November 2008 : Presence and Behavior of Otoacoustic Emissions in Children with Auditory Neuropathy/Auditory Dys-Synchrony
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- Written by Paula Moore MS, Au.D.
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Introduction
Auditory neuropathy (AN) is a hearing disorder clinically characterized by present otoacoustic emissions (OAEs) and absent or grossly abnormal Auditory Brainstem Response (ABR) testing/measures. The diagnostic label “Auditory Neuropathy” was coined by Starr, Picton, Sininger, Hood, and Berlin in 1996, due to the pattern of results that seemingly indicated normal outer hair cell function, as determined by normal OAE recordings, in the presence of abnormal functioning of the vestibulo-cochlear nerve (VIIIth nerve), indicated by abnormal ABR responses and absent middle ear muscle reflexes (MEMRs). Berlin, Hood, and Rose (2001) further investigated the underlying mechanisms of AN in order to ascertain the precise location of dysfunction. From their investigation, Berlin et al. determined that the VIIIth nerve might not always be the site of lesion in AN; consequently, adding the term Auditory Dys-Synchrony in lieu of AN exclusively was recommended in an attempt to be semantically appropriate (Berlin, 2001). OAE and ABR test results of subjects with AN/AD indicate normal functioning of the auditory system up to and including the outer hair cells (OHCs); however, the abnormal responses of the ABR indicate that the auditory signal is not being successfully transmitted to the brainstem. This pattern of abnormality exhibited in the ABR results suggests that the site of dysfunction in subjects with AN/AD could be the inner hair cells (IHCs), the synapse between the IHC and their dendrites, spiral ganglion, VIIIth nerve fibers, or combinations of these sites (Doyle et al., 1998; Varga et al., 2006). While the precise site of dysfunction may be variable in individuals with AN/AD, Santarelli et al. theorize/hypothesize that most cases of AN/AD results from the impaired function of the VIIIth nerve due to demyelinization and to a lesser extent axonal loss (2001).
The known etiologies of AN/AD are diverse; however, approximately 50% of all subjects with AN/AD have no known etiology. The accepted causes of AN/AD appear to be genetic, toxic-metabolic, or immunological (Starr et al., 2000). Due to the relatively recent entrance of AN/AD into the audiological scene, incidence reports of this hearing disorder are sparse. Of frequent investigation are the statistical reports of sensorineural hearing loss in children. It is estimated that approximately 1-2 out of every 1000 children have some degree of hearing loss (Cheng et al., 2005). David and Hirsh (1979) estimated that 1 in every 200 children with hearing loss have AN/AD. Additionally, Berlin et al. (2002) estimated that nearly 10% of the diagnosed deaf population has AN/AD. Kraus et al. (1984) determined that approximately 15% of subjects with no ABR had AN/AD and Madden et al. (2002) reported that 8% of the children with hearing loss had been diagnosed with this hearing disorder, since initiating the AN/AD diagnosis protocol just 5 years before. While prevalence estimations of this hearing disorder vary (ranging from approximately 5 to 15%), it is evident that this disorder occurs consistently enough to be seen in any otodiagnostic center, which employs measures for diagnosing AN/AD.
Auditory Neuropathy/Auditory Dys-Synchrony is diagnosed in patients who present with a very distinct set of clinical findings. The clinical manifestations of AN/AD include variable audiometric data, poor speech discrimination, absent middle ear muscle reflexes, absent or grossly abnormal ABR responses and present OAEs.
