What is tinnitus?
Tinnitus (ringing in the ears) is the perception of a sound without an external source and it is experienced by 10% of the world’s population, an estimated 600 million people. Although only 5% describe it as a problem that interferes with daily life, many people would prefer not to live with a buzzing, humming, hissing, ringing or roaring sound.
What causes tinnitus?
There are many factors that have been associated with an increased risk of developing tinnitus. Some of the factors are not controllable, such as the aging process and an individual’s genetic background. However, there are several factors that clearly are associated with an increased risk of developing tinnitus and that can be prevented or avoided. Noise-induced hearing loss is the most significant of these preventable causes of tinnitus. The link between hearing loss and tinnitus is complex and not completely understood. Only 50% of people with noise-induced hearing loss develop chronic tinnitus. This observation would suggest that the pathology of tinnitus is not the same as the pathology of hearing loss. A critical research question, therefore, is to understand the mechanisms responsible for tinnitus.
What is the trigger that causes tinnitus to develop in some people with hearing loss but not in others?
One hypothesis is that tinnitus develops because of selective damage to the cochlea. When the inner ear is exposed to damaging noise, specific populations of nerve fibers seem to be more sensitive to this exposure and are preferentially damaged. Our research has shown that when these nerve fibers degenerate after noise exposure, tinnitus develops. One potential avenue for preventing tinnitus from noise trauma may involve either preventing the degeneration of these nerves or stimulating regeneration of damaged nerves. An important research question would identify the protective mechanisms that are responsible for preventing tinnitus from developing even in the presence of hearing loss. The photographs show a section of a cochlea at low magnification (A), the intact sensory cells or microphones of the cochlea at higher magnification(B) and a cross section of the nerve fibers that transmit sound information from the cochlea to the brain showing degeneration after loud noise exposure.
Ringing in the ears or ringing in .......the brain ?
Another hypothesis is that tinnitus develops because of changes in how the brain processes sound in response to altered input from the cochlea. Current research is directed towards identifying the pathologic changes in the central auditory pathway that represent the neural signature of tinnitus. The pathology may occur at one or more sites within the auditory pathway. Abnormal neural activity, such as increased spontaneous neural firing, altered frequency selectivity, synchronized activity or bursting activity, has been observed in the auditory brainstem, midbrain and cortex of animals with tinnitus. Damage to the cochlea may trigger an imbalance between the chemicals of the brain that control the electrical activity of the nerves. An increase in the amount of excitatory neurotransmitters (chemicals that cause the nerve cells to be more active) or a decrease in the amount of inhibitory neurotransmitters (chemicals that decrease the activity of nerve cells) or a combination of both could be responsible for the abnormal neural activity that is perceived as tinnitus. It is critical to identify the neurotransmitters that are involved in tinnitus in order to develop successful drug treatments to treat tinnitus. Is tinnitus preventable? The best method for treating hearing loss and tinnitus from noise damage to the ears is prevention and education. Many people realize that even short exposure to very loud sounds, such as gunfire, can be damaging to the ears and cause permanent hearing loss. What is less well known is that long periods of exposure to moderately loud sounds, such as factory noise or music played through ear phones, can result in just as much damage to the inner ear, with permanent hearing loss and tinnitus. Listening to moderately loud sounds for hours at a young age carries a high risk of developing hearing loss and tinnitus later in life. Education on hearing protection at work and in the home is critical.
Table 1 shows some common sources of noise and the loudness of each. Sounds are measured using the decibel scale. An increase of 10 sounds twice as loud but has ten times more power or energy to cause damage. The larger the number, the shorter the period of time for exposure before damage occurs. Avoiding exposure to sounds that are too loud, for too long, or too close is the most effective prevention for noise induced hearing loss and tinnitus.