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ANATOMY AND PHYSIOLOGY OF HEARINGAUDITORY NEUROPATHY SPECTRUM DISORDER AND AUDITORY PROCESSING DISORDER
16. AUDITORY NEUROPATHY SPECTRUM DISORDER AND AUDITORY
PROCESSING DISORDER
Auditory Neuropathy Spectrum Disorder
Auditory neuropathy spectrum disorder (ANSD) is a phenotypically diverse group of hearing disorders that can aect both children and adults. It is characterised by normal outer hair
cell (OHC) function but impaired neural transmission resulting in disruption of coding of
acoustic signals in the auditory system.
Clinical Features
One of the key features of ANSD is hearing diculties greater than would be expected
from behavioral audiometry. e hearing diculties are worse with speech compared with
environmental sounds and are particularly aected by background noise. In children, this
adversely aects speech and language development.
e test battery for ANSD is shown in Table 16.1. ANSD is typied by the following:
1 Speech tests (in quiet and noise) worse than expected from audiometry
2 Presence of OHC function as measured with cochlear microphonics (CM) and
otoacoustic emissions (OAEs)
3 Absence or abnormality of the auditory brainstem response (ABR)
Table 16.1 Audiological test battery for ANSD
Test type Notes
Behavioral audiometry Age-appropriate testing of hearing
thresholds, e.g. visual response
audiometry or PTA
Speech discrimination
tests
Speech in noise tests For example, quick speech in noise
Objective measure of
middle ear function
Objective measure of
cochlear OHC function
Objective measure of
middle ear reex
Electrophysiological
tests
Note: ABR, auditory brainstem response; ANSD, auditory neuropathy spectrum disorder; CI,
cochlear implantation, CM, cochlear microphonics; OAE, otoacoustic emission; OHC, outer hair cell;
PTA, pure-tone audiogram.
88 e Ear
Speech recognition threshold (SRT)
Word recognition score (WRS)
test or hearing in noise test
Tympanogram Usually normal
CM or OAEs Normal OHC function, though
Stapedial reex thresholds Absent stapedial reex in ANSD
ABR Absent or grossly abnormal in
Auditory late response May help guide assessment of
Normal to profound hearing loss
Worse than expected from
behavioral audiometry
Likely markedly impaired
this can worsen with time
If stapedial reex is present but
ABR is absent, this suggests
brainstem neuropathy due to
lesion higher than superior
olivary complex
ANSD
level of hearing impairment;
guide amplication/CI by
serving as marker for auditory
cortical development
AUDITORY NEUROPATHY SPECTRUM DISORDER AND AUDITORY PROCESSING DISORDER
15. TINNITUS AND HYPERACUSIS
ANSD is therefore dierentiated from typical sensorineural hearing loss, in which both OAE
and ABR responses are aected, and pure-tone audiogram (PTA) is usually in keeping with
the level of functional hearing.
Comorbidities such as developmental delays, attention decit hyperactivity disorder
(ADHD), autism spectrum disorders, visual problems, and motor disorders are reported in
up to 54% of children with ANSD.
Epidemiology
ANSD can be congenital or acquired and present at any age. e prevalence of ANSD varies
between 1 and 10% in the general hearing-impaired population, and up to 40% in hearingimpaired patients with a history of admission to the neonatal intensive care unit. e prevalence of
ANSD in a well-baby population is estimated at 1 in 7000. It is bilateral in 75% of cases.
Pathophysiology
ANSD can arise from abnormalities of electromechanical transduction at the inner hair cell
and axons, cell bodies, and myelin sheaths of the auditory nerve.
is results in disruption of temporal synchrony of the auditory neural signals as well as a
reduction of amplitude of neural signals.
Risk Factors for ANSD
A wide range of age-dependent risk factors have been implicated in the development of
ANSD. Perinatal factors including prematurity, hyperbilirubinaemia, respiratory distress
syndrome/neonatal mechanical ventilation, ototoxic drugs, and cerebral palsy are the most
common associations of ANSD in children. A number of genetic mutations causing ANSD
have been identied. Mutations of the otoferl in, pejvakin, and connex in 26 gene are examples .
