There is no precise, short, and distinctive deﬁnition of tinnitus. A patient experience–oriented deﬁnition describes tinnitus as ringing, buzzing, the sound of escaping steam, hissing, humming, cricket-like, or a noise in the ears. Tinnitus as “a phantom auditory perception” represents a physiologic deﬁnition of tinnitus, pointing out a lack of a physical acoustic stimulus related to tinnitus. There is also the deﬁnition proposed by the Committee on Hearing, Bioacoustics and Biomechanics of the US National Research Council, which describes tinnitus as “a conscious experience of sound that originates in the head of its owner.”
Six to 17% of the general population experience tinnitus lasting for a period of at least 5 minutes. About 3 to 7% of people seek help for their tinnitus, and 0.5 to 2.5% report severe effects of tinnitus in their lives. People of all ages experience tinnitus, including children, and the prevalence of tinnitus increases signiﬁcantly with aging. Hearing loss and noise exposure have been correlated with increased prevalence of tinnitus as well. There is no clear effect of gender. Neither smoking, coffee, nor alcohol has been shown to increase the prevalence of tinnitus directly.
The mechanisms of tinnitus generation involve structures of the auditory system (periphery, central auditory pathways) and the central nervous system. Proposed mechanisms responsible for the emergence of tinnitus-related neuronal activity include abnormal coupling between neurons, local decrease of spontaneous activity enhanced by lateral inhibition, discordant damage/dysfunction of outer and inner hair cells, unbalanced activation of type I and type II auditory nerve ﬁbers, abnormal neurotransmitter release from inner hair cells, decreased activity of the efferent system, mechanical displacement within the organ of Corti, abnormalities in transduction processes, various aspects of calcium function, physical/biochemical stress on the auditory nerve, and enhanced sensitivity of the auditory pathways after decreased auditory input.
Tinnitus may be part of more complex medical conditions, such as conductive hearing loss (otitis media, cerumen impaction, ossicular stiffness/discontinuity, otosclerosis), sensorineural hearing loss (Meniere’s disease, presbyacusis, cochlear otosclerosis, vestibular schwannoma, sudden hearing loss), and hormonal changes (pregnancy, menopause, thyroid dysfunction). Tinnitus can be induced by some medications or withdrawal from them.
Somatosounds (sometimes considered as objective tinnitus) are the sounds produced by structures adjacent to the ear (neoplasm, arterial, venous, myoclonus), structures in the ear (eg, spontaneous otoacoustic emissions), or joint abnormalities (temporomandibular joint disorders).
We do not have an objective method to detect and measure tinnitus. Therefore, interview, psychoacoustic characterization (perceptual location, pitch, loudness, maskability, post-masking effects), and physiologic evaluation (otoacoustic emissions, efferent mediated suppression of otoacoustic emissions, spontaneous auditory nerve activity, auditory brainstem responses, late cortical potentials, positron emission tomography, single-photon emission computed tomography, functional magnetic resonance imaging, magnetoencephalography) are typical approaches in clinical practice.
Tinnitus is a symptom that can cause signiﬁcant emotional and somatic distress and signiﬁcantly inﬂuence patients’qual- ity of life, particularly if allowed to become a chronic problem. The list of reported complaints is long and includes emotional problems such as irritation, annoyance, anxiety, and depression; hearing problems such as difficulty with speech comprehension; and somatic problems such as headache, neck pain, and jaw pain. Tinnitus can be very intrusive and may cause difficulty with sleep and concentration and a decreased ability to participate in everyday activities and social events; it may also create problems in relationships. A detailed interview, aimed at characterizing the speciﬁcs and degree of tinnitus impact on patients, coupled with otolaryngologic evaluation, provides the most thorough assessment and allows the practitioner to address all of the issues that need to be considered, including the potential intervention of a psychologist/psychiatrist to accompany the commencement of speciﬁc tinnitus-oriented treatment.
