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Vincent Cumberworth BSc FRCS Consultant Ear,
Nose and Throat Surgeon
Clementine Churchill Hospital Sudbury Hill Harrow Middlesex HA1 3RX
Private Secretary: Tel: 020 8872 3866 Fax: 0208872
3861
Information is supplied only upon the condition that the viewer
will make their own determination as to the suitability of the
information for its purposes prior to use. There will be no
responsibility for information that is misused or misinterpreted and
the information should not be used as a substitute for consultation
with a health care professional. It is not possible to answer
questions regarding individual cases or to enter into individual
correspondence.
HYPERSENSITIVITY OF HEARING
Jonathan Hazell
FRCS, Head of Medical Research, RNID, London UK
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 three components which can contribute to sensitive hearing (hyperacusis,
phonophobia and recruitment) and these will be discussed separately.
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. An equally important but less frequently used
test measures the upper limit of loudness tolerance. You should
indicate when the tones become uncomfortable to the ear (just before
they become painful). For patients who are frightened of loud sounds
this test must be done carefully and with proper instruction. 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 treating
hyperacusis or when fitting a hearing aid to any patient, whether
they have hyperacusis or not.
The mechanism of hyperacusis, recruitment and phonophobia
Hyperacusis is due to an alteration in central processing of sound,
and the cochlea is often completely normal, although patients
frequently wrongly believe it is irreversibly damaged. The
traditional teaching involves only an understanding of ‘recruitment’
due to cochlea damage. Since almost all people with hyperacusis can
be helped by behavioural and ‘sound’ therapy, it has become clear to
us that the symptoms cannot be the result of ear damage. Some people
with hyperacusis also have phonophobia. This means they literally
fear being exposed to a certain sound, sometimes because of the
belief that it will damage the ear. Oten these are normal
environmental sounds like traffic, kitchen sounds, doors closing, or
even loud speech, which cannot under any circumstances be damaging.
In phonphobia certain complex sounds produce discomfort, on the
basis of their meaning or association, but other sounds which are
enjoyed (such as music) can be tolerated at much higher levels. If
there is a discrepancy in the level or different sounds which
produce discomfort, then it is very likely that a degree of
phonophobia exists. Phonophobia can lead to hyperacusis (by changes
in central auditory processing), and a consequent persistence of
abnormal loudness perception.
A common widespread and largely harmless expression of phonophobia
is seen in the fathers of teenage children to ‘modern’ music being
played (even in the distance) and to the dislike for 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
phonophobia!
Recruitment
A normal ear is able not only to hear extremely quiet sounds
(between 0 and 20dB hearing level) but can also tolerate very loud
sound without discomfort (up to levels of 115dB hearing level). With
hearing loss an inability to hear quiet sounds may be coupled with a
paradoxical intolerance for loud sounds due to recruitment. An ear
with recruitment might well be unable to hear sounds, particularly
high frequency sounds, below 50dB, but find any sounds above 80dB
not only uncomfortable but liable to produce distortion. Recruitment
is due to a reduction in neural elements in the inner ear (usually
the hari cells), so that a small change in stimulus intesitiy
produces a very big change in response of the ear. More nerve fibres
are switched or ‘recruited’, for a corresponding sound stimulus.
Another way to look at this problem is to consider the ear rather
like a musical instrument. Think of the ear as an instrument
‘playing sounds to the brain’ where they are perceived in the
auditory cortex. Most musical instruments have what musicians call a
dynamic range. They are able to play very softly (pianissmo) or very
loudly (fortissimo). A normal ear is one not only with good hearing
but with a full dynamic range for different intensities of sound. A
recruiting ear is one in which the dynamic range is narrowed or
contracted. If sounds are heard at all they are heard in musical
terms as ‘fortissimo’.