ANSD can occur in Usher’s syndrome and may be associated with hereditary demyelinating neuropathies such as Charcot-Marie-Tooth syndrome and Friedreich’s ataxia. Acquired
causes of ANSD include multiple sclerosis, autoimmune disease, and hemosiderosis.
Diagnostic Approach
In children, perinatal history and developmental milestone attainment are important considerations, while in adults, family and drug history are key. Clinical examination includes
assessment of developmental milestones, peripheral nerve sensation, cerebellar signs, and
funduscopy. Vestibular assessment and/or testing should be considered as should referral for formal ophthalmological assessment. e audiological test battery is shown in
Table 16.1. Audiometry should be performed on rst-degree relatives. Magnetic resonance
imaging (MRI) of the internal auditory meati and brain is necessary to identify cochlear
nerve hypoplasia/aplasia. Referral to a clinical geneticist should be considered based on
clinical picture and patient consent.
Management of ANSD
A multidisciplinary approach, parental counseling, and timely intervention are crucial.
Broadly speaking, the main strategies involve (1) improving signal-to-noise ratio and (2)
amplication and (3) early language interventions.
Directional microphones, personal frequency modulation (FM) systems, greater support in
classroom, reduction in background noise, and workplace adaptations all play a part.
Judicious hearing aid amplication with close monitoring may be considered once reliable
behaviora l thresholds have been established. However, amplication has been shown to have
variable outcomes, since it can amplify the signal but not overcome the neural transmission
decit. Some studies have also shown a detrimental impact on cochlear OHC function.
Cochlear implantation (CI) can be considered in selected cases with good outcomes.
Implantation can provide consistent neural ring helping to overcome auditory desynchrony
such as in synaptopathy. However, there is some evidence to suggest that CI in patients with
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ANATOMY AND PHYSIOLOGY OF HEARINGAUDITORY NEUROPATHY SPECTRUM DISORDER AND AUDITORY PROCESSING DISORDER
ANSD involving the spiral ganglion and auditory nerve is associated with poorer outcome
compared with pre-synaptic ANSD.
Early language intervention is important in young children with ANSD and will involve input
from speech and language therapy, teachers of the deaf, and signicant parental involvement.
Auditory Processing Disorder
Prevalence and Denition
Approximately 5% of children and 1–10% of adults who present to audiology departments
with complaints of signicant listening diculties in noise or in group conversations have
normal pure-tone thresholds. In a proportion of these patients, listening symptoms are
attributed to functional decits in sound processing within the extended central auditory
nervous system. is clinical presentation is categorised as an auditory processing disorder (APD). APD is a common type of hearing impairment that remains under recognised,
despite its high burden on communication, social, and emotional aspects of life. is clinical
presentation has attracted considerable debate. A recent European consensus proposes that
APD is diagnosed on the basis of the following criteria
3
:
1 Normal audiometric thresholds in both ears
2 Abnormal performance in at least two validated auditory processing tests that assess
dierent processes in at least one ear, including in a non-speech test
3 Presence of listening diculties (Table 16.2) and/or risk factors associated with APD
4 Normal non-verbal intelligence
5 Good ability to follow test instructions
Etiology
In children, as well as in adults, APD may be diagnosed in the background of neurological
disease such as brain tumors, stroke, trauma, prematurity or low birthweight, epilepsy, and
brain infections or demyelinating conditions. APD may also overlap with developmental
disorders such as language impairment, dyslexia, and attention-decit hyperactivity disorder. In such cases, the clinician should carefully consider whether the clinical presentation
is instead due to decits in higher order language or cognitive domains. Another subtype
of APD is the ‘spatial processing disorder (SPD)’ that is attributed to a prolonged history
of chronic otitis media in childhood, giving rise to decits in binaural auditory processing
The Diagnostic Approach
e diagnostic process includes history taking, including patient/teacher/parent questionnaires,
followed by targeted medical examination. Children and adults with APD have speech in noise,
auditory at tention, localis ation of sound, and other aud itory diculties (see Table 16.2). Chi ldren
Table 16.2 Symptoms of APD
Speech understanding difculties In background noise, acoustically challenging/complex
acoustic environments, when speech quality is degraded
Speech discrimination difculties Difculties to repeat or recall similar sounding words
Auditory memory/attention
difculties
Sound localisation/streaming
difculties
Relies on multisensory cues For example, seeking visual/facial cues to better understand
Hyperacusis With or without a diagnosis of autism spectrum disorder
Disproportionate educational/
cognitive/language difculties
Difculties recalling instructions; difculties concentrating in
noise
Difculties identifying the source of a sound; with
separation of auditory foreground from auditory
background
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AUDITORY NEUROPATHY SPECTRUM DISORDER AND AUDITORY PROCESSING DISORDER
Table 16.3 APD test battery (AAA 2010)
Auditory processing domain What the test assesses
Auditory discrimination Ability to differentiate similar acoustic stimuli that differ in
Auditory temporal processing Ability to analyse acoustic events over time
Dichotic listening Ability to separate (i.e. binaural separation) or integrate (i.e.
