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Auditory Neuropathy in Romanian Hearing Impaired Children

 

Simona Serban, Bogdan Mocanu
Institute of Phonoaudiology and ENT Functional Surgery, Bucharest, Romania

 

 

Correspondence:

Simona Serban, MD., PhD
Institute of Phonoaudiology and ENT Functional Surgery
Mihai Cioranu Street 21, Sector 5, Bucharest, Romania
telefon: 021 410 21 70;
fax: 021 410 02 78
e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Abstract

Aim: The goal of the study was to establish the prevalence of auditory neuropathy in a group of children with confirmed hearing impairment ranging from borderline normal hearing to profound hearing loss. This is the first report on the incidence of auditory neuropathy amongst hearing impaired children in Romania.

Material and method: In a retrospective study design involving hearing impaired children attending our clinic, subjects were examined for signs of auditory neuropathy. Sixty-one children with confirmed hearing impairment, aged between 4 to 30 months, tested over a period of 18 months were enrolled. The objective hearing assessment protocol included auditory brainstem responses, auditory steady-state responses and otoacoustic emissions.

Results: Eleven percent (7/61) of the children examined presented with signs of auditory neuropathy. The mean diagnostic age was 16.5 months. The identified subjects presented with variable degrees of hearing loss ranging from borderline normal hearing to profound hearing loss. The risk factors normally associated with this condition were encountered in four of the cases. Genetic disorder with recessive transmission was also considered a possible cause in three cases.

Conclusions: Our data showed a relatively high incidence rate (11%) of auditory neuropathy amongst the children who have not been previously screened for hearing impairment. Pediatric auditory neuropathy cases are likely to increase further with implementation of universal neonatal hearing screening. Auditory neuropathy is a hearing disorder with a variable audiological presentation and as such the therapeutic management of affected patients needs an individual approach and consideration by a clinical team.

Key words: hearing disorder, objective assessment protocol, auditory neuropathy, prevalence 

 

Introduction

Auditory neuropathy (AN) is a disorder that affects the neural processing of auditory stimuli with various consequences on speech perception and ability to detect sounds. This condition coined by Starr and colleagues in 1996 [1] is more of a concept describing a series of audiological signs for its identification. Based on the finding of normal otoacoustic emissions, the outer hair cells in the cochlea are presumed to be normal. Gibson and Sanli [2] investigated 39 children with auditory neuropathy and concluded that the most common cause of this disorder is the selective loss of inner hair cells. Different audiological patterns can be found but the hallmark of auditory neuropathy remains grossly abnormal or absent auditory brainstem responses (ABRs) with presence of cohlear microphonics (CMs). In June 2008, a panel of experts met in Como, Italy at the NHS 2008 Conference to develop Guidelines for the Identification and Management of Infants and Young Children with Auditory Neuropathy. The panel participants recommended the use of auditory neuropathy spectrum disorder (ANSD) to describe the disorder characterized by evidence of normal cochlear outer hair cell (sensory) function and abnormal auditory nerve function. The true prevalence of AN is not known. Some authors [3] estimated that 1 in 20 hearing impaired children have auditory neuropathy, while others [4] considered that 10% of the children with permanent hearing loss suffered from this disorder. The vast majority of patients had significant perinatal risk factors (hypoxia, hyperbilirubinemia, ischemia etc.). A genetic factor is also suspected, particularly in the gipsy population [5].

Approximately one third of patients with AN will ultimately demonstrate loss of outer hair cells function to develop a common sensorineural hearing loss. Thus, OAEs, CMs, and ABRs must be tested early in life to recognize cases of AN. The abnormal ABR may recover if the problem is due to maturation. In such situations recovery may return by 12-18 months. If not another cause of AN can be considered. In some cases perceptual ability seems to improve although ABR remains altered. Otoacoustic emissions which are present at initial assessment may disappear, whether or not the child is aided. The management of auditory neuropathy is still debatable. Some clinicians recommend a trial period with hearing aids coupled with radio frequency modulated (FM) systems, others recommend a cochlear implant from the very beginning.

Material and methods

The purpose of this retrospective study was to identify the cases of auditory neuropathy in a group of Romanian children with bilateral hearing loss. Sixty one children (30 boys and 31 girls) aged from 4 to 30 months (mean = 16.5; standard deviation, SD = 6.64) were investigated over a period of 16 months, between 2007 and 2009. The children were referred to our department for suspicion of hearing loss. None of them had undergone neonatal screening for hearing impairment prior to being discharged from the maternity ward. In Romania a pilot universal neonatal screening program is currently running several maternity units nationwide, however not in the regions represented by the children enrolled in our study. The hearing test protocol used in this study included auditory brainstem responses (ABRs) with both rarefaction and condensation clicks of 21.1 c/sec., auditory steady state responses (ASSRs) and transient evoked otoacoustic emissions (TEOAEs). The status of the middle ear was assessed by tympanometry.

