Tinnitus is defined as a perception of sound when there is no external source of sound. This is a common condition, and affects about 10% of adults in the UK.
In order to understand tinnitus, it is essential to have some idea about the mechanism of hearing. The process of tinnitus generation is complex, and there may be several different underlying causes for this relatively common condition. Furthermore, it is apparent that tinnitus is not always intrusive or distressing. Many people who have tinnitus do not complain about it. In some, the onset of tinnitus can be linked to a stressful event, either emotional or physical.
How we hear
The hearing pathways consist of the ears, the nerves of hearing and the connections within the brain. It is the brain which recognizes and analyses the sound signal. The brain is also able to reject sounds which are meaningless. The organs of hearing are fully developed in the first weeks of pregnancy. Connections are formed between these organs, the hearing nerves and the brain. Links are also formed between the brain’s hearing area and other centres, such as the pathways of emotion (the limbic system), the alarm centre (reticular formation) and the centres of thought and memory (the frontal cortex). It is now known that an unborn baby can hear sounds. After birth, the brain begins to recognize different sounds, especially speech. This recognition process forms the basis of a child’s language development.
Auditory memory and tinnitus
The continued stimulation of the brain by different sounds leads to a process of learning – the auditory memory. While being able to analyze and store information about useful sounds such as speech, the brain has the remarkable ability to reject ‘useless’ sounds by a process of filtration (rejection). This process continues throughout the individual’s life. Occasionally this process of sound filtration may fail. This would result in the perception of an unwanted sound in the higher centres (the cerebral cortex). This is the basis of tinnitus generation. Along with this process, it has been suggested that the brain has the ability to increase its awareness of sound (auditory gain). This can occur in a number of situations, for example if a person is under stress. Here the limbic system becomes active. Increased auditory gain can also occur if an individual is in a room where there is complete silence. In this case, the brain becomes aware of random activity in the hearing pathway which would normally be rejected by the filtration process. This was demonstrated by a well known experiment carried out by Heller and Bergman in 1953. They isolated a group of medical students in a sound proofed room (they all had normal hearing and no perception of tinnitus) and asked them to describe any sounds which they heard. 94% claimed that they heard a variety of different sounds, although no sounds were actually introduced into the room. The silence had increased the students’ auditory gain, resulting in the perception of sound signals generated in the auditory pathway.
Auditory processing disorder
The ability to hear specific sounds such as speech in the presence of other background sounds is the result of a complex process. This is mainly a function of the auditory filter. In addition, we have the ability to improve this by ‘straining to hear’ – increasing the auditory gain. This ability is well developed in certain people e.g. musicians. A mother will normally hear her baby’s cry, even if there are other sounds in the house. However, in many others the brain simply cannot cope with the competing sounds. This is known as an Auditory Processing Disorder (APD).
The impact of trigger factors in tinnitus
It is well known that various emotional and physical factors, such as bad news, head injuries, illness such as colds and flu, injuries of the head and neck may be closely linked to the onset of tinnitus. In all these cases there is increased arousal mediated via the autonomic nervous system, resulting in tinnitus, as described earlier. In such cases, because the tinnitus is often of sudden onset, it can raise fears about potentially serious conditions such as brain tumours. Unfortunately, this only serves to aggravate the situation by increasing the brain’s awareness of the tinnitus.
Increased sensitivity to sound (hyperacusis)
The increase in auditory gain which occurs in tinnitus subjects often results in a general increase in the sensitivity to sound. This is known as hyperacusis. In these subjects loud sounds cause severe discomfort, and also tend to aggravate the tinnitus. Not surprisingly, such people tend to avoid noisy places like pubs and restaurants. This can make matters worse. To achieve a reduction of auditory gain and hence hyperacusis subjects need to be gradually exposed to a variety of sounds. This concept should be explained to patients, so that they can begin the process of auditory retraining.
The role of professionals
Tinnitus is a common symptom, and it is essential for all professionals working in the field to have a thorough knowledge of the mechanisms involved in its generation. Patients with tinnitus who seek advice should undergo a complete medical examination by a doctor to exclude any treatable medical condition. After this, they should be referred to a Specialist Centre where they could receive counselling and advice about the management of their tinnitus.
The final word
It is quite common for tinnitus patients to be told that ‘nothing can be done’ or ‘there is no cure’. This is both unfortunate and incorrect. Research carried out in the UK and other countries have shown the benefits of various treatment measures in tinnitus. These include detailed counselling about the tinnitus, behavioural techniques such as Relaxation Therapy and Cognitive Behavioural Therapy (CBT), together with various forms of Sound Therapy. This can take many forms, such as white noise generators (tinnitus maskers), hearing aids (where appropriate), or other noise generating devices such as personal CD’s. - from the British Tinnitus Association