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70 Disorders of the Auditory System
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comorbid vestibular system abnormalities
that are frequently noted in patients with
hearing loss.
audiologic
assessment
Examination of Hearing
Sensitivity and Speech
Recognition Ability
The audiometer is the primary instrument
used by audiologists in the assessment of
peripheral hearing sensitivity. It allows
the audiologist to evaluate a patient’s
sensitivity for a variety of different sound
stimuli such as pure tones and speech.
When performing a hearing evaluation,
results are displayed on a pure-tone
audiogram (Figure 3–1). The audiogram is
a graphic representation of hearing, which
is plotted as thresholds (i.e., lowest intensity level at which a stimulus is audible)
in decibels (dB hearing level; dB HL) as a
function of frequency, which is measured
in hertz (Hz). The frequencies typically
evaluated include the octave frequencies
from 250 through 8000 Hz. The reason for
this is that most of the sounds critical
for the understanding of speech fall within
this particular frequency range. In some
cases, however, additional frequencies
may be tested. Such cases would include
the assessment of patients presenting with
complaints of tinnitus, those individuals
at risk for noise-induced hearing losses,
and patients who are being monitored for
potential ototoxic effects of medications.
For those patients who report experiencing tinnitus, matching procedures may
be completed in an effort to determine
the frequency and level of the patient’s
tinnitus, which may not occur at one of
the octave frequencies routinely tested.
Testing of the interoctave frequencies of
3000 and 6000 Hz is frequently completed
when a patient presents with a history of
excessive noise exposure, and in the case
of the patient who is being monitored for
potential damaging effects of ototoxic
medication(s), the ultra-audiometric frequencies (i.e., frequencies above 8000 Hz)
are often tested.
Several different classification systems have been recommended for quantifying the degree of hearing loss (see Clark,
1981; Goodman, 1965; Jerger & Jerger,
1980), and many clinicians use a combination of these classification systems to
describe the degree of hearing impairment. An example of the classification
system that we will be using in this text
is provided in Figure 3–2. In addition to
quantifying the degree or severity of hearing loss, the audiologist will determine the
type of hearing loss. It is generally accepted
that there are three types of hearing loss:
(1) conductive hearing loss, (2) sensorineural hearing loss, and (3) mixed hearing loss. Conductive hearing losses are
hearing losses that occur because of a loss
of sound conduction from the outer ear to
the inner ear, whereas hearing losses that
are sensorineural in nature are the result
of cochlear and/or retrocochlear involvement. Mixed hearing losses are exactly
what the name would suggest, that is, a
combination of a conductive and sensorineural hearing loss.
The audiologist’s role is to accurately
measure a patient’s behavioral hearing
sensitivity, and in doing so, to determine
if a hearing loss is present. Once a hearing loss has been diagnosed and the type,
degree, and configuration of the hearing loss is determined, the audiologist is
able to implement appropriate referrals
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Figure 3–1. An example of the audiogram used for the case studies presented in this book.
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Figure 3–2. A schematic representation of the
classification system used in this text for defining degree of hearing loss.
and (re)habilitation efforts. However, one
must keep in mind that the pure-tone
audiogram is only one method for the
evaluation of hearing, which provides
information primarily about peripheral
auditory system involvement. Although
the pure-tone audiogram is the cornerstone of audiologic assessment, its greatest limitation is that it provides limited
information with respect to the processing
and subsequent comprehension of complex auditory information (i.e., it is simply
an auditory detection measure). Patients
can have significant involvement of the
central auditory nervous system (CANS)
and still present with normal audiometric
thresholds (Bocca, Calearo, Cassinari, &
Migliavacca, 1955; Karlin, 1942; Musiek,
Shinn, Chermak, & Bamiou, 2017).