1. Clinical Findings
Variable audiometric data: Variable responses to air and bone conduction stimuli. Hearing thresholds ranging from normal to seemingly no response to tonal stimuli have been reported (Sininger et al., 1995; Berlin et al., 2003; Starr et al., 1996). Poorer thresholds in the lower frequencies as compared to the higher frequencies has been reported (Sininger et al., 1995)
Poor Speech Discrimination: In the auditory system, neurons generate synchronous compound action potentials in relation to presenting frequency stimulation. The firing of these synchronous potentials encode both loudness and pitch pattern percepts, all while extracting spectral peaks and waveform envelopes for speech recognition (Zeng et al., 1999). A disturbance in the synchronous firing of the auditory system interferes with the precise encoding of temporal cues, which impairs speech comprehension abilities (Starr et al., 2003). Individuals with AN/AD are believed to have dys-synchronous auditory systems because of abnormal ABR measures whilst maintaining OAEs. Patient reports, as well as speech recognition measures have been shown to be out of proportion to the pure tone hearing loss. According to Rance et al., speech recognition ability in children with a prelingual form of AN/AD varies. Approximately 50% have little or no speech understanding in spite of sufficient sound detection ability. The other 50% appears to make use to their residual hearing and perform well on both open and closed set tests of speech recognition tests (2002). Although approximately 50% of pediatric patients with AN/AD can understand speech in quiet environments, generally, most patients experience much difficulty understanding speech in the presence of background noise (Hood, 1998; Berlin, 1998). This phenomenon occurs when listening conditions become degraded because an intact neural system is necessary for understanding speech in adverse listening conditions (Tlumak, 2002).
Absent Middle Ear Muscle Reflexes (MEMRs): (Berlin et al., 2003; Lee et al., 2001; Sininger et al., 1995)
Absent or grossly abnormal ABR responses: The synchronous activity of the auditory nerve and auditory brainstem is measured by way of auditory brainstem response (ABR) testing. This measure reflects neural activity synchronized to the onset of the acoustic stimuli. A disturbance in the neural synchrony would disrupt temporal encoding and would thereby affect the ability to obtain ABR responses (Zeng et al., 1999). The occurrence of absent or markedly abnormal ABRs with present cochlear microphonic (CM) potentials in individuals with AN/AD has been reported in the literature (Berlin et al., 2002; Tlumak, 2002; Berlin et al., 1998; Starr et al., 1996). The CM is a gross potential generated by cochlear hair cells, with the predominant response believed to arise from the OHCs. The detection of the CM with surface electrodes is considered a distinctive sign of OHC integrity in individuals with AN/AD (Santarelli et al., 2006). High intensity rarefaction and condensation clicks are used to distinguish the CM from the neural response. With opposite polarities, the CM inverts while the neural response shows only minor latency shifts. (Hood, L. J., et al., Chapter 9). Starr et al. (2000) investigated ABRs obtained from 52 subjects with AN/AD. Their research indicated several possible configurations for the abnormal responses. Seventy three percent of subjects had present CMs with no measurable I, III, or V components, 21% displayed only Wave V of prolonged latency and reduced amplitude, and 6% exhibited Wave V with Wave III of poor morphology. Cochlear microphonics were present in all abnormal ABR configurations (Starr et al., 2000).
Normal Middle Ear Function, as indicated by normal middle ear pressure and mobility. While an individual with AN/AD may have the hearing disorder and temporary middle ear dysfunction concurrently, OAE measures may appear absent in these cases, due to middle ear pathology.
Present Otoacoustic Emissions: Kemp first described otoacoustic emissions in 1978. OAEs are sounds that occur in the external ear canal when the eardrum receives sound pressure vibrations, which are transmitted from the cochlea through the middle ear (Kemp, 2002). These sounds arrive in the ear canal as a result of Basilar Membrane (BM) disturbances that are created by the cochlear amplifier (OHCs) and then travel from the hair cells to the base of the cochlea. This motion of the BM causes the oval and round windows to move in opposition of each other causing vibrations of the ossicles and eardrum, which are recorded as OAEs in the ear canal (Kemp, 2002). According to Kemp et al., (1990) the primary value of OAEs is that their presence indicates normal function of the preneural cochlear receptor mechanism (cochlear amplifier). In general, OAEs are only present in normal or near normal functioning ears. This however is not the case in the OAE results of individuals with AN/AD. While ABR results suggest substantial deficits in hearing ability, OAE measures indicate normal OHC function. Some reports of AN/AD have reported that OAEs were present in all individuals with AN/AD (Santarelli et al., 2002; Shivashankar et al., 2003, Doyle et al., 1998), however, recent reports have indicated partially or completely absent OAEs in these individuals (Marco et al., 2000; Lee et al., 2001; Deltenre et al., 1999; Starr et al., 2001; Raveh et al., Madden et al., 2002). A review of literature was conducted for the purposes of examining the risk factors for AN/AD and determining the presence or absence of OAEs in the AN/AD population.
