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

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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.  Often 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 hair cells), so that a small change in stimulus intensity 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 (pianissimo) 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 hearing 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 strong 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