Low-redundancy speech
recognition (monaural)
Binaural interaction Binaural processes dependent on intensity or time differences
Other tests
Electro-acoustic measures Otoacoustic emissions, acoustic reex thresholds, and acoustic
Electrophysiological measures Auditory brainstem response usually normal (consider
15. TINNITUS AND HYPERACUSIS
frequency, intensity, and/or temporal parameters
binaural integration) disparate auditory stimuli presented to
each ear simultaneously
Recognition of degraded speech stimuli presented to one ear
at a time
of acoustic stimuli
reex decay usually normal
measuring middle latency and auditory late response)
may experience diculties in the classroom and psychosocial diculties. History should also
ascertain risk factors, educational, and professional history, as well as family history of related
disorders (e.g. hearing or neurological disorders). e American Academy of Audiology (2010)
proposes that the central auditory processing test battery should include a number of tests in
addition to PTA and speech-in-quiet tests, as detailed in Table 16.3.
Assessments of other domains such as language and cognition are of paramount importance
as these factors can aect APD test performance. Age-related hearing loss and cognitive
decline is such an example. Assessment of cognitive skills, such as working memory, is therefore useful during assessment.
Management Strategies
e goal for APD rehabilitation is to improve the functional decits of individuals that
impact their communication and well-being. A multidisciplinary team approach should be
employed and should focus on the following areas:
1 Auditory training (AT) to harness brain plasticity and improve neuro-auditory
function (e.g. AT): AT can be a school- or home-based program, as well as therapy
conducted by a speech language therapist or audiologist in the clinic. It involves
predominantly language-based tasks. Examples of informal AT are discriminating
similar sounding notes on a keyboard (temporal or timing skills) and listening to lyrics of songs (speech-in-noise ability). ere are also several commercially available
computer-based AT programs. Post-training improvements are reported on a range of
auditory and non-auditory measures, but long-term benets are unknown.
2 Signal enhancement strategies including environmental modications to reduce the
deleterious eects of noise and reverberation of the acoustic environment: Remote
microphone hearing aids (RMHAs) are personal listening devices that bypass highlevel classroom noise and transmit clearer speech to the child’s ears. A recent metaanalysis presents moderately strong evidence that use of an FM device (a specic type
of RMHA) in the classroom improves children’s listening and attention. ere is casecontrol study evidence that these devices improve speech in noise listening in adults.
3 Teaching children and adults compensatory strategies to overcome functional di-
culties: ese strategies may include ‘active listening’ where the individual is taught
how to take responsibility for their own listening and strategies that aim to enhance
auditory memory/attention. Curriculum modications (e.g. pre-teaching new material, giving breaks to the student during the day) are also widely used.
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ANATOMY AND PHYSIOLOGY OF HEARINGHEARING AIDS AND AUDITORY REHABILITATION
KEY POINTS
• ANSD is a heterogenous group of hearing disorders characterised by normal OHC
function on OAE and CM test but abnormal ABR.
• Both ANSD and APD are characterised by hearing difculties greater than would be
expected from behavioral audiometry.
• Management of ANSD includes improvements in signal-to-noise ratio, amplication,
and early language interventions.
• Management of APD includes improvements in signal-to-noise ratio, AT, and learning
compensatory strategies.
Further Reading
1. Moser, T. and Starr, A. (2016) ‘Auditory neuropathy–neural and synaptic mechanisms’,
Nature Reviews. Neurology, 12(3), pp. 135–149. doi: 10.1038/nrneurol.2016.10.
