Tinnitus

Tinnitus is the hearing of a sound, often a ringing in the ears, without the presence of a corresponding sound source in the environment. This phenomenon occurs in 15–20% of the population. Generally, tinnitus is more common in men than in women and is often seen in the left than in the right ear. It is also more common in people with hearing loss than in normal hearing patients and more common in older people than in young people. Most people with tinnitus can lead a normal life and have only limited problems with the sound, but for 20% of them the condition is unacceptable.

Tinnitus can be divided into two large groups: pulsatile (objective) and non-pulsatile (subjective). The cause of pulsatile tinnitus, which is synchronous with the heartbeat or the respiration, is usually not in a poorly functioning auditory system, but is rather mechanical in nature, such as increased intracranial pressure, calcification of the carotid artery, highly perfused tumors, etc.

With non-pulsatile tinnitus, finding the cause is not always that simple. The most common reason is a reduced entry of information through the auditory system (i.e. deafferentation, hearing loss). Stress, however, can also play an important role in the development of tinnitus. Sometimes tinnitus develops without a demonstrable reason or there are even several causes to show.

Tinnitus is (usually) observed in the ear but is generated in the brain. Auditory information is transferred from the cochlea to the cerebral cortex along several routes. The sound, which consists of vibrations in the air, is transmitted to the cochlea through the outer ear, the eardrum, and the ossicles. There the vibrations are converted by hair cells into electric stimuli that are transported to the brainstem via the auditory nerve.

A first route runs straight from the brainstem to the auditory cortex. This 'lateral route' provides information about the frequency, the direction, and the intensity of the sound. A second path leads to other brain zones than the auditory cortex, such as the insula and the anterior cingulate cortex, areas that are also activated by other sensory stimuli such as pain. This ‘medial route’ gives us more information about the behavioral importance of the sound by showing how pleasant or annoying we find a sound and will determine how much attention we give to it. The insula plays a role in the functioning of our autonomic nervous system (stress or relaxation), while the anterior cingulate cortex is involved in attention. As with pain, not all distressing sounds are truly insufferable and there is a breaking system in the brain that can suppress certain sounds. The front part of the cingulum (i.e. the pregenual part) is responsible for this. This breaking/decelerating system is probably more active in people who suffer less from their tinnitus.

The character and the extent to which the tinnitus is perceived as annoying is therefore mainly related to which brain regions are involved in the 'tinnitus network'.