DECREASED SOUND TOLERANCE
HYPERSENSITIVITY OF HEARING

(Hyperacusis, misophonia, phonophobia)

Jonathan Hazell FRCS, Late Director, Tinnitus and Hyperacusis Centre, 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 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 defence mechanism.

Figure 2 Neuronal networks between ear and brain detect threatening sounds and activate a reflex response involving fear/annoyance, and increase of body functions, to prepare for danger

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

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. This is a normal protective mechanism. These emotional changes can also 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, be it vision, touch, heat, smell, taste or pain are increased greatly in their perceived intensity.
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

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. Nearly all hearing aids have some form of compression,which stops loud sounds entering the hearing aid from being over-amplified.
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 / WNGs)

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.

Children

We are seeing increasing numbers of small children with hyperacusis. This may be on its own, or in association with other processing or behavioural disorders such as A.D.D. autism and Williams syndrome. In some children with hyperacusis it may be the dominant problem and respond quickly to desensitization due to the increased neural plasticity in this age group . Alternatively it may present only as a part of a wider problem, which will not respond to TRT. This does not mean that treating the decreased sound tolerance component with TRT is not worthwhile, and it frequently contributes in a positive way to the overall life quality of the child.

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)

This article may be circulated to patients, their friends and families, freely, provided it is not altered in any way. The authorship and URL of our website www.tinnitus.org should always be acknowledged. JWPH