Download Degeneration of the Hearing Nerve and Its Detection by Distorted

International Tinnitus Journal, Vol. 6, No.2, 112-119 (2000)
Degeneration of the Hearing Nerve and Its
Detection by Distorted-Speech Testing
Hans-Georg Dieroff
Ear, Nose, Throat Clinic-University, lena, Germany (Emeritus)
Abstract: A detailed description of the physiology of the auditory system (including new essential knowledge) is provided, followed by the description of five morphological levels of
acoustic signal analysis, of which four playa significant part in establishing the selection capacity of human hearing. As an essential phenomenon connected with loss of the analyzing
capacity of the ear, ascending degeneration of the afferent nerve fibers of the inner hair cells
and of the hearing nerve (after destruction of the inner hair cells) develops. A distorted-speech
test is submitted that offers a simple and easily performed calculation of a ratio, which value
renders a reliable estimation of the pathological analyzing capacity of the ear caused by ascending degeneration. The test is recommended as a reliable basic component of an audiometric battery of tests.
Key Words: degeneration; distorted speech; hearing nerve; testing
I
n otolaryngologists' offices, simple speech tests using
monosyllabic words and two-digit numbers, such as
the monosyllabic CID-W-22 Silberman and Hirsh
Test [1] employed in the United States or the Freiburg
Hahlbrock Speech Discrimination Test [2] employed in
Germany, have proved to be very effective in routine
examinations, as their designs are easily adapted to the
average patient. They are easily understood and can be
performed quickly . However, they have disadvantages
as regards the many patients suffering from inner ear
hearing impairments in the high-tone frequency range,
such as presbyacusia due to advanced age or noiseinduced hearing loss due to work noise, environmental
noise , or noises connected with military activities. In
these cases, the speech discrimination measurements
carried out in the quiet surroundings of the audiologist's office apparently do not correspond with the patient's hearing sensitivity in real life . The correlations
between the pure-tone threshold levels and the speech
comprehension ability , which are gathered easily from
the alphaJ value for speech hearing loss (gap between
the scores for normal two-digit numbers and pathological numbers on the 50% line) in the speech audiogram,
Reprint reguests: Prof. Dr. H.-G . Dieroff, Texdorfer Weg
1, 07548 Gera, Germany. Phone: 0365-814394.
112
mainly refer to the tone frequencies 500 , 1,000, and
2,000 Hz . However, in cases of minor and medium
basocochlear inner ear hearing loss, the hearing loss of
speech usually is damaged only slightly .
The cocktail-party effect that affects these patientsor of which they generally complain-may reduce the
perimeter of their speech discrimination capacity to a
few centimeters between their ear and the person communicating with them. Therefore, speech discrimination tests with a competing noise are used to establish
the pathological camouflaging effects in the individual patient. As discussions about the most practical
method of testing speech discrimination capacity have
so far not produced satisfactory solutions, we decided
to develop a test that permits us to assess a patient's
impaired selective hearing capacity as compared to
normal hearing, using an easily performed , quick,
and quantitatively reliable test based on tests commonly used in otolaryngologists' offices and by clinical
audiologists .
Several investigations have shown that ascending
degeneration of the hearing nerve , as described by
Spoendlin [3,4], is responsible for the cocktail-party
effect as long as the central auditory functions remain
intact. According to our experience, ascending degeneration can be established best by a supplementary distorted-speech test.
Detection of Hearing Nerve Degeneration
International Tinnitus Journal, Vol. 6, No.2, 2000
DEGENERATION OF THE HEARING
NERVE AND ITS SIGNIFICANCE IN
SPEECH DISCRIMINATION
In the last decades, extensive experimental and clinical
studies of the hearing organ in humans and animals
have disclosed with growing exactitude the multistage
functional efficiency of the auditory system regarding
various damage and possible vulnerability criteria (Fig.
1). According to current knowledge, five separate morphologicallevels of speech perception are distinguished.
They represent principal stages in the hearing and selection capacity of the auditory system (see Fig. 1; Table I) . Of these , level 2 (i.e., the tympanic membrane
and the ossicular chain, being linear amplifiers) may be
ignored as regards selection capacity.
Level 1 (the outer ear) exerts a stereophonic effect
("stereophone coding"). Level 3 represents the cochlea,
including the cochlear amplifier and afferent transmission, and cochlear analyzing functions. Level 4 is the
area of chiasmatic selection, also called the selection of
interconnection. Level 5 represents the primary auditory cortex and the cortical centers responsible for active acoustic selection.