Objective tests were performed during spontaneous sleep or with light sedation. Behavioural observational audiometry (BOA) was also included in the test battery employing standard conditioning and visual response-reward methods. The sound stimuli were presented via intra-aural transducers for both types of audiometry. The equipment used to record the evoked potentials included the Eclipse 25 (Interacoustics) and the Audera (Grason Stadler). The degree of hearing loss in this group were variable: 1,6 % mild hearing loss, 8,1 % moderate hearing loss 32,7 % severe hearing loss and 57,3% profound hearing loss.

Case reports

Patient 1

A premature male infant born at 25 weeks gestation with a medical history of intra-ventricular hemorrhage and assisted ventilation for 6 weeks postnatal was brought to our department at 4 months of corrected developmental age following reports of lack of response to the mother’s voice. The objective tests showed abnormal ABR recordings ( fig. 1), without wave V at maximum intensity level but present cochlear microphonics in both ears. Otoacoustic emissions were recorded bilaterally for the frequencies between 1 and 4 kHz inclusively with waveform reproducibility in at least 3 octave bands of more than 75%. ASSR recordings on both sides suggested estimated thresholds of : 95 dB HL at 2 kHz and 75 dB HL at 4 kHz in the right ear and 75 dB HL at 1 kHz, 95 dB HL at 2 kHz and 75 dB HL at 4 kHz in the left ear. Behavioral observation audiometry elicited threshold responses to sound levels of 70 dB HL between 1 and 3 kHz inclusively. Tympanometry revealed type A tympanograms bilaterally. The child was fitted with digital hearing aids. Referral for neurological assessment of other potential health issues was made.

audio1

Figure 1. ABR recordings in response to 100 dB nHL rarefaction and condensation clicks from patient 1. Cochlear micropfonics present during the first 5 msec. of recording. No activity recorded with clamped tube.

Patient 2

A premature male infant, born at 32 weeks from a twin pregnancy complicated with the death of the other twin, was brought to our clinic at the developmentally corrected age of 16 months due to inconsistent reactions to sounds. Objective tests showed: cochlear microphonics present with no evidence of neural response at maximum intensity ( fig. 2) and absence of ASSR responses on both sides. The otoacoustic emissions were present in both ears. Tympanometry revealed type A tympanograms bilaterally. Behavioral observation audiometry with insert phones indicated bilateral severe hearing loss. The 4 months trial period with digital hearing aids following the evaluation did not provide any improvement of speech development and therefore the patient underwent a cochlear implant at age 22 months. Intraoperative neural response telemetry recordings resulted in no response from the auditory nerve to electrical stimulation. Six months after the implantation, speech development was still poor.

audio2

Figure 2. ABR recordings in response to 100 dB nHL rarefaction and condensation clicks from patient 2. Cochlear micropfonics present during the first 5 msec.

Patient 3

A male child born prematurely at 29 weeks was brought to our clinic around the age of 14 months (corrected age) due to absence of child’s reactions to sounds. The child had retinal detachment at birth. Severely decreased visual acuity and important neuropsychological retardation were identified upon later examination. ABR showed poor responses with cochlear microphonics present in both ears ( fig. 3). No auditory steady state responses were recorded. Otoacoustic emissions were recorded only in the right ear with waveform reproducibility of more than 75% in one octave band. Type A tympanograms were present bilaterally. Behavioral observation audiometry showed profound bilateral hearing loss. The family was counseled to fit the child with powerful hearing aids and take him to a specialized center for patients with multiple handicaps.

audio3 audio4
Figure 3. ABR recordings to rarefaction and condensation clicks from patient 3. Cochlear microphonics present up to 3 msec. after click stimulation and probable small wave V at 100 dB nHL in the right ear Figure 4. ABR recordings to rarefaction and condensation clicks from patient 4. Cochlear microphonics present up to 4 msec. after click stimulation

Patient 4

A premature female infant born at 28 weeks gestation, with evidence moderate motor development delay was referred to the clinic for further assessment at 16 months of age due to auditory reaction to environmental sounds only, with no response observed to voices. ABR tracing without wave V at maximum intensity and consistent cochlear microphonics were recorded in both ears ( fig. 4). No steady state responses or otoacoustic emissions were recorded. The middle ears were normal. Behavioral observation audiometry using insert phones showed a mild hearing loss in the left ear and a moderate hearing loss in the right ear. The initial recommendation was a digital hearing aid for the right ear and auditory verbal rehabilitation under attentive monitoring of child’s performances.