Nevertheless many people, perhaps the majority, who have
hypersensitivity and hearing loss, have hyperacusis rather than
recruitment, or may have some degree of both. Even if recruitment
exists, it is possible to retrain the brain (central processing) to
alter its appreciation of loudness and accommodate the smaller
dynamic range of the eharing impaired and recruiting cochlea. Where
the hearing is normal, or near normal, hypersensitivity is generally
due to hyperacusis (with or without phonophobia) and not due to
recruitment.
Mechanisms of hyperacusis
The brain plays a major part in sensitivity to sound. When sounds
reach the inner ear, they are coded into their individual frequency
components. The 10,000 fibres in the auditory nerve carry
information about the individual frequencies of each complex sound
that we hear and 1/20 of a second later, these reach the sub-cortex
of the hearing part of the brain (in the temporal lobe) where
conscious perception of sound occurs. Until the message reached
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. 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 stong in meaning. An example of this would be the
detection of the quiet sound of a 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 brain, an important signal is
detected on the basis of previously learnt experience. This signal
may then be enhanced, and its passage facilitated in nerve pathways.
These pathways are not inert electrical cables, but complex neuronal
or nerve networks which work by changing electrical resistance
between nerve cells in the pathway. This is somewhat similar to the
switching that occurs in the telephone exchange to allow one person
to speak to another. When the enhanced signal reaches the sub-cortex
(pitches or musical notes) has to be matched with another pattern
that is held in our hearing memory. This pattern matching event may
be very weak, resulting in a weak perception of sound. A strong
pattern match results in a loud and intrusive sound perception. The
strength of pattern matching and consequent sound perception is
governed by the limbic system (the centre of emotion and learning).
The original purpose of this ability to amplify small signals and to
suppress others was to facilitate the detection of potential threats
in the environment.
The Meaning of Loudness
Under normal circumstances we hear more intense sounds as seeming
louder than quiet sounds but our perception of loudness is not
dictated simply by the strength or intensity of the sound arriving
at the ear. Some sounds become loud, intrusive and unpleasant
because of their meaning or association. This is almost universally
true for the sound created by scratching chalk on a slate, or the
sound of a burglar alarm going down the street. In most cases, the
association has some threatening qualities; will the sound damage
the ears? will it disturb sleep? will it reduce life quality by
reducing periods of quiet recreation? will it interfere with
concentration?
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 turned into very loud intrusive and unpleasant
perceptions which become constantly audible whether we like it or
not.
The Limbic System and Emotional Response
Changes in emotional state, particularly mood fluctuations or
anxiety can increase overall arousal and make us more able to detect
potential threats in our environment. These emotional changes can
increase the apparent loudness and irritation of sounds to which we
are already hypersensitive. In some people this results in a
“global” hypersensitivity where all stimuli, whether they be visual,
auditory, olfactory (smell), taste or touch and pain are increased
greatly in their perceived intensity.
The process of developing an increased sensitivity to sound always
involves the limbic system. Where phonophobia already exists it is
not difficult to understand the inevitable association of fear,
anger or irritation with the appearance of the sound whenever it
occurs. However with any situation where there is hyperacusis, a
state of increased arousal becomes filled with the sound, so that
interference with concentration (on another task) occurs. However
with any situation where there is hyperacusis, a state of increased
arousal is induced, because of the enhancement of sound by the
central auditory pathways. The attentional focus becomes filled with
the sound, so that interference with concentration (on another task)
occurs. The repeated appearance of sound which indicates annoyance,
anger or fear, results in the establishment of a subconscious reflex
response with automatic and invariable stimulation of the limbic and
autonomic nervous system. Exactly the same reactions of the nervous
system occur naturally when we automatically take our foot out of
the road we are about to cross on hearing a car horn, only to
discover the car is disappearing in the other direction! Protective
reflexes 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 hear rate, sweating, muscle tension and
other adrenaline mediated body responses.
Treatment of Hyperacusis
With hearing loss
Where there is a hearing loss and a need for a hearing aid fitting,
this must be done without overloading the ear with amplified sound.
