Journal of Otology & RhinologyISSN: 2324-8785

All submissions of the EM system will be redirected to Online Manuscript Submission System. Authors are requested to submit articles directly to Online Manuscript Submission System of respective journal.
bahis siteleri bahis siteleri bahis siteleri casino siteleri

Research Article, J Otol Rhinol Vol: 3 Issue: 6

Long Ringing Cochlear Microphonics - Not Unique to Auditory Neuropathy Spectrum Disorder in Children

Prashanth Prabhu P*, Vijaya Kumar Narne and Animesh Barman
Department of Audiology, All India Institute of Speech and Hearing,Manasagangotri, Mysore, India
*Corresponding author : Prashanth Prabhu P
Department of Audiology, All India Institute of Speech and Hearing, Manasagangothri, Naimisham Campus, Mysore, Karnataka - 570006, India
Tel: (+91) 8904353390
E-mail: [email protected]
Received: April 15, 2014 Accepted: September 23, 2014 Published: December 05, 2014
Citation: Prabhu PP, Narne VK, Barman A (2014) Long Ringing Cochlear Microphonics - Not Unique to Auditory Neuropathy Spectrum Disorder in Children. J Otol Rhinol 3:6. doi:10.4172/2324-8785.1000196

Abstract

Long Ringing Cochlear Microphonics - Not Unique to Auditory Neuropathy Spectrum Disorder in Children

Background: The detection of cochlear microphonics in surface recordings has been considered a distinctive sign of outer hair cell integrity in participants with auditory neuropathy spectrum disorder. The study highlights that this notion could result in misdiagnosis of patients.

Methods: The study reports three children with sloping hearing loss who were misdiagnosed as auditory neuropathy spectrum disorder based on absence of auditory brainstem response and presence of long ringing cochlear microphonics. The study also reports of three children with abnormal cochlear microphonics who have neurological abnormalities.

Results: The abnormal cochlear microphonics (long ringing) with absent auditory brainstem response was noted in sloping hearing loss participants. In addition, abnormal cochlear microphonics was also observed along with auditory brainstem response in children with neurological abnormalities.

Conclusion: Our results show that abnormal CM detection in surface recording is not a distinctive feature of auditory neuropathy spectrum disorder and participants who are diagnosed with auditory neuropathy spectrum disorder based on traditional diagnostic criteria should be reviewed to rule out possible misdiagnosis.

Keywords: Cochlear microphonics; Neurological abnormalities; Tone burst ABR; Polarity reversal

Keywords

Cochlear microphonics; Neurological abnormalities; Tone burst ABR; Polarity reversal

Introduction

The cochlear microphonics (CM) is a gross alternating current (AC) generated by cochlea that can be recorded in humans and experimental animals at several recording sites [1,2]. It is believed to result from the vector sum of the extra-cellular components of receptor potentials arising in outer hair cells (OHCs) and inner (IHCs), with the earlier contributing more to the generation of CM, on account of their greater number [1]. CM recorded at the promontory or in the ear canal is primarily generated from the more basal portions of the cochlea, while the apical regions make a little contribution to its generation at lower intensities [3].
CM has been obtained in humans by recording intra-tympanic or extra-tympanic electrocochleography (ECochG). CM has also been obtained in surface recordings by means of skin electrodes. Until recently, many investigators have considered that CM has extremely limited clinical use and attention has been focused on developing techniques to cancel it from electrocochleographic responses with the aim of extracting the compound action potential (CAP) and summating potentials (SP). Therefore, extensive data concerning CM parameters in normal hearing ears and in ears with various degrees of threshold elevation are not yet available.
Recently, CM recordings have attracted new interest following the identification of auditory neuropathy spectrum disorder (ANSD). ANSD is a disorder characterized by the impairment in the peripheral auditory function with the preserved outer hair cell (OHC) integrity [4-7]. Clinical criteria for diagnosis include absence or marked abnormality of auditory brainstem potentials (ABR) beyond that expected for the hearing loss, preserved cochlear receptor outer hair cell activities indicated by presence of otoacoustic emissions (OAE) and/or cochlear microphonics (CM) [8-10]. The presence of OAE or CM indicates the normal function of the cochlear outer hair cells [10]. Several investigators have also reported the disappearance of OAE or their absence in a large number of ANSD patients. Further, OAE are influenced by middle ear functioning and hence many assessed the OHC integrity and more generally, the diagnosis of ANSD, using CM from surface recordings [4,11,12].
Berlin et al. [4] proposed a simple procedure for detecting CM while recording auditory brainstem potentials for rarefaction and condensation polarity, and when compared they mimic the polarity of the acoustic stimuli used. This is in contrast to ABR and the use of alternating polarity cancels the CM. Similar procedure was employed by Starr et al. [13] for recording CM in children with normal hearing and ANSD. Following these studies, many investigators employed this procedure for detecting ANSD in the absence of OAEs. Berlin et al. [14] reviewed 260 patients with ANSD and the diagnosis was confirmed based on absence of click evoked ABR and presence of OAE/CM. Thus, it is recommended that CM should become a routine test for the infants with abnormal ABR [4,12,13].
The present study attempts to discuss children with sloping cochlear hearing loss who show typical features of ANSD with presence of CM and absent ABR for click stimuli. The study also attempts to discuss long ringing cochlear microphonics in children with neurological abnormality. The study thus focuses that detection of CM in surface recording along with no observable neural components is not a distinctive feature of ANSD and further highlights the importance of ABR using tone burst stimuli in routine audiological evaluation.