2. Literature review
Raveh et al. (2007) described the TEOAE test results of 26 pediatric subjects with AN/AD. The mean age at presentation of the hearing disorder was 13 months, ranging from neonate to 16 years of age. Prematurity, hyperbilirubinemia, ototoxic drug therapy, perinatal asphyxia, family history of hearing loss, parental consanguinity, and meningitis, all factors known to be associated with AN/AD, were present in 18 of the subjects, and are believed to be the etiological factors of the disorders in these cases. The remaining 8 children had no known associated risk factors or pathologies. TEOAEs were present in 18 of the 26 subjects. Three subjects had no OAEs and the remaining 5 subjects were not tested. All subjects without OAEs did have CMs upon electrophysiological testing, thus indicating OHC function, in 100% percent of the subjects tested, although OAE measures did not indicate this finding.
Doyle et al. (1998) conducted TEOAE testing on eight pediatric subjects, aged 4-15 years old, with previously diagnosed AN/AD. These children were initially diagnosed with sensorineural hearing loss (SNHL), but further investigation confirmed AN/AD. Steven’s-Johnson syndrome and Friedreich’s ataxia were diagnosed in two of the subjects; no additional disorders were present in the six other subjects. TEOAEs were present in 15 out of 16 ears. One child presented with unilateral AN/AD and a profound hearing loss in the opposite ear, with no measurable OAEs in that ear. The investigators provided no explanations or theories concerning the absence of OAEs in this subject; additionally, no reports of CM were provided to conclusively rule out AN/AD in this ear.
Santarelli and Arslan (2002) described the DPOAEs of 5 subjects with AN/AD whose ages ranged from 11 months to 19 years old. Complex medical histories were reported in 4 of the 5 subjects. Concomitant medical diagnoses included Kasabeth-Merritt syndrome, optic atrophy, prematurity, tachycardia, polypnea, hepato-splenomegaly and jaundice, for which treatments included blood transfusion and phototherapy. DPOAEs were recorded in 9 out of the 10 test ears. DPOAEs were present from 1-4 kHz in 4 ears, 1-6 kHz in 2 ears, 2-4 kHz in 3 ears (high levels of ambient noise were reported during 2 recordings) and absent in 1 ear, which had significant negative pressure as indicated by abnormal tympanometry measures. This report indicated partial or complete DPOAE responses in all ears tested, which presented with normal middle ear functioning.
Shivashankar et al. (2003) reported the DPOAE results of twenty-four subjects with auditory neuropathy. Their ages ranged from 13 to 49 years old. The onset of AN/AD was during the first and second decade of life for the majority of the subjects. History of noise exposure, infection, and trauma were unremarkable prior to the onset of AN/AD and no family of hearing loss was reported for any of the 24 subjects. DPOAEs were present in all 24 subjects with AN/AD, thus indicating normal OHC function in these individuals and providing evidence that AN/AD can occur in individuals with no other medical conditions or disorders.
Otoacoustic emissions were assessed in 18 subjects with AN/AD by Madden et al. in 2002. Risk factors for AN/AD varied and often appeared in combinations. Hyperbilirubinemia, prematurity, exposure to ototoxic medications, mechanical ventilation, and family history of hearing loss were reported in the subject’s medical histories. Additionally, cerebral palsy was reported in two of the subjects. DPOAEs were reported in 14 out of the 18 subjects. The four subjects with absent OAEs showed present CM potentials upon electrophysiological testing, indicative of OHC function in 100% of the subjects, although DPOAE measures only indicated OHC function in 78% of the subjects.