2. ‘Guidelines for Aetiological Investigation into Auditory Neuropathy Spectrum Disorder
in Children and Young Adults’ (2018). British Association of Audiovestibular Physicians.
https://www.baap.org.uk/uploads/1/1/9/7/119752718/guidelines_for_ansd_
nal_version.pdf.
3. Iliadou, V. V., Ptok, M., Grech, H., Pedersen, E. R., Brechmann, A., Deggouj, N.,
Bamiou, D. E. (2018). European 17 countries consensus endorses more approaches to
APD tha n reported in Wils on 2018. International journal of audiology, 1–2. doi:10.1080/
14992 027.2018.1442937
17. HEARING AIDS AND AUDITORY REHABILITATION
Introduction
Hearing aids partially overcome the decits associated with a hearing loss. ey make audible sounds, and parts of sounds, that would otherwise be inaudible. ey cannot reverse
the reduced resolution with which ears with sensorineural hearing loss analyse incoming
sounds. However, some signal processing algorithms within hearing aids (directional microphones and noise suppression, see later) help compensate for this reduced resolution and the
ensuing reduced understanding of speech in noisy places. Where hearing aids do not fully
meet the needs of patients, additional forms of aural rehabilitation should be considered.
Components of a Hearing Aid
e essential components in hearing aids are a microphone, an amplier with controllable
characteristics, a miniature earphone (called a receiver) to output the amplied signal and a
battery to power the amplier. e ampliers can be controlled so that they amplify signals
at dierent frequencies by dierent amounts so that they amplify so sounds more than they
amplify loud sounds. Frequency-dependent amplication is needed because the amount of
hearing loss usually changes with frequency and because the high-frequency parts of speech
are weaker than the low-frequency parts. Intensity-dependent amplication compensates
for the reduced dynamic range of sounds between threshold and discomfort that inevitably
accompanies sensorineural hearing loss. e rate at which the degree of amplication varies
as the input level varies can be very fast (a few milliseconds) or very slow (a few seconds).
Fast and slow compression have both advantages and disadvantages relative to each other.
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17. HEARING AIDS AND AUDITORY REHABILITATION
HEARING AIDS AND AUDITORY REHABILITATION
Almost all hearing aids use digital signal processing to amplify as it oers more exible
manipulation of the sound, and it is more easily controlled by the computer used to adjust
the hearing aid to an individual patient’s needs.
ese same basic physical components are used in some multifunction, non-professionally
tted, wearable devices, termed hearables. ere is a current merging of these device types,
as the hearables add amplication to their features, and hearing aids add other features such
as fall detection, telephone hands-free operation and step counting.
Most hearing aids also include a wireless receiver so that audio signals can be input to the
device from a mobile phone or other streaming device, a remotely located microphone (such
as worn by a teacher in a classroom) and/or a hearing aid on the other side of the head. e
most common style of hearing aids is behind the ear (BTE), where either the entire hearing
aid, or all the components except the receiver, are positioned between the pinna and the head
surface. ey connect to the ear canal via a sound tube, or via thin wires when the receiver is
in the ear canal. e end of the tube or the receiver are held in place in the ear canal by either
a custom-shaped ear mould, or a compliant tip that deforms to match the cross-sectional
shape of the ear canal. Alternative styles include in the ear, in the canal and completely in
the-canal. While the latter two styles have slight cosmetic advantages over BTE devices,
BTEs can contain directional microphones, which oer performance in noise that the canalstyle devices cannot match. Much less commonly, a contralateral routing of signals (CROS)
hearing aid is used to pick up sounds from the side of the head with a completely deaf ear,
and play an amplied version of it to the other ear.
Hearing Aid Measurement in Couplers and Real Ears
e amplication characteristics of hearing aids can be measured in a standardised way by placing it in a test box, with the hearing aid output connected to a coupler that very approximately
simulates the ear canal acoustic impedance. During tting, amplication characteristics on an
individual are determined by placing a thin probe tube inside the ear canal, and measuring the
sound pressure level in the ear canal with and without the hearing aid present. e increase in
sound pressure level is called the real ear insertion gain. is gain is aected by the way the hearing aid is coupled to the ear canal, including by how open the tting in the ear canal is. Open ttings enable the wearer to perceive their own voice as normal, but limit the frequency range over
which the hearing aid can amplify sounds, and the eectiveness of the noise reduction strategies
in hearing aids. ey are most suited for people with mild or moderate loss.