LevelS --------------------------(
Recent investigations of noise-induced hearing loss,
supported by experimental and histological evidence
from the cochlea, the hearing nerves, and the cortical
acoustic centers, demonstrate the intricate connections
between these specific regions (e.g., the outer and inner
hair cells, the spiral ganglia, the upper sections of the
auditory pathway, and the cortical acoustic centers).
The findings call for new proposals for precise assessment of specific hearing capacity, previously called selection ability. Electron-microscopical reproductions of
the structures of the cochlea, hearing nerves, and spiral
ganglia after long-term dose-controlled noise exposure
show the resulting destruction first of the outer, then
the inner, hair cells and, eventually, of the corresponding afferent nerve fibers, including the spiral ganglia
[5]. In extensive investigations in the late sixties and
beginning of the seventies, Spoendlin [3,4] identified
the entirely different functions of the outer and the inner hair cells and of their various nerve supplies. These
findings and the conclusions drawn from them opened
new dimensions in the field of hearing physiology.
Consequently, the outer hair cells, being by far the
more sensitive hair cells, have been shown merely to
steer the tuning of the sensibility of the inner hair cells.
.
Cortical acoustic
. . --- center
_----Radlatlo acustlca·
Level 4 ---------------------------------
AuditoryCerebrum -- evoked
potentials/EEG •
Primary
slow
auditory
components
cortex
Inferior
Mescencephalon
---quadrigeminal
bodies
- - - Dorsal and ventral lemnisci
-- -- -. - Lateral ganglion
Brainstem -
fast
components
(FAEP)
--. Trapezoid bodies
ventr. auditory pathway
Level 1
-- ---- - - - -Tractus
Trapezoid body
reticulospinalis
'Spiral ganglion
Cartilagi
part
Level 2
Figure 1. Topodiagnostic classification of the hearing system. (EEG = electroencephalography; FAEP = fast acoustic evoked
potentials .)
113
International Tinnitus Journal, Vol. 6, No . 2, 2000
Dieroff
Table 1. Proposal s for Precision Measurements of Specific
Hearing Capacities of the Hearing System
Level
Capacity
I (outer ear)
Stereophonic outer ear effect
("stereophone coding")
Cochlear analyzing function s
Chiasma selection
3 (inner ear)
4 (auditory nuclei and pathways)
Active selection
5 (primary auditory cortex and
superior auditory centers)
NOle : Specific hearing capacities previously were known as seleclion ability"
This is achieved by the specific active motor capacity
of the outer hair cells [6] .
Approximately 30 outer hair cells are supplied with a
single afferent nerve fiber , whereas some 20 afferent
nerve fibers are connected to a single inner hair cell. The
inner hair cells , therefore, are considered the true sensory
cells of the auditory system . Their sensitivity is steered
by the active movement of the outer hair cells and olivocochlear efferent nerve fibers. The interacting function
of the outer hair cell layer has the effect of a "cochlear
amplifier" responsible for the sensibility of the inner hair
cells in the hearing level range of approximately 0 to
40 dB. Functional outer hair cells produce otoacoustic
emissions (OAEs) [7], which are assessed routinely in
an objective test that is both reliable and practical.
Spoendlin [4] further proved that destruction of inner hair cells after overexposure to noise may cause an
ascending degeneration of the hearing nerve - with
devastating consequences for speech discrimination.
Ascending degeneration and its direct effect on speech
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discrimination become apparent in the slowly developing cocktail-party effect. Quantitative assessment, however, is difficult , especially with regard to comparable
data from quantitative audiometric assessment.
Similarly, the assessment of recruitment was a matter of common knowledge [8] until the recruitment phenomenon emerged as corresponding with a dysfunction
of the cochlear amplifier (i .e., of the outer hair cells
only). Discovery of the OAEs has rendered it possible
to abandon widely varying test results.
Oeken [9] recently confirmed and statistically supported the finding that in cases of noise-induced hearing
loss , the distortion product OAEs continually decrease
in correspondence with high-frequency tone hearing
loss (i.e., the basocochlear frequency range) and eventually disappear altogether in relation to duration of exposure. As a consequence, the author recommended
measuring exclusively the distortion product OAEs when
assessing noise-induced hearing loss.
