DECREASED
SOUND TOLERANCE:
HYPERSENSITIVITY
OF HEARING
(Hyperacusis,
misophonia, phonophobia)
Continuous
and loud noise is a source of irritation to most people. However some people
have especially sensitive hearing and are unable to tolerate ordinary levels of
noise. This can occur in people with normal hearing, or in those with a hearing
loss. There are different components which can contribute to sensitive hearing
hyperacusis, phonophobia and misophonia recruitment. Our knowledge based on the Jastreboff
model and
desensitization techniques developed in the 1980s now allows effective treatment
of hyperacusis and misophonia.
The
mechanisms of hyperacusis, misophonia, phonophobia and recruitment.
Hyperacusis
is due to an alteration in the central processing of sound in the
auditory pathways where there is an abnormally strong reaction from exposure to
moderate sound levels. The cochlea is often completely normal, although patients
frequently wrongly believe it is irreversibly damaged. Traditional teaching
involved only an understanding of 'recruitment' , the result of cochlea damage.
Since all people with hyperacusis can be helped by a behavioural approach with
'sound' therapy, it has become clear that the symptoms cannot be the result of
irreversible ear damage. Another reason for sound sensitivity is misophonia.
This means a dislike of being exposed to a certain sound. Here the auditory
pathways may be functioning normally, but there is an abnormally strong reaction
of the limbic (emotional system) and autonomic nervous system (body control
system) to which the auditory system is intimately connected.
Sometimes
because of the belief that it will damage the ear, or makes symptoms
(sensitivity, or tinnitus) worse. If this dislike is very strong we may call it
'phonophobia' literally - fear of sound.
Often normal environmental sounds like traffic, kitchen sounds, doors
closing, or even loud speech, cannot be tolerated, even though under any
circumstances they cannot be damaging to anyone. In misophonia and
phonophobia certain complex sounds produce discomfort, on the basis of their
meaning or association, while other sounds which are enjoyed (such as music) can
be tolerated at much higher intensity levels. If there is a difference in the
intensity of different sounds which produce discomfort, then it is very likely
that a degree of misophonia exists. Misophonia can lead to hyperacusis (changes
in central auditory processing), and a consequent persistence of abnormal
loudness perception. In practice, most people with decreased sound tolerance
have both hyperacusis and phonophobia / misophonia together in varying
proportions.
In
treating these conditions, it is important to diagnose which condition is
present and which is dominant. A common widespread and largely harmless
expression of misophonia is seen in the fathers of teenage children to 'modern'
music being played (even in the distance) and to the dislike of music 'leaking'
from the headphones of portable cassette players on public transport. In
addition some sounds are inherently unpleasant, like the squeak of chalk on a
slate, even though the number of decibels produced by this is very small. This
is 'cultural' or species specific phonophobia!
Mechanisms
of hyperacusis
The
30,000 fibres in the auditory nerve carry information about the individual
frequencies of each complex sound that we hear. 1/20 of a second later, these
reach the cortex of the hearing part of the brain (in the temporal lobe) where
conscious perception of organized sound occurs. Until the message reaches
consciousness, no sound is heard. During
the passage of this coded signal, it
undergoes
a great deal of processing, similar to a computer, but much more complex.
Figure
1. Nothing
is heard until sound patterns generated in the cochlea, reach the cortex of the
brain
The
central auditory system is first of all concerned with extracting important
messages from unimportant background noise. Often the signal is relatively weak
in strength, but strong in meaning. An example of this would be the detection of
the quiet sound of an approaching predator by an animal living in a hostile
environment. Another example would be the ability to detect the sound of one's
name across a crowded room, while other names, even if spoken quite loudly would
go unnoticed. In the subconscious
part of the auditory system brain, an important signal is detected on the basis
of previously learnt experience. This signal may then be enhanced, or suppressed
by these pathways or filters. These pathways are not inert electrical cables,
but complex neuronal or nerve networks. Patterns
of frequencies of sound are enhanced or suppressed to varying degrees on the
basis of their meaning, but nothing od significance is heard until it is matched
with a pattern in auditory memory. The
strength of this pattern matching dictates the loudness and intrusiveness of the
sound perceived. This can be quite
different from the intensity or energy of the sound in the environment outside.
