Degeneration of the Hearing Nerve and Its Detection by Distorted … · 2020. 7. 3. · SPEECH...

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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 es- sential 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 ca- pacity 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 as- cending degeneration. The test is recommended as a reliable basic component of an audiomet- ric 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 noise- induced 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 audiolo- gist's office apparently do not correspond with the pa- tient'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 pathologi- cal 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 patients- or of which they generally complain-may reduce the perimeter of their speech discrimination capacity to a few centimeters between their ear and the person com- municating with them. Therefore, speech discrimina- tion tests with a competing noise are used to establish the pathological camouflaging effects in the individ- ual 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 com- monly 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 degen- eration can be established best by a supplementary dis- torted-speech test.

Transcript of Degeneration of the Hearing Nerve and Its Detection by Distorted … · 2020. 7. 3. · SPEECH...

Page 1: Degeneration of the Hearing Nerve and Its Detection by Distorted … · 2020. 7. 3. · SPEECH DISCRIMINATION In the last decades, extensive experimental and clinical studies of the

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.

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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.

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Detection of Hearing Nerve Degeneration

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.

International Tinnitus Journal, Vol. 6, No.2, 2000

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.

Auditory­LevelS --------------------------( . Cortical acoustic Cerebrum -- evoked

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Figure 1. Topodiagnostic classification of the hearing system. (EEG = electroencephalography; FAEP = fast acoustic evoked potentials .)

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Table 1. Proposals for Precision Measurements of Specific Hearing Capacities of the Hearing System

Level

I (outer ear)

3 (inner ear) 4 (auditory nuclei and pathways)

5 (primary auditory cortex and superior auditory centers)

Capacity

Stereophonic outer ear effect ("stereophone coding")

Cochlear analyzing functions Chiasma selection

Active selection

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) .

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Detection of Hearing Nerve Degeneration

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

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International Tinnitus Journal, Vol. 6, No.2, 2000

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.

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-

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Number of Patients

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Figure 4. Measurement results of reverberation ratios for hearing, conductive hearing loss , combined hearing loss , and sensorineu­ral 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.

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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-

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Detection of Hearing Nerve Degeneration

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 .

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International Tinnitus Journal, Vol. 6, No. 2,2000

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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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,

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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.

REFERENCES

1. Silberman S, Hirsh J . Problems related to the use of speech in clinical audiometry. Ann Otol Rhinol Laryngol 64:1234-1243,1969.

2. Hahlbrock KH. Sprachaudiometrie. Stuttgart: Thieme, 1957. .

3. Spoendlin H. Innervation patterns in the organ of Corti of the cat. Acta Otolaryngol67:239-254, 1969.

4. Spoendlin H . Degeneration behavior of the cochlear nerve. Arch Klin Exp Ohr Nas Kehlk Heilk 200:275-291, 1971.

5. Dieroff H-G , Beck C. Experimentell-mikroskopische Studie zur Frage der Lokalisation von bleibenden Hor­schaden nach Industrielarmbelastung mit tonalen Geraus­chanteilen. Arch Ohr Nas Kehlk Heilk 184:33, 1964.

6. Zenner HP. Modem aspects of hair cell biochemistry, mo­tility and tinnitus. In Proceedings of the Third Interna­tional Tinnitus Seminar, Munster. Karl sruhe: Harsch , 1987.

7. Kemp DT. Evidence of mechanical non-linearity and fre­quency selective wave amplification in the cochlea. Arch Otorhinolaryngol224:37-45,1979.

8. Fowler EP. Measuring the sensation of loudness. A new approach to the physiology of hearing and the functional and differential diagnostic test. Arch Ofolaryngol26:5 14, 1937.

9. Oeken J. Einschrdnkung der Funktionsfdhigkeit dusserer Haarzellen des Hororgans durch Liirmeinwirkung und deren Nachweis durch Messung von Distorsionsproduk­ten otoakustischer Emissionen. Habilitationsschrift Leipzig . Universitat Leipzig: Medizin Fakultat, 1999.

10. Pangert P. Zur Morphologie des Nervus cochlearis des Meerschweinchens nach Impulsschalltrauma. Disserta­tion, Universitat Jena, Medizin Fakultat, Jena , Germany, 1999.

11 . Dieroff H-G. Zur Definition "Selektionsfahigkeit" bei er­schwertem Sprachverstehen als Folge peripherer Perzep­tionsschaden. HNO 40:400-404, 1992.

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