Speech audiometry is another critical component of traditional audiologic
assessment. It provides additional information with respect to the functional
performance of the auditory system. Typically, two speech measures are made during this assessment: (1) speech recognition
threshold (SRT) or alternatively a speech
awareness threshold (SAT), and (2)
word
recognition performance at suprathreshold levels. The SRT is determined by
presenting a closed set of two-syllable
(spondee) words and determining one’s
threshold for speech. In this test procedure, the patient either repeats the stimuli
presented or points to pictorial representations of the presented stimuli. For young
children and other difficult-to-test populations who are not able to complete the
SRT test procedure, SATs are utilized. In
this measure, the individual being tested
does not have to recognize the stimulus
being presented, but, rather, only has to
detect that a signal is being presented (in
this case, the signal is a speech signal as
opposed to a pure-tone signal). The SRT
should be in good agreement (±8 dB) with
the pure-tone average (PTA) and can be
compared to either a three-frequency PTA
(i.e., the average of hearing thresholds
obtained at 500, 1000, and 2000 Hz) for
patients with relatively flat hearing losses,
or to the better two-frequency average for
patients who have steeply sloping hearing losses. However, if an SAT measure is
derived rather than an SRT, the SAT often
is obtained at a lower intensity level than
the SRT would be for reasons explained
previously. As a result, there is likely to be
a greater difference between the SAT and
the pure-tone average when this speech
threshold measure is derived.
In addition to speech threshold measures, speech or word recognition testing
is completed by evaluating the percentage
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of words that patients are able to identify
correctly at suprathreshold levels. This
suprathreshold level is typically between
30 and 50 dB above either the SRT/SAT
or the PTA, but a lower presentation level
may be required for patients who present
with “sensory” hearing losses and significant amounts of recruitment (the abnormal
growth in loudness perception as intensity increases). For a patients with normal
hearing or conductive hearing loss, speech
recognition scores are typically excellent
(90% or greater). For those with sensorineural hearing loss, these scores typically
fall below (90%) and will often decrease as
hearing loss increases. However, although
there is a general trend for speech recognition scores to decrease as the degree of
hearing loss increases, it is important to
recognize that there is some variability
in the performance of individuals with
hearing loss on speech recognition tests.
As a result, there is no way to predict the
speech recognition scores of an individual
based upon his or her hearing sensitivity
as one can observe a patient with a more
severe sensorineural hearing loss having
more favorable speech recognition scores
than a patient with a much less severe
hearing loss. Speech audiometry provides
important information about the patient’s
speech recognition abilities, which can
play a significant role in helping to establish the best rehabilitative approach for a
patient’s care. For example, adult cochlear
implant evaluation candidacy is not based
primarily on pure-tone thresholds, but
rather on poor speech recognition abilities
(Arnoldner & Lin, 2013).
Immittance Audiometry
Immittance audiometry is an important
tool in diagnostic audiology and otol-
ogy. It provides information that can aid
in the detection of a variety of outer and
middle ear auditory disorders as well as
in the documentation of normal/abnormal cochlear and lower brainstem function. For the purposes of this book, we
briefly orient the reader to the two primary measures of immittance audiometry: tympanometry and acoustic reflex
thresholds.
Tympanometry
Tympanometry allows clinicians to measure the amount of compliance of the
tympanic membrane and function of the
middle ear system. Maximum compliance is achieved when the air pressure in
the external ear is equal to the air pressure within the middle ear system. This
measurement is accomplished by sealing
the ear canal with a soft plastic ear tip
attached to a probe assembly and varying the air pressure in the ear canal to
measure the movement of the tympanic
membrane. The results are displayed as a
tracing called a tympanogram that plots
compliance as a function of middle ear
pressure. In addition, information about
the peak pressure, maximum compliance, and equivalent ear canal volume
(referred to simply as “volume” throughout this chapter and in subsequent
chapters) are provided. A normal tympanogram will yield peak pressure, compliance, and volume measurements that
fall within the normal ranges shown in
Table 3–1.
There have been several classification
systems with respect to tympanometry,
but perhaps the most widely used is the
Jerger classification system (Jerger, 1970)
in which a number of tympanograms have
been categorized based on their specific
shapes and characteristics (Figure 3–3).
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Table 3–1. Normative Values for Tympanometry as a Function of Age
Pressure
(daPa)
Adults
Children
*Equivalent ear canal volume.
An equivalent ear canal volume measurement of >2.0 cc/mL with a Type B
Note.
tracing in adults suggests a perforated TM or patent PE tube.
Based in part on normative data provided by Margolis & Hunter, 2000; see
Source:
also Clark, Roeser, & Mendrygal, 2007.