In
misophonia and phonophobia the connections of the auditory pathways leading from
ear to brain interact strongly with the limbic and autonomic nervous systems to
which they are connected. This is
what creates the feeling of unpleasntness, annoyance or fear in the presence of
certain sounds. Such emotions are
not properties of hearing, or of the auditory pathways. The responses are set up because of experiences, or beliefs
about sound which have been previously learned.
Once
the aversive reaction to sound is set up in this way the limbic / autonomic
response ‘talks back’ to the auditory system at a subconscious level,
programming it for future action. This ‘programming’ results in a)
invariable detection of the unpleasant sound – even when so quiet that others
cant hear it, b) invariable limbic and autonomic reaction causing distress c)
enhancement of auditory processing of this and other sounds leading to
hyperacusis.
The
purpose of this ability to amplify small signals and to suppress others is to
facilitate the detection of potential threats in the environment and is a
natural part of our defense mechanism
Figure
2 Neuronal
networks between ear and brain
detect threatening sounds and activate a reflex response involving
fear/annoyance, and increase of
Very
often the over-sensitivity for sounds is begun by an irrational fear which
nevertheless becomes a very strongly held belief. This is commonly the source of
distress in those who believe that their lives are ruined by environmental noise
from nearby factories,generators or low frequency sounds transmitted through the
ground (which other people may be unable to hear). Because the central auditory
processing mechanism is so powerful, it is possible to "train" it by
constantly listening to, and monitoring small sounds. These weak sounds are then
turned them into very loud intrusive and unpleasant perceptions which
become
constantly
audible whether we like it or not.
Hearing
tests
The
standard 'pure tone audiogram' measures the quietest sound you can hear. You
listen through headphones to sounds from a carefully calibrated instrument
(audiometer), and respond (e.g. by pressing a button) whenever you hear a sound,
however quiet. This is called the threshold of hearing, and measures
whether you have a hearing loss. An equally important, but less frequently used
test measures the upper limit of loudness tolerance (loudness discomfort
levels). You should indicate when the tones become uncomfortable to the ear
(before they become painful). For patients who are frightened of loud sounds
this test must be done very carefully and with proper instruction by a TRT
trained professional. None of the sounds from the audiometer are capable of
damaging the ear, even in a sensitive individual. It is very important to have a
good knowledge of the level of loud sound tolerance when diagnosing and treating
decreased sound tolerance, or when fitting a sound generator or hearing aid to
any patient, whether they have hyperacusis or not.
The
Limbic System, Emotional Responses and Global Hypersensitivity
The
process of developing an increased sensitivity to specific sound always involves
the limbic system and autonomic nervous system. Where phonophobia or misophonia
exists there is an inevitable association of fear or dislike, associated with
the appearance of the sound, whenever it occurs.
The attentional focus becomes filled with that sound, so that
interference with concentration (on another task) occurs. These conditioned
responses act like survival reflexes and have to carry a message of unpleasant
emotion, in order to ensure that a response occurs. They also stimulate the
autonomic nervous system to prepare us for 'flight or fight' so there may be
coincident increases in heart rate, sweating, muscle tension, and other
adrenaline-mediated body responses. Check Figure 2
again.
Treatment
of Hyperacusis
With
hearing loss
Digital
and programmable hearing aids frequently make the task of appropriate hearing
aid fitting easier and more appropriate. In fitting hearing aids to sensitive
ears, it is often best to leave the ear canal as un-occluded as possible,
particularly to begin with.
Avoidance
of silence
Many
people seek silence as a way to escape from the pressures of everyday life.
However complete silence is not found in nature, and should be considered
'unnatural'. Consider living in a nest or animal burrow! In the relative silence
of houses with doubled-glazed windows, often hermetically sealed from the
outside world, the absence of sound stimulation leads to an increase in auditory
gain (amplification) in the subconscious auditory pathways. The brain is always
looking the best way it can for auditory signals.
This process is enhanced by silence which is considered to be one of the
signs of possible predator activity . The auditory filters 'open' in an attempt
to monitor the external sound environment. External sounds may then increase
dramatically in their perceived intensity and intrusiveness. Some people take to
wearing ear plugs, perhaps at night, to avoid sounds becoming intrusive, and
this simply worsens the sensitivity. When
hyperacusis develops there is a great temptation to plug the ear to exclude
unwelcome sounds. This is actually making things worse, as it encourages
further increase in the amplification of sounds on their way to the auditory
(hearing) cortex. When these sounds are heard in the absence of plugs, their
perceived loudness is greatly increased.
