Tinnitus and Hyperacusis

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CHAPTER 150 Tinnitus and Hyperacusis

Tinnitus

Tinnitus is the perception of sound without an external source. Although 30 million Americans are estimated to have chronic tinnitus, for most it is not a problem sufficient for them to seek treatment. Tinnitus is a chronic sensation virtually all would prefer not to experience, but for most it is not disabling. Disturbing tinnitus occurs in 3% to 5% of individuals with tinnitus.1 Until more recently, treatments available for disturbing tinnitus were limited. Significant advances in auditory neuroscience have advanced the treatment of tinnitus beyond the traditional recommendation instructing the patient to “learn to live with it.” This chapter reviews current theories and mechanisms of idiopathic subjective tinnitus. It also outlines a clinical strategy for evaluating tinnitus and determining tinnitus subtypes, and reviews current management strategies for tinnitus.

Tinnitus can be classified as objective or subjective. Objective tinnitus can be detected by an observer using a stethoscope or ear canal microphone. Objective tinnitus arises from vascular or muscular sources. Objective tinnitus usually has a pulsatile quality. Table 150-1 lists the causes of pulsatile tinnitus. Many of these causes are uncommon and are not responsible for most instances of tinnitus. Because there are excellent reviews of treatments for objective tinnitus, this uncommon form is not addressed further.2

Table 150-1 Objective Tinnitus

Pulsatile

Nonpulsatile

In contrast to objective tinnitus, subjective tinnitus is not audible to an observer. This form is more common, with a 5-year incidence of 5.7%.3 Estimates of subjective tinnitus prevalence range from 8% to 30%, depending on the definition of tinnitus, tinnitus severity, the population sampled, and assessment methodology.35 In a large population-based study of participants 55 to 99 years old, combining detailed tinnitus questionnaires with audiologic assessment, 30% reported experiencing tinnitus, with prevalence related to audiometric threshold, but not age or gender.5 Mildly annoying tinnitus was reported in 50% of respondents, and extremely annoying tinnitus was reported in 16%. Tinnitus prevalence in individuals with normal hearing was 26.6% compared with 35.1% in individuals with hearing loss. An estimated 20% of individuals with profound hearing loss do not experience tinnitus.6 These data illustrate that chronic tinnitus is associated with, and may be triggered by, hearing loss, but they also indicate that hearing loss is not an invariable cause of tinnitus. The pathology necessary to initiate tinnitus may be quite subtle. The survey data also confirm that as a clinical problem tinnitus is more than the presentation of its sensory features.

Idiopathic Subjective Tinnitus

Idiopathic subjective tinnitus is the most common form affecting adults, and is the focus of this chapter. By definition, this form of tinnitus excludes known objective causes, such as conductive hearing loss, endolymphatic hydrops, and cerebellopontine angle neoplasms. Historically, the pathology underlying idiopathic tinnitus was unknown, and the symptom was generally viewed as refractory to intervention. Advances in tinnitus research have revealed mechanisms likely responsible for some forms of idiopathic tinnitus. Two examples are tinnitus-associated pathology of the cochlea or head and neck, and tinnitus susceptible to somatic modulation. Both mechanisms are discussed subsequently.

Subjective tinnitus can be subtyped based on etiology, the pattern of associated hearing loss, psychoacoustic features (simple versus complex), exacerbating factors, psychological comorbidities, and the presence of somatic modulators. Tinnitus subtype classification schemes can be useful in identifying forms of tinnitus that are responsive to specific targeted treatment programs. Table 150-2 lists some useful features for subtyping tinnitus.

Table 150-2 Subjective Tinnitus Subtypes

Hearing Loss Subtypes

The two most common types of hearing loss associated with tinnitus are noise-induced hearing loss (NIHL) and presbycusis. NIHL is a significant and growing health problem. Although reduction of exposure to occupational noise has been effective in the last several decades, there has been a notable increase in the incidence of NIHL from recreational and leisure activity in children and adolescents, and military combat–related noise exposure in young adults.79 A Web-based survey of 9693 adolescents determined that 61% of respondents experienced hearing loss and temporary tinnitus after attending concerts.10 Acute transient tinnitus is nearly universal immediately after unprotected exposure to loud acoustic stimuli such as gunfire and amplified music. Tinnitus prevalence among Army personnel evaluated for hearing complaints after deployment was 30% compared with 1.5% in personnel not deployed.8

