Portuguese Version

Year:  2003  Vol. 69   Ed. 2 - (22º)

Relato de Caso

Pages: 283 to 288

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Auditory neuropathy: a study case

Author(s): Viviane M. Parra[1],
Carla G. Matas[2],
Ivone F. Neves[3]

Keywords: auditory neuropathy, hearing, hearing tests.

Abstract:
Nowadays, in our clinic, we can complement the conventional audiometry results with electrophysiologic exams. Not only does allow us to diagnose a peripheral hearing loss but also to differentiate cochlear, neural or central hearing loss. It led us to a new group of patients that presented changes in neural synchrony, with normal functions of the outer ear hair cells. This pathology is known as Auditory Neuropathy. Our study purpose is describe the audiologist characteristic of a patient with auditory neuropathy attended at "Centro de Docência e Pesquisa em Fonoaudiologia da Faculdade de Medicina da Universidade de São Paulo", correlating the study data with literature. The patient described is a 24 year-old person, with auditory neuropathy diagnosis, evaluated in our center in 2001. Interview, tonal audiometry thresholds, speech recognition, timpanometry, otoacustic emissions (OAEs) and auditory brainstem responses (ABR) were applied. In this case we observed incompatible results in the tonal audiometric threshold and speech recognition tests, with objectives tests like OAEs and ABR, in which we found a hearing loss with important changes in the speech recognition test, of OAEs present and abnormal ABR. These data suggest a normal cochlear function and abnormal neural synchrony.

Introduction

An adult subject came to the Clinic of Audiology, Teaching and Research Center in Speech and Language Pathology and Audiology, FMUSP, to undergo audiological assessment, but the fact that the results were incompatible between the pure tone audiometry, speech discrimination, evoked otoacoustic emissions (EOA) and Auditory Brainstem Response (ABR) warned us to the fact that such subject presented an Auditory neuropathy.

According to Matas et al. (1998), ABR is an objective hearing test that assesses the integrity of the auditory pathway. The procedure does not intend to replace other audiological routine procedures, but rather complement them for the diagnosis of hearing loss.

The waves found in the ABR tracing are generated by one or more structures along the auditory pathway. One of the most used classifications is by Möller, Jannetta, Bennett & Möller (1981), in which there is the description of the generating sites: wave I - distal portion of the auditory nerve brainstem, wave II - proximal portion; wave III - cochlear nerve; wave IV - superior olivary complex; wave V - lateral lemniscus; wave VI - inferior colliculus; wave VII - medial geniculate body.

EOAs are sub-audible sounds generated in the normal cochlea and collected in the external acoustic canal. The sounds are generated based on the normal cochlear vibration (outer hair cells) spontaneously or after sound stimulation, are efferently retransmitted through the tympanic-ossicle system and captured by the external acoustic canal.

According to Lopes Filho (1997), they studied two basic types of EOAs, the spontaneous and the evoked otoacoustic emissions. Spontaneous EOA are narrow band accumulated acoustic energy, coming from the cochlea, with no specific stimulation. TEOA are responses obtained from brief cochlear stimulation, using clicks that are transient short time stimuli of wide frequency range. Distortion product otoacoustic emissions (DPOAE) are response obtained in the external acoustic canal that originate from the cochlea by the non-linear interaction of two pure tones simultaneously applied.

It is important to bear in mind that EOA allow us to check functional cochlear status, but there is no direct correlation with physiological thresholds. Owens et al. (1993) stated that any abnormality in transmission of the sound energy could result in reduction or absence of responses compromising the analysis of the test. Therefore, the acoustic immitance measures are indispensable for the diagnosis, excluding conductive impairments that can affect the result of the test.

In the past, the diagnosis of hearing loss was based on results obtained in the ABR. After the introduction of the EOAs, complementing the battery of audiological exams, there was the possibility of identifying a new group of patients that presented abnormal neural synchrony with normal function of outer hair cells. This pathology was known as auditory neuropathy.

Hood (1998a) defined auditory neuropathy as a term used to describe conditions that are found in patients of all ages, adults and children, with hearing level compatible with the normal cochlear function and affection of the neural synchrony. Such characteristics are observed in audiological tests with normal OAE and absent or severely impaired ABR.

The purpose of the present study was to describe the audiological characteristics of a patient with auditory neuropathy seen at the Teaching and Research Center in Speech and Language Pathology and Audiology, FMUSP, correlating clinical findings and literature data.

