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Hyperacusis is a rare disorder of loudness perception, where sounds that are ordinarily considered innocuous become intolerable.[rx] Patients may perceive this sensation as painful, frightening, unpleasant, or excessively loud.[rx] Hyperacusis often co-exists with tinnitus and can cause significant distress, with patients regularly reporting impairment in their social, occupational, and recreational activities.[rx] Avoiding sound sources and seeking medical attention are common behaviors.[rx]
Although a definitive cure is yet to be identified, research in this field is accelerating, and there has been a 10-fold increase in the number of peer-reviewed studies on the condition in the past four decades.[rx]
Measured in decibels (dB), loudness discomfort level (LDL) describes the point at which a subject perceives a sound as being uncomfortably loud. In patients with hyperacusis, LDL is decreased by 16–18 dB compared to the general population, in whom the average LDL is around 100 dB.[rx][rx][rx] Regardless of the degree of hearing loss, LDL in hyperacusis is decreased across the whole frequency spectrum, suggesting an increase in auditory gain as the underlying mechanism.[rx] Supported by experimental studies, the central auditory gain model proposes that a reduction in auditory input can lead to a dysfunctional increase in neuronal gain, leading to over-amplification of sound and the symptoms of hyperacusis.[rx][rx] This mechanism is also demonstrated through studies that induce temporary auditory deprivation through wearing earplugs. Even if only worn in one ear, there is a bilateral increase in perceived loudness.[rx] The plasticity of the central auditory system is, therefore, implicated in using sound therapy as a potential treatment for hyperacusis.
There are several other proposed mechanisms. The prevalence of hyperacusis in those suffering from Williams syndrome led to the theory that 5-hydroxytryptamine (5-HT) dysfunction increases auditory sensitivity, however, the high rate of otitis media in Williams syndrome may be more contributory.[rx][rx] Stapes hypermobility is also cited as a cause of peripheral hyperacusis, and conditions that involve paralysis of the facial nerve (i.e., Bell’s palsy, Ramsay-Hunt syndrome, and Lyme disease) are involved in the etiology of the condition.[rx] This cohort responds well to oval window reinforcement, which is reported to improve LDL and subjective symptoms.[rx]
Causes of Hyperacusis
Although there are many associations, a direct underlying cause for hyperacusis is rarely found.[rx] The most common cause of hyperacusis is high noise exposure. The association between hyperacusis and hearing loss is complicated. Occupational and recreational noise exposure and noise-induced hearing loss are commonly cited as major risk factors.[rx] Indeed, studies show that professional musicians are more likely to suffer from hyperacusis, especially those playing pop/rock music and exposing themselves to prolonged periods of amplified sound.[rx][rx][rx] However, in a multivariate analysis of 850 participants, no significant correlation was reported between hearing threshold loss and hyperacusis.[rx] We can reasonably conclude from this that hyperacusis often occurs in conjunction with hearing loss, but it is not essential in the development of the condition.