The prevalence of chronic tinnitus associated with NIHL is 50% to 70%.11 Subjectively, the sensory aspects of acute and chronic tinnitus can be very similar. Chronic tinnitus occurs in a delayed time course for a significant portion of individuals with a history of exposure to damaging sounds. It is unknown if the pathology of transient acoustic trauma–induced tinnitus is the same as the pathology responsible for chronic tinnitus associated with permanent noise-induced threshold shifts. Chronic tinnitus induced by acoustic trauma occurs at a younger age than tinnitus associated with other types of hearing loss. Consequently, acoustic trauma–induced tinnitus is experienced for a longer portion of the life span than other forms of tinnitus.

NIHL and associated tinnitus are preventable. In addition to obvious proactive methods, such as wearing ear protective hardware, intervention in the periexposure period may prove useful in preventing the onset or progression of NIHL, and possibly the incidence of tinnitus. Intense sound exposure triggers a reduction of blood flow and a cascade of metabolic events in the cochlea, with formation of reactive oxygen and nitrogen species that damage cellular lipids, proteins, and DNA, culminating in increased cell death.12 Interventions targeting these molecular mechanisms of NIHL include antioxidant therapy such as vitamin E, salicylate, and N-acetylcysteine.13 Ginkgo biloba extract contains multiple compounds with vasotropic, potential neuroprotective, and antioxidant effects. Although uncontrolled trials and anecdotal reports have suggested the efficacy of ginkgo, a more recent review failed to show efficacy in treating tinnitus.14

Serum magnesium levels have been correlated with NIHL in animals.15 Controlled studies in humans have shown the prophylactic efficacy of oral magnesium in preventing temporary and permanent NIHL.16,17 It is unknown, however, if these interventions are also effective in decreasing the risk of immediate or delayed onset of tinnitus after traumatic injury.

Presbycusis is sensorineural hearing loss related to aging. Most cases of presbycusis cannot be strictly and solely attributed to aging, but rather involve some combination of cochlear injury from additional sources, such as cumulative noise injury, metabolic or vascular dysfunction, and genetic predisposition. Elderly patients with diabetes have significantly higher pure-tone thresholds, lower otoacoustic emission amplitude, and lower speech recognition in noise18 than age-matched individuals without diabetes.19 Interactions between age and other factors affecting the cochlea and auditory pathway make identification of a single mechanism for presbycusis-associated tinnitus and an effectively targeted intervention a challenge.

Somatic Tinnitus Subtype

Somatic tinnitus is a unique form of tinnitus in which the loudness, laterality, or tonality of the tinnitus can by modulated by somatic stimuli. This form of tinnitus was originally observed in a small group of patients after surgical removal of large vestibular schwannomas.20,21 Postoperatively, these patients had the ability to modulate their chronic tinnitus by exaggerated eye movements, leg motion, or gentle cutaneous stimulation of the hands or face.22,23 The presumed mechanism of action for this unusual form of tinnitus is neural sprouting or aberrant reinnervation after auditory deafferentation. Subsequent to these observations, however, a more general form of somatosensory modulation has been found in patients with idiopathic tinnitus. In these cases, tinnitus is modulated by maneuvers or stimulation of the head and neck region. It has been reported that forceful isometric contraction of the head and neck muscles can modify the loudness and pitch of tinnitus in 65% to 80% of patients with mild, idiopathic tinnitus.2426 Tinnitus can be induced by strong contractions of muscles in the jaw, head, or neck in 58% of subjects without a history of tinnitus.25

The association between tinnitus and somatic pathology of the head and neck is underscored by the reported higher incidence of tinnitus in individuals with temporomandibular joint dysfunction and normal audiometric thresholds compared with controls.27 One third of patients with symptoms of temporomandibular joint dysfunction reported modulation of tinnitus with jaw movement or pressure applied to the temporomandibular joint.28 When tinnitus occurs in association with disorders of the head and neck, such as temporomandibular joint dysfunction, unilateral facial pain, otalgia, and occipital or temporal headache, successful tinnitus alleviation may be possible using interventions targeting the somatic dysfunction.

