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Tinnitus

Tinnitus is a functional disorder and not an actual disease

Tinnitus, is a disruptive condition that affects millions of people, and which originates not in the ear but in the brain, and not even exclusively in the brain's auditory centers

-Hearing-

From a physical point of view the ear receives a one-dimensional sequence of sound pressure variations.  
From this one-dimensional sequence of sound pressure variations an individual can obtain a great deal of information.  A listener can select and follow the tune generated by one single instrument in an orchestra, detect the mood in a voice, or get exhilarated by the sounds in a voice or a melody. 
In the inner ear these sound pressure variations affect 15,000 hair cells of which 3,000 are inner hair cells, which are directly involved in the transduction of these pressure variations.  The other 12,000 are outer hair cells and are responsible for frequency discrimination.  The 30,000 nerve fibers of the auditory nerve conduct the acoustic information transduced by the inner ear, to the auditory nuclei as a sequence of electrical currents (action potentials) of the same intensity.  The data reaches the nuclei as a chronological sequence of impulses.   Each successive auditory center manipulates and enhances the information contained in these impulses.  The end result is the transformation of a train of impulses into meaningful and recognizable data crucial to the survival of the individual. 

• Hearing is the detection of acoustic information

• Perception is the identification of what is heard

• The perception of acoustic information carries meaning and generates emotions (sensation). These emotions are generated through connections between the auditory pathways and the limbic system, which is responsible for our mood.

 

Tinnitus Etiology and Management

• All levels of the auditory pathways and several nonauditory systems play essential roles in each case of tinnitus.

• Non-auditory systems are dominant in determining the level of tinnitus annoyance. 

 

The goal of tinnitus habituation is to reach the stage at which, although patients may perceive tinnitus as unchanged when they focus on it, they are otherwise not aware of the tinnitus. 
Habituation is achieved by a combination of:

1. Directive counseling 

2. Environmental sounds

3. Relaxation techniques

4. Judicious use of medications

Incidence

• In the general population 17% suffer with tinnitus.

• In the elderly 33% suffer with tinnitus.

• Majorities of people experiencing tinnitus are not bothered by it.  One quarter of sufferers are annoyed to the extent they actively seek help. 

• 10 million sufferers of which 2 millions are debilitated to the extent of total disruption of the quality of life, and in some cases, suicide. 

 

Neurophysiological model

The neuropsychological model of tinnitus postulates that:

• Tinnitus emerges as the result of the interaction of a number of subsystems in the nervous system with the auditory pathways.

• Other systems, predominantly the limbic system, are responsible for the tinnitus annoyance.

The nervous system has the capacity to be habituated to signals that are:

• Emotionally neutral

• Do not carry important information

• Auditory pathways, particularly at lower levels, exhibit a high level of spontaneous and random activity, which is not perceived as sound 

The presentation of external sounds result in:

• An increase of the average activity

• The emergence of regular patterns in the activity

• An increase in the synchronization in activity among neurons

The auditory system continuously adjusts the threshold of detection of deviations from random spontaneous activity, depending on the level of external sounds.  

When subjects are placed in an environment with an extremely low level of sound, they begin to hear even the slightest sounds within a few minutes (breathing, pulse, cloth rubbing) and start to perceive tinnitus. 

• Experimentally produced temporary or permanent hearing loss in animals, results in increased sensitivity of auditory pathway neurons to stimulation.

 

• The appearance of tinnitus suggests that parallel with a decreased threshold of stimulation, the signals that were previously treated as random and filtered out without reaching the level of awareness start to be perceived.  

• When the deviation from randomness in spontaneous activity at the periphery of the auditory system reaches a certain level, the activity is detected by subcortical centers, transferred to cortical areas and perceived as sound (tinnitus). 

 

• The initial peripheral signal may result from imbalanced activity of type I and type II auditory nerve fibers because of discordant damage of the outer and inner hair cell system or from any other mechanism.  In the majority of people this result in an initial orientation reaction because of its novelty but is not associated with a fear inducing negative emotional state, nor is it interpreted as carrying a significant message, it therefore gradually undergoes habituation. 

• A quiet environment decreases the background-evoked activity within the auditory pathways, therefore enhances the perception of tinnitus.  Even then the tinnitus does not evoke annoyance and is treated as one of many background sounds. 

 

The situation changes dramatically when the initial perception of tinnitus is associated with negative emotions 

• Some people are irritated by the presence of the tinnitus and their lack of ability to control it.  In these individuals the limbic system, which is involved in the emotional association of sensory signals, becomes activated. 

• The limbic system, in turn, facilitates further detection and enhancement of the tinnitus signal by subcortical auditory pathways and induces activation of the autonomic nervous system to prepare the person to respond to danger, resulting in the feeling of annoyance.

Negative reinforcement continuously accompanies the perception of the tinnitus, creating a vicious cycle.  The presence of negative reinforcement prevents habituation from occurring.

Associations of the limbic system and the autonomic system occur at subcortical level and presumably involve defensive and adaptive mechanisms of the body. 

 

 

HABITUATION

• Learning to ignore a stimulus that goes on for an extended period of time, or is repeated frequently without anything bad happening is called habituation

Very early in life, we have to learn how not to pay attention to such things as the feeling of our clothes on our body, the street noise around us, and the sound of our own breathing.  Demonstrated in every organism from protozoa to insect, fishes, rats and humans –

Habituation is vital to survival

An animal constantly distracted by the sound of wind in the trees, for example, is likely to miss the sound of an approaching predator. The rules of habituation are the same for all species:

• The weaker or more frequent the stimulus, the faster the habituation. 

Habituation is defined as the disappearance of reactions to sensory stimulus because of repetitive exposure to the stimulus and the lack of positive or negative reinforcement associated with the stimulus.  Habituation is known as “passive extinction”.

Habituation is a necessity since our brain is able to consciously perform a very limited number of tasks.  We cannot listen to a conversation and read a book at the same time, even though 2 different sensory systems are involved.  The situation is even worse when signals of the same modality are involved: understanding conversation and listening attentively to music. In ordering tasks to be performed, the brain uses the following principles:

• The importance of the signal, particularly if it has negative connotation indicating danger,

• Its novelty.  If a signal has not been associated with any significant event or does not indicate danger, and is not new, it gradually undergoes habituation, and after a number of repetitions, a person does not consciously perceive its presence. 

The speed of habituation depends on the contrast of the signal against a background created by other signals from the same modality.

It is possible to get habituated to even strong yet neutral signals.  The fundamental necessary condition for inducing habituation is to remove the association between tinnitus and the emotional state. 

• Known, even unpleasant, phenomena are less frightening than the unknown.  To facilitate the habituation to tinnitus perception, it is advisable to enhance the level of auditory background surrounding the individual, particularly when in a quiet environment.

• By increasing the background level of spontaneous and evoked activity within the auditory pathways, and by reducing the contrast of the tinnitus signal versus background, the process of habituation is facilitated. 

• It is of the utmost importance not to mask the tinnitus (once a tinnitus is masked, by definition, it cannot be habituated to). 

 

Habituation is basically a reconditioning of connections within subcortical centers; this process cannot occur if the tinnitus is not present. 

40% of tinnitus patients exhibited elements of hyperacusis. 

There is a subpopulation of hyperacusis patients characterized by an enhancement of tinnitus and hyperacusis as a result of exposure to even mild sounds, a long-term constancy of return to preexposure level, and sound induced perception of pain in the ears.