Materials and Methods

Participants
Six participants who reported to the clinic with a complaint of reduced hearing sensitivity has been evaluated using routine audiological evaluation. A routine audiological evaluation consisted of Visual Reinforcement Audiometry (VRA), Immittance evaluation, Otoacoustic emissions (OAE), Auditory Brainstem Response (ABR). All six participants are separated into two groups with first three participants having high frequency sloping hearing loss with presence of CM and the next three participants having neurological involvement with long ringing CM.
The participants 1, 2 and 3 reported with a complaint of reduced hearing sensitivity and limited speech and language. All of them had neonatal hyperbilirubenemia and were kept in NICU for 25 days, 5 days and 3 days respectively. Participant 1 had delayed motor milestones and the other two Participants had normal motor milestones. The participants were 3 years, 4.6 years and 5 years respectively. The behavioral thresholds of all the three participants were inconsistent.
Participant 4 was aged 3.6 years who reported to the clinic with reduced hearing sensitivity and inadequate speech output. The parents reported that the child doesn’t respond for soft sounds. The child had delayed motor milestones and the cognitive abilities of the child were reported to be poor. The child also had history of seizures when she was 3 months old. The behavioral thresholds of the child were inconsistent. Participant 5 was aged 4 years reported to the clinic with inadequate speech and language. The parents reported that child gave inconsistent responses to auditory stimuli. The child had delayed motor and speech milestones and was diagnosed as having cerebral palsy. The behavioral thresholds of the child were inconclusive. Participant 6 was aged 3.6 years who reported to the clinic with a complaint of reduced hearing sensitivity in both ears. Parents reported that child is not responding to soft sounds. The child had history of seizures at the age of one year and the motor milestones were delayed. The speech and language abilities of the child were also reported to be limited.
Procedure
A detailed case history was taken from all the participants. Visual reinforcement audiometry was carried out using loudspeakers at 450 azimuth and participants were seated at distance of 1 meter from the speakers. An illuminated toy was placed above the speakers and the child was initially conditioned to turn towards the reinforcer whenever the sound was presented. The intensity of the warble tones was gradually reduced to find out the threshold at which child provided an appropriate response. Immittance evaluation (tympanometry and acoustic reflex threshold testing) with a 226 Hz probe tone was carried out with a calibrated middle ear analyzer (GSI-Tympstar V 2.0). The transient evoked otoacoustic emissions were measured from calibrated OAE analyzer (ILO V6 DP Echoport) and auditory brainstem responses (ABR) were recorded using the Biologic EP/Intelligent Hearing Systems with ER-3A insert earphones.