-150 to +50
-150 to +50
Compliance
(cc/mL)
Volume*
(cc/mL)
0.3 to 1.7 0.9 to 2.0
0.25 to 1.05 0.3 to 0.9
Figure 3–3. Typical tympanometric tracings associated with normal and
abnormal middle ear function.
Using this classical description scheme,
clinicians have adopted the following universal tympanogram descriptors:
Type A: Type A curves demonstrate
normal pressure, volume, and compliance values.
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Type As: The “s” in As stands for
“shallow.” These tracings typically
demonstrate a normal middle ear
pressure measurement and a compliance peak that is significantly reduced,
but there continues to be some
mobility or compliance in the system.
Type A
: The “d” in Ad stands for
d
“discontinuous” or “deep” and
represents the exact opposite of the
A
tracing. The Ad tracing typically
s
demonstrates normal pressure;
however, there is a hypercompliant
peak. This is typically associated with
too much flaccidity of the tympanic
membrane and can be secondary
to disorders such as ossicular
disarticulation.
Type B:
Type B tympanograms can
occur under several conditions. The
tracings are sometimes referred to
as “flat” because there is no observable compliance peak due to either
a lack of movement of the tympanic
membrane or a measurement error.
An important measure to evaluate
in these cases is the volume. If the
tracing reflects a normal volume,
there likely is middle ear involvement
causing the lack of mobility. This is
typically the result of effusion but may
also be caused by other disorders such
as cholesteatoma. However, if there is
an abnormally small volume present,
it may be that the probe tip has been
occluded or blocked by cerumen,
debris, or the ear canal wall. A tracing
that reflects a large volume typically
suggests a patent pressure equalization (PE) tube or a perforation of the
tympanic membrane.
Type C:
Type C tympanograms
suggest some degree of Eustachian
tube dysfunction. The tracing will
demonstrate normal volume and
compliance with peak compliance
being noted at a negative middle ear
pressure value.
In addition to peak pressure, compliance, and volume measurements, many
clinicians also assess the tympanogram
width or gradient as this can be another
indicator of middle ear pathology. The
tympanometric width is derived by measuring the width of the tympanogram (in
daPa units) at a point that is half the peak
admittance on the positive side of the
tracing. Normative values for children
range between 80 and 159 daPa (Margolis,
Hunter, & Giebink, 1994) and for adults
between 51 and 114 daPa (Margolis &
Heller, 1987). Abnormally large tympanometric widths are indicative of middle
ear dysfunction and can be observed in
middle ear conditions such as otitis media
and cholesteatoma; whereas, abnormally
narrow tympanometric widths have been
noted in cases of ossicular discontinuity
and in some cases of ossicular fixation
(Hunter & Shahnaz, 2014). Other methods
have been introduced for measuring the
tympanometric gradient that express the
gradient as a ratio measure (see Nozza,
2003); however, the tympanometric width
measurement described previously is
more often employed as it is the tympanogram measure provided on many commercial immittance audiometers.
Acoustic Reflexes
Acoustic reflex testing is a direct measure of the acoustic stapedial reflex. It is
defined as the lowest intensity level at
which a reflex can be measured. Because
the stapedial muscle contracts in response
to loud sounds, a direct measure of the
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integrity of the stapedial reflex can be
accomplished through this technique. As
there are both ipsilateral and contralateral inputs within the brainstem, the stapedial reflex can be recorded with either
ipsilateral or contralateral stimulation.
Depending on the pattern of responses
observed, varying inferences regarding
the site of lesion can be made. In most normal hearing individuals, this reflex can be
observed around 70 to 90 dB HL. Reflexes
are elevated or absent when insufficient
intensity is delivered to the cochlea secondary to compromise of the outer and/or
middle ears. In individuals with cochlear
hearing losses, the reflexes may be present at normal or elevated threshold levels,
but they typically are reduced in terms of
their sensation levels (SL). For patients
with profound sensory (cochlear hearing
losses), the reflexes are likely to be absent.