Directional Microphones
Directional microphones provide more amplication for sounds arriving from broadly in
front of the wearer than for sounds arriving from the side or behind the wearer. ey achieve
this by sensing, and then combining the sounds arriving at two closely located sound ports
on the hearing aid. Directional microphones are the major means by which hearing aids
improve the clarity of the sound, in addition to simply amplifying it. Super-directional
microphones, also known as beamformers, achieve a higher level of directivity by wirelessly
transmitting signals sensed on one side of the head, to the hearing aid on the other side of
the head. Each hearing aid thus has access to signals sensed at four dierent locations on the
head. ey can allow people with moderate hearing loss to hear as well in noisy places as
people with normal hearing.
Signal Processing
Digital technology makes available several signal processing schemes. ese include
Adaptive noise reduction, which makes sound more comfortable by de-emphasising
•
frequency bands that are dominated by noise, rather than by incoming speech
Frequency lowering, which moves high-frequency speech information to slightly
•
lower frequencies, where the wearer has less hearing loss, and thus greater ability to
use the information
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ANATOMY AND PHYSIOLOGY OF HEARINGHEARING AIDS AND AUDITORY REHABILITATION
Feedback canceling, which makes whistling (an oscillation) caused by amplied
•
sound leaking back to the microphone less likely to occur
Expansion, which lowers the hearing aid gain applied to so sounds, to make internal
•
hearing aid noise inaudible, even in quiet places
Candidacy
e prime requirements for a person to benet from hearing aids are that they perceive that
they are having diculty hearing and that they are willing to try hearing aids. ese beliefs are
much more important than the actual degree of hearing loss present (which of course is only one
indicator of the extent to which the hearing and auditory processing system dier from normal).
Prescription
Because hearing loss characteristics vary with hearing loss, so too must the amount of amplication that hearing aids provide. e prescription formula describes how the amplication
should vary with frequency and input level to achieve the aim of that formula. e aim of the
widely used National Acoustic Laboratories’ prescription is to maximise the intelligibility
of speech while keeping the loudness to no more than would be perceived by a person with
normal hearing thresholds listening to the same speech signal. Prescription formulae also
specify the maximum output level that hearing aids should provide at any frequency. Apart
from the gain-frequency response and maximum output, other aspects of the tting that
must be considered when choosing the optimal hearing aid for a patient includes the physical
style and size of the hearing aid, user controls, wireless connectivity and types or strength
of signal processing alternatives. Despite the use of prescription procedures, because of individual dierences, there is oen a need for ne-tuning of the hearing aids aer the user has
worn them for a few weeks.
Binaural Hearing
Hearing in two ears enhances our ability to understand speech in noise, and greatly enhances
our ability to localise sounds. is occurs because the brain is able to take advantage of differences in the level and timing of sounds at the two ears created by the head. Bilateral tting
is thus increasingly important as hearing loss increases, so that audibility is achieved at all
frequencies in both ears. For a minority of people, however, binaural interference causes
speech identication ability to be better when unilaterally aided than when bilaterally aided.
Children
e two major ways that hearing aid tting for babies is dierent from adults is that the small
size of their ear canals must be considered, and protection against ingestion of hearing aid
batteries must be provided. More subtle dierences are that whereas adults need hearing to
use language, children need hearing to learn language. e earlier hearing aids are provided,
preferably well before 6 months of age, the better the child’s language develops. Because
children are still learning language throughout childhood, they need a better signal-to-noise
ratio than adults to understand speech in challenging situations, like classrooms. e best
way to achieve this is when the teacher wears a wireless remote microphone that transmits
clear, non-reverberant signals to the child’s hearing aids.
More detailed information about the topics above can be found in the book Hearing Aids.