Tinnitus is frequently accompanied by decreased sound tolerance (oversensitivity to sound), which, in many cases, is a sum of hyperacusis and misophobia. Approximately 40% of tinnitus patients exhibit decreased sound tolerance. There is no generally accepted deﬁnition for decreased sound tolerance to suprathreshold sounds. According to Stedman’s Medical Dictionary, hyperacusis is deﬁned as “Abnormal acuteness of hearing due to increased irritability of the sensory neural mechanism.” Based on neurophysiology, hyperacusis can be deﬁned as abnormally strong reactions to sound occurring within the auditory pathways. At the behavioral level, it is manifested by a subject experiencing physical discomfort as a result of exposure to moderate/weak sound, which would not evoke such a reaction in the average population. Misophonia can be deﬁned as abnormally strong responses to sound of the autonomic and limbic systems without abnormally high activation of the auditory system, resulting from enhanced connections between the auditory and limbic systems. At the behavioral level, patients are afraid (phobic reaction) or have a negative attitude to sound (misophonia). Hyperacusis can lead people to avoid louder environments and therefore preclude them from working and interacting socially. In extreme cases of decreased sound tolerance, patients’ lives are totally controlled by the problem. It is inevitable in all cases with signiﬁcant hyperacusis that misophonia and/or phonophobia will be present and further enhance the effects of hyperacusis.
Hyperacusis has been linked to a number of medical conditions, such as tinnitus, Bell’s palsy, Lyme disease, Williams syndrome, Ramsay Hunt syndrome, poststapedectomy, perilymph ﬁstula, head injury, migraine, medications, withdrawal from benzodiazepines, high pressure of the cerebrospinal ﬂuid, Addison’s disease, and hyperthyroidism. The etiology of hyperacusis includes sound exposure, head injury, stress, and medications.
The mechanisms of hyperacusis are unproven. At the peripheral level, it is possible to speculate that the abnormal enhancement of vibratory signals within the cochlea by the outer hair cells might result in overstimulation of the inner hair cells and subsequently results in hyperacusis. Damage to the cochlea or a decrease in auditory input results in a decrease in the threshold of response in about 25% of neurons in the ventral cochlear nucleus and inferior colliculus, and studies with evoked potentials indicated an abnormal increase in the gain in the auditory pathways after such manipulations. Some of the medical conditions can be linked to the central processing of signals and modiﬁcation of the level of neuromodulators as possible factors inducing or enhancing hyperacusis. Evaluation of loudness discomfort levels provides a good estimation of hyperacusis and misophonia.
The most common treatments for tinnitus include the following:
- Antireassurance . Telling tinnitus patients that “Nothing can be done; go home and learn to live with it” enhances patients’ feeling of hopelessness.
- Pharmacology. All studied drugs for tinnitus (such as lidocaine, tocainide, diazepam, alprazolam, nortriptyline, carbamazepine, clonazepam, baclofen, niacin, nimodip- ine, betahistine, caroverine, furosemide, and ginkgo biloba) have failed to prove their efficacy in a signiﬁcant proportion of patients. Still, various local anesthetics, sedatives, antidepressants, anticonvulsants, vasodilators, calcium channel blockers, and other drugs are frequently prescribed. Lidocaine administered intravenously is the only substance proven to abolish subjective tinnitus for a short period of time, but there is no practical way to use this drug in clinical practice.
- Surgery. Surgery can offer help for some patients with somatosounds, conductive hearing loss, and Meniere’s disease. Neither transection nor microvascular decompression of the auditory nerve, promoted in the past, has proven to be effective.
- Electric suppression. Electrical suppression of tinnitus has been tried since 1855, with mixed results. Only intracochlear (or with electrode on the promontory) stimulation has shown consistent and positive results in approximately 50% of cases. Other approaches were less effective. Two new variants of electrical stimulation for tinnitus have been introduced recently: deep brain stimulation and high-frequency electrical stimulation of the cochlea performed by placing an electrode on the promontory or via cochlear implant. Both approaches are at a very initial stage of investigation.
- Masking . Masking includes the use of an external sound to cover tinnitus and bring immediate relief to some patients. In practice, the approach did not withstand the test of time.
- Psychological approaches. Psychological management of chronic tinnitus, such as cognitive therapies, behavioral modiﬁcations, coping strategies, and minimizing distress protocols, can be helpful for some patients. They are improving patients’ well-being and minimizing tinnitus-caused emotional discomfort.
Treatments of hyperacusis include advising patients to avoid sound (use ear protection) or the desensitization of patients by exposure to a variety of sounds. The ﬁrst approach actually makes the auditory system even more sensitive and further exacerbates hyperacusis. The desensitization approach includes the recommendation of using sound with certain frequencies removed or short exposures to moderately loud sound. The newest approach to tinnitus and hyperacusis involves continuous, prolonged exposure to a gradually increased level of sound.