ABR recordings had been made as follows using a hybrid protocol using clicks and low frequency tone burst: insert phones (ER3A) were positioned in the participants’ ear canals and disposable Ag/AgCl surface electrodes were placed on their vertex (Cz) (non-inverting), and over the mastoid of their test ear (M1 or M2) (inverting) and nontest ear (M2 or M1) (ground). Electrode impedances were maintained at less than 2 kOhms each. ABR waveforms were first recorded by stimulating each ear with tone-burst stimuli (with 2-1-2 cycle envelopes and blackmann ramps) at varying intensity levels at 11.1/sec stimuli using an alternating polarity. The response threshold for each stimulus was defined as the lowest stimulus level at which a repeatable ABR waveform could be obtained. ABR waveforms were then recorded by stimulating each ear with click stimuli (an acoustic click driven by a 0.1 ms square wave electrical click) at 90 dBnHL at 11.1 clicks/s using both rarefaction (two traces per ear), and condensation polarities (two traces per ear). All ABR signals were amplified 100 000 times; recorded for 10.24 ms (click), 20.48 ms (500 Hz tone burst) post-stimulus; averaged over 2048 samples; filtered from 30-3000 Hz; and the non-test ear was masked with white noise at the test ear stimulus level minus 40 dB. The physiological nature of any CM activity had been confirmed by ensuring that it did not start before 0.2 ms post stimulus (corrected for the insert phone delay), that it was clearly differentiated in time from the stimulus artifact, and that clamping the plastic tube of the ER-3A insert phones resulted in its loss.
All CM activity was recorded with surface recordings of ABR and analyzed using the procedure described by Starr et al. [13]:
• The ABR waveforms to condensation (C) and rarefaction(R) click stimuli were superimposed. This identified the CM activity as the short-latency components (beginning at approximately 0.4 ms) that clearly started after the stimulus artifact had finished, and had a phase inversion with polarity reversal of the stimulus.
• The ABR waveforms obtained after clamping the plastic tube were inspected. If the loss of stimulus resulted in the loss of the ABR and CM components, it was assumed that the ABR and CM waveforms were of biologic origin (as opposed to recording artifacts of the electrical input to the earphone).
• The separate averages to C and R stimuli were summed. This attenuated the CM activity and enhanced the neural activity (waves I to V).
• The separate averages to C and R stimuli were subtracted. This enhanced the CM activity and attenuated the neural activity (waves I to V).
• The amplitude of the CM was measured using two methods. The first method involved subtracting the R waveform from the C waveform (C-R). The largest peak-to-adjacent trough amplitude was then calculated for any CM activity occurring between 0.4 and 0.7 ms. The method was as per Starr et al., [13] who did not use out-of-phase components after 0.7 ms to avoid inclusion of wave I that can shift in latency with click polarity to resemble out-ofphase CM components. The second method also involved subtracting the R waveform from the C waveform (C-R), but now the largest peak-to-adjacent trough amplitude was calculated for any CM activity occurring at any latency. The C-R CM amplitudes were twice those of C or R CM amplitudes.
• While the latency of the CM activity was estimated, it was not formally analyzed due to difficulties exactly identifying when the CM activity began and finished.
Ethical considerations
In the present study, all the testing procedures done were using non-invasive technique and all the procedures were explained to the patients and their family members before testing and informed consent was taken from all the patients and their family members for participating in the study. The study followed the principles of Declaration of Helsinki.