Aural Rehabilitation
While some are completely satised with the help provided by hearing aids, others have
needs which cannot be fully met by them. Support for people with hearing loss may be provided by an aural rehabilitation specialist such as a hearing therapist or specialist clinical
psychologist. For many, the experience of losing hearing goes far beyond the frustration of
mishearing speech. It can necessitate changes in lifestyle, both at work and at leisure, make
conversation eortful and tiring and profoundly alter one’s sense of identity. Couples oen
feel that hearing loss places a strain on their relationship. ose having diculty adjusting
94 e Ear
1
17. HEARING AIDS AND AUDITORY REHABILITATION
HEARING AIDS AND AUDITORY REHABILITATION
to life with hearing loss may well benet from an opportunity to discuss their feelings and
explore ways of coping with a rehabilitation specialist. A group in particular need of urgent
referral to rehabilitation services are those who develop sudden hearing loss; they oen
report feelings of utter bewilderment and confusion. In a medical emergency, it is easy for
emotional needs to be neglected, but timely emotional support is just as important as prompt
medical treatment.
Specialist rehabilitative support can also be benecial to people who feel ambivalent about
using hearing aids. ere are many reasons for this, including perceived stigma and lack of
condence. An opportunity to spend time discussing one’s ambivalence in a supportive environment is likely to be more cost-eective than tting hearing aids which remain unused.
Aural Rehab Groups and Lipreading Classes
Some audiology clinics run regular aural rehab groups. ere are many possible formats, but
all have the advantage of bringing people with hearing loss (and sometimes their partners)
together to share experiences and ideas.
Outside the healthcare system, other groups and classes exist for people with hearing loss,
including lipreading classes. ese usually involve a mixture of lipreading exercises and
communication tips (such as asking people to face you and moving away from background
noise). Although eects of class attendance on lipreading ability are equivocal, qualitative
research indicates that people value their classes very highly. Hard-of-hearing groups or
clubs are also available in some areas which oer peer support and an opportunity to take
part in social activities without being restricted by hearing problems.
Auditory Training
Auditory training attempts to improve speech discrimination by presenting a variety of listening tasks involving phonemes, words and sentences. Several computer-based auditory training programs are available, enabling users to practice regularly at home. A systematic review
of 13 computerised auditory training studies2 found evidence that performance on auditory
training tasks improves signicantly with practice. Some (but not all) studies also showed
generalisation of learning to untrained tasks, which of course has more real-world benet.
The Voluntary Sector
Additional support for people with hearing loss and related problems (such as tinnitus and
balance disorders) is provided by charities and voluntary organisations. ese are very
oen an invaluable source of information; many produce fact sheets about a range of topics
and some have telephone and email help lines. ere are several forums available via the
Internet and social media through which people can share information and oer support.
Technology other than Hearing Aids and Cochlear Implants
ere are situations in which many users nd their hearing aids or cochlear implants inadequate. Despite recent advances in digital signal processing, interference from background
noise is still a primary cause of dissatisfaction. Telephone use can also be problematic. People
with more severe hearing loss may nd the television unclear, even when using hearing aids,
and family disputes over TV volume are a frequent source of irritation. Moreover, there are
situations in which most people take their hearing aids or speech processors o (particularly
while bathing or in bed) but still need to be aware of signals such as smoke alarms or alarm
clocks. Additional technology goes some way towards solving these diculties, but awareness of it amongst both patients and clinicians tends to be low. Some of the more common
types of hearing assistance technology is described in Table 17.1.
Using Personal Listening Equipment without Hearing Aids
Many of the devices described can be used with headphones. ese make them accessible
to non–hearing-aid users and to people who need or prefer to be without their hearing aids
temporarily, perhaps due to an ear infection.
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ANATOMY AND PHYSIOLOGY OF HEARINGHEARING AIDS AND AUDITORY REHABILITATION
Table 17.1 Common types of hearing assistance technology
Device Description Additional information
Frequency-modulated
(FM) system
Loop system A looped wire connected to an
Streamer A small device worn around the neck
Amplied phone Landline phone with built-in volume
Pager Unit clipped to clothing vibrates to
Visual alert Bright, ashing light activated by
Receiver connects to user’s hearing
aids by direct audio input or a
neckloop. Transmitter with mic
placed close to any sound source or
clipped to speaker’s clothing.
amplier creates an electromagnetic
eld. Signal is picked up by telecoil
in hearing aids, which must be
activated by audiologist.
enables hearing aids to connect to
Bluetooth
control.
alert wearer to phone, doorbell,
alarm, baby monitor, etc.
doorbell, phone or alarm.
Often used in classrooms and
lecture theatres.