Presented during the last decade, the neurophysiologic model of tinnitus and hyperacusis offers the basis for a new approach, known as tinnitus retraining therapy (TRT), to help patients with tinnitus, hyperacusis, and somatosounds. Several observations led to this model. It is known that tinnitus induces distress in only about 25% of those who perceive it. There is no correlation among the psychoacoustic characterization of tinnitus, tinnitus-induced distress, and treatment outcome. The experiment by Heller and Bergman showed that the perception of tinnitus cannot be pathologic since essentially everyone (tinnitus emerged in 94% of people without prior tinnitus when isolated for several minutes in an anechoic chamber) experiences it when put in a sufficiently quiet environment. These observations strongly argue that the auditory system plays a secondary role and other systems in the brain are dominant in clinically relevant tinnitus (ie, tinnitus that creates discomfort and annoyance and requires intervention).
Analysis of the problems reported by tinnitus patients, who exhibit a strong emotional reaction to its presence, a high level of anxiety, and psychosomatic problems, indicates that the limbic and autonomic nervous systems are crucial in individuals with clinically relevant tinnitus. Sustained activation of the limbic and autonomic nervous systems is essential in creating distress and therefore clinically relevant tinnitus.
Tinnitus-related neuronal activity is processed by different parts of the central nervous system and involves the con- scious and subconscious processing loops working in a dynamic balance scenario. A continuous presence of tinnitus, combined with attention given to it, results in plastic modiﬁcations of synaptic connections, yielding the modiﬁcation of receptive ﬁelds corresponding to the tinnitus signal and its subsequent enhancement.
Although the initial signal provided by the auditory system is needed to start the cascade of events, its strength is irrelevant as the extent of activation of the limbic and autonomic nervous systems depends on the strength of negative associations linked to tinnitus and the susceptibility of the feedback loops to further modiﬁcations. It appears that tinnitus-related neuronal activity may result from compensatory processes that occur within the cochlea and the auditory pathways to minor dysfunction at the periphery.
The neurophysiologic model includes several systems of the brain involved in analysis of clinically relevant tinnitus. All levels of the auditory pathways, starting from the cochlea through the subcortical centers and ending at the auditory cortex, are essential in creating the perception of tinnitus. When subjects are not bothered or annoyed by tinnitus, auditory pathways are the only pathways involved, and tinnitus- related neuronal activity is constrained within the auditory system. Therefore, although subjects are perceiving tinnitus, they are not disturbed by it.
In approximately 25% of those with tinnitus, strong negative emotions are induced, which, in turn, evoke a variety of physiologic defense mechanisms of the brain, mediated by the limbic and autonomic nervous systems. Improper activation of these systems by tinnitus-related neuronal activity results, at the behavioral level, in the problems described by tinnitus patients. Activation of both systems can be achieved through two routes. The ﬁrst includes stimulation of the autonomic and limbic systems from higher level cortical areas, which are involved in our awareness, verbalization, and beliefs. The second arises from the subconscious level and provides stimulation from the lower auditory centers. The activation going through these two routes changes the strength of synaptic connections, enhancing the stimulation of the limbic and autonomic nervous systems by the tinnitus- related neuronal activity during the process when tinnitus becomes a clinical problem.
The question of how the neutral signal of tinnitus can evoke persistent strong distress can be explained by the principles of conditioned reﬂexes linking sensory information with reactions. To create a conditioned reﬂex, the temporal coincidence of sensory stimuli with negative (or positive) reinforcement is sufficient. These types of associations of sensory stimuli are constantly created in normal life. As long as the sensory stimulus is limited in time and there is no functional link between stimulus and reinforcement, this condi- tioned reaction will gradually disappear (habituate) owing to passive extinction of the reﬂex. There are two different types of habituation: habituation of reaction and habituation of perception. Habituation is a crucial characteristic of brain function necessitated by the brain’s inability to perform two tasks requiring complete attention simultaneously.
The central nervous system screens and categorizes all stimuli at the subconscious level. If the stimulus is new and unknown, it is passed to a higher cortical level, where it is perceived and evaluated. However, in the case of a stimulus to which we have previously been exposed, the stimulus is compared with patterns stored in memory. If the stimulus was classiﬁed as nonimportant and does not require action, it is blocked at the subconscious level of the auditory pathways and does not produce any reactions or reach the level of awareness. The reaction to this stimulus and its perception is habituated. In everyday life, habituation occurs to the majority of sensory stimuli surrounding us.
However, if a speciﬁc stimulus was once classiﬁed as important and, on the basis of comparison with the patterns stored in memory, was linked to something unpleasant or dangerous, this stimulus is perceived and attracts attention. Furthermore, the sympathetic part of the autonomic nervous system is activated, inducing reaction that further reinforces memory patterns associated with this stimulus.