Results and discussion

Participant 1
The behavioral thresholds of participant were estimated using visual reinforcement audiometry through sound field condition which revealed a sloping audiogram configuration. Immittance evaluation showed normal middle ear functioning in both ears. TEOAE was absent for 80 dB pe SPL clicks in both the ears. ABR recorded for click stimulus at 90 dB nHL showed absent responses with long ringing cochlear microphonics in both ears. Based on the audiological evaluations, the participant was diagnosed as having auditory neuropathy spectrum disorder (AD) in both the ears considering the current definition of auditory neuropathy spectrum disorder suggested by Starr et al. [8] and Berlin et al. [4].
A re-evaluation was carried out after 3 months on the same patient following the reports from the parents and clinician that child responds differently for high and low frequency sounds. The same audiological tests were used but 500 Hz and 1000 Hz tone burst ABR was also included in the test battery. The results of the evaluations showed that VRA had a sloping audiogram configuration. Immittance evaluation showed normal middle ear functioning with absent acoustic reflexes and TEOAE was absent in both the ears. ABR recorded for click stimulus with condensation and rarefaction polarity showed absent responses at 90 dB nHL with long ringing cochlear microphonics in both ears depicted in Figure 1.
Figure 1: Long ringing cochlear microphonics with absent ABR recorded for a click stimulus at 80 dB nHL for participant 1 and at 90 dB nHL for participant 2 and 3.
The results of ABR recorded for 500 Hz and 1000 Hz tone burst showed identifiable wave V till 80 dB nHL in both ears. The results of the same for right ear are depicted in Figure 2. Based on the reevaluation results, the participant was diagnosed as having bilateral moderately severe sloping sensorineural hearing loss.
Figure 2: ABR responses for 500 Hz and 1000 Hz tone burst stimuli at 60 dB nHL and 70 dB nHL for participant1 and tone burst ABR responses recorded for right ear at 80 dB nHL and 70 dB nHL for participant 2 and 3.
Participant 2
A routine audiological evaluation was carried out which consisted of Visual Reinforcement Audiometry (VRA), Immittance evaluation, Otoacoustic emissions (OAE), Auditory Brainstem Response (ABR). Visual reinforcement audiometry carried out using speakers revealed responses around 70 to 80 dB for 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. Immittance evaluation showed normal middle ear functioning in both ears. TEOAE were absent for 80 dB pe SPL clicks in both the ears. ABR recorded for click stimulus at 90 dB nHL showed absent responses with long ringing cochlear microphonics till 3 ms in both ears shown in Figure 1. Based on the audiological evaluations, the participant was diagnosed as having auditory neuropathy spectrum disorder in both the ears. A re-evaluation was carried out after 6 months and conditioned play audiometry results were obtained for both ears. The results of conditioned play audiometry showed that the participant had severe sloping sensorineural hearing loss in right ear and profound hearing loss in left ear.
Immittance evaluation showed ‘A’ type tympanogram with absent ipsilateral and contralateral acoustic reflexes in both ears. TEOAE were absent for 80 dB pe SPL clicks in both the ears. Click evoked ABR recorded at 90 dB nHL showed absent responses. ABR was present at 80 dB nHL for 500 Hz tone Burst in left ear and the responses were absent at 80 dB nHL for 500 Hz tone burst in right ear shown in Figure 2. Based on the re-evaluation results, the participant was diagnosed as having moderately severe to severe sloping sensorineural hearing loss in left ear and severe hearing loss in right ear.
Participant 3
A routine audiological evaluation was carried out. Conditioned Play audiometry was tried with the child and he showed inconsistent responses. Immittance results showed normal middle ear functioning in both the ears. TEOAE were absent in both the ears. ABR was done for clicks at 90 dB nHL and showed no response with ringing cochlear microphonics in both the ears shown in Figure 1. Based on the audiological evaluations, the child was diagnosed as having auditory neuropathy spectrum disorder in both the ears.
A re-evaluation was carried after 6 months and the results showed that ABR was present at 90 dB nHL in left ear for click stimuli. ABR was absent at 90 dB nHL in the right ear for click stimuli. ABR was also recorded for tone burst stimuli and the responses were present till 80 dB nHL and the responses were absent at 70 dB nHL in both the ears. The tone burst ABR recorded for right ear has been shown in Figure 2. Based on the above results, the child was diagnosed as bilateral moderately severe to severe sloping hearing loss.
The first three participants (participant 1, 2 and 3) in the study clearly show high amplitude and long ringing CM with no identifiable neural components (wave I to V) for click evoked ABR. Based on the traditional diagnostic criteria they were diagnosed as ANSD. In the reevaluation, results of the tone burst evoked ABR and visual reinforcement audiometry confirmed that all the three participants had sloping cochlear hearing loss. These results indicate that long ringing CM can also be observed in surface recording of patients with sloping cochlear hearing loss. The results of the present study support the hypothesis by Sutton et al. [15] that presence of a CM with abnormal/absent ABR does not always indicate ANSD. Similar hypothesis has also been suggested by Wilson et al. [16]. Click ABR threshold around 85 dBnHL with a recordable long ringing CM around the same level might occur in a cochlear loss, for example where the audiogram slopes steeply, either upward or downwards [17]. Thus, CM detection in surface recordings cannot be considered an invariable sign of normal OHCs integrity and further studies need to explore the relationship between CM detection in surface recordings and normal OHC function.
