|Year : 2010 | Volume
| Issue : 2 | Page : 55-61
Study of dead regions in the cochlea: New insight for management of auditory neuropathy/dys-synchrony
Nadia Kamal1, Amani Shalaby1, Khairy Abul Nasr2, Wafaa El Kholi1, Soha Mekky3, Tayseer Taha2, Dalia Helal3
1 Audiology Unit, Otolaryngology Department, Ain Shams University, Abbassia, Cairo, Egypt
2 Audiology Unit, Otolaryngology Department, Saudi German Hospital, Jeddah, Saudi Arabia
3 Audiology Unit, Olaryngology Department Zagazig University , Zagazig, Egypt
|Date of Web Publication||2-Jan-2020|
MD Khairy Abul Nasr
Head of ENT Department, Saudi German Hospital, 4 Batterjee Street, P.O.Box 2550, Jeddah 21461
Source of Support: None, Conflict of Interest: None
Objectives:This research was done to investigate dead regions of the cochlea in auditory neuropathy / dys-synchrony (AN/AD) patients and how much their presence affects the benefit from hearing aids.
Methods:Twenty adolescent and adult patients with auditory neuropathy / dys-synchrony (AN/AD) together with a control group of forty age-matched patients with sensory neural hearing loss (SNHL) were examined. Both groups were evaluated for the presence of dead regions of the cochlea by TEN-HL test (Moore et al., 2004) . TEN-HL test demonstrated positive criteria in 95% of ears in AN/AD patients which was mainly at low frequency bands, while 15% had positive TEN-HL test across all the tested frequency range. On the contrary, SNHL patients showed positive TEN-HL test in 16.5% of examined ears mainly in the mid frequency bands. The number of dead regions was negatively correlated to speech recognition.
Results:Open hearing aid fitting was tried in AN/AD patients aiming to minimize the masking produced by low frequency dead regions and emphasize the high frequency components of speech. Binaural open hearing aid fitting showed improvement in high frequency detection thresholds that was significantly better than monaural fitting. Benefit from amplification was negatively correlated to the number of dead regions in TEN-HL test. Training of AN/AD patients on the use of these high frequency cues was suggested.
Conclusion:Diagnosis of dead regioncan be taken as a guide in hearing aid fittingas well as in cochlear implant programming.
Keywords: Auditory aeuropathy / dys-synchrony, cochlear dead regions, TEN-HL test, Open hearing aid fitting.
|How to cite this article:|
Kamal N, Shalaby A, Nasr KA, El Kholi W, Mekky S, Taha T, Helal D. Study of dead regions in the cochlea: New insight for management of auditory neuropathy/dys-synchrony. Saudi J Otorhinolaryngol Head Neck Surg 2010;12:55-61
|How to cite this URL:|
Kamal N, Shalaby A, Nasr KA, El Kholi W, Mekky S, Taha T, Helal D. Study of dead regions in the cochlea: New insight for management of auditory neuropathy/dys-synchrony. Saudi J Otorhinolaryngol Head Neck Surg [serial online] 2010 [cited 2022 Nov 30];12:55-61. Available from: https://www.sjohns.org/text.asp?2010/12/2/55/274633
| Introduction|| |
AN/AD is a relatively new hearing disorder. The clinical diagnosis of AN/AD has been typically characterized by the presence of otoacoustic emission and/or cochlear microphonics and the concurrent absence of the averaged auditory brainstem responses. This finding reflects the fact that outer hair cells (OHCs)' function is preserved but function of inner hair cells (IHCs) and/or auditory nerve is impaired ,,. Auditory Dys- synchrony is considered a better nomenclature for this condition .
The underlying pathogenesis is not very clear; may be due to multiple underlying causes. One suggested possibility is desynchronized neural discharge that can occur due to demyelination and ion-channel dysfunction in the auditory nerve and/or dysfunctional synaptic transmission between the inner hair cells and the audito- ry nerve. Reduced discharge in the auditory nerve with loss of the neural input to the brain due to inner hair cell loss and/or auditory nerve loss is another possibility ,.
Is it possible that there are non-functioning IHCs/neurons or dead regions (DR) in AN/AD? Dead regions in the cochlea are reported to be present when IHCs are non-functioning at certain places along the basilar membrane in addition to non-functioning auditory neurons making contact with those places,. Recently, Moore et al. , developed the TEN test which is relatively fast and simple, and can be easily applied in clinical practice. This test involves measuring the threshold for detecting a pure tone presented in a background noise called “Threshold Equalization Noise”. Since then, many clinical studies have been conducted on hearing-impaired subjects, and different patterns of dead regions have been described ,,,.