Material AND METHOD

The case described involved a 24-year old adult who was diagnosed with auditory neuropathy, seen at the Teaching and Research Center in Speech and Language Pathology and Audiology, FMUSP.
The patient was initially submitted to an interview so that we could gather information about auditory difficulties, history of hearing loss and impact of hearing loss in quality of life and psychosocial aspects.
The audiological assessment conducted in a soundproof booth encompassed the following procedures:

 Conventional Audiometry, as advocated by Mangabeira Albernaz et al. (1981), using audiometer Model GSI 16, brand Grason-Stadler.
 Acoustic Immitance Measures, conducted with immitanciometer Model GSI-33, brand Grason-Stadler.
 Transient otoacoustic emissions, conducted with Cochlear Emission Analyzer Otodynamics, ILO 88 - Version 4.2, which included recording parameters according to what was proposed by Kemp (1990) and distortion product otoacoustic emissions, with ILO 92 device.

After audiological assessment, we conducted the electrophysiological assessment employing ABR, using the device Model Spirit, brand Nicolet.

Results

History
The complaint presented by the patient was difficulty to understand speech. The patient referred he could hear the words but was not capable of understanding them in other situations, such as for example the conversation between 2 subjects, radio, television, telephone and especially in competitive noise.
According to him, the difficulty to understand speech was progressive, as of the age of 14 years, when he had a head trauma.

Pure Tone Audiometry and Vocal Discrimination
In the pure tone audiometry, there was mild to moderately severe sensorineural loss on the left and mild to severe on the right ear.

The patient had difficulty to understand vocal audiometry, presenting incompatible responses with pure tone audiometry in the Speech Recognition thresholds (SRT) and low Speech Recognition Index (SRI) in both ears.

Speech Discrimination
RE LE
SRTMasking LE 85 dB 55 dB
IPRF(two-syllable) 4% - 90 dB 8% - 90 dB

Acoustic Immitance
Tympanometry: Normal bilateral tympanometric curves.
Acoustic Reflexes: contra and ipsilateral stapedial reflexes were absent bilaterally.

Transient and distortion product otoacoustic emissions
We detected presence of transient and distortion product otoacoustic emissions bilaterally.

Auditory Brainstem Response (ABR)
In the ABR, there was absence of Waves I, III and V at 100dB clicks for the right ear and presence of waves I and III at 100dB for clicks on the left, with absolute latency and increased interpeak intervals.

Temporal Bone Computed Tomography
The patient was submitted to ENT assessment including temporal bone CT scan. We found the following results:
 Normal pneumatization of mastoids;
 External auditory canal, antrum, ossicle chain, cochlea, vestibule, semi-circular canals, and internal auditory canal without affection bilaterally.
 Conclusion: normal exam.

Neurological Examination
 Consciousness level: awaken, conscious, normal temporal and spatial orientation, normal self and external psychological understanding;
 Muscle strength: level V, global and symmetrical;
 Deep reflexes: grade II, symmetrical;
 Superficial reflexes: present and normal;
 Cerebellar tests: within normal range;
 Gait and balance: no abnormalities;
 Cranial nerves: bilateral hearing loss, more marked on the right. Remaining cranial nerves were unaffected.

Figure 1. Transient otoacoustic emissions, patient J.R.



Figure 2. Distortion product otoacoustic emissions, patient J.R.



Figure 3. Auditory Brainstem Response, patient J.R.

Discussion

Considering the results described above, we observed that it was a moderate sensorineural loss on the right and a severe loss on the left, with incompatible results between pure tone audiometry and vocal audiometry, since he had a poor performance in speech intelligibility test. Another important piece of data was the incompatibility in the results obtained by conventional audiological assessment (audiometry and acoustic immitance measures) and those obtained in hearing objective tests (EOA present and abnormal ABR).

Hood (1999) stated the importance of having objective audiological tests for the differential diagnosis of hearing loss and that when we combine pure tone audiometry and vocal discrimination they are more effective, especially for the diagnosis of auditory neuropathy. Therefore, she listed the following tests:

Acoustic Immitance
The response depends on the middle ear, afferent function of the 8th cranial nerve and efferent activation of the facial nerve. Patients with auditory neuropathy normally present normal tympanometry curve and absence of stapedial acoustic reflexes.

Transient otoacoustic emissions and Suspension Effect
The presence of TEOA responses are associated with normal cochlear function. The effect of the suspension occurs as a result of the activation of the descending medial efferent pathways of the olivocochlear bundle. Patients with auditory neuropathy have TOAE responses that are present and absence of suppression.

Auditory Brainstem Response (ABR)
It assesses the auditory part of the 8th cranial nerve up to the lateral lemniscus - brainstem. Patients with auditory neuropathy present absent or severely impaired ABR responses.