Starr et al. (2001) reported the TEOAEs of 33 subjects with AN/AD, whose ages ranged from 4 months to 64 years old. TEOAEs were present in 44 (70%) of the 63 ears that were assessed, and absent in 19 ears (30%). Eight of the subjects had bilateral absent TEOAEs, while 3 subjects had unilaterally absent TEOAEs. Of the 11 subjects with absent TEOAEs, 9 had present OAEs in previous evaluations and all 11 had a history of hearing aid use, indicating possible over-amplification as the cause of absent OAEs.
Rance et al. (1999) investigated twenty pediatric subjects aged 1 - 49 months with AN/AD. Jaundice, hypoxia, cerebral palsy, facial palsy, hydrocephalus, low birth weight, and unilateral middle ear malformation were among the associated medical conditions that were reported in 16 of the 20 subjects. The remaining 4 subjects had no risk factors for the hearing disorder. TEOAEs testing was attempted in 17 of the subjects. TEOAEs were obtained bilaterally in 8 subjects and unilaterally in 1 subject. The unilateral case of present OAEs indicated absent TEOAEs in the malformed ME ear of a subject and present TEOAEs in the anatomically intact ear. The remaining 8 subjects (nearly 50%) had no measurable TEOAEs. All subjects showed normal OHC functioning through the use of electrophysiologic measures of the CM, including those subjects with absent OAEs.
Lee et al. (2001) described the TEOAE results of 2 school-aged children, ages 11 and 12 years old, whose test results met the diagnostic criteria for AN/AD. No known risk factors for hearing loss were noted in the medical histories of either child. TEOAEs and DPOAEs were present bilaterally in one child and unilaterally in the other. In the child with bilateral OAE responses, OAEs were obtained between 1-4 kHz in one ear and at 4 kHz only in the opposite ear. Child 2 had unilaterally present TEOAEs restricted to the 3-4 kHz region and present DPOAEs at 3-6 kHz. Cochlear microphonics were present in all 4 ears, indicating OHC function in both subjects, bilaterally.
Marco et al. (2001) investigated the TEOAE recordings of 2 pediatric subjects, aged 7 months and 20 days old, with AN/AD. Medical history was unremarkable in one child and remarkable for jaundice and intrauterine carbon monoxide intoxication in the other. TEOAEs were recorded initially and were absent upon subsequent testing, approximately 8 months later. This finding gives valuable insight into the time-course for the disappearance of OAEs, indicating that the disappearance of these emissions could happen relatively quickly. Hearing aids were used in one of the subjects; however, the potential cause of the loss of OAEs in the other subject is unclear.
Deltenre et al. (1999) described the TEOAE responses of 2 pediatric subjects with AN/AD. Subject 1 was a twin born prematurely with a complex medical history remarkable for severe respiratory distress syndrome complicated by persistent pulmonary hypertension of the newborn and bronchodysplasia. Subject 2 was born 10 weeks premature and has a history of hyperbilirubinemia, necrotizing enterocolitis, which required ventilation, septic shock with metabolic acidosis, kernicterus and spastic diplegia. Treatments included various antibiotics and exchange transfusions. TEOAEs were present bilaterally in Subject 1 and present unilaterally (above 2 kHz only) in Subject 2. Prior testing of Subject 2 indicated present OAEs bilaterally, which were described as normal, just one-year prior. In follow-up testing conducted at 4 and 7 years of age for Subjects 1 and 2 respectively, OAEs for both children were absent bilaterally; however, CM measures remained.