Examination of the functional characteristics of the peripheral auditory system may help optimize management of AN/AD patients. To date, management of AN/AD is still challenging to professionals and treatment options are limited. Those patients typically do not derive sufficient benefit from conventional hearing aids,,, though recent success has been reported with cochlear implantation,. The purpose of this study was to shed light into the functional status of the peripheral system in AN/AD patients that can be translated to improve management options.
| Subjects and Methods Subjects|| |
Study Group: A total number of 20 AN/AD adolescent and adult patients participated in this study. Their age ranged from 15-50 years; 13 males and 7 females. They all showed the characteristic features of AN/AD disorder in the form of absent ABR with preserved transient evoked otoacoustic emissions . None of the subjects had any systemic neural diseases or auditory disorder other than AN/AD. The diagnosis of AN/AD was based on the following tests: -Auditory Brainstem Response (ABR) was measured using Amplaid evoked potential instrument model MK12. The ABRs were measured at a stimulus level of 90dBnHL using alternating polarity clicks with repetition rates of 21/s. Subjects were tested several times to ensure repeatability. The presence of a detectable peak in the ABRs and repeatability of the waveforms was considered as an indication of the presence of ABRs. Failure to achieve a detectable peak or lack of repeatability was considered as absent ABRs. -Transient evoked otoacoustic emissions (TEOAEs) were measured using an ILO96 Otodynamics analyzer. The emissions were recorded using 240 presentations of click stimuli at 70dBSPL. TEOAEs were considered to be present when the emissions were >3dB above the noise floor.
Control Group: Forty sensorineural hearing loss patients were included with an age range of 18-55 years. They were 29 males and 21 females. Patients have variable etiologies and variable duration of hearing loss. The hearing loss was moderate to severe in degree, which was an important prerequisite for TEN-HL application. The AN/AD group and the sensorineural group were tested at Audiology Unit, Ain Shams University. An informal consent was obtained from all participants in this study.
| Method|| |
Ear, nose and throat examination was conducted to all patients prior to hearing measurements and showed normal findings. Audiometric thresholds were measured for frequencies 250 through 8000 Hz in sound treated booth IAC model 1602 and using two channel audiometer GSI model 61 equipped with TDH39 headphones. Speech discrimination was measured using Arabic 25 CVC monosyllables words (Soliman, 1976 ) at the most comfortable level of the patients (35-40dB) above the speech reception threshold, which was measured using Arabic spondee words . Speech stimuli were presented live voice using microphone and headphones of the audiometer and subjects were instructed to repeat each word they heard. All subjects in the study group had normal middle ear pressure as assessed by immittancemeter Interacoustics model AZ7. Both ipsilateral acoustic reflexes done at 1 & 2 KHz and contralateral acoustic reflexes at 0.5, 1, 2 & 4 KHz were absent in both ears for all AN/AD subjects. Threshold Equalizing Noise-Hearing Level (TEN-HL) test was conducted to all participants in order to measure the dead regions in the cochlea . The test was administered using a compact disc (CD) which was replayed from a Sharp GF500Z CD player connected to a two channel audiometer GSI model 61 where the signal and noise were separately controlled. The right channel, which contains the noise, was first turned off, and the absolute thresholds were measured for each ear at each test frequency (500-4000Hz) using the tone input from the left channel. The noise level was adjusted at 50dB/ERB for near normal hearing and at 70dB/ERB for mild to moderate hearing loss subjects. The noise level was not uncomfortable to any subject. The two channels were then mixed; the masked threshold was measured for each ear at each frequency. The signal level was varied in 2dB steps as recommended by Moore et al. (2004). A dead region at a specific frequency was indicated when a masked threshold is at least 10dB above the absolute threshold and at 10dB above the nominal noise level per equivalent rectangular bandwidth (ERB). Analysis of dead regions was done as regards their number, center frequency, as well as the amount of threshold shift in each ear. AN/AD patients were fitted with two fully digital open fitting hearing aids using receiver in the canal technology model (Bruckhoff Freestyle). The hearing aid’s frequency range was 1500-6600 Hz, with maximum acoustic gain of 37dBSPL and maximum power output of 103dBSPL. This hearing aid was specifically chosen to amplify high frequency sounds; thus making use of the relatively good high frequency discrimination documented in AN/AD patients . The hearing aids were placed in the canals with fixation by pliable plastic strip that was placed in the bowel of the concha. Manipulations were done to change the depth of the sound tube to minimize the feedback together with use of suitable ear tip. Programming was done through NOAH system (software SONIC innovations) and hearing instrument programmer (HI-PRO) with special cable and adaptor. The hearing aids were programmed using manual adjustment to provide no or minimal low frequency amplification while delivering maximum high frequency amplification till feedback occurs or reaching patient’s intolerance. Additional fine tuning of the programming parameters was conducted whenever indicated in order to reach patient’s satisfaction. All AN/AD patients were instructed to wear the hearing aids for one hour before hearing aid evaluation. Short conversation was done with the patients to assess the sound quality. Post-hearing aid fitting evaluation was performed to all patients in the sound treated booth at a distance of one meter from loudspeaker. The patients were oriented at zero azimuth relative to the loudspeakers. Warble tone signals were delivered from front speaker at octave frequencies (0.25, 0.5, 1, 2, 3, 4 KHz), and the unaided sound field thresholds were determined. Then aided sound field thresholds with right mono-aural followed by left mono-aural and binaural hearing aids were also measured.