Animal research has established that multimodal and in particular somatic inputs to the auditory pathway are evident from auditory brainstem to cortex.2933 These inputs allow integration of information from vision, head position, and internal sound (e.g., vocalization) to be used in important functions, such as sound localization and auditory selective attention. It has been hypothesized that aberrations in these multimodal connections can either trigger tinnitus or modulate existing tinnitus. Reduction of normal afferent input to the auditory brainstem (e.g., cochlear nuclei) may trigger inappropriate compensatory up-regulation of somatosensory inputs to these regions. Stimulus-driven or spontaneous activity within the nonauditory somatic pathways may be perceived as an audible “sound.”33

Recognition of somatic tinnitus is important because it may be amenable to targeted intervention. Levine and colleagues34 systematically reviewed the efficacy of treatments targeting somatosensory systems. They defined somatic tinnitus syndrome as tinnitus that is (1) perceived in the ear and (2) ipsilateral to the somatic trigger and (3) not associated with any new hearing complaints. Tinnitus that is strongly lateralized to one ear in the presence of symmetric hearing (including symmetric hearing loss) would theoretically have a somatic etiologic component by the definition of Levine and colleagues.34 The review by these authors presents evidence that somatic tinnitus is often responsive to acupuncture, electric stimulation of the scalp and auricle, trigger point treatment, and treatment of temporomandibular joint dysfunction.34

Tinnitus Treatment Strategies

Auditory Deprivation and Neural Plasticity

Acoustic-based tinnitus treatments encompass a range of delivery systems and types of sound. The rationale for using external sound to treat tinnitus derives from the hypothesis that hearing loss leads to deafferentation-induced reorganization of the auditory pathway. Hearing loss decreases the afferent stream of neural activity from the cochlea to the auditory cortex. Chronic deafferentation alters activity in the auditory brainstem and midbrain, and may change the tonotopic organization of the auditory cortex. Brainstem spontaneous activity may increase, and midbrain patterns may change (e.g., with aberrant synchronization or bursting) because of compensatory down-regulation of inhibition. Altered activity in the auditory pathway may be responsible for the tinnitus percept. Stimulus interventions that restore afferent input to more normal levels may act through improved inhibition. They may also restore cortical tonotopic organization to normative levels and reduce tinnitus.

A related phenomenon that illustrates the concept and treatment of deafferentation-induced cortical plasticity is phantom limb pain.37 The phantom limb syndrome is the continued sensation of the presence of an absent limb, in a distorted or painful manner, after amputation. Amputation of a limb is an extreme form of deafferentation. The parallels between tinnitus and phantom limb pain, in terms of onset rate, persistence, and affected ages, are striking. Greater than 90% of amputees experience a vivid limb phantom immediately after amputation.37 Phantoms are more likely to occur in adults than in children.38 In many cases, the phantom sensation fades after days or weeks, but it persists chronically for decades in 30% of patients.37

It is well known that the adult and the immature brain can undergo plastic change. Neural plasticity is the ability of a neuron or neural network to change its function, organization, and connectivity through long-term alterations in synaptic efficiency.39 Neurons can significantly alter their response to inputs, and receptive field sizes change, as a consequence of either decreased or increased input and training procedures.40 The changes can be extensive. Magnetoencephalographic studies have shown that stimulation of intact body areas distant from the amputation site is perceived, with corresponding central neural activity, at the cortical site of the amputated limb.41 Similar changes of auditory cortical representation may underlie tinnitus.42 Primary auditory cortex steady-state evoked magnetic fields were enhanced in tinnitus patients compared with controls, and the degree of enhancement correlated with the perceived intensity and intrusiveness of the tinnitus.43 Functional imaging studies in humans have suggested that expanded representation of frequency regions in the auditory cortex may underpin tinnitus.4447

Animal models have been important in studying the deafferentation-loss-of-inhibition hypothesis of tinnitus, and have been used to evaluate sound therapy for reversal of pathologic neural plasticity. A key study by Norena and Eggermont48 illustrates the effect of noise trauma and therapeutic sound stimulation on receptive fields of primary and secondary auditory cortex. Cats exposed to a traumatizing noise were evaluated for changes in the location and responsiveness of auditory cortex neurons to acoustic stimuli. The frequency tuning of cortical neurons was altered after the trauma, distorting the overall representation of sound and over-representing frequencies surrounding the exposure sound. A second cohort of cats, exposed to the same traumatizing sound, was subsequently reared in an enriched acoustic environment for several weeks before mapping the auditory cortex. The enriched environment was spectrally composed to compensate for the anticipated frequency distortions induced by hearing loss.