Many hearing aids have some form of compression which stops loud
sounds entering the hearing aid from being over-amplified. Automatic
volume control is available on most hearing aids and non-linear
compression is a more advanced type, that may help some hearing
impaired people with hyperacusis and/or recruitment.
In fitting hearing aids to sensitive ears, it is best to leave the
ear canal as unoccluded as possible. Where there is simply a
relatively small high frequency hearing loss, an “open” mould should
be used. Where more amplification is needed, the mould should be
vented to allow the escape of unwanted high levels of low frequency
sound. Trials with different sorts of ear moulds can often be very
helpful.
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 double-glazed
windows, often hermetically sealed from the outside world, the
absence of sound stimulation leads to an increase in central
auditory gain or ‘amplification’. The auditory filters ‘open’ in an
attempt to monitor the external sound environment. External sounds
may then increase dramatically in their relative loudness and
intrusiveness. Those with long histories of disturbed sleep (often a
symptom of mild depression or anxiety dating from childhood) take to
wearing earplugs to exclude disturbing external sounds to facilitate
getting off to sleep. This further increases the effect of internal
auditory amplification. Some people with hyperacusis have a
life-long aversion to sound intrusion of any kind. This may indicate
a tendency to be easily threatened by external events.
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 (perceptual) cortex, and thus,
when they are detected in the absence of plugs, their perceived
loudness is greatly increased. The loudness of sound depends not
only on the strength of the signal leaving the ear, but on the
electrical voltage or potential reaching the cortex of the brain,
after auditory processing.
The first step we take in the desensitisation is a counselling or
retraining approach designed to remove the need to plug or otherwise
protect the ear from normal levels of environmental sound. Of
course, overloud sounds that CAN damage the ear (e.g. gunshot,
discos, industrial machinery etc) require appropriate protection. 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.
Wide Band Noise Generators (WBN)
Recent research has shown that the use of WBN applied to the ear by
a white noise generator (previously termed a “masker”) can in many
cases help in the eventual abolition of abnormal discomfort for loud
sounds. This is particularly true in the group who have normal or
near normal hearing. WBN needs to be applied very gently and
gradually to the ear beginning at a very low level and under the
supervision of an audiologist with experience in this process of
desensitisation. The effect, which in some cases may be quite
dramatic, results in a turning down of ventral auditory ‘gain’ and a
reduced perception of loudness for previously distressing sounds.
Over a period of months, due to changes in these 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 severed 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
experience professional.
Retraining in Hyperacusis
Where phonophobia (fear of the sound) coexists, no permanent change
in loudness discomfort can be achieved without a successful
behavioural programme aimed at reversing inappropriate beliefs
responsible for the phobic state. This is true for any phobia (eg
claustrophobia, arachnophobia, fear of heights etc). Where there is
an irrational fear that normal environmental noises may be damaging
it is important to “retrain” the auditory system both at a conscious
and subconscious level to respond in more appropriate manner. This
involves the examination and discussion of the reasons behind the
development for the hypersensitivity and the fears held by the
individual, whether felt to be real or imaginary, about the effects
of such noises and the strong emotions that they evoke. In many
cases the fears will have begun, or have been enhanced by
inappropriate negative counselling. This may come from professionals
versed in ‘conventional’ outdated knowledge about central auditory
processing, or from poorly written ‘patient advice’. Much of what is
passed around on the Internet, for example, about hearing
hypersensitivity is based on old wisdom or ‘old wives tales’!
The whole process of desensitisation can take quite a long time,
commonly six months to a year, but is achievable in most cases.
References
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
Jastreboff P J Hazell J W P (1991) A neurophysiological approach to
tinnitus: Clinical Implications Brit J Audiol 27:7-17
Sheldrake JB, McKinney CJ, Hazell JWP, (1995) Practical aspects of
retraining therapy. Proceedings of the Vth International Tinnitus
Seminar Portland Oregon USA July 12-15. Ed G Reich & Vernon, Publ.
American Tinnitus Association, Portland OR 1996, p537-538
© Vincent
Cumberworth 2005 |