Although the proposals for OAE measurements when
examining functional disorders of the cochlear amplifier
are explicit, the fact that failing OAEs also may indicate
ascending degeneration has, until now, not been accepted clinically . However, Pangert [10] submitted some
interesting new findings in this direction. In animal experiments, he exposed guinea pigs to 10 single-spark
impulses with peak values of 164 dB. As a result, an accompanying ascending degeneration of the hearing nerve
was verified by measurements of a reduced action potential density in the hearing nerve that correlated with
noise exposures lasting several weeks to months in three
groups of subjects. In group 1, after 6 and 7 weeks of impulse noise exposure, the result was cross-sectional dam-
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Figure 2. Discrimination curves in age groups 1-7 for (A) nondistorted monosyllables and (B) distorted monosyllables at reverberation grade - 15 dB (Freiburg Hahlbrock Word Test) .
114
International Tinnitus Journal, Vol. 6, No.2, 2000
Detection of Hearing Nerve Degeneration
not only does it measure the speech signal-masking effect (which is of great importance for detecting such
damage) but very often it discloses, in the presence of
an ascending degeneration, a marked impairment in the
time-related analysis of an acoustic signal [11] . This
condition becomes evident in affected patients who are
unable to follow broadcast information in echoing surroundings, such as train stations. Bearing this in mind,
we developed a distorted-speech discrimination test.
age to the hearing nerve of 10.6%. In group 2, after 17
and 19 weeks of impulse noise exposure, the damage
amounted to 27.2% of the cross-section of the nerve,
which is a significant increase. Group 3, measured 9 and
12 months after noise exposure, showed an almost identical value of 26.1 % cross-sectional nerve damage . As
regards the time-related behavior of ascending degeneration, the findings give evidence of an asymptotic process
within given periods of noise exposure.
Two principles for the audiometric assessment of
ascending degeneration are being recommended currently: first, various speech tests in connection with
specific masking noises or with simple background
noises; second, distorted-speech tests with varying but
often too-small reverberation grades. For our experiments , we chose the distorted-speech method because
DETECTION OF ASCENDING
DEGENERATION OF THE HEARING
NERVE BY DISTORTED-SPEECH TESTING
As early as the 1950s, the selection capacity of the ear
was interpreted by Schubert [12] as a specific perfor-
Date of Birth •.........•••.•..••
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speech test: H = words; M = masking noise; reverberation grade, - 15 dB.)
=
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115
International Tinnitus Journal, Vol. 6, No.2, 2000
Dieroff
Number of
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c:::J =Sensorineural hearing loss
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Figure 4. Measurement results of reverberation ratios for hearing , conductive hearing loss , combined hearing loss , and sensorineu-
ral hearing loss .
mance of the inner ear, whereas such authors as Keidel
[13] and Wedel [14,15] classified it as a central process . As depicted in Figure 1, sound processing is not
performed by the inner ear alone but includes several
levels of the auditory system. Taking into consideration
the complicated mechanisms of hearing, Schubert [12]
had recommended several rather intricate speech discrimination tests, among them a distorted-speech hearing test that allowed him to classify selection capacity
as an inner ear performance.
In view of the rather complicated, time-consuming,
and not very practical procedures of Schubert' s speech
tests, we decided instead to develop further the simple
Freiburg Hahlbrock Speech Discrimination Test used
widely in otolaryngologists' offices and, to a large extent, corresponding with the American CID-W-22 Silberman and Hirsh Test, using monosyllabic words and
two-digit numbers. Referring to Schubert's tests, we at
first installed - 5, - 10, and - 15 dB as reverberation
grades and a reverberation time of 5 seconds but, when
put into practice, the findings at the reverberation grade
of -15 dB proved to be the most useful. Only in certain cases do we recommend going back from -15 dB
to - 5 dB.* From the discrimination scores for nondistorted and distorted monosyllables (reverberation
grade, -15 dB) based on the mean values of normalhearing populations in groups of age decades, curve
* The German version of this test is available as a compact
disc under the specification Disc Nr. 10, formerly distributed by Westra Electronic GmbH , Wertingen and, at present,
by their successor Heike Kammermeier, Bergstrasse 16,
89438 Holzheim-Eppisburg, Germany.
116
patterns as shown in Figure 2 were obtained. Group 1
included persons aged 10 to 19 years; group 7 included
persons aged 70 to 79 years. The discrimination curves
derived from the normal Freiburg test material are in
relatively close correspondence up to age 60, whereas
the curves for groups 6 and 7 are slightly diverging. By
comparison, the distorted discrimination scores show
decreased values on the whole but also are in close correspondence up to age 60 . However, a clear curve divergence is evident, especially in relation to the 50%
words line, for groups 6 and 7. Here, the deterioration
of distorted-speech discrimination ability in normalhearing persons beyond age 60 becomes evident.
By calculating the test results as a function of nondistorted and distorted test data, we were able to set up ,
relatively quickly even during examination, a quantitative evaluation of the patient's actual speech discrimination capacity, comparable to real-life conditions even
under aggravating circumstances, such as a noisy party .
