrode), but this
requires anesthesia and physician assistance and is a much more invasive procedure. It can
be done during intraoperative monitoring.
The close placement of the transducer aids in the ability to see wave I. Placement of the
electrodes includes the test ear (nape/mastoid), vertex (forehead), and ground (cheek below
the eye).
n
Filters settings can range from a high-pass filter of 0 to a low-pass filter of 3000 Hz.
n
An ECochG is displayed on a graph with amplitude and latency (Figure 5–14).
n
Assessment occurs by labeling the base, summating potential (SP), and action potential (AP).
Base: placed prior to or at the onset of the stimulus; serves as a reference amplitude for
other measures
209
SP: cochlear response (IHCs, OHCs, spiral ganglion); occurs approximately 0.8 ms after
stimulus onset
AP: synchronous firing of CN VIII; same as wave I of the ABR; occurs approximately
1.5ms after stimulus onset
n
The main result of interest is the amplitude ratio between SP and AP.
AP
SP
Base
AP
SP
Base
-2.0
0.02.0 4.06.0
FIGURE 5–14. ECochG.
8.0
Normal ECochG Abnormal ECochG
-2.0
0.02.0 4.06.0
8.0
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n
Research suggests that normal ratio should be approximately ≤40% (0.4) for tymptrode and
≤50% (0.5) for tiptrodes (Chung et al., 2004).
n
In Ménière’s disease, the ratio is going to be above normative ranges. The difficulty with this
testing and Ménière’s is that if the patient is not actively experiencing symptoms, results are
likely to be within normative ranges.
n
ECochGs are becoming less prevalent in clinical settings due to a lack of reliable norms and
standards and a lack of sensitivity for detecting pathologies.
n
The CM can also be elicited with an ECochG of a constant (either rarefacting or condensing
runs) polarity. The CM represents OHC function, which produces a response that mimics
stimulus polarity. This, plus the remainder of the ECochG, can be important in identifying
the site of lesion for ANSD.
KNOWLEDGE CHECKPOINT
When completing this testing, it is important to note that hearing loss from
2000 to 4000 Hz can lead to an absent ECochG. As such, audiograms to confirm
hearing sensitivity should be completed prior to any testing. ECochG can also
be used for threshold testing, but this is not common.
Middle Latency Response (MLR)
Following the ABR, the middle latency response (MLR) is an auditory evoked potential reflective of
thalamocortical function that is composed of two negative and two positive peaks occurring between
approximately 15 and 75 ms. It comprises four peaks: Na (first negative peak, 15–20 ms), Pa (first
positive peak, 25–35 ms), Nb (second negative peak, 40–50 ms), and Pb (second positive peak,
50–60ms).
n
The Na/Pa complex amplitude is the most commonly used waveform for analysis of the MLR,
specifically comparing between ears.
n
The MLR can be elicited using a click- or frequency-specific stimulus (500 Hz gives very
robust waveforms).
n
Filter settings generally can be set at 5 to 30 Hz for the high-pass and 1500 Hz for the
low-pass filters.
n
The MLR will utilize a much slower stimulus rate (< 10 per second) when compared to the ABR.
n
Electrode placement for the MLR will yield the greatest response at Cz (vertex) with the
reference electrode at the mastoid or earlobe of the stimulated ear. Multichannel recordings can
also be made using site C3, T3, or C5 in the left hemisphere or C4, T4, or C6 over the right.
Clinically, the MLR can serve multiple purposes:
n
Most commonly is for site-of-lesion testing within the central auditory nervous system and for
threshold estimation.
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n
In threshold estimation, the MLR is better utilized for estimating lower-frequency thresholds
due to less dependence upon neural synchronization compared to the ABR. Therefore, in cases
of insult to the central auditory system that may impact neural synchrony, the MLR may be
a viable option for threshold estimation and can generally be obtained within approximately
10dB of behavioral thresholds.
n
In cases with cerebral lesions, the MLR can provide information regarding the physiological
integrity of the underlying neural generators site and inform the clinician regarding possible
insult at anatomical sites. The MLR may provide information in cases of injury, stroke,
neurodegenerative diseases, seizure disorders, and tumors.
n
When analyzing the MLR response, latency and amplitude are both monitored. However,
amplitude, specifically interaural or interelectrode montage, is considered the best indicator
of functional changes in the MLR as the latency of the response has very large variations even
among individuals with normal central auditory systems.
Key concepts of the MLR that clinicians must consider:
n
Response maturation. Like other auditory evoked potentials, the MLR does not fully mature
in terms of latency, amplitude, and morphology until approximately 8 to 10 years of age,
which is much later than the ABR.
n
The Pb component (which is also referred to as the P50 or P1 response) of the MLR may not
reach adult-like maturation until approximately 15 years of age.
n
In younger populations, responses will occur at longer latencies and generally have smaller
amplitudes.
n
In cases of hearing loss, there may be delayed maturation of the MLR.
n
The MLR, unlike the ABR, is susceptible to the patient’s state of arousal and can be influenced
(responses absent/reduced) by sleep, sedation, and various forms of anesthesia. This requires
patient cooperation for testing to be completed and is of particular importance if utilizing the
MLR for threshold estimation, especially in a pediatric population.
211
Auditory Long/Late Latency Responses (LLR)
One of the most common auditory long latency responses (LLR) is the P1-N1-P2 waveform complex
and typically begins around 50 ms in mature adults.
n
The P1 waveform is suspected to be generated by the primary auditory cortex with possible
contributions from the hippocampus, planum temporale, and lateral temporal cortex. It
occurs around 50 ms and is the same wave as Pb in the MLR.
n
The N1 component is likely generated in the auditory cortex in the superior temporal lobe
(Heschl’s gyrus). The N1 response occurs at approximately 100 ms.
n
The P2 waveform is suspected to have multiple generator sites, including the primary and
secondary auditory cortices in Heschl’s gyrus. The P2 waveform occurs at approximately 150
to 200 ms.
n
Similar to the ABR, the P1-N1-P2 response has exogenous characteristics, meaning that the
response will vary based on stimulus characteristics such as tonal or speech stimuli.
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