The
part of the treatment is always a directive counselling, or retraining approach
designed to remove the need to plug or otherwise protect the ear from normal
levels of environmental sound..It is understandably difficult to accept that
sound which can be uncomfortable or even painful to the HEARING, can be quite
harmless to the EAR. A complete understanding of the Jastreboff model is
necessary for both professional and subsequently the patient. The retraining
must be undertaken by professionals trained in TRT.
Hearing conservation remains important in proven damaging noise
situations, (e.g. gunshot, discos, industrial machinery etc) and here
appropriate protection with muffs or plugs is needed, but only when in these
environments.
Damage
is related not only to the intensity of sound, but also to the duration of
exposure, so careful calculations need to be made to establish who is really at
risk.
Wearable
Sound Generators (WSGs)
Research
in the 1980s (Hazell & Sheldrake1991) showed that the use of wide band noise
applied to the ear by wearable sound generators can in help in the treatment of
abnormal hypersensitivity of hearing. This is particularly true in hyperacusis,
where on some occasions, particularly in young children, it is all the treatment
required.
The
sound from the instruments needs to be applied very gently and gradually to the
ear beginning at a low level, always to both ears, and under the supervision of
an audiologist with experience in this process of desensitization and with
training in TRT. The effect, which in some cases may be quite dramatic, results
in a ‘turning down’ of central auditory gain and a reduced perception of
loudness for previously distressing sounds. Over a period of months, due to
changes in the auditory neuronal networks, there is a permanent change in
loudness discomfort, which can be demonstrated by audiometric testing of
loudness discomfort levels. In patients where a severe increase of symptoms
occurs, which genuinely persists after a good nights sleep, very careful use of
sound therapy needs to be applied, under the guidance of an experienced
professional. In severe cases there must be a gradual transition from wearing
ear plugs to using WSGs. Fortunately WSGs 'turn down' the amplification of
external sounds, so many people can immediately tolerate sounds which previously
were distressing. Never undertake any sound therapy without proper advice. Sound
tapes - e.g. pink noise, can make certain hyperacusis and phonophobic patients
considerably worse. In each case carefully explanation of the mechanism of
central processing must be given, so that individuals can understand and believe
what has happened to them, and that the whole process is reversible with time,
and the appropriate therapy.
Where
misophonia (dislike) or phonophobia (fear of sound) exists, no permanent change
in discomfort can be achieved without a successful behavioural programme aimed at
reversing inappropriate beliefs responsible for the conditioned aversive
response. This is true for any phobia (e.g. claustrophobia, arachnophobia, fear
of heights etc). The whole process
of desensitization can take quite a long time, commonly six months to a year,
but is achievable in most cases. In rare cases where phonophobia is present
alone without any hyperacusis, WSGs are not indicated.
Figure
3. Jastreboff model applied to decreased sound tolerance
References
Hazell JWP, Sheldrake JB,
Graham RL. Decreased sound tolerance: predisposing factors, triggers and
outcomes after TRT. In: Patuzzi R. (ed) Proceedings VIIth International Tinnitus
Seminar 2002;255-261.
Jastreboff
P.J., Hazell J.W.P, (1993) A neurophysiological approach to tinnitus: Clinical
Implications. Brit.
J. Audiol. 27:7-17
Jastreboff,
P.J. (1990)Phantom auditory perception (tinnitus): mechanisms of generationand
perception. Neurosci.Res.
8:221- 254
Jastreboff,
P.J. and Hazell, J.W.P. (1993) A neurophysiological approach to tinnitus:
clinical implications. Brit.J.Audiol.
27:1- 11, 1993.
Hazell
J.W.P (1999) The TRT Method in practice. Proceedings of the 6 th International
Tinnitus Seminar. Ed Hazell Publ THC London pp 92-98 (available at www.tinnitus.org)
Hazell
J.W.P., Sheldrake J. (1991) Hyperacusis and tinnitus. Proceedings of the Fourth
International Tinnitus Seminar, Bordeaux, 1991,edited by Aran and Dauman,
p245-248
Sheldrake
J.B, Hazell J.W.P. Graham R.L. (1999) Results of tinnitus retraining
therapy. Proceedings of the 6 th International Tinnitus Seminar. Ed Hazell Publ
THC London pp 292-296 (available at www.tinnitus.org)
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