Elevated or absent acoustic reflexes
can be indicative of retrocochlear involvement (see Wilson & Margolis, 1999). The
auditory nerve, the auditory nuclei in
the low pons (the cochlear nucleus and
the superior olivary complex), and the
facial nerve (both its nucleus and the
nerve itself) must all be intact to provide
a normal acoustic reflex. In the ipsilateral
reflex, the neural response to the stimulus courses from the auditory nerve to
the cochlear nucleus and superior olivary complex and then to the facial nerve
nuclei and back down the facial nerve to
the stapedius muscle ipsilaterally. A lesion
anywhere along this route can affect the
ipsilateral reflex. The contralateral reflex
pathway is similar to the ipsilateral, but
its course crosses midline in the low pons,
ultimately connecting to the contralateral
facial nerve nuclei and then the contralateral stapedius muscle. Therefore, the contralateral reflex can be affected by midline
and contralateral brainstem lesions as
well as by dysfunction of the contralateral
facial nerve (see Chapter 2, “Structure and
Function of the Auditory and Vestibular
Systems”).
Measurement of acoustic reflex decay
can also be utilized clinically. This typically requires a presentation of a 500-Hz
and/or a 1000-Hz stimulus at 10 dB SL in
reference to the pure-tone threshold over
a 10-sec time period. If the amplitude of
the response decreases more than half of
the maximum over the 10-sec time period,
it may indicate retrocochlear involvement
affecting the eighth nerve or low brainstem (Wilson & Margolis, 1999). However, the sensitivity of the acoustic reflex
decay test as an indicator of retrocochlear
pathology is considered to be somewhat
poorer than that of the auditory brainstem
response (Feeney & Schairer, 2015). As a
result, the latter procedure is more commonly used by audiologists to screen for
retrocochlear lesions affecting the auditory nerve and/or the auditory nuclei
located in the low brainstem.
Electroacoustic Measures
Electroacoustic procedures are tests that
evaluate the acoustical responses of the
auditory system. Otoacoustic emissions
(OAEs) are a relatively recent discovery.
They were initially described by David
Kemp in the 1970s (Kemp, 1978), but did
not become clinically integrated until the
mid- to late 1990s. Traditionally, when one
considers how hearing is evaluated, it is
through our oldest measure of hearing,
behavioral pure-tone audiometry. However, OAE assessments play a critical role
with respect to differential diagnosis specifically in regard to the subspecialty of
neuroaudiology. Both the electroacoustic
as well as the electrophysiologic audi-
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tory assessments that are discussed later
in the chapter can provide measurements
of hearing sensitivity. In addition, OAEs
have become a tool often used in the
screening of newborns for hearing loss.
Electroacoustic evaluation as it relates
to OAEs has significantly changed the
face of diagnostic audiology. Otoacoustic
emissions provide clinicians with objective information specifically regarding
the integrity of the outer hair cells of the
cochlea. They are unique in their ability to provide information related to the
active biological process at this level of
the auditory system, which is not readily
available by any other means of assessment. This active process is a result of an
acoustic “echo” caused by stimulation of
the hair cells. That is to say, the recorded
response is shaped by, and similar to, the
eliciting acoustic signal. In order for this
process to occur and emissions to be present, the outer and middle ear systems
must be functioning normally for the
stimulating signal to travel through to the
cochlea and the emission to travel back
through the middle ear to be recorded
by a probe microphone placed in the ear
canal. Although OAEs do not assess central auditory function, they can play an
important role in differentiating between
a cochlear and an eighth nerve or a central
site of lesion. A sensorineural hearing loss
of cochlear origin will typically result in
abnormal OAEs, whereas OAEs will be
normal if the eighth nerve and/or central
auditory system is involved without any
comorbid conductive and/or cochlear
involvement.
In general, OAEs are categorized as
either spontaneous or evoked. For the
purposes of this book, we focus on the
evoked otoacoustic emissions, which
include both distortion product otoacoustic emissions (DPOAEs) and transient
evoked otoacoustic emissions (TEOAEs)
as they are the most widely utilized in
clinical assessment. Both DPOAEs and
TEOAEs are elicited by placing a probe
in the ear of the patient. The probe tip is
placed securely within the external auditory meatus and the stimulus is delivered
either as a pair of tones (DPOAEs) or as a
click stimulus (TEOAEs).