The word discrimination capacity is derived from the
respective discrimination scores for nondistorted and
distorted monosyllables at levels of 50, 70, and 90 dB,
and the reverberation ratio then is calculated as demonstrated in Figure 3.
As is shown for the right ear in Figure 3, the nondistorted discrimination 30% value at 50 dB, the 75% value
at 50 dB, and the 85% value at 90 dB are summed to 190.
For the distorted words, the values 0 at 50 dB , 0 at 70 dB,
and 30% at 90 dB are summed. The resulting ratio of nondistorted speech (190%) to distorted speech (30%) is 6.3.
In the testing of persons with normal hearing, patients with conductive hearing loss or combined hearing impairments, and patients with sensorineural hear-
Detection of Hearing Nerve Degeneration
International Tinnitus Journal, Vol. 6, No. 2,2000
ing loss, the reverberation ratio is pathological almost
solely in persons with sensorineural hearing loss. With
normal hearing, conductive hearing loss, and combined
hearing loss, the reverberation ratios predominantly
show values between 1.0 and 1.49. With sensorineural
damage, however, the values may rise ad infinitum.
The histographic distribution (Fig. 4) shows maximum
ratio values for distorted speech at 2.0 to 2.49 .
Date of Birth: 10.05.25
As can be seen in Figure 4, the reverberation ratios
are significantly higher for sensorineural hearing impairments as compared to conductive hearing loss and normal hearing. Therefore, an inclusion of the distortedspeech test into the audiometric test battery seems essential. In cases of sensorineural hearing loss, it offers a fast
method of verifying a dysfunction of selection capacity,
to establish the degree of damage and possibly even the
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(M = masking noise .)
117
Dieroff
International Tinnitus Journal, Vol. 6, No.2, 2000
level of origin by evaluating all audiometric test results
combined. Routine follow-up examinations of sensorineural hearing loss often reveal a considerable discrepancy between the two ears (Fig. 5), which is of importance when fitting hearing aids. Also, at regular
checkups of patients with noise-induced hearing loss , the
onset and progression of ascending degeneration of the
hearing nerve may be observed by means of the test [16].
In the process of employing the test continually on a
patient, one may find in the reverberation ratio alterations that are generated in the upper brain levels (i .e.,
levels 4 and 5). In addition, with this subjective hearing
test method , the plasticity of the central auditory system may be recorded or assumed. The distorted-speech
test, when employed in the described way , even permits
a quantification to some extent. We found, as an example, in a patient with congenital unilateral deafness an
abnormally reduced reverberation ratio in the contralateral normal-hearing ear, which indicates a certain
central compensation of the unilateral impairment. In
addition, the test permits verification of a "late-onset
auditory deprivation" [17]. Space limitations prevent us
from citing further examples here.
DISCUSSION
The development of modem yet, in many cases, rather
extravagant hearing test methods allows a steadily
growing insight into the localization of damage to the
auditory system. The constantly improving topodiagnostic possibilities compel us now to consider five levels of sound perception and processing as basic components of our audiometric test battery and to draw
audiological evaluations from diverse, meticulously designed test methods .
Various other modem examination techniques, such
as magnetic field audiometry, must be preceded by subtle audiometric examinations because only homogeneous patient cohorts allow conclusions as to specific
alterations in the magnetic field and to the diagnostic
value of the method [18]. In an audiometric test battery,
the distorted-speech test (like any other measuring
method) is especially indispensable in localizing distinct damage levels in the auditory system.
The method seems to be of special advantage in
cases in which an ascending degeneration and damage
to the cortical selection centers are suspected. However, when damage to the outer hair cell layer must be
identified, measurement of the OAEs is the first choice
[9,19 ,20]. With damage to the hearing nerve and to superior brain levels, the aggravated-speech testsspeech tests with background noises and distorted
speech-are, aside from electric response audiometry,
118
brainstem evoked response audiometry, and magnetic
field audiometry, equally useful for diagnostic purposes. Moreover, the best of these tests correspond to
the patient's hearing capacity in real-life situations.
Because the distorted-speech test combined with the
obligatory speech discrimination test renders information both about pathological masked speech results
and about pathological time-related signal-analyzing
mechanisms, it is recommended for incorporation into
every audiometric test battery as a basic component.
Pathological reverberation ratios instantly tell the examiner that specified tests might be necessary in one
case or another.
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Detection of Hearing Nerve Degeneration
International Tinnitus Journal, Vol. 6, No.2, 2000
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