Patient 5

A female child born at full term, with no perinatal risk factors or relatives suffering from hearing loss, was brought to our clinic at the age of 25 months because the family suspected a hearing problem. The objective evaluation showed no reproducible ABR traces with cochlear microphonics on both sides ( fig. 5). Type A tympanograms were recorded. Otoacoustic emissions and ASSR responses were absent. The conditioned audiometry with visual reinforcement showed profound bilateral hearing loss with absent responses above 1 kHz bilaterally. The patient is currently being considered for cochlear implantation.

Patient 6

A female child born at full term, without risk factors for hearing loss, coming from gipsy family, was brought to our department at the age of 36 months, with suspected hearing loss. Auditory tests showed ABR using click stimuli with cochlear microphonics noted in both ears ( fig. 6). OAE and ASSR were absent in each ear. Type A tympanograms were recorded bilaterally. Conditioned audiometry with visual reinforcement showed severe hearing loss in the right ear and profound hearing loss in the left ear. Our recommendation was a bilateral hearing aid trial with close monitoring of the child’s speech and language development over time. Unfortunately, the family abandoned the medical care of our clinic.

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Figure 5. ABR recordings to rarefaction and condensation clicks from patient 5. Cochlear microphonics elicited by 100 dB nHL stimuli Figure 6. ABR recordings to rarefaction and condensation clicks from patient 6. No activity recorded with clamped tube

Patient 7

A male child, born at full term, without perinatal risk factors, coming from gipsy family, was brought for audiological assessment at the age of 24 months because of the child’s delay in speech production. ABR traces showed wave V present up to 40 dBHL. ABR recordings evoked by clicks with reverse polarity showed waves III and V ( fig. 7) but also consistent cochlear microphonics during the first 3 milliseconds after click stimulation. ASSR estimated thresholds between 15 – 25 dB HL from 1 to 4 kHz in each ear. Behavioral observation audiometry elicited clear responses at intensities of 40 dB HL and inconsistent responses at lower intensities (30 dB HL). The child was referred to the neurology department for further investigations.

audio7

Figure 7. ABR recordings to 100 dB nHL rarefaction and condensation clicks from patient 7. Waves III and V are present. Electrical activity reverses in phase with changes in the stimulus polarity up to 3 msec. after click stimulation

Discussion

Reports about auditory neuropathy prevalence are extremely variable, probably because of differences in definitions and baseline population. The purpose of the present study was to establish the occurrence rate of the auditory neuropathy in a Romanian pediatric population with hearing loss. This is the first Romanian report about this condition in hearing impaired children. Our data shows that the incidence of auditory neuropathy in hearing impaired children is not negligible (7/61). In comparison with literature findings on prevalence of auditory neuropathy in children who did not benefit from hearing screening at birth [6], our results indicate a higher incidence of this disorder amongst hearing impaired children. Prematurity was the factor that caused auditory neuropathy in most of the cases (patients 1, 2, 3 and 4). Sometimes severe neurological complications coexisted. No underlying causes were identified in three cases. A genetic factor with recessive transmission may be incriminated in these cases. The severity of the hearing loss was variable, ranging from borderline hearing to profound hearing loss. The objective assessment protocol included ASSR technique in addition to click stimuli ABR with reversed stimulus polarity. Currently ASSR is not used for diagnosis of AN but can be used in addition to ABR especially in the event no ABR thresholds can be recorded or if other tests, such as electrical ABR, are not available. In comparison to click ABR technique, ASSR measurement requires less neural synchrony across a large number of nerve fibers.

The first six cases presented in our report demonstrated either absent neural responses during ABR measures, or poor neural responses showing elevated thresholds or absent waves even at maximal stimulation intensity. Auditory steady state responses were obtained in only two subjects, cases 1 and 7, supporting the hypothesis of only partial neural dys-synchrony. Transient otoacoustic emissions were found in three subjects and in only five ears (cases 1, 2 and 3). In case 7, borderline hearing levels are most likely as threshold testing through behavioral audiometry elicited reactions to stimuli at 30 dB HL, albeit being inconsistent. The audiological pattern was also particular due to presence of waves III and V on ABR traces. Cochlear microphonics were recorded during the first 3 milliseconds following stimulation from both ears. Case 7 shall be reviewed in the near future and monitored in order to distinguish between a permanent neural abnormality from a transient auditory neuropathy although the recovery process should be finished by the age of 18 months.

Conclusions

The study shows that the prevalence of auditory neuropathy amongst Romanian hearing impaired children who have not been screened at birth is 11% (7/61). Pediatric auditory neuropathy cases are likely to increase further after implementation of universal neonatal hearing screening. Auditory neuropathy is a condition with a variable audiological presentation and establishing a diagnosis is sometimes a real challenge. The therapeutic management needs individual approach. Children diagnosed with this disorder must be carefully monitored by serial clinical and audiometric evaluations, while families should have realistic expectations regarding the patient’s development, even if a cochlear implant is considered.

 

 

REFERENCES

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