Two important outcomes were obtained in the cats raised in the enriched environment: (1) Their hearing loss was significantly reduced compared with the cats with similar sound exposure raised in a quiet environment, and (2) there was no evidence of plastic reorganization of the auditory cortex. The tonotopic organization of the exposed and treated cats was similar to that of unexposed control cats. These results suggest that tinnitus may be partly the result of abnormal reorganization of the auditory cortex. These results show that therapeutic acoustic intervention may be effective in reducing or eliminating the abnormal cortical organization associated with, and potentially responsible for, tinnitus. The implications for tinnitus treatment using sound therapy are obvious.

Using Sound to Decrease Loudness and Annoyance of Tinnitus

Acoustic stimulation is an important component of successful management of tinnitus in many patients. A wide range of acoustic delivery strategies are currently available. Most patients with tinnitus, including patients with minimal threshold shift on standard audiometric testing, can benefit from acoustic stimulation. Patients with severe to profound hearing loss that is not amenable to conventional amplification can achieve tinnitus relief from other modes of auditory pathway stimulation, such as cochlear implantation or cortical stimulation (discussed later).

Ambient Stimulation

The simplest method of increasing afferent input to reverse putative central reorganization and tinnitus is environmental sound enrichment. Use of supplemental environmental sound to treat tinnitus has been recommended for more than 50 years.49 Enrichment can be achieved using background music, relaxation tapes or CDs, tabletop nature sound machines, or waterfalls or fountains. Patients are typically instructed to use a source of constant background sound to decrease attention to their tinnitus. Sound enrichment is not intended to mask the tinnitus in the conventional sense of completely eliminating the tinnitus percept (discussed subsequently). Rather, by elevating the level of ambient sound using a constant, spectrally rich stimulus, the tinnitus sensation becomes less noticeable.

Hearing loss is present in more than 90% of patients with tinnitus. Conversely, epidemiologic studies have reported tinnitus prevalence of 50% in hearing-impaired patients.50 Effective use of sound stimulation for managing tinnitus in hearing-impaired patients can be achieved only with appropriate amplification. Typically, amplification is achieved with hearing aids. Even without supplemental sound, the therapeutic effect of hearing aids for tinnitus patients is well documented.51,52 Hearing aids reduce the awareness of tinnitus through amplification of ambient sound and reduce the perception that tinnitus masks hearing and impedes communication.

Surr and coworkers53 reviewed the initial effect of hearing aid use on tinnitus in 124 patients. Approximately one half of patients reported that the hearing aid reduced (26%) or eliminated (29%) the tinnitus. Folmer and Carroll54 reviewed their clinical experience with 50 patients with mild to moderate sensorineural hearing loss fitted with hearing aids for tinnitus management. Patients were re-evaluated 6 to 48 months after initial fitting (mean 18 months), and 70% reported significant improvement in their tinnitus. Similar results were obtained in a larger study of 1440 patients fitted with hearing aids for unilateral or bilateral hearing loss.55 In this study, digital hearing aids provided significantly greater tinnitus relief than analog hearing aids. Of patients fitted with digital hearing aids for unilateral hearing loss, 65% reported greater than 50% improvement in tinnitus compared with 39% reporting a similar degree of improvement after fitting with an analog aid. An even greater therapeutic effect was obtained with bilateral digital hearing aids: 85% of patients reported greater than 50% improvement in tinnitus compared with 30% fitted with bilateral analog aids.

Programmable digital hearing aids can selectively amplify within the high-frequency range, a region where most tinnitus patients typically have some measurable threshold loss. Current-generation digital aids have significant output gain up to 10 kHz, which significantly benefits tinnitus patients with hearing loss in the high-frequency range. An open-fit, nonoccluding ear mold is crucial to minimize the occlusion effect, which can amplify the tinnitus percept.56

Total Masking Therapy

Total masking therapy is the use of sound with spectral characteristics and sufficient volume to render the tinnitus inaudible. This form of sound therapy has likely been used for centuries, and derives from empirical experience that certain environmental sounds are efficient tinnitus maskers. The clinical use of total masking therapy has a long history.59 A formal masking therapy program was first proposed by Vernon and Schleuning.60 Patients are fitted with ear-level devices that generate sound. The devices can be adjusted to have outputs with different frequency spectra and levels. There is no codified scheme for selecting effective masker characteristics. The general fitting principle is to determine the minimum level of broadband noise that masks the tinnitus without interfering with communication. The fitting is empirical because there is a wide range of patient preference in the type and level of sound that masks the tinnitus and is not perceived as annoying.61,62 Two significant benefits likely attend the successful application of total masking therapy: (1) Patients have control over their tinnitus, and (2) patients experience immediate relief with complete elimination of tinnitus.63