Those that suffer from hyperacusis are more likely to be comorbid. Psychiatric conditions, functional diseases (i.e. fibromyalgia, chronic fatigue syndrome), and joint and back disorders are particularly common.[rx] Fifty-six percent of patients referred to secondary care for hyperacusis met the criteria for at least one psychiatric illness, with 47% having an anxiety disorder.[rx] Concurrently suffering from tinnitus is remarkably common, with rates reported to be as high as 86%.[rx] Migraine, post-traumatic stress disorder, Lyme disease, and Williams syndrome are also implicated in the etiology of the disease, with up to 90% of those with Williams syndrome reporting hyperacusis symptoms.[rx][rx]
Diagnosis of Hyperacusis
The subjectivity of hyperacusis has given rise to several definitions in the literature, the simplest of which separates the presentation into four categories: loudness, annoyance, pain, and fear.[rx] Loudness hyperacusis is defined as the perception of moderately-intense sounds as uncomfortably loud. In annoyance hyperacusis, the chief complaint is that of a negative emotional reaction manifesting as irritability and anxiety. Fear hyperacusis is that which results in avoidance behaviors, and pain hyperacusis can present as a stabbing pain felt in the ear. Hyperacusis is almost exclusively a bilateral phenomenon. In patients with unilateral symptoms, one must consider an alternative cause, such as an acoustic shock leading to tonic tensor tympani syndrome.[rx][rx]
Phonophobia and misophonia present similarly (and indeed the terms are often used synonymously), however, an important distinction must be made between these conditions. Hyperacusis is sound sensitivity arising from within the auditory system, and therefore can be triggered by any generic, low-intensity sound.[rx] Contrarily, phonophobia, and misophonia are disorders associated with the limbic system, and there is no abnormality in the peripheral or central auditory system. Phonophobia is a psychiatric disease in which there is a fear of a specific sound, while misophonia is a psychological disease in which a specific sound triggers emotional and physical reactions.[rx]
A full neurotological examination is essential in detecting any associated or underlying causes of hyperacusis, some of which are reversible.[rx] Otoscopy and pneumatoscopy are important for assessing the mobility of the tympanic membrane, and cranial nerve examination can reveal facial nerve dysfunction.[rx] A detailed history will assess for any underlying psychiatric illness and also establish potential risk factors, including noise exposure and acoustic trauma.
The diagnosis of hyperacusis usually involves determining LDL using pure-tone audiometry and using questionnaires to determine disease severity.[rx] Many questionnaires exist, including the Geräuschüberempfindlichkeit (GÜF) and the Multiple Activity Scale for Hyperacusis (MASH). However, the most widely used is the hyperacusis questionnaire (HQ).[rx][rx] When the result of a test is considered alone, there is no agreed consensus on the diagnostic cut-off values for either the HQ or LDL, however, in combination, the results are more sensitive.[rx] Ninety-five percent of patients diagnosed with hyperacusis have LDL ≤ 77 dB (average LDL in non-hyperacusic patients is 100 dB) and an HQ score of ≥ 22.[rx] Unfortunately, these cut-offs only inform of the presence or absence of hyperacusis, and not on its impact on a patient’s life.[rx] Furthermore, it is important to ensure that the evaluation does not cause undue discomfort for the patient and a subsequent breakdown in clinical rapport.
Based on the history and physical examination, clinicians may decide to undertake further investigations if an alternative underlying cause is suspected, such as serological tests for Lyme disease or high-resolution computed tomography or magnetic resonance imaging of the brain for those cases with facial paralysis.[rx]
Treatment of Hyperacusis
Treatment for hyperacusis can broadly be categorized into those which target the physical symptoms, and those which aim to reduce the psychological burden of the condition.
Cognitive-behavioral therapy (CBT) is one of the most effective components of hyperacusis therapy alongside counseling and education.[rx] Through providing patients with the techniques required to manage the emotional reaction to sound, CBT has been shown to increase LDL and reduce hyperacusis severity as assessed by the HQ.[rx][rx] Directive counseling uses a similar approach of identifying and discussing repressed behaviors, although much of the literature is directed towards its use in tinnitus.[rx]
Tinnitus retraining therapy (TRT) involves educating the patient about their condition alongside gradual sound enrichment, and its use in hyperacusis is becoming increasingly popular.[rx] Prolonged low-level noise exposure has been shown to have a reversing effect on the enhanced neural gain, which is thought to be the underlying mechanism of hyperacusis.[rx] Significant improvements in LDL have been seen after 6 months of sound generator therapy. Increasing the mean level of the acoustic environment (i.e. greater auditory stimulation) has been reported to have a beneficial symptomatic effect.[rx][rx]
Surgery may be indicated in select cases, including those refractory to the above treatments or in conductive hyperacusis secondary to superior semi-circular canal dehiscence syndrome.[rx][rx] Round and oval window reinforcement is a simple, reversible procedure that has been shown to have a high success rate and a sustainable reduction in symptoms.[rx][rx][rx]
Alternative treatments often are given special attention for chronic pain and may include supplements and vitamins, acupuncture, exercise, yoga, meditation, massage, relaxation therapy, and hypnosis.