Reports of risk factors associated with AN/AD as well as OAE presence and behaviors indicate that both can be highly variable in individuals with AN/AD. The medical histories reported in the studies of AN/AD ranged from unremarkable to highly medically involved subjects. Risk factor reports suggest that hyperbilirubinemia and kernicterus, ototoxic drug therapy, family history of hearing loss, cerebral palsy, respiratory disorders, and treatments including mechanical ventilation and blood transfusions are among the leading etiological risk factors for the hearing disorder, with several investigations reporting the presence of these conditions in the medical histories of their study participants. The results of OAE testing revealed that while OAEs may be normal in some individuals with AN/AD there are populations of individuals with AN/AD and present with OAEs that disappear, partially present OAEs, and absent OAE responses.
The varying results of OAEs in addition to the contradictory results of present OAEs and absent ABR measures in patients with AN/AD denote the need for further investigation into the presence and behaviors of OAEs in this special population. This paper will report and discuss the risk factors and OAE results of 12 children with AN/AD at A. I. duPont Hospital for Children in Wilmington, DE.
3. Methods
Twelve infants and children (9 males and 3 females) with AN/AD were included in this retrospective study. The subjects were previously diagnosed with AN/AD due to present OAE responses and/or absent or grossly abnormal click evoked ABRs with present CMs. Ten patients had symptoms of AN/AD bilaterally, while the remaining 2 subjects showed symptoms of AN/AD unilaterally, with one subject exhibiting normal auditory function in the opposite ear and the other exhibiting symptoms of sensorineural hearing loss in the opposite ear. The average age of the subjects at diagnosis was 29 months, with a range from 2 months to 8 years of age. The parents or guardians of the 12 subjects gave signed consent for participation in the present study.
The medical records of the subjects were reviewed to determine if any risk factors for AN/AD or hearing loss were present in their prenatal, birth, neonatal, or past medical histories. Additionally, the results of all OAE testing performed on the subjects were reviewed to determine the presence or absence of OAEs. Distortion Product Otoacoustic Emissions were assessed using Bio-logic OAE instrumentation and Transient Evoked Otoacoustic emissions were assessed using the ILO-88 OAE system. The DPOAE at 2f1-f2 was recorded in response to f1 and f2 primary tones of 65 and 55 dB SPL, respectively. The f2/f1 ratio was 1.2. Primary tones were stepped in 6 regular intervals from 2000 to 6000 Hz. The minimum sample size of the DPOAE recording was 40 points with a maximum sample size of 1024 points. Transient Evoked OAEs were evoked using clicks of 80dB, ± 3 dB. The presence of normal DP and TEOAEs was determined by response amplitudes of at least 3 dB above the noise floor. Results of OAE testing were divided into three categories: low, mid and high frequency responses. Low frequency OAEs included responses between 1000-2999 Hz, mid frequency OAE responses were obtained between 3000-4999 Hz, and high frequency responses were recorded between 5000-6999 Hz. A total of 62 OAE recordings were obtained from the 12 patients between June 2005 and December 2007.
4. Results
Risk factors that are known to be associated with hearing loss and AN/AD were found in 8 (67%) of the 12 pediatric subjects (summarized in Table 2). Evidence of Hyperbilirubinemia/jaundice was reported in 3 subjects, prematurity in 2, anoxia/breathing difficulties in 3, and ototoxic medication treatment in 2. Four subjects had associated medical pathologies. Intraventricular Hemorrhage (IVH) in 2, cerebral palsy in 1, and evidence of a peripheral neuropathy was present in 1. Only 4 (33%) of the 12 subjects had no known associated pathologies or risk factors for AN/AD.
The OAE results of the 12 subjects are summarized in Table 2. Otoacoustic emissions response amplitudes of at least 3 dB above the noise floor were recorded in 61 (98%) of the 62 recordings. One recording yielded absent OAEs in the presence of normal middle ear function, as determined by tympanometry measures. Subsequent tests later yielded measurable OAEs in the same subject.