Speech recognition was measured in sound-field using the same setup. Testing was conducted in the four conditions (unaided, aided mono-aural right, aided mono-aural left and binaural). Testing was done first in quiet, using phonetically balanced monosyllabic words at 55dBHL, followed by speech perception in noise which was measured using the Arabic Speech In Noise test for adults . The recorded speech material (25 monosyllabic words) was presented at intensity level of 55dBHL, while the noise (cafeteria noise) was presented at +5 signal to noise ratio and was delivered via the back loudspeaker.
Statistical analysis of the data was done using SPSS statistical software package V5. Data were expressed as mean and standard deviation for quantitative measures. Student’s “t” test was used to compare between means. Mann Whitney Willcoxon U test was used to compare two independent quantitative variables in non-parametric data. Spearman “r” test was used to study correlations between different variables. The probability (p) value was obtained from all tests with (n1+n2-2) degrees of freedom.
| Results|| |
I- Sensorineural Hearing Loss (SNHL) Group
This group consisted of 40 patients suffering from moderate to severe sensorineural hearing loss, of variable etiologies (9 heredofamilial, 7 post-febrile, 5 noise-induced, 1 post-traumatic, I otoxicity, 1 Diabetes Mellitus and 16 unknown). Hearing loss was bilateral and almost symmetrical in all patients. The duration of hearing loss ranged from 1 to 40 years. Pure tone thresholds and speech recognition scores for both ears were grouped together as there was no statistically significant difference between them (p>0.05) (demonstrated in [Figure 1] and [Table 1].
|Table 1: Mean and Standard Deviation (SD) of word recognition scores in SNHL and AN/AD groups|
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II- Auditory Neuropathy/Dys-synchrony (AN/AD) Group
Twenty patients clinically diagnosed with AN/AD disorder were examined in this study. Patients reported hearing loss for a minimum of 4 years and a maximum of 20 years duration. Age at diagnosis ranged from 11 to 23.4 years (mean = 17.8 years). No history of apparent delay in language acquisition as reported by the patients’ relatives. Nine patients, however, had a history of short periods of high fever early in life and 3 other patients had positive family history for hearing loss. Eight patients had tried FM systems while 5 patients had history of use of medical therapy (Tegretol) for short periods with unsatisfactory results. None of the patients was a regular hearing aid user. Neurological examination revealed no clinical evidence of extra-auditory abnormalities. Speech & language assessment showed no significant abnormalities except for distorted nasal tone and disturbed stress in 8 out of 20 patients. Hearing loss was bilateral and almost symmetrical in all patients. Tests of significance revealed no statistically significant difference between right and left ears for pure tone thresholds and word recognition scores (p>0.05).