The results of the present study challenge the widely accepted view that the CM is strictly related to OHC electrical activity with only a minor contribution from IHCs. The true pathophysiology of especially long ringing prominent CM activity in SNHL needs to be fully elucidated. The results obtained were also supported with the presence of long ringing cochlear microphonics with large amplitude that was observed in children with neurologic abnormalities such as cerebral palsy or episodes of seizures. The three participants in whom abnormal CM activity was seen have been reported in the following case examples.
Participant 4
A routine audiological evaluation was carried out. Conditioned Play audiometry was tried with the child and he showed inconsistent responses. Immittance results showed normal middle ear functioning in both the ears. TEOAE were absent in both the ears. The absence of OAE could be because of the cochlear loss present in both the ears. ABR was done for clicks and tone burst stimuli. The results of ABR revealed identifiable wave 5 till 70 dB nHL in both the ears. Cochlear microphonics was recorded for click stimulus recorded for condensation and rarefaction polarity. The results of ABR are depicted in Figure 3.
Figure 3: ABR responses with ringing cochlear microphonics recorded for right ear at 90 dB nHL.
Participant 5
A routine audiological evaluation was carried out. Conditioned Play audiometry was done and the child showed inconsistent responses. Immittance results showed normal middle ear functioning in both the ears. TEOAE were absent in both the ears because of cochlear loss in both the ears. ABR was done for clicks and tone burst stimuli. The results of ABR revealed identifiable wave 5 till 60 dB nHL in both the ears. Cochlear microphonics was recorded for click stimulus recorded for condensation and rarefaction polarity. The results of ABR are depicted in Figure 4.
Figure 4: ABR responses with long ringing cochlear microphonics recorded for left ear.
Participant 6
The result of the routine audiological evaluation using conditioned play audiometry shows bilateral moderate sensorineural hearing loss and the responses were inconsistent. Immittance results showed normal middle ear functioning in both the ears. TEOAE were absent in both the ears. ABR was done for clicks at 90 dB nHL and showed presence of wave V with poor morphology. The waveform also showed long ringing cochlear microphonics till 5 ms in both the ears. The ABR response of left ear with long ringing cochlear microphonics is shown in Figure 5.
Figure 5: ABR response for click stimuli at 90 dB nHL of left ear with long ringing cochlear microphonics.
These three participants (participant 4, 5 and 6) of the study showed abnormal long ringing cochlear microphonics and all of them had cochlear hearing loss. All the participants had ABR present but CM was abnormally long ringing. All the participants had some form of neurological abnormality such has cerebral palsy and/or presence of seizures during infancy. Hence, it could be suspected that the abnormal ringing in CM could be seen in patients with central nervous system abnormality. Santarelli, Scimemi, Dal Monte and Arslan [18] reported that the presence of central nervous system pathology seems to enhance CM amplitude. This effect likely result from dysfunction of the medial efferent system through a reduced inhibitory influence on OHCs, leading, in turn, enhanced outer hair cell movement. The functional changes in the cochlea and changes in the activity of the medial olivocochlear system (MOCS) have been reported as a possible reason for enhancement of OHC activity [2,19,20]. Arslan et al. [4] reported that the abnormal CM in subjects with CNS pathology could be due to interruption of the olivocochlear bundle at the CNS level or a disruption of the CNS mechanism capable of controlling its activation. Thus, the possible mechanism for the long ringing cochlear microphonics in these children could be because of the pathology at the CNS level which alters the cortical control of the efferent system resulting in the presence of cochlear microphonics.

Conclusions

The present study focuses on the importance of using tone burst evoked ABR in the test battery of diagnosis of ANSD. The study clearly shows that if the traditional diagnostic criteria are used for diagnosis, we may misdiagnose many patients with sloping cochlear hearing loss as ANSD. The results of the study also show that long ringing abnormal cochlear microphonics can be seen in children with high frequency sloping hearing loss and in children with neurological abnormalities such as cerebral palsy or history of seizures. Thus, it is recommended that all the patients who are diagnosed with ANSD based on traditional diagnostic criteria should be reviewed to rule out possible misdiagnosis.

Acknowledgements

The authors acknowledge with gratitude Dr. Savithri SR, Director, All India Institute of Speech and Hearing, Mysore for permitting to conduct the study at the institute. The authors also like to acknowledge the participants for their co-operation.

References





















Track Your Manuscript

Media Partners

Associations