Of the 61 recordings with measurable OAEs, 27 (44%) had recordable emissions in all frequency regions assessed (low, mid, and high frequencies), while 35 recordings (57%) had partially present OAEs (OAEs responses in one or two frequency regions). Twenty-two (64%) of the 34 recordings with partially present OAEs exhibited present otoacoustic emissions in 2 frequency regions and the remaining thirteen (40%) had present OAE responses in just one frequency region. Forty-two percent of the partially present OAE recordings were obtained in ears with abnormal middle ear function (e.g. PE tubes, significant negative and positive pressure, and possible effusion).
The presence and absence of OAEs were analyzed as a function of low, mid, and high frequency. This analysis revealed low frequency OAE responses were recorded in 81% of the recordings, mid frequency OAEs responses were recorded in 77%, and high frequency OAEs were recorded in 55% of the recordings. Additionally, no correlation between age of subject and absence and presence of OAEs responses was observed.
Twenty-six OAE recordings were obtained post hearing aid use, in 6 of the subjects involved in the study. Of these 26 recordings, 11 were partially present. Previous OAEs testing revealed partially present OAEs, prior to initiating hearing aid use.
Presence and absence of OAEs were analyzed as a function of ototoxic medication exposure in two subjects. Otoacoustic emissions were partially present in one subject and present at all frequency regions assessed in the other. Additionally, no pattern of OAE presence or absence was observed in the subjects with common etiological risk factors for AN/AD, e.g. EVA, jaundice, and ventilation.
5. Results
The analysis of the data from 12 pediatric subjects with AN/AD reveals risk factors for this hearing disorder. Similar to Madden et al. (2002), the present investigation found these risk factors to be variable and often occurring in combination with one another. Overall, 67% of the 12 pediatric subjects showed evidence of jaundice, prematurity, anoxia, and treatment with ototoxic medication. Other investigations have found these four medical conditions and treatments to be among the leading risk factors of AN/AD (Rance, et al., 1999; Madden et al., 2002). Cerebral palsy was reported in the medical history of one subject in this study. This supports past analyses of children with AN/AD, which indicated cerebral palsy to be among the associated conditions of AN/AD (Rance et al., 1999; Madden et al., 2002). Two cases of Intraventricular Hemorrhage IVH occurred in combination with jaundice and ventilation dependence. The presence of IVH in these cases may be purely coincidental. No etiological accounts of AN/AD due to IVH have been reported, while several reports have described AN/AD due to jaundice and mechanical ventilation (Rance et al., 1999; Madden et al., 2002). As this and other studies indicate, using a combination of OAE and ABR testing to evaluate OHC function, in addition to conventional audiometric techniques will serve to effectively assess the integrity of the auditory system in these individuals.
Classically, AN/AD has been considered a disorder of the auditory nerve with normal functioning of the OHCs within the cochlea. Further investigation of this hearing disorder has confined the potential site of dysfunction to encompass not only the auditory nerve, but also the IHCs, the synapse between the IHCs and VIIIth nerve fibers, or combinations of these sites (Doyle et al., 1998; Varga et al., 2006). The present study indicates that OAEs responses in children with AN/AD can be highly variable. One hundred and eighty two possible responses were measured in sixty-two OAE recordings. Of the 182 possible responses (from 62 recordings), 130 responses were obtained, thus indicating the absence of OAE responses in 29% of the frequency regions. These findings are contrary to early investigations, which report normal OAEs in individuals with AN/AD (Doyle, K. J., Sininger, Y., & Starr, A., 1998; Berlin, C. I., 1998.