Benefits from masking therapy have been shown in a large prospective controlled study of chronic significant tinnitus in U.S. veterans.64 Validated standardized tinnitus severity questionnaires were used to screen and enroll 123 subjects with clinically significant tinnitus. Subjects were quasirandomly assigned to either the total masking group or a tinnitus retraining therapy (TRT) group. Subjects in the total masking group received hearing aids, maskers, or combination instruments, and all received information counseling over an 18-month follow-up period. Subjects in the TRT group received standardized TRT treatment (see later). All subjects were evaluated for response to treatment at 6, 12, and 18 months. Subjects in both treatment groups showed improvement on multiple measures of tinnitus over the course of the study. Subjects in the total masking group who rated their tinnitus as a “moderate” problem had effect sizes between 0.27 and 0.48 compared with effect sizes for the TRT group of 0.77 and 1.26 at 18 months. Mean effect sizes in subjects with tinnitus rated as a “very big problem” were 0.64 in the total masking group and 1.08 in the TRT group. Although both therapies resulted in clinically significant improvement in tinnitus with medium (>0.5) to large (>0.8) effect sizes, optimal outcomes were obtained with long-term TRT treatment.

Acoustic Stimulation Combined with Education and Counseling

TRT combines sound therapy with a formalized program of directive counseling to achieve habituation to tinnitus.65 TRT is based on the assumption that tinnitus distress derives from activation of an emotional and an autonomic response to tinnitus.66 Within this theoretical framework, tinnitus emerges as a result of damage or dysfunction within the auditory pathway, and is detected at subcortical levels of the brain. The critical event that leads to clinically significant tinnitus is not its sensory features, but rather the perception and evaluation of the tinnitus-related neural activity that occurs in the auditory cortex, and subsequent cortical interaction with the limbic system, prefrontal cortex, and cortical association areas.65 According to Jastreboff and Hazell,65 tinnitus becomes clinically significant when a negative affective response to the tinnitus has been established.

The goals of TRT are to remove, decrease, or change the perception of tinnitus by promoting habituation of the reactions to the tinnitus sensation. Habituation of the reaction to tinnitus would reduce the derived annoyance, anxiety, and stress. A key feature of TRT involves behavioral retraining of the associations induced by the tinnitus sensation. TRT uses five distinct levels of therapy, aimed at stratified patient groups. Stratification is based on tinnitus distress, associated hearing impairment, presence of sound-induced exacerbation of tinnitus, and comorbid hyperacusis (see later). The five strata are (1) patients with tinnitus that causes minimal distress (category 0); (2) patients with tinnitus causing distress (category 1); (3) patients with distressing tinnitus and hearing loss (category 2); (4) patients with distressing tinnitus, normal hearing, and hyperacusis (category 3); and (5) patients with distressing tinnitus, normal hearing, hyperacusis, and prolonged sound-induced exacerbation of tinnitus (category 4). Treatment is specific for each patient category, and uses directive counseling, auditory enrichment, hearing aids, and sound generators.

Jastreboff and Hazell65 described directive counseling as a teaching session in which patients are instructed on the mechanisms of tinnitus generation. Directive counseling demystifies tinnitus for the patient, and attempts to eliminate anxiety and fear of tinnitus through education. Auditory enrichment is typically achieved through the use of broadband noise generators. Ear-level devices physically similar in design to maskers are used, but sound output is distinctly lower than that used in masking therapy where sound levels render the tinnitus inaudible. In TRT, the volume of noise is set at a partial masking level, which allows continued perception of the tinnitus. The concept is that habituation would normally occur to the noise, much as one habituates to the background sound of a running air conditioner. Because the external noise and the internal tinnitus are equivalent in loudness, or “mixed,” habituation generalizes to the tinnitus and its attendant emotional reactions.

Retrospective clinical trials evaluating the efficacy of TRT have generally been positive with a consensus that TRT reduces the annoyance and impact of tinnitus within a time frame of 12 to 18 months. Sheldrake and associates67

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