Distortion product otoacoustic emissions are elicited by a nonlinear process
within the cochlea. This process occurs
when two tones that are close in frequency are presented to the cochlea, and
the mechanics of the inner ear create a
distortion of the signals. In a healthy ear
with normal hearing or near-normal hearing sensitivity, this response (i.e., the distortion product) is measured. The tones
used to elicit this distortion are labeled
as f
and f2. The most robust response
1
(the distortion product) is observed at or
around a particular frequency that can
be determined by applying the formula,
f
= 2f1 − f2. For example, if f1 was pre-
dp
sented at 1000 Hz and f
was presented
2
at 1200 Hz, the frequency elicited would
be 800 Hz. Although the most robust
response occurs at the frequency that
results from the application of this formula, other less robust (i.e., less intense)
responses are also generated.
An important concept to understand
is that measurement of OAEs requires
forward and backward sound transmission. That is to say that a signal (eliciting
stimulus) must reach the cochlea, and
the response if generated by the cochlea
must travel back to the outer ear. In order
for responses to be successfully measured, the outer and middle ear must
be free of debris and/or pathology that
could affect this signal transmission. For
example, if there is middle ear effusion
present, the forward transmitting signal
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will be greatly reduced due to the fluid
dynamic impeding the transmission of
the eliciting stimulus. Even in cases of a
healthy inner ear, this will likely result in
a reduced or absent response at the level
of the cochlea due to the reduction in the
stimulus intensity before it even reaches
the outer hair cells. If there is involvement or pathology of the outer or middle
ear systems, often no valid interpretation
can be made regarding cochlear integrity
as the absence or low amplitude of the
response can be due to the presence of the
outer or middle ear pathology rather than
true cochlear involvement.
Interpretation of DPOAEs is performed by measuring the level of the
response (the emission) in reference to
the noise floor or as an absolute value (i.e.,
the sound pressure level of the DPOAE).
The noise floor is the measurable noise
recorded in the ear canal. The DPOAE
response is recorded as a function of its
amplitude above the noise floor for the
particular frequency(s) of interest. If a
response meets clinical criterion for the
level above the noise floor or the absolute
response level (dB SPL), the response is
considered to be present (Figure 3–4).
Transient otoacoustic emissions are
another means by which cochlear function can be measured. TEOAEs are elicited
through the use of a transient signal such as
a click stimulus. The emission onset occurs
approximately 4 msec following stimulation. The TEOAE is a broad-spectrum
response; however, it does provide frequency information as the response follows the tonotopic arrangement of the
basilar membrane. Similar to DPOAEs,
the TEOAE is determined to be present or absent based on the relationship
Figure 3–4. An example of a normal DPOAE response (circles
represent the dB SPL values for the right ear, while squares represent
the noise floor in dB SPL).
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between the response and the noise floor.
If the response is repeatable and exceeds
the noise floor, then it is considered to be
present. Like DPOAEs, TEOAEs are indicative of cochlear outer hair cell function;
however, as was the case for DPOAEs, it
may be impossible to document normal
cochlear function if abnormal outer and/
or middle ear pathology is present. Fig-
3–5 demonstrates an example of a
ure
present TEOAE response.
Although not a direct measure of
hearing sensitivity, OAEs provide general information regarding outer hair
cell function within the cochlea. There
are many applications for OAEs. These
include, but are not limited to, infant
screening, pediatric assessment, ototoxicity monitoring, and differential diagnosis
of cochlear versus retrocochlear involvement. Patients who present with normal
hearing sensitivity typically will demon-
strate normal OAEs. The general clinical
observation with respect to OAE interpretation is that in individuals with present OAEs, hearing is likely to be better
than 30 dB HL (specifically for TEOAEs,
however, DPOAEs may be noted in some
cases with slightly more severe hearing
loss). Therefore, those with absent OAEs
(and normal outer and middle ear function) will likely demonstrate a sensorineural hearing loss with hearing thresholds
greater than 30 to 40 dB HL. Finally, it is
also interesting to note that in some cases,
individuals who have a history of noise
exposure may demonstrate abnormalities
on OAE tests that are not evident on the
audiogram (Attias, Horovitz, El-Hatib,
& Nageris, 2001). In such cases, the early
identification of noise damage to the auditory system may facilitate early intervention measures to prevent more significant
hearing loss effects.
Figure 3–5. An example of a normal TEOAE response (see arrow
in figure) and the black area shows the noise floor.
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