As shown, the duration of hearing loss was significantly correlated to the number of frequency bands (p< 0.001) but not to the amount of TEN-HL test threshold shift. It is worth-noting that patients with extensive form had the longest duration of hearing loss(X=20 years, SD = 3.6 years), while patients with limited frequency band involvement showed the shortest duration (X = 4.1 years, SD = 2.6 years).
| Results of hearing aid fitting|| |
The results of unaided and aided sound field warble-tone thresholds together with speech recognition scores both in quiet and noise using open fitting hearing aids are shown in [Table 2] and [Figure 2]. The monaural aided condition represent averaged data from right and left ears as there was no statistically significant differences using Student’s “t” test (p>0.05). Hearing improvement was demonstrated in the frequency range of 1000-4000Hz in the aided conditions whether hearing aids were monoaurally-or bilaterally-fitted (p<0.001). Moreover, Paired “t” test comparing monaural versus binaural aided thresholds showed statistically significant difference (p<0.001) in the 1000-4000Hz frequency range. Similarly, word recognition scores were better with binaural fitting and the level of significance was p<0.001 in quiet and p<0.05 in noise using Willcoxon sign test. Individual analysis of data showed that 10 patients demonstrated an improvement of 12% or more in speech recognition in quiet, while only 2 patients showed such improvement in speech recognition in noise. In an attempt to identify the potential patients’ characteristics suggestive of improved performance with hearing aids, correlation studies were conducted for aided performance and TEN-HL test results. [Table 3] shows a significant negative correlation (p<0.001) between binaural aided speech recognition in quiet and the number of frequency bands with positive TEN-HL test, duration and degree of hearing loss. Similar correlations were recorded for the binaural aided speech recognition in noise (p<0.05). Considering the patients’ impression, 14 patients (70%) reported acceptable sound clarity. Though all patients tolerated the hearings aids for the duration of testing, only 5 patients requested hearing aids to try for a longer period of time.
|Table 2: Aided warble-tone thresholds and speech recognition scores using open fitting hearing aids|
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|Table 3: Correlation between speech recognition scores and TEN-HL test results, duration and degree of hearing loss|
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| Discussion|| |
Auditory Neuropathy/Dys-synchrony (AN/AD) is not an uncommon disorder. It is estimated to be around 10% of patients with SNHL (Rance et al., 1999 ). The present study focused on adult and adolescent patients with AN/AD and normal language development. It is apparent that hearing loss in this group has developed post-lingually; a situation which is probably different from AN/AD in children. This research addresses the functional characteristics of the peripheral auditory system in a clinical setting. In this essence, a clinically administered test to investigate the presence of dead regions of the cochlea was utilized, namely the TEN-HL test (Moore et al., 2004 ). AN/AD patients demonstrated a continuum from positive TEN-HL test results restricted to low frequencies (500 & 750 Hz) to those with positive TEN-HL test results scanning the whole tested frequency region (500 to 4000 Hz). This probably indicated that most of AN/AD patients could have dead regions with loss of IHCs and/or neuronal connections which are mainly concentrated in the apical region of the cochlea. On the other hand, the picture of extensive dead regions shown in some patients might not be true and could mainly reflect poor signal processing as reported by Moore et al .
A recent systematic study on dead regions in AN/AD patients using TEN-HL test, as contrasted to the gold standard measurement of dead regions namely Psycho-physical Tuning Curves ( PTC), was reported by Vinay & Moore (2007) . They similarly demonstrated that positive TEN-HL test results were more frequently existed at low rather than high frequencies. However, they reported that dead regions identified by TEN-HL test might not be true since PTC did not show similar findings. They concluded that high thresholds for detecting the test tones in TEN-HL test were not the result of dead regions (off-place or off-frequency listening) but instead, resulted from relatively poor detection efficiency. A possible reason was that AN/AD is associated with disruption of neuronal synchrony. In addition, they raised the possibility of poor functioning IHCs or substantial reduction in the number of IHCs but not completely non-functioning over any substantial region along the cochlea. It can thus be stated that, whatever the cause for high thresholds in TEN-HL test, it is shown to be a consistent finding in AN/AD patients with significant involvement of low frequencies. This excessive masking has also been observed in AN/AD patients when simultaneous masking of long-duration tones in noise was used .