In the case of tinnitus, it is impossible to remove the reactions induced by the excitation of the sympathetic autonomic nervous system or even change them in a substantial manner. The solution to achieve the passive extinction of conditioned reﬂex, in which both stimulus (tinnitus) and negative reinforcement are continuously present, is to decrease the magnitude of this negative reinforcement over a period of time. Once activation of the autonomic nervous system is lowered, this decreases negative reinforcement to a signal that is continuously present, gradually decreases the strength of the conditioned reﬂex, and further decreases the reaction. Once tinnitus has achieved a neutral status, its habituation is inevitable.
Tinnitus retraining therapy is a clinical implementation of the neurophysiologic model of tinnitus. It consists of counseling and sound therapy. Retraining counseling (teaching session) in TRT is oriented toward removal of the patient’s negative associations with tinnitus and reclassiﬁcation of tinnitus into a category of neutral stimuli. By activating a naturally occurring mechanism of brain function, habituation, and the plasticity underlying it, it is possible to achieve over time primarily habituation of the tinnitus-induced reaction of the brain and the body and secondarily habituation of the tinnitus perception. The clear goal of achieving an active and selective block of tinnitus-induced reactions is set for the patients.
In addition to decreasing the strength of the activation of the limbic and autonomic nervous systems, initiated during the counseling session, the second component of TRT is sound therapy. By increasing background neuronal activity, we can effectively decrease the strength of the tinnitus signal, which activates the limbic and autonomic nervous systems; achieve a decrease of reactions induced by tinnitus; and, through this, facilitate extinction of the conditioned reﬂex.
Five principles inﬂuence relationships between the physical intensity of sound and its effectiveness on tinnitus habituation: (1) stochastic resonance (enhancement of the signal by adding low-level noise); (2) dependence of the signal ’s strength on its contrast with the background; (3) total suppression of the signal, preventing any retraining and consequently habituation; (4) partial suppression (“partial masking”), which does not prevent retraining but does make it more difficult as the training is performed on a different stimulus than the original; and (5) activation of limbic and autonomic nervous systems by too loud or unpleasant sounds, yielding an increase in tinnitus and contracting habituation.
In most tinnitus patients, the sound level used should blend/mix with the tinnitus signal, but both sounds should still be clearly identiﬁable (below the level of partial suppression or “partial masking”). By decreasing the difference between the tinnitus-related neuronal activity and the background ongoing neuronal activity, the apparent strength of the tinnitus signal decreases, and this weaker signal is passed to the higher level cortical areas and, most importantly, to the limbic and autonomic nervous systems. This helps in initiating and sustaining the process of passive extinction of conditioned reﬂexes that link tinnitus to negative reactions.
Enriched sound background is provided by a number of means. Sound generators are frequently used to provide patients with stable, low-level, broadband noise. The optimal setting of the sound level is different when hyperacusis is the dominant or only problem. Patients start with a sound level, disregarding the effect of stochastic resonance, closer to their threshold but as high as their sound sensitivity allows with- out inducing annoyance.
In the normal acoustic environment, there is a high proportion of low-frequency sounds, below 200 Hz, which provide constant stimulation of the auditory pathways. This yields a strong recommendation for people who have relatively normal low-frequency hearing to be provided with sound generators or hearing aids with ﬁttings as open as possible to preserve stimulation in the low-frequency range. Blocking the ear canal with closed ear molds decreases the auditory input, and many patients experience enhancement of tinnitus when their ears are occluded. Hearing aids for tinnitus patients are used primarily as a part of sound therapy to provide extra ampliﬁcation of background sounds and only secondarily for communicative purposes. To enrich the auditory background, nature sounds, neutral music, or tabletop sound generators may be used.
Typically, patients in the TRT protocol may see improvement in about 3 months, with clear changes in about 6 months. Many patients achieve a high level of control of their tinnitus by about 12 months. Improvement in hyperacusis is typically faster and the success rate higher than that of tinnitus without hyperacusis. The results from centers in the US using TRT show satisfactory results in over 80% of patients.
In conclusion, tinnitus and hyperacusis are still challenging topics to study and symptoms to treat. Many questions remain unanswered. The mechanisms of tinnitus and hyperacusis are speculative and not proven yet. ENT specialists do not have objective methods for detection and evaluation of tinnitus. They believe that the neurophysiologic model of tinnitus and TRT provide a promising approach that may ultimately result in a better understanding of tinnitus and in providing greater help to patients with tinnitus and hyperacusis.