Hood (1999) described cases of auditory neuropathy in order to show the importance of having electrophysiological tests for its diagnosis. One of the cases diagnosed as having auditory neuropathy was of an adolescent with pure tone audiometry indicative of sensorineural hearing loss of moderate degree in high frequencies, poor speech recognition and absence in case of competitive noise, normal tympanometry, ipsi and contralateral reflexes were absent., present TEOA, absent suppression effect, absent ABR responses, presence of cochlear microphonics (inversion of tracing with change of stimulus polarity), absent middle latency potential and normal cortical responses.

In another study, Hood (1998b) reported a case of a child aged 3 years in which they observed bilateral normal EOA and abnormal ABR, confirming the diagnosis of auditory neuropathy.

Starr et al. (1996) studied auditory skills of patients with auditory neuropathy and reported that in 10 patients (5 adults and 5 children) they all presented pure tone hearing loss ranging from mild to severe, with speech intelligibility tests affected, absent or severely abnormal ABR results and normal EOA.

The results found by Starr et al. (1996) and Hood (1988a, 1998b, 1999) were similar to those obtained in the present study, since they all had the basic characteristics of incompatibility of results, including low index of speech recognition, incompatible with pure tone audiometry, presence of EOA and ABR severely impaired.

Conclusion

We observed in the studied case that there was incompatibility of results between pure tone audiometry and speech intelligibility tests and objective tests (EOA and ABR), presenting auditory hearing loss with significant impairment of speech intelligibility tests, present SOAE and abnormal ABR. Such data suggested normal cochlear function and abnormal neural synchrony.

This emphasizes the importance of the conduction of a complete battery of audiological tests, which enables the diagnosis not only of peripheral hearing loss, but also the differentiation between cochlear, neural and central hearing loss.

We tried to described in the present study the results found in the audiological and electrophysiological assessment of a subject with auditory neuropathy, correlating the studies already conducted in order to come to a more effective diagnosis of a disease that is still very little known.

References

1. Hood LJ. Auditory Neuropathy: What is it and what can we do about it? The Hearing Journal 1998a;51(8):10-7.
2. Hood LJ. Update on Clinical Application of the Auditory Brainstem Response and Otoacoustic Emissions. New Orleans, L.A., agosto 1998b. Disponível em: http:/ www. LHood@LSUMC.EDU.
3. Hood LJ. A Review of Objective Methods of Evaluating Auditory Neural Pathways. The Laryngoscope 1999;109:1745-8.
4. Lopes Filho O & Carlos RC. Emissões Otoacústicas. In: Campiotto AR, LevyC, Holzhein D, Vicente LCC, Castiglioni M, Redondo MC, Anelli W. Tratado do Fonoaudiologia. Brasil: ed. Roca; 1997. p.222-37.
5. Mangabeira Albernaz P, Mangabeira Albernaz PL, Mangabeira Albernaz LG, Mangabeira Albernaz Fo P. Otorrinolaringologia Prática. 10ª ed. São Paulo: Sarvier; 1981.
6. Matas CG, Frazz MM, Munhoz MSL. Aplicação do Potencial Auditivo de Tronco Encefálico em Audiologia Pediátrica. In: Basseto MCA, Brock R, Wajntein R. Neonatologia. Um Convite à Atuação Fonoaudiológica. São Paulo:Lovise; 1998. p. 301-10.
7. Möller AR, Jannetta P, Bennett M, Möller MB. Intracranially Recorded Responses from Human Auditory Nerve: New Insights Into the Oringin of Brainstem Evoked Potencials. Eletroencephalography and Clinical Neurophysiology 1981;52:18-27.
8. Owens JJ, Mccoy MJ, Lonsbury-Mrtin BL, Martin GK. Otoacustic Emissions in Children with Normal Ears, Middle Ear Dysfunction, and Ventilating Tubes. Am J Otol 1993;14(1):34-40.
9. Starr A, Picton TW, Sininger Y, Hood LJ, Berlin CI. Auditory Neuropathy. Brain 1996;119:741-58.

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1 Specialization in Clinical Audiology under course, FMUSP.
Ph.D., 2 Professor of Speech and Language Pathology and Audiology, FMUSP.
3 Master studies under course, Speech and Language Pathology and Audiology, FMUSP.
Affiliation: Speech and Language Pathology and Audiology, Medical School, University of São Paulo.
Address correspondence to: Viviane M. Parra - R. Aimberê, 311 Perdizes São Paulo SP 05018-010
Tel (55 11) 3872-9662 - E-mail: viviane.parra@ig.com.br or carlagmatas@globo.com.br or ifneves@usp.br
Article submitted on December 26, 2001. Article accepted on August 26, 2002.

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