Evidence of OHC integrity was previously determined in the 12 pediatric subjects involved in this study, by way of CM presence in absent or grossly abnormal ABR recordings; however, as mentioned, corroboration of normal OHC function was not always indicated by OAEs responses. Although OAEs responses and CM potentials have common generators, certain circumstances can occur which might produce a CM with OAEs response (Rance et al, 1999). One such circumstance involves cases of middle ear pathology, which can prevent the recording of OAE responses. Middle ear dysfunction, as indicated by abnormal tympanometry measures, was present in 37% of the absent OAE responses, suggesting middle ear pathology may have interfered with the measurement of these OAE responses. However, more than 60% of the absent OAEs occurred in ears with normal middle ear function. Another possible cause of present CM potentials with absent OAE responses could be an insult to the OHCs, sufficient enough to disrupt the active process of the hair cells, but not the CM potential. According to Rance et al. (1999), OAEs can be abolished with even minor cochlear insults. Potential cochlear insults in the form of ototoxic medication therapy and hearing aid use were present in 9 of the 12 (75%) of the patients involved in this study and may have accounted for the absence of 36% of the OAEs responses. Potential cochlear insults in combination with middle ear dysfunction could be responsible for as much as 54% of absent OAEs. Starr reports absent OAEs secondary to the progressive nature of AN/AD. The disorder is believed to naturally progress to involve the mechanical properties of OHCs, which consequently abolishes OAE responses, while CMs still persist. This phenomenon is believed to occur in approximately one-third of all patients with AN/AD and could account for the absence of OAEs in the remaining 46% of absent OAE recordings (Starr, A.)
The present report and other investigations of OAE behavior in individuals with AN/AD provides evidence that OHC function may not be “normal” in all cases of AN/AD. This finding has very important implications for the necessity to reevaluate the general consensus regarding the diagnostic criteria for AN/AD, which requires the presence of normal OAEs for diagnosing the hearing disorder. Utilizing the test strategy described by Berlin et al. (2003), which includes tympanograms, ipsilateral and contralateral middle ear reflexes, and OAEs can be used as a quick and objective means of screening for AN/AD and other types of hearing loss. Normal tympanometry results with absent MEMRs, and one or more OAEs (even of low amplitude) may be an indication of AN/AD, and an abbreviated suprathreshold ABR should ensue to aid in proper diagnosis of the type of hearing loss.
Auditory Neuropathy/Auditory Dys-Synchrony presents distinct management challenges compared to other types of hearing impairments (Madden, C. et al, 2002). For individuals with AN/AD, hearing aids have been shown to provide little to no benefit (Raveh, E. et al., 2007), while hearing aids are often very beneficial for individuals with sensorineural type hearing losses. Due to insufficient outcomes of hearing aid use with individuals with AN/AD, frequency modulated (FM) systems are often recommended in lieu of hearing aids. FM systems assist in overcoming the difficulty individuals with AN/AD often experience with understanding speech in the presence of background noise. If FM systems do not provide sufficient benefit, FM system use in combination with low-gain amplification from hearing aids is recommended. Finally, if no benefit, in terms of speech and language development, is achieved from the hearing aid/FM system combination, cochlear implantation is advised, regardless of degree of hearing loss. Generally, cochlear implantation is only recommended in severe to profound cases of sensorineural hearing impairment, where no beneficial use of residual hearing can be obtained through the utilization of hearing aids. Nevertheless, cochlear implantation is a treatment option for individuals with severe AN/AD and recent cochlear implantation of children with AN/AD has shown favorable outcomes (Madden, C. et al., 2002).
Treatment of AN/AD should be managed on a case-by-case basis due to varying audiological factors; consequently, several avenues of rehabilitation have been undertaken for the 12 subjects involved in the current study. Two subjects use FM systems alone, 4 use low gain, digital hearing aids with high quality noise reduction technology, 3 use HA/FM system combinations, and 2 have received cochlear implants. Additionally, in order to achieve maximum development of speech and language skills, 5 subjects receive auditory training and speech-language therapy with their respective hearing devices. These children are currently under continued management and progress for speech and language development will determine the next rehabilitation step.
In conclusion, management for AN/AD and sensorineural hearing loss are markedly different, therefore accurately differentiating between the two hearing disorders is critical to ensure that individuals receive maximum benefit from proposed treatment plans. Clinical manifestations including hearing loss, poor speech discrimination especially in noise, absent middle ear muscle reflexes, normal tympanograms, and present, partially present, or absent OAEs requires the use of condensation and rarefaction clicks to visualize the CM and accurately diagnose or rule out AN/AD.
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