On the contrary, true dead regions indicated by positive TEN-HL test results in SNHL patients were recorded in this study in a limited number of ears (16.25%). Such dead regions occurred mainly in the middle frequency region and involved 1 or 2 frequency bands only. Moore et al. (2004)  supported the existence of true dead regions as tested by TEN-HL test in SNHL patients. A dead region is usually present at or around steeping hearing loss , a configuration similar to that demonstrated in [Figure 2]. In general, dead regions in cochlear disorders are more expected in severe to profound hearing loss .Nevertheless, dead regions recorded in those SNHL patients are tremendously less than those recorded in AN/AD patients in this study. It is worth-noting that signal processing and detection in noise in AN/AD is significantly different from cochlear damage. Similar findings for perceptual tests, namely gap detection, frequency discrimination & temporal processing were reported by Zeng et al . Results from AN/AD patients showed a high negative correlation between positive TEN-HL test results and both duration and degree of hearing loss. Notably, patients with short duration of hearing loss had elevated TEN-HL thresholds confined to low frequencies, while patients who showed a picture suggestive of poor processing of signals had long duration of hearing loss extending up to 20 years. Although most of the researchers have pointed out that the course of AN/AD is unpredictable, results of the present study indirectly suggest the progressive nature of hearing loss and poor signal processing. Berlin et al. (2001)  has demonstrated a wide variability of course in children compared to adults. This would also denote that AN/AD in children may be different than which presents later on in adolescent or adult life.
In their comprehensive report, Moore et al. (2004)  have stated that dead regions with off-frequency listening sounds can be perceived as noise or distortion that could interfere with speech recognition. In the present study, SNHL patients with dead regions showed significantly worse speech recognition scores compared to those who had no dead regions, although both had similar pure tone thresholds except at 3 and 4 KHz). Speech recognition scores in AN/AD patients were significantly worse than in SNHL, though they had better pure tone thresholds. This supports the paradoxical picture repeatedly reported others ,. As shown, speech recognition in the AN/AD group was highly influenced by the degree of hearing loss and the number of dead regions, more so in the quiet condition. Zeng et al. (2005)  reported near normal signal processing at high frequencies. Such finding, together with results shown in this study, urged the authors to investigate the possibility of improved speech recognition using open hearing aid fitting. The aim was to eliminate low frequency amplification and preserve or emphasize high frequency components in speech. This was based on Zeng et al. (2005)‘s report who suggested innovative signal processing algorithms to eliminate low frequencies, emphasize high frequencies and accentuate temporal waveform modulation in speech. They also supported the use of electrical stimulation as cochlear implants and brainstem implants for improved performance in AN/AD.
Open fitting hearing aids successfully enabled AN/AD patients in this study to hear amplified high frequency sounds with improvement in their detection thresholds while low freq sounds remained unchanged. When patients were binaurally-fitted, they had significantly better high frequency detection thresholds as compared to the monaural aided condition. It is postulated that, in this way, the near normal processed frequencies are amplified and the excessive masking produced by low frequency dead regions is minimized. Typically, patients with dead regions extract little or no information from frequency components of speech that fall within a dead region, even when those components are amplified sufficiently to make them audible. These components are received through the “wrong” place in the cochlea, detected and analyzed via the same neural channels that are used for other frequencies; thus may compromise speech recognition . This would justify the selective high frequency amplification proposed for and provided to AN/AD patients in this study. The question is: how much these audible sounds can contribute to speech understanding? Our short-term benefit analysis showed a modest improvement in speech understanding in quiet, with less than favorable results for speech understanding in background noise. Moreover, aided performance was significantly better in the binaurally-fitted condition. Binaural hearing with either diotic acoustic stimulation or combined acoustic and electric stimulation produced significantly higher intelligibility than monaural stimulation in quiet but not in noise . Training to utilize high frequency cues needs to be considered for these patients for better use of the available information. It is supposed that more benefit is expected where neural plasticity can take place. Unfortunately, this was not addressed in the present work.
In reference to TEN-HL test results, benefit from amplification was shown to be negatively correlated to the number of frequency bands with positive TEN-HL test together with degree and duration of hearing loss. In other words, patients with preserved processing at mid frequencies, in addition to high frequencies, showed better results in speech recognition. It remains to be seen whether technologies such as frequency transposition or frequency compression recently-implemented in hearing aid industry would allow better use of information in the functionally-dead or poor regions of those patients. In summary, this study highlights the importance of measuring the functional characteristics of the peripheral system in AN/AD patients. The extent of non or poor- functioning cochlear/neural spots and hearing loss duration had an influence on speech recognition. Selective amplification of high frequencies may aid to maximize performance. Though a certain pattern of abnormality has been recorded in adult AN/AD patients, such results should not be applied on children with AN/AD patients until thoroughly investigated given the differences in clinical presentations, heterogeneity and central plasticity.
The support of Amplifon Middle East Company and The patients who shared in the study are gratefully acknowledged.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]