Fitted in many theatres, cinemas
and service counters. Smaller
versions available for home use
and travel.
Can be used with any Bluetooth-
enabled device (mobile phones,
MP3 players, tablets, etc.).
Additional features (e.g. extra-
large buttons) also available.
Under-the pillow vibrating unit
available for nighttime.
May be portable or
wall-mounted.
A personal amplier with headphones can be particularly helpful in hospital. Many hearingimpaired patients on a ward will not be wearing their hearing aids. is sometimes results in
sensitive information being spoken at high volume by hospital sta and being clearly audible
to all those around. Speaking to the patient via a simple amplier can make all the dierence
to condentiality and dignity.
Hearing Dogs
People with severe or profound hearing loss can apply for a hearing dog. Such dogs are
trained to alert their owners and lead them to the source of sounds like doorbells, phones
and timers. ey are also taught a ‘danger’ signal in response to a smoke alarm. Hearing dogs
are identied by a special coat and can accompany their owners in public places. Many deaf
people feel more condent with a hearing dog by their side.
Language Service Professionals
Language service professionals (LSPs) are sometimes employed to enable participation in
meetings, conferences, training courses or court proceedings. Examples of LSPs are sign language interpreters, speech-to-text transcribers and lip speakers (who repeat what a speaker
is saying voicelessly to enable lip reading). However, improvements in voice recognition soware mean that it is becoming increasingly easy to provide real-time transcription without
the need for a third party.
Conclusion
e consequences of hearing loss are far reaching. Modern hearing aids can provide great
benets, but many people with hearing loss also need additional or alternative rehabilitation
services. While it is not the responsibility of otorhinolaryngologists to provide such services,
it is important to be aware of what is available in the local area and to be able to make appropriate referrals and recommendations.
96 e Ear
17. HEARING AIDS AND AUDITORY REHABILITATION
COCHLEAR IMPLANTS AND AUDITORY BRAINSTEM IMPLANTS
KEY POINTS
• Advances in digital technology mean that as well as being able to adjust hearing aids
to meet individual hearing loss characteristics, hearing aids now automatically adapt in
various helpful ways to the environment in which they are being used.
• Super-directional microphones, and streaming of wireless signals originating from
mobile phones, remotely located microphones, televisions and other audio sources are
amongst the advances that enable hearing-impaired people to hear well, even in noisy
places.
• Despite the huge benet that hearing aids can provide, they may not completely
reverse the negative impact of hearing loss on well-being.
• Emotional and behavioural support for people struggling with hearing loss can
be provided individually and in groups by clinicians with specialist training in
rehabilitation.
• Hearing assistance technology and LSPs can help in many situations in which hearing
aids or cochlear implants do not provide adequate benet.
Further Reading
1. Dillon, H. 2012. Hearing Aids. ieme, New York.
2. Henshaw, H. & Ferguson, M. A. 2013. Ecacy of Individual Computer-Based Auditory
Training for People with Hearing Loss: A Systematic Review of the Evidence. PLoS
One, 8.
18. COCHLEAR IMPLANTS AND AUDITORY BRAINSTEM IMPLANTS
Cochlear Implants
Cochlear implants (CIs) are neuroprosthetic devices that directly stimulate the auditory
nerve. is technology has proven revolutionary in restoring hearing to individuals with
severe to profound hearing loss, enabling speech and language development in children, and
improving speech perception in adults.
How a CI works
e main components of CI devices are shown in Figu re 18.1. e electrode array consists
of multiple electrode contacts (between 12 and 22 depending on the manufacturer), each
of which is intended to stimulate a distinct population of auditory neurons. CIs attempt to
mimic natural tonotopic encoding by representing high frequencies at the basal and low
frequencies at the apical end of the array. e array is ideally placed in the lower compartment of the cochlea, the scala tympani, where it lies closer to target auditory neurons. Perimodiolar electrodes are pre-curved and sit closer to target auditory neurons in the modiolus,
while lateral wall electrodes are further from these neurons but are associated with less traumatic insertion.
Sound processing refers to how the acoustic signal is transformed into an electrical stimulus
and varies between device manufacturers. Most strategies decompose the signal into frequency bands and extract the envelope information (slow amplitude uctuations) and use
this information to modulate electrical pulses at corresponding electrode contacts.
e Ear 97
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